Rosemary olive oil is one of the most popular herb-infused oils. It is richly aromatic and has a very pronounced savory pine flavor.

Herb-infused oils are a wonderful way to capture the benefits of herbs for many uses. I always have a bottle of herb-infused oil in my kitchen. It’s my go-to ingredient when I roast vegetables, make pasta sauces, or prepare salad dressings. Herb-infused oils are also great as an appetizer with great artisan bread.

However, until recently only commercially produced, infused oils were safe.

The primary concern with infused oils is extremely dangerous and sometimes deadly microorganism Clostridium botulinum, which can cause botulism. That is because the ideal environment for the botulism toxin to be activated is low-acid, low-oxygen, moist, and warm (held at temperatures between 38-118°F/3-48°C). Those are exactly the set of conditions that are created when fresh herbs (low-acid foods containing water) are surrounded by oil (oxygen-free environment) and stored at room temperature (~68°F/20°C).

There are only two scientific methods for safely infusing oils with herbs at home and keeping the oils (with herbs) at room temperature*:

• Infusing oils with dried ingredients: Clostridium botulinum bacteria need water to grow. Dried herbs add no water into the oil, and so bacteria can’t grow. This is the safest way to prepare flavored or infused oils at home.
• Infusing oils with acidified ingredients: researchers at the University of Idaho developed guidelines for infusing oils with fresh basil, oregano, and rosemary. The fresh herbs need to be completely submerged in a 3% solution of citric acid at room temperature for 24 hours to allow the acid to fully penetrate the ingredients and bring the acidity beyond the growth limit for the botulism bacteria. The soaking ratio is one part of fresh herb (stems with leaves attached) to 10 parts of 3% citric acid solution, by weight. Less-concentrated acid solutions or shorter soaking times can result in an unsafe product. After soaking for 24 hours, the acid is drained away and the acidified herbs are ready for infusion. 

Because flavors will continue to intensify with time, it is best to remove the herbs from the oil when it has reached the desired flavor. However, it is acceptable to leave the herbs in the oil, particularly rosemary, for an attractive look.

All that said, there is another (safe!) way to make herb-infused oil. It allows for fresh herbs, doesn’t require acidification, and is really quick and easy. The only caveat is that the herbs need to be strained out out of the oil. 

Tips for Making Herb-Infused Oil (Strained)

Ingredients

• Oil: the best carrier oils are avocado oil and olive oil. These oils have a buttery, sweet profile and are suitable for many applications. Avoid oils with strong, assertive flavor (e.g. coconut oil, sesame oil, or walnut oil).
• Herbs: you can use one herb or a blend of herbs to make herb-infused oil. The choices are endless and completely up to you! I like to keep it simple and only use one herb at a time. Whole, fresh herbs work the best.

How to Make Herb-Infused Oil

There are many techniques for making herb-infused oil, but the stove-top method is the most common for culinary herb-infused oils. 

1. Clean the herb. Choose sprigs of herbs that do not require washing. If you must clean your herbs, rinse them off and dry them thoroughly before using. Any water left on the herbs will impede their contact with the oil.
2. Slightly bruise the herbs. Each cell that makes up each leaf or stem of herbs contains oils. To release the oils, either the temperature of the cell must grow high enough to rupture the cell wall or the cell wall needs to be structurally disrupted. Bruising is an easy thing to do – simply bend or gently rub the leaves so they show a wet crease, denoting the broken cells inside. This will begin to release the wonderful scent of the herb.
3. Heat the oil. Place the herb of your choice into a small saucepan and cover it with oil. Heat the oil over medium-low heat to 150°F/65.5°C. Be careful not to exceed the temperature too much or it can burn the delicate flavors of both the oil and the herb. Heat the oil for about 10 minutes and then turn the heat off.
4. Cool and strain the oil. Allow the infusion to cool to room temperature. Then strain the oil into a clean, dry, ideally dark glass bottle. Since the oil is stored without any flavoring ingredients, there is no danger of Clostridium botulinum bacteria growing. 

Herb-Infused Oil Variations