Chemical and physical deterioration of frozen foods. Chapter 20.

ZARITZKY NOEMI.

Chemical Deterioration and Physical Instability of Food and Beverages .Editors: Leif Skibsted, Jens Risbo and Mogens Andersen,

Woodhead Publishing

Lugar: Copenhague; Año: 2010; p. 561 - 607

Living cells, biological materials (plant and animal tissues) and food derived from them contain a large proportion of water. Freezing involves lowering the temperature of a system below its initial freezing point converting water to ice, by elimination of the latent heat associated to the change of phase. During freezing water is removed from the food matrix by forming ice crystals. In consequence the concentration of the dissolved substances in the unfrozen regions increased lowering the water activity (aw) of the product.Freezing is therefore, a very efficient method for food preservation not only because the effect of low temperatures on chemical reactions and microbial growth, but also due to the decrease of water activity.Freezing is recognized as one of the best methods for preserving food quality; it inhibits the growth of deteriorative and pathogenic micro-organisms, and retards biochemical and enzymatic reactions that would otherwise occur in unfrozen food. In the present chapter the basic principles of food freezing including the description of the properties of water and ice, the concepts of supercooling and glass transition, the ice formation mechanisms (nucleation and crystal growth) and the presence of intra and extra cellular ice in food tissues are discussed.However the formation of ice involves different physico-chemical modifications during freezing, frozen storage and thawing, that affect food quality; case studies are commented. The most important physical changes induced by freezing are: modifications of cell volume, water dislocation during freezing, mechanical damage, freeze- cracking, moisture migration during storage, freezer burn, recrystallization of ice and exudate production.Regarding the chemical changes occurring during freezing and frozen storage, enzymatic reactions, degradation of pigments and vitamins, flavor deterioration, protein denaturation and lipid oxidation are described.  The storage temperature has a marked effect on the quality of frozen foods; recommendations to increase their shelf life are presented. Besides, the concept of cryostabilization (that means storing frozen food below the glass transition temperature of the maximally freeze concentrated matrix) and its influence on the physical deterioration processes are discussed.