11 Wine Spoilage by Fungal Metabolites 629
Fig. 11.5Hydroxycinnamic acids and their esters (obtained from Monagas et al. 2005, permission
to be obtained)
reduced by a vinylphenol reductase, originating the ethylphenol derivatives (Heresz-
tyn 1986a and Fig. 11.4). In grape juices, hydroxycinnamic acids are esterified,
mainly to tartaric acid (Fig. 11.5). In wines they may be present in the free or ester-
ified form, either with tartaric acids or other polyphenols (Table 11.4). Recent work
has shown that most of the free acids from tartaric esters appear after the malolactic
fermentation (Hern ́andez et al. 2006, 2007; Cabrita et al. 2007). The other esterified
forms of hydroxycinnamic acids are cinnamoyl-glucoside anthocyanins (Romero
and Bakker 2000; Monagas et al. 2005; Oliveira et al. 2007) andtrans-p-coumaric
acid hexoses (Monagas et al. 2005; Hern ́andez et al. 2006, 2007). Boselli (2006)
also mentioned the existence of hydroxycinnamic acid esters with ethanol, like ethyl
caffeoate, in white wines.
The above-mentioned release of hydroxycinnamic acids from anthocyanin esters
during wine maturation may be only due to chemical reactions but conversions of
acid precursors to volatile phenols are typically dependent on enzyme or microbial
activity.
In grapes or grape juices, the tartaric esters may be hydrolysed by enzymes
from contaminant fungi or from commercial pectolytic preparations, both with cin-
namoyl decarboxilase activity, releasingfree hydroxycinnamicacid forms (Dugelay
et al. 1993; Gerbaux et al. 2002). However, the tartaric esters are mostly hydrolysed
after malolactic fermentation (Hern ́andez et al. 2006, 2007), it being hypothesised
that the hydrolytic activity of lactic acid bacteria follows the completion of malic
conversion to lactic acid (Cabrita et al. 2007) (see Table 11.4).
It is accepted that yeasts only metabolise the free acid forms, although brewing
S. cerevisiaewas supposed to possess feruloyl esterasic activity (Coghe et al. 2004).
Then the availability of freehydroxycinnamic acids appears to be crucial for the
production of VPs either by yeasts or bacteria.