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• Reaction between the sugars and proteins (Maillard reactions).

• The release or formation of sulfur-containing components.

Degradation of Precursors. The triglycerides (lipids) in milk fat are formed from complex as well as simple fatty acids of many types, including substituted acids such fi-keto acids and hydroxy acids. These acids originate in the diet of the animal and also may be the result of normal metabolism. Methyl ketones are formed from the /?-keto acids (5). Although they are formed in fresh milk, maximum yield occurs at prolonged elevated heating (140°C for 3 h). The liberation of methyl ketones is generally below their individual odor thresholds, yet they contribute to the aroma of milk because of their synergistic interaction due to their combined concentration (6). Methyl ketones are formed from the decarboxylation of the /?-keto acids, whereas ring closure of <5- and t-hydroxy acids form the corresponding lactones.

The Maillard Reaction. Reaction between the lactose and the milk protein can lead to many different compounds of flavor significance. These reactions are referred to as Maillard reactions. Discussion of the Maillard reaction is complex and outside the scope of this article, but its basic chemical pathway is the reaction between aldose and ketone sugars and a-amino acids resulting in the formation of aldosylamines or ketosylamines. These materials then undergo further rearrangement, resulting in the creation of many key flavoring materials, such as acetyls, furans, pyrroles, aldols, and pyrones. The normal heat treatment of milk does not appear to produce enough of the Maillard reaction product to be a flavor contributor. However, the cooked flavor of milk is noted when it is heated to above 75°C (7). At that point many Maillard reaction products start to contribute to the flavor of the milk. Further heating above the cooked-flavor range produces a carameliza-tion flavor, due to the intense amount of Maillard reaction products being formed and the liberation of many sulfur-containing chemicals.

Formation of Sulfur-Containing Compounds. Dimethyl sulfide is produced from the S-methyl methionine sulfo-nium salt (from the vegetable matter in the animal's diet) that is decomposed on heating (8). At high concentrations this chemical produces a malty or cowy flavor defect in the milk (9). The flavor from the caramelization of milk has found great use in various culinary schools of fine food preparation.

Flavor Defects in Milk. Caramelization is a wanted defect in many food applications; yet, there are some defects that are not wanted. Extended storage of dry milk causes the stale off-flavor where the release to the fatty acid contributes to the further development of methyl ketones above their threshold value, but below the quantity seen in certain cheeses; for example, the family of blue cheeses (10).

The oxidation of milk causes the cardboardy or cappy defect that is evaluated as the milk being metallic, tallowy, oily, or fishy. The oxidation of the unsaturated fatty acids leads to the methyl ketones and series of aldehydes. Very low levels of eis-4-heptenal, hexenal, and 1,3 octenone contribute to the oxidized flavor defect (11).

The lipolysis of the milk by lipases remaining active after the heat treatment can cause rancidity of the milk. This is due to the increase of both the short-chain fatty acids and the increased acidity of the milk. In normal milk the total free acid content is about 360 mg/kg, but in rancid milk it rises to 500-1500 mg/kg (4).

One final major flavor defect is known as the sunlight defect. It is believed to be produced from the degradation of methional (a common amino acid found in milk) to several sulfur-containing compounds, including mathane-thiol, dimethyl sulfide, and dimethyl disulfide. These contribute to a flavor characterized as burnt or cabbagey. The extent of the flavor defect is related to the length and strength of exposure to light (12).

Milk's Basic Flavor. Milk's basic flavor, then, is due mainly to the taste components found in the milk as noted in Table 3. The fats, sugars, salts, and protein all contribute to give milk a subtle, but enjoyable, flavor and taste. Unless the milk has been mistreated or subjected to high temperature for prolonged time, no significantly predominate flavor is noted. In contrast, products made from milk have characteristic and, in some cases, strongly predominate aroma and tastes.

Cultured Dairy Products

Among the first products of milk to be developed were cultured products. Cultured products are produced throughout the world, as seen in Table 4. Originally, fermentation occurred naturally to milk set aside for later use. The spoilage organisms in the milk produced the proper conditions for the preservation of the cultured product. The lowering of the pH of the milk by the organisms allows for the preservation of the product and contributes to its characteristic aroma and taste.

The preparation of these cultured products became an art that survives today. The consumption of these products varies around the world, with the northern European countries consuming the greatest amount (—20 kg/y/ person); and the United States consuming the least (—1.2 kg/y/person) (14). However, the United States is a 300-fold

Table 3. Fat Content of Cow's Milk

Type of fats

Range of occurrence (%)

Triglycerides

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