Front Matter

1.3 Diglycerides

1.3.1 Introduction

Diglycerides are known for their retarding effect on fat crystallization, e.g., as anti-

blooming agent in cocoa butter (Okiy, 1978; Wa ̈hnelt et al., 1991), and have large

potential as building block for the chemical synthesis of phospholipids, glycolipids

and drugs (El Kihel et al., 1996). However, their actual application is rather limited

due to the lack of a selective low-cost production process. Conventional chemical

production as described above lacks any positional selectivity; however, their use as

pro-drug building block requires the diglycerides to be available as pure 1,2(2,3)- or

1,3-isomers.

Many processes for the preparation of concentrated partial glycerides have been

developed in recent years. Using various different lipases they are all based on the

three principal reactions catalyzed by lipases, i.e., hydrolysis, esterification or trans-

esterification. Various researchers have been investigating partial glyceride produc-

tion, focusing on product yield and purity enhancement. Mostly the reaction condi-

tions such as temperature, solvents system, molar ratio of substrates have been var-

ied. However, also lipase selectivity’s have been exploited to optimize target product

formation (Table 4).

1.3.2 1,2-Diglycerides

The most straightforward route for the production of 1,2(2,3)-diglycerides is the

hydrolysis of triglycerides using a 1,3-regioselective lipase, such as porcine pancrea-

tic lipase. Using this lipase in free powdered form,>90 % diglycerides (fraction of

the total of partial glycerides) were produced at an overall conversion of 50 % (Plou

et al., 1996). However, this process is based on kinetic resolution and hence the

degree of conversion and incubation time are important parameters determining

product composition and purity. At low degree of conversion, high levels of trigly-

cerides still remain, whereas at high conversions significant amounts of monogly-

cerides will form.

The latter can be avoided using a specific lipase fromPenicilliumsp. strains.

These fungi are known to produce several lipases, one of which displays a reduced

hydrolytic activity on partial glycerides in comparison with triglycerides (Penicil-

sp. UZLM-4 (Gulomova et al., 1996),P. cyclopium(Ibrik et al., 1998),P. expansum

(Sto ̈cklein et al., 1993) andP. roquefortii(Millqvist Fureby et al.,1997). Thus, in the

hydrolysis of triolein byPenicilliumUZLM-A lipase a 1 : 1 w/w mixture of mono-

and diglycerides was obtained with 64 % degree of conversion (Gulomova et al.,

1996). It was shown that the specific hydrolytic activity of the lipase on diolein

and mono-olein was about a factor of 5 to 10 respectively lower as compared to

its activity on triolein.

Further optimization of the reaction may perhaps enhance the selectivity of the

lipase to discriminate between diglycerides and monoglycerides. Optimization of the

water activity during glyceride synthesis was shown the enhance the selectivity of a

1.3 Diglycerides 11