EFFECTS OF FREEZING PROCESS AND FROZEN STORAGE ON THE NUTRITIONAL VALUE OF SOME VEGETABLE PRODUCTS

Abstract

The main goal of this study was to investigate the effects of freezing process and frozen storage on the nutritional value of some fresh vegetables products namely; okra, green beans, peas, spinach and molokhia. The results obtained in this study indicated that, all the investigated vegetables contain high levels of moisture, available carbohydrates and crude fiber which ranged between 83.86 - 86.52%, 4.13 - 6.83% and 5.64 - 6.42%, respectively, but, with very low levels of protein, fat and energy which ranged between 1.42 - 1.93%, 0.77 - 0.87%, and 27.66 - 39.70 kcal/100g, respectively. Also, all the vegetables were found very rich in potassium, magnesium, calcium, sodium and ascorbic acid which ranged between 596.81 - 823.83, 564.45 - 783.26, 208.65 - 603.57, 127.03 - 269.42 and 38.13 - 56.32 mg/100g, respectively. Moreover, the activities of peroxidase, lipoxygenase and polyphenol oxidase enzymes were found at the maximum levels (100%) in the different fresh vegetables. On the other hand, after the blanching process, the moisture content increased slightly by 1.43%, 1.70%, 1.84%, 1.98% and 2.25% in green beans, molokhia, spinach, peas and okra, respectively. While, the total carbohydrates, energy value, ascorbic acid, sodium, potassium, calcium, magnesium contents and the activity of peroxidase, lipoxygenase and polyphenol oxidase enzymes were significantly decreased. The decrease as per-cent was estimated to range between 5.64 - 10.13%, 2.04 - 11.76%, 11.61 - 20.37%, 11.42 - 26.59%, 9.53 - 20.79%, 11.08 - 25.79%, 90.03 - 91.32%, 90.03 - 91.46% and 90.08 - 90.97%, respectively. After the freezing processing step, the moisture content in the different vegetables slightly decreased and therefore, the energy value and the available carbohydrates, increased by 6.02 - 16.23% and 10.61 - 26.91%, respectively. While, sodium, potassium, calcium, magnesium and ascorbic acid contents continued to decrease by 8.44 - 16.76%, 3.00 - 11.49%, 3.27 - 13.53%, 0.87 - 1.62% and 4.59 - 14.35%, respectively in comparison with their values after the blanching step. In addition to that, the freezing processing step slightly affected the activity of peroxidase, lipoxygenase and polyphenol oxidase enzymes. After a frozen storage period of 12 months at -18˚C, the energy values and available carbohydrates in the different vegetables gradually increased during the frozen storage period. The increment as per-cent by the end of the period was estimated to range between 19.75 - 20.54% and 29.90 - 48.28%, respectively. Whereas, sodium, potassium, calcium, magnesium and ascorbic acid in the different vegetables were decreased by 8.12 - 23.02%, 2.91 - 8.19%, 8.60 - 18.36%, 5.19 - 13.82% and 3.46 - 4.83%, respectively by the end of the frozen storage period compared to their values after the freezing processing step (zero time). Moreover, the enzymes inactivation in the different vegetables as per-cent by the end of the frozen storage period was estimated to range between 33.33 - 40.79%, 31.95 - 42.96%, and 28.85 - 37.20%, for peroxidase, lipoxygenase and polyphenol oxidase enzymes, respectively.