By Nick Houtman
In a provocative study of food and human evolution, Harvard primatologist Richard Wrangham did things that some people might find a bit extreme. He ate raw goat meat mixed with tree leaves. He sampled the uncooked fruit of the African pepper-bark tree — food favored by monkeys and chimpanzees. Its hot taste, he wrote, makes it “impossibly unpleasant for humans to digest.”
Partly as a result of his work, Wrangham and others have proposed that cooked foods suit our taste buds better than raw. Cooking, they argue, may have helped to fuel human development by giving our ancestors access to more calories.
Heating goat meat may indeed make it more flavorful, but scientists have long debated the role of taste in human health. Our love of sweets reflects the critical importance of glucose, the sugar that flows in our blood and fuels our bodies. An adult requires a minimum of about 150 grams, or five ounces, of glucose a day. We tend to reject foods with a bitter taste, which is often associated with toxins. Salty, sour and savory (aka umami) round out the five well-known taste categories. Now, Juyun Lim at Oregon State University has discovered a possible sixth.
At first, she thought it was a mistake, but the associate professor and her collaborators in Food Science and Technology are learning that taste is about more than preferring bananas to strawberries or being a picky eater.
The Taste of Starch
At the Center for Sensory and Consumer Behavior Research, Lim explores the mechanisms of what food scientists call the human sensory system: taste and smell. As often happens, she stumbled onto her discovery in the course of studying something else. In this case, she was looking at a question near to the heart of parents everywhere: How do we learn to like the flavor of foods, such as kale? Can parents really help kids to finish their broccoli?
“Some people dislike a vegetable like broccoli at first. But after eating it many times, they may start to like it,” says Lim. “What’s that process like? Are they just getting used to it? Or does it have to do with getting calories, feeling satisfied and associating the experience with the flavor? What is it?”
Lim and her research team set up experiments and gave volunteers vegetable juice with and without a starch-based food supplement that is considered tasteless: maltodextrin. “My graduate research assistant came back to me and said her subjects could taste maltodextrin, and I said, ‘no, I don’t think so,’” says Lim. “I thought it must have some impurities that cause a taste.”
The starches we usually eat — whether in corn tortillas, whole-wheat bread or rice pilaf — are comprised of extraordinarily long and complex glucose molecules. If an individual glucose were the size of a twig, a starch molecule would be as big as a tree. “It’s so big and insoluble, no one thought it would activate any taste receptors,” says Lim. (See What’s That Taste?)
However, maltodextrin is made of much shorter starch molecules, the equivalent of small trees with only a few branches. Is it possible, she wondered, that our taste buds can detect these shorter molecules through the well-known sweetness receptor on the tongue? Or was another unrecognized taste receptor at work? After all, the receptors on our taste buds get triggered by a variety of chemicals, such as salt or sugar.
So, in a second round of experiments, Lim and her team decided to purify their samples and go one step further. They added a commonly used food additive called lactisole. Food processors use lactisole to reduce the sweetness of jams and jellies, so more of the fruit flavor can come through. Lim gave lactisole to her subjects because it blocks the sweetness receptor on the tongue.
In blind taste tests, the volunteers were again able to taste the purified samples. Clearly, one or more previously unknown taste receptors were being activated.
Lim and her team refined their samples further and determined that their subjects were detecting a specific component of maltodextrin. It’s likely, she says, that when we chew a piece of bread and mix it with saliva, we break up the starch into smaller compounds that we can taste.
“Our volunteers told us that the samples we gave them taste like root vegetables or that it was pasta-like, or bread-like,” says Lim. “From the standpoint of human evolution, this makes sense. We get 60 to 70 percent of our calories from carbohydrates, which include starches.”
The Spit Test
Last summer, in the journal Chemical Senses, Lim and her team published their potential discovery of a sixth sense of taste, setting off a flurry of publicity around the world. But the researchers haven’t stopped there. Lim wants to understand exactly what happens when we put foods composed of starch in our mouths. Does it make a difference if they are cooked or raw, barely chewed or thoroughly masticated?
The question gets back to Richard Wrangham’s suggestion that cooking produces more of the compounds that we can taste. By enabling us to get more calories out of what we eat, cooking may also play a fundamental role in human evolution. Lim’s experiments with maltodextrin offer one way to study these questions.
Last fall, Lim and her team gave corn starch — both raw and cooked — the spit test. Saliva contains an enzyme that breaks down starches into the short molecules that we can taste. So volunteers were asked to spit into a cup for one minute (in case you are wondering, we vary a lot in how much saliva we produce, but we typically generate two to four pints of saliva every day).
Lim’s team then mixed samples of raw and cooked starch with saliva. To consider how long we chew, they analyzed some mixtures after a few seconds and others after 30 seconds. The samples that had been cooked or mixed with saliva for a longer time generated a lot more of the short molecules than those that were uncooked or mixed for shorter times.
“By cooking, we are able to generate more of the compounds we can taste. And the longer you chew, the more of them are produced in your mouth,” says Lim.
But the real payoff, she adds, doesn’t come just from enjoying the flavor of a baked potato or cornbread. It’s possible, she says, that the taste system may play an important role in health by prompting the body to release insulin even before these foods arrive in the stomach.
To test this hypothesis, Lim is working with a Corvallis physician to determine how tasting carbohydrates affects insulin levels in blood. She and her team are gathering data this spring.
