How to infuse oils, butters and vinegars with herbs, spices, garlic and chilli, hot or cold, and how to avoid the real risk of botulism when fresh garlic or herbs sit in oil.
Safety first¶
⚠️ Botulism risk
Fresh garlic, fresh herbs, fresh chillies and other moist, low-acid ingredients can carry spores of Clostridium botulinum. Submerged in oil, they sit in an airless environment where, at room temperature, the spores can germinate and produce botulinum toxin, which is odourless, tasteless and potentially fatal. Heating the oil does not make it safe to store; ordinary cooking temperatures do not destroy the spores.
For home-made oils containing fresh garlic, fresh herbs or other fresh, moist ingredients:
- Refrigerate immediately and use within 4 days (the figure widely given by food-safety authorities such as US extension services), or
- freeze in small portions and use straight from frozen, or
- acidify the ingredients first using a tested recipe, which is how commercial garlic-in-oil products are made safe.
Never leave these oils at room temperature, and never give them as gifts on the shelf.
Oils infused with thoroughly dried herbs, dried chillies or whole dried spices carry much lower risk because there is little water for bacteria. Keep them in a cool, dark place and use within a month or two, before the oil turns rancid.
Choosing an oil¶
Use a neutral oil (sunflower, rapeseed, groundnut) when you want the flavouring to dominate, or a mild olive oil where its fruitiness helps. Strong extra virgin olive oil can overwhelm delicate infusions and turns bitter if heated hard.
Hot infusion (quick)¶
- Put the oil and aromatics in a small pan. Allow roughly 250 ml oil to 4–6 sprigs of rosemary or thyme, or 6–8 sliced cloves of garlic, or 2 tbsp chilli-flakes.
- Warm gently to about 70–80°C, well below frying heat: small bubbles form around the aromatics but they should not sizzle hard. Hold for 5–10 minutes.
- Take off the heat and leave to steep until cool, 30 minutes to 2 hours.
- Strain through a fine sieve or muslin into a clean bottle.
- Refrigerate if any fresh ingredients were used, as above.
Garlic cooked until golden gives a sweeter, roasted flavour; stop before it browns or it turns bitter.
Cold infusion (slow)¶
Suitable only for dried ingredients. Combine dried herbs, dried chillies or whole spices with oil in a clean jar, seal, and leave in a cool, dark place for 1–2 weeks, shaking occasionally. Strain and bottle.
Flavoured butters¶
Compound butters are softened butter beaten with chopped herbs, garlic, spices, citrus zest or chilli: parsley and garlic for garlic bread, tarragon and shallot for steak, chipotle and lime for corn. Roll in baking paper into a log and chill.
Butter made with fresh garlic and herbs is a perishable fridge item: keep it chilled and use within about a week, or freeze slices for up to 3 months. For clarified, spice-infused butter, see tempering-spices and the spiced clarified butter niter kibbeh of ethiopian-cuisine.
Flavoured vinegars¶
Vinegar's acidity makes it a far safer medium than oil. Use a vinegar of at least 5% acidity (most commercial wine and cider vinegars qualify).
- Pack a clean, sterilised jar loosely with the herbs or spices: tarragon (the classic), chives and their flowers, rosemary, dill, thyme, chillies, peppercorns or raspberries.
- Warm the vinegar to just below simmering (about 80°C) and pour over. Heat speeds extraction; cold vinegar works too but slower.
- Seal with a non-metallic lid (vinegar corrodes metal) and leave in a cool, dark place for 2–4 weeks, tasting weekly.
- Strain, bottle and label.
Strained herb vinegars keep for several months; store in the fridge if they contain garlic, as a precaution.
🛟 Cloudy or bitter oil
Cloudiness in the fridge is just the oil solidifying and clears at room temperature. Bitterness means the aromatics were overheated; keep below frying temperature next time. Any oil that smells off, fizzes, or has a bulging lid must be thrown away without tasting.