FOOD SCIENCE AND PROCESSING 3 (2+1)

Lesson 26 : Processing and preparation of Milk and Milk products

Fermented milk products

• Butter
  The fat content of butter is generally about 80 per cent. The non-fat components of butter consist of moisture, milk solids not fat and salt if added. Most of the butter produced in our country is converted to ghee.

  Butter is made from sweet or sour cream. The cream is pasteurised at 62.8oC for 30 minutes after which it is immediately cooled. Then a culture of desirable micro-organisms is added. The culture consists of bacteria of two types. Streptococcus cremoris or lactobacillus lactis ferments lactose producing lactic acid which in turn curdles the milk. Leuconostic citrovorum principally attacks the citric acid of milk-producing volatile acids and products such as diacetyl which give the desirable flavour and aroma to the butter. The cream is then allowed to ripen at 21.1o C for several hours for the fermentation to take place.

  The ripened cream or dahi is next churned. Churning brings about the denaturation, by violent agitation of the fat globule surface; the membrane material is eliminated in buttermilk. The fat globules clump together causing a change from an oil-in-water emulsion to a water-in-oil emulsion . For this change to be effective the temperature should be such that there is a blending of solid and liquid fat. As churning proceeds the cream becomes granular and a definite separation of the butter milk takes place.

  The colour of the butter varies from yellowish white to deep yellow. Yellow colour is due to carotene derived from the feed of the animal. Sometimes synthetic permitted carotene is added. Pas­teurised table butter should be stored at 80°F to maintain good quality and prevent deterioration.

  When butter spoils as a result of hydrolysis of triglyceride molecules, the disagreeable odour and flavour are due primarily to the release of free butyric and caproic acids from the triglyceride molecules.


• Cheese
  Commercially sometimes curdling is done using rennet berries. Cottage cheese is called paneer. Cottage cheese is made using curd for curdling milk and removing the whey with weight. It does not undergo any further processing. It is highly perishable and must be kept refrigerated. Recipes like peas and paneer curry are made from cottage cheese.The varieties of cheese are differentiated according to their flavour, body and the texture which in turn depends upon the type of the milk used, manufacturing and processing method adopted salts and sweetenings added and the type of bacteria and mould species used in the ripening. Cheese is classified as hard or soft, depending on their moisture content and whether they are ripened by bacteria or moulds or unripened. Cheese is made of casein.

Figure gives classification of cheese.




• Curd formation
  Pasteurised whole milk is brought to a temparature of 31oC and lactic acid producing starter culture and required colouring matter are added. After about 30 minutes to the mildly acidic milk, rennin solution is added, stirred and allowed to set the curd for 30 minutes.


• Curd cutting: The curd is cut into small cubes. The removal of whey from small cubes is easy. For different type of cheese the curd is cut into different sized cubes.



• Curd cooking: Curd it is heated to 38°C and held at that temperature for about 45 minutes. During this period, the curd is stirred to prevent matting.


• Curd drainage: Heating squeezes out whey from the cubes. Heat increases the rate of acid production which makes the curd cubes shrink. Whey is drained off and the curd is allowed to mat.


• Cheddaring: Curd , it is subjected to the process of cheddaring. This consists cutting the matted curd into blocks turning the block at 15 minute intervals and then piling the blocks on one another. During the cheddaring operation which takes about 2 hours, acid formation continues. The cheddared curd is passed through a curd mill which cuts the slabs into strips.


• Salting the curd: Whey is eliminated during this process . Salt is added to draw whey out of curd by osomosis and also acts as a preservative. Salt also holds down spoilage of organisms and adds flavour.


• Pressing: The cheese cubes are pressed under pressure overnight. This pressing determines the final moisture content of finished product and then the cheese is ripened from 60 days to 12 months depending on the strong or mild flavour cheese required.


• Ripening: Ripening takes place under controlled conditions of temperature and humidity for varying periods of time and makes cheese easily digestible. During this process, cheese changes from a bland tough rubbery mass to a full-flavoured soft product. During ripening, rennin splits the protein of cheese into peptones and peptides. Enzymes formed by the micro-organisms act on these and other substances to form products like amino acids, amines, fatty acids, esters, aldehydes, alcohols and ketones that give its characteristic flavour. There is also increase in B-vitamins during ripening. Ripening also improves the cooking quality. The increased dispersibility of the protein of ripened cheese is a factor in the ease of blending cheese with other food ingredients. The type of starters used also contributes to flavour and consistency of cheese.
  
  Cheese has limited keeping quality and requires refrigeration, should be kept cold and dry i.e., it should be wrapped in wax paper or metal foil.


• Cooking quality: Hydrolysis of the proteins of the cheese occurs during ripening. Cooking quality of cheddar cheese of normal fat content improves as the product of protein hydrolysis is increased.

  Moisture content is also related to cooking quality since cheese of high moisture content is superior compared to low moisture cheese.

  When fat content is low the tendency for mat formation is exaggerated. The fat of normal samples melted during cooking contributes to the consistency and tenderness of the heated product and pre­venting the formation of a continuous protein mass. The cooking quality of processed cheese is better than the natural cheese (cottage cheese). The presence of emulsifying agent not only helps in keeping the fat emulsified but increases the alkalinity of cheese which in turn increases the solubility of casein. As casein is rendered more soluble it is less likely to be tough or form firm strings on cooking. It is possible that both ageing and the addition of emulsifying salts improve the cooking qualities of cheese, because they make the protein more soluble.


• Curd
  This is India's most commonly used milk product and 7 per cent of milk produced is used for curd.

  Nutritive value almost remains the same during curd making. The digestibility is better when compared to ordinary milk. Calcium and phosphorus content of curds are more easily assimilated. Curds contain more B vitamins than milk.

  During curd formation the lactose of milk is converted into lactic acid. Acid curdles the milk protein. The fat globules coalesce and distribute themselves on the top. Organisms involved in curd formation belong to the group of lacto bacillus and streptococcus -L. Casei, L. brevis, L. bulgaris, S. Lactis and S. Thermophilus. Starter culture containing a combination of lacto bacillus bulgaris and streptococcus, organisms is good. Starters containing yeasts, moulds and gas-containing organisms spoil the quality of curd.

  Curd is prepared by initially boiling the milk to destroy viable organisms. It is cooled to the body temperature and starter, 10 ml per litre, is added. Lesser quantities are needed during summer. After 6-8 hours an acidity of 0.9-1 per cent acid is formed. Due the drop in pH, the casein is coagulated and the curd is set. Curd is enjoyed as such and also used in the preparation of kadhi, raihta, lassi and shrikhand.


• Yogurt
  Yogurt is a type of curd. Whole, low-fat and skim milks and even cream can be used to make yogurt. Often, non-fat dry milk solids are added. There is increase in folic acid concentration during the fermentation process. In the production of yogurt, a mixed culture of lactobacillus bulgaricus and streptococcus thermophilus, lactobacillus acidophilus is usually added to the pasteurised milk and incubated at 42 to 46°C. Often, yogurt is marketed with sweetened fruit added. Yogurt can be used in lactose intolerance. The fermentation lowers the lactose content by 20-30 per cent.
  
  By churning curd and removing butter, butter milk is obtained. It is used as a beverage for quenching thirst. Powdered butter milk can be made by spray drying. Metchnikoff postulated that the bacteria involved in yogurt fermentation, lactobacillus bulgaricus and streptococcus thermophilus suppress the putrefactive type fermentation of intestinal flora. Fer­mented milk has been reported to be useful for a wide variety of disorders like colitis, constipation, diarrhoea, gastroenteritis, diabetes and hypercholesteremia.


• Shrikhand
  It is another fermented product which is made by concentrating dahi by removing whey and to which sugar, flavour and condiments are added.