One of the most important tips for making chili oil is to be sure that all safety precautions are taken, including protecting the skin from the oil in the chilies, making sure the oil jars are sterile, and storing the oil properly and using it in a timely fashion. There are different types of chili oil, so knowing what the final use for the oil will be can help to make chili oil that has the correct level of heat and is not overwhelming or under-flavored. The way in which the chilies are prepared before being added to the oil also can affect the final flavor; whether they are dried, roasted or fresh can each impart a slightly different flavor to the oil. Choosing the right combination of oil and chili types can make a huge difference and possibly make chili oil that is appropriate for one use but not another.
It is important to follow all safety guidelines when attempting to make chili oil. The cans or jars used to store the oil should be sterilized in boiling water before use, and the oil should be fresh and clean. If cold infusion is used to make chili oil, then the oil should be kept in the refrigerator and used as soon as possible to prevent harmful organisms from developing in the oil. For a hot-infusion method, the oil should be stored in the refrigerator once it is done, and all the peppers should be removed from the oil if it is not going to be used the same day. Nearly all commercially produced chili oil contains citric acid and other ingredients that prevent harmful bacteria from growing in the oil, while homemade oils do not have these and can quickly become dangerous to eat if not handled correctly.
The type of oil that is used should be considered carefully. For general-purpose use and cooking, neutral-flavored oil such as canola or grapeseed can work well. Olive oil can be used, although it has a very low smoking point for hot-infusion methods, and most of the olive flavor will be removed both by the cooking process and the heat of the chilies. Asian oils such as sesame or peanut oil generally are good choices, especially if dried hot Thai peppers are used.
In general, dried chilies normally are used to make chili oil, although fresh chilies can be roasted in an oven to dry them out and develop flavors. Fresh chilies tend to impart a fruitier flavor, sometimes with less heat than the dried chilies, although the moisture content in fresh chili peppers increases the risk of harmful bacteria developing. Most of the heat from chilies will be imparted to the oil as soon as they are added to the hot oil, not over time, so leaving out some of the seeds and primarily using the flesh of the peppers can make chili oil that has a more floral taste with less heat.