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The occurrence of apiculate yeasts in grape and must samples from the Robertson area
Neil Jolly
Keywords: apiculate yeasts, Robertson, wine
INTRODUCTION
The predominant non-Saccharomyces yeasts on grapes and in must at the start of fermentation are the apiculate yeasts (Kloeckera apiculata/Hanseniaspora uvarum) (Parish & Caroll, 1985; Bisson & Kunkee, 1991; Frezier & Dubourdieu, 1992; Jackson, 1994; Granchi et al., 1998; Fleet, 2003). These yeasts with their distinctive apiculate or lemon-shaped cell morphology are associated with the production of undesirable volatile acidity. Apiculate yeasts were also isolated by Jolly et al. (2003) during their investigation of grape and must samples from Constantia, Stellenbosch and Slanghoek. However, no apiculate yeasts were found in the Robertson samples during the 1997 and 1998 vintages. During the 2000 vintage apiculate yeasts were isolated, but only 10% of the isolates in the must sample belonged to this group. To elucidate whether the absence and/or low presence of apiculate yeasts in the Robertson samples was specific to the location investigated or was a phenomena of the wider Robertson area, further sites (vineyards) and their corresponding cellars needed to be investigated. The aim of this study was therefore to investigate five vineyard and cellar samples from one vintage for the presence of apiculate yeast.
Areas sampled
Five Chardonnay vineyards (Sites 1 to 5) and their accompanying commercial cellars in the wider Robertson area were selected for sampling. The first vineyard (Site 1) was the same as that sampled by Jolly et al. (2003) while the second vineyard (Site 2) was close (1 km) to Site 1. The other vineyards viz. Sites 3, 4 and 5 were approximately 14, 24 and 28 km from Site 1, respectively (Fig. 1).
Sample collection, isolation and characterisation of yeast isolates
Grapes were randomly and aseptically sampled directly from the vineyard (vineyard sample), one day prior to commercial harvesting, and the corresponding must samples were taken aseptically in the cellar after processing and sedimentation (cellar sample). Yeast isolation was done as reported by Jolly et al. (2003) and a random selection of 30 yeast colonies was made for each vineyard and cellar sample per site. Apiculate yeasts were identified microscopically (400X magnification) and the remaining yeasts were morphologically characterised from colony appearance on YPD agar (Biolab, Merck). The yeasts were subsequently classified as either apiculate, pigmented (orange/red; non-apiculate) and other yeasts (non-pigmented; non-apiculate).
RESULTS AND DISCUSSION
Sampling sites, yeast isolation and identification
The five sites sampled represent ten sampling points (i.e. five vineyards and five cellars). Although this still does not represent an exhaustive study of the Robertson area, it is more representative than the limited previous study (Jolly et al., 2003).
Aseptic sampling of the grapes and the subsequent laboratory procedure avoided contamination of samples by yeasts not present on the grapes. The cellar sample was taken after sedimentation to obtain a representation of vineyard yeasts carried over to the must, as well as yeasts resident on cellar equipment and surfaces that came into contact with the grapes and must during processing, but excluding any yeast that normally would be removed during must clarification.
Random selection of 30 colonies represents from 100% to only 10% of the possible isolates. While it is obvious that some minor species may have been overlooked, the random selection ensured that predominant species were selected. As apiculate yeasts have a distinctive cell morphology, it was easy to distinguish them from the other yeast isolates microscopically. Their colony morphology is also somewhat distinctive (smooth, greyish) and for the purpose of this investigation it was not necessary to do any further biochemical and/or karyotypic identification.
Apiculate yeasts in vineyard and must samples
The apiculate yeasts generally did not predominate in the vineyard samples and were mostly present in low numbers (Table 1). Site 4, the exception, showed in contrast to the other sites, a high percentage (83%) of apiculate yeasts. None of the cellar samples had apiculate yeast predominance. This was against expectations, as there should have been a carry over of yeasts to the must during processing. However, the reduction in numbers of a specific group of yeasts originally present in the vineyard sample has been noted before (Jolly et al., 2003) and can be due to the influences of must clarification techniques and other cellar practices detrimental to yeast growth e.g. addition of SO2. The reduced presence of apiculate yeasts in must samples has also been noted by Yanagida et al. (1992) who used a similar method of yeast isolation as in this study.
Table 1 - Click to enlarge
The relative close proximity of the vineyards to each other means that they shared similar macro- and meso-climatic conditions so climatic influences on the yeast microflora between the sites should not play a major role. However, individual viticultural practices (affecting the micro-climate) and methods of harvesting (hand vs. mechanical) can vary. This together with cellar practices can impact on the yeast microflora.
It was also evident that with the reduced apiculate yeast presence, the pigmented yeasts e.g. Candida pulcherrima were generally present in higher numbers. This may be due to a number of factors such as pigmented yeasts suppressing the growth of the apiculate or they may be purely opportunistic and filling the "gap" left by the absence of the apiculate yeasts. It has been reported that the pigmented yeast Metschnikowia pulcherrima/Candida pulcherrima has a killer effect against Saccharomyces cerevisiae and some non-Saccharomyces yeasts (Nguyen & Panon, 1998). However, they did not test the effect against apiculate yeasts.
Apiculate yeasts are generally implicated in the production of volatile acidity to the detriment of wine quality (Romano et al., 1992). The absence of apiculate yeasts should therefore imply a smaller risk for winemakers who allow spontaneous fermentation to take place. During this type of fermentation the growth and activity of the indigenous yeasts is not controlled or suppressed by a large inoculum of cultured S. cerevisiae yeasts. Subsequently, the apiculate yeasts, if present, can proliferate, with the concomitant production of volatile acidity. This strengthens the necessity of knowing the non-Saccharomyces species profile and population numbers in a given vineyard and cellar and also the effect these species will have on wine fermentation. Utilisation of this knowledge will enable the wine producer to maximise the desired regional characteristics of his/her wines and minimise any potential spoilage.
One of the cellars investigated in this study (Site 3) makes exclusive use of spontaneous fermentation for their Chardonnay wine, despite the high risk linked to this practice. The results are successful and these wines are sold at premium prices indicating consumer acceptance. Together with this cellar’s winning combination of ‘terroir’, viticultural and oenological practices, the low levels of apiculate yeasts in the Robertson area can be contributing to their success. The belief of the old world wine producers concerning indigenous yeast contribution to the regional characteristics and quality of wines (Amerine et al., 1972; Jackson, 1994) seems to be substantiated in this case.
CONCLUSIONS
This study achieved its purpose of elucidating the occurrence of apiculate yeasts in the broader Robertson region. These yeasts are generally not predominant in the Robertson area and this could have a positive impact on the quality and regional characteristics of the wine produced there, especially if spontaneous fermentation practices are followed.
LITERATURE CITED
Amerine, M.A., Berg, H.W. & Cruess, W.V, 1972. The technology of winemaking. The AVI Publishing Company, Inc., Connecticut.
For further information please contact Neil Jolly at jollyn@arc.agric.za.
Summary
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