It could be dated from the beginning of 19 centuries of the bleaching earth firstly used in edible oil refining, and till not, it is still the most important utilization of bleaching earth as its high efficient and commercial costs.
The mainly methods to purify edible oils is the traditional alkali process and physical refining. With the traditional alkali process by means of diluted lyes and soda solutions while fatty acid are removed by distillation as the physical refining.
To identify the purification degree after bleaching earth, we need to learn some parameters.
Content of free fatty acid
In the entirely bleached and refined oil, free fatty acid content should be lower than 0.01% (normally within 0.01%-0.05%). The detailed data is depended on the crude oil quality. Good quality of crude oil with free fatty acid no higher than 5%, if more than 10%, it is not suitable for human health and edible.
Yellow and red colors are measured as one of the standards of bleaching earth working. Bleached and refined oil should exhibit color values of between 1.0 and 3.0 Lovibond red and 10-30 Lovibond yellow. Good bleached oil should be yellow color.
Degree of oxidant
The absorbing oxygen directly influences the edible oil quality. The peroxide value means the quantity of peroxides formed in oils; while Anisidine value indicates the content of oxidant products (like Aldehydes, Ketones). High Peroxide value and high Anisidine value mean the high oxidation level of edible oil.
UV extinctions provide the information on if the oils or fats have been refined
The Totox value is being increasingly used as a characteristic value for the overall degree of oxidation
Totox=2*PV+AnV, more than 30 are indicative oxidized oils and fats,
Gums are present in most oil plants and production; they pass out of the cell walls and membranes into the crude oil. In some vegetable oil (especially soybean oil) the content of phosphatide could be as much as 2.5%,normally it should be controlled within 0.1--1,3%.
Because of the negative effects on stability, color and taste, gums need to be removed in most. The good quality bleached oil should be with the phosphorous contents below 5 ppm,(normally 1-3 ppm)
Various color bodies and pigments may cause the dark color of crude oil, the most important is carotene and carotenoids (yellow-red color to dark red) as well as chlorohpyll and its derivates (deep green color). Carotene could be regarded as a valuable substance as antioxidant pro-vitamin, but chlorohpyll is a pro-oxidant and impair the quality, so it must be removed with the value at 50ppb
Heavy metal contents
Heavy metal present in gums and pigments or as metallic soap, those as the powerful oxidising catalysts and results the formation of radicals, so it should be removed. Arsenic has the effect of a powerful poison on hydrogenation catalysts, the content in well bleached oil should be lower than 0.1 ppm.
Also some other values should be existed, like iodine value, saponification value, melting point, the solid fat content etc, they are of less significance of the refining process, but nevertheless play an important role in the hydrogenation of fats.