15 MAY 2008
Meet the mechanical mouth
You crunch a clove of garlic, and it releases that distinctive bouquet. You savor a sliver of French cheese, the aromas drift into your nose, and you can taste the cheese.
The nose knows: Most of what we call "taste" starts when volatile chemicals get picked up by the olfactory apparatus inside the nose.
From original garlic photo by South Dakota Department of Health
There may be no accounting for taste, but a French research group is trying to quantify taste with a mechanical masticating machine that can chew food (er, perform "in vitro mastication") and then collect the vapors that get released for chemical analysis.
The motivation, says Gaiîle Arvisenet, a researcher with the National Teaching and Research Institute in Food Science and Technology (Nantes, France), "is to understand the perception of aroma in food." Chewing both liberates and changes the volatile compounds in food, which number at least 30 in an apple.
To understand the link between food's chemical composition and our perception of its taste, we need to understand how the volatiles are released during eating, Arvisenet says.
Existing techniques leave him hungering for a better one:
You can extract and analyze volatile compounds directly from food samples, but "not all the volatiles are released in the mouth during consumption," Arvisenet says, "and we need to know exactly which compounds reach the olfactory receptors, and in what amount."
You can collect and analyze the breath of people while they eat, but he adds, that is, "both difficult and expensive, and it requires working with several people to be reliable."
Opening the artificial mouth
Arvisenet and company have chosen a third alternative: standardizing the mouth by creating a knockoff. Their artificial mouth is a cylinder housing a circular set of chompers mounted on a rotating shaft that can slide to compress the food. Water and saliva seep through the mush, and a flow of inert helium gas removes the volatile compounds for analysis.
Courtesy American Chemical Society.
By controlling the time, rate of rotation and compression, the researchers found a process that reduced gobs of golden delicious apple to a goo similar to one that human volunteers spat out after performing "in vivo mastication" (chewing) of apples.
Chewing, Arvisenet assures us, has a major impact on odor release. "When the food is crushed and mixed with saliva, its structure is modified and the diffusion of its volatiles to the headspace [nasal passages] is affected. With mastication, the food surface area exposed to the air increases and the food matrix is separated from the water it contained initially. The temperature of the mouth also increases the volatility of volatile compounds, increasing their release."
Photo by Scott Bauer, USDA
So to quantify how something smells, you need a realistic experimental setup. You need an artificial kisser.
We observed that the artificial mouth had to chew quite a while to reduce the apple to a pulp. Does this mean the human mouth is a lot more efficient? No, wrote Arvisenet, but he added that "human olfactory receptors are more efficient. Very small quantities of volatile compounds are needed to interact with them to induce a perception, and such small quantities can hardly be detected by instrumental methods. By increasing the time of the mastication, we increased the release and then the quantity of volatile compounds to be analyzed."
In your place, machine!
It's not every day we get to talk (actually, e-talk) with a chewing expert, so we asked for instructions on chewing to maximize flavor. "The more one chews foods, the more he perceives flavor," Arvisenet said. "But that is not the reason why we are told to chew our foods thoroughly, which is to improve digestion."
Now that the artificial mouth has realistically munched apple, Arvisenet wants to "teach" it to reproduce the sequence of release of odor chemicals that occurs during in vivo mastication.
Naturally, we had more questions. Why did he test apple instead of something more scrumptious? "We wanted to study a foodstuff that was difficult to analyze," Arvisenet said. "Apple is at the same time a hard food (so difficult to crush by the artificial mouth) and it has a low aroma."
But does the man-made mouth brush its teeth twice a day? Does it talk with its mouth full? Does it get garlic breath?
- David ("It's always time to eat") Tenenbaum
• Release of Apple Volatile Compounds in an Artificial Mouth, GAËLLE ARVISENET et al, J. Agric. Food Chem., Vol. 56, No. 9, 2008.