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Chemistry Of Milk In Relation
To Manufacture Of Indian Dairy Products
Bimelsh Mann, Rajesh Kumar, Ram Bhagat Sangwan and Shilpa Vij*
Dairy Chemistry Division *Dairy Microbiology Division
National Dairy Research Institute, Karnal
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Milk is an extremely complex biological fluid with scores of nutrient. These nutrients exist in milk in three physical phases: a dilute emulsion, colloidal dispersion and a solution. The emulsion can be broken by low speed centrifugation and the milk separates into lipid and aqueous phases or compartments, each with a characteristic composition. With ultracentrifugation the casein micelles precipitate, bringing some other proteins such as lactoferrin with them. The supernatant remaining after this process has the characteristics of a true solution. The chemical makeup of milk and its physicochemical behavior provide scientific basis for process of milk and manufacture of products. In India, milk from buffalo and cow (desi, cross breed and exotic) is commercially important. Depending on its characteristics, each type of milk is eminently suitable for certain types of region specific indigenous traditional milk products. These Indian traditional dairy products can be classified into the following broad categories:
Heat Desiccated Milk Products
Heat desiccated milk products such as khoa–an intermediate concentrate that is the base for a wider range of mithais including gulabjamun, burfi, peda, kalakand, rabri, kulfi and their variants. The heat processing and accompanying reduction in water activity result in substantial destruction of pathogenic and spoilage micro organisms as well as inactivation of enzymes. Besides, desirable heat induced chemical interactions among milk constituents result in reduction of redox potential and water activity which extend the product shelf life. The desirable flavours, textures and overall acceptability of these products are enhanced by heat processing. The main reaction in the preparation of khoa is the heat denaturation and coagulation of milk proteins. Most of the albumins and globulins are rapidly denatured, the protective properties of the other colloids are destroyed early in the boiling process, and the process is accelerated by the incorporation of air and the frothing during stirring. Total heat coagulation of the proteins occurs when the boiling mixture thickens to a buttery consistency in the pan. Coagulation of milk proteins is brought about by heating to 132-136ºC. Albumin and globulins are coagulated below 100ºC, while casein is coagulated above 100ºC. During desiccation whey proteins are almost fully denatured from colloidal state of non-dispersible state. The factors that influence the heat coagulation are temperature and time of holding, concentration of casein, acidity of milk, salt balance
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and precise heat treatment. The color of milk changes from light to a more intense shade of the color due to denaturation and coagulation of protein. Due tovigorous agitation of milk at high temperature the fat globules are appreciately sub-divided. Almost half of the globular fat is released as free fat-the extent of which depends upon the type and fat content of milk and manufacturing process. Usually 44.8 to 62.8 % of fat appears as free fat in khoa. Patel et al (1990) have found a positive and significant correlation in total solids and hardness. The fat level is usually adjusted to 4 % for cow milk and 5 % for buffalo milk. The yield of khoa depends upon the type of milk used when milk is standardized to 4.5 % fat and 8.5 % SNF; its yield is about 20 % for buffalo milk. Khoa from buffalo milk is considered distinctly superior, being whiter in color and having smooth body and granular texture. These characteristics are considered desirable for making high quality sweets.
The quality of khoa is better when made from buffalo milk as khoa from cow milk is inferior due to its moist surface, sticky and sandy texture which is not considered suitable for the preparation of sweetmeats (De and Ray, 1953). The higher emulsifying capacity of buffalo milk fat due to the presence of larger proportion of butyric acid containing triglycerides (50 %) compared to only 37 % in cow milk fat may be responsible for smooth and mellowy texture of its khoa (Sandhu, 1996). Good quality khoa can be prepared from cow milk with the incorporation of whey protein concentrates (5 %) and keeping the total solids of the product low, as higher total solids in WPC containing khoa adversely effect the rheological parameters (Patel et al, 1993). Rajoria et al, 1990 studied the effect of milk quality on the chemical, sensory, rheological properties of khoa. They observed that khoa from slightly soured milk (0.2 % lactic acid) did not have much adverse effect on the flavour of khoa while khoa prepared from excessively sour milk had acidic smell, which has improved but not totally suppressed by neutralization of excessively soured milk had a salt taste. The coarse structure of the khoa is increased with increase in the acidity of milk. Increase in size and hardness of the grain lead to poor body and texture of khoa. Smoothness of khoa was also adversely affected by the developed acidity. The hardness of khoa was observed to increase. Gumminess and chewiness of khoa was also increased due to developed acidity in milk and was reduced by neutralization of the same. Khoa from excessively high acidic milk is sour in taste.
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87
Dairy Year Book 2008 |
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