Expression, or cold pressing, best extracts oils from citrus peels. Expression applies pressure to citrus peels to break open the oil glands under the peels' surface. Unlike distillation extraction, expression takes place at room temperature, requiring no extraneous heat. Citrus oils respond poorly to other extraction methods because heat damages the oils' chemical properties and creates undesirable odors. Lime oil represents the one exception to this rule; either steam distillation or expression works best for lime oil.
Cut the fruit in half and remove the pulp. Soak the peels in warm water. Turning the peels inside out, squeeze the peels by hand over a sterilized sponge which absorbs the oil. Turn the pressed peels right side out to sponge up any residual oil. Wring oil from the sponge. To remove peel particles and other impurities, pour oil through a fine sieve. Bottle the oil in a clean, dark, glass container. Stored in a cool, dark location, the oil will keep for six months.
Additional Expression Tips
To expedite the process of manual expression, puncture the peels' oil glands with a meat cleaver or another sharp tool prior to pressing. Likewise, grating citrus peels also succeeds in breaking open the oil glands. Once pulverized, press the grated peels to extract oil. Be aware that manual expression, while fun and informative, is a slow and laborious process, with very low oil yield per fruit. For this reason, modern oil extraction facilities employ grand-scale expression machines to extract oil more efficiently.
Many citrus oils cause increased sensitivity to the sun. If using these oils externally, dilute heavily, and wear protective clothing and sunscreen. Likewise, adding these photo-toxic expressed oils to tanning oils will not improve tanning, but will only result in burning under sun exposure. Before heavy external use, test the diluted oil on a patch of skin on the inside of your wrist or elbow. Leave the patch uncovered and unwashed for 24 hours. If irritation occurs, cease usage.