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How Does the Chemistry of Cooking Work?

 
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Mr.Endo



Joined: 20 Jan 2009
Posts: 2
Location: Central New York State, US

PostPosted: Tue Jan 20, 2009 1:59 am    Post subject: How Does the Chemistry of Cooking Work? Reply with quote

My wife asked me why chocolate changes texture after you bake chocolate chip cookies, and why baking soda makes bread rise. I answered, "It's science of course!" I figured that a website as awesome as Cooking For Engineers would tell me how cooking actually works. All I know about cooking is from 7th grade Earth Science: cooking is a chemical change.

We have an article on Cooking for Engineers on heat transfer that claims it's not going to answer the "chemistry" questions of cooking. There is no article that does claim to answer those questions though. Maybe it's too big a question for something in the "Kitchen Notes," but it's definitely something that belongs on the website.

Until we have an article, can anyone answer the question?

Why can you melt chocolate, and it will return to its original hard state, but if you cook it, it remains soft?
What makes bread rise?
Why does the texture of meat change when you cook it?
How does browning veggies really work?
There are plenty of questions, but maybe someone can help me answer the first two specifically.

Nathan
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Dilbert



Joined: 19 Oct 2007
Posts: 971
Location: central PA

PostPosted: Tue Jan 20, 2009 3:25 pm    Post subject: Reply with quote

oh wow, that would be one gigantic 'article' - very large amount of territory to cover in those questions.

making bread rise: more generally, leavening. there's chemical, biological and 'other'

on the biological side - yeast. it is actually a living thing - technically a fungus. the stuff you buy in little packets is in 'suspended' state so to speak. when it is moistened and has food (simple sugars) the yeast consumes the sugar and emits carbon dioxide gas as part of that digestive process. the carbon dioxide gas gets trapped in the physical structure of the bread, making 'holes' - big or small - depends on technique and recipe. the bubbles cause the bread to rise - or increase in volume. when the bread is baked the heat causes the dough to get stiff - and the bread does not 'collapse'

on the chemical side, baking powder and baking soda. actually, baking powder is baking soda with an additional dry acid ingredient.

if you put a put a spoon of baking soda in a bowl and add some vinegar, you'll see the 'leavening' action very clearly. the acid in the vinegar reacts with the baking powder producing carbon dioxide gas.

in a similar experiment, mix the dry yeast into a quarter cup of warm (body temp) water and after a few minutes you'll see it making a 'foam' - those are the carbon dioxide bubbles produces as the yeast 'comes back to life.' often call "blooming the yeast" - historically a 'required' step, now largely eliminated by the advent of the "instant yeast" products.

baking powder baking soda have the same effect as yeast - the bubbles get trapped in the dough and cause it to increase in volume.
most baking powders sold today are 'double acting' - act one is the baking soda+vinegar reaction, act two is heat from baking causing another chemical reaction which releases additional carbon dioxide.

the 'other': used less for breads, more often for cake type goodies, a strictly 'mechanical' approach. example: beaten egg whites - makes for a big pile of little tiny bubbles inside "egg white balloons" - mix the beaten egg whites into the batter, heat. heat expands the air in the balloons, batter rises.

the behavior of sugar:
it's the sugar behind the hard/soft/sticky chocolate issue.
at normal temps, sugar is a crystalline structure, additionally it is fond of water (hygroscopic).
when heated, sugar melts - this 'destroys' the crystalline structure and depending on how high a temperature is reached, and how quickly it cools, or cools and is held at a slightly lower temp (tempering) - produces 'hard'/brittle to soft results. additives like cocoa butter and cocoa powder (=chocolate) affect that process, as is the case in chocolate candy bars - but even 'pure' sugar ranges from rock candy to a caramel confection depending on the heating and cooling - which are the result of how the sugar re-crystallizes and the amount of moisture it contains. heating sugar to a high temperature, driving out all the water, gets you a hard candy, for example.
working with sugar can be a demanding task - temperature control is critical and environmental factors play a huge role. using exactly the same methods on a low humidity winter day, you may get entirely different results than a warm high humidity summer day.
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Michael Chu



Joined: 10 May 2005
Posts: 1606
Location: Austin, TX (USA)

PostPosted: Tue Jan 20, 2009 11:03 pm    Post subject: Reply with quote

Most of the textural changes that occur to meats when cooking are the tightening of proteins. Muscle fibers contract (which eventually forces out juices when cooked to well-done levels). Initially this is a good thing since raw meat can be a bit squishy and not firm enough to bite through easily (depending on cut). As the meat cooks you can often see the individual muscle fibers become visible as they tighten then shrink. I believe this is caused due to the unraveling of proteins caused by the introduction of energy in the form of heat. The protein chains then rebond with neighboring proteins resulting in a shrinking/tightening on the macro level.
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Mr.Endo



Joined: 20 Jan 2009
Posts: 2
Location: Central New York State, US

PostPosted: Thu Jan 22, 2009 12:18 am    Post subject: Reply with quote

I knew this was the right place to come for these kind of answers! Thanks!
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epicuriosityfilledthekat



Joined: 25 Jun 2009
Posts: 3
Location: NY

PostPosted: Thu Jun 25, 2009 6:33 pm    Post subject: Browning Reply with quote

Browning is often a result of oxidation in vegetables... basically, enzymes (polyphenol oxidase) spark the formation of quinones (oxidizers/electron receptors for photosynthesis and aerobic respiration), which then polymerize and make melanins (the same pigments in our skin/hair/eyes), thereby browning the veggies in question.

This isn't always a bad thing: a plant is most at risk to insect infestation/disease when ripe and, by browning, it helps protect the fruit from being prematurely destroyed. The same thing happens in response to bruising to stave off further damage. On a more appetizing note, oxidation also yields the delicious, distinct flavors found in coffee, tea, chocolate, and some wines.
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julythermostats



Joined: 18 Jul 2009
Posts: 1

PostPosted: Sat Jul 18, 2009 3:51 pm    Post subject: Reply with quote

Temperature control is critical and environmental factors play a huge role. using exactly the same methods on a low humidity winter day, you may get entirely different results than a warm high humidity summer day.


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