Last April Swedish scientists discovered high levels of a potentially cancer-causing chemical called acrylamide in wide range of starch-containing foods that are fried or baked, particularly french fries, potato chips and crackers. The announcement received worldwide publicity. But at the time, no one knew where the acrylamide came from, how it was formed, or, indeed, if there is a link between acrylamide in food and cancer. The findings were quickly confirmed by the British Food Standards Agency. Earlier this autumn the source of the acrylamide was identified independently by researchers at the University of Reading in England, Nestlé in Switzerland and Procter & Gamble in the United States. They showed that acrylamide is produced when asparagine, an amino acid abundant in cereals and grains, is heated above 100 degrees Centigrade (212 degrees Fahrenheit) with either of two sugars, glucose or 2-deoxyglucose.
Now Bruce Ganem, a professor in Cornell University's Department of Chemistry and Chemical Biology, has offered a more-detailed chemical explanation about how acrylamide is produced when starch-containing foods are fried or cooked at high temperatures. His theory is proposed in a letter, "Explaining acrylamides in food," in a recent issue of the journal Chemical and Engineering News (Dec. 2, 2002).
Acrylamide is a polymer that is widely used in the treatment of drinking water. It also is used in the manufacture of plastics. It was first evaluated as probably carcinogenic to humans in 1994 by the International Agency for Research on Cancer. But it was not known to occur in high levels in fried or baked foods before this year's Swedish study.
"The organic chemistry of what happens is not very well understood," Ganem says. "Everyone agrees that a molecule of carbon dioxide must be lost in order to form acrylamide, but it was unclear how that might happen." The British and Swiss research teams invoked the Maillard reaction to explain the formation of acrylamide, but they did not propose any chemical details. The Maillard reaction, also known as non-enzymatic browning, was first observed in 1912 by Louis Camille Maillard. It involves the reactions between proteins and carbohydrates that cause food to turn brown when cooked. The reactions result in the formation of many products, most of which have some impact on the flavor and appearance of cooked food.
Procter & Gamble scientists noticed that acrylamide also was formed from a combination of the amino acid asparagine and the sugar 2-deoxyglucose. "This is interesting because 2-deoxyglucose lacks a key molecular feature needed for the Maillard reaction," says Ganem.
"That's where my letter to Chemical & Engineering News comes in. By focusing on explaining how carbon dioxide might be released, I recognized another plausible reaction pathway -- not involving the Maillard reaction -- that could account for the formation of acrylamide. My idea was based on how some biological systems achieve decarboxylation, which means the loss of carbon dioxide. That connection provided a big clue that led to the step-by-step chemical mechanism I present in my letter."
Instead of undergoing the Maillard reaction, fried or baked foods, Ganem suggests, undergo an alternative chemical pathway that results in the loss of carbon dioxide through natural metabolic processes, known as enzymatic decarboxylation.
"The asparagine is the actual source of acrylamide," Ganem says. "The pathway I presented probably would not occur under normal biological conditions, but it's important to recognize that we're talking about temperatures well above 100 degrees Centigrade while the food is being cooked."