Uses of Condensed Milk and Evaporated Milk

The concentrated milk is used in a variety of food products. The uses of condensed milk and evaporated milk are listed below.

i.    Condensed Milk finds the following use

•  It is easily reconstituted by dilution with water to the equivalent concentration of raw milk and used in the preparation many sweetened milk drinks.
•  It is used as sweetening and whitening agent in tea, and coffee beverages.
•  It is used in ice cream preparation.
•  It is used extensively in the preparation candy, chocolate and other confectionery products.
•  It is used in many types of puddings and prepared foods

ii. Evaporated Milk finds the following uses

•  Because, in its homogenization and sterilization evaporated milk finds use infant feeding due to formation of soft curd in the stomach.
•  It is used for reconstitution into milk with water.
•  It is used as an ingredient in the preparation of many types of pudding,sauces and gravies because of its smooth consistency.
•  It is used in the manufacture of ice cream.
•  It is used in the preparation of chocolate, bakery and confectionery products.
•  It is used in lieu of cream in coffee, tea, cocoa or chocolate.
•  It is diluted with milk and cream to produce coffee cream.

Defects Their Causes and Prevention

Condensed milk and evaporated milk are the products suitable for prolonged storage.They have typical properties after production by which they are identified. These properties must be such that the product is fit for sale immediately after production and does not alter during reasonable period of storage. They should, therefore be physically, chemically and bacteriologically fit for human consumption by the end of storage period. Routine examination of the product soon after manufacture as well as during storage may be carried out to judge the quality of the product. If any defect is noticed, proper care should be taken to climate the defect in the subsequent batches. It is, therefore, important to know what type of defects may occur in the product, the reasons for their occurrence and the preventive measures to be used to avoid these defects.The defects which may occur in condensed milk may be divided into two categories as follows:

i.    Microbial Defects

These defects are due to presence of abnormal bacteria in the milk or in products.

The common defects are as follows:

a) Gassy fermentation/Bloats: Formation of gas takes place in cans and barrels of condensed milk causing bulging or bursting of containers. Gas producing yeasts are the cause of many types of gaseous fermentation. The source of contamination may be the raw milk, or inferior quality sugar or unhygienic factory conditions especially not properly washed and sanitized equipments

and filling machine.

To avoid this defect good quality raw milk and properly preheating temperature should be used. Only good quality sugar without any yeast contamination should be selected. Proper sanitary conditions should be maintained during manufacture and packaging of condensed milk. The containers should be filled fully with little space for air or oxygen.

(b) Bacterial thickening: Condensed milk gets thickened progressively during storage. This is due to microorganisms which produce rennin like enzyme.These organisms are easily destroyed during preheating process. Optimum sugar ratio (64.5) may inhibit the growth of microorganism. Low temperature storage also helps in reducing the bacterial thickening.

(c) Mould buttons: This defect occurs during storage due to mould contamination. Small reddish brown pieces of curd about ¼” to ¾” in diameter are formed on the surface causing localized coagulation. This defect occurs after storing the product for some time. The causative mould “Aspergillus repeno” produces a“clotting enzyme” which causes localized clotting. High temperature storage also helps in the growth of the organism. The milk may be infected with the

organism during concentration process.

To avoid this defect, scrupulous cleaning and care of dairy equipments is essential.Since this organism does not grow at low temperature storing of condensed milk at lower temperature will prevent this defect.

ii.   Non Microbial Defects

The non-microbial defects are of chemical or physical origin. These are listed below.

(a) Sandiness: Good quality condensed milk should possess a smooth homogenous texture and be pleasant to palate. Some times however the milk may be gritty containing large number large sized hard lactose. The solid particles are of such size that the product lacks smoothness and grittiness is noticeable, as the sample is being tasted. This defect is readily detected by an average consumer.Sandy, rough grainy, granular, and gritty are the term used to describe this defect.

The sandiness may be due to presence of relatively large size crystals of lactose.Also if excess amount of sugar is used in manufacture of condensed milk, sugar particles may also crystallize out and cause sandiness. If manufacturing conditions are not conducive to the formation of small lactose crystals, large and coarse crystals are formed. Cooling of condensed milk must be carried out in such a manner that smooth texture is obtained by the formation of large number of minute sugar crystals.High viscosity also delays the crystal formation. It is, therefore, necessary to rapidly cool the condensed milk in the initial stages.Correct cooling and induced rapid crystallization with correct amount of seed lactose will help to avoid this defect. Optimum storage temperature is also essential to avoid this defect.

(b) Age thickening: Thickened condensed milk is the most common defects seen in the sweetened condensed milk. This defect varies markedly in its intensity from slight jelly to a firm and consistency. The defect becomes progressively more intensive upon storage, especially at room temperature or above.Preheating temperature of milk and degree to which the milk is concentrated have been observed to have profound effect on age thickening. With high prepackaging ,heating temperatures, there is a greater tendency upon the product to thicken early. With increasing concentration of milk solids the thickening tendency becomes more marked. In order to avoid early thickening of condensed milk,optimum pre heating temperature should be maintained. Sugar should only be added at the end of condensing of milk. The product should preferably be stored at temperature below 15°C with the addition of proper type of stabilizers age thickening may be decreased to a great extent.

(c) Brown colour: usually brown colour discolouration is associated with age thickening, both of which become progressively more intense on storage. The acidity and temperature of storage are factors, which determine the rapidity of change. This defect may be avoided if the condensed milk cans are stored at reasonably low temperatures. Other defects in condensed milk include some flavour defects such as rancid, tallowy, metallic etc. Following good manufacturing practices, good quality raw material, good quality milk, good packaging and storing the product at low temperature may help to avoid the defect.

(d) Fat separation in condensed milk is rare.

Defects in Evaporated Milk: Evaporated milk is sterilized product. If sterilization is properly carried out the product will remain in good condition without any bacterial spoilage during storage. However, if sterilization process is improperly carried out, some microorganisms and spore may, survive and cause spoilage, during storage, Gassy fermentation, coagulation, bitterness and fishy flavour are some of the defects noticed in improperly sterilized evaporated milk. If the bacterial defect in evaporated milk is to be eliminated, it is essential that the sterilization process should be carried properly and adequately with strict cleanliness in the factory.

Chemical/Physical defects in Evaporated Milk

Age thickening

Age thinning

Fat separation

Sediments/mineral deposit

Brown colouration

(a) Age thickening: As in the case of sweetened condensed milk thickening or high viscosity of the product is due to improper preheating of milk, higher total solids concentration, inadequate homogenization and storage at higher temperature.

To eliminate this defect the milk should be heated at optimum preheating temperatures. The concentrated milk should be homogenized at proper pressure and the product should be stored at low temperature.

(b) Age thinning/low viscosity: Milk like consistency of evaporated milk indicates its low viscosity. This defect may be due to insufficient concentration of milk and inadequate or no homogenization of milk or improper sterilization. This defect is eliminated by following correct/adequate processing conditions and storing the product at low temperature.

(c) Fat Separation: A thick, heavy cream layer is noticed at the top of the can when it is opened.

The cream layer is very dense and will not easily mix with remainder of milk. Even after mixing creamy chunks of butter particles are noticed floating in the milk of relatively low viscosity. The reasons for this defect to appear are in adequate homogenization high storage temperature, long storage period and improper handling during storage. Proper homogenization, low temperature storage and correct handling of the product will eliminate this defect. A more viscous is desirable from fat separation point  of view.

(d) Sediments/Mineral deposit: Sediments noticed in evaporated milk cans may be due to crystallization of some of the calcium and magnesium salts, and denatured proteins. This gritty sediment formation takes place during prolonged storage of evaporated milk. The rapidity with which the sediments are formed is influenced by the nature of the milk, conditions of manufacture and temperature of storage. Selection of good quality milk, correct processing conditions and low temperature storage will help to eliminate this defect. Higher viscosity product is less prone for this defect.

(e) Brown Colour: The brown colour in evaporated milk is associated with high sterilization temperature, high storage temperature and longer storage period.The sterilization temperature should be as low as possible with germicidal efficiency. The brown colour defect in evaporated milk is eliminated by employing proper preheating temperature, correct sterilization process and low temperature for storage of the product.

Judging and and Grading

Good and acceptable quality food products are made by selecting good quality raw materials and following the established manufacturing techniques. This is also true with the manufacture of condensed milk and evaporated milk. Proper care should be taken to produce good quality product. Manufacturer should routinely examine the product to judge its quality and to maintain the grade that is acceptable to customers.

Judging refers to the act of evaluating the dairy product for its “Eating quality” on the basis of various attributes. Grading refers to its classification into different categories or grades. The eating quality of a dairy product is generally determined by organoleptic/sensory tests, which include all the five senses of sight, smell, taste,touch and sound. Of these taste and smell are the most important in judging and grading. There are various methods available for judging and grading of dairy products. Some of the subjective tests based on organoleptic examination such as flavour, taste etc. make use of the hedonic scale or variation of it.

i. Hedonic Scale: This method of rating for individual attributes is simple and gives a clear indication of the particular attribute of the product. For example the flavour of a dairy product may be evaluated on a hedonic scale rating from 1-10 points as suggested in the following Table Modification of hedonic scale using narrow range 1-5 may also be used in evaluating the product.

 

Hedonic scale for flavour

ii. Score card: The scorecards for condensed milk and evaporated milk proposed by the American Dairy Science Association (ADSA) were most useful in establishing standards and obtaining uniform high quality products

Score Card for Condensed Milk (ADSA)

Score Card for Evaporated Milk (ADSA)

These score cards considered such items as (a) flavour and odour; (b) body and texture; (c) colour; (d) fat content; (e) milk solids; (f) bacteria and (g) sugar in case of condensed milk only). Numerical values were assigned to each item. Flavour/ odour and body/texture were given higher ratings (30 and 25 points in case of condensed milk and 30 and 25 points in case of evaporated milk). Colour rated 5 points while all other items were treated equally being allotted 10 points each.

The following procedure should be followed for judging a product.

i) Sampling: Select a can of the product at random for examination.

Sequence of observations: Avoid undue agitation when transporting to the laboratory. Place it on the table for examination in the same upright position as before. Cut more than three fourths of the top of the can and turn it back.Then examine in the following order.

ii) Appearance of the can: Look for signs of rust etc., both outside and inside (when emptied).

iii) Appearance of the product: Examine uniformity of colour; look for absence of lumps in condensed milk and cream layer/butter/particles/curd in evaporated milk.

iv) Body and texture (viscosity): Observe whether the viscosity is high, normal or low while pouring the contents into a beaker.

v) Sediment: Watch for presence or absence of sediment at the bottom of the container when emptied.

vi) Flavour and odour: Note defects if any, by placing a small spoonful of condensed milk or diluted evaporated milk (1:1 with distilled water) on the tongue.

vii) Laboratory Tests: Take a sample aseptically and then test for fat, total solids,bacteria, sugar, adulterants and preservatives.

iii. Requirements for High Grade Condensed milk and Evaporated milk:The person who is judging the product should be familiar with the desirable qualities of the product as well as the possible defects, which may occur in both condensed milk and evaporated milk.

a) Condensed milk: It should have a clean, pleasant aroma, a pronounced sweet taste, smooth and uniform body and texture and uniform light colour, which should be yellow for cow milk and light greenish white for buffalo milk.

b) Evaporated Milk: It should have mild pleasant flavour, a relatively viscous body, uniformly smooth in texture and uniform colour.

Stroage of Condensed Milk

Both condensed milk and evaporated milk are expected to be stored for longer period. Hence conditions should be maintained such that the product should not deteriorate in quality during storage. The room temperature at which the product is stored is one major factor, which will have definite effect on the keeping quality. The present trend is to store both condensed milk and evaporated milk around 10 to 15°C. Very low temperature such as 0°C or below may cause sugar separation in condensed milk leading to a defect called sandiness in the product. The sandiness is due to the presence of very large crystals of lactose. Viscosity of the product also will increase due to low temperature storage. Increase in viscosity may be beneficial up to a certain level, but very high viscosity changes due to low temperature storage may also affect the body and texture characteristics of the product. It has been shown that commercial evaporated milk remains acceptable even after two years when stored below 15°C but deteriorates rapidly when stored at 21°C or above.Humidity of the storage space should also be kept low (below 50%) to check the

spoilage of cans and labels. Periodic inversion of cans in case of evaporated milk during storage will help to minimize fat separation.

Packaging

Packaging essentially means placing a commodity into a protective wrapper or container for transport or storage or both. Thus package must perform the following important functions.

• Contain the product
•  Protect the product
• Help in selling the product

 Concentrated milk is packed in suitable containers for storage and transport.Sweetened condensed milk is packed at the end of the manufacturing process.Evaporated milk is packed in containers immediately after concentration and then sterilized with the container. In case of Ultra High Temperature (UHT) treated milk, the product is sterilized and then aseptically packed in suitable containers.

i.                    Packaging of Condensed Milk

Two types of packages are used. For large quantity or for bulk transport, the condensed milk is filled in barrels of various sizes ranging from 50kg to 300 kg. The barrels are made either from wood or metal. Barrels are coated internally with wax or provided with polythene liners. Before condensed milk is run into barrels, they should be sterilized by means of steam, condensate allowed to drain and then filled by means of a funnel placed in the bunghole. When the barrels are full, the bungs are driven in, thus effectively, sealing the content. The room in which the barrels are filled should be in sanitary condition and its atmosphere should preferably be one of filtered air. Condensed milk is usually filled in barrels, at around 15.5°C (60°F).For retail sale the condensed milk is packed in cans of various sizes from 200 gms to 5 kg or even more. The method used for can filling varies from hand filling to continuous automated filling. In big factories tin sheets are shaped into cans by machines, which are then filled and sealed immediately. In small factories the cans are bought readymade and filled and sealed either manually or by machines. Sealing is carried either by means of solder or by crimping on the tops of the cans to form an airtight seal without the use of solder. The cans should be sealed immediately after they have been filled in order to prevent contamination of the product by exposure to atmosphere. Since cans filled with condensed milk do not undergo subsequent sterilization, strict sanitary conditions should be observed during filling process so as to prevent contamination which will otherwise affect the keeping quality of the stored product. The filling and sealing area should be enclosed and only filtered air should be circulated to avoid direct exposure to atmospheric dirt or dust during filling. The filling machines and accessories should be thoroughly washed and sanitized before start of the filling operation. The cans and lids on their passage to the filler should be suitably sterilized. This is usually done by passing the cans under or over a battery of suitable gas jets. Personnel operating the filling and sealing machines should observe sanitary habits. It is important to fill cans fully so as to exclude as much air from the container as possible and seal them promptly after filling.

ii.                  Packaging of Evaporated Milk

Several machines have been designed for this purpose. In one type, which is known as “Vent hole type” concentrated milk is filled through a small aperture (3 mm of 1/ 8 inch diameter). After cans have been filled they are immediately sealed. Sealing process requires care, as the seal has to with stand the heat of sterilization process.It is also essential that cans are filled as quickly as possible to avoid contamination of milk. The opening in the cans is usually soldered by mechanical finger. But hand soldering is also practiced while handling smaller number of cans. When the cans leave the sealing machine they are tested for leaks by being plunged into a hot water tank. If any of the tins rise or give off air bubbles, they are discarded as unfit for sterilization. Modern sealing processes are so rapidly carried out that a large number of cans may be sealed in one minute. In this process open or sanitary type cans are used. The concentrated milk is filled into the open can one end of which is already seamed and after filling the other end of the can is seamed by an automatic seaming machine.

Packaging,Stroage and Common Deffects in Condensed Milk

It is absolutely essential to maintain the quality of good grade concentrated milk during storage until it reaches the intended customer. Selection of proper packaging materials and storage conditions will ensure prolonged keeping quality of the product.In the manufacture of different types of concentrated milks care should be taken to produce uniform acceptable quality product. Routine examination of the product is important, not only to assure improvement in the product, but also to see that it reaches the customer in good condition. In this respect, it is helpful to know and understand how to evaluate and grade good quality product. If however, certain defects are noticed in the product the reasons for the same and the preventive steps to be taken to eliminate defects should also be known to the manufacturer.

Frozen Condensed Milk

This is plain condensed milk, frozen to give longer storage life. It is used largely in ice cream factories.

Super Heated Condensed Milk

This is plain condensed milk superheated by blowing live steam directly into it towards the end of condensing period. The major purpose of superheating is to increase the viscosity. It is used in ice cream factories and bakeries.

Plain Condensed Milk

This is unsweetened condensed milk made from whole milk partly skim milk or entirely skim milk and condensed to 2.5 to 4:1 ratio. It is used in ice cream factories and bakeries. Its keeping quality is similar to that of good quality pasteurized milk.

Manufacture of Evaporated Milk

Unsweetened concentrated milk is known as evaporated milk and manufactured practically in the same manner as the sweetened condensed milk. In this product no sugar is added as preservative. To achieve preservation the product is sterilized by heat after concentration and sold in the hermetically sealed container in which it has been sterilize.

Flow Diagram of Manufacture of Evaporated Milk

i) Receiving milk/filtration/clarification/standardization: The raw milk used in the preparation of evaporated milk is treated in a similar manner to that which applies in case of sweetened condensed milk as discussed earlier. It is important that the raw milk should be of very good quality in order to ensure that it is free from any heat resisting microorganisms. Only fresh milk can be used since, if the acidity of evaporated milk is above normal it will clot or curdle during sterilization and will thus be rendered un-saleable.

ii) Prheating/Forewarming of milk: To improve the heat stability of concentrated product and for imparting optimum viscosity to the finished product the fluid milk is preheated before it is condensed. Milk may be preheated either at 95-1000 C for few minutes (5-10) or at 140-1450 C with not holding time.

iii) Concentration: Preheated milk is concentrated in an evaporator. Multiple effect evaporators are used for handling large amount of milk to have continuous operation.

iv) Homogenization: After concentration the milk is homogenized. Fat separation in evaporated milk during storage is a major defect. This defect can be reduced or eliminated by homogenization of condensed milk. Homogenization refers to a process of forcing milk under pressure through equipment called homogenizer.This is an essential part of production of evaporated milk. The homogenizer reduces the mean size of the fat globules in the milk so that they are uniformly distributed in milk and do not rise to the top to form the creamy layer during storage. In raw milk, the diameters of the fat particles (globules) vary from 1 to 20 micron while a diameter of about 2 micron or less is required to keep the fat distributed uniformly. In this process milk is forced through a small orifice at a high pressure by means of a positive displacement pump. The mechanical forces, which are set up as the fat particles pass quickly through the orifice,cause the particles to split. Condensed milk removed from evaporator is passed through two stage homogenizer at about 50°C with a pressure of 175 kg/sq. cm at first stage and 35 kg/sq cm at second stage.

v) Cooling: After homogenization concentrated milk is cooled to 5°C and held in storage tank. Stabilizing salts may be added to concentrated milk in tank as indicated by pilot sterilization test.

vi) Pilot Sterilization: The purpose is to determine the amount and type of chemical stabilizers to be added to any given batch of condensed milk for most satisfactory heat stability. Heat coagulation of milk is influenced by many factors such as initial quality of raw milk, fore warming, concentration and sterilization. In raw milk it has been suggested that salt balance of milk i.e. ratio of calcium +Magnesium to citrate + Phosphate is important for the stability of milk towards heat. If the ratio of Ca + Mg/Citrate + Phosphate is disturbed the heat stability may be affected. The more common cause of heat coagulation is the disturbed salt balance. Depending on the location of pH of maximum stability with respect to natural pH of concentrated milk either sodium salt of orthophosphate (disodium phosphate) or calcium chloride/monosodium phosphate are used as stabilizers. This is the basis of a process of pilot-sterilization in which various quantities of selected stabilizers are added to the concentrated milk in 170 ml (60 oz) tins which are then sterilized and the condition of the product carefully examined in order to determine whether any stabilizer is necessary and if so in what amount it should be used. Depending upon the type of milk and the processing condition used 100 to 300 gm of stabilizing salts per 100 kg of evaporated milk may be needed to improve the heat stability. Calculated amount of salt is added to the evaporated milk in the form of a solution using just enough water to dissolve it.

vii) Packaging: The evaporated milk is filled in suitable cans or containers either manually or mechanically. It is essential that the packaging is done as quickly as possible. After cans have been filled they should be sealed immediately. The sealing process requires care, as the seal has to withstand the heat and pressure of sterilization.

viii) Sterilization: The object is to destroy all bacteria their spores and enzymes thereby preserve the product for a longer time. In addition sterilization process is also used to increase the viscosity and improve body and texture to give a creamy consistency to the product.

Two important factors in sterilization process are:

•  The temperature attained
•  The period during which this temperature is maintained.

The temperature time of heating during sterilization should be such as to ensure sterility consistent with sufficient body and texture without causing objectionable discolouration or excessive cooked flavour.

Two systems are in common use for sterilization:

Batch Sterilization: This method is suitable for small scale operation and is also useful for cans of various sizes. In this system the cans leaving the filling machine are placed in racks. The racks are then loaded into a steam-tight boiler. The racks are moved by a revolving mechanism, which keeps them in motion throughout the process,the speed of rotation varies between 6- 12 revolutions per minute. Steam is admitted in to the sterilizer until the temperature reaches 118 °C and the sterilizer is maintained at this temperature for not less than 15 min. After sterilization is completed, the steam is shut off and cans are cooled to 25- 30 °C either by means of water spray or by immersing in cold water. After cooing the cans are passed through an oscillating machine, which breaks up any curd formed during the process of sterilization and retains the smooth homogeneous texture of evaporated milk.

Continuous Sterilization: This is used for large scale continuous operation. This system consists of many compartments, which are maintained at different temperatures/ pressures. The filled cans entering the apparatus roll through the chambers by means of spiral track, which ensures that the heat distribution to each can is uniform. This rolling motion of cans takes the place of the methods of agitation provided in the batch method. The filled cans are preheated before they pass to the main sterilizing section. The movement of cans is controlled so that they remain in the sterilizing chamber for 15 min. After sterilization process is complete the cans pass to the cooling section, which under similar pressure conditions to those of the sterilizer.Both systems have their merits and demerits. During sterilization the temperature of evaporated milk is raised rapidly to 116-118°C and held at this temperature for 15 minutes and their cooled.

Storage: After sterilization cans of evaporated milk are unloaded from the sterilizer, and cooled by water spray. The cans are then shaken mechanically to break any curd or lump which might have formed during sterilization to have homogenous consistency. Excessive shaking is avoided as it may decrease viscosity. As evaporated milk is a sterilized product it may be stored at or below room temperature. The present trend is to store at below room temperature to check deterioration in quality and thereby prolong keeping quality.

Types of Evaporation

Condensing is a process of removal of water from milk in the manufacture of concentrated milks. Evaporators are the equipment used for carrying out the process of evaporation. Evaporator is a major equipment used in the dairy factories for condensed milk. There are two types of evaporators:

• Vacuum pan/evaporator: Batch operating type evaporator is preferred when relatively small quantities of milk are to be handled in a batch operation for product like sweetened condensed milk.
•  Continuous flow evaporator: Continuous flow evaporator is preferred for large scale operation working in conjunction with milk drier.

i) Vacuum Pan: This type of evaporator is employed in the majority of condenseries for the manufacture of condensed milk (Fig 9.1). The principal parts of vacuum pan are: i) Evaporator body, ii) Heating surface,iii) Condenser and trap and iv) Vacuum pump. The tubular calandria pans, usually made of stainless steel, are provided with either internal or external heating systems and work under reduced pressure. The pans are internally fitted with steam coils and the lower portion is steam jacketed.The steam pressure is usually 1.4 to 1.5 kg/cm. The operation is usually carried out at 54-63°C, the milk boils at this temperature owing to reduced pressure (63.5 cm of mercury) which is maintained in the pan. A vacuum pump is used to maintain the vacuum in the pan. Condenser is one of the important parts of the system. When the water vapour, which rises from boiling milk inside the pan, comes in contact with cold metal surface or cold spray of water in the condenser, it is condensed and carried off as water by means of pumps or other means.

Vacuum Pan Evaporator

ii) Horizontal Tube Evaporator: In this type of evaporator, the material to be evaporated boils outside the horizontal tube and steam condenses inside the tubes. These types of evaporators are not considered efficient as the heat transfer is lower especially if the liquid is viscous.

iii) Vertical Tube Evaporator: These are distinct improvement over horizontal types. In this type the liquid boils inside vertical tubes with heating medium, which is usually condensing steam held in a chest through which the tubes pass. In this type of evaporator there is a large diameter hole in the centre through which the liquid to be evaporated flows down. Heating and boiling of liquid cause it to rise and flow upwards and un-evaporated liquid flows down through the central large diameter tube. Thus natural circulation is promoted in this type of evaporator.

iv) Forced Circulation Evaporator: This type of evaporator is a modified form of vertical tube evaporator. In this type a pumps is used to force the evaporating liquid through the tubes. This helps to further improve the evaporating capacity.

v) Long Tube Vertical Evaporator: This type of evaporator consists of one pass vertical shell and tube heat exchanger discharging the product to be evaporated into a relatively small vapour head. Normally no liquid level is maintained in the vapour head and the residence time of liquid is only few seconds. The tubes are usually 2 to 5 cm in diameter. The length normally varies from 6-12 meters. In this type the feed enters at the bottom of the tube, starts boiling as it moves up the tube and the mixture of liquid and vapour leaving at the top at high velocity strikes against a deflector placed above the tube. This deflector is effective both as a vapour separator and as a foam breaker. When the ratio of feed to evaporation is low, recirculation of liquid is provided. The liquid is recirculated by a re-circulating pump.

vi) Climbing Film Evaporator: In this type the liquid to be evaporated is fed at the lower end of the vertical tubes, heated by the condensing steam on the outside of the tube. The liquid starts boiling at the bottom, the vapour along with the liquid tends to rise further up the tube in the form of a thin film. As the film rises further evaporation occurs until they reach the top of the tubes. The mixture of vapour and milk then passes into a separate chamber from which the vapour passes to a condenser. The concentrated milk is removed by a pump.The main disadvantage of this type of evaporator is that a comparatively large volume of liquid is present in the evaporator at one time and most of it located at the bottom. Because of this, severe scale formation occurs more rapidly in this region.

vii) Falling Film Evaporator: This type of evaporator also consists of vertical tube bundles heated by condensing steam on the outside (Fig ). The liquid to be evaporated is fed to the top of the tube and allowed to flow downwards on the inside of the tubes. Arrangements are provided at the top to distribute the liquid evenly to all tubes in the bundle. As the liquid flows in a thin film rapid evaporation takes place. The resulting vapour and concentrated liquid flow into a separator at the foot of the evaporator.The vapour is drawn off from the top of the separator and liquid from the bottom by a pump.

Falling Film Evaporator

The advantages claimed for the falling film evaporator are short residence time of liquid, good performance with small temperature difference across the heating surface. Also more effects can be used with falling film evaporator because it will operate with a lower temperature difference than climbing film evaporator.

viii) Plate Evaporator: The tubular heaters described above are longer and occupy more space. Development of plate type heat exchangers indicates clearly the advantages of better heat transfer that could be obtained between metal surfaces and liquids when arranged in layers so that the liquid flows in highly turbulent manner in thin layer between the metal plates. Among the advantages claimed for this type of evaporator are compact nature of equipment, requirement of low head- room and the comparative ease for addition of plates or their removal from the assembly for increasing or decreasing the capacity.

ix) Centri-therm Evaporator: This type may be described as mechanical evaporator, the operation of which is based on number of conical surfaces rotating about a vertical axis, the gap between the cones providing the steam condensing and evaporating surfaces. Feed is introduced centrally from the top and steam centrally from the bottom. Boiling takes place on the conical surface and centrifugal force throws the concentrated product radially outwards to the rim of the cones from where it is expelled. Condensate likewise is thrown out radially and discharged in a similar manner. The concentric conical construction gives a compact form of heating surface and the high radial velocity obtained from the rotating action results in a thin high speed film moving across cones.

x) Mechanical Recompression Evaporator: In this system of evaporation process, a part or all of the evaporated vapour from an evaporating unit is compressed and this compressed vapour making up a large percentage of the heat-required for further evaporation. A mechanical recompression evaporator is generally limited to single effect, compressing the vapour by means of a positive displacement pump or centrifugal compressor which can be run by electric motor, steam turbine or diesel engine. All the vapours from single effect are compressed and returned to calendria with no vapour going to the condenser. This eliminates the cooling water requirement normally associated with conventional evaporators.In thermo-compression evaporator the compression of vapour is affected by the use of high steam pressure and jet type steam ejector. The vapour evolved from boiling milk are partially entrained by a jet of high- pressure steam and the vapours arising from this mixture of steam and vapour is injected into the calendria and forms the heating medium for incoming milk. This system enables a considerable saving in quantity of steam and water required for evaporator operation.

xi) Multiple Effect Evaporator: Considerable saving in steam and water can be effected by using multiple effect evaporator. The vapours from one effect have considerable latent heat and may thus be used for heating second effect and vapours from second effect to heat a third effect and so on. Thus great economies of operation may be made in large installations with multiple- effect evaporator. For example in a double effect unit, the steam and water consumption are just about one-half that required in a single effect unit. With double effect evaporator, the first effect operates at a lower vacuum than the second effect, which makes the boiling temperature higher in the first effect than the second effect. Since the vapour given off from the first effect are hotter than the boiling point of the milk in the second effect, they are used to heat the milk in the second effect. This cuts the steam and condensing water requirement to about half of what is required for a single effect evaporator. Units having as many as four effects are in use for evaporation milk with corresponding saving in steam and water. A four-effect unit will require only about one-fourth of steam and water per kilo-gram of water evaporated as compared to ordinary single effect. For a single effect evaporator steam required to evaporate one kilogram of water from milk is approximately 1.2 kilogram.

Manufacture of Sweetened Condensed Milk

Different steps involved in the manufacture of Sweetened Condensed Milk are given in the Figure .

Flow diagram for preparation of sweetened condensed milk

i) Receiving Milk : Raw milk received for the manufacture of condensed milk should be of good quality. This is important because the quality and marketability of final product depends upon the initial quality of raw milk. Great care, therefore, must be taken to ensure that only good quality milk is received for processing into sweetened condensed milk. After the milk has been accepted on the basis of standards platform tests, it is weighed, sampled and further processed.

ii) Filtration/Clarification: The objective is to remove visible foreign matter.Simple filtration may remove suspended particles by straining process while clarification removes the same by centrifugal sedimentation. Filters generally,contain cloth or pad of described pore size, which retain smaller particles,while clarification removes sediments/slime much more efficiently than filtration.Clarifiers remove even finer particles that escape filters. The clarified milk is then chilled and stored in large storage tanks until further processing.

iii) Standardization: The object is to standardize fat and solids-not-fat in milk to obtain a final product, which meets the legal standards. This operation is of considerable importance as the yield and keeping qualities of the final product are very much dependent on it. Further it also helps in maintaining the requirements of legal standards in the final product. Standardization establishes the desired ratio of fat: SNF in milk . Standardization of milk involves addition of sufficient quantity of cream or skim milk to ensure the resultant product has the correct fat: SNF ratio to yield the final product of desired composition. The following steps are followed to know whether cream or skim milk is to be added to milk for standardization purpose.

Find the ratio of SNF/Fat desired in the final product. (e.g., 22/9 = 2.44).

Accurately calculate the Fat and SNF percent of milk in the storage tank.

Find the ratio of SNF/Fat in milk, i.e., 8.9/3.6 = 2.47 or 9.58/6.5 = 1.47

If the ratio in milk is higher than the ratio desired in the final product, SNF is in excess in milk and hence more source of fat (i.e. cream) should be added to milk.

If the ratio is less, fat is more in milk and hence SNF source (i.e. skim milk)should be added to get the final desired ratio.The quantity of cream or skim milk to be added to milk for standardizing purpose may be determined by the following general formula:

To calculate the amount of cream to be added to milk:

(SNF)/F  (Qxs1 + Cxs2)/(Qxs1 + Cxs2)

Where

(SNF)/F=SNF:F ratio desired in condensed milk

Q = Quantity of milk in Kg available for standardization

C = Amount of cream in Kg required

F1 = Percent fat in milk

S1 = Percent SNF in milk

F2 = Percent fat in cream

S2 = Percent SNF in cream

To calculate the amount of skim milk to be added to milk.

(SNF)/F  ((Q*S1)+(S*S3))/((Q*F1)+(S*F3))

Where

(SNF)/F = F ratio desired in condensed milk.

Q = Quantity of milk in Kg required

S = Amount of skim milk in Kg required

F1 = Percent fat in milk

S1 = Percent SNF in milk

F3 = Percent fat in skim milk

S3 = Percent SNF in skim milk

Standardized milk is now ready for further treatment.

iv) Fore warming/Preheating: Fore warming or preheating refers to heating of milk before it is condensed. The object of this process is to reduce or eliminate bacterial and mould contamination and to destroy undesired enzymes present in milk which encourage slow chemical deterioration in the final product during storage. This treatment exerts an important influence in controlling the viscosity to avoid age-thickening or age-thinning in the finished product. Fore-warming also helps in dissolving the sugar which is added during the next stage of operation. Sugar dissolves much more readily in hot milk than in cold milk. Further, evaporation takes place more rapidly if heated milk is fed into vacuum pan or evaporator during concentration. In brief fore-warming helps in the following:

•  Destruction of microbial contaminants and enzymes• Control of storage defects e.g. Age-thickening and Age-thinning.
•  Dissolving sugar
• Un-interrupted boiling during evaporation process.
• Improve the heat stability of condensed milk.

Fore-warming temperature is determined as to provide optimum viscosity in the condensed milk without inducing thickening or thinning during storage. The temperature-time of fore-warming extends over a wide range, such as 82-93°C for 5-15 minutes; or 116-149°C for 0.5 to 5 minutes. The modern trend is towards high temperature short time heating such as 115-118°C without holding. Tubular heat exchangers are commonly used for fore warming or preheating; either a double-tube or shelf-and-tube heat exchangers are preferred. Plate heat exchangers may also be used.

v) Addition of Sugar: Sugar is added for the purpose of preserving condensed milk without resorting to sterilization by heat. Sucrose is generally referred as sweetening agent. Refined cane sugar or beet sugar is used in the manufacture of sweetened condensed milk. Other sweetening agents such as glucose,dextrose and corn syrup have been tried to partially replace sucrose. The disadvantages of these sweetening agents are their reduced sweetening capacity compared to sucrose and their adverse effects on colour and the rate of thickening during storage. Hence they are not suitable.

Amount of Sugar/Sugar ratio: The quantity of sugar added to milk should be enough to preserve the milk. It has been recommended that a sugar ratio of 62.5 would protect the condensed milk from bacterial defects. Generally sugar ratio of 62.5 to 64.5 % is considered adequate to protect condensed milk against bacterial spoilage and to protect against causing sugar crystallization. Sugar ratio is the sugarin-water concentration of condensed milk.

The following formula is used for thin purpose.

% Sugar ratio =( %Sugar in condensedmilk)/( 100 –%Totalmilk solids in condensesmilk)*100

Example:

Condensed milk contains 31.0% total milk solids and 43.1% added sugar. What is the sugar ratio?

Solution: % Sugar ratio =(43.1)/( 100 – 31)* 62.5%

The following formula is used to determine percentage of sugar required in condensed milk for desired sugar ratio.

Percent Sugar in condensed milk =((100 –%TMS)* SR)/100

Where, TMS = Total milk solids in condensed milk

SR = Sugar ratio

Example:

If, TMS in condensed milk = 31.0%

SR = 62.5

% Sugar in condensed milk =( (100 – 31) 62.5)*100 = 43.1

Amount of sugar to be added to milk to get the desired % Sugar in condensed milk.

First determine ratio of concentration by dividing Percent total milk solids in condensed milk by % total milk solids in fresh milk. Then divide the percentage of sugar in condensed milk by ratio of concentration.

Ratio of concentration =(%TMSin condensedmilk)/ %TMSin freshmilk

Sugar in milk = (%Sugar in condensedmilk)/( Ratio to concentration)

Method of adding sugar: The temperature and time of addition of sugar to the milk in the batch has definite effect on keeping quality and physical stability of the finished product. In one method part of fore warmed milk is run into a sugar mixing pan where sugar is mixed in a fine stream with proper agitation. The sweetened milk is then drawn into the vacuum pan. If sugar is added before condensing milk, an increase in viscosity and greater difficulty in the evaporation of water is experienced.Further the presence of added sugar in the fresh milk during fore warming increases the heat resistance and survival capacity of microorganism thereby adversely affecting keeping quality of the product. Another method is to dissolve sugar by boiling in water in a separate tank called sugar well. This mixture is then added to milk in the vacuum pan towards the end of the evaporation process. The disadvantage of this method is additional water has to be evaporated from added sugar solution. But boiling of solution destroys practically all microorganisms, which the sugar may contain and sugar solution can be filtered to remove any extraneous matters.

(vi) Condensing: Sweetened condensed milk is concentrated to a greater degree than is the case with the unsweetened variety. Vacuum pan method being usually employed in most condenseries for condensing milk.

Operation of Vacuum pan: The vacuum pan is sterilized with steam before the operations commence and vacuum pump is operated until 50-55 cm vacuum is obtained. The milk inlet valve of the pan is opened slightly and milk is drawn by the action of reduced pressure. When each section of the heating coil is covered with milk then the steam is gradually admitted. When the milk level is sufficiently high to cover the heating surfaces the milk inlet is partially closed in order to maintain constant level of milk. During operation heat is so applied as to ensure that the milk boils vigorously and a working vacuum 63-65 cm is attained. The speed with which the milk is condensed to the desired consistency depends upon the following factors:

a) Quantity of milk in the pan

b) Area of the heating surface

c) Capacity of the vacuum pump

d) Temperature of condenser

Striking the batch: The term refers to the end of concentration operation in the vacuum pan as determined by specific gravity or density tests. The precise point at which the batch should be “struck” depends largely upon the experience of vacuum pan operators. Samples are taken at regular intervals by means of the sampling cocks with which the pans are provided. Baume’hydrometer or a viscometer is used to determine the progress of concentration. Baume’Hydrometer test is most commonly used for density tests of vacuum pan samples.The hydrometer scale may record the density either directly or indirectly. For condensing milk the Baume’Hydrometer ranges from 30-37° be at 49°C (120°F). When the desired concentration as indicated by the Baume’ reading (32°Be’) is reached, the batch of milk is ready for removal from the pan. This operation is called striking the batch. The steam is shut-off, the vacuum broken and the milk run off to cooling tank.

vii) Cooling: Cooling of condensed milk is essential to control the texture of condensed milk. Sweetened condensed milk is a highly concentrated solution of lactose and sucrose. The quantity of sugar which milk can carry in solution varies according to the temperature of milk. As the condensed milk is cooled, lactose which is less soluble than sugar crystallizes out and if the lactose crystals are small enough these crystals will not be detected but if they are too large they will impart sandy texture to condensed milk. If the crystal size of lactose grows bigger during cooling or during subsequent storage, gritty texture becomes apparent in sweetened condensed milk. To avoid this defect it is necessary to create conditions favorable for maximum crystallization of lactose.

viii) Forced crystallization: The purpose of this is to produce mass crystallization of lactose. It is the period in the cooling process when sweetened condensed milk reaches a temperature, which is most favorable for rapid crystallization of lactose.For sweetened condensed milk of average composition the temperature of maximum rapidity of crystallization is around 30°C. This temperature is optimum for seeding of condensed milk.

Seeding: Crystallization of lactose is usually assisted by the addition of fine powder of lactose or small quantity of condensed milk from previous batch in which the lactose crystals are in minute form. Other alternative is to use spray dried skim milk powder as seed material. Seeding refers to the introduction of lactose in a very fine powder form during cooling process to provide nucleic for crystallization. The purpose is to give lactose, present in the super saturated state, an added incentive to crystallize. Seeding at an optimum temperature for mass crystallization with properly prepared lactose and with vigorous agitation yields large number of small crystals of uniform size. The recommended amount of finally pulverized lactose for seeding purpose is 375-500 gm per 1000 kg of initial milk taken or 0.1 to 0.3 percent of condensed milk. For best result of forced crystallization the hot condensed milk should be cooled from pan temperature as rapidly as possible to the seeding temperature.

Method of Adding Seed lactose: The required amount of seed lactose is blended with small quantity of condensed milk and added to the batch with vigorous agitation. After seeding and forced crystallization, further cooling of the product is continued slowly to 24-25°C for about an hour or longer when the product is ready for packaging. The packed condensed milk is then stored until further use.

Cleaning and Sterilization of Plant: As with all other plants used in the preparation of food products the equipments used in the production of condensed milk should be cleaned and sterilized immediately after use. All internal surfaces of the plant should be washed first with cold water, then with detergent solution, hot water and finally sterilized by means of live steam. If cleaning-in-place (CIP) system is available,recommended procedures should be followed for cleaning and sterilization of the plant.

Sweetened Condensed Skim milk: The manufacture of sweetened condensed skim milk is similar in all respect to that of sweetened condensed whole milk.