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I've been making a chili oil and green onion oil for my friends for months with a lot of interest in buying. I want to start selling it at farmers markets & stores but want to ensure it is properly preserved. Can anyone advise on how to do so? How can I find the expiration date?

You don't realize it, but you've asked a hot-button question. Expect to get lots of comments about botulism, etc. This is a result of a report a few years back about folks getting botulism from homemade garlic oil. I'll keep my answer practical.

First, depending on where you live, your state, city, county, or other regional government may already have health codes for infused oils. You need to research these and follow them; regardless of the actual safety of your oils, if you don't follow the rules you could be facing a hefty fine and a court order to never sell food products again.

Second, contamination of infused oils by anaerobic bacteria (of which botulism is only one) is a real danger, so you need to treat your oils to prevent it. These methods include dehydrating the seasonings before adding them to the oil, acidifying them, or even pressing out the oils from the seasonings and using those instead of the whole seasoning.

You can also make the oil safe through pastuerization. Heating the oil (ideally dehydrated first) up to at least 121C/250F for 4 minutes or more should kill even botulism spores (the hardiest of the anaerobic bacteria). You need to make sure the water is gone from the solids before you heat the oil, which means either using dehydrated seasonings, or straining out the solids and then pasteurizing the oil. Also, depending on the type of oil you use, this pastuerization may harm its flavor; certainly unfiltered extra virgin olive oil will change flavor if heated this way.

Hope that helps, and good luck with your new business!