Losses of thiamine, ascorbic acid and lysine in thermally sterilised foods
The major objective of this investigation is the comparative nutritional evaluation of thermosterile foods in pouches and in cans during sterilization treatment and during storage. To achieve this objective, four products (whole brussels sprouts, whole new potatoes, pork casserole and rice pudding) were developed and produced in pouches and in cans. All variables were maintained constant in the production of these food products and equivalent heat sterilization treatments were applied. Available lysine, ascorbic acid and thiamine (vitamins C and B1) were selected as the nutrients for this investigation. Those nutrients were determined immediately before and after the sterilization treatment and at several intervals during storage. Pouched processing improved the retention of ascorbic acid and thiamine in relation to canning, Storage conditions produced mainly the same effects in ascorbic acid and thiamine retention independ ently of type of package for pork casserole, but larger losses were obtained for the canned products for brussels sprouts and new potatoes. Available lysine losses due to the sterilization treatment are small (= 15%). Difficulties with the sterilization process control and the poor sensitivity of the analytical technique employed seriously question the significance of the comparison between the canned and pouched products. For all samples the use of longer processing times to obtain higher sterilization treatments produced larger destruction of nutrients. Also the use of lower storage temperatures increased the nutrient retention. Large losses of ascorbic acid were obtained in pouched brussels sprouts during storage at three storage temperatures (5°C, 20°C and 37°C), and the ascorbic acid losses were greatly increased at 37°C storage conditions. A series of experiments were conducted to investigate these results. Peroxidase activity was found to be responsible for these losses. It was found to be present in the pouched samples but not in the canned products. It was determined that processing times of over 20 minutes at 121.1°C and 116°C were required to inactivate the enzyme. The results suggest that the improved heat penetration of pouches which permit the achievement of a microbiologically safe sterilization treatment in short times can also be insufficient for the inactivation of heat resistant peroxidases.