Ch.15: The Four Basic Food Molecules
Chapter 15 of the book "On Food and Cooking: The Science and Lore of the Kitchen" by Harold McGee (1984) Water Fats, Oils, and Relatives: Lipids Carbohydrates Proteins
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[The Four Basic Food Molecules] What are the four basic food molecules?
Water The main component of nearly all foods, and a cooking medium. Its acidity or alkalinity affects flavor and the behavior of the other three. Lipids (fats and oils) Do not mix with water. A cooking medium for frying (crisping, browning); fat droplets thicken sauces; fats carry and produce aromas. Carbohydrates Mostly from plants: sugars give sweetness; starch and cell-wall carbohydrates give bulk and texture. Example: sugar, starch, cellulose, pectin. Proteins Mostly from animal foods; thei…
[Water] Roughly what percentage of each is water: the human body, raw meat, fruits and vegetables?
Body 60%, raw meat about 75%, fruits and vegetables up to 95% (by weight).
[Water] Why is a water molecule ( H2O\mathrm{H_2O}H2O ) polar?
Oxygen pulls the shared electrons harder than hydrogen, and the molecule is bent. So the oxygen end is slightly negative and the two hydrogens, on one side, slightly positive. Left to right: the formula, the molecule with its charges, and water molecules joined by hydrogen bonds (dashed lines).
[Water] What is a hydrogen bond between water molecules?
The attraction between the oxygen of one water molecule and a hydrogen of another. The oxygen end is slightly negative, the hydrogen slightly positive; the attraction holds the two molecules close together.In liquid water each molecule takes part in 1 to 4 hydrogen bonds at a time, and these bonds are constantly broken by molecular motion and formed again.
[Water] What does it mean, at the molecular level, that a substance has dissolved in water?
Each of its molecules is surrounded by water molecules, which keep it apart from the others. The water molecules are hydrogen-bonded to the dissolved molecule's polar regions.Only substances with some electrical polarity dissolve. Of the other food molecules, carbohydrates and proteins have polar regions; fats do not.
[Water] Why is ice less dense than liquid water, unlike most solids?
In ice, hydrogen bonds hold the molecules in an open lattice with more space than in liquid. The crystal arranges the molecules so that the hydrogen bonds are spread evenly, which spaces them out.Water expands by about one-eleventh of its volume when it freezes.Example: a soup container filled to the brim can crack in the freezer; a forgotten beer bottle in the freezer can burst.
[Water] Why do raw meat, fish, fruits and vegetables leak liquid after being frozen and thawed?
Growing ice crystals tear the cells open, so their fluid runs out on thawing. Water expands as it freezes and ruptures cell membranes and cell walls.
[Water] What makes tap water hard?
Dissolved calcium and magnesium, mostly as carbonate and sulfate salts.
[Water] What problems does hard water cause in the kitchen?
Scum with soap, crusty deposits in kettles, and changes in vegetables and bread dough. The calcium and magnesium cause all of these; in vegetables they affect color and texture, in bread the consistency of the dough.
[Water] What are the two usual ways to soften hard water?
Adding lime Precipitates the calcium and magnesium out of the water. Ion exchange Replaces the calcium and magnesium ions with sodium ions.
[Water] What is the specific heat of a substance?
The amount of energy it takes to raise the substance's temperature by a given amount. Example: heating an ounce of water by 1° takes about 10 times the energy needed for an ounce of iron.
[Water] Why does water have such a high specific heat?
Some of the heat added to water first has to break the hydrogen bonds between water molecules. Only after that can the energy make the molecules move faster, which is what a temperature rise is.Example: in the time an iron pan on the stove becomes too hot to touch, water in it gets only lukewarm.
[Water] A covered pan of water and a covered pan of oil are put on equal burners. Which reaches a given temperature first?
The oil. The water takes more than twice as long. Water absorbs much more energy per degree of temperature rise (it has a high specific heat).
[Water] A covered pan of water and a covered pan of oil at the same temperature are taken off the heat. Which stays hot longer?
The water. Water has to give off much more energy than oil to cool by each degree (it has a high specific heat).
[Water] What is water's latent heat of vaporization?
The energy water absorbs while changing from liquid to gas, without rising in temperature. Water's is unusually high, because the hydrogen bonds must be broken. So evaporation removes a lot of heat.Example: evaporating sweat cools the body; porous clay jars keep drinking water cool.
[Water] Why does a custard cook more gently in an open water bath than directly in the oven?
Evaporation from the bath uses up heat, so the bath stays cooler than the oven air. The same principle: slow roasting at low oven temperatures, simmering stock in an open pot.
[Water] You can hold a hand in a 212°F/100°C oven for a while, but steam at the same temperature scalds in a second or two. Why?
Steam condensing on the skin releases a lot of heat (its heat of vaporization). So steam passes far more heat to the skin than air at the same temperature.For the same reason steam cooks food quickly. In bread baking, a burst of steam at the start increases oven spring and gives a lighter loaf.
[Water] Even pure water contains a few ions besides H2O\mathrm{H_2O}H2O molecules. Which ions?
H3O+\mathrm{H_3O^+}H3O+ and OH−\mathrm{OH^-}OH− . Only about two ten-millionths of a percent of the molecules are in this split state at any moment.
[Water] How do the ions H3O+\mathrm{H_3O^+}H3O+ and OH−\mathrm{OH^-}OH− form in pure water?
Now and then one water molecule passes a proton (hydrogen ion) to a neighbor. The molecule that lost the proton becomes OH−\mathrm{OH^-}OH− ; the one that gained it becomes H3O+\mathrm{H_3O^+}H3O+ . Left: water acting as a weak acid. Right: acetic acid releasing a proton to water.
[Water] What is an acid?
A substance that releases protons (hydrogen ions) into a solution. Many free protons can make otherwise stable molecules unstable, which is why acidity affects the color, texture and flavor of food.
[Water] What is a base (alkali)?
A substance that takes up protons (hydrogen ions) from a solution and so neutralizes them.
[Water] Which taste sensation tells us the proton concentration of a food?
Sourness. The word "acid" comes from Latin acere, to taste sour.
[Water] How is pH defined?
pH=−log10[H+]\boldsymbol{\mathrm{pH} = -\log_{10}[\mathrm{H^+}]}pH=−log10[H+] [H+][\mathrm{H^+}][H+] is the hydrogen ion concentration in moles per liter.Introduced by the Danish chemist S. P. L. Sørensen in 1909.Example: pure water has [H+]=10−7[\mathrm{H^+}] = 10^{-7}[H+]=10−7 mol/L, so its pH is 7.
[Water] On the pH scale (0 to 14), what do a pH of 7, a pH under 7, and a pH over 7 mean?
7 is neutral (pure water); below 7 is acidic, above 7 is basic (alkaline).
[Water] How many times more hydrogen ions does a solution of pH 5 contain than one of pH 8?
1,000 times. Each step of 1 in pH is a factor of 10 in proton concentration: 10−5/10−8=10310^{-5} / 10^{-8} = 10^310−5/10−8=103 .
[Water] Roughly what is the pH of: yogurt, milk, lemon juice, black coffee, orange juice?
Yogurt 4.5, milk 6.9, lemon juice 2.1, black coffee 5.0, orange juice 3.0. Nearly every food is at least slightly acidic. Human stomach juice: pH 1.3 to 3.0.
[Water] Name two kitchen liquids that are alkaline (pH greater than 7), with their pH.
Egg white (7.6 to 9.5) and baking soda dissolved in water (8.4). Household ammonia is far more alkaline: 11.9.
[Fats, Oils, and Relatives: Lipids] What do fats and oils do for food in the kitchen?
Flavor and smoothness They give a rich, lasting smoothness in the mouth, and carry aromas. Tenderness They permeate a food's structure and weaken it. Example: shortening in pastry. A cooking medium hotter than boiling water Frying above 212°F/100°C dries the food's surface, which gives crispness and browning flavors.
[Fats, Oils, and Relatives: Lipids] Which substances belong to the lipid family?
Fats, oils, phospholipids, carotenoid pigments, vitamin E, cholesterol and waxes. Example: lecithin is a phospholipid; beta-carotene is a carotenoid.
[Fats, Oils, and Relatives: Lipids] What molecular structure do all lipids share?
A backbone of linked carbon atoms, each carrying hydrogen atoms. A carbon inside such a chain is usually bonded to two neighboring carbons and two hydrogens.
[Fats, Oils, and Relatives: Lipids] Why don't lipids dissolve in water?
Their carbon-hydrogen chains are nonpolar, so they cannot form hydrogen bonds with water. Carbon and hydrogen pull about equally on their shared electrons, so neither end of the chain is charged.Water molecules bond to each other, the lipid chains bond to each other by weaker van der Waals bonds, and the two separate. Oil gathers into large drops to minimize its contact with water.
[Fats, Oils, and Relatives: Lipids] Substances that do not dissolve in water, like lipids, are called hydrophobic.
From Greek: "water-fearing".
[Fats, Oils, and Relatives: Lipids] Why do carrots, tomatoes and green vegetables color cooking fat much more strongly than cooking water?
Their pigments are lipids or partly lipid, and lipids dissolve in each other. Beta-carotene (carrots) and lycopene (tomatoes) are carotenoids; chlorophyll has a lipid tail.
[Fats, Oils, and Relatives: Lipids] Why do all natural fats and oils, solid or liquid, float on water?
Water is denser: hydrogen bonds pack its small molecules tightly, while fat molecules are bulky. Fats feel viscous and clingy because their long chains form many weak bonds with each other.
[Fats, Oils, and Relatives: Lipids] How does the energy content of fat compare with that of sugar or starch, per weight?
Fat holds about twice as many calories. So animals and plants store fat as a compact energy reserve.
[Fats, Oils, and Relatives: Lipids] What is the chemical difference between a fat and an oil?
There is none: both are triglycerides. An oil is simply a fat that is liquid at room temperature.
[Fats, Oils, and Relatives: Lipids] What is a triglyceride made of?
One glycerol molecule (a 3-carbon frame) bonded to three fatty acids. Glycerol with one fatty acid is a monoglyceride, with two a diglyceride. Three fatty acids (top) attach to glycerol to form a triglyceride; the chains can rotate into a chair-like shape (bottom).
[Fats, Oils, and Relatives: Lipids] What is a fatty acid?
A long hydrocarbon chain with an acidic group at one end that can release a hydrogen ion. The acidic end is the part that bonds to glycerol. In a free fatty acid it is polar, so free fatty acids are partly water-compatible; in a triglyceride it is capped, so the whole molecule is nonpolar.
[Fats, Oils, and Relatives: Lipids] How long are fatty acid chains?
4 to about 35 carbons; in foods mostly 14 to 20.
[Fats, Oils, and Relatives: Lipids] What determines the properties of a fat?
Which fatty acids its triglycerides carry, and where on the glycerol they sit. A fat is a mixture of many different triglycerides.
[Fats, Oils, and Relatives: Lipids] What do saturated, unsaturated and polyunsaturated mean for a fatty acid?
Saturated No carbon-carbon double bonds: every carbon in the chain carries the maximum number of hydrogens (two). Unsaturated At least one carbon-carbon double bond; each double-bonded carbon has room for only one hydrogen. Polyunsaturated More than one double bond.
[Fats, Oils, and Relatives: Lipids] Why are saturated fats firmer than unsaturated ones at the same temperature?
Straight saturated chains pack closely into crystals; kinked unsaturated ones don't. A double bond puts a rigid kink in the chain. The orderly packing of straight chains has been called "zippering". Saturated (straight), cis monounsaturated (one kink), cis polyunsaturated (curled) and trans polyunsaturated (nearly straight).
[Fats, Oils, and Relatives: Lipids] Roughly how much of animal fat is saturated, compared with vegetable oil?
Animal fat: about half saturated. Vegetable oil: about 15%. That is why animal fats are solid at room temperature and vegetable oils liquid.Example: beef and lamb fat are harder than pork and chicken fat, because more of their fatty acids are saturated.
[Fats, Oils, and Relatives: Lipids] Besides double bonds, what else lowers a fat's melting point?
Short fatty acid chains They pack less readily into crystals than long chains. Variety The more different the fatty acid structures in a fat, the more likely the fat is an oil.
[Fats, Oils, and Relatives: Lipids] Why do unsaturated fats turn rancid faster than saturated fats?
Oxygen attacks the chain at its double bonds and breaks it into smelly fragments. At a double bond, one side of each carbon has no hydrogen, which leaves room for oxygen to react.Water and metal atoms from other ingredients also promote this.Example: beef keeps longer than chicken, pork or lamb: beef fat has a higher proportion of saturated fatty acids.
[Fats, Oils, and Relatives: Lipids] Name some pleasant aromas that come from fragments of unsaturated lipids.
Crushed green leaves, cucumber, and part of the aroma of deep-fried food. The leaf and cucumber aromas come from membrane phospholipids broken by plant enzymes; the fried aroma from fatty acid fragments formed at high temperature.
[Fats, Oils, and Relatives: Lipids] Which common fats are the most saturated?
Coconut oil (86% saturated) and palm kernel oil (81%). Then butter (62%) and cocoa butter (60%). Beef fat: 50%; canola oil is the least saturated: 7%.
[Fats, Oils, and Relatives: Lipids] Which kind of fatty acid makes up most of olive oil?
Monounsaturated fatty acids (74%). Canola oil is also mostly monounsaturated (55%).
[Fats, Oils, and Relatives: Lipids] Which kind of fatty acid makes up most of safflower, corn and soybean oil?
Polyunsaturated fatty acids (58 to 75%). Safflower 75%, corn 59%, soybean 58%.
[Fats, Oils, and Relatives: Lipids] What is hydrogenation of an oil?
Adding hydrogen to the double bonds of its fatty acids, which makes the oil solid. It also makes the fat slower to go rancid.Example: shortenings and margarines are made from seed oils this way.
[Fats, Oils, and Relatives: Lipids] How is an oil hydrogenated industrially?
Heated with hydrogen gas under high pressure, with nickel as a catalyst. The nickel is filtered out afterwards.
[Fats, Oils, and Relatives: Lipids] What do cis and trans mean for a carbon-carbon double bond in a fatty acid?
Cis: the two hydrogens are on the same side of the double bond. Trans: opposite sides. Latin: cis "on this side of", trans "across from". Natural unsaturated fatty acids are mostly cis, which gives the sharp kink.
[Fats, Oils, and Relatives: Lipids] How do trans fatty acids arise during hydrogenation?
Instead of receiving hydrogen, some cis double bonds get twisted into the trans form. The chain stays unsaturated (the double bond remains), but it is much less kinked.
[Fats, Oils, and Relatives: Lipids] How do trans fatty acids behave, compared with the cis fatty acids they come from?
Like saturated fatty acids: they make the fat firmer and slower to go rancid. Their chains are nearly straight, like saturated ones.Also like saturated fats, they raise blood cholesterol, which can contribute to heart disease.
[Fats, Oils, and Relatives: Lipids] What is an omega-3 fatty acid?
One whose first double bond is at the third carbon from the chain's end. Counted from the end away from the acidic group.The most common unsaturated fatty acids are omega-6. Omega-3s are essential in the diet, e.g. for the immune and cardiovascular systems. Linolenic acid: 18 carbons, 3 double bonds (green leaves, some seed oils). Eicosapentaenoic acid: 20 carbons, 5 double bonds (almost only in seafood).
[Fats, Oils, and Relatives: Lipids] Why do most fats soften gradually over a range of temperatures instead of melting at one sharp point?
A fat is a mixture of different triglycerides, each melting at its own temperature. This is why butter is spreadable at room temperature, and it matters in pastry and cake making. Cocoa butter is an exception: it melts sharply.
[Fats, Oils, and Relatives: Lipids] At about what temperatures do fats boil?
500 to 750°F / 260 to 400°C, far above water's boiling point.
[Fats, Oils, and Relatives: Lipids] Fats cannot form hydrogen bonds. Why do they still boil at much higher temperatures than water?
Their long chains form very many weak bonds with each other, which add up. These are van der Waals bonds; breaking them all takes a lot of energy.
[Fats, Oils, and Relatives: Lipids] What is a fat's smoke point?
The temperature at which the fat breaks down into visible fumes. Most fats reach it well below their boiling point.
[Fats, Oils, and Relatives: Lipids] Why should frying fat stay below its smoke point?
The breakdown products spoil the food's flavor, and the fumes can catch fire. The fumes are also unpleasant, and can ignite on a gas flame.Among those products are reactive free fatty acids.
[Fats, Oils, and Relatives: Lipids] What mainly determines a fat's smoke point?
Its content of free fatty acids: the fewer it has, the higher the smoke point.
[Fats, Oils, and Relatives: Lipids] Which fats have higher smoke points: vegetable or animal, refined or unrefined, fresh or old?
Vegetable, refined and fresh fats, because they contain fewer free fatty acids.
[Fats, Oils, and Relatives: Lipids] At about what temperature do fresh refined vegetable oils and animal fats begin to smoke?
Refined vegetable oils: about 450°F/230°C. Animal fats: about 375°F/190°C. Fats containing other substances smoke lower: butter (with its proteins and sugars), and fats with emulsifiers or preservatives.
[Fats, Oils, and Relatives: Lipids] Why does a deep-frying fat smoke at a lower temperature each time it is reused?
Each use breaks down some of the fat and leaves food particles that speed further breakdown. This happens even at moderate frying temperatures.
[Fats, Oils, and Relatives: Lipids] What kind of pan slows the breakdown of deep-frying fat?
A tall, narrow pan, because less of the fat's surface touches the air.
[Fats, Oils, and Relatives: Lipids] What is the molecular structure of an emulsifier such as a monoglyceride or a phospholipid?
A water-compatible polar head and a fat-soluble hydrocarbon tail. In monoglycerides and diglycerides (such as phospholipids), glycerol carries only one or two fatty acids, and the free position holds a small polar group of atoms.
[Fats, Oils, and Relatives: Lipids] How does an emulsifier keep oil droplets in a sauce like mayonnaise from merging?
It coats each droplet, and its charged heads, facing the water, keep the droplets apart. The emulsifier's fatty tails dissolve in the oil droplet. Phospholipid molecules (top) coating two oil droplets.
[Fats, Oils, and Relatives: Lipids] Which emulsifier is the most abundant in egg yolk?
Lecithin, a phospholipid. It makes up about a third of the yolk's lipids.
[Fats, Oils, and Relatives: Lipids] How do phospholipids form a cell membrane?
As a double layer: heads facing the water on both sides, tails together in the middle. One layer's heads face the watery inside of the cell, the other layer's heads the watery outside.
[Fats, Oils, and Relatives: Lipids] Why do commercial bakers add monoglycerides to bread?
To slow staling. Apparently they bind to amylose (linear starch) and block starch retrogradation (the starch molecules re-bonding to each other).
[Carbohydrates] Which elements are carbohydrates made of?
Carbon, hydrogen and oxygen.
[Carbohydrates] Where does the name carbohydrate come from?
From the early belief that these molecules were made of carbon and water ("hydrate"). They do contain hydrogen and oxygen, but not as intact water molecules.
[Carbohydrates] Which carbohydrates serve plants for energy storage, and which for structure?
Energy: sugars and starch. Structure: cellulose, hemicelluloses and pectins. The structural ones are the carbohydrates of plant cell walls.
[Carbohydrates] Why are 5-carbon sugars especially important to all life?
The 5-carbon sugars ribose and deoxyribose form the backbones of RNA and DNA. RNA and DNA carry the genetic code.
[Carbohydrates] Why is the 6-carbon sugar glucose especially important to living things?
Most living things get the energy to run their cells from glucose. Sugars matter so much as nutrients that we have a taste for detecting them: sweetness.
[Carbohydrates] What are oligosaccharides, such as the raffinose in beans?
Short chains of a few sugar units. Raffinose, stachyose and verbascose have 3, 4 and 5 units; seeds such as beans store them as part of their energy supply.They are too large to trigger our sweet receptors, so they have no taste.
[Carbohydrates] Why do the oligosaccharides of beans (raffinose, stachyose, verbascose) cause intestinal gas?
We can't digest them, so they reach the colon, where bacteria feed on them and make gas. We lack the enzymes that would split them into single sugars. The bacteria produce carbon dioxide and other gases.
[Carbohydrates] What is a polysaccharide?
A long chain of sugar units, up to several thousand of them. Usually of only one or a few kinds of sugar.Example: starch and cellulose, both made only of glucose.
[Carbohydrates] Polysaccharides have many exposed oxygen and hydrogen atoms. Does that make them dissolve in water?
Not necessarily: it depends on how strongly the chains bond to each other. They always form hydrogen bonds with water and absorb some.
[Carbohydrates] What is starch?
A chain of glucose units: the compact form in which plants store sugar. In the plant, starch molecules are laid down in concentric layers, forming microscopic solid granules.
[Carbohydrates] What are the two forms of starch molecule?
Amylose, a straight chain of glucose units, and amylopectin, a highly branched one. Each can contain thousands of glucose units. A glucose ring (top left), glucose units linked into a chain (top right), amylose and amylopectin (bottom).
[Carbohydrates] What happens to starch granules when starchy food is cooked in water?
They absorb water, swell, and release starch molecules.
[Carbohydrates] Starchy food has been cooked in water, so its granules have released starch molecules. What can happen as the food cools?
The starch molecules bond to each other again and can form a moist but solid gel.
[Carbohydrates] What is glycogen?
"Animal starch": a chain of glucose units like amylopectin, but even more branched. It is a fairly minor part of animal tissue.
[Carbohydrates] Why does the glycogen in an animal's muscles at slaughter matter for the meat?
It affects the meat's final pH, and with it the meat's texture.
[Carbohydrates] Cellulose and amylose are both linear chains of glucose. How do they differ?
In how the glucose units are linked to each other. That difference is why cooking dissolves starch but not cellulose, and why most animals digest starch but not cellulose.Cattle and termites digest cellulose only thanks to bacteria in their guts. For humans, cellulose is indigestible fiber.
[Carbohydrates] What are hemicelluloses and pectic substances?
Jelly-like carbohydrates in plant cell walls that glue the cellulose fibers together. They are made from several sugars, e.g. galactose, xylose and arabinose.
[Carbohydrates] Why do vegetables and fruits soften when cooked, while their cellulose stays intact?
The cell-wall "glue" (hemicelluloses and pectins) partly dissolves in water. That glue holds the cellulose fibers of the cell walls together.Pectin is plentiful enough in citrus fruits and apples to be extracted and used to set jams and jellies.
[Carbohydrates] What is inulin?
A polymer of fructose, with a few to hundreds of units per molecule. It stores energy for the plant and acts as antifreeze (sugars lower water's freezing point). We cannot digest it, so bacteria in the large intestine feed on it and produce gas.
[Carbohydrates] In which foods is inulin found?
Plants of the onion and lettuce families, especially garlic and the sunchoke.
[Carbohydrates] What are plant gums used for in food?
Thickening and gelling liquids, and stabilizing emulsions. They also make frozen desserts and candies smoother.They are complex polymers of several different sugars or related carbohydrates.
[Carbohydrates] Where does the food thickener xanthan gum come from?
Bacteria make it, in industrial fermentation. Other gums: agar, alginates and carrageenans come from seaweed cell walls; gum arabic oozes from cuts in Acacia trees; guar and locust-bean gum come from seeds.
[Proteins] Why are proteins much more sensitive to cooking conditions than water, fats and carbohydrates?
Proteins are the active machinery of living cells, so their structure is made to change. They build and break down molecules, move them around the cell, and contract muscles.Fats and carbohydrates are mainly passive energy stores or structural materials.McGee: proteins are "the most challenging and mercurial" of the food molecules.
[Proteins] What is an amino acid?
The building block of proteins: a small molecule with a nitrogen-containing amine group. It has 10 to 40 atoms, mainly carbon, hydrogen and oxygen, plus the amine group NH2\mathrm{NH_2}NH2 .A couple of amino acids contain sulfur. About 20 kinds occur in significant amounts in food.
[Proteins] How many amino acids long is a typical protein?
Dozens to hundreds. A protein often contains many of the about 20 kinds of amino acids found in food.
[Proteins] What is a peptide?
A short chain of amino acids.
[Proteins] What are the three reasons amino acids matter to the cook?
Browning They take part in the browning reactions that create flavor at high cooking temperatures. Taste Individual amino acids and small peptides do taste of something: most are sweet or bitter, a few savory. Example: aged cheese, cured ham, soy sauce, where proteins have been partly broken down. Protein behavior Their different chemical natures determine how the protein they belong to folds and bonds.
[Proteins] Which amino acid gives the savory umami taste?
Glutamic acid, sold in concentrated form as MSG (monosodium glutamate). Some peptides also taste umami. Umami is Japanese for "delicious".
[Proteins] Which foods are rich in the savory umami taste of glutamic acid?
Tomatoes, certain seaweeds, and salt-cured and fermented foods.
[Proteins] What aroma do sulfur-containing amino acids produce when heated?
Eggy and meaty aromas.