338 The nutrition handbook for food processors 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 leach- ing and chemical destruction. Alternative heating systems have been develope such as those using steam and microwaves. However, as pointed out by Bender (1993), consistent evidence for nutritional benefits from these alternative blanch- ing procedures has not been observed. Part of the reason lies in the inherent vari- ability 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%0 15.5.2 Frozen storage in the glassy state As pointed out above, natural foods stored at -18C 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 pos- sibility of freezing vegetables and fruits without the need for blanching and suffering the associated nutrient losses The effects on ascorbate retention of stor ing unblanched peas at different temperatures compared with conventionally blanched and frozen peas are shown in Fig. 15.2. The temperature at which peas 100 -55°C 38°C -24°C 20°C 米-B| arched Time of storage(months) Fig. 15.2 Effe perature on ascorbate retention of peas: Ascorbate reten frozen at different temperatures compare with commercia tored at@.(from Sharp, unpublished15.5.1 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 leach￾ing 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 blanch￾ing procedures has not been observed. Part of the reason lies in the inherent vari￾ability 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 pos￾sibility of freezing vegetables and fruits without the need for blanching and suffering the associated nutrient losses. The effects on ascorbate retention of stor￾ing unblanched peas at different temperatures compared with conventionally blanched and frozen peas are shown in Fig. 15.2. The temperature at which peas 338 The nutrition handbook for food processors 50 60 70 80 90 100 110 0 3 6 9 12 Time of storage (months) Ascorbate (% retention) -55°C -38°C -24°C -20°C Blanched 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)