Bleaching earths have been used in the refinement of edible oils and fats since the end of the 19th century, while our Tonsil® bleaching earths have been used for this purpose for about 100 years. Even today, this application is still one of the most important areas of use for bleaching earths. The entire oil and fat production worldwide currently stands at more than 100 million metric tons per year, while reliable forecasts indicate a further increase in production and demand over the decades to come. These figures alone indicate the importance which bleaching earths have in the supply of nourishment for the world’s increasing population.
For reasons of consumption capability, and especially storage stability, by far the larger proportion of crude oils and fats have to be subjected to processing: i.e. they undergo industrial refining. With the current state of the art, a distinction is drawn between two different processes: the traditional alkali process and physical refining. The key difference between the two processes is the method by which the free fatty acids contained in the oil are removed.
Fully refined oils of good quality (refined, bleached, and deodorized products of high quality) feature ffa contents of less than 0.1% (normal range 0.01 – 0.05%, in most cases related to oleic acid content). The ffa content of crude oils depends to a high degree on their quality. In very general terms, the quality deteriorates as the ffa content rises. Good crude oils have ffa ranges of less than 5%, mostly in the range of 0.5 – 3%. Crude oils with higher contents than 10% ffa are to be regarded as spoiled and, in most cases, are no longer suitable for use in the food industry. As a rule, they are difficult to refine.
In the edible oil industry, the Lovibond-scale is generally used. Different cells are available (1 1/4”, 2”, and 5 1/4”). Yellow and red colour values are measured (blue and white values are available, but are seldom used). Bleached and deodorized oils should exhibit colour values of between 1.0 and 3.0 Lovibond red and 10-30 Lovibond yellow (5 1/4” cell); in other words,
a pure yellow colour. The colour of crude oils is, in most cases, of little information value.
Degree of oxidation
The quality of edible oils depends directly on the quantity of oxygen absorbed. The peroxide value (PV) determines the content of peroxides formed in the oil, while the anisidine value (AnV) indicates the content of oxidation products (aldehydes, ketones). A high content of peroxides (PV>10) and a high anisidine value (AnV>10) are typical of oils with a high oxidation level.
The UV absorption of the oil (UV extinction) characterises the content of isomerised, polyunsaturated fatty acids, from which conclusions can be drawn as to the peroxide content of the oil prior to refining. As a rule, UV extinctions provide information on whether oils or fats have been refined (lard, olive oil).
(phosphatides) and need to be removed as far as possible from the oil because of their negative
effect on stability, colour and taste. Refined, bleached, and deodorized products of high quality should
show phosphorous contents which are well below
5ppm (normal is 1 – 3 ppm P).
Colour bodies and pigments
Most triglycerides (which are in fact the main constituents of oils and fats) feature only a slight natural colour (colourless to pale yellowish). The deep colour of many crude oils results from small quantities of various colour bodies and pigments, the most important of which are carotene and carotenoids (yellowish-red to deep red colour), as well as chlorohpyll and its derivates (deep green
colour). While carotene can be regarded as a more or less valuable accompanying substance (antioxidant, provitamin), with numerous attempts accordingly being made to obtain it, chlorophyll is a pro-oxidant, and, because of its intense colour, severely impairs quality and must be completely removed.
Heavy metal contents
Heavy metals are present in oils and fats in small quantities (mostly Fe, Cu, Ni and As), either complexed in gums and pigments or as metallic soaps (especially after hydrogenation). These act as powerful oxidising catalysts, causing the formation of radicals, and therefore need to be removed in their entirety. Arsenic frequently occurs as a natural impurity in fish oils, and has the effect of a powerful poison on hydrogenation catalysts. Refined, bleached and deodorized products of high quality should not exceed the threshold value of 0.1 ppm for individual metals.
A series of other characteristic values exists, such as the iodine values (IV), the saponification value, melting points, the solid fat content (S.F.C.), etc., which are of less significance for the refining process, but which nevertheless play an important part in the hydrogenation of fats.