Monoacid triacylglycerols

Only oils that are rich in one fatty acid contain much monoacid triacylglycerol, for example, olive (Table 5), sunflower, and linseed oils containing OOO, LLL, and LnLnLn, respectively. Compilations of the triacylglycerol composition of commodity and other oils are available (8, 9). 

Figure 10. Onset temperature of nucleation and polymorphic form of monoacid triacylglycerols with different chain lengths (nj at slow cooling rate (0.4°C/min). o.mp, P p and p p represent the melting temperatures of the different polymorphs (2).

In contrast, scission is a minor process in neat lipids or aprotic solvents at room temperature where background levels of scission products range from about 1 % in monoacid triacylglycerols (337) to 10-20% in free esters of O, L, and Ln (14, 335)... 

The triacylglycerols are named in various ways. For example, unsaturated fatty acids sometimes are indicated as U and the saturated as S. If glycerol is completely esterified with stearic acid, the resulting monoacid triacylglycerol may be designated as SSS, or, more descriptively as StStSt, tristearin, tristearoyl-glycerol, or glycerol tristearate. 

When a monoacid triacylglycerol is cooled slowly and without mixing, it preferentially assumes the P crystal form. However, the required tight packing of crystals requires time for alignment, and may be thwarted by increased viscosity as the fat/oil mixture cools. However, if the heated oil is cooled rapidly, it may first be rushed into an amorphous (vitreous, glasslike) sub-a state. Upon exposure to heat, the fat then may pass directly to the P crystal form, or stepwise through the a and p forms until it reaches the P form. 

Fig. 34.1. Drawings of two- and three-chain triacylglycerol layers (a) saturated monoacid SSS-type (b) saturated symmetrical PSP-type where 2 chain differs from 1 and 3 chains in length (c) symmetrical POP-type where 2 chain is unsaturated and (d) symmetrical OPO-type where 1 and 3 chains are unsaturated. Crystals grow in bilayer units.

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