Ch2ocor

CHOCOR Phosphatidylethanolamine

I (cephalin)

CH2OCOR

CHOCOR Phosphatidylserine

CH2-PO(OH)—OCH2CH(COOH)NH2

CH2OCOR

CHOCOR OH_OH Phosphoinositides

OH OH Figure 2. Structure of the major phospholipids.

fats. Hydrogénation is most often used to change oils into fats. Another use for hydrogénation is to improve the oxidative stability by partial hydrogénation to remove most of the linolenic acid.

The separation of oils and fats from animal tissues is done by rendering, either dry rendering or steam rendering. The separation of oils from oilseeds usually involves a pretreatment, crushing or flaking of the seeds, followed by pressing (11). This is usually followed by solvent extraction to remove the remainder of the oil and yield a residue with less than 1% residual oil (12).

The crude oils obtained by rendering, pressing, and/or extraction are purified by a series of operations designed to remove impurities that may detract from the quality of the oil. Removal of phospholipids is achieved by degum-ming (13). The crude oils are treated with steam, which hydrates the phospholipids and makes them settle out. De-gumming can also be achieved by using solutions of phosphoric or organic acids. The soybean "gums" are purified and used as food emulsifiers, known as soy lecithin.

Free fatty acids in crude oils are removed by alkali refining (13). Solutions of caustic soda are used to reduce the level of free fatty acid to 0.01 to 0.03%. Care is required to prevent saponification of neutral oil. Removal of free fatty acids can also be achieved by physical refining. This involves treatment of the oils under vacuum with steam. The advantage of physical refining is that the process is similar to deodorization and these processes can be combined. A possible disadvantage of physical refining relates to the high temperatures (up to 270°C) employed in this process. This may cause randomization of the glyceride structure, formation of dimers and conjugated fatty acids (positional isomerization) and cis-trans isomerization (14).

Bleaching is used to remove colored impurities, such as carotenoids and chlorophyll. In the bleaching process the oils are treated with bleaching earth or activated carbon. The yellow-red color of most vegetable oils, mostly carotenoids, is easily removed by bleaching earth. The green and brown pigments are more difficult to remove.

After refining and bleaching, vegetable oils are further processed into margarines, shortenings, and frying and bilking fats. Two-thirds of all liquid oils produced in North America are used in the form of fats. Hydrogénation is used to change oils into fats, and involves the reaction of gaseous hydrogen, liquid oil, and solid catalyst under pressure and at high temperature (15). The catalyst used for edible oil hydrogénation is invariably of the activated nickel metal type (16). The hydrogénation reaction can be represented by the following scheme, in which the reacting species are the olefinic substrate (S), the metal catalyst (M), and hydrogen:

The intermediates 1, 2, and 3 are organometallic species and are labile and short-lived and cannot usually be isolated. In heterogeneous catalysis the metal surface performs the catalytic function. In theory, the finer the particle size, the more active the catalyst will be. In practice, however, particle size has to be balanced against filtera-bility, since removal of the catalyst at the end of the process should not be too difficult.

When hydrogen is added to double bonds in natural fats and the reaction is not carried to completion, a complex

Table 7. Sterol Content of Fats and Oils

Table 7. Sterol Content of Fats and Oils

Lard

Living Gluten Free

Living Gluten Free

A beginners guide that will reveal how living "G" free can help you lose weight today! This is not a fad diet, or short term weight loss program that sometimes makes you worse off than before you started. This is a necessity for some people and is prescribed to 1 out of every 100 people on earth by doctors and health professionals.

Get My Free Ebook


Post a comment