Effect of Freezing Rate on Vegetables

Abstract

VIuch confusion appears in the literature concerning the effects of fast and slow freezing on the quality of vegetables. In this study, five different rates of freezing were employed, ranging from very rapid, by means of liquid air, to very slow, in an insulated box. Peas and snap beans were blanched, frozen, and stored at -6°F. for six months. Analyses were made for ascorbic acid, carotene, and thiamine.Riboflavin was run on peas only. Analyses were madfe on raw, blanched, and frozen samples, and again after six-month storage and after cooking. Significant differences in vitamin content could not be detected among the samples from the rates of freezing studied. Significant differences in taste and texture were not observed, with the exception of the texture of those frozen in liquid air. These were somewhat softer, probably because of cracking that took place during freezing. Photomicrographs show that the slower the rate of freezing, the larger the ice crystals; but in the corresponding thawed samples, these differences disappeared, and damage was not apparent.A NUMBER of articles have been published relative to the effect of freezing rate on the quality of quick-frozen vegetables. Woodroof {12, IS) indicated that slow freezing of nonstarchy vegetables resulted in flabbiness; in the case of starchy vegetables, such as peas and shelled beans, less change was noted, apparently because of the support lent to the cell walls by the starch grains.Joslyn and Marsh (4) "do not find that there is a direct relation between loss in weight of treated fruits and vegetables and change in texture as judged by the degree of retention of original shape, turgidity, and crispiness" as reported by Woodroof. Table 14 of their article shows that steam-blanched asparagus, frozen in still air at 0°F., lost 25.2 % in weight when thawed one year later. The same lot of asparagus, steam-blanched but frozen in dry ice at -110°F ., showed a loss of 14.4% when thawed a year later. The figures for corresponding treatments of peas were 6,6 and 0.6%, respectively, and for string beans, 13.8 and 6.8%, respectively. They, further stated: "It has been our experience that with the possible exception of asparagus, increasing the rate of freezing by using solid carbon dioxide did not appreciably improve the texture of fruits and vegetables examined." Diehl and Berry {l) reported: " ...

Effects of Freeze Preservation on Nutrients

“…The vitamin data in Table 11.10 are in reasonably good agreement with other comparable studies dealing with cooking losses of (3-carotene in peas and green beans (Lee et al 1946), thiamin in green peas (Barnes and Tressler 1943;Lee et al 1946;Martin et al 1960), and riboflavin in green beans (Lee et al 1946;Phillips and Fenton 1945;Van Duyne et al 1950). Lee et al (1946) reported that riboflavin losses during cooking of frozen green peas range from 0 to 39%, which is contrary to the results in Table 11.10.…”

“…Lee et al (1946) reported that riboflavin losses during cooking of frozen green peas range from 0 to 39%, which is contrary to the results in Table 11.10. Studies by Dawson et al (1949), Lee et al (1946),…”

“…b Barnes and Tressler (1943), Dawson et al (1949), Derse and Teply (1958), Lee et al (1946, Tinklin and Filinger (1956). f Barnes and Tressler (1943), Derse and Teply (1958), Guerrant and O'Hara (1953), Lee et al (1946), Stimson and Tressler (1939), Van Duyne et al (1950). et al (1950).…”

Physiological Aspects of Low Temperature Preservation of Plant Products

“…This result was noted in the preliminary observations made by Joslyn & Marsh (68). Lee et al (64) reported no difference in texture, microscopic ap pearance, or palatability in strawberries, raspberries, and sliced peaches packed in sirup whether frozen very slowly, very rapidly, or at intermediate rates.…”

“…MacArthur (60,61) agreed with him that slow freezing re sulted in more tissue destruction by large, internal ice crystals than more rapid freezing, particularly in asparagus. Lee et al (62) found that although slow freezing resulted in the formation of large veins of ice in peas and snap beans, no injury was evident when the product was thawed and no signifi cant differences in palatability could be detected between produds frozen slowly and those frozen rapidly. Similarly, the rate of freezing had no effect on the palatability of spinach, corn, and lima beans although with aspara gus, differences in texture were found with different rates of freezing (63).…”