December 31, 2004
Several web visitors have emailed with chemistry comments and questions relating to freezing olive oil: What are the clouds in my olive oil, will olive oil freeze in the refrigerator and does the way it freezes say anything about its quality?
Refrigerator temperatures are preset to around 37 degrees F by most manufacturers. Chemistry texts list the freezing point of pure oleic acid at around 39 degrees F. Olive oil manufacturers don't generally list a freezing temperature because it is very variable depending on the olive variety and ripeness of the olive at processing. Unlike the properties of an element or simple compound like H2O, olive oil is made up of hundreds of chemicals, many of which change with every pressing.
Like most fruit, olives have waxes on their epidermis (epicarp) to protect them from insects, desiccation and the elements. These natural waxes are what allow an apple to be shined. If an oil is sent to a cold climate or will be used in a product like salad dressing where it will be stored in the refrigerator, it is often "winterized". The oil is chilled and filtered to remove the waxes and stearates. A standard test to determine if olive oil has been sufficiently winterized is to put it in an ice water bath (32 degree F) for 5 hours. No clouding or crystals should occur.
Oil which has not been winterized will clump and form needle-like crystals at refrigerator temperatures as the longer chain fats and waxes in the oil congeal, but the oil will not usually harden completely unless chilled further. Some olive varieties form waxes which produce long thin crystals, others form waxes which congeal into rosettes, slimy clumps, clouds, a swirl of egg white like material, or white sediment which the consumer may fear represents spoilage. These visual imperfections may form outside the refrigerator during the winter when oil is exposed to cold temperatures during transport. Chilling or freezing olive oil does no harm and the oil will return to its normal consistency when warmed.
The ideal temperature to store olive oil to reduce oxidation but to avoid clouding is around 50 degrees F.
To determine the actual freezing temperature I put some oils in the freezer with a thermometer. At 40 degrees most of the oils had not hardened or formed any crystals. At 35 degrees F most were firm enough that they could not be poured but were as soft as butter at room temperature. As the temperature lowered, more components of the oil solidified. At 10 degrees F the oils were hard enough that a fork could not penetrate them. Determining at what point to call the oil "frozen" is a matter of semantics. This slow increase in hardening as the temperature is lowered is in sharp contrast to a pure substance such as water which switches from a liquid to solid phase at an exact temperature.
Eric from Montreal writes that he has heard that to determine whether an olive oil is Extra-Virgin, place a small quantity of the oil in a glass bowl and refrigerate it for a few days. If it becomes crystalline, the chances are good that it is a true extra-virgin olive oil. If it forms a block, it is most likely chemically refined oil that has had some first-pressed oil added to it.
I don't think this is a valid observation. While refined or pomace oils will usually be stripped of their waxes and it is more common for a refined oil to be winterized to be used in a cheap dressing, many excellent extra virgin oils do not form "crystals". I have watched the production process of many premium oils from olive to bottle and they form a solid block when frozen. Unfortunately, detecting fraud is more difficult than just freezing the oil.