Rosemary oil is olive oil that has sat with rosemary at a gentle heat until it tastes of it. Here is how to make rosemary oil in one line: dried rosemary in oil, held around 100 °C for 2 to 3 hours, strained, cooled, and kept in the fridge for up to 4 days.
That is the whole recipe. The rest of this page is how to get it right the first time: which rosemary, which oil, how hot, and the one storage rule you cannot skip.
This is rosemary oil you eat. It goes on roast potatoes, focaccia and white beans.
What you need
- Dried rosemary. Up to 30 g for a 350 ml batch. Start nearer half that if you want it subtle.
- 300 to 330 ml of oil. Olive oil if you want the oil to taste of itself too. Sunflower oil if you want only rosemary.
- A fine sieve or a filter, and a clean glass bottle or jar with a lid.
- A kitchen thermometer, if you use the hob.
- Masking tape and a pen for the date.
The 30 g and 350 ml figures come from the NOIDS Herb Lab, which takes up to 30 g of herbs in a 350 ml glass. They work as a ratio in a pan too.
Dried or fresh rosemary
Use dried rosemary for your first batch.
Fresh sprigs carry water. Water and hot oil do not mix: the oil spits, and the water stays in the finished batch. Dried rosemary gives you a cleaner oil with a stronger, more resinous flavour.
If you only have fresh rosemary, strip the needles, spread them on a tray and let them dry out fully before they go near the oil. Wash them first if they came from the garden, and dry them completely after.
Either way, dried or fresh, the storage rule is the same. More on that below.
Which oil
Two good choices.
Olive oil. Rosemary and olive oil taste like they were made for each other. Use an everyday extra virgin, not your best bottle. A long infusion takes the edge off its peppery finish.
Sunflower oil. Neutral. The rosemary is all you taste. Better if you want the oil for dressing or for frying an egg.
Avoid oils with a strong flavour of their own, such as toasted sesame. They fight the rosemary and win.
How to make rosemary oil on the hob
You need a heavy pan, the lowest flame you have, and a thermometer.
- Put the rosemary and the oil in a cold pan.
- Warm it slowly. Aim for around 100 °C and do not go over.
- Watch the rosemary. A few lazy bubbles around the needles are fine. A steady fizz means it is frying. Turn the heat down or lift the pan off for a minute.
- Hold it there for 2 to 3 hours, stirring now and then.
- Take it off the heat and let it cool until you can hold the pan.
- Strain through a fine sieve into a clean bottle. Press the rosemary gently to get the last of the oil out.
- Close it, date it, and put it in the fridge.
The hard part is step 3. A hob drifts. Leave the kitchen for ten minutes and you can come back to brown needles and a bitter oil. If that happens, there is no rescuing it. Start again with less heat.
How to make rosemary oil in the NOIDS Herb Lab
The NOIDS Herb Lab does step 3 for you. It holds 100 °C within 1 °C for the whole run, then beeps.
- Put the dried rosemary in the glass, below the MAX mark, and pour in the oil.
- Fit the sleeve and the lid, and press the glass into the base until it touches the probes.
- Turn the dial to Oil. It beeps after 3 seconds and starts heating.
- The programme runs 120 to 180 minutes. You can leave the room.
- When it beeps again, lift the glass out, fit the filter set and strain.
- Cool, bottle, date and refrigerate.
The glass and the filter set go in the dishwasher afterwards.
Hob or machine: what changes
| Hob | NOIDS Herb Lab | |
|---|---|---|
| Temperature | Drifts with the flame | Held within 1 °C |
| Your time | Watching for 2 to 3 hours | Set it and leave |
| Straining | Sieve by hand | Filter set in the glass |
| Risk of frying the herbs | Real | Low |
| Cost | A pan you own | A machine |
If you make herb oil once a year, the hob is fine. If you make it every couple of weeks, the hours of watching add up. Browse the rest of the herbal extraction shelf for the full kit.
How long rosemary oil keeps
This is the rule that matters, so here it is plainly. Keep homemade rosemary oil in the fridge and use it within 4 days. For longer, freeze it.
Oregon State University Extension gives this rule for homemade flavoured oils, fresh or dried herbs alike. Oil shuts out oxygen, and herbs are a low-acid food. Together, at room temperature, that is the setting where Clostridium botulinum can grow. The heat of the infusion does not make the oil safe to keep on the counter.
Three habits make it work:
- Write two dates on the tape: the day you made it and the day four days later. OSU suggests exactly that.
- Make small batches. 350 ml is about right for four days of cooking.
- Freeze the rest. Pour cooled oil into an ice cube tray, freeze, then tip the cubes into a bag. One cube dresses a tray of potatoes.
Olive oil goes cloudy and thick in the fridge. That is normal. Take it out a little before you need it, or warm the bottle in your hands.
Never leave a decorative sprig of rosemary in the bottle on a sunny shelf. It looks good in a photo and breaks every rule above.
What to cook with rosemary oil
- Roast potatoes. Toss them in the oil before they go in, and spoon a little more over as they come out.
- Focaccia. Pour it into the dimples before baking.
- White beans. Warm a tin of cannellini in rosemary oil with a pinch of salt and a squeeze of lemon.
- Lamb. Brush it on before the grill.
- Soup. A few drops on top of a tomato or a bean soup, at the table.
- Fried eggs. Fry them in it, then pour the oil from the pan over them.
The mistakes that ruin a batch
Too hot. Brown needles mean bitter oil. Stay at or below 100 °C.
Wet rosemary. Water spits and stays. Dry it first.
On the counter. Fridge, 4 days, or the freezer. No exceptions.
One huge batch. You cannot use a litre in four days. Make 350 ml and freeze what you will not use.
Store the dried rosemary properly
Dried rosemary loses its smell to air, and that smell is the flavour you are paying for. Keep it in a jar that seals. A Terp Seals glass jar pumps the air out through its lid and shows how many days the jar has been closed, so you know how old the batch is.
FAQ
How long does homemade rosemary oil last?
Four days in the fridge. For longer, freeze it in portions. Do not keep it at room temperature.
Can I use fresh rosemary?
Yes, but dry it first. Fresh sprigs carry water into the oil, which spits in the pan and stays in the batch. The 4 day fridge rule applies either way.
What temperature should rosemary oil be infused at?
Around 100 °C, for 2 to 3 hours. Hotter fries the needles and turns the oil bitter.
Why does my rosemary oil taste bitter?
It got too hot, or the rosemary was in too long. Brown needles are the sign. Start a new batch with less heat.
Which oil is best for rosemary oil?
Olive oil if you want the classic flavour. Sunflower oil if you want the rosemary to taste of nothing but rosemary.
