Source: Ref. 1.

product. Although the heat treatment can have some influence on the taste and flavor of fresh milk, there is a much greater relationship to the time of year, nature of the animals' feed, and the species of animal from which the milk is derived.

Milk is a very complex substance based on fats, sugars (mainly lactose), proteins (such as lactalbumin), and salts (such as calcium phosphate). The taste of milk is mainly affected by the change in this mixture and the degree to which the fat is emulsified (homogenized) in a continuous phase with the aqueous phase. The sensory perception determines the physical nature of the emulsion, with a slightly salty/sweet taste from the salts and lactose. Although some slight flavor changes can be detected in even the mildest of heat-treated milk, the flavor defects are generally considered to be notes associated with good quality, pasteurized milk.

Of the various constituents of the milk, the lipid (fat) fraction has the greatest effect on milk's flavor. It is also the precursor of many chemical components considered important in all types of dairy flavors. The fatty acid composition of the milk lipids is also very complex and unique among food products. More than 60 fatty acids have been reported in cow's milk (2). Quantities of butyric acid and caproic, caprilic, decanoic (fatty acids with 6,8, and 10 carbon atoms) acids are unique to milk and of great importance in the development of flavors in products based on milk. Oxidation of these lipids gives rise to some key flavor components. For example, the oxidation of unsaturated oc-tadecadienoic acid leads to the formation of 4-cis-heptenal, which has been identified as the cream-like flavor component in milk and butter (3). The free fatty acid distribution of milk and butter is shown in Table 2.

The heat treatment of milk is divided into two types of processing: pasteurization in which the milk is heated for 15 s at 72°C and ultra high-temperature treatment (uht), where it is heated at 135-150°C for a few seconds. The flavor changes that occur during the heat treatment may be formed from three distinct chemical mechanisms:

• Degradation of thermally labile precursor found in milk.

Table 2. Free Fatty Acids in Milk

Fatty acid

(carbon no.: unsaturation)

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