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  • Where does bread get its fluffiness?

  • Swiss cheese its holes?

  • And what makes vinegar so sour?

  • These foods may taste completely different,

  • but all of these phenomena come from tiny organisms chowing down on sugar

  • and belching up some culinary byproducts.

  • Let's start with yeast.

  • Yeast are single-celled fungi used to make bread, beer, and wine,

  • among other products.

  • Yeast break down carbohydrates, like sugar,

  • to get energy and the molecules they need to function.

  • They have two different ways to do this:

  • the oxygen-dependent, or aerobic, pathway,

  • and the oxygen-independent, anaerobic pathway,

  • which is also called fermentation.

  • When you bake bread, yeast can use both pathways,

  • but they normally prefer to start with the anaerobic process of fermentation.

  • In this process, ethanol is produced in addition to CO2.

  • No, bread isn't alcoholic.

  • Small amounts of alcohol that are secreted evaporate during baking.

  • In the aerobic, or oxygen-dependent pathway,

  • the yeast consume some of the sugar

  • and produce carbon dioxide gas, or CO2, and water.

  • In both processes, the CO2 accumulates and creates tiny bubbles.

  • These bubbles get trapped by gluten and create a sponge-like structure

  • that gives the bread its soft texture.

  • Wine also relies on yeast.

  • But a wine-making set-up keeps the oxygen levels low

  • so that yeast consume sugar using fermentation,

  • the anaerobic pathway.

  • The process often starts with wild yeasts already hanging out on the grapes.

  • But to get consistent results,

  • most winemakers also add carefully selected strains of yeast

  • that can tolerate high levels of alcohol.

  • The yeast consume the sugar in the grape juice,

  • and as the sugar level drops,

  • the alcohol level rises.

  • This doesn't necessarily mean that sweeter wines have less alcohol.

  • Different types of grapes start with different amounts of sugar,

  • and sugar can also be added.

  • What happens to the carbon dioxide?

  • It just bubbles away through a vent.

  • In carbonated alcoholic beverages, like champagne and beer,

  • sealed containers are used in primary or secondary fermentation

  • to keep the carbon dioxide in the bottle.

  • Wine also introduces us to our second type of food-producing microorganism:

  • bacteria.

  • A special strain of bacteria

  • turns a tart compound in grape juice into softer tasting ones

  • that are responsible for some of the flavors in red wines and chardonnays.

  • Another type of bacteria, called acetic acid bacteria,

  • isn't so desirable in wine,

  • but they have their function, too.

  • If there's oxygen around,

  • these bacteria convert the ethanol in wine into, well, acetic acid.

  • Let this process continue and you'll eventually get vinegar.

  • Bacteria are the key for cheese, too.

  • To make cheese, milk is inoculated with bacteria.

  • The bacteria gobble up the lactose, a kind of sugar,

  • and produce lactic acid, along with many other chemicals.

  • As the milk gets more and more acidic,

  • its proteins start to aggregate and curdle.

  • That's why spoiled milk is clumpy.

  • Cheesemakers usually add an enzyme called rennet,

  • naturally found inside of cows, goats, and some other mammals

  • to help this process along.

  • Eventually, those little curdles turn into bigger curds,

  • which are pressed to squeeze out the water,

  • and create a firm cheese.

  • Different strains of bacteria make different kinds of cheese.

  • For example, a species of bacteria that emits carbon dioxide

  • is what gives swiss cheese its characteristic holes.

  • Some cheeses, brie and camembert, use another kind of microorganism, too:

  • mold.

  • So your kitchen functions as a sort of biotechnology lab

  • manned by microorganisms that culture your cuisine.

  • Yogurt, soy sauce, sour cream, sauerkraut,

  • kefir, kimchi, kombucha, cheddar, challah, pita, and naan.

  • But maybe not all at the same dinner.

Where does bread get its fluffiness?

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B2 US TED-Ed yeast bacteria fermentation wine cheese

【TED-Ed】The beneficial bacteria that make delicious food - Erez Garty

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    黃于珍 posted on 2016/04/14
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