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Improving the yield and quality of blackcurrant (Ribes nigrum L.) extracts

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Garland, SM (2007) Improving the yield and quality of blackcurrant (Ribes nigrum L.) extracts. PhD thesis, University of Tasmania.

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Abstract

Aspects of harvest, post-harvest storage and extraction technology were investigated
with view to improving yield and quality of blackcurrant bud extracts. Sieving
extraneous material from machine harvested buds increased yield by 20 % consuming
8% less solvent. New extraction technology in which buds were steeped in ethanol
prior to partitioning against petroleum ether increased the yield of volatiles by 29 %
whilst reduced the yield of acids by 61 %. The extracts had a more pungent catty
aroma reminiscent of the European product. The chemical profile of extracts from
new clonal material selected from cv. White Bud was found to contain higher levels of
4-methoxy-2-methyl-2-butanethiol. This component confers a 'catty' note associated
with quality extracts. The new selection differed from White Bud by having higher
levels of sabinene, myrcene, bicyclogermacrene and hardwickic acid.
Studies focused on the levels of the endogenous thiol which was depleted by 85 % in
extracts stored for less than a month. Freezing of buds also resulted in a loss of 50%
of thiol and 18 % of other volatiles after 24 hours. Research on harvest technology
showed that cutting buds by hand increased thiol concentration (4.6 mgkg-1 DMB)
compared to those extracted from machine-harvested (`3.5 mgkg^-1 DMB`). Post-harvest
thiol production was most rapid in fresh hand-cut buds. Mechanical harvesting
resulted in a loss ofthiols and reduced post-harvest production in fresh buds. Volatile
concentrations were reduced when machine-harvested buds were further damaged by
rolling.
Damage to bud structure by rolling resulted in depletions of mono and sesquiterpenes
and had a significantly greater detrimental effect in the retardation of post-harvest
production than did freezing. Rolling did not stop synthesis of oxygenated
sesquiterpenes and diterpene acids whilst freezing deactivated these processes.
Disparities in the behaviour amongst terpenoid families were consistent with subcellular
compartmentalisation.
The dry weight of buds and yield of volatiles decreased throughout dormancy.
Diterpene acids increased as bud burst approached. Commercial scale-incubations for 72 hours produced a 29 % increase in the level of 4-methoxy-2-methyl-2-butanethiol
and an 8 % decrease in oil yield. A novel synthetic method using methyl-3-
methoxypropionate to produce a butanol intermediate with thiolation using
Lawesson's reagent was developed. A non-volatile cysteine thiol conjugate thought to
be the precursor to volatile thiols was successfully detected in blackcurrant. Levels of
the conjugate and the thiol in the new clonal buds were 3.3 and 4.6 fold higher,
respectively, than that detected in the White Bud. An inverse relationship in the levels
of the thiol and those of the conjugate during the final stages of dormancy up until bud
burst was elucidated.

Item Type: Thesis (PhD)
Keywords: European black currant
Copyright Information:

Copyright 2007 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Date Deposited: 07 Apr 2008 15:54
Last Modified: 14 Mar 2017 01:58
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