Nutritional implications of new developments in freezing

In considering the introduction of new developments in the freezing of foods and in associated technologies it is clear that they are unlikely to be driven solely by the motivation to improve nutritional value. If processed according to current good practice and consumed within their recommended storage lives, frozen foods already often have a nutritional value equivalent to foods available as fresh in the retail supply chain. Nevertheless, new developments designed to improve the organoleptic properties of frozen foods or to reduce the costs of production may have significance for nutrient retention.

15.5.1 Developments in blanching of vegetables and fruits

More rapid blanching of vegetables and fruits, and alternatives that do not use hot water immersion would be expected to preserve labile nutrients from leaching and chemical destruction. Alternative heating systems have been developed, such as those using steam and microwaves. However, as pointed out by Bender (1993), consistent evidence for nutritional benefits from these alternative blanching procedures has not been observed. Part of the reason lies in the inherent variability in plant raw materials. For example, ascorbate levels may differ by as much as two-fold in freshly harvested vegetables and the improved ascorbate retention to be achieved by alternative methods to conventional blanching may be only within the order of 5-10%.

15.5.2 Frozen storage in the glassy state

As pointed out above, natural foods stored at -18°C to -24°C contain significant amounts of liquid water in which reactions leading to quality and nutrient loss may occur. If the temperature of foods is further lowered, the remaining liquid eventually enters a so-called 'glassy state', i.e. a non-crystalline solid (for reviews, see Levine and Slade, 1989; Goff, 1997). In this state, rates of reaction, including enzyme mediated reactions become insignificant or greatly reduced. This gives rise to the possibility of storing frozen foods for longer periods than currently used without the risk of significant oxidation. There is also the possibility of freezing vegetables and fruits without the need for blanching and suffering the associated nutrient losses. The effects on ascorbate retention of storing unblanched peas at different temperatures compared with conventionally blanched and frozen peas are shown in Fig. 15.2. The temperature at which peas

110 100

Fig. 15.2 Effects of frozen storage temperature on ascorbate retention of peas: Ascorbate retention in unblanched peas stored frozen at different temperatures compared with commercially blanched and frozen peas stored at -24°C. (from Sharp, unpublished)

0 3 6 9 12 Time of storage (months)

Fig. 15.2 Effects of frozen storage temperature on ascorbate retention of peas: Ascorbate retention in unblanched peas stored frozen at different temperatures compared with commercially blanched and frozen peas stored at -24°C. (from Sharp, unpublished)

are estimated to be in the glassy state is approximately -30°C and below this temperature they do not lose significant amounts of ascorbate. The temperature at which foods enter the glassy state varies and depends on the type and concentration of molecules in solution. Generally, the glassy state transition temperatures for foods are well below those used in the commercial supply chain and the costs entailed in modification of freezer operation would delay widespread uptake of this procedure.

15.5.3 Use of anti-freeze peptides

Anti-freeze peptides (AFP) are a class of compound that both depress the freezing point of water and prevent ice crystal enlargement during frozen storage (Lillford and Holt, 1994; Griffith and Ewart, 1995). If incorporated into frozen foods they may potentially prevent the structural and mechanical damage caused by ice crystal enlargement, thereby improving the sensory properties of food and potentially reducing drip loss from frozen food when it is thawed. This is illustrated by the finding that fish naturally containing AFPs suffer a lower amount of drip loss on freezing and thawing than those without such peptides (Payne and Wilson, 1994). Widespread applications of AFPs in frozen foods are currently limited by their cost and the need to produce them on any commercially relevant scale by using biotechnology.

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