UPDATE: Please see You Don’t Want Fries With That! Gluten-Free Foods Cooked In Shared Fryers With Wheat: A Pilot Study Assessing Gluten Cross Contact. The abstract presented at the AOAC conference along with a power point presentation with audio, a consumer flier about the study, and a white paper are available on Gluten Free Watchdog at https://www.glutenfreewatchdog.org/news/you-dont-want-fries-with-that/
While I avoid “coated” fries like the plague, there are those rare occasions when traveling, starving, and about to faint that I really don’t want to know what else might be lurking in the oil used to fry fries.
There are various theories on fries and whether it matters if a dedicated fryer is used. Some wonder whether any lingering gluten from the frying of a breaded product would be broken down from the heat of the oil; others wonder if any breading would sink to the bottom of the fryer instead of being dispersed throughout the oil.
Tackling the latter question first, while I am no expert on commercial frying operations, from what I’ve read, heavier food particles, such as chunks of batter may sink to the bottom of the fryer but smaller particles, such as flour may remain suspended in the oil. Some restaurant fryers may have a filtration system to remove suspended particles and increase the “shelf life” of the oil. This is because any particles left to linger too long will cause the oil to develop an off flavor. Regardless, according to Thomas Grace of Bia Diagnostics in Burlington Vermont, “the gliadin fraction of gluten, which is considered the most toxic fraction of the gluten protein complex for individuals with celiac disease, is also the most hydrophobic (i.e., oil loving) and soluble (i.e., dissolvable) in oil.”
As far as gluten being broken down by the heated oil, this is unlikely. According to my Joy of Cooking cookbook, the best temperature for deep frying is 365 degrees F. According to Thom, this is not a high enough temperature to completely hydrolyze gluten, at least in the short term.
“Gluten proteins are extremely resilient and can’t be broken down easily with temperature or time. The oil might have some effect on the protein tertiary (i.e., 3-dimensional) structure, BUT remember some breads are cooked at 500 degrees F for 10-15 minutes (pizza) and the gluten remains intact. So the short answer is that hot oil for the most part can not be trusted to completely hydrolyze gluten.”
If this all sounds confusing, it may help to keep in mind that soluble does not mean hydrolyzed. As Thom says, “hydrolyzed, in regards to protein, is simply a process by which the protein is chemically broken down into its basic parts (amino acids). Soluble or solubility is the process by which a compound or substance is dissolved into something else homogeneously (i.e. distributed evenly,) usually into a liquid.” Think sugar added to tea or coffee. The sugar is still there, it has just dissolved into the hot beverage.
All of this said, we are eating fries not oil. We have to consider how saturated the oil may be with gluten and how much oil ends up in the fries. Saturation depends on several factors, including how frequently the oil is changed and whether there is a filtration system. The amount of oil the fries absorb depends on several factors, including frying time and surface area of the potato. It may be the case that while the oil contains greater than 20 parts per million of gluten, the fries may contain less. The only way to know is to test several batches of restaurant-prepared fries for gluten. When we decide to do this we will let you know.
Thanks for your help, Thom!
Best bet: Until we are able to test, eat fries from dedicated fryers only!!