Chlorophyll degradation in ripening canola seed (Brassica sp.) and effects of processing and storage on chlorophyll pigments in canola oil
Ward, Kerry A.
High levels of chlorophyll in harvested canola seed result in the downgrading of the crop. These studies have investigated several aspects of chlorophyll breakdown in ripening canola seed and oil. Both the genotype of the plant and the location at which it was grown affected the chlorophyll content of the seed at harvest. Minimal genotype by environment interaction occurred with the four cultivars tested. Three cultivars of B. napus had significantly different rates of seed chlorophyll breakdown. Lower temperatures resulted in slower chlorophyll breakdown. Moisture and chlorophyll levels were positively correlated in ripening canola seed. All cultivars had the same rate of moisture loss from the seeds and this rate was not affected by environment. The rate of ethylene evolution from siliques containing seed was positively correlated with seed chlorophyll content, but the peak of ethylene production began after the initiation of chlorophyll breakdown and seed moisture loss. This suggests that endogenous ethylene production is not the trigger for these events. Chlorophyll pigments in ripening canola seed were monitored using high performance liquid chromatography (HPLC). The major pigments detected were chlorophylls A and B and pheophytins A and B, with low levels of pheophorbide A, methylpheophorbide A and pyropheophytin A. pigment composition was dependent on seed maturity, with green seed containing both chlorophylls and pheophytins, but mainly chrorophylls remaining in ripe seed. The pheophytins and minor components were breakdown intermediates, formed from the chlorophylls and subsequently degraded. The ratio of chlorophyll A:B increased during seed ripening. The "B" derivatives appeared to degrade faster than the "A" derivatives, suggesting enzymatic reactions. The initial steps in the chlorophyll breakdown pathway appear to be: chlorophylls -- - - pheophytins - - - pyropheophytins ! ! pheophorbides - - - - methylpheophorbides Chlorophyll pigments in canola oil were characterized by HPLC immediately after commercial extraction and following oil storage for one month. The main pigments in canola oil were pheophytin A, pyropheophytin A and chlorophylls A and B. The "A" derivatives comprised 81 to 100% of total chlorophyll pigments. During degumming, chlorophylls were converted to pheophytins and pyropheophytins. During oil storage, both exposure to light and high temperatures resulted in the conversion of chlorophylls to pheophytins and pyropheophytins.