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Exposure to heat alters the structures of proteins, fats, and carbohydrates, so it changes food's texture (the way food particles are linked to make the food feel hard or soft). In other words, cooking can turn crisp carrots mushy and soft steak to shoe leather.

Protein

Proteins are made of very long molecules that sometimes fold over into accordion-like structures (see Chapter 6 for details about proteins). Although heating food doesn't lower its protein value, it does

1 Break protein molecules into smaller fragments

1 Cause protein molecules to unfold and form new bonds to other protein molecules

1 Make proteins clump together

Need an example? Consider the egg. When you cook one, the long protein molecules in the white unfold, form new connections to other protein molecules, and link up in a network that tightens to squeeze out moisture so that the egg white hardens and turns opaque. The same unfold-link-squeeze reaction turns translucent poultry firm and white and makes gelatin set. The longer you heat proteins, the stronger the network becomes, and the tougher, or more solid, the food will be.

To see this work, scramble two eggs — one beaten and cooked plain and one beaten with milk and then cooked. Adding liquid (milk) makes squeezing out all the moisture more difficult for the protein-network. So the egg with the added milk cooks up softer than the plain egg.

Heat melts fat, which can run off food, lowering the calorie count. In addition, cooking breaks down connective tissue — the supporting framework of the body, which includes some adipose (fatty) tissue — thus making the food softer and more pliable. You can see this most clearly when cooking fish. The fish flakes when it's done because its connective tissue has been destroyed.

When meat and poultry are stored after cooking, their fats continue to change, this time by picking up oxygen from the air. Oxidized fats have a slightly rancid taste more politely known as warmed-over flavor. You can slow — but not entirely prevent — this reaction by cooking and storing meat, fish, and poultry under a blanket of food rich in antioxidants, chemicals that prevent other chemicals from reacting with oxygen. Vitamin C is a natural antioxidant, so gravies and marinades made with tomatoes, citrus fruits, tart cherries, or blueberries slow the natural oxidation of fats in cooked or stored foods.

Grains: Split personality performers

In cooking, grains, such as corn, exhibit split personalities — part protein, part complex carbohydrates. When you boil an ear of corn, the protein molecules inside the kernels do the break-unfold-network dance (the molecules break their links, the protein unfolds, and the molecules form new links). At the same time, carbohydrate starch granules begin absorbing moisture and then soften.

The trick to boiling perfect corn is controlling this process, removing the corn from the water when starch granules have absorbed enough moisture to soften the kernels but before the protein network has tightened.

That's why cookbooks advise a short stay in the pot. But if you're a person who likes corn chewy, just let it boil away, 15 minutes, 30 minutes—you be the judge.

Carbohydrates

Cooking has different effects on simple carbohydrates and complex ones (if you're confused about carbohydrates, see Chapter 8). When heated

1 Simple sugars — such as sucrose or the sugars on the surface of meat and poultry — caramelize, or melt and turn brown. (Think of crème caramel.)

1 Starch, a complex carbohydrate, becomes more absorbent, which is why pasta expands and softens in boiling water.

1 Some dietary fibers (gums, pectins, hemicellulose) dissolve, so vegetables and fruits soften when cooked.

The last two reactions — absorption and dissolved cell walls — can improve the nutritional value of foods by making the nutrients inside previously fiber-stiffened cells more available to your body.

A less-beneficial effect of heat on carbs surfaced early in 2002 when Swedish researchers set off a nutritional hoo-ha with the announcement that exposing starchy carbohydrate foods — such as potatoes and bread — to the high heat of baking or frying produces acrylamides, a family of chemicals known to cause cancer in rats. Then things got worse when scientists at the City of Hope Cancer Research Center (Los Angeles) said that acrylamides could trigger cell changes leading to cancer in human beings. But a 2003 analysis of data from a study of 987 cancer patients and 538 healthy "controls" conducted by researchers at Harvard School of Public Health and the Departments of Oncology-Pathology and Medical Epidemiology at Karolinska Institutet in Stockholm shows no evidence of an increased risk of bowel, bladder, or kidney cancer among fans of fries and toast. And in June 2004, an expert panel at the National Toxicology Program of the National Institute of Environmental

Health Sciences said the level of acrylamides in a normal American diet — even one that includes baked and fried carbs — is too low to be carcinogenic. Fries on toast, anyone?

At a follow-up U.N.-sponsored meeting in Geneva, a group of really important international food scientists confirmed the Stockholm discovery of acrylamide in carbs but couldn't agree on what to do about it other than to call for another study, which showed up pretty quickly. By the end of the year, kitchen scientists agreed that moderate amounts of fries are safe. Nutritious, too. The most healthful potato, it's true, is one that hasn't been fried, but even when crisped to a fare-thee-well, one ounce of potato chips nevertheless may still deliver up to 12 percent of the RDA for vitamin C, up to 7 percent of the RDA for folate, up to 4 percent of the RDA for iron, and more than a gram of dietary fiber. In other words, as one part of a varied diet, the chip may still be okay to dip. The problem is: Are you sure you can have just one? Or two? Or . . .

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