Hydrolysis and transformation of terpene glycosides from muscat must by different yeast species
Introduction
Most studies dealing with microbiological aspects of wine-making have been mainly focussed on yeasts belonging to the Saccharomyces genus, usually employed in alcoholic fermentation. However, during the early stages of wine-making there is a substantial growth of non-Saccharomyces yeast species which could give special characteristics to wines (Ciani and Picciotti, 1995).
Terpenes as well as other volatiles compounds such as straight-chain alcohols, norisoprenoids and benzenoids, contribute to improve wine aroma. These odorous compounds may occur as free forms in grapes, or bounded to sugar molecules (glucose, disaccharides) to form odourless non-volatile glycosidic complexes (Cordonnier and Bayonove, 1974; Di Stefano, 1982; Williams et al., 1982a; Gunata et al., 1988).
Some authors Bayonove et al. (1984), Cordonnier et al. (1989), Gunata et al. (1990a) observed that monoterpenes could be released from sugars by the action of several enzymes. The activity and functionality of these enzymes are different, depending on their origin. Therefore, the aromatic profile obtained will depend both on the varietal glycosides composition and the origin of the enzyme employed.
Wine yeasts were studied for the presence of useful hydrolytic activities mainly β-glucosidase as well as other glycosidases. While S. cerevisiae, the main wine yeast, is not believed to be a significant producer of extracellular enzymes, non-Sacccharomyces wine yeasts are described as potential sources of glycosidases (Charoenchai et al., 1997; Zoecklein et al., 1997; Fernández et al., 2000; Úbeda and Briones, 2000; Mendes-Ferreira et al., 2001; Strauss et al., 2001).
Also, the production of a particular terpene alcohol was not only related to the hydrolysis of the corresponding glycoside, but to other reactions involving this terpenic aglycons, such as chemical isomerization, hydration or reduction conducted by wine yeasts metabolic activity (Di Stefano et al., 1992).
The aim of this paper was to determine the capability of different species of wine yeasts to relase aroma compounds from the glycoconjugated forms to increase the flavour of wines, through the complexity of their aroma composition.
Section snippets
Yeasts
A total of 9 yeast strains belonging to the species Saccharomyces cerevisiae var. cerevisiae UCLM 325, S. cerevisiae var. cerevisiae 301, S. cerevisiae var. cerevisiae 39, M. pulcherrima 132, M. pulcherrima 9, Debaryomyces hansenii 1, Kluyveromyces thermotolerans 85, Hanseniaspora uvarum 31, Pichia kluyveri 84, isolated from grapes and wines, were issued from Ubeda and Briones wine yeast collection and identified by PCR-RFLP (Esteve-Zarzoso et al., 1999; Fernández et al., 2000) and by
Results
In Table 1, the capability of the 10 yeast strains tested to hydrolyse the p-NPG can be observed. Most of them, including S. cerevisiae strains, were able to hydrolyse weakly p-NPG. When all the strains were grown in pure cellobiose as the sole source of carbon, only the strains of M. pulcherrima, D. hansenii, C. molischiana and H. uvarum species, were able to utilize it.
Table 2, Table 3 show the role played by yeasts in releasing volatile compounds from a glycosidic extract of Muscat grape
Discussion
In the preliminary study, when p-NPG was used, most strains showed β-glucosidase activity. However, this substrate is unreliable since 1,3-glucanase (showed by most yeasts) can also hydrolyse it (Strauss et al., 2001). In addition yeasts belonging to S. cerevisiae species do not possess β-glucosidase enzyme (Adam et al., 1995).
Moreover, the comparison of data obtained for the Muscat glycosides and the p-NPG, showed that the synthetic glucoside seems not to be the ideal substrate for the
Acknowledgements
M. Fernández González wishes to express her gratitude to The Junta de Comunidades de Castilla La Mancha for “Jose Castillejo” Grant. Thanks are also due to the Istituto Sperimentale per L’Enologia, Asti, Italy.
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