The impact of reducing fatty acid desaturation on the composition and thermal stability of rapeseed oil

Abstract

Oilseed rape (Brassica napus) is the third largest source of vegetable oil globally. In addition to food uses, there are industrial applications that exploit the ability of the species to accumulate the very-long-chain fatty acid (VLCFA) erucic acid in its seed oil, controlled by orthologues of FATTY ACID ELONGASE 1 (Bna.FAE1.A8 and Bna.FAE1.C3). The proportion of polyunsaturated fatty acids (PUFAs) in rapeseed oil is predicted to affect its thermal stability and is controlled by orthologues of FATTY ACID DESATURASE 2, particularly Bna.FAD2.C5. Our aim was to develop rapeseed lines combining high erucic and low PUFA characters and to assess the impact on thermal stability of the oil they produce. The new type of rapeseed oil (high erucic low polyunsaturate; HELP) contained a substantially greater proportion of erucic acid (54%) compared with high erucic rapeseed oil (46%). Although the total VLCFA content was greater in oil from HELP lines (64%) than from high erucic rapeseed (57%), analysis of triacylglycerol composition showed negligible incorporation of VLCFAs into the sn-2 position. Rancimat analysis showed that the thermal stability of rapeseed oil was improved greatly as a consequence of reduction of PUFA content, from 3.8 and 4.2 h in conventional low erucic and high erucic rapeseed oils, respectively, to 11.3 and 16.4 h in high oleic low PUFA (HOLP) and HELP oils, respectively. Our results demonstrate that engineering of the lipid biosynthetic pathway of rapeseed, using traditional approaches, enables the production of renewable industrial oils with novel composition and properties.

Keywords: Brassica napus; erucic acid; oilseed rape; polyunsaturated fatty acids; rapeseed; thermal stability.

Figure 1

A simplified overview of the fatty acid biosynthesis pathway in A. thaliana seeds. In the Brassicaceae, the predominant product of fatty acid biosynthesis in the plastid is oleic acid (denoted C18:1 to indicate an acyl chain comprising 18 carbon atoms and containing 1 double bond). Further elaboration of acyl chains is catalysed by microsomal enzymes that either extend the acyl chains, for example to produce the very‐long‐chain fatty acids (VLCFAs) C20:1 and C22:1, or increase the extent of desaturation, that is the number bonds, for example to produce the polyunsaturated fatty acids (PUFAs) C18:2 and C18:3.

Figure 2

Seed fatty acid composition. Composition classified by fatty acid type: saturated fatty acids (SAFAs) = sum of C16:0, C18:0, C20:0, C22:0 and C24:0; polyunsaturated fatty acids (PUFAs) = sum of C18:2 and C18:3; oleic acid = C18:1; very‐long‐chain fatty acids (VLCFAs) = sum of C20:1, C22:1 and C24:1. Mean values plotted for 10 biological replicates each of K0472, K0047, Maplus, K0472‐HE and K0047‐HE, respectively. Standard deviation is shown as error bars.