Smoke curling out of an open window or from under a car hood is a bad omen, and smoky chimneys and cigarettes emit toxins. It often means bad news, and we made a metaphor of it, There’s no smoke without fire. But occasionally it seems almost wholesome.
I used to enjoy it gently billowing over a wok before I threw vegetables on the hot oil to stir-fry. Perhaps smoke from cooking is welcome because the kitchen is a sanus sanctuarium— provided the oil doesn’t catch fire! I wonder if different notions about smoke rarely cross-fertilize because they are processed in separate registers on the right and left sides of my brain. No doubt there’s a lot more mental subtlety involved, but one day when smoke from a wok on the kitchen hob made me cough my right brain woke up to ask why. Wasn’t it a healthy smoke? And, if not, what kind of chemical brew was I creating?
It’s a question that has been studied more by biological chemists than you might think from the few media reports, although some food warriors have posted smoke signals. When I started reading tidbits from scientific journals I understood why.
When any kind of vegetable oil is heated a chain reaction starts to evolve aldehydes like acrolein, 4-hydroxynonenal, and a bevy of other unfamiliar molecules. Heated above the ‘smoke point,’ the rates of reactive aldehyde formation climb ever higher. Some of these molecules are known mutagens that can form adducts with DNA, making them candidate carcinogens. Polyunsaturated fatty acids in oils recommended by the authorities are more vulnerable to oxidation than the monounsaturated types because they have more double bonds. Saturated fat, which by definition have none of them, are much more stable. Suddenly, it dawned on me that home cooking can make my home more polluted than the street at rush hour.
If vapor was the sole danger I could avoid it by turning up the extractor fan, but my meal was bathing in a toxic slush. Since higher temperatures and long cooking times increase the problem, I wondered how many times cooking oil is re-used at the fast food joint where I sometimes buy a burger and fries? I also thought about my neighbor who saves oil from his deep fryer after cooking a Christmas turkey? Perhaps it’s safer dining at home.
Restaurant chains can reduce oil oxidation under a blanket of non-reactive nitrogen, or at least test the quality of any reused oil, but home cooks don’t have that option until there is a mass spectrometer for the kitchen countertop. Meantime, we blithely follow recipes regardless of chemistry, and have to rely on our senses and common sense for safe cooking.
The first precaution is to watch for smoke, which is a warning that the oil has spoiled and should be replaced. The second is to sniff over the pan because an acrid smell points to acrolein in the vapor. And the final test is to taste the oil.
When I compared several cooking oils by heating them to 400 °F. (204 °C.), the extra virgin olive oil and walnut oil were smoking before they reached this temperature, and their fruity notes were gone and tasted bitter. These oils had already gone bad. But refined oils like canola had higher smoke points, and though they started tasteless they remained that way. Lastly, I opened a large bottle of olive oil which had just passed its sell-by date. Since it smelled rancid, oxidation was happening at room temperature and, reluctantly, it had to go. Vegetable oils are a kitchen doctor’s dilemma that Mum never had to worry about. How did it happen?
As heart disease grew to epidemic proportions, the American nutritionist, Ancel Keys, led a charge against saturated animal fat over fifty years ago. He was followed by an omnibus of researchers, medical societies, and government officials who transformed the national diet which we still have (but I won’t say enjoy). The arguments were based on apparently unassailable reason and facts—energy-rich fat makes us fat, and dietary cholesterol deposits in atheromatous plaques. Clinical surveys (occasionally) helped to prove the low fat revolution was sound.
But when animal fats were thrown out of the diet the food industry had to fill the gap. Where would the missing calories come from, and what would replace the fat needed as shortening for pastries, cookies, etc? The industry turned to vegetable oils rich in polyunsaturated fats. This decision seemed a safe bet, especially because they were already marketed as margarines created by chemical conversion of liquid oils to partially hydrogenated trans fats. These products had the dual virtues of remaining solid at room temperature (like butter) and having a high smoke point for deep-frying (like lard or beef tallow). As the demand for more vegetable oils bloomed, farmers sowed fields with ever larger crops of rapeseed and soybean. However the safety of trans fats was not sufficiently scrutinized until recently when were they banned in restaurants, food labeling was mandated and the F.D.A. posted a health warning. The complex mixture of unnatural isomers in trans fats are implicated in a wide range of diseases, and now it’s their turn to be condemned.
The food industry is reluctant to return to animal fat because of the legacy of research stretching back to Dr. Keys and the stamp of officialdom. Although a search for substitutes continues, refined vegetable oils have found a new role in deep frying. They are presented as a healthy choice because they don’t raise the ‘bad’ type of cholesterol (LDL), although a rather weak predictor of heart disease in most people. You seldom read about the downside.
The public continues to be subjected to a vast dietary experiment that fails to conquer cardiovascular disease, which still beats cancer to the top of the C.D.C. list of causes of mortality. Switching from animal fat to trans fat and from trans fat to refined vegetable oils have been hailed, in turn, as advances in public health, but we may have ended up with a new cancer risk, or at least a concern that deserves more research. Moreover, as dietary calories from fat have declined over the years they have been replaced by refined carbohydrates, which are now held responsible for obesity and diabetes emerging in children. It’s hard to grasp the scale of disease, disability, and death that can be traced back to dietary recommendations over the last two generations.
I hope people are thinking more about their diet, and listening less to official prescriptions. The relationship between our food and health is far more complicated than the experts were wont to believe. I hope that people fret less about fat in their diet than about the environment in which crops are grown and farm animals are husbanded. There are far too many vested interests in our food choices. Mum’s cooking probably wasn’t as unhealthy as the next generation assumed when they squirmed at the larded memory. I’m sure her pastry, cakes, and cookies tasted better than those served today.
I have turned the tables on cooking fats and oils at home. Margarine and polyunsaturated spreads are out, butter is in. Butter can be good even at high temperatures if the solids and water are removed by clarification. And if lard ever reappears on supermarket shelves I may pull out one of Mum’s old recipes.
For high temperature cooking, I often use refined oils, like safflower, which have high smoke points; olive oil and tasty nut oils are reserved for salad dressing and other cool uses. The pros and cons of different oils otherwise confuse me because the composition of healthy fatty acids (usually meaning omega-3s) and antioxidants vary with the source, season, and storage conditions. Of course, we choose those that taste best (the unprocessed kinds), but I am more focused now on safe cooking to avoid nasty radicals generated by oxidation. And I am more careful about storage conditions, meaning for shorter periods in the cool and dark. I even squeeze drops of vitamin E from capsules into bottles of vegetable oil to help preserve them.
These concerns about dietary fats are new to me, though they shouldn’t be. As long ago as the late 1950s, Denham Harman (1916-2014), a gerontologist at the University of Nebraska, was expressing doubts about the wholesale adoption of vegetable oils in the national diet. As a former chemist at Shell Oil, he knew that unsaturated fats are unstable and susceptible to oxidation. In one of my books published in the 1990s (Cheating Time), I mentioned his free radical theory of how oxidized lipids cause cell aging and disease. But the penny didn’t drop in a lifetime of cooking until I leaned over a smoky wok one day.
[In coming weeks I will post about sugar and salt]
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