We burn food routinely for flavour — a crispy grilled (or barbecued) lamb chop or seared steak for example. Mostly it is animal proteins that we treat this way. But not entirely — burnt toast for example. Still, we deliberately burn animal proteins but generally shy away from this for vegetables.
However, there is a trend these days to explore burning vegetables. Why? To add (mostly) bitter notes and textural contrast to a vegetable and lift its flavour profile. For example, a cabbage has a familiar and comforting flavour but you can take that in a new direction if you do something like this to it:
It has been basted with kelp-infused butter while burning in the pan on one side, served with thai basil and a squirt of vinegar (to adjust the butter). The flavours added are savoury (kelp/MSG), bitter (burnt), aromatic (basil), sharp (vinegar) and unctuous (butter). The recipe comes from Christian Puglisi. Even Michelin-starred restaurants are serving burnt vegetables these days.
Another example. Take a pile of herbs and vegetables and cook in a high oven until dehydrated, burnt, smoking and turned to ash. Powder the remains, sieve and use to coat a fillet that was cooked sous vide to tenderness. The powdered ash supplies smokey-seared flavours to the fillet, which in turn does not need to be seared (keeping it uniformly tender). An adaption of a Massimo Bottura recipe.
Burning protein has always carried a health concern, but we do it anyway because it’s tasty. The two main concerns are Heterocyclic amines (HCAs) and Polycyclic Aromatic Hydrocarbons (PAHs).
HCAs are created by the reaction of amino acids in muscle proteins and creatine (a chemical also found in muscle) under high heat. They can form on the surfaces of meats cooked to high temperature (grilled, fried, barbecued) and in pan drippings (roasts, pan-fries) that are often then used to make a sauce. Eating vegetables (and fruits) with the meat might be protective (they can bind to HCA and possibly lessen potential damage).
PAHs are formed by incomplete combustion of hydrocarbons (principally from reactions among the carbon atoms). Hydrocarbons are not a significant component of meat flesh, but the fat is high in carbon (fatty acids are essentially chains of carbon atoms with hydrogens attached). Even lean meat, if grilled over a wood-fired barbecue (flame-grilled), can accumulate PAHs on its surface from the burning wood (carried to the meat by smoke and rising hot air). Fat dripping from fatty meat can combust on contact with the coals and form PAHs that are also carried to the meat.
HCAs are not a concern with burnt vegetables, because amino acids and creatine are not significant components of vegetables (or fruits), however as vegetables are primarily composed of carbohydrates, PAHs may be present.
To keep a perspective, PAHs are one of our most widespread organic pollutants. They are released by burning any fossil fuel (oil, coal, gas) either industrially for power or by motor vehicles (or related engines), or by burning wood or other plant matter (barbecues, wood fires, bushfires, tobacco, incinerators). They are found in air, water, soil and the food-chain. The main concern with PAHs is prenatal exposure and childhood development, rather than effects on the mature adult. The contribution of PAHs from occasionally consuming burnt food as an adult needs to be weighed up against these chronic sources of contamination.
Finally, it is also about dosage. The renaissance physician and occultist Philippus Aureolus Theophrastus Bombastus von Hohenheim (1493–1541), who thankfully adopted the shorter name Paracelsus, is regarded as the father of toxicology for his axiom: “The dose makes the poison”. This is still the driving principle for health regulators. Much of what we eat is toxic at some dose, including water (consumed in sufficient quantity it upsets our chemical (ionic) balance and is fatal) and, most vegetables come with toxins that they use for chemical defence. This includes the staples that we have eaten for centuries.
PS: PAHs are everywhere, including outer space — mostly from dying stars. There is so much of it about that it can cling to astronauts’ suits after a space walk — they have reported the smell of burned or fried steak when getting out of their suits on return to the International Space Station (the ‘A’ in PAH, after all, stands for ‘Aromatic’). Apparently the moon smells like spent gunpowder. PAHs turn up on comets, space dust and meteors, and may have had a role in the very first stage of life on Earth.