Spoilage occurs mainly due to the high water content inherent in the structure of plants laid in oil. Using jars that have not been cleaned thoroughly or improperly cared for also provokes spoilage in the steeping process. You can check on your herb periodically by opening the jar, checking for mold and letting out any excess air. I don’t like to disturb the oils during the infusion period but it’s a good way to spot spoilage early on and chuck it, saving you a lot of time by starting over before the season is over.
I’ve had a lot of good luck using the crock-pot of infusing plant material with oil but I find my harvests of each specific plant are not large enough to warrant a crock-pot infusion. My calendula flowers every 3-4 days and I just can’t seem to be able to process that much material in high harvest season. Which is why I tend to use the jar infusion method more often because its less labour intensive when I need it.
This summer was not great for herb harvesting. Very late spring meant that everything was put in 2 weeks late and wet summer made collecting herbs difficult. Fall was nearly non-existent and it seems we went straight from summer to winter. Since my seeds were so late getting in and growing, my calendula patch was very late blooming – not a bloom until the beginning of October! Our first snow came before our first frost!? Yup! I managed to collect just enough calendula to fill one mason jar before the frost came in thick. Today I opened my one and only jar of calendula oil and took a deep breath in. UGH. The one thing about spoiled oil is that theres no mistaking it. When its off, it is definitely off. Chickweed was the same way. I am so sad the calendula is off because I just didn’t get a chance to even dry any – winter came on that quick! So now im stuck without any calendula of my own.
Infusing dried herb lessens the chance of spoilage (dehydrators are best for making sure as much water is evaporated as possible), but as you can see, quality is greatly affected in steeping fresh vs dried plant material. (plantain oils shown here – guess which is which!)