Abstract:
Food-borne pathogens and in particular the emergence of new food-borne bacterial pathogens,
which includes Listeria monocytogenes, are a significant threat to public health. This thesis
investigated a commercial red grape juice, derived from Vitis vinifera variety Ribier grapes, with a
strong inhibitory action against Listeria monocytogenes. The commercial grape juice caused a 6-
log reduction in Listeria monocytogenes numbers within 10 minutes exposure to the grape juice.
The inhibitory effect was specific to all Listeria species. The grape juice was bactericidal and
inhibitory at pH ≤ 5.0. The addition of protein (BSA), PVPP, and metal ions (Fe3+ and Mg2+)
suppressed the antilisterial activity of the grape juice. The antilisterial factor in the commercial
grape juice was identified as pigmented condensed tannin (proanthocyanidin oligomers). The
antilisterial pigmented condensed tannin in the commercial grape juice was likely derived from the
skin of Ribier grapes. Fractionation of the commercial grape juice pigmented condensed tannin by
molecular weight revealed that the antilisterial activity was distributed over a range (mean degree of
polymerisation 2.8 through 8.5) of tannin fractions. Condensed tannin fraction (G) had a mean
degree of polymerisation of 5.9 and was found to have the strongest antilisterial activity. The
molecular weight of oligomers identified in fraction G ranged from MW 867 to MW 2905.
Fraction G contained the highest percentage of polymeric colour with 90% the pigmented moieties
(anthocyanins) in the fraction incorporated into the polymer structure. Fraction G was comprised of
the flavonoid subunits catechin, epicatechin, epigallocatechin, and epicatechin-3-O-gallate.
Ribier grape seed condensed tannin had strong antilisterial activity. The antilisterial condensed
tannin derived from Ribier grape seed was comprised of the flavonoid subunits catechin,
epicatechin, and epicatechin-3-O-gallate and increasing antilisterial activity was associated with
increasing mean degree of polymerisation of the tannin polymers. The efficacy of the pigmented
condensed fraction of the commercial grape juice and Ribier grape seed condensed tannin as natural
food preservatives were assessed. In the presence of Listeria monocytogenes contaminated
cabbage the inhibitory activity of the pigmented condensed tannin from commercial grape juice was
suppressed. The antilisterial activity of the Ribier grape seed condensed tannin was reduced but not
eliminated when Listeria monocytogenes was associated with cabbage. The grape seed tannin may
be effective at eliminating Listeria monocytogenes from food.