Almost all foods undergo some form of processing before they are eaten. This may be done to increase the palatability of food, increase its storage life, minimise the risk of food-borne disease, or produce a novel product from basic ingredients. Processing can range from simple washing, trimming, milling or cooking, to freeze drying, canning, irradiation or extrusion. All forms of processing, whether done in the home or a factory, will affect nutrient content. Even basic processing such as washing some foods or toasting bread can remove nutrients but, in the context of a total diet, such losses are not likely to be nutritionally significant.

There is no intrinsic reason why a factory-processed food should be nutritionally inferior to the same food prepared in the home. It is difficult to make such comparisons, as much depends on the processing method used and on the quality and freshness of the original ingredients. Another difficulty is that not all food factories use the same methods to process food. Nor do all home cooks. Differences in initial preparation techniques, cooking methods, and holding times all influence the retention or loss of nutrients. In some instances, commercially processed foods such as breakfast cereals and fruit juices have nutrients added to partly offset those lost during processing.

To illustrate the importance of the original ingredients used in processing, consider the nutrient content of bread produced from different flours. Milling wheat produces a flour with varying proportions of the original nutrients. Wholemeal flour contains most of the original wheat nutrients whereas white flour will have lost some of the original vitamins, minerals and trace elements depending on the particular milling practice. The most important factor in the nutrient content of any bread baked from these flours will depend more on the type of flour used rather than whether it was factory or home baked.

In general processing reduces nutrient content, but some processes are more wasteful of the original nutrients than others and some nutrients more prone to loss than others. There is little likelihood oi nutritional deficiency if foods are selected from the wide range available and chosen using the principles of the Commonwealth Department oi Health's Dietary Guidelines For Australians.