Blanching may induce both physical and biochemical changes in the structure and textural properties, depending on blanch time and temperature, and type and state of the vegetable. Blanching green beans at 90°C for 30 to 240 s did not seem to influence the degree of cell damage (54), and no tissue disruption in carrots was observed after blanching for 3 min at 100°C or cooking for 10 min at 100°C. This suggested that physiological and chemical rather than physical changes may have occurred to cause softness of the tissues. Subsequent freezing did cause cell disruption (55).
It has been observed that the effects of moisture and heating during blanching lead to a swelling of the cell walls, for example, of green bean pods (150-200%), and to a beginning of separation of the individual cells. This is attributed to the extraction of protopectin from the middle lamella. It was found that not only short blanching but also boiling to doneness preserved the histological structure well. No evidence of any cell wall rupture was observed during cooking, and it was concluded that earlier suggestions that cell wall rupture did occur probably arose through poor methodology in preparing sections (56).
The effects of boiling, steaming, and pressure cooking on the loss of pectic substances, and also the effect of water hardness on potato structure, have been reported (57). Steaming and pressure cooking gave similar results, but boiling resulted in significantly greater losses followed by a characteristic leveling off with time. Steaming probably caused less cell wall damage than boiling because of the lack of water to wash away degraded pectic substances. Boiled potatoes retained significantly more pectic substances when cooked with hard water. Surface to volume ratio was increased by slicing, thus increasing the rate of calcium diffusion into the tissue and of degraded pectic substance out of the tissue.
French fries have been preblanched in disodium dihy-drogen pyrophosphate to sequester ions, such as calcium, and prevent discoloration, and this treatment did not reduce protopectin stability or cause deterioration of French fry texture. Low-specific-gravity potatoes were also studied because these gave a soft texture after frying. After 15 min at 70°C in 0.5% calcium chloride the best firming results were obtained, although still not equal to the firmness of high-specific-gravity potatoes (58). It was shown that blanching increased shear value and springiness but decreased hardness of asparagus. The optimum blanch times at 100°C were 135 s for stalk diameters of 16-25 mm, 120 s for 12-15 mm, 105 s for 9-11 mm, and 90 s for 6-8 mm (59).
The texture and color of beans were improved by a stepwise blanch treatment involving high and low temperatures (60). Others found that low-temperature blanching at 74°C for 20-30 min resulted in firmer carrots. Their studies confirmed the conclusion that the increase in firmness was caused by the effects of pectin methyl esterase, which was activated at low temperature and inactivated at a higher temperature (61). The rate of thermal softening of these vegetables follows first-order kinetics (62). Mung bean shoota were blanched at 75°C for 30 s to activate pec-tinesterase and then held at 55°C for 30 min to maximize deesterification. Such firming has been applied to tomato, potato, and cauliflower. Subsequent addition of calcium salts to the canning liquor caused insoluble calcium pec-tate gels in the cell walls, thus firming the cell walls (63). The noncooking effects of salts added after cooking on the texture of canned snap beans has been studied (64). Blanching may also affect the dietary fiber content of vegetables (65).
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