The blanching operation promotes the thermal degradation of the blue-green chlorophyll pigments to the yellow-
green pheophytins (47,48). This has been demonstrated in peas, and it has been shown that the addition of sodium carbonate or calcium oxide or alkalinization with sodium hydroxide during a 3-min blanch at 90°C increased chlorophyll stability. Addition of calcium chloride had the opposite effect. Iron and tin ions promoted, and copper and tin inhibited, chlorophyll decomposition; chlorophyll b was more resistant to heat than chlorophyll a (49).
Steam blanching of ground red peppers gave a major improvement in color in the first 10 min. This effect was greatest in material that had not been stored, in which the ripening process was not terminated. Because of changes in the conjugated system of carotenes, the red proportion increased and the yellow decreased (50). The effect of different types of blanch on the color of sliced carrots using hot water, in-can steam, direct steam, and microwaves resulted in similar color changes. These arose from changes in ultrastructure, especially in chromoplasts, from which liberated carotenes dissolved into lipids, and some were lost into the water (51).
The use of sodium acid pyrophosphate has been reported in blanch solutions, especially for potato and cauliflower to prevent discoloration during storage of the processed products. A cause of the discoloration may be the reaction between oriAo-dihydroxyphenols with ferrous ions, forming a pigment in the ferric form on oxidation. Pyrophosphate is unique among condensed phosphates because of its strong ferric ion binding ability.
When peaches were microwaved a uniform browning of the skin resulted because the high levels of polyphenoloxidase in the outer fruit parts received the least heating effect. To prevent this a combination of microwave heating to blanch the inner parts and lye peeling to scald the outer parts was proposed (52).
Soluble or volatile flavor components may be lost during blanching. However, the resulting enzyme inactivation and oxygen removal may aid greater flavor retention during subsequent frozen storage. Unblanched carrot, cauliflower, and french bean developed off-odor after 9, 3, and 6 months, respectively, at — 30°C (53). Unblanched onion, leek, and swede did not develop detectable off-flavor or off-odor, and no changes in total lipid content were found. In onion, no lipoxygenase or peroxidase has been found.
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