Cellulose, a complex carbohydrate, is the chief constituent of the cell walls of vegetables (and fruits). The higher its proportion in the cells, the firmer the raw vegetable will likely be. To tenderize the cellulose, and, therefore the vegetable, the cook generally uses a combination of heat and moisture.
Some foods present a special problem. Consider broccoli and asparagus. The stems have a higher cellulose content than the tips and will therefore still be undercooked by the time the tops are tender unless remedial action is taken. Suitable solutions include cooking the stems and tips separately or cooking them together but giving the stems a head start. You can also shorten the cooking time required for the stems by paring them (a significant percentage of the cellulose resides on or near the surface) or by cutting the stem into smaller pieces.
The difference in cellulose content is the principal reason brown rice has to be cooked approximately twice as long as white rice to make it palatable and digestible. Unlike white rice, brown rice still retains the outer bran layer, which has a high cellulose content. (The endosperm - the white inside part of the cereal grain - has far less cellulose).
Apples like Red or Golden Delicious are unsuitable for cooking because, lacking sufficient cellulose, they become mushy and lose their shape. In addition, they do not contain enough acid to balance the sugar often added by the cook. York Imperial, Rome Beauty, and crap apples are among those that cook well.
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