Identifying the factors determining the chlorophyll content of UK rapeseed

Abstract

During ripening of oilseed rape seed, chlorophyll pigments are steadily broken down. Any chlorophyll remaining at harvest is extracted along with the oil during the crushing process. Since the chlorophyll interferes with processing, it must be removed during refining. Chlorophyll concentrations in oil extracted from UK-produced rapeseed are usually higher (50%) than those from seed imported from continental Europe. In recent years, they have been especially high (up to 3 fold higher), leading to problems during refining and increasing refining costs. The higher concentrations in UK-produced seed, compared to seed from continental Europe, threatens the competitiveness of the UK industry, as importing seed or crude oil with a lower chlorophyll concentration becomes an increasingly attractive option for the crushers and refiners. Research was undertaken to investigate the possible causes of the high concentrations in the UK. The objective was to determine the extent of any variation between varieties, variety types (hybrids versus conventional lines), sites and seasons.

Seed samples from 13 varieties harvested in 2001 were collected from the recommended list trials at 5 sites, and analysed for chlorophyll. Varieties differed significantly in their seed chlorophyll concentration. The most popular UK variety, Apex, had the highest concentration. Collectively, restored hybrids had lower concentrations than conventional lines or varietal associations. Differences in maturity class between varieties did not account for much of the variation in seed chlorophyll concentration. There were differences between sites, but no clear effect of latitude on the chlorophyll concentration.

Large seasonal fluctuations in the chlorophyll concentration of commercial crude oil were observed. 2001 was a relatively low chlorophyll year (30 mg per kg oil) whereas 1999 and 2000 were high years (60 and 50 mg per kg respectively). Concentrations of chlorophyll in samples of seed from commercial crops in 2000 and from recommended list trials in 1999, were correspondingly high. The high concentrations appear to be related to periods of particularly dry weather between swathing and harvesting. It is possible that premature harvesting in periods of unsettled weather may also contribute to the problem.

A brief survey of oilseed rape agronomy indicated that the major differences between the UK and continental Europe are the varieties grown and the method of harvest. In Europe most crops are directly combined, very few are swathed or desiccated beforehand. In France, warm dry weather leading up to harvest is the norm indicating that dry weather does not inevitably lead to high seed chlorophyll concentrations.

There may be scope for reducing the chlorophyll concentration to values found in European seed, through use of lower chlorophyll varieties (both hybrids and conventional lines) and through possible improvements in harvest timing or method. Further research is needed to determine the effects of harvest method and timing on the process of chlorophyll breakdown during seed ripening under UK conditions before firm recommendations can be made.