Variation during dormancy and the effect of freezing and postharvest incubation on the chemical composition of blackcurrant buds (Ribes nigrum L.)

The yield and chemical composition of extracts of buds collected from blackcurrant stems (Ribes nigrum L.) throughout dormancy were investigated. Selections from 'White Bud', which produce higher levels of the odorous thiol, 4-methoxy-2-methyl-2-thiol-butane, were compared with the standard 'White Bud'. The dry bud weight of all variants decreased by 16% throughout dormancy until immediately before bud burst. The amount of volatiles extracted also decreased. The most discriminating feature distinguishing the new clones from 'White Bud' were higher levels of 4-methoxy-2-methyl-2-thiol-butane, myrcene, bicyclogermacrene, caryophyllene oxide and hardwickic acid. β-Caryophyllene and the combination of β-phellandrene and limonene were higher in 'White Bud'. The effects of post-harvest storage conditions on the volatile components of solvent extracts of blackcurrant buds were investigated. Fifty four per cent of the 4-methoxy-2-methyl-2-thiol butane and 18% of other volatile components were lost within 24 h of freezing. Longer term freezing resulted in a steady overall loss of volatile components. The effects of storage of fresh and previously frozen, hand-cut and machine-harvested buds at 10°C were investigated. Incubation of hand-cut buds in air and nitrogen for 48 h resulted in a 2.6 and a 1.4 fold increase of thiol content respectively. Levels declined thereafter. Thiols were also produced in buds which had been frozen prior to incubation with a 1.2 fold increase recorded in both hand-cut and machine-harvested buds following storage for 72 hours in air. The other terpene-based volatiles in hand-cut fresh buds increased slightly in all incubations, compared with a small decline in fresh machine-harvested buds. The volatile content of hand-cut buds which had been frozen prior to incubation remained relatively constant. In contrast, machine-harvested buds (previously frozen) showed a 1.9 fold increase in volatiles when incubated for up to 72 h. However, a decrease in volatiles was evident over a similar incubation period if machine-harvested buds were rolled for freezing.