Why infuse olive oil at all?

A good extra virgin olive oil is already a flavoured thing — peppery, grassy, sometimes with a pleasant bitterness at the back of the throat. Infusing it with herbs simply takes that character somewhere else. A bottle of rosemary oil drizzled over roasted peppers, or thyme oil spooned onto a bowl of white beans, does work that no plain oil can. It also makes a genuinely useful gift, and it's a fine way to use up a glut of herbs from the garden.

The good news is that infusing is easy. The less good news is that oil is a friendly home for some rather unpleasant bacteria if you treat it carelessly. The rules aren't complicated, though, and once you know why they exist you'll follow them without thinking.

The one rule that matters: keep water out

Everything that follows comes down to moisture. The bacterium that causes botulism, Clostridium botulinum, needs three things to produce its toxin: an oxygen-free environment, a low-acid environment, and moisture. A bottle of oil gives it the first two. All you have to do is deny it the third.

Dried herbs contain very little water, so an oil infused with them is a low-risk product. Fresh herbs, fresh garlic and fresh chillies carry plenty of moisture, and that moisture settles into the oil where oxygen can't reach it. That's the danger zone.

So the practical rule is this: dried herb infusions can live in a cool, dark cupboard; anything made with a fresh ingredient must be refrigerated and used quickly.

Infusing with dried herbs: the safe, simple method

This is the method to start with, and for most people it's the only one they'll ever need.

• Use roughly 2–3 tablespoons of dried herbs per 500ml of oil. Rosemary, thyme, oregano, bay and dried chilli flakes all work well. Crush them lightly in your palm first to release the oils.
• Warm the oil gently with the herbs in a small pan. Aim for 60–70°C — warm to the touch but nowhere near bubbling. Use a thermometer if you have one. Never let it fry; high heat flattens the flavour and damages the oil.
• Hold that temperature for one to two hours, then take it off the heat.
• Cool the pan quickly by sitting it in a bowl of cold water. Bacteria multiply fastest in warm oil.
• Strain through a fine sieve lined with muslin into a dry jug, then funnel into your bottle.

If you'd rather not heat anything, cold infusion works too: combine the oil and dried herbs in a clean jar, leave it in a dark cupboard for two weeks, shaking daily, then strain. The flavour is softer and more herbal.

Fresh herbs: refrigerate or don't bother

Fresh basil, thyme or rosemary in oil is a lovely idea, but it needs handling with respect. Make small batches, keep them in the fridge and use them within a week. If you want to keep them longer, freeze the strained oil in ice cube trays and drop a cube into the pan whenever you need it.

A useful trick for leafy herbs such as basil or parsley: blanch them for ten seconds, plunge into iced water, then dry them thoroughly in a tea towel before blending with oil. This sets the green colour and reduces the bacterial load, though it does not make the result shelf-stable. It still goes in the fridge.

Never leave a jar of fresh herb oil on the windowsill to "develop". It isn't developing. It's fermenting.

Garlic, chilli and other tricky additions

Fresh garlic is the classic offender and the cause of most home infusing mishaps. A peeled clove dropped into a bottle of oil and left at room temperature is exactly the environment botulism spores enjoy. If you want garlic oil, either use dried garlic granules in a room-temperature infusion, or make a fresh-garlic version that lives in the fridge and is used within four days. Commercial garlic oils are acidified to make them safe; that's an industrial process, not something to improvise at home.

Dried chilli flakes are fine at room temperature. Fresh chillies behave like fresh herbs — refrigerate and use within a week. Lemon zest, dried tomato and fresh ginger all carry moisture, so treat them the same way. Adding salt does not preserve the oil; it simply sinks and looks unattractive. Leave it out.

Bottles, labels and knowing when to throw it out

Start with genuinely clean glassware. Wash bottles in hot soapy water, rinse well, then dry them in an oven at around 140°C for fifteen to twenty minutes. Let them cool completely before filling, and make sure there is not a single droplet of water inside. Fill to within a couple of centimetres of the top and seal.

Label every bottle. Write the ingredients, the date you made it, and "Keep refrigerated" if it contains anything fresh. A strip of masking tape and a pencil is perfectly adequate, and future you will be grateful.

Bin the bottle if you notice any of the following:

• Bubbles rising through the oil, or pressure when you open it
• A sour, fermented or yeasty smell
• Any fuzzy or blue-green growth — mould means moisture got in
• A putty, crayon or old-nut smell, which signals rancidity

Cloudiness in the fridge is normal, by the way. Olive oil solidifies at cold temperatures and clears again as it warms. As a rough guide, dried herb infusions keep for two to three months in a cool cupboard, refrigerated fresh infusions for a week, and frozen cubes for three months. Start with 250ml batches, keep notes on what worked, and you'll soon have a shelf of oils you reach for without thinking.