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 cookingis 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 pri￾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￾ditions 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 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￾etables 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. 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 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￾ability. This means that while frozen vegetables may have lost some nutrients during blanching they will probably suffer reduced losses during cooking. Freezing 335