Info

Source: Refs. 24 and 25.

Source: Refs. 24 and 25.

rate of deterioration is determined by the storage temperature and the water activity of the food.

In dried milk the oxidation of lipids produces rancid flavors owing to the formation of secondary products, including ¿-lactones. Most fruits and vegetables contain only small quantities of lipid, but oxidation of unsaturated fatty acids to produce hydroperoxides, which react further by polymerization, dehydration, or oxidation to produce aldehydes, ketones, and acids, causes rancid and objectionable odors. Some foods, (eg, carrots), may develop an odor of violets produced by the oxidation of carotenes to (i-ionone. These changes are reduced by vacuum- or gas-packing, low storage temperatures, exclusion of ultraviolet or visible light, maintenance of low moisture contents, addition of synthetic antioxidants, or preservation of natural antioxidants.

The technical enzyme glucose oxidase is also used to protect dried foods from oxidation. A package that is permeable to oxygen but not to moisture and that contains glucose and the enzyme is placed on the dried food inside a container. Oxygen is removed from the headspace during storage. Milk powders are stored under an atmosphere of nitrogen with 10% carbon dioxide. The carbon dioxide is absorbed into the milk and creates a small partial vacuum in the headspace. Air diffuses out of the dried particles and is removed by regassing after 24 h. Flavor changes, due to oxidative or hydrolytic enzymes, are prevented in fruits by the use of sulfur dioxide, ascorbic acid, or citric acid, by

Particles

Figure 12. Agglomerated powder.

pasteurization of milk or fruit juices, and by blanching of vegetables.

Other methods used to retain flavors in dried foods include

1. Recovery of volatiles and their return to the product during drying.

2. Mixing recovered volatiles with flavor-fixing compounds, which are then granulated and added back to the dried product, eg, dried meat powders.

3. Addition of enzymes, or activation of naturally occurring enzymes, to produce flavors from flavor precursors in the food (eg, onion and garlic are dried under conditions that protect the enzymes that release characteristic flavors). Maltose or maltodextrin are used as a carrier material when drying flavor compounds.

Color

Drying changes the surface characteristics of food and hence alters the reflectivity and color. Chemical changes to carotenoid and chlorophyll pigments are caused by heat and oxidation during drying. In general, longer drying times and higher drying temperatures produce greater pigment losses. Oxidation and residual enzyme activity cause browning during storage. This is prevented by improved blanching methods and treatment of fruits with ascorbic acid or sulfur dioxide. For moderately sulfured fruits and vegetables the rate of darkening during storage is inversely proportional to the residual sulfur dioxide content. However, sulfur dioxide bleaches anthocyanins, and residual sulfur dioxide is an important cause of color deterioration in stored dried fruits and vegetables.

The rate of Maillard browning in stored milk and fruit products depends on the water activity of the food and the temperature of storage. The rate of darkening increases markedly at high drying temperatures, when the moisture content of the product exceeds 4—5%, and at storage temperatures above 38°C (26).

Nutritive Value

Large differences in reported data on the nutritive value of dried foods are due to wide variations in the preparation procedures, the drying temperature and time, and the storage conditions. In fruits and vegetables, losses during preparation usually exceed those caused by the drying operation. For example, losses of vitamin C during preparation of apple flakes are reported to be 8% during slicing, 62% from blanching, 10% from pureeing, and 5% from drum drying (27).

Vitamins have different solubilities in water, and, as drying proceeds, some (eg, riboflavin) become supersaturated and precipitate from solution. Losses are therefore small (Table 7). Others, (eg, ascorbic acid) are soluble until the moisture content of the food falls to very low levels and react with solutes at higher rates as drying proceeds. Vitamin C is also sensitive to heat and oxidation. Short drying times, low temperatures, and low moisture and oxygen levels during storage are necessary to avoid large losses.

Thiamin is also heat sensitive, but other water-soluble vitamins are more stable to heat and oxidation, and losses during drying rarely exceed 5-10% (excluding blanching losses).

Oil-soluble nutrients (eg, essential fatty acids and vitamins A, D, E, and K) are mostly contained within the dry matter of the food, and they are not therefore concentrated during drying. However, water is a solvent for heavy-metal catalysts that promote oxidation of unsaturated nutrients. As water is removed, the catalysts become more reactive, and the rate of oxidation accelerates (Fig. 12). Fat-soluble vitamins are lost by interaction with the peroxides produced by fat oxidation. Losses during storage are reduced by low oxygen concentrations and storage temperatures and by exclusion of light.

The biological value and digestibility of proteins in most foods does not change substantially. However, milk proteins are partially denatured during drum drying, and this results in a reduction in solubility of the milk powder, aggregation, and loss of clotting ability. A reduction in biological value of 8-30% is reported, depending on the temperature and residence time (30). Spray drying does not affect the biological value of milk proteins. At high storage temperatures and at moisture contents above approximately 5%, the biological value of milk protein is decreased by Maillard reactions between lysine and lactose. Lysine is heat sensitive, and losses in whole milk range from 310% in spray drying and 5-40% in drum drying (31).

The importance of nutrient losses during processing depends on the nutritional value of a particular food in the diet. Some foods, eg, bread and milk, are an important source of nutrients for large numbers of people. Vitamin losses are therefore more significant in these foods than in those that either are eaten in small quantities or have low concentrations of nutrients.

In industrialized countries, the majority of the population achieve an adequate supply of nutrients from the mixture of foods that is eaten. Losses due to processing of one component of the diet are therefore insignificant to the long-term health of an individual. In one example, complete meals that initially contained 16.5 jug of vitamin A lost 50% on canning and 100% after storage for 18 months. Although the losses appear to be significant, the original meal contained only 2% of the recommended daily allowance (RDA), and the extent of loss is therefore of minor importance. The same meal contained 9 mg of thiamin and lost 75% after 18 months' storage. The thiamin content is 10 times the RDA, so adequate quantities therefore remained. Possible exceptions are the special dietary needs of preterm infants, pregnant women, and the elderly. In these groups there may be either a special need for certain nutrients or a more restricted diet than normal. These special cases are discussed in detail in Refs. 32, 33 and 34.

Reported vitamin losses during processing give an indication of the severity of each unit operation. However, such data should be treated with caution. Variation in nutrient losses between cultivars or varieties can exceed differences caused by alternative methods of processing. Growth conditions, or handling and preparation procedures before processing, also caiuse substantial variation in nutrient loss. Data on nutritional changes cannot be di

Table 7. Vitamin Losses in Selected Dried Foods

Table 7. Vitamin Losses in Selected Dried Foods

Food

Vitamin A

Thiamin

Vitamin B2

Niacin

Vitamin C

Folic acid

Biotin

Fruits0

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