## 4 Discussion
However, there are few studies that would confirm or refute the influence of this process on the sensory quality of wine. Our study complements this part by showing that the spontaneous alcoholic fermentation has an influence on the sensory characteristic of wine. However, further research is needed to investigate how this can be integrated into the understanding of wine typicity regarding geographical indications or terrori, as was demonstrated for Douro wines [57] and discussed by other authors [8; 11]. On the other hand, it was not possible to distinguish the wines with cluster thinning from the wines without crop regulation by means of a sensory triangle test, a result that contradicts the result of the one-year and one-location study on the Syrah variety [33] and the three-year and one-location study on the Pinot Noir variety [37]. Thus, it cannot be confirmed that yield regulation between 27 and 54% of the original yields in two vineyards and two years had a significant influence on the sensory characteristics of the Merlot wine in our study. It is true that in our experiment we could not ensure fully repeatable yields per vine in two consecutive years, as in 2021 the yield at location B was lower due to spring frost, and in the same year at location M, there were differences in the number of clusters between the control and thinned vines even before thinning. However, based on the results of other experiments on the influence of cluster thinning on the sensory quality of the wine, we can conclude that thinning could have a linear effect over a larger range of yield per vine [65]. The chemical parameters showed a tendency toward an increase in the content of ethyl acetate and volatile acids, as well as \(\gamma\)-butyrolactone, in the cluster thinned wines. These differences were smaller than those observed by other authors for the Syrah grape variety, where the esters were found in higher concentrations in the cluster thinned wines [34]. The cluster thinning in the Welschriesling grape variety showed an effect on the significantly increased content of 3 MHA in one of the three years of the trial and in some cases also a tendency to increase the content of 3 MH [36], which we could not confirm in the current study. At the same time, we also confirmed the results of the meta-study [31] and specific studies [30; 34; 37] that cluster thinning influences increasing the pH value, with a simultaneous tendency toward a slight decrease in the content of total acids in the wine. These results and the result confirming that CT has no influence on the total anthocyanin content of wine are also consistent with the results of another study on the Merlot grape variety [30] and are in contrast to those obtained for the Pinot Noir grape variety [37]. Our results are also in line with the conclusions of the authors of the recent review on cluster thinning [66] and other studies, in which the authors concluded that the effect of this technical factor on wine composition is an interaction between the variety [67], environment, vintage and intensity of cluster thinning [37], and therefore, the outcome of the intervention is difficult to predict. It should also be emphasised that in our experiment, the vintage effect outweighed the effect of cluster thinning and the type of alcoholic fermentation. The interaction between the type of alcoholic fermentation and the vintage and also the cultivar was already shown by others [68]. However, spontaneous alcoholic fermentation reduced the influence of vintage on the wine composition in our study, while the opposite was true for cluster thinning when we considered the first two PCA plots, one for each location. The importance of vintage has already been emphasised, with its effects on the metabolic profiles of grapes outweighing the effects of soil [3] as an important factor in determining wine quality.