The influence of dehydration of foods on the digestibility and the biological value of the protein

The nutritive value of 12 cooked foods was compared with that of the corresponding cooked products after subsequent dehydration by hot air, or freeze-, vacuum-or spray-drying. Biological value (B.V.), true digestibility and net protein utilization were estimated by the Thomas and Mitchell N balance method, with the modification that endogenous N was measured in separate groups of rats on a diet with 4% protein provided by freeze-dried whole egg. The test materials were given as the sole source of protein to groups of 10 ten-day-old male Sprague Dawley rats for a 10-day experimental period, urine and faeces being collected during the last 7 days. Hot air drying somewhat reduced the B.V. of cabbage, Lima beans and green leaves and the digestibility of red beans, but not of kale and turnip greens, while freeze-dried beef, chicken, fish, fish patties and maize, and spray-dried cheese and a product made from whole egg, condensed skimmed milk and cottonseed oil, showed no noticeable change in protein quality. Rehydration of fish patties, green beans and kale before feeding hardly affected protein utilization. Canning of green beans caused a considerable loss of protein quality. In general, dehydration in hot air showed more indications of slight protein damage than did spray drying, or freeze drying. In contrast with dehydration, the canning of green beans produced a considerable decrease in protein quality. A slight damage to the protein was caused by frying fish patties in deep fat for 2½ min at 200°C. The utilization of protein was hardly, if at all, affected whether or not dehydrated fish patties, green beans, and kale were rehydrated prior to feeding. The importance of equalized water content of diets for accurate determinations of nitrogen balances was stressed.