Influence of Food Preparation on Nutritional Quality; Introductory Remarks

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1 J. Nutr. Sci. Vitaminol., 36, S1-S6, 1990 Influence of Food Preparation on Nutritional Quality; Introductory Remarks Institute of Social and Preventive Medicine, University of Zurich, Zurich, Switzerland Key Words household handling of food, vitamins, minerals, food prep aration, heated food fats, nutritional quality The topic of this workshop "Nutritional losses due to food preparation and possibilities of their prevention" is rather important. Such losses occur between harvesting and distribution, during household and industrial handling as well as catering and during storage. Some of the foods have to be processed so that they remain storable for a longer period of time. Only in this way can the population be provided with a regular and constant supply. It is estimated that 60-70% of all foods are handled industrially. There are losses in nutrient content due to these processes, to a lesser or to a greater extent, depending on the manner of production. Industrial procedures are now known which cause only slight changes in quality. Losses in nutritional value can also result from the household handling of foods and even more so from the preparation of meals in catering. Their magnitude depends in both cases on the cooking method. We carried out investigations in this field some years ago which should be briefly summarized. To receive comparable results it is important to standardize the experimental conditions in all phases. It was investigated how various cooking influence the vitamin C and the thiamin content of potatoes and various vegetables. These experiments show that the extent of vitamin C losses in potatoes and vegetables differs according to the method of preparation. The results of the experiments concerning the decrease in vitamin C content of potatoes by various cooking methods are summarized in Table 1. The smallest reduction of vitamin C content was by the steaming method, i.e. cooking with very little water (so called "waterless" cooking) and the highest losses when cooked in a pressure cooker (1). One probably finds the results surprising that the vitamin C losses in potatoes prepared in a pressure cooker are higher than when boiled in water. These findings confirm our previous experiments (2), in which the vitamin C losses in whole, half and one eights potatoes were determined after cooking in much water and in S1

2 S2 Table 1. Vitamin C losses in potatoes due to various cooking methods. * The vitamin C losses are calculated on dry weight basis. Table 2. The decrease of the vitamin C content in potatoes in relation to the size of the pieces and cooking methods. pressure cooker (Table 2). There is a considerable difference in vitamin losses between the two cooking methods. Statistical analysis shows that the difference is highly significant. The p value is less than The same results were obtained when the experiment was carried out with pealed potatoes. If they were cut in half, the vitamin losses of potatoes in boiling water were higher than in whole potatoes but still lower than in those which were prepared in a pressure cooker. The situation changed only when the potatoes were cut into eight pieces. Now, due to the leaching out effect, the vitamin C decrease in potatoes boiled in water was greater than when cooked in a pressure cooker. A possible explanation why the potatoes in a pressure cooker lose more vitamin C than in boiling water is perhaps the following. According to our measurements the temperature in the middle of the potatoes prepared in a pressure cooker is about 5 Ž higher than those cooked in water. But if the temperature increases by 5 Ž the speed of reaction as well as the degree of oxidation of vitamin C will rise by 30%. The changes in vitamin C content in a leaf vegetable such as spinach by various cooking methods are shown in the Table 3. J. Nutr. Sci. Vitaminol.

3 FOOD PREPARATION AND NUTRITIONAL QUALITY, VITAMIN CONTENT S3 Table 3. Vitamin C losses in spinach due to various cooking methods. * The vitamin C losses are calculated on dry weight basis. Table 4. Thiamin losses in potatoes due to various cooking methods. * The thiamin losses are calculated on dry weight basis. Table 5. Thiamin losses in spinach due to various cooking methods. * The thiamin losses are calculated on dry weight basis. The smallest losses were again observed when steaming the spinach (cooking with very little water) and the highest losses when cooking in much water (1). Similar results were obtained when the thiamin content was determined in potatoes and spinach before and after cooking by various methods (Table 4 and Table 5) (3). The results of these experiments (1, 3) were confirmed by several authors (among others 4-7). I do not go further on this theme because the papers by Rumm-Kreuter and that by Itokawa and Kimura deal with this topic. Leinert, Becker, Somogyi and Hotzel (7) have carried out investigations about the influence of various cooking methods on the mineral content of potatoes and vegetables. The changes in iron, copper, potassium and magnesium content were determined. The losses in the potassium-, magnesium and copper-content of potatoes and spinach than lower cooking in very little water, than preparing them Vol. 36, Suppl. I, 1990

4 S4 in pressure cooker or in much water. As is known from many investigations, losses in the nutritional value of foods by preparation of meals are generally higher in catering than in household han dling. The nutritional value of these foods is, however, of significance because these meals contribute considerably to the daily food intake for a greater part of the population in industrialized countries. But here are various systems of catering with which the losses in nutritional quality can be decreased. Paulus is dealing with the changes of nutritional quality of food in catering in his contribution (s. page S35). Not only is the quality of the micronutrients influenced by household and industrial handling but also that of the macronutrients. Proteins are of great interest here but the changes in the quality of fats and, to some extent, of carbohydrates also play a role. During processing, protein can react with other proteins or interact with carbohydrates (Maillard reaction), with lipids or polyphenols. The various reac tions may influence the nutritive value of proteins by changes in digestibility and in biological value. These reactions and interactions of proteins due to processing cause changes in certain foods, such as milk, legume seeds, cereal flours, etc. Mauron as well as Yasumoto and Suzuki are discussing this important theme (s. page S57, S71). In conclusion let us discuss briefly-just the most important facts as high lights-the influence of household and industrial handling on the quality of food fats. Oils and fats of plants as we get them after pressing or extraction of oil seeds are not suitable for human consumption due to their taste, flavour and colour. Therefore, the raw oil has to be refined, which is a rather laborious process. What are the changes in food fats caused by household handling? Figure 1 gives an overview about the temperature which result by various cooking meth ods (8). With boiling and steaming the temperatures are not higher than 100 Ž. Under these conditions the fats are not affected. The situation is quite different with roasting. The temperatures reach here values of 250 Ž and more. A degradation of the fat takes place, however, only to a small extent. A cooking method at which relatively high temperature are reached is grill ing. Inspite of this fact, the changes in fat quality should be small according to Billek (8). The temperatures reached by deep frying are lower and lie between 160 and 180 Ž. In spite of the low temperatures, a destruction of the fats by this cooking method cannot be excluded, because these fats are used several times in contrast to the other cooking methods mentioned here. The degree of the decomposition of fats can of course be determined, a question with which I shall not deal. It is an important problem whether and to what extent heated fats can be J. Nutr. Sci. Vitaminol.

5 FOOD PREPARATION AND NUTRITIONAL QUALITY, VITAMIN CONTENT S5 Fig. 1. harmful to the health. This question can be only clarified by animal experiments. The German Society for Fat Science consider a heated fat as spoiled if it contains more than 27% polar fraction. Such a fat will be refused already because of its bad taste and flavour. Finally I hope, that this workshop has clarified some questions in this field and will stimulate further research on this important and interesting topic. REFERENCES 1) Somogyi, J. C. (1975): Einfluss der Zubereitungsweise auf den Vitamin C-Gehalt von Kartoffeln and Gemuse. Ernahr. Umsch., 22, ) Somogyi, J. C. (1966): Die Bedeutung der Fruchte, Gemuse and Kartoffeln in der heutigen Ernahrung in Neue Erkenntnisse in der Gemeinschaftsverpflegung. ed, by Somogyi, J. C., Forster-Verlag, Zurich, pp. 105, ) Somogyi, J. C., and Kopp, P. M. (1978): Einfluss der Zubereitungsweise auf den Thiamin-Gehalt von Kartoffeln and Gemusen, Ernahr. Umsch., 25, ) Blumenthal, A., Scheffeldt, P., and Dunnenberger, G. (1981): Zu den Nahrstoff gehalten tischfertiger Nahrungsmittel. Alimenta, 20, ) Augustin, J., Johnson, S. R., Teitzel, C., True, R. H., Hogan, J. M., Toma, R. B., Shaw, R. L., and Deutsch, R. M. (1987): Changes in the nutrient composition of potatoes during home preparation II. Vitamins. Am. Potato J., 55, ) Meier-Ploeger, A., Badelt-Vogt, A., Stettler, H., Riedel, G., and Nau, L. (1981): Einfluss verschiedener Garverfahren auf den Ascorbinsaureverlust von Kartoffeln and Kohlrabi. Ernahr. Umsch., 28, ) Leinert, J., Becker, D. P., Somogyi, J. C., Hotzel, D. (1981): Einfluss der Garweise Vol. 36, Suppl. I, 1990

6 S6 auf Mineralstoffverluste. Ernahr. Umsch., 28, ) Billek, G. (1985): Einfluss der industriellen Verarbeitung and der haushaltsmassigen Zubereitung auf die Nahrungsfette, in Handling of Food without Changes of Quality Dream or Reality? ed. by Somogyi, J. C., Bibliotheca Nutritio et Dieta 34, S. Karger, Basel, pp J. Nutr. Sci. Vitaminol.