6.1: Mouth

The digestive tract begins with the mouth. Food is macerated by chewing, and is made more liquid and slippery by saliva. Just thinking of food can make the mouth water, starting the digestive process even before eating. Saliva has a slightly basic pH, and we secrete about a quart and a half each day. Saliva contains amylase, an enzyme that digests starch (amylose, amylopectin). But very little starch is digested in the mouth because food is usually there so briefly.

Taste

Taste buds are buried on the surface of the tongue. We even have some in the lining of the mouth and at the back of the throat. Though plentiful, they recognize only five sensations— sweet, salty, sour, bitter, and umami (“savory”). All other “tastes” are really smelled. We acknowledge this when we bemoan the tastelessness of food when we have a stuffy nose.

The different tastes can be tasted all over the tongue, and may have evolved from an ancient need to differentiate between nutritious foods and noxious ones. Tasting sweetness encourages selecting sweet and nutritious foods like fruit. Poisons are often bitter, and bitter foods are often rejected. Salt (sodium chloride) is a required nutrient and used to be scarce, so it was important to identify and select salty food.

Sweetness

Sugar is sweet. Starch is made of sugar (glucose), so why isn’t it sweet too? Sweetness depends mostly on how a substance fits into the taste bud. Starch is too big. But if you chew a cracker and hold it in your mouth, it turns sweet. Saliva’s amylase enzyme breaks down the starch, releasing some maltose (the double sugar made of two glucoses).

A spoonful of sugar taken straight from the sugar bowl doesn’t immediately taste sweet. Sugar crystals are too big to fit into taste buds. As the crystals dissolve in saliva, you taste the sweetness. (Whether sweet, salty, sour, bitter, or umami, “tasty” molecules must be dissolved in a liquid before they can be tasted.)

How the arrangement and composition of atoms in a molecule affect its sweetness isn’t fully understood. There are no instruments to measure sweetness, so it’s usually measured by dissolving sweeteners in water and having people taste them. Taste tests are carefully controlled, because sweetness is affected by concentration, temperature, etc. Melted ice cream, for example, tastes sweeter than frozen ice cream.

Sweeteners are found mostly by trial and error or accidentally. A chemist found aspartame (NutraSweet) when he licked his fingers and noticed they were sweet. Sweeteners aren’t necessarily sugars. Aspartame is made up of two amino acids (aspartate, phenylalanine) linked together. The sweetest known substance is thaumatin, a protein from the berry of a West African plant.

When aspartame is sold in food, it’s called NutraSweet; when sold in powdered form, its brand name is Equal. The body treats it as protein: the 2 amino acids are broken apart (digested), absorbed as amino acids, and have 4 cal/g. NutraSweet does have calories, but it’s so much sweeter than sugar that very little is needed. This is why cans of NutraSweet-sweetened sodas say Less than 1 calorie per serving.

NutraSweet is about 160 times sweeter than table sugar (sucrose). A 12-oz soda having 10 teaspoons of sugar is as sweet as one with 1/16 of a teaspoon of NutraSweet. The next time you see cans of diet and regular sodas chilling in a tub of ice water, note that the diet versions float higher in the water. Carbon dioxide in the carbonated drinks makes the cans buoyant, but the weight of the sugar dissolved in regular sodas makes them sink deeper.

Changing a carbohydrate can change its sweetness, as in the earlier example of maltose being released from starch in saliva. Vegetables like corn and peas lose their sweetness as they age because their sugar converts to starch. To preserve sweetness, producers of frozen vegetables blanch (briefly heat) the vegetables soon after harvest to denature (inactivate) the enzymes that catalyze this conversion. Some fruits, like bananas, get sweeter as they ripen. Starch in the fruit is converted to sugar. (Sugar in fruits is mostly a mix of glucose, fructose, and sucrose.)

Honey is sweeter than table sugar (sucrose), although both have glucose and fructose in a 1:1 ratio. Glucose and fructose are “loose” as single sugars in honey, whereas they’re combined as a double sugar in sucrose. The combined sweetness of glucose and fructose is sweeter than an equal amount of sucrose. Honey is also sweeter because its sugar is dissolved in water, so a teaspoon of honey has more sugar (6 g) than dry table sugar (4 g).

High-fructose corn syrup is made from cornstarch, which is composed entirely of glucoses linked together (Figure 5.1). The cornstarch is broken down into its component glucoses, to become corn syrup. About half of this glucose is then coverted to fructose by rearranging a few of its atoms (Figure 5.1). Fructose is sweeter than glucose (Table 6-1), so less syrup is then needed to sweeten food products.

Table 6-1: Sweetness Relative to Sucrose

Consumers are confused about high-fructose corn syrup. It’s been demonized because of its fructose content, although its sugar content (about 50% fructose, 50% glucose) is the same as honey and table sugar. Fructose in large amounts has adverse health effects, but substituting highfructose corn syrup with honey or regular sugar doesn’t lower the fructose content. To lower fructose intake, cut back on all sugar-sweetened drinks, candy, and desserts, whether the sugar is high-fructose corn syrup, honey or regular sugar.