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54 WINES&VINES July 2017 WINEMAKING PRACTICAL WINERY & VINEYARD I nterest in uninoculated primary fermenta- tion is increasing in wineries around the globe. This trend has various drivers in- cluding consumer preference for a per- c e i v e d " n a t u r a l " p r o d u c t a n d q u a l i t y considerations. The latter motivation is likely the most significant, as research from institu- tions around the world highlights the potential positive impacts of vineyard- and winery-asso- ciated non-Saccharomyces yeasts (or wild yeasts) on the flavor, aroma and perceived complexity of the wines produced. Research at the Institute for Wine Biotechnol- ogy at Stellenbosch University, South Africa, has demonstrated the ability of several non-Saccha- romyces yeasts isolated from a local vineyard to reduce wine ethanol levels by up to 1.5% and produce higher concentrations of desirable aro- matics such as esters and monoterpenes. 1 One important concept is that of "microbial terroir," a term now fully entrenched in the wine dictionary. Until fairly recently, the po- tential contribution of microbes such as yeast and bacteria to terroir had been ignored. 2 Recent studies (mostly in the past five years) have revealed that microbial popula- tions contribute to the perceived geographic character and sensorial signatures of wines produced by fermentation with native-selec- tion yeasts. 3,4,5 Spontaneous fermentations are thus an ideal means to exploit the natural microbial biodiversity (unique in many ways) and enhance the regional identity of wines. 6 There is a reason why yeast inoculation from dry active cultures of S. cerevisiae revo- lutionized winemaking in the first place: The risks associated with spoilage and stuck or sluggish fermentation in a spontaneous fer- mentation made controlled inoculation an attractive option for winemakers in the past. Fortunately, our understanding of non-Saccha- romyces yeasts and ability to monitor and control fermentations have improved signifi- cantly over the years, affording today's wine- makers the best of both worlds: a predictable and successful fermentation outcome com- bined with the sensory complexity associated with spontaneous fermentation. For winemakers considering a first foray into spontaneous fermentation, here are some helpful hints to guide you on your journey. Quality is key Compromised grapes may lead to a compro- mised spontaneous fermentation. If the quality of incoming grapes is questionable, pursue a safer inoculated fermentation (that would be to sequentially inoculate with commercial non- Saccharomyces yeast, then Saccharomyces yeast for risk mitigation and desired aroma profile). Go easy on the SO 2 : Trying to start a spontaneous fermentation with a vineyard selection yeast by killing off most of the native species yeasts is probably not the best ap- proach. Certain non-Saccharomyces yeasts are very sensitive to SO 2 compared to S. cerevisiae. The anti-microbial effect of SO 2 is greatly in- fluenced by must pH. It must also be kept in mind that some yeasts can form SO 2 as a result of sulfate reduc- tion and that residual vineyard sprays can have an effect on SO 2 concentration in the must. Much less SO 2 (than the norm of 30-50 ppm) or no addition is recommended for spontane- ous fermentation. Working without SO 2 brings new challenges to the winemaking process that must be carefully managed, such as potential oxidation and microbial spoilage. Know your starting material Not all musts are equal; some are less suit- able for spontaneous fermentation than others. Most important is the initial sugar content of the grapes, as grapes harvested with extremely high sugar levels (above 24° Brix) could pose a high risk for stuck fermen- tation in the absence of sequential inocula- tion with a strong fermenting wine yeast strain. There are various reasons for this, but an important one is the lower alcohol toler- ance of most species of non-Saccharomyces yeast. More sugar equals more alcohol, equals unhappy wild yeasts. Juice that has been extensively clarified (less than 50 NTU) are likely to have a lower lipid and sterol content than musts with higher turbidity. In the absence of oxygen that stimu- lates ergosterol production by yeast, unsatu- rated fatty acids and sterols derived from grapes are essential to anaerobic yeast growth (2-4 mg/L phytosterols is required for maximal growth). 8 They are particularly important in ensuring cell membrane integrity and fluidity (and thus yeast viability) in the face of increas- ing ethanol concentration. Yeast nutrients with a high sterol and lipid content can be added to the must to enhance fermentation. The addition of oxygen to the must to stimulate ergosterol production could pose risks due to the low SO 2 regime followed for spontaneous fermentation. However, oxygen additions during the rapid exponential growth phase of yeast is a rea- sonable option, as sufficient yeast growth and active biomass production means the yeast population present in the must will absorb most of the oxygen and remove it rapidly before oxidative reactions occur. Uninoculated Fermentation New insight into a traditional technique By Debra Rossouw