Introduction
Dairy processing involves conversion of raw milk into fluid milk products, and an array of dairy products such as butter, yogurt and fermented milks, cheeses, dry milk powders, dry whey products, ice cream, and frozen desserts, and refrigerated desserts.
Factors related to the cow such as breed, intervals of milking, stages of milking, different quarters of udder, lactation period, season, feed, nutritional level, environmental temperature, health status, age, weather, estrus cycle, gestation period and exercise are known to cause variations in fat, protein, lactose and mineral levels in milk derived from individual cows. In general, these variations tend to average out and display an interesting pattern in commercial milk used by the processors. However, the seasonal variations in major milk constituents are relevant to the processor since they impact important properties of finished products. In general, in the United States, approximately 10% variation in fat and protein is observed in milk received in JulyâAugust (lowest level) as compared to milk delivered in OctoberâNovember (highest level). Subsequently, functional contribution of milk proteins (viscosity in yogurt, buttermilk as well as curd firmness in cheese manufacture) also follows similar trend. Furthermore, cheese yield and whey protein production are also negatively affected by seasonal variations in milk composition.
The concentration of minerals such as chloride, phosphates, and citrates of potassium, sodium, calcium, and magnesium in milk is important in processing, nutritive value, and shelf life of dairy products. Their concentration is <1% in milk but are involved in heat stability of milk, alcohol coagulation of milk, age-thickening of sweetened condensed milk, feathering of coffee cream, rennin coagulation, and clumping of fat globules on homogenization. All the minerals considered essential for human nutrition are found in milk (Chandan, 2007a).
From consumer standpoint, quality factors associated with milk are appearance, color, and sensory attributes such as aroma, flavor, and mouthfeel.
The color of milk is perceived by consumer to be indicative of purity and richness. The white color of milk is due to the scattering of reflected light by the inherent ultramicroscopic particles, fat globules, colloidal casein micelles, and calcium phosphate. The intensity of white color is directly proportional to size and number of particles in suspension. Homogenization increases the surface area of fat globules significantly as a result of breakup of larger globules. Accordingly, homogenized milk and cream are whiter than nonhomogenized counterparts. After the precipitation of casein and fat by the addition of a dilute acid or rennet, whey is separated, which possesses a greenâyellow color due to the pigment riboflavin. The depth of color varies with the amount of fat remaining in the whey. Lack of fat globules gives skim milk a blue tinge. Physiological disturbances in the cow make the milk bluer.
Cow's milk contains pigments carotene and xanthophylls, which tend to give golden yellow color to the milk. Guernsey and Jersey breeds produce especially golden yellow milk. Milk from goats, sheep, and water buffalo tends to be much whiter in color because their milk lacks the pigments.
The flavor (taste and aroma) of milk is critical to its assessment criterion of quality by the consumer. Flavor is an organoleptic property where both odor and taste interact. The sweet taste of lactose is balanced against the salty taste of chloride, and both are somewhat moderated by proteins. This balance is maintained over a fairly wide range of milk composition even when chloride ion varies from 0.06 to 0.12%. Saltiness can be detected organoleptically in samples containing 0.12% or more of chloride ion...
