Production of vegetable oils
Cold pressing, hot pressing, refining, extraction
In the case of cold pressing, the raw materials are extruded in a single operation and then filtered. The name cold pressing does not say anything about the temperature at which the oil is pressed. Temperatures of 75 °C can also be reached, which is not expected when you hear the word. However, 45 °C should not be exceeded in order to ensure gentle pressing. The modern oil mills are usually equipped with a screw press. This must be adjusted to maintain the low temperature. The yield obtained depends on the chosen die. The crude oil obtained here still contains particles which are removed by filtration. The gentle process allows all ingredients to be retained. This has an influence on important criteria such as taste, smell, color and vitamins. Cold pressed oil has the taste determined by the fruit / seed.
During the hot pressing, considerably higher temperatures are produced, since heat is additionally supplied. As a result, the yield is higher. Hot-pressed oils are refined to remove residues. This reduces the quality of the oil in terms of taste and vitamins. However, the odorless and flavor-neutral oil is significantly longer lasting.
The by-product in the production of vegetable oil by hot-pressing is called extraction meal, while the by-product is referred to as a press cake during cold pressing.
During refining, unwanted substances are removed from the crude oil (eg pigments, odor, flavoring and bittering substances). These have an influence on the quality (taste, durability, technical processing, smell and color). The refining process (chemical refining) consists of the steps degumming, neutralization (only if the acid number > 4), bleaching (if the color values require it), deodorization (approx. 1.5 h at 210 °C). The last stage of the refining process is therefore deodorization.
Another method to get oil is the extraction (Latin "extrahere" = extract, leach, wash out). The comminuted fruits or seeds are mixed with a solvent (extraction agent). This solvent should dissolve the oil. For example, after the oil has dissolved by hexane, it is distilled to separate oil and solvent. Now only the free fatty acids have to be neutralized and the turbid materials removed.