Mint and Peppermint Oil Extraction and/or Expression Methods
The best way to distill the oil is to take the crop directly to the plant. "Some distillers prefer the plants to be previously dried or steamed, but the general opinion is that it's best to distill as soon as cut. Many farmers let the herb lie on the ground and dry in small bundles." The loss by partial drying in the field is "not regarded by growers as sufficient to offset the increased cost of handling and distilling the herb". Some essential oil is lost during drying but, "fermentation may occur if the herbs lie on the ground in heaps for an excess amount of time". This will reduce the quality and quantity of the oil.
During the plants flowering stage, approximately six weeks, the stills will operate 24 hours a day. Stills are made of copper and can "hold up to 5 cwt of the herb". The process is as follows: "Before putting the peppermint into the still, water is poured into a depth of about 2 feet, at which height a false bottom is placed, and on this the herb is trodden down by men. The lid is then closed, and under pressure the distillation is conducted by the application of direct heat at the lowest possible temperature, and is continues for about 4.5 hours." These vapors that occur from the direct heat are "led through pipes into cooling chambers, where they are condensed and deposited as crude peppermint oil". The false bottom on the still is then raised from a windlass and the steamed peppermint herb is returned to the field and used as compost.
Source:http://www.wsu.edu/~gmhyde/433_web_pages/433Oil-web-pages/essence/essence-oils.html
