In 1986, I commenced the oenology degree as a distance education student at Charles Sturt University (CSU). One of my first subjects was vineyard establishment and I was intrigued at the then practice to ‘dip–rip’ the soil followed by extensive spraying with a weedicide/herbicide, essentially to ensure that vine growth would not be restricted through competition with the previous growth in the parcel of land. Such a strategy was perhaps understandable, given the then urgent need to expand grape production. The practice would of course be detrimental to the natural soil microflora, something that would detract from the concept of the site’s terroir potential.
During my time as Director of the National Wine and Grape Industry Centre, the plant pathology researchers carried out a one-year study of the microbiological population of a biodynamic vineyard. A more active and disperse population was found in the biodynamic vineyard than in vineyards maintained by conventional crop management methods. This was in the early days of biodynamic cultural practices in Australia, and the trial was not continued owing to lack of funding support.
In 2012, researchers at Stellenbosch University (South Africa) described a detailed study of the ‘vineyard yeast microbiome’ on three adjacent vineyards, each planted to Cabernet Sauvignon but managed by different cultural practices: conventional, integrated pest and disease management, and biodynamic (PLoS ONE 2012, vol. 7(12), e52609). The results demonstrated that the least cultivated vineyard (biodynamic) had significantly higher species richness, with some yeasts having what the authors termed ‘biocontrol potential’. The data analysis also showed that the species distribution was not uniform with marked intra-vineyard fluctuation.
The fundamental question is whether there is any evidence that the vineyard yeast population has any effect or influence on the fermentation and final wine character. In Australia and most new-world wine countries, it is common practice to inoculate the grape juice or must with a selected strain of Saccharomyces cerevisiae to minimise any activity from the indigenous yeasts on the grapes. Otherwise, it was argued that the wine might show unusual aromas, ‘like a European wine’! On the other hand, a winemaker of a top shelf Chardonnay from Burgundy, when asked if he ever inoculated a ferment with a commercial strain of S. cerevisiae, responded ‘if I want to drink Coca-Cola, I would go to Australia … or Bordeaux’!
The argument over inoculated versus indigenous fermentation often overlooks that S. cerevisiae can be found in the indigenous yeast population. In a parallel Stellenbosch study to the one described above, the authors examined the changing indigenous yeast population and fermentation kinetics during small-scale fermentations (S. Afr. J. Enol. Vitic. 2015, vol. 36, pp. 243–51). The initial total yeast population for the biodynamic vineyard was higher than for the conventional and integrated vineyards, and indigenous S. cerevisiae was detected, albeit at a low percentage of the total. Fermentation started immediately for the biodynamic vineyard, whereas there was up to a two-day lag with the conventional and integrated vineyards with the fermentation taking about twice as long to complete. S. cerevisiae was only detected two days after the onset of the fermentation.
Exploring the proposal that indigenous populations of S. cerevisiae display biogeographic patterns, researchers from the University of Basilicata and the University of Florence collected several isolates from Aglianico del Vulture (Basilicata) and Sangiovese (Tuscany). The isolates were inoculated into the red grape must from both varieties. Distinct regional or biogeographic patterns were found. For example, some yeasts from Aglianico did not complete the fermentation when inoculated into Sangiovese. In essence, the best results were obtained with yeast strains that were adapted to each viticulture area (Front. Microbiol. 2016, vol. 7, article 1018).
In a separate study, the University of Basilicata researchers selected three indigenous strains of S. cerevisiae from three separate regional vineyards and inoculated Primitivo must from each vineyard with the strain relevant to that vineyard. A parallel fermentation was carried out in each cellar using a commercial active dried S. cerevisiae yeast. The fermentations were monitored for effectiveness and influence on chemical composition, and sensory analysis or liking preference was also performed. Generally, the 84 consumers preferred the combination of winery/indigenous yeast as reflecting the ‘typical traits of Primitivo wine’, opening up the feasibility of utilising cellar-specific indigenous strains to produce a wine that is unique to the winery (Fermentations 2019, vol. 5, article 87).
A recent review by researchers at the University of Melbourne (Front. Microbiol. 2019, vol. 10, article 2679) provides a succinct summary of this expanding field of endeavour. The review clearly underpins the role that microbial biogeography plays in influencing wine characteristics. One question that has always intrigued me is the time it takes for a vineyard to develop and maintain a stable microbial system. Our vineyards are ‘young’ in comparison to many in Europe. The impact of a changing climate on this microbial diversity needs also to be considered.
Research is slow, with several years needed to check the stability of the site’s microbiology. Recently, I came across a proposal for a microbiome-on-a-chip as a possible means for evaluating plant-microbiota research (Trends Microbiol. 2017, vol. 25, pp. 610–13). I would be happy to chat about this with anyone who may be interested.