Screening for new brewing yeasts in the non-Saccharomyces sector with Torulaspora delbrueckii as model.

This study describes a screening system for future brewing yeasts focusing on non-Saccharomyces yeasts. The aim was to find new yeast strains that can ferment beer wort into a respectable beer. Ten Torulaspora delbrueckii strains were put through the screening system, which included sugar utilization tests, hop resistance tests, ethanol resistance tests, polymerase chain reaction fingerprinting, propagation tests, amino acid catabolism and anabolism, phenolic off-flavour tests and trial fermentations. Trial fermentations were analysed for extract reduction, pH drop, yeast concentration in bulk fluid and fermentation by-products. All investigated strains were able to partly ferment wort sugars and showed high tolerance to hop compounds and ethanol. One of the investigated yeast strains fermented all the wort sugars and produced a respectable fruity flavour and a beer of average ethanol content with a high volatile flavour compound concentration. Two other strains could possibly be used for pre-fermentation as a bio-flavouring agent for beers that have been post-fermented by Saccharomyces strains as a consequence of their low sugar utilization but good flavour-forming properties.

Torulaspora indica a novel yeast species isolated from coal mine soils.

Four yeast strains (APSS 805, APSS 806, APSS 815 and AP-18) belonging to a novel Torulaspora species were isolated from coal mine soils of Singareni in Andhra Pradesh state, India. Another strain (PBA-22) was isolated from agricultural field soil from Gujarat state, India. The vegetative cells of all these strains were round, haploid and produced asci by conjugation between independent cells or mother cell and bud, with rough ascospores, suggesting their possible relation to ascomycetous yeast genus Torulaspora. Phylogenetic analysis of the D1/D2 domain of the large subunit (LSU) rRNA gene and Internal Transcribed Spacer (ITS) regions revealed that, among the five strains, three viz. APSS 805, APSS 806 and APSS 815 have identical sequences. The other two strains (AP-18 and PBA-22) differed from the other three strains in less than 1% nucleotide substitutions in the combined D1/D2 domain and ITS sequences, indicating that all of them (five strains) may belong to the same species. These five strains were closely related to Torulaspora globosa, but showed more than 3-7% sequence divergence from T. globosa and all other species in the genus Torulaspora in the combined sequence analysis of D1/D2 domain and ITS region of rRNA gene. In addition, these strains also showed distinct microsatellite finger-printing pattern from related species and differed in several physiological responses suggesting that these strains belong to a novel species of Torulaspora. We propose to name these strains as Torulaspora indica sp. nov., and designate APSS 805(T) = MTCC 9772 (T) = CBS 12408 (T) as the type strain of this novel species. The Mycobank number of the novel species is MB 563738.

Torulaspora quercuum sp. nov. and Candida pseudohumilis sp. nov., novel yeasts from human and forest habitats.

Strains XZ-46A, XZ-105, XZ-129 and XZ-281(T) isolated from the oral cavities of healthy Tibetan volunteers were revealed to represent two novel ascomycetous yeast species by molecular taxonomic characterizations. Strain XZ-281(T) was most closely related to Candida humilis, but differed from the type strain of the species by eight (1.2%) substitutions in the 26S rRNA gene D1/D2 domain and by >100 (>20%) mismatches in the internal transcribed spacer (ITS) region. Strains XZ-46A, XZ-105 and XZ-129 had identical or similar D1/D2 and ITS sequences with each other and with strain 17YF(T) isolated from a leaf of an oak tree (Quercus sp.). The closest relative of this group was Torulaspora microellipsoides. They differed from the type strain of the species by five (0.9%) substitutions in the D1/D2 domain and >70 (>15%) mismatches in the ITS region. A sexual state was observed in strain 17YF(T), but not in the other four oral strains. An anamorphic name Candida pseudohumilis sp. nov. is proposed for strain XZ-281(T) (=AS 2.3956(T)=CBS 11404(T)) and a teleomorphic name Torulaspora quercuum sp. nov. is proposed for strain 17YF(T) (=AS 2.3768(T)=CBS 11403(T)) and the other three oral strains.

Effect of mixed Saccharomyces cerevisiae Y10 and Torulaspora delbrueckii Y22 on dough fermentation for steamed bread making.

The use of non-Saccharomyces yeasts in dough fermentation has become increasingly popular because of their effects on product composition, texture and flavour. These yeasts are co-cultured with Saccharomyces cerevisiae. In this study, the characteristics of dough fermentation with combined Torulaspora delbrueckii Y22 and S. cerevisiae Y10 were investigated. In the dough containing co-cultures, S. cerevisiae Y10 cell populations increased rapidly and reached approximately 8.5 Log CFU/g wet dough, which is comparable to the monoculture after 24 h of fermentation. However, the cell number of T. delbrueckii Y22 did not significantly change throughout the dough fermentation (p > 0.05). When co-culture was used, the gas holding capacity and CO2 production profile of the dough improved, and a high maltose concentration reaching 5.93 mg/g dry dough was observed after 12 h of dough fermentation. The mixed inocula of S. cerevisiae Y10 and T. delbrueckii Y22 enhanced the production of succinic acid, acetic acid and essential amino acids in the fermented dough. These results revealed the synergistic behaviour between S. cerevisiae Y10 and T. delbrueckii Y22 during dough fermentation and suggested the potential use of mixed yeast cultures in dough fermentation for steamed bread making.