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FST 151 FOOD FREEZING FOOD SCIENCE AND TECHNOLOGY 151 Food Freezing – Quality Aspects Lecture Notes Prof. Vinod K. Jindal (Formerly Professor, Asian Institute of Technology) Visiting Professor Chemical Engineering Department Mahidol University Salaya, Nakornpathom Thailand Food Freezing Quality Aspects Prof. Vinod Jindal 1 Food Freezing Quality Aspects Prof. Vinod Jindal 2 Food Freezing Quality Aspects Prof. Vinod Jindal 3 Food Freezing Quality Aspects Prof. Vinod Jindal 4 Food Freezing Quality Aspects Prof. Vinod Jindal 5 Food Freezing Quality Aspects Prof. Vinod Jindal 6 Food Freezing Quality Aspects Prof. Vinod Jindal 7 Food Freezing Quality Aspects Prof. Vinod Jindal 8 Food Freezing Quality Aspects Prof. Vinod Jindal 9 Effect of freezing on plant tissues: (a) slow freezing; (b) fast freezing. Food Freezing Quality Aspects Prof. Vinod Jindal 10 Changes in foods Effect of freezing • The main effect of freezing on food quality is damage caused to cells by ice crystal growth. Freezing causes negligible changes to pigments, flavors or nutritionally important components, although these may be lost in preparation procedures or deteriorate later during frozen storage. Food Freezing Quality Aspects Prof. Vinod Jindal 11 Food Freezing Quality Aspects Prof. Vinod Jindal 12 Food Freezing Quality Aspects Prof. Vinod Jindal 13 Food Freezing Quality Aspects Prof. Vinod Jindal 14 The main changes in frozen foods during storage are: • Degradation of pigments. Chloroplasts and chromoplasts are broken down and chlorophyll is slowly degraded to brown pheophytin even in blanched vegetables. In fruits, changes in pH due to precipitation of salts in concentrated solutions change the color of anthocyanins. Food Freezing Quality Aspects Prof. Vinod Jindal 15 • Loss of vitamins. Water-soluble vitamins (for example vitamin C and pantothenic acid) are lost at sub-freezing temperatures . • Vitamin C losses are highly temperature dependent; a 10ºC increase in temperature causes a sixfold to twentyfold increase in the rate of vitamin C degradation in vegetables and a thirtyfold to seventyfold increase in fruits. Losses of other vitamins are mainly due to drip losses, particularly in meat and fish (if the drip loss is not consumed). Food Freezing Quality Aspects Prof. Vinod Jindal 16 • Residual enzyme activity. In vegetables which are inadequately blanched or in fruits, the most important loss of quality is due to polyphenoloxidase activity which causes browning, and lipoxygenases activity which produces off-flavors and off-odors from lipids and causes degradation of carotene. Proteolytic and lipolytic activity in meats may alter the texture and flavour over long storage periods. • Oxidation of lipids. This reaction takes place slowly at 18ºC and causes off-odors and off-flavors. Food Freezing Quality Aspects Prof. Vinod Jindal 17 Food Freezing Quality Aspects Prof. Vinod Jindal 18 Food Freezing Quality Aspects Prof. Vinod Jindal 19 Food Freezing Quality Aspects Prof. Vinod Jindal 20 Food Freezing Quality Aspects Prof. Vinod Jindal 21 Food Freezing Quality Aspects Prof. Vinod Jindal 22 CROSS-SECTION OF ASPARAGUS SPEARS FROZEN BY THREE METHODS (A) Immersion freezing (B) Contact freezing (C) Still air freezing Food Freezing Quality Aspects Prof. Vinod Jindal 23 The ice crystal location in cod muscle, blast frozeen at -30OC Food Freezing Quality Aspects Prof. Vinod Jindal 24 Food Freezing Quality Aspects Prof. Vinod Jindal 25 Food Freezing Quality Aspects Prof. Vinod Jindal 26 Food Freezing Quality Aspects Prof. Vinod Jindal 27 Food Freezing Quality Aspects Prof. Vinod Jindal 28 Recrystalization of ice in beef tissues. Effect of time and storage tempetarures on mean equivalent diameters of te ice crystals. -5OC, o -10OC, -15OC, 20OC Food Freezing Quality Aspects Prof. Vinod Jindal 29 Change in ice crystal size distribution in vanilla ice cream during accelerated recrystallization with and without oscillations (a and b) in the storage temperature. 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