Flavoured Milk

i. Definition

Flavoured milk is milk to which some flavour has been added. When the ‘milk’ is used, the product should contain a milk fat percentage at least equal to the minimum legal requirement for market milk. But when the fat level is lower (1-2 per cent),the term ‘drink’ is used.

 

ii. Types of Flavoured Milk

The main types of flavoured milk are as follows:

•  chocolate milk/drink
•  fruit flavoured milk/drink
•  sterilized flavoured milk/drink

  iii. Preparation of Chocolate Milk Drink

The milk on receipt is standardized to 2% fat level for preparation of drink.Standardized milk is then pre-heated to 35-40OC and filtered; alternatively, after standardization it is pre-heated to 60OC, homogenized at 2500 psi and then clarified.To the warm milk, cocoa powder (1 to 1.5%), sugar (5 to 7%) and stabilizer(sodium alginate – 0.2%) are slowly added and stirred to dissolve them properly.The mixture is then pasteurized at 71OC/30 min., cooled rapidly to 5OC, bottled and kept under refrigeration (5OC) until used. The detailed flow diagram for the manufacture of chocolate milk/ drink is given below:

iv. Preparation of Fruit Flavoured Milk

The method of preparation of fruit flavoured milk is similar to that used for chocolate milk/drink. Instead of cocoa powder, permitted fruit flavours/essence, together with permitted (matching) colours and sugar are used. The common flavours used are strawberry, orange, lemon, pineapple, banana, vanilla, etc. In order to obtain good results, the following precautions should be taken:

•  No acid (citric or tartaric) should be added to the fruit syrup, as this may cause curdling of milk.
•  Excessive sweet syrup should be avoided. The best sugar content of the syrup is 45-55 per cent.
•  Add 1 part of fruit syrup to 5 parts of milk.
•  Care should be taken to see that there is a pleasant blend of sweet, fruity and milky flavours (together with an appealing colour)

v. Preparation of Sterilized Flavour Milk

These combine the advantages of both sterilized and flavoured milk/drinks. The method of preparation is given below:

The raw milk, upon receiving, should be strictly examined by the prescribed physico-chemical and bacteriological tests and only high quality milk should be used for production of sterilized milk. The incoming milk should be promptly cooled to 5OC for bulk storage in order to check any bacterial growth. Next, it should be pre-heated to 35-40OC for filtration, so as to remove visible dirt, etc. Flavour/ essence,permitted (matching) colour and sugar (syrup) are added to clarified milk and mixed well. The fruit flavoured milk is now filled in cleaned and sterilized bottles and then capped properly. The filled bottles are then sterilized at 108-111oC for 25-30 min.The sterilized milk bottles should be gradually cooled to room temperature. Finally,the sterilized milk is stored in a cool place.

Reconstituted Milk

i. Definition

Reconstituted milk refers to milk prepared by dispersing whole milk powder in water (approximately in the proportion of 1 part powder to 7-8 parts water). During the lean season, reconstituted milk is the main source of milk supply in cities.

 

ii. Advantages

•  It helps in making up the shortage of fresh milk supplies.
•  It is used by the military forces

 iii. Preparation

The calculated amount of potable water is received in pasteurization tank equipped with agitator. The water, is heated to 38-43oC and then calculated amount of spray dried whole milk is slowly added at the point of agitation. The mixture is thoroughly mixed, filtered and pasteurized at 63oC/30 min. and promptly cooled to 5oC or below until distribution. Detailed flow diagram of the process is given below:

Recombined Milk

i. Definition

Recombined milk refers to the product obtained when butter oil (also called dry/anhydrous milk fat), skim milk powder and water are combined in the correct proportion to yield fluid milk. The milk fat may also be obtained from other sources,such as unsalted butter or plastic cream. However, production of recombined milk is currently not in practice.

Under PFA rules, recombined milk should contain a minimum of 3.0 per cent fat and 8.5 per cent solids-not-fat throughout the country.

 

ii. Advantages

•  It helps in making up the shortage of fresh milk supplies in developing countries.
•  Helps prevent price rise of liquid milk in cities.

iii. Preparation

A stepwise process for preparation of recombined milk is given below:

A calculated amount of potable water is received in pasteurization tank and it is heated to a temperature of 38O-43OC, while the agitator is kept in motion. A proportionate amount of dried skim milk is slowly added at the point of agitation.When the water reaches a temperature of 43-49OC, proportionate amount of butter oil is added. The mixture is thoroughly mixed, filtered and pasteurized at 63OC for 30 min. It is then homogenized at 2500 psi pressure and cooled to 5OC.

The detailed flow diagram for manufacture of recombined milk is given below:

Skim Milk

i. Composition

The average percentage composition of skim milk is given in the following table:

ii. Utilization of Skim Milk for Making Different Dairy Products

Skim milk is a by-product of cream separation process. It is mainly used for standardization of milk and cream. The broad principles for utilization of skim milk,together with the names of commonly made dairy products is given in following table:

Standardized Milk

i. Definition

Standardized milk is a product, whose fat and/or solids-not-fat (SNF) content have been adjusted to a certain pre-determined level. Under the PFA Rules (1976), the standardized milk for liquid consumption should contain a minimum of 4.5% fat and 8.5% SNF throughout the country. The standardization can be done either by partially skimming the fat in the milk with a cream separator, or by admixture with fresh or reconstituted skim milk in proper proportions.

 

 ii. Advantages

Standardized milk offers several advantages such as:

•  It ensures a milk of practically uniform and constant composition and nutritive value to the consumers.
•  The surplus fat can be converted into butter and ghee.
•  It becomes possible to supply cheaper milk as compared to the full cream milk.
•  It is more easily digestible because of less fat content as compared to full cream milk.

iii. Preparation

The detailed step-wise method of manufacture of standardized milk is given below:

First of all milk should be received, and tested for fat and SNF levels. It is to be pre-heated to 35-40OC, followed by filtration/clarification. Milk should be standardized to 4.5% fat and 8.5% SNF levels after calculation of required quantity of skim milk or cream to be added. Upon standardization, milk should be homogenized (2500 psi/65OC) and then it must be pasteurized (72OC/15 sec). After pasteurization, milk must be packaged either in glass bottles or polypacks and then stored below 5OC till distribution. The detailed flow diagram for preparation of standardized milk is given below:

Toned and Double Toned Milk

i. Definition

Toned milk means the milk obtained by the addition of water and skim milk powder to whole milk. In practice, whole buffalo milk is admixed with reconstituted spray dried skim milk for its production.

Under the PFA Rules (1976), toned milk should contain a minimum of 3.0 per cent fat and 8.5 per cent solids-not-fat throughout the country, whereas double toned milk should contain a minimum of 1.5 per cent fat and 9.0 per cent solids-not-fat throughout India.

 

ii. History

Toned milk is the brainchild of D. N. Khurody (an Indian Dairy Pioneer), who is also credited with coining its name. Under his auspices, it was first produced in 1946 in the Central Dairy of the Aarey Milk Colony and marketed in Bombay city.Soon other cities, notably Calcutta, Madras and Delhi started producing and marketing toned milk.

 

iii. Preparation

The calculated amount of potable water is received in the pasteurizing vat/tank equipped with an agitator. The water is heated while the agitator is kept in motion to 38 – 43OC. Then a proportionate amount of spray dried skim milk is slowly added at the point of agitation and the mixture thoroughly agitated till it dissolves completely.A calculated amount of whole buffalo milk is now added and the mixture again agitated thoroughly till a homogenous mixture is obtained. The mixture is then filtered, pasteurized at 63OC for 30 min, rapidly cooled to 5OC, packaged and kept at 5OC or below until distribution. The detailed flow diagram for manufacture of toned and double toned milk is given below:

Full Cream Milk

i. Definition and Standards

Full cream milk means milk, or a combination of cow or buffalo milk or a product which has been prepared by a combination of both, which has been standardized to fat percentage of 6.0 and solids-not-fat (SNF) percentage of 9.0 by adjustment or addition of milk solids. Full cream milk must be pasteurized. It should show a negative phosphatase test. Upon pasteurization, it should be packaged in clean and sanitary containers and should be properly sealed in order to prevent subsequent contamination.

Preparation of Desinated and Special Milk

When natural constituents of whole milks have been altered by addition, removal,exchange and/or treatment, the resultant milk is designated as special milk. Recent years have witnessed a large increase in the market penetration of special types of milk into the total fluid milk market. In India, there is great seasonal fluctuation in the production of milk on account of which many milk plants have to run below their installed capacity, particularly during the lean season. Besides, the cost of whole milk generally remains very high throughout the year. Production of recombined milk and low fat toned milk have greatly helped in extending the market supply and reducing the cost of milk to the consumers. The machinery and manpower of a market milk plant can be fully utilized all the year round by such diversifications.

Aseptic Packaging

Aseptic packaging can be defined as the process in which UHT processed or sterilized milk is filled in pre-sterilized containers under aseptic/sterile environment.This ensures that there is no post processing contamination of the milk so that the product has longer shelf life. Since aseptic packaging systems are complex, great care is needed to prevent contamination. Before the start of product packaging, trial runs are routinely conducted with sterile water. Critical parts of the filling machine and carton forming systems are thoroughly checked. The seal integrity of the package and overall microbial quality of the packaging material are monitored properly. Generally, for a good processing plant permissible spoilage rate is one in every 5000 sterilized, filled and sealed package of one litre carton.

 

i. Types of Sterilizing Medium

Sterilizing mediums to be used in aseptic packaging systems could be broadly classified under two categories: physical sterilization mediums and chemical sterilization mediums.

Physical sterilization mediums: Steam under pressure or hot water is the most simple and reliable sterilant for high sterilization efficiency in short time. In aseptic packaging, its use is however restricted to sterilization of the milk tubes and valve and fittings coming into product contact.

Dry heat/ super heated steam: Hot air is generally used to sterilize the closed space where the filling of milk takes place. Air heated to 300OC may be taken to the areas surrounding electric resistances used for sealing the packages. Dry air at 330-350OC is also used for sterilizing the milk filling tubes. Sterilized air (180-200OC) is used for evaporating residual H2O2 (chemical sterilant) from the package.

Ultraviolet radiation: UV rays (optimum wavelength 250 nm) alone are not a very effective sterilizing medium for aseptic packaging units. Two major reasons for this are: (i) intensity of radiation is not uniform on the entire package surface(ii) bacteria adhering to packages could be protected by dirt/ dust particles present on the surface. UV radiations are therefore used as a complementary sterilizing medium.

Ionizing radiations: Gamma rays are often used for sterilizing packaging materials unable to withstand high temperature. Usually 2.5 Mrad intensity is suitable for sterilizing plastic laminates used in aseptic bag-in-box.

 

ii. Chemical Sterilization Mediums

Ethylene oxide: Ethylene oxide has slow sporicidal action. It is sometimes used as a pre-sterilization agent to reduce microbial load on packaging films so that a shorter time is required for final sterilization.Hydrogen Peroxide (H2O2): H2O2 has poor sporicidal effect at room temperature. However, with increasing application temperature and concentration, sterilization performance improves. H2O2 is the most popular sterilant for aseptic packaging system. H2O2 is applied on the package surface by either dipping or spraying. As its boiling temperature is slightly above 100OC, supply of heat by either sterilized hot air or infrared elements can evaporate the residual H2O2 from the package surface. Thus there is little H2O2 left for contaminating the product. Safety regulation recommended by IDF requires that atmospheric concentration of H2O2 in the packaging hall should not exceed 1 ppm. Further more, residual concentration in milk immediately after filling should not exceed 100 ppb and should reduce to 1 ppb within 24 hours. The most successful combination of sterilizing medium being used in commercial aseptic packaging units are H2O2 coupled with heat supplied by radiant heating element. Some packaging systems also use a combination of H2O2 and UV radiation.

Other sterilizing agents which are rarely used in such applications are sodium hypochlorite and per acetic acid. These agents leave the residues of chloride and acetic acid on the package, which may finally contaminate the product.

 

ii. Type of Packaging Materials

Metal container: Cans made of tin plate or drawn aluminium are generally used for packaging of condensed milk, viscous liquids and chunk-in-gravy type of products.These are expensive and unsuitable for low cost products like liquid milk. They are bulky and require large storage and shipment space. The empty cans are carried in a conveyor to a tunnel for sterilization with steam super heated with gas flame at atmospheric pressure and require about 40-45s. The cans then move to filling chamber for product filling. The can lids are separately sterilized, placed on the cans and seams sealed. The can sterilizing, filling and sealing zones are sterilized before the filling begins with the same mixture of superheated steam and flue gas,which fills them during operation. Cans have been used for in-package sterilization for a long time. Manufacturers of UHT milk who want to impress the consumers with the advantages of the new technology therefore do not prefer to use cans which are so identified with a old technology

Laminates/cartons: Different layers of flexible films of different materials viz.paper, polyethylene and aluminum foil are co-extruded to form a laminate. These materials have specific properties viz. water vapour transmission, burst strength,etc; and hence when co-extruded form an ideal packaging film. Such laminates could be 3, 4 or 5 ply and are generally used for products like, milk, cream, fruit juices, soups, etc. These laminates are supplied as film rolls, which can be mounted on FFS (form-fill-seal) machines. Alternatively, cartons made of laminates are supplied as preformed blanks, which are assembled into cartons for filling and sealing at the top. Plastic films: Black and transparent polyethylene films are co-extruded for packaging of UHT processed milk intended for 2-3 weeks shelf life. The co-extruded film protects the product against light but not oxygen. The packaging machines also need to operate at not more than 45-50OC filling environment. A co-extrusion of polyvinylidine chloride (PVDC) or ethylene vinyl alcohol (EVOH)with black or white polyethylene film is also used as packaging film. Such a combination imparts protection against oxygen as well as light and shelf life of milk can be extended upto 3 months.

Other forms of packaging materials : Preformed packages of different shapes and sizes are also used for aseptic packaging of value added dairy products. Blow-moulded plastic bottles of polyethylene or polypropylene are used as cheap substitutes.However, these are transparent and permeable to oxygen. Multilayer materials with better light and oxygen barrier properties have also been developed. Pre-formed plastic cups of polypropylene (PP) or polystyrene (PS) are now gaining popularity.Bulk filling bags are made of laminates of 3 or 4 layers of which one will be barrier material such as metalized polyester (polyester with a coating of aluminium particles) or ethyl vinyl alcohol (EVOH). The bag with filling valve is sterilized by r-radiation(2.5 Mrad dose) before shipping. Bags remain sealed and internal surface therefore remains sterile. At the filling station, the sterilized bags are opened, filled and sealed under aseptic condition. All product contact surfaces in the filler however need to be sterilized with steam before the filling operation begins.

 

iii. Description of the Packaging System

Most of the aseptic packaging machines being used in the country are of form-fill-seal (FFS) type. Packaging material used generally is laminate of polyethylene –paper – polyethylene – Al foil – polyethylene. Packaging film in the form of a roll is mounted on the packaging machine. The film moves continuously downward in the form of a strip and a shaping roll gives it a cylindrical shape. Heat sealing forms an overlapping longitudinal seal. Simultaneously extra polythene strip is heat bonded along inside of longitudinal seam. This is done to prevent filled product penetrating the paper layer. As this continuous cylinder moves downward, jaws at the bottom make transverse heat seal. The product is filled instantly and another jaw seals the package at the top. Depending on the type of machine, different shapes can be given to the package. The most popular is brick shaped package. Tetrahedron shapes were also being used some times back. Some new innovations that are now being used for packaging of fruit juices are Fino packs. To cut down on costs some dairies have introduced pillow packs for packaging of milk.

UTH Processing

i. Definition

UHT milk can be defined as a product obtained by heating milk in a continuous flow to a temperature in excess of 125oC for not less than two seconds and immediately packaging in sterile packages under aseptic conditions. In India, UHT milk is generally processed at 140oC for 2 seconds.

 

ii. Theoretical Basis

Heating of milk results in death of microorganisms. While some bacteria are destroyed by pasteurization (71.7OC/15 s) only, some survive this thermal treatment.Bacillus subtilis and Bacillus stearothermophillus spores are very heat resistant.Of the two, Bacillus stearothermophillus spores are most heat resistant. It is therefore, considered index organism for evaluating performance of UHT processing.

Heating of milk at higher temperatures also result in undesirable changes in chemical quality. Browning reactions are particularly important. Higher thermal load results in more browning and therefore loss of flavour and quality. In the temperature range of 100-120oC, time required for death of almost all B. stearothermophillus spores are more. This may therefore result in more browning in the product.However, if milk is treated in the UHT range i.e. 135-150oC for only few seconds,almost all spores may get killed and browning would be minimum. Loss of nutrients and total quality also will be minimum. A product processed in this temperature range will be thus microbiologically safe and yet superior in terms of overall quality.

 

iii. Types of Sterilization Plants

There are two types of UHT plants: Direct type and Indirect type. In direct type plants, heating is done by mixing product and steam. In indirect type plant, product is heated by steam or hot water without the two coming in direct contact. Heating in direct type plant is very rapid particularly between 80-140oC and total heat load is less. Changes in the product quality are therefore minimum. In indirect plant, rise in temperature is very gradual. Therefore, heat load on the product is more. Changes in chemical quality are comparatively more in indirect type than in direct type plants.

(i) Direct Heating Plant: There are two types of direct heating plants (a) Injection type and (b) Infusion type.

Injection type: Processing is through steam-into-milk arrangement. Steam injector is the heart of this plant. Preheated milk at 80-90OC enters the injector nozzles from one side. Steam at slightly higher pressure enters the injector from the other side.As the steam mixes with milk, steam condenses and the product is rapidly heated.Rapid condensation of steam prevents entry of air in holding tube. Air in holding tubes results in improper heating. Backpressure is maintained on the discharge side.Backpressure ensures that product does not boil in holding tube. Boiling may result in fouling and improper heating of milk. Several designs of injector are available.

Infusion type: In this system, milk is heated by milk-into-steam arrangement. The processing unit consists of a chamber filled with pressurized steam. Milk enters the chamber from the top. There are two alternative arrangements for distribution of milk. In the first type, milk flows to a hemispherical bowl with loose circular disc closing the top. When the bowl is full, milk overflows and falls in droplets through the steam environment. In an alternative arrangement, milk flows through a series of parallel and horizontal distribution tubes. These tubes have slits along the bottom and milk flows like a thin film through the chamber. As milk reaches the bottom of the chamber, it is heated to desired temperature. This system is particularly suitable for thicker liquids and for liquids suspended with smaller chunks.

Advantages and Disadvantages of Direct Heating System: During processing in direct type heating systems condensing of steam coming into product contact results in dilution of the product. To remove this excess of water from the product,cooling is done in an expansion cooling vessel. In expansion vessel, along with the evaporating water incondensable gases and undesirable flavour volatiles produced during heating are also removed. The product therefore tastes better. Steam injection induces formation of casein aggregates, which give “chalky” or “astringent” mouthfeel to the product. Aseptic homogenizer, which can safely homogenize the product after final heating section, is generally preferred with direct heating systems to overcome such defects in the product.

Rate of heating is very high (takes less than 1 sec to attain sterilization temperature).Thick/viscous liquid can also be easily processed. Deposit formation is minimum,hence plant can be operated for longer time without cleaning. Undesirable flavours are removed during flash cooling. Oxygen is removed during cooling, hence oxidized flavour defects are delayed during storage.

Cost of processing per unit volume of milk is high. Requires additional equipment(vacuum expansion chamber and aseptic homogenizer) – cost of plant is twice that of indirect type plant. Heat energy requirement is very high. Water and electricity(25-50% more than in direct type) consumption are high. Requires culinary steam and hence special boiler. Creates greater noise during operation.
(ii) Indirect Type Heating System: There are three types of indirect heating systems: (a) Plate heat exchangers (b) Tubular heat exchanger (c) Scraped surface heat exchanger.

Plate heat exchanger: This resembles plate heat exchanger of HTST plants.Several rectangular stainless steel plates with corrugations are arranged in sequence.These plates are then mechanically tightened to hold together. Corrugations on the plates induce turbulence and therefore result in high heat transfer. High temperature processing generates high internal pressure. The gaskets are therefore made of heat resistant materials such as medium nitrile rubber or resin cured butyl rubber. A major advantage of this plant is therefore simple design and comparatively less cost. If deposit formation is more, plates can be removed and manually cleaned.

Tubular heat exchanger: There are two types of tubular heat exchangers – (a)concentric tube, (b) shell and tube type. Concentric tube type heat exchangers comprise two or three stainless steel tube lengths put one inside another. Spacer is placed in each inner tube space to maintain them concentric. Several such multiple tubes are bound together and placed into an outer cylindrical housing. Two tube heat exchangers are used for simple cooling and heating. In triple tube heat exchanger, available heat transfer area is doubled. It is generally used in final cooling section. It is also suitable for processing of thick liquids, which generally reduces heat transfer rate. Product flows through the middle annular space. Heating or cooling medium passes through inner tube and outer annular space. In shell and tube type heat exchangers, 5-7 straight lengths of smaller tubes (10-15 mm internal diameter) are assembled in an outer tube. The smaller tubes are connected to large outer tube at both ends by a manifold. Product passes through the smaller tubes.Heating or cooling medium passes through the space around them in a counter current flow. Tubular heat exchangers are mechanically very strong and can withstand even very high internal pressure generated during homogenization (200-300 bar). Therefore the need for acquiring an aseptic homogenizer to be placed after heating section is totally eliminated. Instead, the high pressure reciprocating pump of an ordinary homogenizer can be placed before the sterile section. The homogenizing valve can be put at any point on the downstream side (even after final heating section). The problem of product contamination arises from the homogenization pump and not the valve. Therefore, with tubular heat exchangers,the product can be homogenized before sterilization, after sterilization or on both the occasions. Fat rich products like cream require homogenization after final heating to prevent re-association of fat globules due to high temperature processing after homogenization.

Scraped Surface Heat Exchanger (SSWE): It is a very specialized type of heat exchanger. It consists of a jacketed cylinder. A shaft passes along the axis of the cylinder. The shaft is supported by bearings at both ends of the cylinder. The shaft also carries several scrapper blades. As shaft rotates, scrapper blades provide turbulence and physically remove the product from the surface of the wall. The colder product subsequently replaces the heated product and the cycle continues.SSHE is used only for heating very thick liquids. SSHE units are very expensive and have poor energy conversion efficiency. The cost of processing is therefore very high.

Advantages and Disadvantages of Indirect Heating System: It is simple in design and requires less pumps and controls. It can regenerate 90% of the thermal energy requirement. It does not require aseptic homogenizer, which is very costly.It does not require culinary steam and therefore special type of boiler. The indirect type plant is less noisy. It requires low initial capital and operational cost is also comparatively less.

In indirect type heat exchanger, rate of heat transfer is low. More heat load results in less acceptable product quality. Deposit formation is more and therefore plant requires frequent cleaning. For removal of dissolved oxygen from milk, additional equipment ‘deaerator’ is required.

 

iv. Changes in Milk during Processing

UHT processing does not cause reduction in biological value of proteins. There is only small loss of available lysine (6-7%). UHT processing changes the casein micelle structure. This slows rennet action during cheese manufacture. Serum proteins are denatured (direct processing – upto 50-75%, indirect processing upto 70-90%). Denatured serum proteins interact with casein and increase casein micelle size. This reflects more light and UHT milk appears whiter. Aggregates of denatured serum proteins and casein also give ‘chalky’ mouth feel to the product.There is no physical or chemical change in milk fat. The total mineral content also does not change during UHT processing. The vitamin content of UHT milk is comparable to pasteurized milk. Losses in B-complex vitamins are not more than 10%. Folic acid and ascorbic acid are destroyed up to 15% and 25%, respectively.Fat soluble vitamins A, D, E and K are not affected by UHT processing. Fresh UHT milk has slightly cooked flavour. The cooked flavour is due to oxidotion of the SH (Sulphydryl) groups from the denatured serum proteins.

 

v. Changes in UHT Milk during Storage

Chemical, physical or sensory changes in stored UHT milk are dependent on storage temperature. Changes are rapid if storage temperature exceeds 30OC.Browning reactions between protein and lactose progress during storage. At higher storage temperature (>30OC) UHT milk may become little brown after 3-4 months.Refrigerated storage of raw milk before UHT processing favours growth of psychrotrophs. They liberate heat resistant proteases and lipases. Proteases that survive UHT treatment act on proteins during storage. Bitter peptides are released causing bitterness in the product. Extensive proteolysis and other physico-chemical changes occurring as a result of interaction of proteins and salts during storage may cause thickening or sweet curdling also referred as age thickening after longer storage (more than 6 months). Lipases surviving ultra-high-temperature treatment act on lipid fraction. Short and medium chain free fatty acids are released. Short chain fatty acids particularly butyric acids contribute to development of rancid flavour in the product. Air in the product or in the packet reacts with unsaturated fatty acids. This auto oxidation reaction causes formation of aldehydes and ketones.These compounds cause oxidative rancidity (flavour defect) in the product. The cooked flavour in UHT milk disappears in first few days and milk tastes best after this period. Few weeks after this, depending on the temperature of storage, oxidized flavour defects appear which becomes more pronounced with progressive storage.In milk stored for considerable period of time, which could be 3-4 months at >30OC,stale flavour is a common defect. Several compounds that form during the progress of Maillard reactions in stored milk are associated with the appearance of this defect. Sometimes coconut like flavour defect also appears in UHT milk stored for longer period. Compounds such as d–dodelactone and d–dodecalactones are responsible for this.

Sterilization

i. Definition

Sterilized milk refers to a product obtained by heating milk in a container in a commercial cooker/ retort to temperatures of 110-130OC for 10-30 min. The process is also referred as in-container sterilization. Sterilized milk is generally intended for prolonged storage at room temperature (up to 6 months). The major objective of heat sterilization is to destroy microbial and enzymatic activity. The length of time and magnitude of temperature employed during processing depend on the type of the product, number and heat resistance of microorganisms and enzymes present in milk. The heat resistance of microorganisms or enzymes is generally evaluated in terms of D-value or Z-value. Sterilization load or heat load for sterilization is generally expressed in terms of Fo value.

 

ii. Theoretical Basis

Clostridium botulinum is considered as the index organism for assessing thermal sterility in foods. Under anaerobic conditions, inside a sealed container, it can produce botulin, a toxin, which can be 65% fatal to humans. Therefore, destruction of this organism is a minimum requirement of heat sterilization. As milk is a low acid (pH>4.5) food, it is recommended to achieve 12 decimal reductions for C.botulinum. This can be achieved by heating the product at 121OC for 3 min(Fo = 3). However, this minimum treatment may produce milk that is safe but not necessarily commercially sterile. This is so because there are more heat-resistant spores present in milk. There is B. stearothermophilus or B. sporothermodurans.These spores are not pathogenic. Their presence may require heat treatment equivalent to two (2) or more decimal reductions. This may correspond to an F0 value of 8.Target spoilage rates should be less than one survivor in every 10,000 containers.

 

iii. Types of Sterilization Plants

Sterilizing retorts are either batch type or continuous in operation. Batch type sterilizers may be either vertical or horizontal. Horizontal retorts are easier to load or unload. They have facilities for agitating containers/cages. However, they require more floor space. Typically such horizontal retorts contain concentric cages. Cans are loaded horizontally into the annular space between the cages. When cages are full, the retort is sealed. The cages are supported by guide rails, which slowly rotate them. This stirring of the contents in cans facilitate proper heating. Continuous retorts are generally equipped with better controls. They cause very gradual change in pressure inside the cans. Thus products are heated more uniformly. Can seams are also subjected to less strain in comparison to batch process.

Continuous sterilizers: They are mainly of three types: (a) cooker-coolers; (b)hydrostatic sterilizers; and (c) rotary sterilizers. Cooker-coolers carry cans on a conveyor which pass through three sections of a tunnel. These sections are maintained at different pressures for preheating, sterilization and cooling. The hydrostatic sterilizer consists of a chamber equipped with provision for steam injection. The chamber that is partially full of water is connected to two water columns (12 to 18 meter tall, barometric leg) which are used to adjust pressure in the chamber. If the height of the water columns is changed, the steam pressure is changed and therefore the maximum attainable temperature changes. For example,to get a temperature of 116oC, a difference in height between the two water columns should be 10.7 m while for attaining 121oC temperature in the chamber,the water column difference should be 13.7 m. A conveyor with provision to accommodate cans of different sizes moves through the steam chamber carrying the food cans. The heating time could be regulated by varying the speed of the conveyor. Hydrostatic sterilizers are very flexible and suitable for large capacity plants. However, size of the structure and high capital costs are the major disadvantages of this system.

Continuous rotary sterilizer consists of several horizontal inter linked cylinders which allow for preheating, heating, precooling and cooling in upto four continuous stages.The vessel has a spiral track on the inner wall. A spoke or reel within the centre of the cooker causes the cans to roll along the spiral track. Rotary valves used to interconnect the shells, maintain pressure in the heating and cooling sections. Sealed cans are introduced directly from the sealing machines. The contents inside the cans are mixed as cans travel along the helix and therefore enhance heat transfer and ensure less heat damage to the product. Cans coming out of the cooker are directly taken to labelling and palletizing machine. Rotary sterilizers are particularly suitable for processing of milk and milk based products, which are extremely heat sensitive and susceptible to browning.

 

iv. Description of the Canning Process

Basic operations in conventional retorting/canning process include: preparation of the raw material, filling of the container, exhausting, sealing of container, sterilization,cooling of the cans, labelling and storage.The preparation of raw materials refers to washing, peeling, cutting, blanching, pre-cooking, etc. in case of fruits, vegetables, meat, etc; and preheating, mixing, homogenization, etc; in case of milk. Filling of containers can be carried out either manually or mechanically. Correct and accurate filling is important from economic standpoint as well as for prevention of entrapment of large volume of air/ gas inside the can, which might decrease the intensity of heat treatment. Exhausting is an essential operation in the canning process and involves removal of air/ oxygen from the container before it is closed. Removal of air ensures minimum of strain on the can seams or pouch seals through expansion of air during heat processing. Removal of oxygen is essential to prevent internal corrosion of the container through oxidation and creation of vacuum inside the container while cooling. Absence of oxygen inside the container also delays oxidative deterioration of the product besides destruction of ascorbic acid.

After exhaustion, containers are sealed. Depending on the type of containers (metal cans, glass bottles, flexible pouches); sealing machines are chosen. Glass jars are normally vacuum-sealed while tins are closed with a double sealing on the seal side and may also be vacuum-sealed. Flexible retortable pouches are sealed by fusion of two thermoplastic materials through application of heat by heated plates or jaws.

Product in the closed containers is heated in the sterilizer in an atmosphere of saturated steam or hot water or air-steam mixture. The sterilizing action of steam depends on its latent heat of vaporization as it condenses on the surface of the can.Saturated steam condenses readily and is therefore an efficient sterilizing medium.Displacement of all air present in the retort by steam before the sterilizer is brought to operating temperature is a very essential step. This is also known as venting.The purpose of this processing step is to maintain uniform steam-air mixture in the sterilizer and prevent under processing. Sterilization temperature – time combination in retorts may vary from 110 – 130OC for 10-30 min. Sterilized containers are then cooled and brought to room temperature for labelling and storage. Turbidity test developed by Aschaffenburg is conducted to ensure sterility of the product. This is an indirect test and it measures denatured whey proteins. Complete denaturation indicates that the milk is adequately sterilized.

 

v. Quality of Sterilized Milk

Sterilized milk has a rich creamy appearance and a distinct cooked flavour (rich,nutty, caramelized). It is considerably browner in colour than raw milk. The brown colour develops due to formation of coloured pigments resulting from interactions between free amino groups of proteins and aldehyde group of lactose through Maillard reactions. The intensity of cooked flavour and brown colour depends upon the severity of heat treatment. In-container sterilization causes loss of nearly half of the ascorbic acid (Vitamin C) and sizeable loss of thiamine (30-40%). Vitamin B12 is almost completely destroyed. Fat soluble vitamin A, carotene, riboflavin and nicotinic acid are not affected. Biological value of proteins is only marginally affected.Sterilized milk cannot be coagulated with rennet unless calcium chloride is added externally.

Sterilization and Ultra High Temperature Processing

We know milk is a highly perishable commodity. Its myriad nutrients makes it extremely favourable medium for the growth of microorganisms. It is, therefore,essential that milk is subjected to certain processing treatments for enhancing its keeping quality and ensuring safety to consumers.

Thermal processing is the most prevalent preservation process employed in the dairy and food industry. Starting from pasteurization, which is a mild heat processing technology, in-bottle/in-package sterilization emerged as a means of extending shelf life of milk for several weeks at room temperature. Considerable changes in nutritional and sensory quality due to severity of heat treatment in this process, restrict its application to only special milks. Ultra-high-temperature processing, a relatively new processing know-how, became popular as it uses very high temperature (140OC) for short time (2 s) to sterilize milk. Such a time-temperature combination ensures minimal change in the product quality. Sterilized milk is then packaged in sterile container under aseptic conditions to prevent post-processing contamination. The product thus obtained has very long storage life.

Influence of Process Variables on the Processing Efficency and Product Quality

i. Factors Affecting Homogenization Efficiency

Type of Homogenizing Valves: Design of homogenizer valve affects homogenization efficiency. Grooved valves require less homogenization pressure to attain same degree of homogenization pressures as compared to either simple valve with flat seat or needle valve.

Homogenization Pressure: The recommended pressure ranges for homogenization of milk is 140-175 kg/cm2. If the homogenizer is in perfect working condition i.e.the homogenizer valves are not worn out and are well seated, a homogenizer pressure of 175 kg/cm2 should give good homogenization efficiency. Some modern valves may, however, give satisfactory performance at lower homogenization pressure as well. Higher pressure of homogenization however does not improve the efficiency any further.

Single or Two Stage Homogenization: Two stage homogenization is often recommended because broken fat globules after first stage homogenization (175 kg/cm2) may have a tendency to agglomerate. In order to re-disperse them,homogenization at reduced pressure (35 kg/cm2) may be thus necessary in the second stage. A homogenization process of two or more stages does not however affect the mean particle size of the fat globules in any significant way. Modern homogenizer designs permit two stage homogenization with a single machine.Effect of Fat Content in Milk: Homogenization becomes less effective with increasing fat content. When high fat milk is homogenized, the newly created total fat globule surface becomes so large that materials required to form new membranes for all the fat globules is not sufficiently availably in the serum phase. Thus the newly formed fat globules may have a tendency to agglomerate and rise to the surface during storage.

Effect of Temperature of Homogenization: Milk can be homogenized over a wide range of temperature provided the homogenization temperature is above the melting point of milk fat (32OC). However, a temperature in excess of 50OC is often recommended which is necessary to inactive nature lipases. It lipase is not inactivated;it acts as a surface active agent and becomes incorporated into the newly formed membranes thereby causing hydrolytic rancidity in the product. Raw milk is therefore not to be homogenized. The recommended temperatures for attaining high degree of homogenization (80-90%) are therefore between 60 and 70 OC. Higher homogenization temperatures are also recommended for high fat milk. This is so because at higher temperatures, less protein is adsorbed during the formation of a new fat globule membrane. Furthermore, the membranes are formed more rapidly and thus the tendency of the fat globules to agglomerate is significantly reduced.

 

ii. Effect of Homogenization on Physico-Chemical Properties of Milk

Effect on Fat: Homogenized milk drains more freely out of the glass container leaving less milk sticking to the sides. This lack of adhesion is attributed to the reduction in size of the fat globules and the protection provided to these globules by the adsorption of higher proportion of casein. Homogenized milk with normal fat content does not have marked clustering of fat globules. This lack of clustering is attributed to:

•  destruction of natural agglutinin of milk during homogenization.
•  resurfacing of the fat globules.
•  increased brownian movement resulting from greatly increased number of fat globules.

Proper homogenization however, does not cause any change in important fat constants or physico-chemical properties.

Effect on Protein: The fat globule membrane is composed of approximately 1/3 phospholipids and 2/3 protein. The membrane acts as an emulsifier to keep the emulsion stable. During homogenization, the original membrane is destroyed and the surface active agents in the serum phase get adsorbed on the fat globules to form a new membrane. The new membrane consists mainly of casein as well as serum proteins. While only 2% casein in milk is adsorbed on the fat globules in un-homogenized milk, in homogenized milk almost 25% of casein is adsorbed as part of fat globule membrane. Homogenization is often associated with destabilization of proteins. This destabilization effect is reflected in reduced alcohol stability, increased feathering of cream in coffee and in coagulation during the manufacture of evaporated milk. This destabilization effect is partly attributed to adsorption of citrates and phosphates on the newly formed fat globule membrane, which lowers their concentration in the serum phase thereby adversely affecting the protein stability.

Colour of Milk: Homogenization results in more uniform, opaque and whiter milk which make the product more acceptable to the consumers. The increased whitening is due to the increase in number and total surface area of fat globules, which reflect and scatter more light.

Emulsion Stability: It is practically not possible to churn homogenized milk.However, with increasing fat content, the emulsion stability decreases.

Curd Tension: Homogenized milk has greater tendency to form coagulum and requires less coagulating agent. The resultant coagulum has lower curd tension and a soft, spongy body. Homogenization at recommended pressure of 175 kg/cm2 causes the curd tension to be lowered by more than 50%. The possible reason for this effect of homogenization on curd tension is attributed to the increase in the number of fat globules, which serve as the points of weakness in the coagulum.Further, nearly 25% of the casein get adsorbed on the fat globules during the formation of new fat globule membranes as against only 2% of the total casein adsorbed on the surface of the fat globules in un-homogenized milk. This results in lower casein concentration in the serum phase thereby lowering the curd tension.Fat losses in the cheese whey are however low as the finely divided fat globules are retained in the curd due to adsorption of casein micelles on their surface.

Viscosity: Single stage homogenization causes increase in viscosity. This is brought about by formation of fat clusters, which results from membranes of newly formed fat globules joining together although fat itself is not in contact. When the milk is subjected to second stage homogenization, the fat globule clusters are disintegrated/broken down resulting in decrease in viscosity. The degree of clustering of the fat globules is directly proportional to the viscosity. A high fat content, a high homogenization pressure and a low homogenization temperature can significantly increase the fat clustering and hence the viscosity of milk. Preheating of milk at temperatures that promote whey protein denaturation also reduces membrane formation and hence increases agglomeration of fat globules.

 

iii. Problems/ Defects Associated with Homogenized Milk

 

Curdling During Cooking/Sterilization: Homogenized milk is some times more susceptible to curdling when it is used in certain food preparations requiring cooking.This is in part related to reduced protein stability of homogenized milk as also to the seasoning salts added as an ingredient in the new food formulation.

Recovery of Fat During Centrifugal Separation: Milk fat is difficult to separate from the homogenized milk. If the milk has been homogenized at the generally accepted homogenization pressure of 175 kg/cm2, a significant portion (50-90%) of the fat remain in the skim milk after centrifugal separation. Even addition of homogenized milk with un-homogenized milk and then centrifugal separation does not yield a satisfactory result. The recovery of fat from homogenized milk is a serious problem for commercial dairies which receive significant quantities of processed milk as ‘returns’ from the market and need to salvage fat for economic operation of the plant.

Formation of Cream Plug: Appearance of scum or buttery particles on the surface of the homogenized milk is objectionable. Sometimes, fat rising in homogenized milk is to such an extent that a compact ring of creamy material is visible under the container closure often referred as cream plug. Several factors such as worn out or poorly maintained homogenizer valve, improper homogenization pressure, excessive foaming, improper cleaning of processing lines and failure to recycle the first few liters of milk coming out of the homogenizer lead to such defects in the product.

Sedimentation: Appearance of sediments in homogenized milk upon storage could be a serious problem. This defect is often ascribed to settling of the extraneous matters such as body cells and dirt as also to destabilization of proteins during homogenization. However, clarification of milk before homogenization reduces the amount of deposits significantly whereas clarification after homogenization prevents this defect entirely.

Foaming: Though not a serious problem, excessive foaming in homogenized milk poses handling difficulties. The two possible reasons for this could be inclusion of air as a result of splashing or excessive agitation of the homogenized milk or homogenizing the air into the milk during processing. However, improving the handling procedure during homogenization can largely eliminate this problem.

Flavour Defects of Homogenized Milk: The most important flavour defect associated with homogenized milk is ‘sunlight flavour’, sometimes referred as ‘tallowiness”, ‘burnt like’ or ‘activated flavour’. This develops due to oxidation of free methionine and formation of free SH compounds from sulfur containing amino acids. Development of ‘sunlight flavour’ also requires riboflavin. Probably, all of these compounds are together responsible for ‘sunlight flavour’. The possible reasons for sensitivity of homogenized milk for development of these flavour defects could be the effect of the light upon the increased protein surface following homogenization.Homogenized milk is however resistant to development of oxidized flavour defect.This could be attributed to the formation of new fat globule membranes resulting in ‘dilution’ of catalytic metals viz., copper and iron, which are concentrated in the native MFGM, thereby minimizing direct contact between the fat and the metal ions.

Homogenization: Theories and Process Description

i. Definition of Homogenized Milk

United State Public Health Service has proposed one of the most comprehensive definitions for homogenized milk. This has been the most widely accepted and referred definition. It states that “Homogenized milk is milk which has been treated in such manner as to ensure break-up of the fat globules to such an extent that after 48 hours of quiescent storage no visible cream separation occurs in the milk and the fat percentage of the milk in the top 100 ml of milk in a quart bottle (946ml),or of the proportionate volumes in containers of other sizes, does not differ by more than 10 per cent of itself from the fat percentage of the remaining milk as determined after thorough mixing.

 

ii. Theories of Homogenization

The principle underlying the process of homogenization is to subject the fat globule to enough severe conditions, which disrupts it into smaller globules. The newly formed fat globules are maintained in dispersion for sufficient time to allow milk fat globule membrane (MFGM) to be formed at the fat-serum interface. The following theories have been proposed to be responsible for the entire phenomenon.

Shearing or Grinding: As milk is passed at high pressure (velocity ~ 200-300 m s-1) through the homogenizer valve (~ 100 mm gap), fat globules undergo shearing action. The shear between fat globules and the surface of the homogenizer wall coupled with wire drawing effect results in elongation of the fat globules which progressively becomes unstable. These phenomenon result in subdivision of the fat globules. Furthermore, the difference in velocity of the faster moving serum phase at the centre of the liquid stream as compared to the liquid near the edge of the stream causes the fat globules to grind against each other. The turbulence created by the difference in velocity and eddy currents of the liquid add to the shear effects and thus enhance the process of disruption of the fat globules.

Exploding: This theory suggests that during homogenization, there is build up of tremendous pressure. When this pressure is suddenly released, the internal pressures within the fat globules pull the globule apart with exploding effect. This results in disintegration or subdivision of fat globules into smaller globules.

Splashing/Shattering: As the high homogenizing pressure is attained in the homogenizer, the homogenizing valve releases the highly compressed milk at very high velocity. The liquid suddenly strikes a retaining wall/ perpendicular surface.This causes splashing or shattering effect on the fat globules resulting in break down of globules into smaller sizes.

Acceleration and Deceleration: This theory relates sudden change in velocity of milk as it passes through homogenizer to the homogenization effect. When milk enters the homogenizer valve, velocity of milk changes from almost static to very high velocity. As it comes out of the valve, there is sudden deceleration at a rate at which it was accelerated. This sudden change in velocity results in shattering effect leading to division of fat globules.

Cavitation: It is postulated that as the milk passes through the homogenization valve, the initial homogenization pressure decreases sharply due to sudden increase in the velocity of milk. Depending on the back pressure that exists outside the homogenizer valve, the pressure can drop to as low as the saturated vapour pressure of liquid. This leads to formation of vapour bubbles due to cavitation. Cavitation generates shock waves, which could be in excess of 1600 kg/cm2 in intensity. Due to overlapping of these shock waves, disintegration of the fat globules may occur.

 

iii. Advantages and Disadvantages of Homogenized Milk

Advantages

•  Prevents removal of fat/cream from milk
•  Homogenized milk results in softer curd and therefore easily digested by infants
•  Churning of fat does not occur during bulk transportation
•  Fat is uniformly distributed and therefore gives uniform consistency
•  Homogenized milk is comparatively resistant to development of oxidized flavour defect

Disadvantages

•  Homogenization offers possibility of incorporation of foreign fat into milk
•  Homogenized milk is prone to development of ‘sunlight’ or ‘activated’ flavour defect
•  Homogenized milk if returned unsold from the market is difficult to salvage as centrifugal separation of fat is not possible

iv. Viscolised Milk

Viscolised milk refers to a product, which has unusually deep cream/fat layer resulting from admixing of homogenized cream, skim milk and/or whole milk. The homogenized fat forms very loose clumps with the unhomogenized fat globules and rise to the surface giving an appearance of deep cream layer. The unfair traders who first separate the cream, homogenize it and then remix with the skim milk sometimes practice it. This gives the remixed milk a very rich and creamy surface appearance and thus deceives the consumers.

 

v. Design and Operation of Homogenizers

There are several types of homogenizer valves and therefore designs of homogenizers vary depending on the manufacturers. However, many homogenizers used in the dairy industry have been developed based on the principles introduced by Gaulin.Homogenizers essentially consist of two components – a piston pump to generate high pressure and a homogenizing valve.The homogenizer pump is generally a positive displacement pump with at least three and sometimes five or seven pistons, which operate consecutively to generate steady pressure. Single piston pumps generate pulsating output with fluctuating pressure thereby resulting in poor homogenization. The pump block is generally made of stainless steel but the piston seal rings are of a soft composite material.

Homogenizer valves, used for milk may be either a ‘poppet type’ or ‘ball type’. A poppet design has relatively large contact surfaces and provides close fitting seal.If maintained properly, ‘poppet’ valves give better performance with low viscosity liquids like milk. ‘Ball’ valves can exert greater pressure on the much small seal area and are therefore, suitable for high viscosity liquids or suspensions with smaller particles.

Milk from high pressure manifold enters into the centre of the valve seat. The internal diameter of the valve seat is smaller than the manifold. As it passes into the narrow gap between the fixed and the adjustable faces of this valve, milk velocity gets accelerated. The gap is maintained against the feed pressure by a counter force exerted by an adjustable heavy duty spring. Shear effects result from the high velocity gradients between the liquid and the surface of the homogenizing valve. Turbulence also results from the high velocity of the liquid in the valve,causing eddy currents within the flow. Liquid which passes across the valve at about 200-300 m s-1 suddenly drops in pressure to below saturation vapour pressure.

This permits microscopic bubbles to form for a few microseconds before collapsing.The high velocity jet of milk then impinges on a perpendicular impact ring. These effects contribute to the disruption of the fat globules. Homogenizer valves are made of very tough corrosion resisting alloys such as stellite. Better resistance to corrosion can be achieved by using tungsten carbide and ceramic valves, which are used by many manufacturers in modern homogenizers.

Sectional view of single-stage Gaulin type homogenizing valve

As the fat globules are subdivided into smaller globules, there is increased surface area of the newly homogenized fat globules. The original milk fat globule membrane(MFGM) material is not sufficient to cover this. Proteins, particularly casein micelles migrate from serum phase to form new membrane material with the existing MFGM.This may result in sharing of the casein micelles and therefore some aggregation of fat globules could take place thus defeating the purpose of homogenization. A second stage homogenization therefore, becomes essential at reduced pressure(almost 20% of the first stage pressure (175 kg/cm2) or upto 35 kg/cm2). This enables aggregated fat globules to be disrupted for formation of stable emulsion of finely dispersed fat globules.

 

vi. High Pressure Homogenization Technology

With significant improvement in understanding of machine design, material strength and fluid mechanical knowledge, homogenizers with higher pressure capabilities have been developed. Such high pressure homogenizers could be operating based on two principles: (1) Conventional valve type homogenizer operating at a far higher pressure; (ii) Micro-fluidization based on the principle of collisions between high speed liquid jets.

Conventional Valve Type High Pressure Homogenizers (HPH): Such high pressure homogenizers work on the principle of conventional ball-and-seat type homogenizer valve. Highly abrasive resistant and durable components of high pressure homogenizers are made from best quality stainless steel, high alloy compositions and new ceramic materials. This allows these systems to operate at pressures upto 2550 kg/cm2 or more. Besides the regular use in emulsion formation, these high pressure homogenizers find applications in inactivation of enzymes and bacteriophages and also in destruction of micro-organisms. Destruction of bacterial cells by HPH is due to several physical phenomenon viz., pressure drop, cavitation, shearing,turbulance and collision. These systems can be therefore, used as a combined process for pasteurization and homogenization.

Microfluidization Technology: The microfluidizer operates under a different principle as in this case the liquid being processed is divided into micro streams that are so projected that these collide with each other. The essential design features of micro-fluidizers include a double acting intensifier pump and an interaction chamber. The intensifier pump, which is either air-driven or electric-hydraulic driven,forces milk/product at high pressure through the interaction chamber. The interaction chamber has fixed-geometry micro-channels, which divides the product into streams.These streams, which accelerate to a very high velocity, are made to collide against each other. Shear and impact that occur lead to homogenization effect. These micro-fluidizers are capable of generating pressures upto 2800kg/cm2. As in case of conventional valve homogenizers, microfluidizers too bring about changes in the fat and protein fractions of milk thereby altering some physico-chemical properties of milk.

 

vii. Vacuum Homogenization

This innovation in the homogenization technology is based on the discrete pulse energy input theory. In vacuum homogenization, energy is introduced discretely into the liquid (milk) through powerful short-time impulses. The homogenizer unit, placed in the HTST pasteurization line has two condensing chambers. The chilled milk is first pumped to condenser-I where it is heated to 20oC and subsequently to 30oC in condenser-II. Milk then enters regeneration section of the pasteurizer where it is heated to 65oC. It is then delivered to 1st stage vacuum (homogenization) chamber through a special nozzle. As a result of flashing effect, bubbles are formed in the milk as it falls in the vacuum chamber maintained at 0.15 to 0.2 kg/cm2. Due to the pressure changes taking place, the bubbles either show high frequency pulsation and release energy or collapse producing shockwave effect in the product. The bubbles therefore burst into smaller units and the fat globules are divided into smaller globules. As the milk enters 2nd stage homogenization chamber, further breakdown of fat globules takes place. The outgoing milk passes through regeneration section followed by chilling section to finally attain 5oC temperature. Besides the generally accepted homogenization effect, the other major advantages offered by this vacuum homogenizer are deodourization, reduced acidity and partial suppression of microbial activity. These systems also claim to be economical as it consumes almost 2.5 times less power than the valve type high pressure homogenizer.

 

viii. Checking the Efficiency of Homogenization

The method recommended by the United States Public Health Service has been the most widely used for checking the homogenization efficiency. It is performed by subjecting a specified volume (one quart) of milk to quiescent storage for 48 hours and then testing the fat in upper 100 ml and the remainder of milk. For properly homogenized milk, the percent difference in both the top 100 ml and the remainder milk should not be more than 10 per cent.

Homogenization

Milk is an oil-in-water emulsion. Fat globules in the milk are dispersed in a continuous water phase (skim milk) and normally vary in sizes ranging from 1 mm to 22 mm,with a mean size of approximately 3-4 mm. As the density of milk fat is less than that of skim milk, the fat globules tend to rise to the surface during storage and form a cream layer. The rise of fat globules follows Stoke’s law where the velocity of rising fat globules is expressed as:

V a(d2 (rs - rf))/18h

Where, d = diameter of the fat globule, rs = density of the serum phase, rf = density of milk fat and n = viscosity of milk serum.Homogenization 

Very small fat globules (<1 mm) remain suspended in the serum phase due to brownian motion and adversely affect the creaming phenomenon. The presence of cryoglobulins in the raw milk causes agglomeration of fat globules, which subsequently have increased tendency to rise to the surface.

Homogenization is a mechanical process in which milk is forced through a homogenization valve under very high pressure. The milk is thus deflected at right angles through a narrow opening of about 0.1 nm (100mm). As the milk comes out of this valve opening, there is sudden drop in pressure and the milk is subjected to impact against an impact ring. This complete process results in disruption of fat globules leading to decrease in the average diameter (typically from 0.2 to 2 mm) and an increase in the number and surface area of fat globules.

Homogenization with reference to milk/ dairy applications thus refers to a mechanical process that is used to reduce the size of fat globules such that milk fat does not rise to form a cream layer during storage of milk. Although homogenization renders fat globules uniformly distributed in the body of the milk, upon prolonged storage it does not remain completely dispersed.

Test for Pasteurization Efficiency

Phosphatase Test: Phosphatase test is done to determine whether milk has been properly pasteurized or not immediately after pasteurization of milk. The test is based on the principle that alkaline phosphatase, a natural enzyme present in raw milk, is simultaneously deactivated by heat treatment as specified for pasteurization.When milk-containing phosphatase is incubated with p-nitro phenyl di-sodium ortho phosphate, it hydrolyses the substrate and, as a result, para-nitro phenol is liberated which gives a yellow colour under alkaline condition of the test. The amount of the yellow colour present is directly proportional to the amount of phosphatase present in milk. The presence of yellow colour indicates inefficient pasteurization or post-pasteurization contamination of the milk. The intensity of the colour is compared with standard and lavibond comparator disc.

Operation of Pasteurization Plant

We have studied the importance of pasteurization process and the important components of a pasteurization plant. Now let us see the operations involved in running of a pasteurization plant and how to cope with operational problems.

 i. Starting the Plant

The following steps should be followed to start the plant:

a) Start the air compressor;
b) Switch on the control panel mains;
c) Fill the hot water tank, start the hot water pump;
d) Open the air vents
e) Start flow of the milk to the float controlled balance tank
f) Start the milk pump
g) Close the air vents when the milk coming out from them indicates that all air has been displaced.
h) Set the temperature controller to maintain the milk at 72°C.
i) Turn on cold water and chilled water and hot water set.

 

ii. Shut down of the Plant

For shutting down the plant, at the end of the milk run:

a) Make available in the storage tank a sufficient quantity of water (approx. equal
to the capacity of the plant).
b) As the last milk is leaving the float balance tank, tip in the water from the tank.
c) When the last of the water is leaving the float balance, turn the three-way cock
at the finished milk outlet so that the flow is diverted to the floor.
d) Place a hose in the float balance tank and flush the plant thoroughly with water
until the discharge from the finished milk outlet becomes clear.
e) Turn off the cold water, brine or chilled water in the cooling sections.
f) Shut off the steam supply to the hot water set.
g) Admit cold water to the hot water tank and run until the plant is cold.
h) Stop the milk and hot water pumps.
i) If brine is used, flush out with running water.
j) Turn off the air supply and the main electric switch at the panel.

Thereafter, the plant must be thoroughly cleaned.

 

iii. Cleaning and Sterilization of the Plant

•  Cleaning the plant: Cleaning is done after completion of pasteurization process.The milk supply is stopped to constant head tank by turning off the valve of Raw Milk Storage Tank. Clarifier and homogenizer are stopped. The water is added to the constant head tank. Hot water temperature is set at 70°C. Primary detergent solution is circulated for 20-30 minutes. Flush the system with lukewarm water. Secondary detergent solution is circulated for 20-30 minutes. Flush the plant with water.
•  Sterilization: The plant can be sterilized by hot water or sodium hypochlorite solution. The raw milk tank and pasteurized milk tank are bypassed and hot water (87-90°C) is circulated for 10 minutes. The sterilization is done before running the plant with milk for pasteurization.

iv. Pasteurization of milk

The operation of plant with the milk is called running of the plant. The plant is started. It is sterilized. The plant is run on water. The standardization is done to check the flow, operation of flow diversion valve, heating temperature and cooling temperature. The homogenizer pressure is also set in according to requirements.The flow of milk from raw milk tank to pasteurized milk tank is monitored.

 

v. Trouble shooting

Pasteurizing problems may occur during start up procedures or during the run.When a problem occurs it is important to be able to identify the problems from the symptoms, identify the cause of the problem and take the appropriate action towards solving the problem. If the problem causes a delay in processing it is advisable to turn off essential services such as steam and heating and cooling system to prevent burn on in the heating section and a freeze up in the cooling system. The common problems and their remedial measures are given in practical exercises. These could be grouped in three broad areas: (i) inadequacy in achieving temperature, (ii)chocking of plates and (iii) leaking plant assembly. The broad reasons for these are given here.

(i)Inadequacy in achieving temperature : The possible reasons are: inadequate steam supply, faulty temperature controllers, air in milk and improper assembly of plates.
(ii) Chocking of plates : Fouling, high milk temperature, high milk acidity and inadequate filtering of milk could be the reasons for chocking of the plant.
(iii) Leaking plant assembly : The reasons are: damaged and worn gaskets,damaged plates and wrongly fitted plates.

 

vi. Preventive maintenance

Preventive maintenance will help to control damage, excessive wear and tear and occurrence of accidents. Preventive maintenance can be divided into two areas, (i)avoiding damage to the plant and equipment and (ii) observations and inspection of plant and equipment. Avoiding damage consists of basically the careful handling of machinery and equipment. The regular inspection of the plant and equipment is important as a part of preventive maintenance, and may include:

a) Periodical tests may be made to check the flow rates of heating medium,cooling medium and milk.
b) The recording instruments such as thermometers, etc must be periodically checked for accuracy.
c) Air operated instruments should be supplied with clean air.
d) The plate surfaces and gaskets must be checked during the manual cleaning of plants.
e) Filter cloth/filters must be changed at regular intervals.
f) The faces of the plate bar and tightening spindle should be lightly coated with grease.

Hist Pasteurizer Plant and its Components

The HTST system is the most common method used by the dairy plants for pasteurization of milk. The main advantage of HTST pasteurization is its capacity to heat treat milk quickly and adequately with built-in safeguards that prevent improper pasteurization due to under heating of milk. The HTST system employs plate heat exchangers for heating, regeneration and cooling. The system consists of feed pump, plate heat exchanger, holding section, flow diversion valve,instrumentation, essential services and piping system. The entire process is automatic and is ideal for handling of 5000 litres per hour (lph) or higher quantity of milk. This is a continuous flow process and also saves energy due to regeneration section(Figure).In order to understand a pasteurizer let us go systematically for:

•  Flow diagram of process;
•  Different compartments/sections;
•  Plate heat exchanger, which is the main part; and
•  Instrumentation

i. Flow diagram of pasteurization process

The schematic flow diagram of HTST pasteurization is given in below Figure.Raw milk enters the constant heat tank (balance tank), passes to the milk pump and then through a flow controller to the plate heat exchanger. The plate heat exchanger consists of regeneration section, heating, holding and cooling sections.

 Flow Diagram of Pasteurization

The raw milk enters the pre-heating (regeneration section), where hot pasteurized milk (72°C) flows counter current to the raw cold milk, within adjacent plates,transferring heat for pre-heating of raw milk and pre-cooling of pasteurizing milk resulting in energy saving. The partially heated raw milk passes through a filter or clarifier and homogenizer. It then enters the heating section where it is heated to at least 72°C. The hot milk then passes through the holding section to ensure that the fastest moving particles of milk are held at 72°C for at least 15 seconds.

The flow diversion valve diverts the milk to constant head tank if it is not properly heated to pasteurization temperature. Properly pasteurized milk passes forward through the flow diversion valve into the regeneration section where it is cooled by incoming cold raw milk passing in the opposite direction on the other side of the plates. Milk enters the cooling section and is cooled at 4°C before storage.

An indicating thermometer situated at the outlet of the holding section measures the temperature of the hot milk and this is recorded on a revolving thermograph. If the temperature of the milk falls below 72°C, the hot milk-recording pen drops past the set pointer on the thermograph and this activates the flow diversion value, the safe-guard pen and an alarm bell. The flow diversion valve diverts the unheated milk into the constant head tank for re-circulation until the milk reaches the correcting temperature.

 

ii. Components of a HTST Pasteurization Plant

The complete pasteurizer plant consist of:

•  Constant head tank
•  Milk feed pump
•  Flow controller
•  Filters
•  Clarifier
•  Homogenizer
•  Plate heat exchanger consisting of bank of plates compartmentalized into regeneration, heating, holding and cooling sections,
•  Flow diversion valve
•  Instruments associated with indicating controlling and/or recorded functions,
•  Systems for providing steam, air, water, heating and cooling arrangements, and
•  Piping system to link various components

 iii. Plate Heat Exchanger (PHE)

The Plate Heat Exchanger consists of a bank of plates inter-connected (sections) held in a rigid frame (figure). The main function of the PHE is the exchange or transfer of heat from a hot liquid (hot water or hot pasteurized milk) to a cooler one (cold water, chilled water brine or raw milk) across a metal plate. Let us see how the heat is transferred through plates.

Plates: The plates are thin stainless steel sheets usually rectangular in shape. The plates are corrugated and cause a turbulent flow, which increases rate of heat exchange. The rate of heat exchange also depends on the surface area of the plate,the thickness and type of metal used in the plates, the rate and direction of flow of the liquids and the difference in temperature between the two liquids involved in the heat exchange process.

An approximate 3-8 mm space is maintained between the plates by a non-absorbent rubber seal, which is bonded around the edges of the plate. The liquids, which are sandwiched among the plates, enter and leave the interspaces through holes in the corners of the plates. Open and blind holes route the liquids from one set of plates to another. The capacity of the pasteurizer is secured by a corresponding number of plates.

 Plate Heat Exchanger

Regeneration sections: The bank of plates is usually divided into four sections separated by connector grids with inlet and outlet bosses. In the regeneration section, the incoming cold milk is heated by the hot pasteurized milk and the pasteurized milk is cooled by transferring heat to the cooling medium. This heat transfers process work most effectively when the two liquids involved flow in
opposite direction, i.e. counter current flow on either side of the plates. Regeneration section raises the raw milk temperature from 4°C to 67°C and cools the pasteurized milk from 72°C to 10°C. Thus, PHE saves about 92% of heating and cooling energy. Theregeneration efficiency is calculated by using the following formula:

% Regeneration = temperature increase due to regeneration/ total temperature increase

For example: The cold milk enters the pasteurizer at 4°C and attains a temperature of 60°C after regeneration. The final pasteurization temperature is 72°C. Calculate the regeneration efficiency.

Increase in Temperature due to regeneration: 600C-40C=560C

Total Temperature Increase: 720C-40C= 680C

% Regeneration efficiency: 560C/680C = 82.36%

Steam-heated hot water or vacuum steam is used in heating section to raise the partly heated raw milk to pasteurization temperature. The holding section is either plate type or tube type. The plate type will have a number of plates. The partly cooled pasteurized milk is further cooled in cooling section to 4°C.

 

iv. Instrumentation

The instruments associated with the pasteurization plant are used for performing three functions in below Table

Instruments associated with pasteurizer and their functions