Effect On Foods

Nutritional changes take place during processing, the extent varying with the type of food, the process, the plant in use, and the degree of control exercised. Many losses are inevitable, particularly if the process involves heating. Part of the water-soluble nutrients such as the B vitamins, together with lesser and less-important amounts of mineral salts, protein, and even carbohydrate will be precipitated out. Vitamin C is oxidized in air and accelerated by heat, whereas vitamins A and D and niacin are unaffected (20). A comparison of nutrient losses in milk preserved by evaporation and UHT sterilization is shown in Table 4.

Aroma compounds that are more volatile than water will be lost during evaporation. With some products such as fruit juices, the retention of taste and aroma is important, yet in other foods such as cocoa and milk, the loss of unpleasant volatiles improves the product quality. The color of foods darkens, partly due to an increase in the solids content and partly because the reduction in water activity promotes chemical changes (for example, Maillard browning) (12). As these changes are time and temperature dependent, short residence times and low boiling points produce concentrates with higher qualities. For in-

Liquid refrigerant

Refrigerant vapor return

Figure 18. System of concentration using refrigerant cycle evaporators. Source: Reprinted with permission from Ref. 25.

Liquid refrigerant

Refrigerant vapor return

Figure 18. System of concentration using refrigerant cycle evaporators. Source: Reprinted with permission from Ref. 25.

Hot water

Expansion valve

Hot water

Expansion valve

Table 4. Vitamin Losses in Concentrated and UHT Sterilized Milk

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