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conjugated dienes isooleate —»- stearate

Hydrogenation of linolenate is more complex and is greatly dependent on reaction conditions. The possible reactions can be summarized as follows:

linolenate?

, isolinoleate-- linoleate -

isooleate stearate

'conjugated dienes-

mixture of reaction products results. Hydrogenation may be selective or nonselective. Selectivity means that hydrogen is added first to the most unsaturated fatty acids. Selectivity is increased by increasing hydrogenation temperature and decreased by increasing pressure and agitation. Selectively hydrogenated oil is more resistant to oxidation because of the preferential hydrogenation of the linolenic acid.

Another important factor in hydrogenation is the formation of positional and geometric isomers. Formation of trans isomers is rapid and extensive. The isomerization can be understood by the reversible character of chemi-sorption. When the olefinic bond reacts, two carbon-metal bonds are formed as an intermediate stage. The intermediate may react with an atom of adsorbed hydrogen to yield the half-hydrogenated compound, which remains attached by only one bond. Additional reaction with hydrogen would result in formation of a saturated compound. There is also the possibility that the half-hydrogenated olefin may again attach itself to the catalyst surface at a carbon on either side of the existing bond, with simultaneous loss of hydrogen. Upon desorption of this species a positional or geometric isomer may result. The proportion of trans isomers is high because this is the more stable configuration. Double bond migration occurs in both directions, but probably more extensively in the direction of the terminal methyl group. The hydrogenation of oleate can be represented as follows:

| stearate isooleate—

The change from oleate to isooleate involves no change in unsaturation but does result in a considerably higher melting point. This is why the hardening effect of hydrogenation is only partly the result of saturating double bonds; trans-isomer formation has a major effect on hardness.

For example, olive oil with an iodine value of 80 is liquid at room temperature. When soybean oil is hydrogenated to the same iodine value it is a fat with the consistency of lard.

Hydrogenation of linoleate first produces some conjugated dienes, followed by the formation of positional and geometric isomers of oleic acid, and finally stearate:

In the nonselective hydrogenation of seed oils, polyunsaturated fatty acids are rapidly isomerized or reduced and trans isomers increase to high levels (Fig. 3).

Interesterification is a process whereby fatty acid radicals can be made to move from one hydroxyl of a glycerol moiety to another one, either within the same glyceride or to another glyceride. The reaction pattern has been described (17):

RCOOR2 + R1COOR3 RCOOR3 + RxCOOR2

The reaction is used in industry to modify the crystallization behavior and the physical properties of fats. The catalysts are usually alkaline and consist of sodium meth-oxide or alloys of sodium and potassium. At temperatures above the melting points of the reactants, several raw materials may be interesterified together so that new products are produced. If the reaction is carried out below the melting point, so that only the liquid fraction reacts, the process is called directed interesterification. Industrially, lard has been interesterified to improve its properties. Lard has a narrow plastic range, creams poorly, and gives poor cake volume. After interesterification these properties are greatly improved. Acetoglycerides can be prepared

Figure 3. Change in fatty acid composition during hydrogenation of canola oil.

Figure 4. Fat crystals in a fat as seen in the polarizing microscope.

by interesterification of natural fats with glyceryl triacetate. The resultant products are waxy, translucent materials that can be used as edible coatings and plasticizers.

Interesterification may provide an alternative to hydrogénation for the production of margarine and shortening fats. Hydrogénation has the disadvantage of forming trans isomers and of losing essential fatty acids. Interesterification of liquid oils with highly saturated fats, obtained by complete hydrogénation or by fractionation, may result in nutritionally more desirable fats.

Ester interchange of fats with a large excess of glycerol, at high temperature, under vacuum, and in the presence of a catalyst, results in an equilibrium mixture of mono-, di-, and triglycerides. After removal of excess glycerol, the mixture is called technical monoglyceride. Technical mono-glycerides are used as emulsifying agents in foods. Molecular distillation yields products with well over 90% 1-monoglycerides; these are also widely used in foods.

Figure 4. Fat crystals in a fat as seen in the polarizing microscope.

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