Many unit operations, especially those that do not involve heat, have little or no effect on the nutritional quality of foods. Examples include mixing, cleaning, sorting, freeze-drying, and pasteurization. Unit operations that that intentionally separate the components of foods alter the nutritional quality of each fraction compared with the raw material. Unintentionally separation of water-soluble nutrients (minerals, water-soluble vitamins, and sugar) also occurs in some unit operations (for example blanching, and in drip losses from roast or frozen foods).
Heat processing is a major cause of changes to nutritional properties of foods. For example, gelatinization of starches and coagulation of proteins improve their digestibility, and anti-nutritional compounds (for example a trypsin inhibitor in legumes) are destroyed. However, heat also destroys some types of heat-labile vitamin, reduces the biological value of proteins, (owing to destruction of amino acids or Maillard browning reactions) and promotes lipid oxidation.
Oxidation is a second important cause of nutritional changes to foods. This occurs when food is exposed to air (for example in size reduction or hot-air drying) or because of the action of heat or oxidative enzymes. The main nutritional effects of oxidation are:
1.The degeneration of lipids and subsequent reactions to form a wide variety of carbonyl compounds, hydroxy compounds and short chain fatty acids, and in frying oils to toxic compounds.
2.Destruction of oxygen-sensitive vitamins.
The importance of nutrient losses during processing depends on the nutritional value of a particular food in the diet. Some foods (for example 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, which either are eaten in small quantities or have low concentration 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 the processing of one component of the diet are therefore insignificant to the long-term health of an individual. In an example, complete meals, which initially contained 16.5 micro-grams 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 intake (RDA) and the extent of loss is therefore of minor importance. The same meal contained 9 mg of thiamin and lost 5% after 18 months’ storage. The thiamin content is ten times the RDA, and adequate quantities therefore remained. Possible exceptions are the special dietary needs of pre-term 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.
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 cause substantial variation in nutrients loss.
