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Studies of ileal digestibility show that we use cooked starch very efficiently:
Home-cooked kidney beans: Noah et al. (1998); flaked barley: Livesey et al. (1995); cornflakes, white bread, oats: Englyst and Cummings (1985); bananas: Langkilde et al. (2002), Englyst and Cummings (1986), Muir et al. (1995); potatoes: Englyst and Cummings (1987); wheat: Muir et al. (1995). For review see Carmody and Wrangham (forthcoming).
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The principal way cooking achieves its increased digestibility is by gelatinization:
Eastwood (2003) and Gaman and Sherrington (1996) provide textbook accounts; Olkku and Rha (1978) give a detailed review; Svihus et al. (2005) and Tester et al. (2006) discuss frontline research. For an example of effects of baking on starch (i.e., heating in the absence of water), see Karlsson and Eliasson (2003). Lee et al. (2005) illustrate how increasing degrees of gelatinization lead to increased hydrolysis and increased glucose absorption in rats, an example of incomplete starch digestion in animals.
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The granules are . . . too small to be seen with the naked eye:
Despite the granules’ small size, people may be able to detect them in their food, because foods containing particles as small as two micrometers in diameter (two thousandths of a millimeter, or 0.08 thousandths of an inch) feel rougher when rubbed against the top of the mouth, or between tongue and lips, than foods without any particles. So people may be able to use “mouth-feel” to detect the presence of starch granules. Engelen et al. (2005a) tested human perception of particle size by adding silica dioxide and polystyrene spheres of known size to custard desserts. Until their research, it was thought that perceptions of food smoothness, slipperiness, and roughness were affected only by lubricative properties of the food, like oiliness. That people perceive food with particles the size of even very small starch granules as being rough suggests that we can detect (and avoid) raw starch through its effect on texture.
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the glucose chains are unprotected, and gelatinize:
The chains of glucose come in two types, or molecules. Amylopectin is the “good” one. Amylopectin is a huge molecule made up of as many as two million glucose units linked to each other in a rambling, open, branching framework. Following gelatinization, amylopectin offers easy access to digestive enzymes. So starches that are mostly made up of amylopectin satisfy quickly, providing a highly digestible food with a high glycemic index.
The difficult component of starch is amylose because this molecule is resistant to digestion even after gelatinization. On average, amyloses compose 20 percent to 30 percent of starch granules by weight, but their concentration can vary from zero to 70 percent. Amylose is a small molecule made up of only fifty to five hundred glucose units. The units line up in relatively short unbranched chains that can wrap about themselves, sometimes together with lipids, to form hydrophobic structures resistant to penetration and therefore readily protected from amylases and related enzymes. So amylose-rich starches are a good food for someone trying to lose weight or worried about diabetes. Especially at higher concentrations, their presence is a major reason for starch being resistant to digestion. Brown et al. (2003) showed that cooking makes amylose more digestible, though above 60 percent amylose, even cooking did not completely remove the resistance of starch.
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The effect of eating cornstarch:
Collings et al. (1981).
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Cooking consistently increases the glycemic index of starchy foods:
Brand-Miller (2006).
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Even the effects on proteins are a matter of debate:
Reviewed by Carmody and Wrangham (forthcoming).
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“An egg should never be cooked”:
Christian and Christian (1904), p. 159.
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This kind of argument:
Roach (2004) describes controversies among bodybuilders about the value of raw eggs.
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When aborigines on the beaches of Australia’s tropical north coast are thirsty:
Isaacs (1987), p. 166.
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hunter-gatherers prefer to cook them:
Emu eggs: Basedow (1925), p. 125. Yahgan: Gusinde (1937), p. 319.
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a Belgian team of gastroenterologists tested the effects of cooking:
Evenepoel et al. (1998, 1999).
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in the large intestine bacteria and protozoa digest the food proteins entirely for their own benefit:
Rutherfurd and Moughan (1998), p. 909: “Amino acids do not appear to be absorbed to any significant extent by the large intestinal mucosa of large mammals.”
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they were able to check their results with healthy subjects as well:
After feeding the labeled egg meals to the ileostomy patients, the researchers not only collected ileal effluent every thirty minutes but also took samples of their exhaled breath. They found that the course of digestion (monitored from ileal effluents) was closely correlated with the appearance of stable isotopes in the breath. This taught the researchers that breath tests alone would reveal how well the labeled protein was digested. Breath tests were accordingly used to study egg digestion in healthy volunteers.
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Cooking increased the protein value of eggs by around 40 percent:
The discovery that we do not digest raw egg protein nearly as well as cooked egg protein is the first to identify the effects of heat on the digestibility of protein in the human gut. But the evidence that raw eggs are a relatively poor food has been hinted at by other studies. For example, allergy researchers collected breast milk from women who had eaten either raw or cooked eggs for breakfast. They found that the concentration of ovalbumin rose in breast milk after eating eggs, and that the rise was about twice as fast when eggs were cooked as when they were raw. Again, the cooked eggs appeared more digestible. Allergy study: Palmer et al. (2005). The recent data about the effects of cooking on digestibility of eggs were anticipated by at least two groups. Hawk (1919) claimed his research team had evidence that raw egg white is used less completely than cooked egg white. Cohn (1936) showed that rats grew poorly on diets rich in raw egg white compared to those eating cooked egg white. She attributed this partly to the anti-trypsin factor and partly to raw egg proteins being passed more rapidly than cooked egg proteins from the stomach to the small intestine, an effect also found by Evenepoel et al. (1998). Cohn’s suggestion that a rapid gastric emptying rate might be responsible for the poor energy supply from raw eggs is not supported by modern data. First, in recent decades the idea that the stomach was responsible for a large proportion of digestion has given way to the orthodoxy that most digestion occurs in the small intestine. Second, Evenepoel et al. (1998) found no difference in transit time to the ileocecal junction (the half-time averaged 5.3 hours in both cases). This meant that raw eggs spent more time than cooked eggs in the small intestine, where digestive processes are most active, so they should have been better digested than the cooked eggs.
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Denaturation occurs when the internal bonds of a protein weaken:
McGee (2004), Wandsnider (1997).
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In 1987 researchers chose to study a beef protein:
Davies et al. (1987) studied the degradation of bovine serum albumin by trypsin with and without heating. Proteins were four times more easily digested in an experiment in which they were lightly heated. This suggests that in real life when they have been properly cooked they would be much easier to digest.
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Acid is vital in the ordinary process of digestion:
The pH of the empty stomach is usually less than 2. This intense acidity is not always regarded by digestive physiologists as important with respect to denaturation. Johnson (2001) and King (2000) both cite the function of gastric acid as bactericidal and converting pepsinogen to pepsin; neither mentions denaturation. By contrast, Sizer and Whitney (2006), p. 81, report that “Stomach acid works to uncoil protein strands and to activate the stomach’s protein-digesting enzyme. Then the enzyme breaks the protein strands into smaller fragments.”
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Marinades, pickles, and lemon juice . . . can contribute to the denaturing of proteins in meat, poultry, and fish:
Gaman and Sherrington (1996).
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Hunter-gatherers have likewise been reported mixing acidic fruits with stored meats:
Tlingit: Emmons (1991), pp. 140, 143; pemmican: Driver (1961), p. 71; Australians: Berndt and Berndt (1988), p. 99.
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Animal protein that has been salted and dried, such as fish, is likewise denatured:
Sannaveerappa et al. (2004) found that when Indian milkfish were salted for twenty-four hours, their large muscle proteins were substantially denatured. Sun drying exacerbated the effect.
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“A large portion of the side was blown off ”:
Beaumont (1996), p. ix.
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“At 12 o’clock, M., I introduced through the perforation, into the stomach”:
Beaumont (1996), p. 125.
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“The rugae gently close upon it”:
Beaumont (1996), p. 77.
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“gradual appearance of innumerable, very fine, lucid specks”:
Beaumont (1996), p. 104.
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“Vegetable, like animal substances, are more capable of digestion”:
Beaumont (1996), p. 47.
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“Fibrine and gelatine . . . are affected in the same way”:
Beaumont (1996), p. 35.
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“Pieces of raw potato”:
Beaumont (1996), p. 48.
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“Of all the attributes of eating quality”:
Lawrie (1991), p. 199.
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the cook’s main goal has always been to soften food:
“Softness” is an elusive quality. Hardness is the force needed to initiate a crack. Toughness measures the force needed to keep it going. Springiness tells how fast a deformed food returns to its original shape. Chewiness is the number of times it has to be chewed to make it fit to be swallowed. All those factors contribute to the ordinary perception of softness, or food that “melts” in the mouth. Others matter too, such as juiciness (the rate at which moisture is released) or greasiness (the difficulty of removing a fatty film coating the mouth). Different cuts of meat vary in each of these aspects, and cooking affects each kind of texture in different ways. Lucas (2004) discusses the physics of food. Ruiz de Huidobro et al. (2005) discuss meat textures.
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“The central theme is that cooks assist the bodily machine
”: Symons (1998), p. 94.
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“to render mastication easy”:
Beeton (1909), p. 108.
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“it is so tender that the sinews will fall apart”:
Tanaka (1980), pp. 38, 39.
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The island-living Yahgan:
Gusinde (1937), p. 325.
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a delicacy shared by the Tlingit:
Emmons (1991), p. 141.
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Game animals have a few soft parts:
Utes: Pettit (1990), p. 44. Australians: Dawson (1881), p. 17. Inuit (intestines): Jenness (1922), pp. 104, 106. Inuit (kidneys and liver): Jenness (1922), p. 100. Chimpanzees: personal observation. Philbrick (2000) reports the eating of raw liver by marine cannibals. Cannibalism normally involved cooking, however.
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fat-tailed sheep:
Fernandez-Armesto (2001), p. 88.
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While some foods are naturally tender
: Gaman and Sherrington (1996).
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the tensile strength of tendons can be half that of aluminum:
Lawrie (1991), chapter 3.
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Three left-handed helices of protein twirl:
Woodhead-Galloway (1980).
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good cooking tenderizes every kind of meat:
The most widely used index of meat toughness is the Warner-Bratzler shear force, measured by the effort needed to penetrate meat with a steel blade. Warner-Bratzler measurements tend to match consumers’ perception of “hardness,” but hardness is only one of several components of consumer preference. So taste panels of consumers who chew meat samples provide the best assessment of texture even though they are time-consuming, expensive, and somewhat variable in their results. For instance, consumer perceptions vary across countries. Warner-Bratzler shear force: Harris and Shorthose (1988), Tornberg (1996). Variation across countries: Lawrie (1991). Meat tenderized by cooking: shrimp: Rao and Lund (1986); octopus, Hurtado et al. (2001); rabbit, Combes et al. (2003); goat, Dzudie et al. (2000); beef, de Huidobro et al. (2005).
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Steak tartare requires:
Rombauer and Becker (1975), p. 86.
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Brillat-Savarin recorded an enthusiastic testimony:
Hunt (1961), p. 17, citing Brillat-Savarin’s
Gastronomy as a Fine Art
(1826).
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A team of Japanese scientists:
Oka et al. (2003). The average yield forces of hard and soft pellets were 85.5 newtons and 41.8 newtons, respectively.
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Secor and his team have shown repeatedly:
Pythons: Secor (2003); toads: Secor and Faulkner (2002). Overview of the costs of digestion: Secor (2009).