One day, a boy (let's call him "Jake") decided to cook a lovely breakfast for his special lady friend (let's call her "Katie"). He planned out the perfect meal, and he knew Katie would love it. (They had been dating almost ten years, after all, and Jake knew what foods Katie liked.) He set about to frying a mess of sliced potatoes, a thick slab of maple-flavored bacon, and a panful of the most glorious scrambled eggs ever invented with tomatoes, peppers, green beans and extra-sharp cheddar cheese. Oh, the cheese! The kitchen was filled with a harmonious collaboration of breakfast smells, clanking pots, and banjo music streaming from a laptop on the kitchen table.
Now, being as it was early morning, the potential for wildlife-based distractions was high. And sure enough, Jake looked casually out the window toward his lush garden oasis. What did he see? A ne'er-do-well cottontail rabbit working his way through the rows of green beans that had just begun to ripen. Didn't that varmint know better than to work in the beans while there was still dew on them?! This situation simply could not be allowed to unfold without intervention. Jake grabbed his trusty home-made blow gun and snuck out to the garden, intent on adding some rabbit to the breakfast menu.
Despite his best efforts, Jake got to the garden just in time to see the rabbit disappear through a hole in the fence at the back corner. "So that's how he'd gotten in! Well, I better fix up this fence right away or he'll be in there again by the time I get back to the house," thought Jake to himself. He went to the garage to get the pliers and wire cutters, and a shovel to re-bury the bottom part of the fence. Fifteen minutes later, there's no way that rabbit's getting back in here! Hey, what's that smell?
The potatoes, eggs, and bacon were barely recognizable. The harmonious scents of breakfast had turned to an acrid cloud billowing from the stove; the banjo music from the laptop was now part of a cacophony that also included a roommate's alarm clock (forgotten to turn off before leaving for the weekend) and every smoke alarm in the house. So much for surprising Katie with breakfast in bed! She's probably standing outside in the designated meeting area waiting for the firetruck to show up.
If you've been cooking for more than a few months, you might have a similar story about burning food (maybe without the banjo music and blow guns). After finally convincing the smoke alarms to shut up and airing out the house, the typical follow up question is, "How am I going to get this mess out of these pans?" A quick web search reveals that people have probably been trying to answer this question since the beginning of time.
It seems that there are five commonly attempted routes to get food residues off pots and pans:
Vinegar (acetic acid)
Cream of Tartar
Barkeeper's Friend (oxalic acid)
Soda (carbonic acid)
Baking soda (sodium bicarbonate)
Oven or drain cleaner (sodium hydroxide, or lye)
Add solvent (can't recommend the last one)
Alcohol (as in deglazing a pan with wine)
Detergent such as soap or dishwashing liquid
Nail polish remover (acetone)
Gun-cleaning solvent (hydrocarbon cocktail)
Add oxidant (also can't recommend the last two)
High heat (above 900 °F or 500 °C)
Bleach (sodium hypochlorite)
Add abrasive (and scrub)
Another one that shows up occasionally is rapid heating and cooling of the soiled pan, which is likely an attempt to take advantage of the difference in coefficients of thermal expansion between the metal pot and the carbonaceous film coating it. Since the difference is probably pretty small, and most films are somewhat malleable anyway, this technique is probably not going to be more efficient than the above mentioned ones.
Explanations of why each of these methods work chemically are few and far between, but there are plenty of anecdotal descriptions of each not working in some cases and working like a charm in others. The variety of experiences probably comes from the varying combinations of food types and pans, conditions of forming the residue (temperature, aerobic/anaerobic conditions, cook time), and patience level of the anecdote suppliers. In truth, the exact mechanism of each route probably hasn't been conclusively determined, but we can get some insight by considering what types of chemistry are likely going on.
When food 'burns' on a stove, it normally starts (in our experience) as a few spots on the bottom of the pan that darken. What reactions produce this darkening depends on the type of food in the pan. The food is undergoing a complex process called pyrolysis, which happens in limited oxygen and consists of food molecules simultaneously breaking down into smaller compounds and building up (polymerizing) into larger compounds. The temperature and oxygen availability help to determine what the ratio of 'breaking down' to 'building up' is. Higher temperatures and more oxygen lead to more 'breaking down.'
For the potatoes in the story above, which are mainly composed of starch (a carbohydrate polymer), the reactions are mainly hydrolysis and dehydration reactions, converting the carbohydrate polymers first into simple sugars like glucose (or glucose anhydrides), then dehydration products (oxygen-functionalized aromatic molecules (e.g., phenol) and re-polymerized sugars), and eventually, carbon. For the eggs and bacon, which contain a lot of proteins, an analogous depolymerization and further reaction of the amino acid chains occurs, but incorporates many nitrogen-containing functional groups in the aromatic molecules (e.g. benzonitrile) as well. The bacon grease, which is made up of triglycerides, breaks down first into glycerol and fatty acids; the glycerol dehydrates like a carbohydrate, and the fatty acids can polymerize or break down into smaller hydrocarbons. The proteins and carbohydrates can also react together through the Maillard reaction, which makes that nice golden brown color, but also less healthy compounds. There are also minerals like potassium and calcium carbonates and hydroxides involved, but these components are probably not the main concern in your carbonaceous mess.
|Example of how milk protein can go from a nutritious component of a white sauce to a useless black glob stuck to the bottom of your pan. Fortunately, you can remove it with an alkaline solution (pH > 7, but easier with 9 < pH < 12), e.g. with baking or washing soda. Image credit: Wikipedia.|
The general trend is that the higher the heat and the longer the cook time, the closer to pure carbon you get. Concomitantly, the closer to carbon you get, the less effective each cleaning technique becomes. In fact, pure carbon is typically insoluble in acids, bases, and most common solvents, so the only real options if you get to that point are oxidation and abrasive cleaning. In each case, the problem to solve (ba-da-bum!) is the chemical bonds formed between the food and the metal pan (e.g., iron or aluminum). You have to convince the metal and the food that they should not like each other. The trick is that the different types of food respond differently to the different treatments. For example, the Arizona Department of Health Services discusses that protein residues respond well to addition of basic components, while adding an acid can actually make the problem worse. They offer this guide to clarify what will typically work to clean your pans based on what type of food you were trying to cook.
Why do basic solutions work well for multiple types of food? The key lies in the versatility of the hydroxide ion, OH-. When bases dissolve in water, they release a hydroxide ion into solution, which can react with many types of organic molecules. With fats and greases, the hydroxide ion is able to catalyze the saponification reaction, which essentially turns the grease into soap. (...and which is also why lutefisk can tend to taste soapy). So in this case, you get both a detergent and a dissolution effect from the base. The carbohydrate-based residues can be removed by the saponified grease. The hydroxide ion also interferes with the chemical bonding in protein residues (especially of sulfur-sulfur linkages), which denatures the proteins into water-soluble pieces. Triple whammy!
|Table with recommendations on how to clean up food messes depending on their origin. Adapted from Arizona DHS.|
The acids typically work best on mineral deposits, but can also act as a solvent (think of the acetic acid in vinegar as kind of a hybrid between water and acetone). Both acids and bases may also serve to corrode a thin layer of the metal pan, which can release the food residues. Also for both acids and bases, the rate at which they work their magic goes up with temperature. So if you heat them in boiling water, they'll work faster. Stronger acids and bases work faster than weak ones, so lye will work faster than baking soda, for example. (Ok, it should formally depend on the concentration of each, but practically speaking, lye will work faster).
Oxidation converts the deposits mainly to CO and CO2, using either an aqueous oxidant, such as in bleach or Oxyclean, or using O2 with either a photochemically-induced (bright sunlight) or thermally-induced (high temperatures) reaction. The mechanism of abrasion is pretty self explanatory--you use something harder than the deposit to grind it off. Of course, be careful with abrasives on nonstick surfaces.
|General trends for making burnt food messes and how to clean them up.|
Essentially, what it comes down to is that for most food residues, your most practical (read cheap, safe, and easy) approach will probably be to make a concentrated solution of baking soda in water in the pan with the residue, bring it to a boil, and let it sit overnight. In the morning, try to scrub the residue off. If it doesn't work, add some kind of abrasive (pending the hardness of your cooking surface) and scrub some more. If you still can't get it, go the oxidation route.
Do you have any other techniques for cleaning burned food residue off pots and pans? Tell us about it in the comments section below!