15 Freezing J M. Fletcher, Unilever r& D Colworth 15.1 ntroduction The modern frozen food industry was started by Clarence Birdseye in America in 1925. As a fur trader in Labrador Birdseye had noticed that fillets of fish left by the natives to freeze rapidly in arctic winters retained the taste and texture attrib utes of fresh fish better than fillets frozen in milder temperatures at other times of the year. Frozen foods were available before Birdseyes pioneering innovations but they were of poor and uncertain quality. Birdseye's insight was that speed of freezing is crucial to retain quality and he was the first to develop machinery that could freeze foods rapidly on an industrial scale. Quick freezing allowed the trans- port of produce over long distances and the year-round consumption of seasonal produce that was of very superior quality compared with alternative preservation methods such as canning and drying. Although Birdseye was probably unaware of this particular advantage, quick freezing, if combined with appropriate treatment prior to freezing, also has the potential to ensure excellent preservation of nutri- tional value for a wide range of foods. In the context of the nutritional value of vegetables and fruits, the US Food and Drug Administration has recently(1998) approved frozen produce to be labelled as healthy. Based on presented data the Food and Drug administration concluded that .. because frozen fruits or veg etable products are nutritionally comparable to the raw versions, they would likely have the same inherent beneficial effects as the raw version In the years since 1925 the application of freezing has become globally an important aspect of food processing technology. In the year 2000 total world wide sales of frozen foods(excluding ice cream) was estimated as 13.6 million tonnes with a retail value of US$ 58.5 billion( Euromonitor). The process of quick freez ing was first applied to a limited range of fish, meat, fruits and vegetables; today
15 Freezing J. M. Fletcher, Unilever R & D Colworth 15.1 Introduction The modern frozen food industry was started by Clarence Birdseye in America in 1925. As a fur trader in Labrador Birdseye had noticed that fillets of fish left by the natives to freeze rapidly in arctic winters retained the taste and texture attributes of fresh fish better than fillets frozen in milder temperatures at other times of the year. Frozen foods were available before Birdseye’s pioneering innovations, but they were of poor and uncertain quality. Birdseye’s insight was that speed of freezing is crucial to retain quality and he was the first to develop machinery that could freeze foods rapidly on an industrial scale. Quick freezing allowed the transport of produce over long distances and the year-round consumption of seasonal produce that was of very superior quality compared with alternative preservation methods such as canning and drying. Although Birdseye was probably unaware of this particular advantage, quick freezing, if combined with appropriate treatments prior to freezing, also has the potential to ensure excellent preservation of nutritional value for a wide range of foods. In the context of the nutritional value of vegetables and fruits, the US Food and Drug Administration has recently (1998) approved frozen produce to be labelled as healthy. Based on presented data the Food and Drug Administration concluded that ‘. . . because frozen fruits or vegetable products are nutritionally comparable to the raw versions, they would likely have the same inherent beneficial effects as the raw version’. In the years since 1925 the application of freezing has become globally an important aspect of food processing technology. In the year 2000 total world wide sales of frozen foods (excluding ice cream) was estimated as 13.6 million tonnes with a retail value of US$ 58.5 billion (Euromonitor). The process of quick freezing was first applied to a limited range of fish, meat, fruits and vegetables; today
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 stability
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. Currently, 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 approximately 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 increasing 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 processing 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 industrial and domestic storage of frozen foods (typically -24°C and -18°C respectively), 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 intraand 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 structures 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
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 for
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 oxidation 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 macromolecular 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 incorporated 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 vegetables, 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
334 The nutrition handbook for food processors 100四 Ambient 50 Chil‖l △= Frozen 0 Time since harvest( days) Fig. 15.1 Effects of storage and freezing on ascorbate retention in spinach: typical values for retention of ascorbate in spinach stored at either ambient or chill temperature (4C) compared with blanched and frozen spinach. All samples were taken from the same field and time zero levels were obtained from freshly harvested spinach. Blanching and freez- ing were carried out in a commercial factory. (from Favell, 1998) canning are usually harvested at a more mature stage than those selected for freez- ing and consequently have approximately 10% lower ascorbate concentration. The type of cultivar may also influence the amount of nutrient lost during pro- cessing, reflecting differences between culitvars in morphology and mechanical stren 5.3.2 Storage after Many vegetables, and to a lesser extent fruits, are relatively unstable after har vesting and undergo rapid chemical changes that result in significantly reduced levels of some nutrients. For example, concentrations of ascorbate in spinach may all to 50% of their initial, pre-harvest level, after two days of storage as shown in Fig. 15.1(Favell, 1998). The magnitude of nutrient losses during storage prior to freezing is highly variable and depends on the crop, the method of harvesting and the duration and conditions of storage. To preserve the nutritional value of fresh vegetables and fruits it is clearly desirable to minimise the time in blanch ing and freezing and to cause minimal mechanical damage 153.3 Washing and blanching The need for washing of vegetables and fruits may cause some loss of water- soluble nutrients, particularly from cut surfaces. As noted above, oxidation is a key factor influencing stability in the frozen state and this is particularly a concern with vegetables and fruits because they contain many enzyme systems that give rise to reactive oxygen species. It is to prevent enzyme-mediated oxidation reac- tions that most vegetables and fruits are blanched before freezing. Another reason
canning are usually harvested at a more mature stage than those selected for freezing and consequently have approximately 10% lower ascorbate concentration. The type of cultivar may also influence the amount of nutrient lost during processing, reflecting differences between culitvars in morphology and mechanical strength. 15.3.2 Storage after harvest Many vegetables, and to a lesser extent fruits, are relatively unstable after harvesting and undergo rapid chemical changes that result in significantly reduced levels of some nutrients. For example, concentrations of ascorbate in spinach may fall to 50% of their initial, pre-harvest level, after two days of storage as shown in Fig. 15.1 (Favell, 1998). The magnitude of nutrient losses during storage prior to freezing is highly variable and depends on the crop, the method of harvesting and the duration and conditions of storage. To preserve the nutritional value of fresh vegetables and fruits it is clearly desirable to minimise the time in blanching and freezing and to cause minimal mechanical damage. 15.3.3 Washing and blanching The need for washing of vegetables and fruits may cause some loss of watersoluble nutrients, particularly from cut surfaces. As noted above, oxidation is a key factor influencing stability in the frozen state and this is particularly a concern with vegetables and fruits because they contain many enzyme systems that give rise to reactive oxygen species. It is to prevent enzyme-mediated oxidation reactions that most vegetables and fruits are blanched before freezing. Another reason 334 The nutrition handbook for food processors 0 50 100 0 7 14 21 Time since harvest (days) Ascorbate (% retention) Ambient Chill Frozen Fig. 15.1 Effects of storage and freezing on ascorbate retention in spinach: typical values for retention of ascorbate in spinach stored at either ambient or chill temperature (4°C) compared with blanched and frozen spinach. All samples were taken from the same field and time zero levels were obtained from freshly harvested spinach. Blanching and freezing were carried out in a commercial factory. (from Favell, 1998)
Freezing 335 is to ensure microbiological safety but this can be achieved by other means. The advantages of blanching can be illustrated with reference to cauliflower and spinach. If they are frozen without blanching they become unpalatable after only a few months due to the development of off flavours and odours caused pI marily by oxidation of membrane lipids. If these vegetables are blanched before freezing they have a storage life of 18-24 months. Commercial blanching con litions typically involve heating in water or steam at 95-100oC for 3-10 minutes, depending on the type and size of material to be blanched. The conditions are chosen so as to ensure inactivation of the enzymes responsible for oxidation During blanching, nutrients may be lost by leaching and by chemical degrada- tion. A great deal of information has been published on losses of labile nutrients during blanching(for review see Clydesdale et al, 1991). Ascorbate is often used as an indicator of potential nutrient loss because of its high solubility, sensitivity to heat and ease of measurement. Typical losses of ascorbate from vegetables during blanching are of the order 5-40%(Favell, 1998; Bender, 1993). In general, it may be concluded that nutrient losses are minimised if the raw material is as little damaged as possible during handling and if processing conditions are chosen that keep the temperature, duration of heat exposure and product to water ratio as low as is consistent with denaturing the enzymes responsible for oxidative spoilage 15.3. 4 Frozen storage Bender (1993) has summarised the contradictory results of published studies designed to estimate the magnitude of vitamin loss during frozen storage of veg- tables and fruits. Even for a particular vegetable, processed and stored under apparently similar conditions, the extent of ascorbate loss has been reported as negligible, or up to 40% after a year of frozen storage(Bender, 1993). As Bender comments, there are many possible sources of experimental variation that may lead to these different conclusions, most notably incomplete denaturation of oxidative enzymes during blanching. Since the review by Bender no large scale systematic study addressing this issue has been published. It may be concluded that if vegetables and fruits are adequately blanched and stored at conven- tional freezer temperatures without undue temperature fluctuations they will still possess valuable levels of potentially labile nutrients for a period of at least 12-18 months When comparing the nutritional value of different processing methods it is also necessary to consider the ways in which consumers handle these different prod ucts. Cooking methods may have important effects on the quantity of nutrients within a food. Because frozen vegetables have already been blanched, they require less cooking time than fresh vegetables to reach the same levels of palat bility. This means that while frozen vegetables may have lost some nutrients during blanching they will probably suffer reduced losses during cooking
is to ensure microbiological safety but this can be achieved by other means. The advantages of blanching can be illustrated with reference to cauliflower and spinach. If they are frozen without blanching they become unpalatable after only a few months due to the development of ‘off’ flavours and odours caused primarily by oxidation of membrane lipids. If these vegetables are blanched before freezing they have a storage life of 18–24 months. Commercial blanching conditions typically involve heating in water or steam at 95–100°C for 3–10 minutes, depending on the type and size of material to be blanched. The conditions are chosen so as to ensure inactivation of the enzymes responsible for oxidation. During blanching, nutrients may be lost by leaching and by chemical degradation. A great deal of information has been published on losses of labile nutrients during blanching (for review see Clydesdale et al, 1991). Ascorbate is often used as an indicator of potential nutrient loss because of its high solubility, sensitivity to heat and ease of measurement. Typical losses of ascorbate from vegetables during blanching are of the order 5–40% (Favell, 1998; Bender, 1993). In general, it may be concluded that nutrient losses are minimised if the raw material is as little damaged as possible during handling and if processing conditions are chosen that keep the temperature, duration of heat exposure and product to water ratio as low as is consistent with denaturing the enzymes responsible for oxidative spoilage. 15.3.4 Frozen storage Bender (1993) has summarised the contradictory results of published studies designed to estimate the magnitude of vitamin loss during frozen storage of vegetables and fruits. Even for a particular vegetable, processed and stored under apparently similar conditions, the extent of ascorbate loss has been reported as negligible, or up to 40% after a year of frozen storage (Bender, 1993). As Bender comments, there are many possible sources of experimental variation that may lead to these different conclusions, most notably incomplete denaturation of oxidative enzymes during blanching. Since the review by Bender no large scale systematic study addressing this issue has been published. It may be concluded that if vegetables and fruits are adequately blanched and stored at conventional freezer temperatures without undue temperature fluctuations they will still possess valuable levels of potentially labile nutrients for a period of at least 12–18 months. 15.3.5 Cooking When comparing the nutritional value of different processing methods it is also necessary to consider the ways in which consumers handle these different products. Cooking methods may have important effects on the quantity of nutrients within a food. Because frozen vegetables have already been blanched, they require less cooking time than fresh vegetables to reach the same levels of palatability. This means that while frozen vegetables may have lost some nutrients during blanching they will probably suffer reduced losses during cooking. Freezing 335