Milk Treatments Pasteurization Along with correct cooling, the pasteurization of milk is one of the most important processes in the treatment of milk. If carried out correctly, these processes will supply milk with the longest possible shelf life. The temperature and the pasteurization time are very important factors which must be specified precisely in relation to the quality of the milk and its shelf-life requirements, etc. The pasteurization temperature for homogenized, HTST pasteurized, regular grade milk is usually 72 73 C for 15 20 seconds. The pasteurization process may vary from one country to another due to different legislations. Common for all countries is that the heat treatment must guarantee the destruction of unwanted microorganisms without the product being damaged. As described in 'General Milk Treatment', one of the following procedures can be used: LTLT (Low Temperature Long Time): 63-65 C for 30 minutes. HTST (High Temperature Short Time): 72-75 C for 12-40 second HTST: 80-90 C for 10 20 seconds (mainly used fr cream). One important aspect of partial homogenization concerns the heat treatment of the homogenized cream. Homogenization will have destroyed the membranes of the fat globules and exposed the fat. Therefore, this fat will have no protection against attack by the lipases which are present in the skim milk after remixing. The standardized milk must be pasteurized as quickly as possible after remixing. This does not cause any problem in the milk processing line, as remixing and pasteurization are performed rapidly and continuously in a closed process system. The problem increases where, for example, the pre standardization method is used. It then becomes necessary to design the 1
process in such a way that there is no possible risk of the homogenized milk or cream being mixed with unpasteurized milk or cream. If this should occur, the resulting mixture must be pasteurized immediately. Homogenization The purpose of homogenization is to disintegrate of finely distribute the fat globules in the milk in order to reduce creaming. Homogenization may be total or partial. Partial homogenization is a more economic solution as a smaller homogenizer is used. During partial homogenization the cream should have a maximum fat content of 12%, but a lower fat content, 10 11 %, is normally used. Cream with a higher fat content should not be homogenized as this will result in cluster formation, mainly due to lack of material (casein) to cover the newly formed small fat droplets. A sufficiently good homogenization effect requires approximately 0.2g casein per 1g of fat. The homogenization temperature is normally 60 70 C at homogenization pressure of 150 250 bar. Two stage homogenization is recommended except in the production of some milks where one stage homogenization should be used. Determining the Homogenization Efficiency Homogenization must always be sufficiently efficient to prevent creaming. The homogenization of the milk can be checked by determining the homogenization index. The following may serve as an example of how to determine this index. A milk sample is kept for 48 hours at a temperature of 4-6 C. The fat content in the upper layer (1/10 of the volume) and in the lower layer (9/10 of the volume) is then determined. The difference in fat content between the upper and lower layers, expressed as a percentage of the upper layer, is referred to as the homogenization index. 2
An example: If the fat content in the upper layer is 3.65% and 3.35% in the lower layer, the homogenization index will be (3.65 3.35) 100/3.65 = 8.2. The index for homogenized milk should be in the range 1 to 10. Production of Cream Cream sold to consumers is produced with different fat contents. The thinner type of cream is often referred to as coffee cream, but is used less and less for that purpose today and more and more for desserts and in the preparation of food. Cream with a higher fat content is usually considerably thicker. It can be whipped into a thick foam, and is therefore reffered to as 'whipping cream'. Whipping cream is used whipped or unwhipped as a dessert, for cooking, etc. Whipping Cream In addition to tasting good and keeping well, whipping cream must also have good 'whippability', i.e., the cream must be easy to whip and produce a fine cream foam with a good increase in volume. The foam must be firm and stable, and must not be disposed to syneresis. A condition for good whippability is that the cream has a sufficiently high fat content. Whipping cream with 40% fat is usually easy to whip, but the whippability decreases as the fat content drops to 30% and below. However, it is possible to produce good whipping cream with a low fat content (about 25%) by adding substances which improve the whippability, e.g., powder with a high lecithin content, made from sweet buttermilk. Air inclusion must be avoided in the manufacture of whipping cream. Air pickup leads to formation of foam and destabilization. Exposed to excessive mechanical treatment, specially shortly after having left the cooling section, the fat- globule membrances will be damaged, 3
resulting in fat amalgamation and formation of clusters. Creamlining takes place when roughly treated cream is stored in the package. The layer of cream will be dense and sticky. This 'homogenization effect' will greatly impair the whipping characteristics of the cream. Air is beaten into the cream when the cream is whipped. This produces a foam full of small air bubbles. The fat globules in the cream collect on the walls of these air bubbles. The mechanical agitation destorys the membrance of many fat globules and a certain amount of liquid fat is liberated. This fat makes the globules stick together. The fat globules must contain the correct proportions of liquid and crystallized fat in order to obtain a firm foam. Warm cream contains liquid fat which makes whipping impossible. Cream for whipping must therefore be stored at a low temperature (4-6 C) over a relatively long period of time in order to obtain a proper crystallization of the fat. This storage period is called ripening time. Cream is usually ripened in jacketed process tanks with scraper agitators. Heat is released during crystallization. The cream must be cooled in order to prevent a temperature increase. It should be agitated gently during ripening and cooling. Slightly lower ripening temperatures may be used in the summer, when the milk fat usually is softer than during the winter. Comprehension Exercises A. Put "T" for true and "F" for false statement. Justify your answers. 1. The pasteurization process varies in different countries due to different legislations. 2. Homogenization protects the fat droplets against heat. 3. Partial hmogenization is cheaper than tatal homogenization. 4
4. For more effective homogenization, the milk should contain approximately 0.2 g casein per g of fat. 5. The homogenization temperature is usually 60-70 C at a pressure of 150 250 bar. 6. When cream is wapped, the fat globules collect on the wall of air bubbles in it. B. Choose a, b, c, or d which best completes each item. 1. The temperature and time in the HTST process for pasteurizing milk are.... a. 63-65 C and 30 minutes b. 72 75 C and 10 12 seconds c. 80-90 C and 10 12 seconds d. 72-75 C and 12 40 seconds 2. Homogenization of milk... creaming. a. increases. c. decreases. b. May either decrease or increase d. has no effect on. 3. When milk is homogenized, the newly formed small fat droplets will be covered with.... a. a membrane c. casein b. lactalbumin d. albumin 4. Homogenization of milk can be checked by determining the.... a. size of the fat droplets c. homogenization index b. amount of cream in the milk d. amount of casein in the milk 5. As the fat content of cream decreases, its whippability.... a. decreases c. increases b. changes d. will not change 5
C. Answer the following questions orally. 1. What are the pasteurization temperature and time for homogenized, HTST pasteurized, and regular-grade milk? 2. What are the three pasteurization procedures? 3. What is common for all countries with regard to milk pasteurization? 4. What is the purpose of homogenization? 5. What is whipping cream? 6. Why should cream with a high fat content not be homogenized? 7. How is the homogenization index determined? Part 3. Translation Activities A. Translate the following passage into Arabic. Market Milk Products Market milk products are liquid products made from milk and cream, intended to be used directly by consumers. This group of products includes whole milk, skim milk, standardized milk, and various types of cream. Cultured products are also included in the consumer milk products. These are made with special bacterial cultures and are dealt with separately under 'Cultured Milk Products'. All the stages of processing which have been described under 'General Milk Treatment' are, in principle, used in the processing of market milk products: Chilling Clarification and fat separation Fat standardization Deaeration Homogenization 6
Pasteurization In most countries, chilling, clarification, and pasteurization are compulsory stages in the processing of consumer milk products. The fat is homogenized in many countries, whilst the highest possible cream content is wanted in other countries a good 'cream line' is regarded as evidence of quality milk in these countries. Deaeration is used in certain ceses when the milk has a very high air content, and also when highly volatile, off-flavor substances are present in the product. This may occur if, for example, the cattle have eaten foods containing plants of the onion family. Processing of market milk products requires a first class raw material and correctly designed process lines if finished products of highest quality are to be attained. Gentle handling must be ensured so that the valuable constituents are not negatively influenced. B. Find the Arabic equivalents of the following items and write them in the spaces provided. 1. Booster pump... 2. Buttermilk... 3. Chilling... 4. Clarification... 5. Creaming... 6. Creamline... 7. Cultured products... 8. Dairy... 9. Displacement pump... 10. Filling machine... 7
11. Flow diversion valve... 12. Flow rate... 13. Flow transmitter... 14. Heat exchanger... 15. Homogenizer... 16. Low fat milk... 17. Off-flaver... 18. Pasteurizer... 19. Preheat... 20. Process line... 21. Production line... 22. Regular-grade... 23. Remix... 24. Ripening time... 25. Separator... 26. Shelf life... 27. Skim milk... 28. Syneresis... 29. Whip... 30. Whippability... 31. Whipping cream... 32. Whole milk... 8