## 1 Introduction
However, their effects on wine aroma appear to be a strain-specific trait, as only certain _Hanseniuspora_ strains result in wines with increased levels of acetate esters such as 2-phenyl acetate and ethyl acetate, which are associated with fruity and floral aroma descriptions of wines [18; 20]. The production of other secondary metabolites, including glycerol, acetaldehyde, ethyl acetate, and hydrogen sulphide, also differs among strains [21; 22]. _Sarmerella_/_Candida_ spp. (_Candida zemplinina_/_Candida stellata_) strains, on the other hand, strongly influence the analytical profile of the wines, with a consistent increase in the levels of glycerol, succinic acid, terpen, and lactones, and a decrease in the levels of acetic acid, aldehydes, 3-mercaptoethanol-1-ol (3 MH), 3-mercaptoethanol acetate (3 MHA), and acetate esters compared to the values for _S. cerevisiae_ fermentation, but the result depends on whether pure or mixed cultures are used, on the yeast strain, and on the grapevine variety [13; 23; 24; 25; 26]. The use of non-_Saccharomyces_ strains can also intensify the colour of red wines after fermentation through the formation of large amounts of stable pyranoanthocyanin pigments [27], and _S. cerevisiae_ strains also differ in their influence on the formation of stable pigments [28]. However, an important management technique in the vineyard is yield regulation, which influences the ratio between the leaf area and the fruit mass of the vine and has a considerable influence on the composition of the fruit and the quality of the wine [29; 30]. To improve this ratio, winegrowers often use cluster thinning (CT), a management method in which clusters are selectively removed from the vines. There is still no consensus on the optimal timing and intensity of thinning to improve fruit composition [31]. Later authors summarised the results from the literature in a meta-analysis and reported, firstly, that the timing of thinning had little effect on fruit composition, giving growers more flexibility in applying this practise. Secondly, the severity of CT was important for improving fruit composition (TSS and pH), with only the moderate range (36-55%) being effective. The authors concluded that fruit composition is influenced more by the severity of CT than by the timing. In this meta-analysis, CT was also found to affect TSS accumulation on a variety-dependent basis, and the authors observed a significant but marginal increase in pH in grapes from vines exposed to CT. The increase in pH was similar in absolute terms to the average decrease in TA [30; 32]. However, the decrease in TA was not significant, although several individual studies reported occasional significant decreases in TA in vines exposed to CT [33; 34]. The meta-analysis and individual studies also showed that the concentration of anthocyanins and phenols in wine did not increase significantly because of CT [30; 31]. However, many individual studies have indicated a positive effect of CT on the anthocyanin and phenol concentration of the berries [31; 32; 33; 35]. The influence of CT on the composition of volatile organic compounds (VOCs) in wine and the sensory characteristics has been studied less than other parameters of wine composition. The positive effect of cluster thinning (approximately 40% of the vine's clusters and 27% in the yield per vine) on the VOCs in the wines and the clear separation of control and thinned wines was also demonstrated with the Syrah grape variety in a Mediterranean climate [34]. In this study, different esters were found in higher amounts in thinned wines. Our earlier study on Welschriesling in a cool climate region also showed that the concentration of the varietal thiols tends to be higher in thinned wines [36]. In addition, studies on cluster thinning in the Syrah and Pinot Noir grape varieties showed that the wines obtained from the thinned vines differ sufficiently from the control wines to be significantly distinguished by a trained panel in a triangle test [33; 37].