So I heard that you could get oil extract from the peppermint leaves by basically leaving them in a ziplock bag and letting them sweat the oil out. The only problem is that I left it outside for 2 whole days and now the leaves are all brown and sludgy and are spoiled. So my question is, is the oil extract still good?? Can I boil it and make it usable?
To extract peppermint oil you must do a distillation of peppermint leaves and collect the essential oil, in a distillator. There will be also water condensing, that is the hydrolate. But the hydrolate is water, and comes before the essential oil (which is oil). Both smell but the essential oil is much stronger, nicer but much less amount.
When the leaves sweat they might let some oil escape but it's way too less, and mixed with too much water, obviously it will still smell but it will rotten quickly. Not to be used, as it will have plenty bacteria.
in Portugal, sheltered terraces facing eastwards, high water table, uphill original forest of pines, oaks and chestnuts. 2000m2
in Iceland: converted flat lawn, compacted poor soil, cold, windy, humid climate, cold, short summer. 50m2
Mint oils are steam distilled. You are kind of doing that in principle with the plastic bag, but distillation is a controlled process where the steam passes through a biosolid and is then condensed and collected. You can make a home steam distiller with some equipment from a lab supply house for not a whole lot of money if this is something you want to do routinely.
You can make a weaker version at home -- peppermint extract -- by soaking the leaves in a 50% ethanol alcohol solution, like cheap 100 proof vodka, for about 4 weeks in a cool, dark spot. Use a sterile glass container, like a mason jar that's been boiled. Use about a 1:3 ratio of fresh leaves to alcohol and shake up the mixture every few days.
As to whether you can salvage what you have... I supposed you could sterilize the liquid by boiling, but I personally wouldn't bother. You don't have essential oil there so much as you have mint bong water.