The main objective of this study was to improve fermentation of yellow passion fruit (YPF) juice by an adequate choice of commercial Saccharomyces (S.) yeast strains and nutrient supplements. Comprehensive information has been gathered on the basis of the experimental results obtained from the research on grape wines of the varieties Scheurebe and Sauvignon blanc. The optimal choice of commercial Saccharomyces yeast strains and nutrient supplementations in YPF juice was studied in detail based on the results, which were obtained from grape wines.YPF puree was used for fermentation trials. The puree has a high acidity (4.0-5.5 % citric acid). Therefore, it was diluted by the addition of water and then this prepared YPF juice showed a lower amino nitrogen level. Consequently, there is a need for nutrient supplementation, because nitrogen deficiencies are linked to slow and sluggish fermentations and the formation of SO2-binding compounds and reduced sulphur compounds. The prepared YPF juices were fermented at 20 °C under controlled conditions and the fermentation characteristics, fermentation-derived and varietal aromas which impact YPF wine quality were investigated. In order to select adequate commercial yeast strains for the improvement of YPF wine, 14 Saccharomyces yeast strains were examined. The results revealed that when 0.5 g L-1 diammonium hydrogen phosphate (DAP) was added as sole source of nitrogen in the YPF juice, three S. cerevisiae (var. bayanus) strains, two S. cerevisiae strains and two Saccharomyces spp. strains seemed to be an optimal choice for the YPF wine production.Two S. cerevisiae (var. bayanus) strains, three S. cerevisiae strains and one Saccharomyces spp. strain were then selected to study the yeast fermentation behaviour and the production of metabolic compounds as well as volatile thiols, which contribute to the varietal aroma of YPF wines. In addition, different nutrient supplements were tested to select an appropriate nitrogen source and concentration. The results clearly showed that nutrient supplements like the nutrient mixtures of DAP and thiamine (NM1) as well as DAP, thiamine and yeast cell walls (NM2) at the levels normally recommended for grape wine production (0.4-0.5 g L-1) were the best nutrient condition to improve the fermentation characteristics and the formation of desirable non-volatile and volatile compounds in the YPF wines for most yeast strains mentioned above.The results also showed that one strain of S. cerevisiae (var. bayanus) with the addition of 0.25 g L-1 DAP and 0.5 g L-1 NM1 tended to a higher production of varietal volatile thiols, 3-sulphanylhexanol (3SH) and acetic acid 3-sulphanylhexyl ester (3SHA). Nevertheless, it has to be taken into account that DAP addition resulted in excessive formation of keto acids by one S. cerevisiae strain. The other strain of S. cerevisiae (var. bayanus) also formed the highest amounts of 3SHA in the variant with 0.5 g L-1 NM1 treatment. One S. cerevisiae strain with the addition of NM1 and NM2 at 0.4 g L-1 also produced the YPF wines having the greatest concentration of 3SH. Neither the yeast strains nor the nutrient sources influenced the amount of 3SHA in YPF wines. Concentrations of 3SH and 3SHA were quite beyond their aroma threshold value contributing to typical varietal aromas. It can be concluded that certain yeast strains and optimal nutrient supplementations had a great impact on successful YPF wine fermentation, minimizing the formation of SO2-binding and undesirable volatile sulphur compounds and improving desirable aroma compounds in final YPF wine products. Nevertheless, the choice of nutrient supplements and their concentrations is yeast strain-dependent.This study has an important implication for the YPF wine as well as for the tropical fruit winemaking industry, particularly in Thailand, where a better understanding of the nutritional requirements of Saccharomyces yeast is necessary to reduce fermentation problems and to improve the final product quality. It is worthwhile pointing out that this work is an interesting new observation for the YPF wine fermentation, although the fermentation trials were only done in the laboratory scale. Nevertheless, further studies for a better understanding of the effect of nutrient supplementation on yeast metabolic products as well as sensory properties is still required in both the laboratory and pilot scale.
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