Differences in oenological characteristics of autochthonous non-Saccharomyces yeasts based on multivariate statistical analysis

Non-Saccharomyces yeast (NS) strains can be adapted to generate diverse secondary metabolites, in order to alter the flavor and aroma characteristics of wines. Particularly, the NS strains are co-inoculated with Saccharomyces cerevisiae in the terroir of the production area. Therefore, it is of great significance to identify the differences in the oenological characteristics of autochthonous NS strains, in order to regulate the wine flavor. In this experiment, the test strains were selected as Hanseniaspora uvarum (HX-1, HX-3), Metschnikowia pulcherrima (HX-2, HX-4), and Torulaspora delbrueckii (HX-6, HX-7) isolated from spontaneously grape juice in Hexi Corridor wine region. A systematic measurement was performed on the growth curves of these 6 non-Saccharomyces yeast strains, together with their tolerance to vinification factors. The results showed that the NS strains belonging to the same genus showed similar growth curves, and all strains were well tolerant to the high concentrations of glucose and SO₂, among which HX-2, HX-6, and HX-7 still grew normally at a glucose concentration of 300 g/L. Except for the HX-6 strain, the growth of the rest strains was only slightly inhibited at an SO₂ concentration of 300 mg/L. Notably, there were significant differences among the tested strains under different culture temperatures, pH, and ethanol levels. Among them, HX-1, HX-3, HX-6, and HX-7 strains showed excellent low-temperature tolerance, where their OD₆₀₀ values reached more than 0.6 under the fermentation temperature of 15℃. In addition, HX-1, HX-2, HX-3, and HX-4 strains grew normally under low pH conditions. HX-6 and HX-7 strains showed the strongest tolerance to ethanol. In particular, the growth of HX-6 was slightly inhibited, where the absorbance value reached 0.569 when the ethanol level reached 9%. All the test strains were inoculated into Merlot grape juice for pure fermentation, in order to investigate the vinification performance of these NS strains. The oenological parameters of the wine samples were determined to detect the volatile aroma compounds using HS-SPME-GC-MS. Pure culture fermentation experiments showed that the HX-6 and HX-7 strains exhibited strong fermentation ability and high production of ethanol and glycerol during Merlot wine making. Torulaspora delbrueckii strain HX-7 was closely related to the formation of volatile compounds, such as 1-heptanol, isoamyl alcohol, isoamyl acetate, isoamyl acetate, and ethyl acetate, while the HX-6 strain was effectively improved the content of ester compounds, such as ethyl caprate and ethyl caprylate in wine samples. Hanseniaspora uvarum strains HX-1 and HX-3 significantly enhanced the content of total acidity and malic acid, with great contribution to the formation of C13-norisopentadiene in the wines. The limited reducing sugar by the HX-1 strain resulted in the alcohol fermentation stagnation on the 8th day after inoculation. The HX-4 strain shared a high yield of volatile acidity. The HX-2 strain significantly increased the content of varietal aroma compounds, such as citronellol and linalool in Merlot wine. Hierarchical clustering of the oenological parameters combined with the aroma compounds revealed that there were significant intraspecific or interspecific differences in the fermentation characteristics of the tested NS strains. Comprehensive analysis showed that the oenological parameters and aroma profile of the autochthonous Torulaspora delbrueckii and Hanseniaspora uvarum strains during pure culture fermentation showed intraspecies similarity, while the fermentation characteristics of Metschnikowia pulcherrima showed strain dependence. The finding can provide a scientific reference to regulate the flavor quality of wines using mixed fermentation.