Contaminants

Blanching can significantly reduce the level of contaminating microorganisms, which is especially important prior to freezing or dehydration. A 3-min blanch of soy sprouts in boiling water reduced the total bacteria to =sl4,000/g, coliforms to <10/g, and salmonella to zero (35). However, the numbers of thermoduric organisms may not be reduced, and Bacillus stearothermophilus was isolated from blanch water used at 90°C for 5-min treatments prior to canning peas in an investigation of the cause of spoiled packs (36). Because blanching may render the vegetable more easily infected by microorganisms, hygienic handling is important.

The content of pesticide residues in vegetables has been studied recently. Commercial blanching was shown to remove 50% of DDT (dichlorodiphenyltrichloroethane) and 68-73% of carbonyl residue from green beans (37). The fate of di-syston has been studied during the processing of potatoes (38), and a 15-min blanch at 100°C reduced the levels of Aldrin, heptachlor epoxide, and Endrin in Irish and sweet potatoes (39). Blanching for 3 min at 100°C reduced levels of DDT isomers in turnip greens (40). The effects of water cooking on the pesticide residues in spinach have been studied (41).

Laboratory canning operations were highly effective in reducing strontium and cesium concentrations in beans and kale. However, blanching of sweet potatoes appeared to result in a transfer of radioactivity from the peel to the core, suggesting that skins of contaminated potatoes should be removed prior to thermal treatment (42).

Enzymes

Peroxidase has often been used as a blanching efficiency indicator enzyme because it is the most heat resistant enzyme and is easy to measure (43). Recent work has shown that a significant but not well-defined proportion of active peroxidase can be left in many vegetables after blanching and a long storage life can still be achieved (44). Less than 5% residual peroxidase activity did not affect quality during storage of carrot, cauliflower, french bean, onion, leek, and swede stored at -20 or -30°C for 15 months (20). Good quality was retained in carrots after storing at — 20°C when palmitoyl-CoA hydrolase was inactivated by blanching, even though catalase and peroxidase activity was present. Although this hydrolase was a better indicator enzyme, there was no cheap, easy method of measurement. The complete inactivation of peroxidase does correlate well with the achievement of best quality in peas, but the best sensory quality of green beans was achieved prior to complete peroxidase inactivation.

A study on brussels sprouts found that the activities of polyphenoloxidase and peroxidase were related to the size of the sprouts, and increased from the center to the outer layers. Reducing activity to about 1% of the initial level required 7 min at 98°C for large sprouts, 5 min for medium, and 4 min for small. Residual activity increased toward the center of the sprout (45). The inactivation of lipid-degrading enzymes has also been shown to be very important to final sensory quality (46).

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