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 reasoncanning 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 strength. 15.3.2 Storage after harvest 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 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 blanch￾ing 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 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 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 freez￾ing were carried out in a commercial factory. (from Favell, 1998)