Blanching induces the expansion and removal of intercellular gases from within vegetable tissues. This reduces the potential for subsequent oxidative changes and permits the attainment of adequate headspace vacua in cans. Air removal from the surfaces of a vegetable may change the apparent color shade. When peas of various sizes were blanched between 66 and 95°C for a few seconds up to 10 min it was found that removal of internal gases had largely occurred within the first minute. Gas volume varied from 0.47 mL/100 g grade 2 peas to 1.21 mL/100 g grade 7 peas (66).
The gas and volume changes in sliced green peppers and beans were measured during blanching in steam at 1.5 atm and in water at 100,90, and 80°C. Initially the peppers contained 14% gas, which was reduced to 2.8-3.8% by steam or water at 100°C, but only to 3.2-5.8% at the lower temperatures. Beans initially contained 19.3%, which was reduced to 1.1-4.1% by steam or water at 100°C, and to 4.2-6.4% at the lower temperatures. Maximum gas reduction was achieved after 2 min with beans and after 1 min with peppers (67). Further data are available for red cabbage and carrots (68).
Several early blanching studies reported that steam blanching was slower than water blanching. Since the heat transfer coefficient of condensing steam is at least an order of magnitude greater than that of hot water to the same surface, it was postulated that the noncondensable gases mixed with steam accumulated at the surfaces being heated, thus interfering with the heat transfer from the steam. Blancher designs where the noncondensable gases are not swept away continually and removed from the system might be prone to a stagnant layer of gas accumulating at the surfaces, thus reducing heat transfer rate. Studies using pure steam revealed that the most important source of noncondensable gases was from the interior of the vegetables being blanched. Foaming was observed, demonstrating that heat transfer had to occur through an insulating layer of foam from the steam to the vegetable. A surface heat transfer coefficient for carrot in steam at atmospheric pressure was given as 1136 W/m2-K (69). The work showed that the rate of heat transfer was increased by vacuum pretreatment and decreased by pressure pre-treatment. Results indicated that some means of degassing vegetable particles prior to steam blanching would reduce the heating time much more effectively than increases in steam velocity.
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