The short version

Smoke flavour comes from pyrolysis, the thermal breakdown of wood between roughly 200 and 500 °C, which releases mostly phenolic compounds from lignin. Wood that smoulders in that window perfumes your food. Wood that flares up, or that suffocates, just dirties it. Two numbers frame everything else in this article. First, charcoal-grilled vegetables carry considerably more polycyclic aromatic hydrocarbons (PAHs) than the same vegetables raw: a 2025 study measured between 7.5 and 21.8 µg/kg of PAH4 depending on the vegetable, roughly 25 to 50 times the levels the same team had found in raw produce. Second, an acidic marinade genuinely changes that: red grape vinegar cut total PAH4 formation by an average of 75 % in the same study, cider vinegar by 68 % and lemon juice by 52 %.

So a marinade is not only about flavour, and the wood species matters less than how the wood burns. The rest is method: dry the surface of your food, aim for thin bluish smoke rather than thick white smoke, hold the temperature, and never bring a still-warm barbecue into an enclosed space.

What smoking actually is, chemically

Smoking exposes food to the products of incomplete wood combustion. Heat breaks down the three main polymers in wood. Cellulose and hemicellulose give organic acids, ketones, aldehydes and furans, much as heated sugar does. Lignin produces the phenols that carry the identity of smoke.

A gas chromatography–olfactometry analysis published in Molecules in 2025 characterised condensed hardwood smoke. The authors identified 27 odorants with a meaningful flavour dilution value, 19 of which exceeded the perception threshold at a 0.3 % dosage. The most powerful were 2,3-butanedione (buttery), guaiacol (clove, vanilla), 4-methylguaiacol (toasted, vanilla, ashy), 3-ethylphenol (ashy), 4-methylsyringol (burnt, clove) and butyric acid (cheesy). The sensory panel described eight dominant attributes, among them ashy, burnt-sulfurous, creosote, green-woody and spicy-sweet.

That detail has a direct practical consequence. The pleasant notes and the unpleasant ones come out of the same wood; what changes is the proportion. A cold, starved fire pushes towards creosote and acridity. A fire that is too fierce burns off the phenols before they ever reach the food and leaves only soot. The useful window is narrow, and holding it is the whole skill.

The part most articles leave out: PAHs

PAHs form during incomplete combustion and settle on the surface of food along with the smoke. The US National Cancer Institute makes a point that matters directly to vegetarians: heterocyclic amines, the other major family of concerning compounds from high-temperature cooking, form from muscle creatine and are therefore not found in significant amounts in foods other than meat. In vegetarian cooking that problem disappears. PAHs do not, because they come from the smoke itself rather than from the food.

The key study here was published in Food Science & Nutrition in 2025 by Sibel Kacmaz Ozcetin and Levent Artok. Six common vegetables were charcoal-grilled and then assayed for PAH4, the indicator group chosen by EFSA (benzo[a]anthracene, chrysene, benzo[b]fluoranthene and benzo[a]pyrene). Unmarinated controls ranged from 7.52 µg/kg for onion to 21.79 µg/kg for green pepper, with potato at 16.39 µg/kg and aubergine at 10.95 µg/kg.

Two caveats matter. These figures come from direct charcoal grilling with the food 20 cm from the coals, not from low-temperature indirect smoking. And EU Regulation 2023/915 sets maximum PAH levels for commercial smoked meat and fish (2.0 µg/kg benzo[a]pyrene, 12.0 µg/kg PAH4), but nothing of the sort applies to what you cook at home. No inspector will ever check your grill. The method is entirely on you.

Marinade as a technical lever

This is the most useful finding in that study. The same vegetables, marinated before grilling, showed a clear drop in total PAH4:

• red grape vinegar: 75 % average reduction (68 to 82 % depending on the vegetable);
• cider vinegar: 68 % (62 to 72 %);
• lemon juice: 52 % (47 to 56 %);
• garlic powder: 34 %;
• black pepper: 30 %.

The amounts used were modest: 2 ml of vinegar or lemon juice, 1.5 g of garlic powder or 0.6 g of pepper per 100 g of vegetables. The authors link the effect to the phenolic content and radical-scavenging capacity of each ingredient, with red grape vinegar showing both the highest phenolic content and the strongest antioxidant activity. For lemon juice they also point to the lower pH, which slows some Maillard pathways.

That mechanism remains a plausible explanation rather than a demonstrated causal chain, and the study covers six vegetables in a single set-up. But the measurement itself is clear, and it echoes what had already been observed on other grilled foods. In practice: put vinegar in your barbecue marinades, and not only for taste. One tablespoon of red wine vinegar per 300 g of vegetables is already in the tested range.

Choosing wood without kidding yourself

Barbecue writing tends to sort woods by presumed gentleness: mild fruit woods on one side, oak and hickory on the other. That ranking describes the aroma profile, not the contamination level. A 2024 study in Foods compared three hot-smoking woods: beech, plum and oak. Plum, a fruit wood, produced the highest PAH levels, ahead of oak, with beech lowest. The authors worked on fish, so the absolute values do not transfer to vegetables, but the ordering is worth remembering: a wood that smells sweet and fruity tells you nothing about how cleanly it burns.

What matters more, in order:

• Wood moisture. Dry wood burns cleaner. Commercial chips sit around 10 % moisture.
• Air supply. Combustion starved of oxygen smokes a lot, and smokes badly.
• Wood cleanliness. No pallets, no plywood, no chipboard, nothing painted, varnished or glued: adhesives and treatments do not volatilise politely.
• Species, last. Conifers such as pine and spruce give resinous, acrid smoke.

For vegetables and tofu, beech remains the safest default: neutral, widely available, forgiving. Apple and cherry bring rounder notes. Oak suits seitan and tempeh, which can take heavier smoke. Mesquite and hickory are strong enough to flatten any vegetable; if you want them, use a small handful blended into beech rather than a full load.

The soaked-chips myth

Soaking wood chips is the single most repeated piece of barbecue advice. The physics only half supports it. Until the water has boiled off, the wood is capped near 100 °C and cannot reach the 200 to 500 °C pyrolysis range where aroma compounds form. Soaking therefore does not produce more smoke: it produces steam first, then smoke, later. On a covered barbecue that has already stabilised, that delay is sometimes convenient. It is not magic, and it does not replace setting the vents correctly.

No published controlled trial establishes that soaking chips never helps in domestic barbecues. What is clear is that if your smoke is white, thick and sharp, the cause is almost always too little air or too much wood, not too little water.

Temperatures: three regimes, three uses

Hot smoking, 90 to 130 °C. The normal working range on a covered barbecue. Food cooks and takes smoke at the same time. Allow 20 to 40 minutes for vegetable slices, 30 to 45 minutes for centimetre-thick slices of pressed tofu.

Warm smoking, 30 to 60 °C. Useful for flavouring something that will be cooked conventionally afterwards, or for a plant-based cheese that would melt. Needs a smoke generator rather than coals.

Cold smoking, below 25 to 30 °C. It flavours and does not cook. One point rarely spelled out: food sitting for hours between 8 and 63 °C is outside the safe range implied by UK chilling guidance, which puts the fridge at 0 to 5 °C. Cold smoking moist food should therefore be done in cool weather, kept short, and followed immediately by refrigeration. Our article on cold smoking for tofu and vegetables covers the equipment.

One note about potatoes and root vegetables: above 120 °C, a starchy low-moisture food forms acrylamide. The Swiss food safety office classifies it as probably carcinogenic to humans and sums up its advice in one phrase, golden rather than charred. On a barbecue that means par-boiling potatoes and finishing them in the indirect zone, instead of letting them blacken over the coals.

Preparing food: water is the main obstacle

Smoke compounds deposit on a surface, and a streaming wet surface pushes them off. Vegetarian ingredients happen to be very wet. According to the USDA FoodData Central database, raw aubergine holds 92.3 g of water per 100 g, portobello mushroom 92.8 g, and even nigari extra-firm tofu 82.3 g alongside 9.98 g of protein and 5.26 g of fat.

Hence the routine:

1. Press the tofu. Twenty to thirty minutes under a one-kilo weight, between two tea towels. Unpressed extra-firm tofu smokes poorly and breaks.
2. Salt in advance. Half a teaspoon of salt per 400 g of aubergine or courgette, thirty minutes, then blot. Water drawn out now is water that will not have to evaporate on the grill.
3. Dry the surface before the smoke. Five minutes on the hot grate, uncovered, before adding wood.
4. Oil lightly, not generously. Fat dripping onto coals pyrolyses and sends PAHs back up onto the food. That is exactly the mechanism the National Cancer Institute describes for grilled meat, and olive oil behaves no differently.
5. Cut wide slices rather than tiny cubes. More surface exposed to smoke, less risk of drying out.

Tempeh benefits from ten minutes of steaming before marinating: it softens and absorbs better. Seitan, being very neutral, takes smoke faster than tofu, so cut the time by a third to avoid bitterness. If you make your own, our guide to homemade seitan without a machine gives the ratios.

Three marinades worth making

The quantities below cover 400 to 500 g of food and keep the acid fraction within the range tested in the PAH study.

Basic acidic marinade. 2 tablespoons red wine vinegar, 1 tablespoon olive oil, 1 crushed garlic clove, 1 teaspoon sweet paprika, salt, pepper. This is the version that ticks both the flavour box and the PAH box.

Soy and maple marinade. 2 tablespoons soy sauce, 1 tablespoon maple syrup, 1 tablespoon cider vinegar, 1 teaspoon sesame oil, grated ginger. The sugar caramelises fast, so keep this one for indirect smoking, well away from the coals.

Lemon and herb marinade. Juice of half a lemon, 1 tablespoon oil, thyme, rosemary, a pinch of chilli. Particularly good with mushrooms and fennel.

In all three cases allow at least thirty minutes in the fridge, two hours for tofu and tempeh. Past twelve hours the acid starts to break down the texture of tender vegetables. For timings and different substrates, see our complete guide to marinating tofu and tempeh.

On smoked paprika, one regulatory detail is worth knowing: Regulation 2023/915 sets maximum PAH levels for dried spices but explicitly excludes smoked Capsicum from that limit. Smoked paprika is therefore not regulated like other spices on this parameter. That is not a reason to avoid it, but it is a reason not to treat it as the obviously cleaner alternative to real smoke.

As for liquid smoke, EFSA published an opinion in November 2023 on the eight smoke flavourings on the EU market whose authorisation was up for renewal: based on the available evidence, the experts could not rule out genotoxicity concerns for any of the eight. That is a statement of uncertainty rather than a ban, but it is enough to stop presenting bottled smoke as the obviously wiser option.

Setting up a charcoal barbecue

1. Light half a chimney of charcoal and wait until the coals are covered in grey ash.
2. Push all the coals to one side of the kettle. That is the indirect set-up.
3. Put a water pan under the grate on the empty side. It stabilises the temperature and limits drying.
4. Add a small handful of chips, not a full one. Too much wood at once means white smoke.
5. Close the lid with the bottom vent half open and the top vent wide open above the food, so the smoke is drawn across.
6. Wait until the smoke turns from opaque white to translucent blue before adding food. That change is visible to the naked eye and does more for the result than the species of wood.
7. Place the food on the side away from the coals. Turn halfway through.
8. Top up the chips once at most over a 40-minute cook.

On an electric barbecue the logic is identical, but there are no coals to ignite the chips: you need a smoker box sitting on the element. Our comparison of electric barbecues for greener grilling covers compatible models.

Safety: the two real risks

Carbon monoxide. This is the most serious risk and the most casually ignored. The NHS explicitly lists using a barbecue indoors among the causes of carbon monoxide build-up, a gas that is both colourless and odourless. A barbecue that has stopped flaming still produces it. Never bring a barbecue, even one that looks out, into a tent, a garage, an enclosed porch or a flat. The symptoms to know are headache, dizziness, nausea, weakness and confusion, and they ease when you go outside.

Storage after smoking. A smoked vegetable is not a preserved vegetable. Home smoking reliably lowers neither water activity nor pH. The WHO notes that foodborne botulism cases are mostly linked to improperly processed home preserves, particularly low-acid foods. In practice: never store smoked vegetables in oil at room temperature. Refrigerate at 0 to 5 °C in a clean container, eat within two or three days, or freeze.

None of this makes a vegetarian barbecue dangerous. These are procedural habits, the same order of thing as washing a chopping board.

Where the dish fits in a meal

A plate of smoked barbecue is not a nutritional problem in itself, but it is often unbalanced by construction: plenty of fat, plenty of salt, and fewer vegetables than anyone counts. Swiss recommendations for adults set five portions of fruit and vegetables a day, three of them vegetables, with one portion at 120 g, plus one daily portion of a protein-rich food, meaning 120 g of plain tofu, tempeh or seitan. The WHO sets at least 400 g of fruit and vegetables and at least 25 g of fibre a day for everyone over ten.

Translated to a barbecue: two generous vegetable skewers per person already cover two portions, one thick slice of smoked tofu covers the protein portion, and there is room left for a cold pulse salad rather than a third piece of bread. If you are planning the whole menu, our article on organising a vegetarian barbecue gives quantities per guest, and the complete guide to plant proteins sets out the equivalences.

Three preparations that work

Paprika-smoked tofu. 400 g extra-firm tofu, pressed 30 minutes, cut into 1.5 cm slices. Marinade: 2 tablespoons cider vinegar, 1 soy sauce, 1 maple syrup, 2 teaspoons sweet paprika, 1 teaspoon mustard. Two hours in the fridge. Indirect smoking at 110 °C over beech chips, 35 minutes, turned once. The tofu should stay yielding under a finger.

Whole portobello mushrooms. Four large caps, gills up. Brush with a tablespoon of red wine vinegar and one of olive oil, plus garlic and thyme. Twenty minutes to rest, then 25 minutes indirect at 120 °C with cherry wood. They release a lot of liquid, so keep them gills up rather than losing it.

Smoked aubergine to crush. Two whole aubergines, pricked with a fork, placed indirect at 130 °C over beech for 45 minutes, until the flesh gives. Peel them hot, crush with tahini, lemon juice and salt. Here the smoke replaces charring over a naked flame and gives a far softer texture.

Common mistakes

1. Dumping a full handful of chips in at once and ending up with white, bitter smoke.
2. Adding food before the smoke has settled into thin blue.
3. Oiling heavily, which multiplies the drips onto the coals and therefore the PAHs.
4. Smoking waterlogged vegetables that were never drained or dried.
5. Lifting the lid every five minutes, which drops the temperature and restarts a fierce burn.
6. Treating fruit wood as a guarantee of mildness, when combustion matters more than species.
7. Using salvaged wood of unknown origin.
8. Letting raw potatoes blacken on the grate instead of par-boiling them.
9. Storing smoked vegetables in oil at room temperature.
10. Bringing the barbecue under a closed shelter because it started raining.

Frequently asked questions

Does smoking preserve vegetables? No. Industrial smoking combines salting, drying and smoke. Forty minutes of home smoking delivers none of those three to a useful degree. Treat the result as a fresh cooked dish.

Is smoky flavour bad for you? The question is badly framed. The flavour comes from phenols, the compounds of concern are mainly PAHs, and the two are not mechanically tied. You can get a lot of flavour with little deposit by managing the fire and marinating, and you can get little flavour with a lot of soot by choking the fire.

Do I need a dedicated smoker? Not to start. A kettle barbecue with a lid and two adjustable vents covers every case described here. A vertical smoker becomes interesting once you regularly smoke for more than two hours.

How long does home-smoked tofu keep? Two to three days in the fridge at 0 to 5 °C in a sealed container. It freezes well; the texture turns slightly spongier, which is no problem in a stir-fry.

Can I smoke on a balcony? The barbecue has to stay in open air, never under a closed canopy or indoors. Smoke bothers neighbours far more than plain grilling does, and many building rules address this specifically.

Does smoked paprika replace smoking? It gives a smoky note, not the same one. Real smoke deposits dozens of different phenols and acids; paprika delivers a narrower profile. In a tahini sauce with smoked spices it does the job perfectly; on a slice of tofu you can hear the difference.

Sources

• Kacmaz Ozcetin S., Artok L., “Effect of Marination on the Formation of Polycyclic Aromatic Hydrocarbons in Grilled Vegetables”, Food Science & Nutrition, 2025: europepmc.org/article/MED/40630425
• Savin R.-L. et al., “Influence of Fish Species and Wood Type on Polycyclic Aromatic Hydrocarbons Contamination in Smoked Fish Meat”, Foods, 2024: europepmc.org/article/MED/38928732
• Vazquez T., Tello E., Peterson D. G., “Identification of the Key Aroma Compounds in Condensed Hardwood Smoke”, Molecules, 2025: europepmc.org/article/MED/39942823
• EFSA, smoke flavourings topic page, November 2023 opinion: efsa.europa.eu
• Regulation (EU) 2023/915 on maximum levels for certain contaminants: eur-lex.europa.eu
• National Cancer Institute, “Chemicals in Meat Cooked at High Temperatures and Cancer Risk”: cancer.gov
• Swiss Federal Food Safety and Veterinary Office, acrylamide factsheet: blv.admin.ch
• Swiss Federal Food Safety and Veterinary Office, dietary recommendations for adults: blv.admin.ch
• WHO, “Healthy diet” fact sheet: who.int
• WHO, “Botulism” fact sheet: who.int
• NHS, carbon monoxide poisoning: nhs.uk
• GOV.UK, “How to chill, freeze and defrost food safely”: gov.uk
• USDA FoodData Central, composition values: fdc.nal.usda.gov