Toxic Constituents

In addition to the leaching of nutrients, toxic constituents naturally present in the vegetable may also be leached. The level of nitrates in foods has caused concern because of potential toxicity, and high levels of nitrites may cause methaemoglobinaemia in infants. Nitrate levels in spinach petioles have been shown to be as much as 5 to 10 times greater than in leaves, but water blanching subsequently reduced levels by more than 50% (32). Any nitrite formed by reduction was eliminated, and it was concluded that a combination of leaf selection and blanching could reduce nitrate levels to 25% of the original content. Nitrate levels in carrots were reported for raw carrot, 339 mg nitrate/kg; water washed at 17°C, 226 mg/kg; blanched at 65°C, 187 mg/kg; blanched at 80°C, 181 mg/kg; and blanched at 95°C, 165 mg/kg (33).

The calcium uptake from blanch water influences the ratio of soluble and insoluble oxalates (32). Although excess calcium chloride in the blanch water reduced soluble oxalate in spinach, it also adversely affected color. The influence of washing and blanching on the cadmium content of several vegetables has been studied. Contents initially were 0.121-0.379 mg % for spinach, 0.029 mg % for green beans, and 0.038 mg % for peas. Although 0.1 ppm cadmium was recommended as the maximum in the fresh weight, some spinach still contained higher levels after blanching (34).

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