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I will sheepishly tell you that I set off the fire alarm in my office when I was preparing my morning ritual of avocado toast and didn’t notice that one of the slices touched an element in the toaster until I smelled smoke. It seems the smoke detector is very sensitive! Luckily it was early morning and only a few people had to evacuate the building. Then I had a quandary: Eat the toast or chuck it? That question was raised because I was familiar with the scientific literature that had assessed the risks of eating burned foods, particularly as it pertains to acrylamide, a purported carcinogen that forms when foods containing both carbohydrates and the amino acid asparagine are heated to temperatures that exceed about 120 C. Bread, essentially made of starch with small amounts of asparagine falls into that category.
When heated, some of the starch breaks down to release glucose, and proteins decompose to release amino acids to join the amino acids already present. Glucose and amino acids can then engage in what is known as the “Maillard reaction,” named after French chemist Louis Camille Maillard, who in 1912 described the reaction between sugars and amino acids that results in an array of compounds that give browned foods their distinctive flavour. When that amino acid is asparagine, the compound that forms is acrylamide, classified by the International Agency for Research on Cancer (IARC) as “a probable human carcinogen.” This is based on feeding large doses to animals and making an educated guess about the effect of smaller doses on humans.
Different science agencies make different guesses, varying from 25-195 micrograms, about the maximum amount that an adult can safely ingest every day. These are based on animal data, because obviously, humans cannot be fed different amounts of acrylamide and be monitored for decades to determine the incidence of cancer. The closest one can come are studies that follow the health status of groups of people who periodically fill out food frequency questionnaires from which acrylamide intake can be estimated. The majority of such studies have found no association with cancer.
Nevertheless, it is prudent to try to minimize exposure to any substance that causes cancer in animals, so we can take a look at acrylamide content of specific foods and compare it to the guesses for maximum recommended daily intake. When calculations are made taking all foods into account, an adult consumes a daily average of 30 to 40 micrograms, well below the average of the guesses. Potato chips and French fries are at the high end, with a serving containing about 50 micrograms. A serving of cereal has about 7 micrograms and a cup of coffee less than 1. And toast? That’s roughly 5 micrograms per slice. Burned toast would have more but still below the most stringent daily recommended intake of 25 micrograms.
Although the effect of acrylamide on humans is tenuous, researchers have investigated various methods to reduce exposure. An obvious goal is the reduction of asparagine that is present in foods that also contain sugars and free amino acids. If there is no asparagine, acrylamide cannot form. Asparaginase is an enzyme that catalyzes the conversion of asparagine to unreactive aspartic acid and can be isolated from a variety of fungi and bacteria. The common mold Aspergillus niger and a strain of E. coli are typical examples. Asparaginase added to flour should then be able to reduce the amount of acrylamide that forms when dough made from this flour is heated.
Italian researchers explored this possibility by looking at, what else, pizza. The amount of acrylamide in the final product was reduced by 50 per cent! Other scientists demonstrated a 90 per cent reduction of acrylamide in toast made from dough treated with asparaginase and a close to 60 per cent reduction in french fries from potatoes that had been soaked in an asparaginase solution.
Another attractive approach is to reduce the amount of asparagine that is naturally present in wheat and potatoes through genetic engineering. The production of asparagine in grains and potatoes requires the activity of several genes, suggesting that if these genes are silenced to some degree, the amount of asparagine is reduced. Two possibilities arise: Either prevent the genes from giving the signal that triggers the production of asparagine, or remove these genes from the wheat or potato’s DNA.
The first can be accomplished by RNA silencing. The message to produce asparagine involves transferring the information needed for its formation from DNA in the cell’s nucleus to messenger RNA (mRNA) that then uses this information to direct the cell to make asparagine. In 2006, Andrew Fire and Craig Mello were awarded the Nobel Prize in Physiology or Medicine for discovering that short strands of RNA that can be synthesized in the lab can bind to and inactivate a specific mRNA. This technology has already been used to develop potatoes that reduce the potential formation of acrylamide. Since no foreign genes are introduced, there is no labeling requirement. Interestingly, french fry marketers have not jumped to promote their use of such potatoes because it would suggest that the fries they were selling before had an element of risk.
For wheat, the technology that holds promise is based on a tool, CRISPR/Cas9, that garnered the Nobel Prize in Chemistry for Emmanuelle Charpentier and Jennifer Doudna in 2020. This is usually described as a type of “molecular scissors,” allowing for specific genes, such as the ones responsible for producing asparagine, to be edited out from DNA. Again, no foreign genes are introduced. Professor Nigel Halford at the Rothamsted Research Center in the UK has used the CRISPR/Cas9 technique to develop wheat that has a greatly reduced asparagine content. The research has emerged from the laboratory into field trials that have demonstrated that the technology works.
There is no question that the methods to reduce our intake of acrylamide through these technologies are of significant academic interest. However, their practical significance is questionable, given that the evidence of acrylamide being a human carcinogen is less than compelling. The average Canadian adult consumes about 25 micrograms of acrylamide a day, an amount that, even by the strict standards of the European Food Safety Association’s and California Proposition 65, is not a problem.
Now back to my burned toast. Knowing that I wouldn’t be consuming anything further that day with any significant acrylamide content such as chips or French fries, I scraped off the black stuff, spread the slice with avocado and ate it. But I am now more careful not to burn my toast. And more importantly limit my consumption of chips and French fries.
