Sulfur biochemistry of
garlic: the biosynthesis of flavour precursors
HA Collin1, J Hughes1, A Tregova1, Jonothan GC Milne1, G van der Werff1,
M Wilkinson1, R. Cosstick3, L Trueman2, T Crowther2,
L Brown2, MG Jones1, B Thomas2, and AB Tomsett1
1 School of Biological Sciences, The Biosciences Building, The University of Liverpool
, Liverpool. L69 7ZB. UK.
2 Warwick HRI, Wellesbourne,
Warwick. CV35 9EF. UK.
3 The Department of Chemistry, Donnan Laboratories,
The University of Liverpool, Liverpool, L69 7ZD
Abstract of poster and talk presented at Alliums 2004,
Beijing China 21 - 26 April 2004
The major flavour precursors in the genus Allium include alliin and isoalliin,
members of the alk(en)yl cysteine sulfoxide group (CSOs). There are also large amounts of S-alk(en)yl
gamma-glutamyl-cysteine sulfoxide compounds. It has been suggested that alliin is derived from serine and an
allyl thiol source, or from glutathione and an allyl source, while isoalliin is synthesised via glutathione
and a series of gamma-glutamyl peptides. We have combined several approaches to investigate the intermediates
and enzymes involved in the biosynthetic pathways. These include profiling and tracking sulfur components in
garlic leaf, clove and root tissue as bulbs develop in the greenhouse. This has allowed us to build up, for
the first time, a picture of the dynamic uptake, allocation and distribution of flavour precursors during the
life cycle. This indicated that CSOs are remobilised to the new cloves in a late stage of development, following
uptake of the bulk of sulfur by the plant. Experiments involving feeding potential intermediates to onion and
garlic callus tissue, that generally lack flavour precursors, indicated that several alk(en)yl thiols could be
converted to alk(en)yl cysteine and alk(en)yl cysteine sulfoxide by callus. This may indicate that synthase and
oxidase enzyme(s) with broad substrate specificity may exist in Allium. Using data derived from the scientific
literature, we have searched for genes and enzymes that may be involved in the biosynthetic pathway. Cysteine
synthase (CSase) and serine acetyltransferase (SATase) may play a role in flavour precursor biosynthesis.
Sequences with significant homology to two groups of plant CSases and to a SATase have been obtained from garlic
(cultivar Printanor). The sequence of one further CSase has been identified, using partial peptide sequences from
a protein that showed allyl cysteine synthase activity. Our results provide further insight into the biosynthetic
pathway of garlic flavour compounds.
Assessment of sweet onions in the UK.
MG Jones1,
HA Collin1, B Smith2, T Crowther2, L Trueman2
AB Tomsett1 and D O'Connor.3
1 School of Biological Sciences, The Biosciences Building, The University of Liverpool
, Liverpool. L69 7ZB. UK.
2 Warwick HRI, Wellesbourne,
Warwick. CV35 9EF. UK.
3 The Allium and Brassica Centre, Wash Road, Kirton, Boston, Lincs, PE20 1QQ, UK.
Abstract of poster and talk presented at Alliums 2004, Beijing China 21 - 26 April 2004
Fresh sweet onions, that sell at a premium price, are a new market segment in the UK, but one that has shown rapid growth in other countries. Consumer confidence in a new product, through assured quality control, is essential. Flavour in onion derives from the enzymatic cleavage of flavour precursors to yield volatile sulfur compounds and a more stable product, pyruvate. We have tested two measures of onion pungency, namely pyruvate and flavour precursor levels, in parallel with flavour testing by taste panels. Comparison of the two chemical analytical methods indicted a direct relationship between either the total level of flavour precursors and pyruvate. Statistical analysis of the responses of the taste panels, using a standardised protocol and record sheet, showed reproducible reaction to both commercial and breeding-line varieties. Production of mild sweet onions within the UK requires both appropriate genetic material and also agricultural practices. Field-trials have been carried out to assess the response of several cultivars to different planting, irrigation and cultivation regimes, and also to several soil types and seasons. As a consequence of this study it is considered possible for mild sweet onions to be grown in the UK..
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