Freezing 333 Probably the most important reaction leading to both quality and nutrient losses in frozen foods is oxidation. The consequences of oxidative instability are the key factors that limit the storage life of frozen foods. Just as in foods kept at more normal ambient temperatures, unless they are stored in a vacuum, or in an inert gas, atmospheric oxygen will diffuse through frozen food and may react with many of the soluble and insoluble components. One consequence of oxida tion on sensory quality is the development of ' off flavours'and rancidity, usually caused by oxidative breakdown of membrane and storage lipids(Erickson, 1997 Other adverse consequences of oxidation may include colour loss and/or change and in fish and meat foods a toughening of muscle structures. Although macro- molecular components such as carbohydrates and protein may undergo limited oxidation, any infuence on nutritional value is likely to be small. However, several vitamins such as ascorbate and folates are particularly susceptible to oxidative damage A feature of the quick freezing of foods is the formation of a large number of relatively small ice crystals that cause minimal damage to cellular and tissue structures but on prolonged frozen storage, and particularly in conditions where temperatures fluctuate, crystals of ice grow in size. Although at any temperature below 0C, the total amount of ice in a food will remain constant, large crystals grow instead of a larger number of smaller crystals, a process known as Ostwald ripening. The growth of larger ice crystals may break delicate food structures and compress others On thawing of frozen foods these changes may have serious effects on texture leading to poor sensory quality: vegetables and fruits may lose their characteristic crispness and meat or fish may become tougher and drier. An adverse consequence for nutritional value is the reduced water-holding capacity of structurally damaged foods, leading to increased ' drip loss. Significant amounts of water-soluble nutrients may be discarded if this drip loss is not incor- porated into the food to be consumed. 15.3 Vegetables and fruits There are several factors that potentially contribute to differences in nutrient levels between vegetables and fruits in the frozen format and those supplied as fresh or preserved by other processes. Any differences are likely to be in the loss nd preservation of vitamins; it has been shown that compared with fresh veg- etables, there are negligible differences between the mineral and fibre contents of equivalent frozen vegetables(Polo et al, 1992: Nyman, 1995) 15.3.1 Selection of cultivar and time of harvesting Particular cultivars and harvest times are chosen to optimise sensory quality and these may differ between those selected for freezing and those that are consumed in fresh, canned or dried formats. The cultivar and harvest time may have some effects on nutritional value (Shewfelt, 1990); for example peas selected forProbably the most important reaction leading to both quality and nutrient losses in frozen foods is oxidation. The consequences of oxidative instability are the key factors that limit the storage life of frozen foods. Just as in foods kept at more normal ambient temperatures, unless they are stored in a vacuum, or in an inert gas, atmospheric oxygen will diffuse through frozen food and may react with many of the soluble and insoluble components. One consequence of oxida￾tion on sensory quality is the development of ‘off flavours’ and rancidity, usually caused by oxidative breakdown of membrane and storage lipids (Erickson, 1997). Other adverse consequences of oxidation may include colour loss and/or change, and in fish and meat foods a toughening of muscle structures. Although macro￾molecular components such as carbohydrates and protein may undergo limited oxidation, any influence on nutritional value is likely to be small. However, several vitamins such as ascorbate and folates are particularly susceptible to oxidative damage. A feature of the quick freezing of foods is the formation of a large number of relatively small ice crystals that cause minimal damage to cellular and tissue structures but on prolonged frozen storage, and particularly in conditions where temperatures fluctuate, crystals of ice grow in size. Although at any temperature below 0°C, the total amount of ice in a food will remain constant, large crystals grow instead of a larger number of smaller crystals, a process known as Ostwald ripening. The growth of larger ice crystals may break delicate food structures and compress others. On thawing of frozen foods these changes may have serious effects on texture leading to poor sensory quality; vegetables and fruits may lose their characteristic crispness and meat or fish may become tougher and drier. An adverse consequence for nutritional value is the reduced water-holding capacity of structurally damaged foods, leading to increased ‘drip loss’. Significant amounts of water-soluble nutrients may be discarded if this drip loss is not incor￾porated into the food to be consumed. 15.3 Vegetables and fruits There are several factors that potentially contribute to differences in nutrient levels between vegetables and fruits in the frozen format and those supplied as fresh or preserved by other processes. Any differences are likely to be in the loss and preservation of vitamins; it has been shown that compared with fresh veg￾etables, there are negligible differences between the mineral and fibre contents of equivalent frozen vegetables (Polo et al, 1992; Nyman, 1995). 15.3.1 Selection of cultivar and time of harvesting Particular cultivars and harvest times are chosen to optimise sensory quality and these may differ between those selected for freezing and those that are consumed in fresh, canned or dried formats. The cultivar and harvest time may have some effects on nutritional value (Shewfelt, 1990); for example peas selected for Freezing 333