CHIRAL AND DIETARY DIACYLGLYCEROLS

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CHIRAL AND DIETARY DIACYLGLYCEROLS

P. T. Gee and S. H. Goh*

Palm Speciality Products Sdn Bhd, Masai, Johor, Malaysia and

*Forest Research Institute of Malaysia, Kepong, Kuala Lumpur, Malaysia

[Presented in part at the OFIC2000 Conference, Sept. 4, 2000, Kuala Lumpur]

Abstract: It is well known that crude palm oil contains about 4-7.5% of mixed diacylglycerols (DAG) or diglycerides but the potential value of these and the originally formed 1,2-diacylglycerols have yet to be exploited. Fresh palm oil provides chiral 1,2-diglycerides in contrast to random or partially equilibrated mixtures obtained from undirected synthesis. When processed to be a chiral intermediate, 1,2-DAG’s value may rival or even exceed those of carotenes and vitamin E. Interestingly, randomized diacyglycerols provide the thermodynamically more stable 1,3-diglycerides which are now in vogue as a dietary fat. The specific action of pancreatic lipase on such DAG oils ensures that there is little 2-monoglycerides to be absorbed and reconstituted for body fat.

Introduction

Chirality (spatial distribution of molecules causing right- or left-handedness) continues to be marvelled and the phenomenon prompts lipid scientists to design their oils and fats for desirable dietary purposes. Much of the chemistry depends on the action of enzymes which synthesize molecules in a stereospecific manner and provide chiral compounds. The fat molecule is a triacylglycerol (TAG) which when suitably substituted by different fatty acids can exist in chiral forms. Major pathways that are now recognized to be used by nature to biosynthesize, via enzymes, triacylglycerol and a key intermediate is the 1,2-diacylglycerol (1,2-DAG). Crude palm oil is unique to provide as much as 4-7.5% of DAG1.

One present preoccupation of food specialists is to try to utilize the exotic properties of oils and fats to make food glorious, both for the palate and the figure. One of the crude ways is to use unnatural fatty acid derivatives of sugars which being totally indigestible are not absorbed but remain to provide a semblance of natural fat (TAG) properties. In the pursuit to provide desirable natural fats or synthetic “natural” fats, a common practice is to take advantage of the inherent properties of pancreatic (or similar lipase) enzymes which are predominantly 1,3-specific in their reactions. This property can be exploited by drugs (e.g. Xenical, a lipase enzyme inhibitor) to limit the digestion of fats. Or, alternatively, materials are designed to absorb the hydrolysed fatty acids and make them less available for absorption (e.g. chitosan, etc). Further, it is possible to structure the fat molecule (TAG) and position the chosen fatty acids for either easy or poor absorption. Structured or designer fats can have 1,3-long chain (e.g. C20-C22), saturated fatty acids which are poorly absorbed. A good percentage of palm and cocoa butter fats can be considered naturally structured fats as they have significant saturated fatty acids at the 1,3-positions (but the 2-position is mainly mono- or poly-unsaturated) and this leads to good nutritional attributes, e.g. their low hypercholesterolemic properties2 despite the relatively high overall saturation. Designer fats can be optimized to have the desirable fatty acids at the 2-position of TAG while the 1,3-positions are attached with other fatty acids to provide organoleptic characteristics but remain poorly absorbable and so reduce serum TAG (and fat deposits). On the other hand, when the saturated palmitic acid is required as for infant nutrition, palmitic acid can be made to occupy the 2-position while the 1,3-positions can be mono- or poly-unsaturated (e.g. 1,3-dioleoyl-2-palmitoyl glycerol).

Chiral 1,2-DAG

The presence of diglycerides1 in palm oil can sometimes be considered a problem in oil fractionation but it is also known that they can be beneficial as they can stabilize b’- to b-crystal transformation (e.g. in margarines). Most of the DAG in CPO end up as mixtures during processing due to isomerisation or are increased from enzymatic or non-enzymatic hydrolysis. Another unknown benefit is that DAG can potentially be exploited as a high valued minor component if recovered. DAG in the admixture form (see FOSHU below) is already a premier dietary cooking oil. The original DAG in palm oil is not a product of hydrolysis of the TAG, rather it is an intermediate in the biosynthesis of palm TAG. But it is important to note that this DAG in the fresh oil is a chiral 1,2-DAG.3 Chiral intermediates similar to 1,2-DAG such as R(+)-glycidol and D(+)-glyceraldehyde command a market price of about US$25,000/kg; this would be equivalent to about US$3,500/kg of palmitic-type 1,2-DAG. Disregarding the market requirements, 10 million tonnes of CPO could provide for a theoretical RM$5,320 billion if 4% of 1,2-DAG is assumed to be present in fresh CPO without FFA. As with carotenes and vitamin E in palm oil, such theoretical potential is of course not realistic in the marketplace. The processing for optically pure 1,2-DAG is unknown but if 99% of the cost is assumed for processing, the following conservative estimates can be arrived at and be compared to those for carotenes, vitamin E mixture and DAG oil:

Components Content in CPO Unit Price (RM/kg) PotentialValue (RM/MT)

1,2-DAG ~4% 133 5,320

Carotene 500 ppm 1,000 for 30% conc. 1,667

Tocotrienol 800 ppm 1,444 for 30% conc. 3,850

Mixed DAG ~6% 10 600

CPO 100% 1.3 1,300

The DAG experimentally isolated3 in derivatised form from fresh crude palm oil has been determined to be chiral 1,2-DAG by comparison with synthetic samples. This chirality is also expected to be derived from the biosynthetic pathway (Scheme below) and not a product of lipolytic hydrolysis. Randomisation will be expected from normal processing of CPO where acyl-migration (thermal and acid-catalysed) occurs to give the more stable 1,3-DAG. Normal CPO contains therefore as much as a 70/30 mixture of predominant 1,3-DAG and 1,2-(2,3)-DAG.

!,3-Diacyglycerols (1,3-DAG)

The spread of Japanese DAG oils (FOSHU or Food for Specified Health Use) as a dietary fat to reduce body fat and serum triacylglycerols is as remarkable as it is swift, especially into US which has strict FDA rules but has a major problem of obesity in the population. As discussed above DAG cooking oils are probably constituted mainly as 1,3-DAG through processing to this more thermodynamically stable isomer. As from the discussion above the action of pancreatic lipase on 1,3-DAG will not provide the 2-monoglyceride intermediate required for re-esterification to TAG.2 The subsequent lack of serum TAG helps to reduce fat deposits while absorbed fatty acids are utilized for energy metabolism. There is therefore tremendous opportunity to design structured DAG oils for dietary purposes just as it has been for TAG oils.

References

1.  Siew WL and Ng WL (1990) “Influence of diglycerides on the crystallization of palm oil” J Food Sci Agric 79: 722-726.

2.  Goh SH (1999) “Cholesterol” Mal Oil Sci Tech 8:59-59.

3.  Leong Hsiao Tung (1998/99) “Aspects of Some Lipid Natural Products” Honours Thesis, NUS; H.T. Leong, P.T. Gee and S.H. Goh “Studies in Lipid Natural Products” Abstracts, Chemistry Honours Symposium, NUS, 1999.

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Malaysian Oil Science and Technology 2001 Vol. 10 No. 1