332 The nutrition handbook for food processors in addition to these still important basics there is a very wide range of processed foods, meal components and whole meals available in the frozen format. Cur- rently, the sectors of red meat, poultry, fish/seafood and vegetables make up approximately 10%0 each of the total tonnage of the total frozen food market as well as frozen potatoes at 15%o and ready meals at 20%(Euromonitor). As might be expected, there are considerable geographical differences between regions and countries in usage of frozen food. Whereas in the US and in Europe approxi- mately 13 kg and 10kg of frozen food are consumed per capita per year, in Africa and Asia the amounts consumed are only 0.3 kg and 0.9kg respectively. In the future it is anticipated that freezing as a processing option will take an increas ing share of the food market. In both developed and undeveloped nations the increased demand for frozen foods will come from consumers' wishes for high convenience, high organoleptic quality and high nutritional value Although freezing on its own has a negligible impact on nutrient levels in food, the associated pre-freezing processing, storage in the frozen state and structural damage evident in some thawed frozen foods may have significant detrimental effects. The early literature describing the effects of freezing and associated pro- essing on nutrient content and nutritional value has been reviewed by Bender in 1978, and more recently in 1993. This chapter will summarise the key principles, review newer findings and highlight areas of continuing uncertainty in assessing he nutritional impact of freezin 15.2 Change and stability in frozen foods The defining step in freezing is the removal of heat. This lowers the temperature of foods so that microbial and chemical changes are prevented or minimised. By toring in the frozen state it is possible to prolong greatly the length of time that many foods may be maintained with an excellent sensory and nutritional value It is, however, important to realise that at the typical temperatures used for indus- trial and domestic storage of frozen foods(typically -24oC and -18 C respec- tively), chemical reactions that can lead to a reduction of quality and nutrient los may continue to occur. Many of these reactions take place in solution and even at-24oC, natural foods such as fruit, vegetables and meats may still contain 2-5%0 of their total water content in the liquid phase As the temperature of natural foods is reduced below 0C ice crystals begin to form and the solutes present in intra- and extra-cellular fluids become more concentrated in the remaining liquid water, thereby lowering the freezing point of this water. Therefore, although the rates of most reactions will be substantially reduced by the lower temperature of frozen foods, the increased solute concentration may to some extent counteract this effect. Another effect of the increased solute concentration is to move water by osmosis between compartments. The formation of ice may also rupture cell struc tures causing mixing and reactions between components previously held apart The complex nature of the changes that take place when foods are frozen makes it difficult to predict effects on quality and stabilityin addition to these still important basics there is a very wide range of processed foods, meal components and whole meals available in the frozen format. Cur￾rently, the sectors of red meat, poultry, fish/seafood and vegetables make up approximately 10% each of the total tonnage of the total frozen food market as well as frozen potatoes at 15% and ready meals at 20% (Euromonitor). As might be expected, there are considerable geographical differences between regions and countries in usage of frozen food. Whereas in the US and in Europe approxi￾mately 13 kg and 10 kg of frozen food are consumed per capita per year, in Africa and Asia the amounts consumed are only 0.3 kg and 0.9 kg respectively. In the future it is anticipated that freezing as a processing option will take an increas￾ing share of the food market. In both developed and undeveloped nations the increased demand for frozen foods will come from consumers’ wishes for high convenience, high organoleptic quality and high nutritional value. Although freezing on its own has a negligible impact on nutrient levels in food, the associated pre-freezing processing, storage in the frozen state and structural damage evident in some thawed frozen foods may have significant detrimental effects. The early literature describing the effects of freezing and associated pro￾cessing on nutrient content and nutritional value has been reviewed by Bender in 1978, and more recently in 1993. This chapter will summarise the key principles, review newer findings and highlight areas of continuing uncertainty in assessing the nutritional impact of freezing. 15.2 Change and stability in frozen foods The defining step in freezing is the removal of heat. This lowers the temperature of foods so that microbial and chemical changes are prevented or minimised. By storing in the frozen state it is possible to prolong greatly the length of time that many foods may be maintained with an excellent sensory and nutritional value. It is, however, important to realise that at the typical temperatures used for indus￾trial and domestic storage of frozen foods (typically -24°C and -18°C respec￾tively), chemical reactions that can lead to a reduction of quality and nutrient loss may continue to occur. Many of these reactions take place in solution and even at -24°C, natural foods such as fruit, vegetables and meats may still contain 2–5% of their total water content in the liquid phase. As the temperature of natural foods is reduced below 0°C ice crystals begin to form and the solutes present in intra￾and extra-cellular fluids become more concentrated in the remaining liquid water, thereby lowering the freezing point of this water. Therefore, although the rates of most reactions will be substantially reduced by the lower temperature of frozen foods, the increased solute concentration may to some extent counteract this effect. Another effect of the increased solute concentration is to move water by osmosis between compartments. The formation of ice may also rupture cell struc￾tures causing mixing and reactions between components previously held apart. The complex nature of the changes that take place when foods are frozen makes it difficult to predict effects on quality and stability. 332 The nutrition handbook for food processors