Making SAFE Herbal Infused Oils

Part 8 of the Series, Methods to Preserve Your Herbal Harvest

Series HeaderPlease note: This instructional is from the most current information put out by the USDA National Center for Home Food Preservation. It has therefore been tested for the safest way to make these infused oils. I urge you to follow all the instructions and precautions that are given here for the safety of yourself AND your loved ones.

Flavored oils have been a popular and much discussed topic of conversation for many years. To my mind, there is just nothing better than pulling a fresh loaf of bread out of the oven, grabbing my herbs and spices, mixing with a bit of olive oil, and dipping in! However, making infused oils ahead of time for storage has NOT been something I have embraced because of the potential for food-borne illness. This has been a controversial topic since it is only borderline safe at best, even when refrigerated.

Oil infusions are easy to make, but they have a real potential to become unsafe when not properly prepared and stored. The problem of safety lies in the fact that a number of cases of botulism, a debilitating and potentially fatal form of food poisoning, have occurred as a result of improperly stored, home-prepared, garlic- or herb-in-oil mixtures. Garlic and herbs can be a source of Clostridium botulinum, widespread bacteria that produce the botulism toxin under certain conditions.

The danger with food packed in oil is that the oil creates an airless condition; (fresh food and herbs have the higher pH AND the higher water activity) so the toxins can be produced making the fresh food dangerous. Botulism requires a pH greater than 4.6, an anaerobic environment and a water activity of 0.92 or greater to produce toxins. Essentially if you have pieces of fresh low-acid product surrounded by oil, the oil creates a perfectly insulated little biosphere for the development of the toxins. It’s not the oil per se, it’s the vegetable matter in it.

When garlic or herbs are placed in oil, the low-acid, oxygen-free environment favors the growth of these bacteria. Commercially available oils flavored with garlic and herbs either have been acidified to prevent the growth of bacteria or they contain specific levels of microbial inhibitors. (When purchasing from boutique or small scale producers, check the label or ask the producer to ensure this required safety treatment has been applied.)

Because I have learned to cook in my kitchen with safe, shelf stable, preservative-free food products (using my refrigerator only as a vehicle for storing my week-to-week foods), flavored oils have not really been a storage option for me. To be able to store flavored oils I have to know that they are safe for my family and friends.

Enter in the University of Idaho Extension team in the Spring of 2014. They developed a safe, tested method of making flavored oils, researching, testing, and publishing a procedure for consumers to acidify garlic and herbs for making flavored oils safely at home!  The team conducted extensive research on the acidification properties of fourteen different cultivars of garlic and of three herbs (basil, rosemary, and oregano), even growing some of the plants themselves. My new heroes!

The research identified the conditions necessary to prevent growth of the botulism bacteria when garlic and herbs are immersed in oil. Refrigeration of these infused oils is recommended for quality, but not required for safety. There is no safe, tested procedure for home canning these infused oils.

The quality issue is just one that we deal with in all products containing oils/fats/lipids. Rancidity is the issue here; foods become rancid when they oxidize. Oxidation of lipids is one common and frequently undesirable chemical change that may impact flavor, aroma, nutritional quality, and, in some cases, even the texture of a product.

If your oils have any tell-tail signs of an “off” odor that resembles stale, grassy, chemicals or paint, toss it. If you see a thickening of the oil or a milkiness, do not use it. If you are ever unsure, don’t take any chances; rancid food can make you sick. Throw it out and make fresh oils. Also, look for signs of mold, moisture or “sweating” in your oils before you use it. All indicate improper exposure of the food to bacteria, causing it to spoil. Safety first!


Oils. Olive oils often are infused with flavoring materials. Garlic and herbs offer a nice complement to olive oil’s distinct flavor. While monounsaturates (like olive and peanut oil) also can go rancid after about a year or so, I find they are still much more stable than polyunsaturates.

If you are interested in having the garlic or herb flavor predominate, the Idaho team says you can use a blander oil, such as canola oil. Canola and olive oils are nice because they contain fewer polyunsaturated fatty acids than many other vegetable oils and thus oxidize and turn rancid less quickly. I, myself, would choose to use rice bran oil which has a mild flavor and is neutral in taste.  It may lend a mild nutty flavor and can be used for sautéing, grilling, marinades, and salad dressings. It’s light, quite versatile, and cheap if you have an Indian or Asian store close by.

The more exotic oils (macadamia, walnut, sesame, sunflower, fish, flaxseed, etc.) are major candidates for fast rancidity, and should all be stored in the refrigerator or freezer. They should not be used for this type of shelf-stable application.

Citric acid. Citric acid imparts less flavor than lemon juice or vinegar. It is often available at health food stores, pharmacies, grocery stores, and other retail outlets that carry canning supplies. Shop around because this is one of those items where the price can vary widely. Lemon juice and vinegar have not been tested for acidifying the garlic and herbs for making infused oils and cannot be substituted for citric acid. It is important not to confuse citric acid with ascorbic acid (vitamin C); ascorbic acid does not have the same acidifying properties as citric acid. You will be making a 3% citric acid solution, the strength called for in the Idaho monograph.

Garlic and herbs. The acidification procedure was developed for FRESH garlic, basil, oregano, and rosemary. Do not use it with other vegetables or herbs or fruits until the appropriate research has been conducted. Research to determine acidification procedures for ingredients other than garlic, basil, oregano, and rosemary have not been conducted at this time.

You cannot use dried garlic or herbs to flavor these oils. If you season oil with dried garlic, dried herbs, or both, you must refrigerate the mixture and use it within 4 days or freeze it for long-term storage. In theory, dried garlic and herbs cannot support the growth of bacteria because they contain too-little moisture. However, without specialized laboratory equipment, it is not possible to determine if the garlic and herbs are truly dry enough. Even a very small pocket with sufficient moisture can allow bacteria to grow and produce toxin.

While I can agree that whole garlic is more attractive than chopped garlic when left in the garlic-infused oil, don’t do it. Whole garlic acidifies much more slowly than chopped garlic and does not reach the required level of acidity within 24 hours.

You can have an herb blend though by acidifying two herbs at the same time; for example, use 3/4 cup each of basil and rosemary in 2 cups of 3% citric acid. Yes. The herbs tested so far (basil, oregano and rosemary) can be acidified together as long as you use the correct ratio of total herbs to acid solution. Garlic must be acidified separately because the ratio of garlic to acid solution is different. However, as long as you acidify the garlic, basil, oregano, and rosemary according to the instructions provided here, you can use them in any combination you like to produce flavor-infused oil.

And Now for the Fun Part!

The first step to making your oils is the acidification of garlic and/or herbs. Raw, chopped garlic or fresh herbs (basil, oregano, or rosemary) are immersed in a 3 percent solution of citric acid. After soaking for 24 hours, the acid is drained away and the acidified garlic or herbs are ready for addition to your vegetable oil of choice.

Acidified Garlic for Oil InfusionsInfused-oils04

  • 2 cups water
  • 1 tablespoon granular citric acid
  • 2/3 cup chopped garlic (about 8 ounces of garlic bulbs)
  1. To make the 3% citric acid solution, pour 2 cups warm water into a mixing bowl. Add 1 level tablespoon of granular citric acid and stir gently, dissolving the citric acid completely.
  2. Place 2/3 cup chopped garlic into the acid solution and stir gently. Cover and hold at room temperature for 24 hours to allow garlic to become fully acidified.
  3. After 24 hours, remove the acidified garlic from the solution; draining well.
  4. Add the acidified garlic to your choice of oil and allow its flavor to infuse the oil to taste. Start with 1 part garlic to 10 parts oil, by weight; add more acidified garlic to taste if you wish.

Acidified Herbs for Oil Infusions

  • 2 cups water
  • 1 tablespoon citric acid
  • 1-1/2 cups (47 grams or 1.7 ounces) fresh rosemary, basil, or oregano, loosely packed, leaves attached to stems
  1. To make the 3% citric acid solution, pour 2 cups warm water into a mixing bowl. Add 1 level tablespoon of granular citric acid and stir gently, dissolving the citric acid completely.
  2. Wash the fresh herbs thoroughly and pat dry.
  3. Place 1-1/2 cups of herb into the acid solution. Make sure the acid solution completely covers the herb. Use a clean dish to weigh down the herb and keep it submerged. Cover the bowl, and allow the herb to soak up acid solution for at least 24 hours.
  4. After 24 hours, remove the herb from the solution, drain well, and gently pat dry. The herb is now safe to place in an oil of your choice.

Note: Soaking the garlic or herbs a few hours longer than 24 is fine, but leaving them in the acid longer will result in a less-desirable appearance and flavor.

Infusion Ratio, Time, and Temperatures

The proportion of flavoring material to oil and the temperature of the infusion affects how quickly the flavoring material will infuse into the oil. Experiment to determine the conditions that produce a flavored oil most suited to your taste. The ratio of flavoring material to oil used in the Idaho team’s research was 1 part acidified garlic or herb to 10 parts of oil, but the ratio may be increased or decreased to suit personal tastes. My experiments yielded three pints of oils with the acidified herbs and garlic.

Successful infusions were conducted at room temperature (about 70°F) for 1 to 10 days, with the intensity of the infused flavor increasing over time. Oils may also be heated to more quickly infuse the flavoring materials. Our research successfully used acidified herbs to flavor oil at an infusion temperature of 140°F for 5 minutes. Significantly hotter temperatures would damage oil flavor. I infused my jars for one week and that was plenty for me as far as taste infusion. I did not heat my oil, but left it at room temperature. I gave the jars a gentle shake each day of the infusion process.

Because flavors will continue to intensify with time, it is best to remove the acidified garlic or herb from the oil when it has reached the desired flavor. However, it is acceptable to leave the garlic or herb in the oil, particularly rosemary, for an attractive look, a practice sometimes used with commercial flavored oils. I removed the plant material after one week, strained the oil into my sterilized bottles, and stored them away with labels to identify what was in them. I also dated the labels.

Storage of Infused Oils

While oils infused with flavors from acidified garlic, basil, oregano, and rosemary can be safely stored at room temperature, oil flavor quality is maintained for a longer period of time with refrigerator or freezer storage. It is also best to protect infused oils from light by storing them in dark-colored bottles.

These items will usually keep for about 1 month stored in a cupboard, 3 months in the refrigerator.

In the Freezer, they will keep their same consistency. Try freezing measured amounts in a ice cube tray. Freeze the trays for an hour or until the portions are frozen through, then pop the cubes out of the tray and place them in a plastic freezer bag or glass container with an airtight lid. They’ll last in the freezer for up to a year, but try to use them within six months for the best flavor.

Make sure the bottles are sterilized, and food grade with a lid that seals well. All vegetable oils retain quality better at cold temperatures and when protected from light.

As light and heat are detrimental to the oil, keep your oil in a dark, cool place, away from the stove. Dark glass protects the oil from light, including halogen and fluorescent lights, much better than clear glass. Beware of clear bottles of olive oil on the top shelf at the store.


Note: The procedure in this article is based on work reported in the following journal article: Abo, B., Bevan, J., Greenway, S., Healy, B., McCurdy, S.M., Peutz, J., and Wittman, G. 2014. Acidification of garlic and herbs for consumer preparation of infused oils. Food ProtectionTrends 34(4):247-257.

Methods to Preserve Your Herbal Harvest, an 8 Part Series
To see the other articles in the series, click on the links:

Part 1, Harvesting, Preparing, and Seeding Herbs & Spices
Part 2, Freezing Methods for Herbs & Spices
Part 3, Drying Methods for Herbs & Spices
Part 4, Extracts/Tinctures from Herbs & Spices
Part 5, Preserving Herbs & Spices with Infused Vinegars
Part 6, Making Teas from Herbs and Spices
Part 7, Herbal Infused Honey

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