Deffcts in Cream and Their Control

The requirements of high quality table cream or manufacturing cream are the same as given here under.

i) Clean, sweet, pleasant flavour.

ii) Smooth, uniform and reasonably viscous body and texture.

iii) Low titratable acidity.

iv) Packaged in clean, leak-proof containers under hygienic conditions.

Defects in cream develop during storage. These defects may be due to poor initial quality of milk from which it is separated or faulty methods of production, processing,packaging and storage. As the storage period advances, the titratable acidity of cream also may increase. This may lead to defects like the curdling/serum separation.During storage, usually cream is prone to the development of flavour defects more rather than body and texture defects.

 

i. Flavour Defects and Control Measures

Common flavour defects likely to develop during storage are given here with their probable causes and control methods.

Acidic flavour may develop due to all or any of the following causative factors:

i) Use of sour milk for cream separation,

ii) Holding raw cream for longer period at ambient temperature,

iii) Using less than recommended time-temperature combination for pasteurization,

iv) Prolonged storage,

This requires strict maintenance of sanitary condition in the plant, reception of only good quality milk, using proper processing methods and storage temperature.

Cooked flavour: Uncontrolled excessive heating at any stage of processing of cream may lead development of cooked flavour. Hence, it requires check on pasteurization temperature in particular.

Oxidized/Oily/Metallic/Tallowy flavour: These kinds of flavour develop due to oxidation of fat caused by exposure of cream to light, metals, like copper/iron.Hence, its control requires proper packaging material and avoiding any metallic contamination during processing of cream.

Rancid or Bitter flavour: This is due to hydrolysis of fat by the action of enzyme lipase. Lipase may come from initial under heating and high microbial population.Entrance of moisture through packaging enhances the problem. Therefore, proper heat treatment, prevention of post processing contamination and proper packaging may serve as control measures. Bitter flavour may also come from milk if the animal is fed on some bitter weeds.

Cheesy flavour: The flavour of cream resembles that of cheddar cheese. Presence of proteolytic bacteria in cream leads to breakdown of protein causing this defect.Prolonged storage may allow psychrotrophic bacteria to grow at low temperatures and cause the defect. Prevention of post pasteurization contamination and quick disposal of the product may help reduce the problem.

Yeasty/Fruity flavour: Microbial contamination, use of improper processing methods,post processing contamination, unhygienic conditions prevailing in the plant lead to the development of undesirable flavour like that of vinegar, etc. It requires proper hygienic control at every stage of processing besides plant hygiene.

Utensils flavour: Use of dirty utensils imparts this objectionable flavour to cream.Therefore all the utensils and equipment should be properly cleaned and sterilized before and after use.

Feed/Weed flavour: This type of flavour defect comes from milk from the milch animals fed on some objectionable flavour imparting feed or weeds. Therefore strict control should be exercised while receiving the milk to control the defect.

 

ii. Body and Texture Defects

Curding: This defect is indicated by the appearance of flakes or coagulated particles in cream. This defect may result due to the following reasons:

i) Initial poor quality.

ii) Development of acidity.

iii) High temperature of storage.

iv) Post processing contamination.

Phase Separation: When this defect develops the watery portion and coagulated particles or lumps form two layers in the containers. It mostly happens due to thawing of frozen cream. It is, therefore, better to avoid frozen storage of cream to control the problem.

Undesirable fermentation: It may result in flavour as well as body & texture defect. This is mainly caused by the contamination of cream at any stage of processing particularly after pasteurization. It is, therefore, necessary to take all the precautionary measures to control contamination during and after processing including packaging to avoid the problem.

Packaging and Stroage

i. Packaging Machines

Packaging machines used for milk or condensed milk can also be used for packaging of cream. These include

•  Bottle filling machine (wide mouthed bottles)
•  Capping machine
•  Can filling
•  Can sealer
•  Form/Fill/Seal machine for Sachet filling
•  Heat-sealing machine

ii. Physical Characteristics of Cream

We know that several types of cream are available in the world market. They differ in their physical characteristics from each other. Some of them are free flowing and easily pourable (light cream), while others are highly viscous, difficult to pour or non-pourable but spoon able (heavy cream). The physical characteristics of different types of cream demand different forms of packaging material. Therefore,it is essential to use the most suitable form of packaging, which protects the product and at the same time has appealing influence on the customers.

The physical characteristics are influenced by the following factors:

i) Method of production.
ii) Fat content
iii) Treatment of cream
iii. Packaging Materials

Traditionally, fresh cream was packed in metal cans, glass bottles/jars, waxed paperboard cartons with press-in-lids of the same material, etc. The paperboard cartons have a tendency to absorb moisture and are damaged during storage under refrigerated temperature. Now many packaging materials of different characteristics are available in the following forms, which are commonly used for cream.

a) Plastic Cups/Collapsible tubes/Plastic tubs with crimped-on-skirted caps of aluminum-foil (Al-foil).
b) Plastic tubs with Al-foil sealed to the rim of the container.
c) Polystyrene tubs with heat- sealed Al-foil closures.
d) Rectangular gable topped waxed or plastic coated paper board packages.
e) Plastic bag-in-box for bulk packaging of cream.
f) Tinned steel cans with slip-on lids (Small capacity).
g) Wide mouthed omni a-sealed opaque glass jars.
h) Heat -sealed collapsible plastic tubes.
i) Cartons of tetrahedral types.
j) Aerosol packaging system.

Earlier pasteurized cream was packed in glass-bottles or in waxed or polyethylene-coated cartons. Now the most common form of packaging is the injection moulded polyethylene tubs or flat-topped round containers. The containers are filled with the product and closed with heat sealed polyethylene-Al-foil laminate. A clear plastic lid is often provided to consumers to reseal the containers once opened. Bag-in-box system of packaging is used for bulk packaging of pasteurized cream.

Sterilized cream is packaged in cans and in containers like crown-capped bottles using machines similar to that used for sterilized milk. Wide mouthed omni-a-sealed opaque glass jars, and hermetically sealed tinned steel cans are also used for sterilized cream.UHT cream requires aseptic packaging. Various packaging formats exist for packaging UHT cream. The form-fill-seal type laminated cartons are one option in which case the presence of an Al-foil layer in the laminate is essential to protect the product from the influence of light and initiation of auto oxidation during storage. Thermo formed containers made from co-extrusion of polyethylene and poly-vinylidene chloride are also used.These containers are closed with heat-sealed polyethylene-Al-laminate foil.
Blow moulded polyethylene or polypropylene containers formed at the point of filling may also be used. Bag-in-box packages made from metalized plastic laminates may be aseptically used for bulk packaging of UHT cream.

Pasteurized heavy cream requires wide mouthed containers while pasteurized,homogenized cream does not require such containers. Sterilized or UHT low fat cream requires opaque containers so that the phase separation (serum separation) is not visible to the customers. Phase separation usually occurs in high heat treated low fat cream on prolonged storage.Clotted cream is packed at 4-7°C in polystyrene flat-topped round containers.These are filled and closed with a polyethylene-Al-laminate foil. Alternatively, it is often packed in square or rectangular shallow high-density polyethylene containers closed with a clear plastic film and inserted in a paper sleeve carrying the product information. Small capacity tinned steel cans with slip-on lids are also used for clotted cream. Aerosol cans and PE tubs with-snap-on lids are suitable for packaging whipping or whipping cream.

 

iv. Packaging Methods

Packaging methods for cream are similar to those used for packaging of milk. It requires correcting dosing of cream by volume in the containers and then properly closing with lids or treat sealing. It is important to remember that all the operations involved in supply, filling and sealing of containers must be carried out in highly hygienic conditions. The containers, machines, pipelines, etc. must be thoroughly cleaned and properly sterilized before they are used.

 

v. Storage

Cream is considered as a short shelf life product. In general, pasteurized cream packed in a closed container is expected to remain whole some for 5-7 days under refrigeration storage (5-10°C). UHT aseptically packaged cream is stored at ambient temperature and is expected to remain acceptable for 1-6 months. Sterilized cream might remain good for 6-12 months at ambient temperature. However, once the containers containing UHT or sterilized cream is opened it should be treated as only pasteurized cream. Storage of cream in frozen state destabilizes the emulsion (oil in water type) of cream. On thawing of frozen cream, separation of fat phase and serum phase takes place. Such cream is not useful for whipping but can be used for some products like soups to impart flavour.

 

vi. Distribution

It is advisable to distribute pasteurized cream to the consumers as early as possible preferably within 3 hrs of removing it from the cold store. Always fresh cream should be delivered to the dairy plant for manufacture of products

Defination and Packaging Requrirements

The protection offered to a product against any damage caused by the outside influences of environment (humidity, light, oxygen, temperatures), foreign odour,chemicals, and insects, physical hazards, etc. during storage, transportation, distribution and retailing is referred to as packaging. In other words, enclosing a product in specially designed containers to achieve the desired protection against spoilage is packaging. Packaging is different than packing. Packaging is the primary package,

which is in direct contact of the product. It provides initial and major barrier against spoilage of the product. It is the primary package in which the consumers purchase the product from a retail shop. Packing refers to enclosing several individual items packed in primary packages in a container for shipment from one place to another.Thus, packing forms a secondary package and contains a number of primary packages.

The following factors must be considered if the product is to be protected against spoilage and remain acceptable until it is consumed.

a) Composition of the product.

b) Storage conditions (temperature, humidity, light, gases, dust, microorganisms).

c) Type and probable cause of spoilage.

d) Physical hazards during storage and subsequent handling and transportation.(Shocks, vibrations, compression).

e) Consumer’s requirement in terms of capacity, and functional properties.

f) Availability.

g) Interaction between product and package.

h) Economics of packaging.

 

i. Advantages of Packaging

•  Contains and protects the product against damages caused by mechanical,biological and environmentl agents.
•  Informs about the product, e.g. nutritional and compositional quality, date of manufacture, weight, cost, conditions for storage, shelf life, uses, etc.
•  Convenience of handling to the retailers and buyers both.
•  Facilitates storage, transportation and distribution of product.

ii. Requirements of Packaging

Cream is that part of milk which is rich in milk fat and has been obtained by gravity separation (hand skimming) or by mechanical separation of milk. It may be considered as an intermediate product between milk and butter. The aqueous phase (water) in cream is reduced but the oil-in-water emulsion is not broken unlike butter, which is a water-in-oil type emulsion. Cream contains all the constituents of milk but the solids-not-fat content is inversely proportional to its fat content. Thus with the increase in fat content of cream the proportions of protein, lactose, mineral salts and also water soluble vitamins decrease while the proportion of fat soluble constituents such as phospholipids, cholesterol and fat soluble vitamins increase.Cream has unique natural flavour. But the flavour of cream is very susceptible to spoilage because of high percentage of fat (normally 18-80%). Thus it requires protection. For example, the presence of light may initiate auto-oxidation of milk fat and produce flavour defect like rancidity in cream. Homogenized cream is more susceptible to the action of light than un-homogenized cream. Cream may also absorb odours from outside environment like gases, moisture or fat and develop flavour defects. Thus, it requires a packaging material, which provides protection against all the agents causing spoilage of cream.

The cream requires protection against light, gases, and absorption of outside odour and gain or loss of moisture. The packaging material, therefore, must be impermeable to all these agents. In addition it should have sufficient mechanical strength, tear strength, burst strength and wet strength to provide protection against mechanical hazards during storage, and subsequent handling to prevent the loss of cream. The packaging material should not produce any toxic substance or impart its own flavour or colour to the product. Metal cans, coloured glass jars or bottles, plastic containers,incorporating one or more layers of nylon, containers made of laminated materials in which one layer is of aluminum foil or similar material are common which meet most of the desired requirements for cream packaging. The packaging material may contain compounds like printer’s ink, dyes or monomers from plastic materials.These materials may migrate to or interact with the cream constituents causing gradual deterioration in quality. Hence, proper selection of packaging material is very important.

Packaging,Stroage and Common Deffcts in Cream

Packaging is a technique used to contain, protect and identify a product. It facilitates its transportation, distribution and safe delivery of a product to the consumers.Thus, it contributes significantly to marketing and sale of a product. The packaging requirements to preserve natural quality of a product vary with the products. For example, skim milk and cream require materials with different characteristics. Skim milk does not require a material, which is impermeable to light whereas for cream or any other fat rich product impermeability to light is an essential requirement for packaging material. Consequently, a large number of packaging materials like polymers(single layer polymer, multi-layered polymer), metallic foils, metal cans, glass-bottles and coatings have been developed to meet the requirements of different products.In addition, different packaging machines and methods are also available to pack the product in different forms and shapes. In this unit, we will study the packaging requirements for cream and the common defects, which may develop during its storage. This until will also deal with the probable causes of such defects and their preventive measures.

Composition and standards

Cream is not a specific product. It can be produced with different fat content.Other constituents in cream vary in their proportion depending on the fat content of cream. As the fat content in cream increases, the other constituents (solids-not fat) decrease in their proportion. The average chemical composition of cream containing different fat levels and of different types is given in the following tables

The solids-not fat (SNF) content of cream can be calculated by the formula given below:

Percent SNF = (100 - % fat in cream)/(100 - % fat in milk) x (% SNF in Milk)
               

Average chemical composition of cream of different
fat content

Average chemical composition of different
types of cream

 

i. Standards Requirements

It is the responsibility of dairy industry and the Governmental agencies to safe guard the interest of the consumers with respect to the safety and quality of the product. Cream is not a product of any fixed composition. Hence, it becomes necessary to set standards of quality to meet the above responsibility effectively.In India, we have standards of quality set by Prevention of Food Adulteration Act and Bureau of Indian Standards. It is essential to meet the quality standards set by the PFA before marketing of any dairy product.

PFA specifies that cream, excluding sterilized cream, is the product obtained from cow milk or buffalo milk or a combination thereof, which contain not less than 25 per cent fat.

United Nations Food and Agricultural Organization (FAO) and World Health Organization (WHO) have also specified quality standards for different products.As regards cream FAO a and WHO have specified standards for fat content in designated cream for international marketing Table

Standards for cream for international marketing

Uses of cream

The unique natural flavour of cream imparts richness to many food products and enhances flavour quality. Cream contains milk fat, which has melting points in a narrow range. This facilitates quick release of flavours. Cream also imparts smoothness and viscous mouthful to many food products. Milk fat in cream serves as a carrier of fat-soluble vitamins like A, D, E and K. Hence it enriches the food products with vitamins. The presence of protein in cream acts as an emulsifier and helps in emulsification, aeration, foaming. It helps in whipping of cream giving over-run to food products. Cream adds a whitening effect to the product and improves
the appearance and colour. It can also impart browning to cooked food due the reaction between proteins and lactose present in cream. The functional properties of cream mentioned above make it suitable for use in many food products. It may be used directly from the container as an ingredient in the preparation of several food products or serve an intermediate product. Some of the common uses of
cream in which it used in food industry are listed below:

i) Directly in tea/coffee hot chocolate/other beverages to impart richer characteristics colour, texture, flavour and preparation of table/coffee/ whipping cream to used directly as toppings to fresh fruits, desserts and cereals.

ii) Preparation of some special dishes like fruit cream, cream salad, sauces cheese cakes, etc.

iii) Production of plastic cream, frozen cream, whipping cream, sour cream, as intermediary product.

iv) Manufacture of butter, ghee, cream cheese, ice-cream, butter-oil.

v) Creaming cottage cheese

vi) Standardization of milk and milk products.

vii) Sweet-syrups like caramels, fudges, fruit syrup.

viii) Bakery products and confectionary products.

ix) Alcoholic cream beverages.

x) Piping cream-whipping cream is often piped onto cakes and desserts.

Whipping cream

It may be defined as a cream product containing air bubbles captured in a network of fat globules. Cream containing more than 30% fat can be converted into whipping cream by beating it with air to nearly double the volume of cream. Beating cream produces foam by incorporating air in the cream. The foam thus produced has remarkable stability. If whipping is continued it yields butter because of the joining of the fat globules together. Homogenization of high fat whipping cream produces excessively viscous product and hence not desired. It also decreases the functional property of whipping cream. Homogenization, however, is used for the production of spoon-able whipped cream and in UHT pasteurized whipping cream to prevent fat separation.

Whipping cream is used in cakes, ice creams and for decorative toppings. It is available in small bottles, plastic cans or large cans. It is also marketed as in-can sterilized cream and even supplied with sugar and a driving gas in an aerosol-can that delivers a ready-made whipped cream.Cream is usually whipped immediately prior to use. Whipping cream should posses a pleasing flavour, good keeping quality, good whipping ability and stability after whipping. The whipping quality of cream is influenced by the following factors:

i) Fat content 35-36% is preferred

ii) Composition of fat i.e. proportion of crystalline fat and liquid fat. More crystalline fat is preferred.

iii) Combination of cream-protein: It is needed in the composition to help formation of air cells.

iv) Surface-active substances: Addition of surface-active substances like mono-glyceride or Tween helps.

v) Temperature of cream: Higher temperature lowers the whipping quality.

vi) Separation temperature of cream.

vii) Ageing: Ageing at 4°C is desirable to produce more crystalline fat in cream.

viii) Acidity of cream: If the acidity of cream is higher it reduces whipping quality.

ix) Addition of stabilizers: It reduces whipping ability.

Cream obtained from pasteurized milk is standardized to contain 35-36% fat. The standardized cream is stored for 24 hours at 4°C so that all the fat globules contain some crystalline fat. A thickening agent like k-carrageenan @ 0.01% is added to cream to prevent creaming or serum separation during storage. The aged cream is then pasteurized preferably by holder method at a temperature of 74°C for 30 min. In-can sterilization of whipping cream is not advisable as it causes coalescence of fat globules unless it is homogenized. But homogenization impairs the whipping ability of cream. UHT heating of cream homogenized at low pressure (e.g. 2 and 0.7 M Pa at 35°C) and containing some emulsifiers is preferred. However, fluctuation of temperature during UHT treatment may increase the viscosity, decrease whipping properties and cause churning.

The pasteurized and cooled cream is stored in cold store to effect crystallization of liquid fat. Whipping i.e. beating of cream with air is done prior to use of whipped cream. Whipping produces stable foam by enclosing air bubbles in network of fat.Slow beating of cream may cause churning of cream while vigorous beating results in high over-run and finally smooth foam. Prolonged whipping may also cause churning and yield butter and butter milk. Whipping effect can also be produced in aerosol can without clumping of fat globules and beating. The foam is formed when the gas pressure in cans is released. The foam thus produced is unstable. Therefore,it requires addition of some surfactants or presence of proteins to impart some degree of stability by protein. The process of preparation of whipping cream involves the steps outlined in Fig.

Flow diagram for preparation of Whipping cream

Sour Cream

It is a cream product, which is prepared by ripening or souring of sweet cream to a titratable acidity of 0.60%, L.A with desirable lactic acid bacteria. It has slightly acidic flavour and smooth texture. Sour cream is usually used in the preparation of different foods and sometimes in drinks and beverages. The following steps are involved in the preparation of sour cream Fig

Flow diagram for preparation of Sour cream

Sour cream is prepared from fresh and sweet cream standardized to contain 18-20 % fat. The standardized cream is pasteurized by LTLT or HTST Method and cooled to 60-65°C. At this temperature it is homogenized employing two-stage homogenization process. Low temperature homogenization is preferred to promote formation of fat clusters, which during ripening flocculate and also increase the viscosity of the product and improve the texture. Homogenized cream is then inoculated with 2-4% fresh and active starter culture containing lactic acid and flavour producing microorganisms, namely, Streptococcus lactis, Streptococcus cremoris and diacetyl lactis. The inoculated cream is then incubated at a temperature of 22°C and allowed to develop titratable acidity and desired body and texture.Usually, it takes 10-12 hrs to attain the desired acidity. Sometimes some thickening agent or rennet is also added to sweet cream to increase the firmness of the sour cream. When the cream has attained the desired acidity (0.60% LA or pH 4.5) it is cooled with gentle stirring and chilled to 2-4°C followed by packaging.Alternatively, the cream after homogenization and inoculation is filled in the containers and the packages are incubated at 22-25°C. The acidity develops. Then the packages are transferred to cold rooms where cooling of the product is achieved.

Frozen Cream

As the name suggests the cream in frozen state is called frozen cream. Freezing of cream inhibits bacterial growth. It is prepared to improve keeping quality of cream so that it can be transported without spoilage over a long distance in a refrigerated van. Freezing also helps to store surplus cream for use during shortage.Ice-cream manufacturers use most of the frozen cream. It is also used in recombined milk and cream soups. Addition of 10-15% sucrose by weight to cream before freezing is helpful in preventing oiling-off after thawing of frozen cream. During freezing ice crystals are also formed. These crystals rupture the membrane enclosing the fat globules. Frozen cream, therefore, tends to oil-off on thawing especially at higher temperature. Oiling-off in cream also impairs whipping property of the product. Preparation of frozen cream also starts with collection of good quality of milk, its filtration and separation. Resulting cream is then standardized to contain 40-50% fat and then pasteurized. Pasteurization is done by heating cream at 74°C for 30 minutes in a batch type of pasteurizer or plate heat exchanger. The heated cream is cooled after the holding period to 4°C. The pasteurized and cooled cream is then filled in paper/plastic containers or cans and sealed. These containers are then transferred to a freezing chamber maintained at a very low temperature (-20°C). The cream freezes after a sometime in the containers. The frozen cream is stored below -12°C. Preparation of frozen cream involves the following steps as shown in Fig.

Flow diagram for preparation of frozen cream

Freezing of cream could also be achieved by employing any of the following methods:

i)Blast freezing chamber- Bulk containers, containing cream, are allowed to pass through a chamber where they meet a current of chilled air effecting freezing.

ii) Plate freezers

iii) Rotating drum freezers

iv) Cryogenic freezing tunnels

Plastic Cream

Plastic cream is a highly viscous product than any other type of cream. It resembles to paste in texture. It contains fat between 60-85 %. However it differs from butter in that it is still a fat in water type emulsion in spite of containing fat near or equal to butter. The plastic cream is used directly for the manufacture of butter oil or in the preparation of standardized milk. It may also replace use of butter oil in the preparation of recombined milk. Plastic cream can be prepared by any of the following methods.

i. Re-separation of normal cream containing 30-40% fat in a normal cream separator.

ii. Separating milk in a specially designed plastic cream separator to yield high fat cream.

For preparation of plastic cream by any of the above two methods, it is essential to pasteurize the initial product i.e. cream and milk. In case of milk it is heated before separation to about 50°C and then separated. Pasteurization of cream is done by heating to a temperature of about 74°C for 30 minutes followed by cooling to 60-66°C before re-separation. Resulting products of the separation will be plastic cream and skim milk. The cream is then packaged and stored for further use.

Sterilized Cream

It is also known as Table cream. The objective of preparation of sterilized cream is to prolong the shelf life of cream and make it available to the consumers in a ready-to-use form and in good condition. Sterilized cream should be smooth and free from lumpiness and separation of serum. The high heat treatment, however imparts it a peculiar flavour. Homogenization increases its viscosity. The whipping quality of sterilized cream is also poor. The fat content in sterilized cream ranges between 20-25 % and solids-not-fat content between 6.5-9.5 %.

 

Manufacturing steps

Flow diagram for preparation of sterilized cream

 The steps involved in preparation of sterilized cream are given in fig. Selection of cream: Fresh and sweet cream is collected for the preparation of sterilized table cream. The percent titratable acidity (TA) of freshly separated cream is always lower than that of the milk from which it has been separated.

The T.A can be calculated by the following formula:

Percent T.A. of cream=(Percent serum in cream)/(Percent serum in milk) x Percent T.A of Milk
                                      

                    = (100 – Percent fat in cream)/(100 – Percent fat in milk) x Percent T.A of Milk
                                        
Standardization: Standardization refers to the adjustment of milk fat level in cream to the desired level i.e. conforming to the standards requirement. The fat level(percent) in cream is usually adjusted or decreased to the desired level by addition of calculated quantity of skim milk. The quantity of skim milk to be added to cream is calculated by the Pearson’s square method as explained below (Fig)

Pearson’s Square

Let us assume that the fresh sample of cream contains 40 percent fat. It is required to reduce its fat to 20 percent. The available skim milk for standardization contains only 0.1 per cent fat.

The required quantity of skim milk to be added to cream is calculated as follows by drawing a square and writing the fat percentage of cream (40%) and skim milk(0.1%) on the two corners on the left side of the square.

(i) The desired fat percent level (20%), in cream is written in the centre of the square.

(ii) Now subtract the number in the centre (20) from the larger number (40) at the left-hand side of the square and place the remainder (20) at the diagonally opposite right-hand corner.

(iii) Again subtract the smaller number (0.1) on the left hand side from the number in the centre (20) and place the remainder (19.9) at the diagonally  opposite right-hand corner.

The numbers on the right-hand side now represent the number of parts of cream and skim milk to be mixed to obtain cream containing 20% fat. It means that if 19.9 parts by weight of cream containing 40% fat is blended with 20 parts by weight of skim milk (0.1% fat) will yield cream containing 20% fat.Pre-heating: The purpose of pre-heating of cream is to delay its spoilage during subsequent processing steps. Pre-heating inactivates undesirable micro-organisms and the enzymes particularly lipase. It also helps in efficient homogenization of cream. Standardized cream is pre-heated by heating it to 80-90°C without holding.

Homogenization: The purpose of homogenization of cream are as follows:

i) To prevent rising of fat during sterilization and storage.

ii) To prevent formation of a fat plug in the container.

iii) It also improves taste, whipping power and flocculation stability of the product.Flocculation of coffee cream is a major problem. When cream is added to hot coffee, flocculation occurs mainly because of the precipitation of casein.Homogenization of casein free cream enriched with whey protein and pre-heated to 90°C for 5 min. improves the flocculation stability probably because of whey-protein denaturation.

Homogenization of pre-heated cream at 80-90°C is carried out in two stages employing homogenization pressure of 25-30 kg/sq cm in first stage and 5kg/sq cm in the second stage. In the first stage the fat globules are sub-divided in smaller size globules and distributed uniformly in the whole mass of cream; it increases viscosity. The second stage reduces viscosity and prevents the fat globules from clumping together and forming lager clusters. Clumping refers to the tendency of fat globules to loosely adhere to one another to form clusters. The tendency of clumping is influenced by the following factors.

i) Fat Globule size - Large fat globules clump more readily than smaller ones.

ii) Temperature - Clumping tendency decreases with increase in temperature.

iii) Agitation - It increases clumping tendency at lower temperature (7°C) while at higher temperature (60°C) the tendency of clumping decreases.

iv) Separation Method - Gravity separation yields cream with increased tendency of clumping than that obtained by centrifugal separation with the same fat content.

v) Adhesive - Addition of adhesive helps in clumping.

Cooling: Since filling of cream in the containers requires holding of cream for sometime there is every possibility of increase in the titratable acidity of cream.This may cause problems during sterilization process. Therefore, it is essential to cool the cream to about 15-16°C to prevent any change in the acidity of cream if the manufacturer requires enough time to fill the cream in containers in good condition and seal them. However, if filling and sealing do not require holding of cream, it can be filled directly from the homogenizer in to the containers and transferred immediately to the sterilizer. Thus, it does not require cooling.

Filling and Sealing: Homogenized cream is filled in containers (lacquered tin cans,glass bottles, retortable pouches etc.) leaving minimum head space and sealed.These containers are then transferred to the sterilizer.

Sterilization: Sterilization of cream improves the shelf life of cream by making it free from viable organisms. In-can sterilization of cream is done in retorts. The filled cans or bottles are packed in retorts/batch sterilizer and then heated allowing 15 minutes for coming up time to desired temperature (118°C), 12-15 minutes holding time at 118°C and 15 minutes for cooling to room temperature. Thus, it takes 45 minutes to complete one cycle of sterilization process for one batch. After completion of the process the tins/bottles are cleaned and tested for leakage by a immersing them in clean water. Leaking tins/bottles are then discarded separately.

Storage and Distribution: The leak proof tins/bottles or packages are staked properly in cardboard cartons and stored at room temperature. Properly sterilized cream is expected to keep good for few months.

Preparation of Different Types of Cream

Cream is classified on the basis of its fat content. The fat content in cream may vary over a wide range (10.0 % to 85.0 %). The heavier the cream, the higher is the proportion of fat contents. The fat content in half and half cream is approx.10% fat. Various grades of cream include half-and-half cream, light cream, medium and heavy cream, etc. Low fat cream is also called as market cream which includes table cream, sterilized cream, light-cream, coffee cream etc. and usually contains 12-25% fat. The market cream is used for direct consumption. The cream containing more than 40% fat is known as manufacturer’s cream and include whipping cream, heavy cream, plastic cream, etc. Whipping cream usually contains fat between 28-35% while plastic cream between 60-85%. In this unit, we will learn various grades/types of cream, their method of preparation and uses.

Processing of Cream

The processing of cream depends on the purpose for which it is required. In general following processes are involved for different types of cream.

 

i. Standardization

The fat content of cream is adjusted to the desired level either by the addition of calculated quantity of water or skimmed milk. This step is referred to as standardization. The use of water or skim milk depends on the purpose for which the cream is required. If the buttermilk is to be used for drying or standardization of milk for products making or for beverage preparation the standardization is done with skim milk. The purpose for which cream is to be used and the suggestion for use of skim milk or water are indicated below in Table

Standardizing agents for cream

The cream after standardization is subjected to homogenization and appropriate thermal processing prior to packaging for retailing.

 

ii. Homogenization

Cream is an oil-in water emulsion. Therefore the cream standardized to various fat levels with skim milk must be homogenized to reduce the fat globule size and increase the stability i.e., prevent fat separation. Homogenization is a process whereby cream is forced through a narrow orifice under considerable pressure.This breaks up the fat globules to smaller size and ensures they are evenly distributed throughout the cream. By homogenization various grades of viscosity in cream products can be obtained. Usually lower pressures are used for cream than that used for milk products. Whipping cream is rarely homogenized as this process greatly reduces the whipping ability of cream. Stabilizers such as mono-glycerides are added to improve whipping ability. On the other hand, homogenization of high fat cream is utilized in the production of spoon-able whipped cream products, which are used as dessert toppings. Homogenization is employed for UHT pasteurized whipping cream in order to prevent fat separation, which increases with UHT pasteurization.

 

iii. Thermal Processing

The thermal processing of cream involves either pasteurization or sterilization. The cream could be pasteurized by either batch or continuous method. The sterilization could also be done by either of the two methods, i.e., by batch method using counter pressure autoclaves referred to as retorts or by continuous method followed by aseptic packaging. The time-temperature combination for cream is higher in comparison to milk processing. This is simply because it contains more total solids than milk. The possible time-temperature combinations are indicated below.

Batch Pasteurization - 74°C/30 minutes
Continuous Pasteurization - 85°C/25 seconds
Batch Sterilization - 115-120°C/15 minutes
UHT Processing - 135°C/1-3 seconds

As soon as the heat processing of cream is completed it is cooled to less than 10°C to avoid the growth of heat resistant micro-organisms during storage. In practice it is cooled to 4-5°C.

 

 iv. Packaging and storage

The cream after processing is packaged in suitable containers and stored at low temperature (below 5 °C). If the cream is sterilized or UHT cream it can be stored at ambient temperature.

Separator slime and its Compostion

During the course of operation of cream separator a slimy mass consisting of foreign matter, milk proteins, fat calcium phosphate, leucocytes, bacteria and red blood cells are accumulated in the bowl shell of the cream separator. This accumulated mass is called separator slime. As the accumulation of slime increases,the efficiency of fat separation from milk decreases. The reduction in efficiency of separation is ascribed to the decreased effective diameter in the bowl due to accumulation of sludge. In the modern separators there is a provision to discharge this slime automatically and maintain the efficiency of the cream separation. The average composition of the slime is presented in the Table
 

Composition of separator slime

The quantity of slime depends on quality of milk. Under Indian conditions this is observed to vary between 0.05 – 0.20%.Definition, Composition,Standards and Processing of Cream

In certain parts of Southern India, the slime is coagulated with commercial acids and casein of industrial grade is extracted.

Yield of Cream

The yield of cream from milk can be calculated by the formula given below:

Quantity of cream =(Fat per cent in milk - Fat per cent in skim milk)/(Fat per cent in cream - Fat per cent in skim milk) X Quantity of milk

In a dairy plant for practical purpose approximate yield of cream can also be calculated in the following manner:

i) Calculate the total quantity of fat in milk.

ii) Deduct the fat loss amounting to about 3% of total fat, which is lost in skim milk under standard operating conditions.

iii) The approximate yield of cream containing different fat percentage per kg of available fat is obtained by multiplying it by a multiplication factor given in Table below. However the multiplication factor for cream containing fat percentage other than that mentioned in the table has to be calculated separately.

Let,

Quantity of milk = M

Fat per cent in milk= fm

Total fat in milk = M fm /100 = F

Loss of fat at the rate of 3 % of total fat = 3 F / 100= 3 M fm / 10,000

Available fat = F 1= Total fat in milk – Fat in skim milk

= F - 3F / 100 =100 F – 3 F / 100

(100 F – 3 F)/100

= 97 F / 100 =0.97 x M x fm /100 = .0097 x M x f

Yield = Y = K x M x fm

Where,

M= quantity of milk

fm = Fat percent in milk

K = Constant=0 .0097

In order to estimate the yield of cream the total fat is multiplied by the multiplication factor as shown in Table depending on the fat content desired in cream.

Multiplication factor for calculating yield of cream

 If 4.0 kg milk fat is available from 100 litres of milk after loosing in skim milk (i.e.,3% of fat available in skim milk), then the yield of cream will be:

4 x 2.5 = 10.0 Kg (40% fat cream)
4 x 2.0 = 8.0 Kg (50% fat cream)
4 x 1.67 = 6.68 Kg (60 fat cream)
                                
Percent fat recovered in cream = (Kg fat in cream)/(Kg fat in milk) x 100
                                  
The yield of skim milk can be calculated by the following formula:

Quantity of skim milk =(Fat per cent in cream- Fat per cent in milk)/(Fat per cent in cream- Fat per cent in skim milk) X Quantity of milk
                                    
Per cent fat loss in skim milk =  (Kg fat in skim milk)/(Kg fat in milk)  x 100
                                    
                                      

Fat Losses in skim Milk

There are several factors, which influence the efficiency of cream separation process. Consequently the entire fat present in milk is not recovered during centrifugal separation. The percent of total fat recovered in cream from milk is referred to as skimming efficiency. The skimming efficiency is calculated by the equation given below.

                         
Skimming Efficiency (%) = (Total fat in cream)/(Total fat in milk) x 100
                                                                 
Factors affecting the skimming efficiency are listed here:

i) Temperature of milk: The temperature of milk at the time of separation should be around 37-50°C. Separation at lower temperatures results in higher fat loss in skim milk and may lead to partial clogging of the bowl due to the increased viscosity of cream.

ii) Speed of bowl: The fat loss in skim milk will be higher at lower speed of the bowl. This loss is ascribed to the insufficient centrifugal force generation.Hence, milk should not be fed in to the cream separators unless the cream separator attains its full speed.

iii) Rate of milk inflow: The flow of milk to the cream separator should be at optimum level. If the flow is at higher rate it will result in greater loss of fat in skim milk.

iv) Size of fat globules: Smaller the size of fat globules in milk higher will be the fat content in skim milk. Due to this reason, it is observed that generally cow milk and goat milk have lower separation efficiency in comparison to buffalo milk.

v) Presence of air: Greater the amount of air in milk higher will be the fat loss in skim milk. The entrapped air reduces the efficiency of hermetically sealed separators more than that of the normal cream separators.

vi) Acidity of milk: Higher acidity of milk reduces the efficiency of separators.This is mainly due to the partial coagulation of milk, which in turn, increases the sludge formation in the bowl affecting the efficiency of separation.

vii) Mechanical condition of separator: Vibration in the machine, use of deformed/dirty/scratched/rough discs and accumulation of separator slime causes increased losses of fat in skim milk.

viii) Fat percentage in cream: Production of cream containing more than 50-60% fat causes more losses of fat in skim milk.

ix) Degree of agitation and temperature of milk: Higher temperature of milk and more agitation cause higher losses of fat in skim milk.

x) Position of cream screw: As high fat in cream causes more losses the cream screw should be appropriately adjusted.

Factors Influencing Fat Percentage of Cream

The following factors influence the fat percentage in cream

a) Position of cream or skim milk screw: Any of the screws can be moved IN or OUT. Thus it comes nearer to or moves away from the center of rotation. Adjusting the cream screw towards IN position or Skim milk screw towards OUT position yields high fat percentage in cream or vice-versa.

b) Fat percentage of milk: Milk containing high fat yields rich cream.

c) Bowl speed: The higher and recommended speed of the bowl yields cream of higher fat content.

d) Rate of milk in-flow: Faster rate of inflow of milk to the separator produces cream with lower fat percentage.

e) Temperature of milk: Lower temperature of milk during separation yields cream with higher fat percentage.

f) Quantity of water: If the amount of water used to flush the bowl is more the fat percentage in cream will be low.

Types of Cream Separators

Common types of cream separators are listed below:

a) Cold milk separator: It produces skim milk and viscous cream of high quality with less foam. But it may cause partial churning of milk during separation and has low capacity.

b) Warm milk separator: It facilitates close skimming, and has high capacity but produces low viscosity in cream and also foam.
Definition, Composition,Standards and Processing of Cream

c) Hand driven cream separator: It is a low capacity machine suitable for farm scale operation and is economical.

d) Power driven cream separator: It is high capacity machine suitable for dairy plants but are expensive.

e) Open bowl cream separator: It has low capacity and so suitable for small dairy plants. But it produces foam during separation.

f) Hermetically sealed/Air-tight/Foamless tri-process separator: As the name indicates it does not produce foam and performs three operations. It can clarify,separate and standardize the milk or cream. It can deliver cream or skim milk to respective tanks without any additional pumps, produces cream of high viscosity and of desired fat content without stopping the machine. Since it is airtight there is no contact of cream or skim milk with outside atmosphere and hence the quality is better. But it is expensive and regular maintenance is required.

g) Domestic cream separator: Domestic food processors or mixers are usually provided with a speed variation from 1400 to 18000 rpm. However they lack a rigid foundation and sufficient mechanical strength of the driving unit. Therefore an attachment has been developed at the National Dairy Research Institute,Karnal. The attachment consists of raw milk, cream and skim milk pans of the matching size of the mixer. The lowest pan has built-in- power transmission assembly and is fixed with the mixer to give a fairly rigid base to the bowl. The bearings are so designed that they take care of the vibrations and overheating of the mixer. It has 8-9 discs and operates at 3250 rpm. It can separate about 4 litres of milk in 10 min and produces cream containing 40% fat.

Principle of Separation

The fat is present in milk as fat-in-water type emulsion and is lighter than the skim milk. The mean density of milk fat is 0.93 as compared to 1.036 of skim milk at 16°C. Separation of cream is based on the principle that milk fat,because of its lower density is lighter than the skim milk portion. Hence it tends to rise to the surface and separates from the serum (skim milk). This principle is applicable to both gravity method and mechanical method of cream separation.

 

i. Gravity Method

In gravity method when milk is allowed to stand undisturbed for sometime the fat being lighter will rise and float on the surface under the influence of gravitational force. The fat floating on the surface will form a layer and can easily be removed or separated. It is common method of fat separation, which is governed by this principle and adopted at household level. However the rate at which the fat separates is governed by the Stoke’s law as given by the following equation:

v= (2G (ds – df)r2)/9n

Where

V = Rate at which fat globule rises
G = Acceleration due to gravity
ds = Density of skim milk
df = Density of fat
r = Radius of fat globule
η = Viscosity of skim milk

Thus, it can be inferred from the above equation that rate of separation of fat from milk increases with the following factors:

a) Increase in radius of the milk fat globule (clumping)

b) Increased difference in the densities of milk and fat (dilution)

c) Decrease in viscosity of milk

The gravity method of separation is a slow and simple process. It is followed at household and cottage level. It separates fat from milk in the form of cream. This is the easiest way to separate cream. Fresh or boiled and cooled milk is left to stand in a vessel and after a while the fat globules cluster together or aggregate and rise to the surface forming a layer of fat rich portion (cream). The layer can be removed by hand or ladle.

If the milk is not properly heat-treated it may undergo undesirable fermentation,sometimes leading to curdling of milk. The separation of fat will occur but the resulting cream and the skim milk will be of very poor quality. The fat separating on the surface of milk in the form of skin is scooped and the process is repeated at least two times to extract fat. By this method it is possible to extract about 50% of fat present in milk. The fat thus obtained is preserved with lactic culture to avoid putrefaction. The rate of separation of fat by gravity method is affected by the following factors:

i) Size of fat globules: If the size of fat globules is bigger, the separation will be faster. This is the reason separation is faster in buffalo milk as compared to cow milk.

ii) Temperature: It affects the viscosity, which in turn, influences the flow or velocity with which the fat will separate. Hence separation at higher temperature is faster because of the reduced viscosity of milk.

iii) Clumping: Clumping of fat globules together increases the size and thus increases the velocity and rate of separation.

ii. Mechanical Method

In the mechanically operated cream separator, which is used commercially, the separation of fat is achieved by application of the centrifugal force. The centrifugal force acting on the system is about 3000- 6000 times greater than the gravitational force. Hence the separation of fat, which is governed by the Stoke’s law, is faster than the gravity method of separation. Fat globules of smaller size separate 6500 times faster in a centrifuge rotating at 5400 rpm than they do by gravity. The Stoke’s law as applied to centrifugal separation process is expressed by the following equation:
 
v=(r2 (ds – df)K.R.N2))/9η

Where,

V = Velocity of movement of fat globule
r = Radius of fat globule
ds = Density of skim milk
df = Density of fat
N = Speed of bowl (rpm)
R = Distance of fat globule from the axis of rotation
K = Constant
= Viscosity of skim milk

The mechanized process is also known as centrifugal method of separation. It is a faster process. It is adopted as an industrial method of cream production. In this case it is very essential to fix the mechanized cream separator firmly to avoid vibration during the course of its operation. The large capacity separators (250 litres milk / hr or above) are installed on the ground while the smaller ones or laboratory models on wooden stool or tables. To run the separator the electrical connection is switched on. The separator is allowed to acquire the requisite speed.Generally the milk is preheated to 37-50oC before separation for optimum results.This makes the process easier and more efficient as the warm milk is less viscous than the cold milk. Milk is fed down as an inlet tube into the separator bowl where it rotates and moves up through a series of aligned holes in the disc stack. In-flow of milk, is regulated by adjusting the milk in-let valve to the separator. As the disk stack revolves the cream moves towards the center of the bowl and the skim milk is directed outwards by the centrifugal force. Thus milk is separated in two streams,a highly concentrated milk fat stream termed as cream and a nonfat stream of skim milk. Under normal conditions it produces skim milk and cream in the ratio of 90:10

Nutritive Value

Fresh cream contains all the constituents of milk but will invariably contain at least 5 times more fat than milk. Hence the energy value of cream will be several times higher than that of milk. The energy value can be calculated by assessing the composition of cream and taking into consideration the individual contribution of the constituents to the energy value as given below:

Milk Fat 9.3 k cal/g

Milk Sugar 4.1 k cal/g

Milk Protein 4.1 k cal/g

In addition to the calorific value the cream is richer in fat- soluble vitamins like A,D, E, and K than milk as it has more fat content.