Front Matter

partial-glyceride selective lipase fromPenicillium camembertii(Yamaguchi and

Mase, 1991) (see Section 1.4.3).

Though selective enzymes were used in the processes described above, product

yields were rather low. Much better results were obtained usingP. roquefortiilipase

(Lipase R) for hydrolysis in an organic solvent. This lipase is 1,3-regiospecific and

also displays a low activity towards diglycerides. Hydrolysis of trilaurin using this

lipase was shown to yield nearly pure 1,2-diglycerides (Millqvist Fureby et al.,

1997). Similar results were also obtained by alcoholysis instead of hydrolysis. Using

ethanol at 25 8 C, approximately 80 % diglycerides were obtained, 95 % of which

were the 1,2(2,3)-isomer (Millqvist Fureby et al., 1997).

It should be noted that the relatively low reaction temperature applied above is

essential with respect to the suppression of acyl migration. At temperatures> 408 C,

acyl migration of fatty acids in diglycerides (Kodali et al., 1990; Millqvist Fureby et

al., 1996; Xu et al., 1998) as well as monoglycerides (Boswinkel et al., 1996) will

result in significant amounts of the 1,3-isomers. Moreover, at thermodynamic equi-

librium the 1,3-isomer will be the dominant diglyceride at a ratio of 1.3 : 1 to 2 : 1

over the 2-position isomer (Kodali et al., 1990; Millqvist Fureby et al., 1996).

Solvent interactions also appear to effect the rate of acyl migration. The lower the

polarity, the more important acyl migration (Sjurnes and Anthonsen, 1994). Alco-

holysis in ethers (diisopropylether, methyl-tert-butylether) thus appeared to give the

highest product yield, whereas in alkanes (hexane, isooctane) low yields were ob-

tained due to acyl migration and subsequent hydrolysis of the 1,3-position isomers

(Millqvist Fureby et al., 1997).

Apart from the typical selectivities of theP. roquefortiilipase described above, this

lipase also appears to discriminate between the two diglyceride isomers themselves.

During ethanolysis a very low activity was found on the 1,2(2,3)-isomer as compared

to the 1,3-diglyceride. Surprisingly this difference was lost during hydrolysis. This

phenomenon was therefore attributed to a specific interaction of the ethanol with the

active site of the lipase, inhibiting the binding of the 1,2(2,3) diglyceride (Millqvist

Fureby et al., 1997).

1.3.3 1,3-Diglycerides

Several routes have been described for the production of 1,3-diglycerides. Starting

from triglycerides as the substrate, high-yield production was reported by directed

glycerolysis (Yamane et al., 1994). Starting with a molar ratio glycerol/hydrogenated

beef tallow of 1 : 2, approximately 90 % diglycerides were obtained, 95 % being the

1,3-isomer. During this process temperature programming was applied. Thus, start-

ing at 62 8 C the temperature was stepwise decreased to reach an end temperature of

488 C, thereby promoting preferential crystallization of the diglycerides (Yamane et

al., 1994). Comparable results were obtained for the production of dipalmitin at

408 C, starting with palm stearin at 60 8 C (McNeill and Berger, 1993).

Although very successful, this principle can be applied only for diglycerides with

high melting point, i.e., those consisting of long saturated fatty acids (Rosu et

al.,1999). Using oils of lower melting point and hence higher levels of unsatura-

1.3 Diglycerides 13