Are you curious about the art and science of cooking? WHAT CHEMICAL CHANGES OCCUR IN AN EGG DURING COOKING? WHAT CHEMICAL REACTIONS OCCUR WHEN YOU MARINATE MEAT? DOES THE PH OF FOOD CHANGE WHEN YOU COOK IT? WHY DO COOKED VEGETABLES BURN WHEN DEEP FRIED AND RAW VEGETABLES FRY NICELY? WHAT IS THE DIFFERENCE BETWEEN BAKING POWDER AND SODA, AND WHAT ARE THEY USED FOR? Q:WHAT CHEMICAL CHANGES OCCUR IN AN EGG DURING COOKING? The best reference I have on the egg is "On food and Cooking" by Harold McGee. The cooking of an egg is all related to coagulation... "The behavior of eggs in the kitchen is mostly a matter of protein chemistry, and in particular the chemistry of coagulation. The albumen proteins are long chains of amino acids that are folded up onto themselves in a compact, roughly globular shape. Each protein molecule is held in this shape by various kinds of bonds between different parts of its chain. Different molecules are prevented from bonding to each other because of the native chemical environment of the albumen, each one accumulates a net negative charge: and particles with like charge repel one another. Both of these conditions-the bonding that shapes a molecule, and the mutual repulsion of the different molecules-are very easily disturbed by changes in acidity, salt content, temperature, or even air. Such changes can cause the protein molecules to bond together into a solid mass: that is to coagulate. Take as an example the changes that occur during cooking. When eggs are heated, the increased energy of all the molecules in the albumen breaks some of the shaping bonds in the proteins, and the individual protein molecules begin to unfold. This unfolding exposes more of each molecule's length to others, and so bonding between different molecules more likely. As the temperature of the egg rises, then, the proteins unfold, bump into parts of each other, and bond to each other. Eventually, the initially separate, globular molecules dispersed in water form a mass of partly extended, intricately interconnected proteins, with water molecules held in the interstices of the mass. The liquid has become a solid, and because the proteins have clustered together densely enough to deflect light rays, the clear matter has become opaque." Q: WHAT CHEMICAL REACTIONS OCCUR WHEN YOU MARINATE MEAT? A: This depends on the method. I can think of three chemical methods to tenderize meat by marinating it in acids, salts, or alcohol. All three will denature the proteins in the meat via a different chemical reaction. Acid hydrolysis of peptide bonds in the meat proteins can be accomplished by marinating meat in fresh citrus-citric acid, vinegar-acidic acid, or yogurt-lactic acid. This process results in denaturing of the proteins and a more tender meat product. You can also denature the proteins using a high salt solution ("brine") by interfering with the hydrophilic intra molecular interactions, which is used a lot for pork. Alcohol also denatures proteins by interfering with the intra molecular interactions, but the hydrophobic ones. Wine or hard liquor (vodka) can be used, but is concentration dependent (wine takes longer due to the lower concentration of alcohol). Q: DOES THE PH OF FOOD CHANGE WHEN YOU COOK IT? A: When making sauces, especially tomato or citrus (orange, lemon, lime) based sauces, pH is critical. As you concentrate the sauce, the pH drops. Care is needed if cream or other acid sensitive ingredients are added. When cooking with butter milk or sour cream (both contain lactic acid), care must be taken not to increase the acidity (lower the pH) too much or the milk solids will curdle and separate. Eggs tend to be very sensitive to pH. Whipped egg whites tend to build better with a lower pH (this is why cream of tarter or tartaric acid is added during the whipping of stiff egg whites). Egg yolks can be tricky, as with a classic bernaise sauce-addition of the lemon juice is critical for flavor, but can be disastrous if done wrong resulting in "breaking" or separating of the sauce. A: Whole milk is about 87 percent water, with the remainder being about 4.9 percent carbohydrate-lactose, 3.5 percent fat, and 3.5 percent protein. The fatty acids in milk fat are oleic, palmitic, and stearic acids; butyric and other fairly short-chain fatty acids contribute to the flavor of milk, as well as to its energy value. Q: WHY DO COOKED VEGETABLES BURN WHEN DEEP FRIED AND RAW VEGETABLES FRY NICELY? A: The cooked vegetables burn due to the decreased amount of water content. The water in the fresh vegetables has a high specific heat of activation that absorbs a lot of the energy from the oil. In addition, the cooked vegetables have already had a lot of the proteins and sugars broken down-so when they are put into 325-375 degree oil, they complete cooking VERY fast-and thus burn. Q: WHAT IS THE DIFFERENCE BETWEEN BAKING POWDER AND SODA, AND WHAT ARE THEY USED FOR? A: Baking powder and soda are two LEAVENING AGENTS. Leavening agents provide a source of gas (air, steam, or carbon dioxide) to make air pockets in baked goods.
Q: WHY DOES MY PEE (URINE) SMELL AFTER EATING ASPARAGUS? A: There are actually two reasons. First, you have the ability to metabolize some components (the exact source remains an open question) in asparagus to S-methyl thioacrylate (1) and S-methyl 3-(methylthio)thiopropionate (2). (1) CH2=CHC=OSCH3 and (2) CH3SCH2CH2C=OSCH3 (both the oxygens are off of the carbon) Second, you have the ability to smell the molecules, which is a specific smell hypersensitivity. Not all people produce the odorous substances, and not all people can smell them either. References: White, R. H.: Science, 1975, 189, p810. Lison, M. British Medical Journal, 1980, 281, p20. Back to the Kitchen Chemistry Home Page updated |