Thanks for contacting Ask an Expert.

This is an excellent question due to the risk of botulism in flavored oils made at home. Properly dried foods (See our publication Drying Fruits and Vegetables https://catalog.extension.oregonstate.edu/pnw397) do not have risk of botulism since moisture is needed to grow, but changing the storage conditions may be risky. Tomatoes should be dried until they are leathery or crisp.

Risk conditions include: a low acid food in an anaerobic environment - no oxygen, stored at room temperature. Olive oil is an anaerobic environment where Clostridium botulinum spores can turn into toxin.

There are two ways you can make Tomatoes in Oil safely.

If you dehydrated your tomatoes halved (no more than 3/4-inch wide) with the skins removed you can make a safe product that is shelf stable. Dip the slices in bottled lemon juice, put them in clean sterilized jars, cover with your preferred olive oil, put on clean sterilized caps. No other ingredients may be added. Then use them after a week or so or cure them in a cool, dark, dry cupboard for one to two months to develop a lovely earthy aroma and flavor, and a brick red color with a chewy texture. With this method, tomatoes in oil can be stored safely for months. Over time, rancidity can occur, so test the aroma before you use them. Use the tomatoes first on pizza or pasta or on crackers with pesto or cream cheese. Use any remaining flavored oil in salads or stir-fry dishes. If you use some of the flavored oil it can drop the level of oil in the jar. This will result in some tomatoes that are exposed to the air and mold can develop. Do not use Tomatoes in Oil when spoilage is detected.

If you dehydrated your tomatoes whole or halved with the skins removed you can roll them quickly in a small bowl with bottled lemon juice, put them in clean sterilized jars, cover with your preferred olive oil, put on clean sterilized caps. Store them in the refrigerator for up to 4 days.They will plump up by the second day and have a bright red color. Refrigeration will solidify olive oil, so to use it, gently warm the jar in a saucepan with about 1" of water until the oil liquefies. Cool and refrigerate any leftovers immediately. Unfortunately, the product will not develop the earthy aroma, taste and color with this short storage time.

Dried whole tomatoes with the skin on cannot be preserved in oil, safely. The skins are too dense for the acid to penetrate.

Our friends at University of Idaho recently completed research for safe preservation of shelf-stable Garlic and Herbs in Oil (https://catalog.extension.oregonstate.edu/pnw664) with specific acidification procedures. Follow the directions exactly.

You are my hero. I can't thank you enough for such a thorough response!

I now know that it won't be safe to store my dried tomatoes in oil at all because we dried them with the skins on.

Thank you also for the link to the OSU publication on Drying Fruits and Vegetables and for the link to further reading from the University of Idaho.

Take care, and keep up this good work!!

Kris