• What is the effect of cooking on digestibility of various foods? Clearly, a food can have a nutritive value only if it is digested. Hence, it is necessary to investigate the effects of cooking on digestibility. We'll see that there is no general rule, and that there are many examples of foods for which cooking improves digestibility, and many examples for which it does not.

• Postulated effects of dietary "food enzymes." One of the main arguments of raw versus low-temperature cooking is that heating above 104°F (40°C) results in progressive enzyme damage; hence, even if you steam your vegetables, your food is "dead." However, we will contend that such arguments are invalid, and that dietary enzymes have a very minor role in our health, if any.

• Degree of vitamin and mineral losses. It is well-known that cooking may result in some vitamin and mineral losses. We try below to give an estimate of these losses. Not all vitamins are lost by heating; other factors (such as exposure to air or light) may result in vitamin loss; and heating by itself doesn't destroy minerals (however, some minerals may be leached in water). In short, we try to determine how much vitamins and minerals can be lost by heating a food.

• Evaluating the pros and cons. As with the question of toxicity that was assessed in Part 1, the consideration here cannot be reduced to black and white. Instead, the various factors must be weighed against each other. We conclude that cooking does not represent huge nutritional losses, but of course, when the choice is given, it is best to eat raw whatever is palatable in that form, and to cook as little as possible when heating is necessary to improve digestibility or to improve taste.

On the other hand, some resistant (indigestible) starch is formed by cooking. Resistant starch is present [Englyst 1985] in smaller amounts in (dehulled, rolled, steamed) oats than in cornflakes or white bread. However, about 94% of the carbohydrates are digested. Uncooked oats don't contain resistant starch, so their starch is totally digestible if left long enough in test tubes, but in practical terms it is less digestible than in cooked form. (Any reader not convinced of that should try two comparison meals for themselves, both consisting of 200 grams of oats, one raw and the other cooked.)

Experimental results show cooked starch to be 2 to 12 times more digestible than raw starch. Kataria and Chauhan [1988] provide a direct comparison of starch digestibility in raw vs. cooked mung beans. Here, starch digestibility was measured in milligrams (mg) of maltose released per gram of food. The data from this study indicate that digestion of mung beans soaked 12 hours yields 25.3 mg maltose/gm, mung beans sprouted 24 hours yield 75.0, mung beans soaked and subjected to ordinary cooking yield 138, while mung beans soaked and pressure-cooked (for 5 minutes) yield 305 [Kataria and Chauhan 1988, Tables 1-3, pp. 54-56]. This is solid evidence that the starch in cooked mung beans is much more readily digested than the starch in soaked or sprouted beans, by a factor ranging from about 1.8 (i.e., 138 ÷ 75.0) to 12 times more efficient (305 ÷ 25.3), depending on differences in methods of preparing raw vs. cooked starch. We thus conclude here that overall, certainly at least in the case of mung beans, cooking greatly improves starch digestibility.

One final point regarding starch digestion needs discussion. Some raw advocates claim that their ability to digest raw starch increases when raw starches are a regular part of the diet. This assertion is based on close examination by the raw vegan of his/her stools (fecal matter; a topic of intense interest for some rawists). While there are reasons to believe that the human digestive system adapts, to a certain degree, to the diet eaten, it appears that there are no studies or experiments to quantify the degree of adaptation in comparisons of raw vs. cooked starch digestion. Furthermore, the limited qualitative anecdotal evidence available does not support a claim that the human digestive system can become 1.8 to as much as 12 times more effective in digesting raw starch--i.e., the magnitude of the effect seen from cooking of mung beans, as seen in the above-cited research.

Protein

We won't try to be exhaustive here, but our intention is to show by means of examples that, depending on the food and on the temperature and duration of cooking, a food can become more, or less, digestible, so that no general rule can be inferred.

• Legumes: African yam beans (soaking + cooking/autoclaving), moth beans, mucuna seeds, mung beans, soybeans.
  
• Cereals: wheat.
  
• Seeds: sunflower seed.

Processing conditions may also alter the food structure on a cellular level, enabling digestive-tract enzymes to gain physical access to the protein.

Some native legume proteins resist proteolysis (breakdown of proteins into smaller components), but are degraded after heating.

Destruction of antinutrients of primary importance. But perhaps the major reason for the improved digestibility that can result from cooking is the destruction of antinutrients, such as protease inhibitors, polyphenols (including tannins and saponins), hemagglutinins, phytates, and dietary fiber. Polyphenols and phytates are, however, not reduced by heat alone. (For instance, soaking, sprouting, and fermentation also reduce phytates.)

From Bradbury [1984], the protein digestibility of the aleurone layer and grain coat from raw rice was only 25%, but increased to 65% from cooked rice, due to the disruption of the cellulose cell walls at 100°C (212°F), which was shown by electron microscopy.

From Oste [1991], heating (above 100°C, or 212°F) decreases meat protein digestibility. Frying chickpeas, oven-heating winged beans, or roasting cereals at 200-280°C (392-536°F) reduces protein digestibility.

Seidler [1987] studied the effects of heating on the digestibility of the protein in hake, a type of fish. Fish meat heated for 10 minutes at 130°C (266°F), showed a 1.5% decrease in protein digestibility. Similar heating of hake meat in the presence of potato starch, soy oil, and salt caused a 6% decrease in amino acid content.

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GO TO PART 1 - Is Cooked Food "Toxic"?

GO TO PART 2 - Does Cooked Food Contain Less Nutrition?

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