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FST 151FOOD FREEZINGFOOD SCIENCE AND TECHNOLOGY 151

Food Freezing – Quality AspectsLecture Notes

Prof. Vinod K. Jindal(Formerly Professor, Asian Institute of Technology)Visiting ProfessorChemical Engineering DepartmentMahidol UniversitySalaya, NakornpathomThailand

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Effect of freezing on plant tissues: (a) slow freezing; (b) fast freezing.

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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.

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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.

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• 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).

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• 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.

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CROSS-SECTION OF ASPARAGUS SPEARS FROZEN BY THREE METHODS

(A) Immersion freezing(B) Contact freezing(C) Still air freezing

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The ice crystal location in cod muscle, blast frozeen at -30OC

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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

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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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