216 Meat refrigeration 10.3 Frozen storage This chapter is a brief summary of a full review by James and Evans(1997) The factors that influence the storage life of frozen meat may act in any one of three stages: prior to freezing, during the actual freezing process and postfreezing in the storage period itself. 10.3.1 Oxidative rancidity The importance of fat oxidation in frozen meat is illustrated by a short quo tation from a paper published by Lea(1931); it is often the deterioration of the fat which limits the storage life- from the point of view at least of palata bility-of the meat. This view has been reiterated many times, and as freez ing technology has improved it is true to say that fat oxidation remains the obstacle to very long-term storage of frozen meat. Early studies on fat oxi- dation and freezing were reviewed by Lea(1938)and Watts(1954) 10.3.1.7 Mechanism of oxidation The reaction of oxygen with fatty acids produces peroxides. It is the break- down products of the peroxides that produce the characteristic objection able odour and flavour of rancid meat. The development of oxidative rancidity in meat is affected by two groups of factors, one group consisting of the built-in characteristics of the meat and the other group consisting of those factors involved in the treatment of the meat. The former are mainly under the control of the farmer or are innate characteristics of the living animal, whereas the latter can be controlled by the abattoir, the meat packer or the cold store operator. Although the first group cannot be changed by the meat processor it is necessary to consider their effect so that procedures may be modified to limit them. Before discussing either group it is neces- sary to look at the process of fat oxidation in the hope that knowledge of the process will indicate the ways in which it may be controlled. The reaction of gen with fat is an autocatalytic process. Once the reaction starts, the products of the reaction stimulate it to go faster. The initial reaction is between a molecule of oxygen and a fatty acid to form a peroxide. This is a slow reaction but like any other chemical reaction its rate is increased by raising the temperature. The rate is also influenced by the type of fatty acid. Saturated fatty acids react slowly, but unsaturated fatty cids react more rapidly, and the more double bonds that a fatty acid con- tains, the more reactive it is. The presence of peroxides in fat does not change the flavour, it is the breakdown products of the peroxides which produce the rancid odour and favour. The breakdown of peroxide is accel erated by heat, light, organic iron catalysts and traces of metal ions, espe- cially copper and iron. The breakdown products of the peroxides cause the oxygen to react more rapidly with the fatty acids, thus producing the auto- catalytic effect.10.3 Frozen storage This chapter is a brief summary of a full review by James and Evans (1997). The factors that influence the storage life of frozen meat may act in any one of three stages: prior to freezing, during the actual freezing process and postfreezing in the storage period itself. 10.3.1 Oxidative rancidity The importance of fat oxidation in frozen meat is illustrated by a short quo￾tation from a paper published by Lea (1931);‘it is often the deterioration of the fat which limits the storage life – from the point of view at least of palata￾bility – of the meat’.This view has been reiterated many times, and as freez￾ing technology has improved it is true to say that fat oxidation remains the obstacle to very long-term storage of frozen meat. Early studies on fat oxi￾dation and freezing were reviewed by Lea (1938) and Watts (1954). 10.3.1.1 Mechanism of oxidation The reaction of oxygen with fatty acids produces peroxides. It is the break￾down products of the peroxides that produce the characteristic objection￾able odour and flavour of rancid meat. The development of oxidative rancidity in meat is affected by two groups of factors, one group consisting of the built-in characteristics of the meat and the other group consisting of those factors involved in the treatment of the meat. The former are mainly under the control of the farmer or are innate characteristics of the living animal, whereas the latter can be controlled by the abattoir, the meat packer or the cold store operator. Although the first group cannot be changed by the meat processor it is necessary to consider their effect so that procedures may be modified to limit them. Before discussing either group it is neces￾sary to look at the process of fat oxidation in the hope that knowledge of the process will indicate the ways in which it may be controlled. The reaction of oxygen with fat is an autocatalytic process. Once the reaction starts, the products of the reaction stimulate it to go faster. The initial reaction is between a molecule of oxygen and a fatty acid to form a peroxide.This is a slow reaction but like any other chemical reaction its rate is increased by raising the temperature. The rate is also influenced by the type of fatty acid. Saturated fatty acids react slowly, but unsaturated fatty acids react more rapidly, and the more double bonds that a fatty acid con￾tains, the more reactive it is. The presence of peroxides in fat does not change the flavour, it is the breakdown products of the peroxides which produce the rancid odour and flavour. The breakdown of peroxide is accel￾erated by heat, light, organic iron catalysts and traces of metal ions, espe￾cially copper and iron. The breakdown products of the peroxides cause the oxygen to react more rapidly with the fatty acids, thus producing the auto￾catalytic effect. 216 Meat refrigeration