Effects of Botrytis cinerea and Erysiphe necator fungi on the aroma character of grape must: A comparative approach
Introduction
Botrytis cinerea and Erysiphe necator (previously known as Uncinula necator) are two of the most important fungi in viticulture. They have a great economic relevance since they are widespread and are responsible for serious diseases of the vineyard, which can be prejudicial for yields and wine quality (Calonnec et al., 2004, Steel et al., 2013).
Numerous studies have dealt with the influence of Botrytis cinerea on the aroma of wine, either in its damaging form, or studying its positive effects when it produces noble rot. The last occurs under special climatic conditions, when moist nights and foggy mornings are alternated with dry and warm days. In this case, it can result in highly valued sweet dessert wines; some distinctive descriptive terms associated with botrytized wines are floral, tropical fruit, honey and caramel. The aroma of the most relevant examples has already been studied in Sauternes (Bailly et al., 2006, Bailly et al., 2009, Campo et al., 2008, Sarrazin et al., 2007, Sarrazin et al., 2007), Tokaji Aszú (Miklosy et al., 2004, Miklosy et al., 2000, Miklosy and Kerenyi, 2004) and Amarone wines (Fedrizzi et al., 2011, Genovese et al., 2007, Tosi et al., 2012). On the other hand, Botrytis cinerea can also be very detrimental to wine quality when it causes botrytis bunch rot also known as gray mold. Besides, the role played by this disease is becoming more important since an increase of its frequency has been observed in some wine growing regions in the last years being most likely related to climate change. Although one of the most important consequences of bunch rot is the generation of earthy off-odours, with several studies having dealt with this issue (Darriet et al., 2000, La Guerche et al., 2005, La Guerche et al., 2006, La Guerche et al., 2007), the precise nature of these taints remains in some cases still unknown. In addition, there is a significant knowledge gap on how this disease influences the overall aroma profile of wine in general.
Erysiphe necator is responsible for powdery mildew. Grapes affected by this disease show lower total soluble solids, less juice colour and higher acidity in comparison with healthy grapes (Calonnec et al., 2004, Gadoury et al., 2001, Pool et al., 1984, Stummer et al., 2003, Stummer et al., 2005). It is therefore responsible for wines with poor quality. In addition, powdery mildew has been related with development of off-odours reminiscent of mushroom (Darriet et al., 2002, Stummer et al., 2005). However, studies dealing with this disease found opposing results regarding how intensively it affects aroma character; on the one hand, Calonnec et al. (2004) found that the negative organoleptic effects are perceivable only when the proportion of affected grapes used to produce wine is high, since experts were able to recognize wines produced with 25% of affected grapes, but the value was raised to 50% when the test was performed by non-experts. On the other hand, Stummer et al. (2005) observed that sensory changes could be detected in juice and wine made with only 1–5% affected bunches.
Aroma is the dominant quality parameter of wine, and this is dependent on the condition of grapes, among other factors. Accordingly, it was considered necessary to analyse the influence of these fungi on one of the earliest stages of the production, the must, in order to differentiate between effects of fungal infections to must composition and deepen the understanding of the changes that may finally also lead to changes in wine. Therefore, this study aims to characterize in a model situation the aroma differences caused by the infection of White Riesling, Red Riesling, Gewürztraminer and Gm 8622-3 grapes by Botrytis cinerea and Erysiphe necator, thereby comparing must produced from healthy and infected grapes.
Section snippets
Basic oenological parameters
Brix were measured with a standard refractometer, pH and titratable acidity with a TitroLine alpha plus titrator (SI Analytics, Mainz, Germany). Data were evaluated with TitriSoft 2.73 software (SI Analytics, Mainz, Germany).
Chemicals
Dichloromethane and anhydrous sodium sulphate were from VWR (Darmstadt Germany). Dichloromethane was freshly distilled in order to improve its purity. The following reference compounds were obtained from the suppliers shown: ethyl isobutanoate, 2-methyl-3-buten-2-ol,
Basic oenological parameters
Botrytis cinerea infection produced a pronounced decrease in berry water content indicated by the increase of sugar level (°Brix). The musts affected by Botrytis cinerea (botrytized must, BM) encountered on average a 20% increase in °Brix value in relation to the healthy samples (healthy must, HM) collected at the same day from the same vineyard. In addition, an increase of titratable acidity was also observed while the pH did not undergo any clear variations. Regarding the effects of Erysiphe
Acknowledgments
The project was funded by the Bavarian Research Foundation in the project 152-12.
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