Read Online Catching Fire: How Cooking Made Us Human by Richard Wrangham - Free Book Online
Authors: Richard Wrangham
Tags: General, science, Social Science, History, Evolution, Cooking, Political Science, Anthropology, Cultural, Technology & Engineering, Life Sciences, Popular Culture, Public Policy, Agriculture & Food, Cultural Policy, Fire Science
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by gelatinization. Starch inside plant cells comes as dense little packages of stored glucose called granules. The granules are less than a tenth of a millimeter (four-thousandths of an inch) long, too small to be seen with the naked eye or to be damaged by the milling of flour, and they are so stable that in a dry environment they can persist for tens of thousands of years. However, as starch granules are warmed up in the presence of water they start to swell—at around 58 o C (136 o F) in the case of wheat starch, a well-studied and representative example. The granules swell because hydrogen bonds in the glucose polymers weaken when they are exposed to heat, and this causes the tight crystalline structure to loosen. By 90 o C (194 o F), still below boiling, the granules are disrupted into fragments. At this point the glucose chains are unprotected, and gelatinize. Starch does not necessarily stay gelatinized after being cooked. In day-old bread the starch reverts and becomes resistant. This might help explain why we like to toast bread after it has lost its initial freshness.
Gelatinization happens whenever starch is cooked, whether in the baking of bread, the gelling of pie fillings, the production of pasta, the fabrication of starch-based snack foods, the thickening of sauces, or, we can surmise, the tossing of a wild root onto a fire. As long as water is present, even from the dampness of a fresh plant, the more that starch is cooked, the more it is gelatinized. The more starch is gelatinized, the more easily enzymes can reach it, and therefore the more completely it is digested. Thus cooked starch yields more energy than raw.
This effect is detected easily in blood measurements. Within thirty minutes of a person eating a test meal of pure glucose, the concentration of glucose in his or her blood rises dramatically, before returning to base levels in just over an hour. The effect of eating cornstarch is almost identical as long as it is cooked. But following a meal of raw cornstarch, the value of blood glucose remains persistently low, peaking at less than a third of the value for cooked cornstarch.
The effects of cooking are captured by comparing the glycemic index of cooked and raw foods. Glycemic index (GI) is a widely used nutritional measure of a food’s effect on blood sugar levels. High-GI foods, such as pure sugar, white bread, and potatoes, are good sources of energy after exercise, but for most people they are poor foods because they easily lead to excessive weight gain. In addition, the calories they offer tend to be “empty,” being low in protein, essential fatty acids, vitamins, and minerals. Low-GI foods, such as whole-grain bread, high-fiber cereals, and vegetables, reduce weight gain, improve diabetes control, and lower cholesterol. Cooking consistently increases the glycemic index of starchy foods.
Animal protein has been almost as important as starch in diets throughout our evolution, and it remains a strongly preferred food today. Yet the effects of cooking on the energy derived from eating meat have never been formally investigated, particularly the effects due to meat’s complex structure. Even the effects on proteins are a matter of debate. Until recently some scientists, such as David Jenkins, saw cooking as reducing protein digestibility. Others claim cooking protein is beneficial or has no effect. Recent studies of the digestion of eggs are starting to resolve the argument, showing for the first time that cooked protein is digested much more completely than raw protein.
In contrast to the new finding, in the past raw eggs have often been claimed to be an ideal source of calories, for reasons that sound logical. “An egg should never be cooked,” wrote raw-foodists Molly and Eugene Christian in 1904. “In its natural state it is easily dissolved and readily taken up by all the organs of digestion, but the cooked egg must be brought back to liquid form before it can be
Gelatinization happens whenever starch is cooked, whether in the baking of bread, the gelling of pie fillings, the production of pasta, the fabrication of starch-based snack foods, the thickening of sauces, or, we can surmise, the tossing of a wild root onto a fire. As long as water is present, even from the dampness of a fresh plant, the more that starch is cooked, the more it is gelatinized. The more starch is gelatinized, the more easily enzymes can reach it, and therefore the more completely it is digested. Thus cooked starch yields more energy than raw.
This effect is detected easily in blood measurements. Within thirty minutes of a person eating a test meal of pure glucose, the concentration of glucose in his or her blood rises dramatically, before returning to base levels in just over an hour. The effect of eating cornstarch is almost identical as long as it is cooked. But following a meal of raw cornstarch, the value of blood glucose remains persistently low, peaking at less than a third of the value for cooked cornstarch.
The effects of cooking are captured by comparing the glycemic index of cooked and raw foods. Glycemic index (GI) is a widely used nutritional measure of a food’s effect on blood sugar levels. High-GI foods, such as pure sugar, white bread, and potatoes, are good sources of energy after exercise, but for most people they are poor foods because they easily lead to excessive weight gain. In addition, the calories they offer tend to be “empty,” being low in protein, essential fatty acids, vitamins, and minerals. Low-GI foods, such as whole-grain bread, high-fiber cereals, and vegetables, reduce weight gain, improve diabetes control, and lower cholesterol. Cooking consistently increases the glycemic index of starchy foods.
Animal protein has been almost as important as starch in diets throughout our evolution, and it remains a strongly preferred food today. Yet the effects of cooking on the energy derived from eating meat have never been formally investigated, particularly the effects due to meat’s complex structure. Even the effects on proteins are a matter of debate. Until recently some scientists, such as David Jenkins, saw cooking as reducing protein digestibility. Others claim cooking protein is beneficial or has no effect. Recent studies of the digestion of eggs are starting to resolve the argument, showing for the first time that cooked protein is digested much more completely than raw protein.
In contrast to the new finding, in the past raw eggs have often been claimed to be an ideal source of calories, for reasons that sound logical. “An egg should never be cooked,” wrote raw-foodists Molly and Eugene Christian in 1904. “In its natural state it is easily dissolved and readily taken up by all the organs of digestion, but the cooked egg must be brought back to liquid form before it can be
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