Question of the Week

How does food change when we cook it?

Fri, 20th May 2016

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Question

Sol asked:

What role does cooking play in digestion?

Answer

We put this question to geneticist Giles Yeo.cooking

Giles - Cooking had a very important role in the evolution of humankind because what it did was increase the availability of calories. Now what do I mean by that?

If you assume 100 calories of sugar - thatís 100 calories, okay, because there's no processing required. If you assume 100 calories of celery or 100 calories of sweet corn, then I think you can tell in the loo the next morning after youíve had the sweetcorn, not all the corn is going to get absorbed into you.  What cooking does is to actually increase this availability. So, if you take the corn and put it in a stew, you end up for any given mass, any given amount of food is get more calories from it. And this clearly then played a huge role because you put in the effort to pick food, gather food, hunt food and clearly the more you get from that effort, the more likely you are to survive. So, what cooking does is take the same amount of food and allow you to get more calories out of it.

Emma - So does cooking actually make food more digestible?

Giles - Cooking makes certain types of food more digestible by beginning the breaking-down process. Some foods will never be digestible by human beings. Grass is probably a good example. You need a rumen for that, you need specific type of bugs. But yes, cooking does make certain types of food more digestible.

Emma - Which foods would we be unable to digest if we didnít cook them?

Giles - Thatís an interesting question. I think an interesting thing to that is, actually, sweet corn is a very good example. Where, in the kernel form, most of it doesnít get digested. The interesting thing about that is that if you then actually break it down into flour and bake and eat it you can actually digest a lot more of it. So thatís a perfect example where you make cornbread and clearly you donít poop out cornbread the other side. Corn is a very good example where cooking, processing, turning it into cornbread makes it digestible whereas the other is not.

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I don't know too much about cooking chemistry, but I'll share some of what I do know, regarding digestibility...

Proteins have very specific ranges of shapes that they adopt in living organisms, and probably aren't significantly different in fresh, unspoiled, uncooked food. At elevated temperatures (say 50 įC and up, being especially significant above 100 įC) proteins begin to "denature." This doesn't necessarily break any covalent bonds within the protein, but does change how the protein is folded. This can make particularly robust proteins somewhat easier to digest, and it can also decrease toxicity.

For instance many funguses contain a wide variety of enzymes that help them digest their food sources, be it cellulose, lignin, humins (not humans) or animal tissues... Eating raw funguses could lead to a scenario in which they are digesting you while you are digesting them. Once cooked, however, the enzymes completely loose their activity, and are just as harmless and nutritious as eating cooked egg whites. The same goes for polypeptidic toxins contained in animals (various venoms), plants (like ricin in castor) and in funguses (some particularly nasty toxins in amanita funguses are harmless once denatured)

Another class of toxins, cyanohydrins also known as alpha-hydroxy nitriles, release cyanide when digested or when being cooked ("cyanogenic"). These compounds are responsible for a large number of plant-derived foods which are toxic when raw, but harmless when cooked (cassava/yucca/manioc/tapioca, apricot seeds, bamboo shoots; see: http://www.foodsafety.govt.nz/elibrary/industry/Cyanogenic_Glycosides-Toxin_Which.pdf)

Glycosides (some of which are cyanogenic) are also a large class of toxins, of which most are decomposed during the cooking process. chiralSPO, Fri, 1st Apr 2016

A recent paper suggests that cooking greatly reduces the mechanical strength and elasticity of food, allowing extraction of far more calories.
The authors of the paper ran experiments which included chewing raw goat meat. Human teeth make almost no impression on it, and you can extract almost no calories from it (but you can get some nutrition by slicing it into small pieces with stone tools).

I assume that cooking breaks open cell walls, and denatures the connective tissues, making it much easier to bite off pieces, chew and swallow. Digestive enzymes would then have easier access.
See: http://www.nature.com/news/food-processing-1.19513 evan_au, Fri, 1st Apr 2016

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