WINETECH Technical Yearbook 2020

Torulaspora delbrueckii and its role in winemaking

TECHNICAL CHARACTERISTICS Torulaspora delbrueckii is a moderate fermenter with less fermentation vigour and slower growth rate than S. cerevisiae. Most strains cannot ferment to dryness and produce varying amounts of ethanol ranging from 5-11% (v/v) and can tolerate up to 10% ethanol and ferment to dryness in 50 mg/L SO 2 . A sequential inoculation with S. cerevisiae at a ratio of 10:1 (i.e. 10 7 cells/mL T. delbrueckii : 10 6 cells/mL S. cerevisiae ) is recommended to allow significant contribution of T. delbrueckii (Morata et al ., 2020; Ramírez & Velázquez, 2018; Zhang et al ., 2018). Torulaspora delbrueckii establishes better in clarified juice, which has less diversity and density of wild yeasts than unclarified must. NITROGEN METABOLISM Torulaspora delbrueckii displays similar nitrogen consumption and nitrogen source influence profile to S. cerevisiae (Kemsawasd et al ., 2015; Roca-Mesa et al ., 2020). Most importantly, production of acetic acid in the yeast is influenced by the nitrogen composition with NH 4 + resulting in the lowest acetic acid production compared to media with a mix of NH 4 + and amino acids (Roca-Mesa et al ., 2020). IMPACT ON WINE AROMA Low production of acetate esters has been attributed to low enzymatic activity of esterases and a lack of ATF1-2 genes re-

sponsible for the alcohol acetyltransferas- es required for the production of volatile aroma-active esters (Mecca et al ., 2020; Tondini et al ., 2019). Desirable oenolog- ical properties include low production of acetic acid, ethyl acetate, acetaldehyde and acetoin, medium production of H 2 S, and medium to high resistance to SO 2 (Az- zolini et al ., 2015; Benito, 2018; Ramírez & Velázquez, 2018; Zhang et al ., 2018). Furthermore, some strains of T. delbrueckii have been shown to enhance volatile thiols in Sauvignon blanc (Ramírez & Velázquez, 2018; Renault et al ., 2016). EFFECT ON MALOLACTIC FERMENTATION Most T. delbrueckii strains consume malic acid. The consumption level is strain dependent and vary from 10-30% of malic acid consumption (Mecca et al ., 2020; Du Plessis et al ., 2017). Supplementation with complex nutrients, such as Fermaid ® E-blanc and OptiMUMWhite™ (both from Lallemand), was shown to stimulate malic acid consumption (Mecca et al ., 2020). COMMERCIAL PRODUCTS Several commercial starter cultures of T. delbrueckii are available as active dry yeast in monoculture (table 1). Mixed cultures with S. cerevisiae (Oenoferm ® Wild & Pure) and with S. cerevisiae and Lachancea thermotolerans (Viniflora ® Melody™) are also available.

SEPTEMBER 2020

EVODIA SETATI: Department of Viticulture and Oenology, Stellenbosch University, Stellenbosch KEYWORDS: non- Saccharomyces yeasts, malic acid, bioprotectant

kefir, mezcal, colonche, olive, and tequila and cider production. Strains associated with grapes and wine form a genetically distinct group from those derived from other bioprocesses (Albertin et al ., 2014). WHAT IT LOOKS LIKE Torulaspora delbrueckii forms spherical to ellipsoidal cells which are 2-5 x 3-7 µm in size, slightly smaller than S. cerevisiae. On Wallerstein nutrient agar, the colonies of T. delbrueckii are knoblike with a smooth surface and a cream to light green pigmen- tation similar to S. cerevisiae (Azzolini et al ., 2015) . Torulaspora delbrueckii repro- duces asexually by multilateral budding and sexually with the formation of up to four spherical ascospores (Benito, 2018).

T. delbrueckii is generally regarded as a low producer of aroma compounds, in particular higher alcohols and esters, however, this is strain dependent. CURRENT AND PREVIOUS NAMES Torulaspora delbrueckii has had many synonyms, amongst them Saccharomyces rosei or Saccharomyces roseus and Saccharomyces delbrueckii. Its anamorph is Candida colliculosa. WHERE IT IS FOUND Torulaspora delbrueckii is ubiquitous and has been frequently isolated from natural environments (plants, soil and insects) and from various habitats associated with human activities, such as winemaking,

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