It's a very familiar scenario to those of us who haven't given up on greasy, spectacularly unhealthy fast foods: You step up to the counter of your preferred burger joint and, thinking only of how extremely hungry you are, you order the large fries. You are given a disturbingly huge mound of french fries that any normal human would be extremely unwise to consume at one sitting, so you either throw out the leftovers or -- since throwing away food is wrong because of all the starving children in Africa/China/India -- you take the uneaten fries home so you can stick them in the fridge... and throw them away at a more convenient time. The fries are inevitably destined for the trash because, as everyone knows, cold, leftover fast food french fries taste like rubber. This raises the question: Is there another way this scenario might play out?
You could just stop buying french fries because they're nothing but empty, fat-laden, delicious calories that won't get burned off because your exercise routine is pretty much just a figment of your imagination... but who are we kidding? What's really needed is a strategy for rescuing those cold, dead fries and making them tasty again. The first thing a fry-rejuvenator might be tempted to try is simply sticking them in the microwave for a few seconds, but if you know anything about frying, you'll already know why such a plan is destined to yield less than appetizing results.
There is, in fact, a huge amount one might know about frying. Humans, starting with the Chinese, have been deep-fat frying for the last 3,600 years or so and we still don't know all there is to know about the process. When raw potatoes hit sufficiently hot oil, so much goes on at the molecular level that one could easily spend an entire semester of college studying the subject. Let's see what I can do in about a thousand words...
Start with the fact that frying is absolutely nothing like boiling. Somewhat counterintuitively, frying is actually a dry cooking method; heat is transferred to the food via the oil, not moisture in or around the food. When you place raw fry-cut potatoes into preheated oil, moisture on and near the surface vaporizes and comes bubbling out. We all know that you don't get GBAD ("Golden Brown And Delicious") from boiling food in water. You get GBAD when you heat up some oil, dunk your food in and get a Maillard reaction.
Every cooking student learns about the Maillard reaction (pronounced "My yard"), named after the French chemist, Louis Camille Maillard who studied the process in the early years of the 20th century. "Maillard reaction" is a bit of a misnomer since it gives the impression that only one reaction is taking place when the term really describes a series of reactions involving amino acids and reducing sugars, chemical compounds found in nearly all of the plants and animals we like to eat. When enough heat is added, the amino acids and reducing sugars combine to form new molecules... molecules that taste good. That "enough heat" part is important; if you want GBAD you need a Maillard reaction, but you don't get a Maillard reaction when your food is cooking at less than 300º Fahrenheit (150º Celsius). Assuming your kitchen isn't under curiously high air pressure, water can only reach 212º F (100º C) before it turns into steam while the temperature of oil can exceed 370º F (188 º C). When the surface temperature of foods we like to fry gets over 300º F/150º C, our favorite flavor molecules start to form.
When it comes to frying, once you go beyond the Maillard reaction, things get super complicated. So complicated that even the experts haven't learned what all those molecules are doing when potatoes and hot oil meet. Scientists have found over 200 different chemical compounds formed by deep-fat frying. Stuff like...
and, of course,
If the chemistry doesn't sound particularly appetizing, it probably shouldn't. Take, for example, acrylamide. If anything about the science of frying will make you give up on french fries, it's acrylamide. Basically, in large doses, it's toxic. It's one of the nastier toxic compounds that result from deep-fat frying. Acrylamide is a neurotoxin that's found in tobacco smoke as well as fried foods. The compound has been used in the industrial manufacture of plastics for decades, so, yeah, it's nasty stuff that you probably shouldn't be eating. The exact mechanism that produces high levels of acrylamide in food is kind of a mystery, but it is known that one can reduce the amount of it that forms by frying foods at the lower end of the temperature range and cooking foods for less time. By the way, acrylamide might also be carcinogenic.
Frying creates other toxic compounds as well including aldehydes and benzene. And don't get me started on the heterocyclic aromatic amines.
That so many different compounds are formed during frying makes sense when you consider how many different types of chemical reaction are taking place. As I mentioned, "Maillard reaction" is kind of an umbrella term for just some of what's going on in the fryer. To really understand what's happening in that oil, and why microwaving your old fries doesn't make them more edible, you want to know a little something about hydrolysis, oxidation and polymerization.
Hydrolysis is when water molecules break up and form new compounds, and potatoes are full of water. Water is what is known as a "weak nucleophile" meaning it breaks up in chemical reactions pretty easily. The two hydrogen atoms and the one oxygen atom (H2O) break up into ions (H+ and OH-) that react with the oil to form new compounds... that get into the oil and will eventually ruin it and make it bad for frying.
Oxidation causes the formation of free radicals (hydroxide ions, which are bad), but the oxidation of linoleic acid -- a key component of most cooking oils -- is generally what produces the flavor molecules we like so much (which is good).
You can't nuke your leftover fries back to great tasting life because the chemistry of deep-fat frying that made them so good in the first place isn't replicated by microwaving. A microwave works by agitating water and sugar molecules (and some fat molecules) with oscillating electromagnetic waves that cause molecules within the food to move around with a lot more energy, heating the food from within. But the heating is uneven and doesn't duplicate the high heat, rapid dehydration and the chemical reactions you get with frying. After the timer on the microwave goes off telling you your fries are done, you find you're left with rubbery bits of starchy stuff that lack the smell, taste and texture you look for in a french fry. But all is not lost.
Here's what you want to do: Say you have about half of a large serving of McDonald's fries left over in your fridge. (Funny aside: You've heard that McDonald's fries aren't vegetarian, haven't you? It caused a minor kerfuffle when a chemist working for a vegetarian lawyer named Harish Bharti discovered meat proteins in the "natural flavor" that McD's adds to their fries.) Leftover fries need four things to become edible again: fat, flavor, dry heat and a baking sheet lined with foil.
FAT: 3 or 4 tablespoons of olive oil (enough to coat the fries really well).
FLAVOR: A few generous shakes of salt, pepper, garlic powder, onion powder and a little dried parsley.
DRY HEAT: Preheat your oven to around 365º F.
A BAKING SHEET LINED WITH FOIL: A baking sheet lined with foil.
Put the fries into a bowl. Pour on the oil and shake on the seasonings. Toss everything together until the fries are well coated. Spread the fries out in a single layer on the cooking sheet and put them into the oven (top rack) for about 10 minutes. That's just an estimate... home ovens vary so keep an eye your fries and don't let them start to turn black! When the fries look reasonably GBAD, take them out, let them cool a little, and enjoy. They won't taste the same as when you first got them, but they'll still make a tasty snack. And instead of landfill, you can digest the leftovers into easily dissolved, environmentally friendly fecal matter.
Upon reflection, I offer this final advice: forget the french fries. Have some nice broccoli. Steam some kale. Your body's a temple. Stop filling it with french fries, you pig.
- Journal of Agricultural Food Chemistry, Issue 38, 1990. "Deep Fat Frying of Frozen Prefried French Fries: Influence of the Amount of Linoleic Acid in the Frying Medium" by Jean-Louis Sebidio, Anne Bonpunt, Andre Grandgirard, and Jacques Prevost.
- The Science of Good Cooking by the editors at America's Test Kitchen. 2012.
- Townsend Letter for Doctors and Patients, Oct., 2001. "McDonald's French Fries." Klotter, Jule. Academic OneFile. Web. 1 July 2013.
- Townsend Letter for Doctors and Patients, July, 2004. "Fear of frying." Gaby, Alan R. Academic OneFile. Web. 1 July 2013.
- "Reducing acrylamide levels in french fries." September 26th, 2012. http://phys.org/news/2012-09-acrylamide-french-fries.html
- Journal of Food Science, 72: R77–R86. doi: 10.1111/j.1750-3841.2007.00352.x (2007). "Chemistry of Deep-Fat Frying Oils." Choe, E. and Min, D.B. http://onlinelibrary.wiley.com/doi/10.1111/j.1750-3841.2007.00352.x/full
- Chemistry and Physics of Lipids, Volume 165, Issue 6, September 2012. "Chemical alterations taken place during deep-fat frying based on certain reaction products: A review" by Qing Zhang, Ahmed S.M. Saleh, Jing Chen, and Qun Shen.
- "Chemical Reactions of Deep-Fat Frying of Foods," a PowerPoint Presentation from the Ohio State University Department of Food Science and Technology. No year or attribution, but it was very useful and the information seemed to check out.
- "Fats and Oils 2000: Challenges and Opportunities" by Bob Wainwright. June 2000. http://www.foodproductdesign.com/articles/2000/06/fats-and-oils-2000--challenges-and-opportunities.aspx