WINETECH Technical Yearbook 2020

Metschnikowia pulcherrima yeast and its role in winemaking

TECHNICAL CHARACTERISTICS ● Temperature requirements/ tolerance Metschnikowia pulcherrima strains can grow well under low temperature (15-20°C), but cannot grow at high temperature (e.g. 37°C). They can also grow over a wide pH range (3-6), but stability below pH 4 is low (Morata et al ., 2019). Most strains can tolerate up to 40 mg/L total SO 2 (Morata et al ., 2019; Barbosa et al ., 2018; Comitini et al ., 2011), with a few strains including M. pulcherrima Flavia ® MP346 resistant to higher levels (Morata et al ., 2020). ● Alcohol tolerance Most strains of M. pulcherrima can tolerate at least 6% v/v ethanol, with only a few exceptions showing an ability to grow above 9% v/v (Barbosa et al ., 2018). ● Oxygen requirements Metschnikowia pulcherrima dis­ plays a highly aerobic respiratory metabolism and requires high levels of oxygen to persist. Indeed, M. pulcherrima falls under the group of non- Saccharomyces yeasts that under suitable aeration conditions more than 40% of the sugar they consume is expected to be respired, whereas in S. cerevisiae only 25% of

diploid cells into thick-walled, refractile, spherical chlamydospores (also referred to as pulcherrima cells) that contain lipid globules. Some strains may form pseudohyphae under anaerobic conditions (Vicente et al ., 2020; Morata et al ., 2019; Lachance, 2016). When cultivated on YPD (yeast extract, peptone and dextrose) agar, M. pulcherrima cells form cream-coloured colonies that produce a reddish-brown soluble pigment (pulcherrimin) visible from the bottom. NITROGEN METABOLISM Metschnikowia pulcherrima can utilise a variety of nitrogen sources. It can use lysine as the sole nitrogen source and can use proline more efficiently than Saccharomyces cerevisiae (Su et al ., 2020) . Metschnikowia pulcherrima is one of the non- Saccharomyces yeast species that display low yeast assimilable nitrogen (YAN) uptake and is unable to complete wine fermentation regardless of the nature of the nitrogen source, despite displaying a shorter lag phase on most nitrogen sources. The presence of ammonia in the media does not greatly affect amino acid uptake (Su et al ., 2020; Roca-Mesa et al ., 2020). Overall, M. pulcherrima is least influenced by must nitrogen composition compared to S. cerevisiae and other non- Saccharomyces yeasts frequently encountered in wine fermentation (Roca-Mesa et al ., 2020).

OCTOBER 2020

EVODIA SETATI: Department of Viticulture and Oenology, Stellenbosch University, Stellenbosch KEYWORDS: non- Saccharomyces yeasts, glycosidases, β -lyase

WHERE IT IS FOUND Metschnikowia pulcherrima is an ubiqui­ tous yeast primarily associated with fruit- feeding insects. However, it has been found on grapes and is commonly isolated from grape juice and grape must. It has also been found in various fresh and spoiled fruit, flowers, nectar and tree sap fluxes (Morata et al ., 2019). WHAT IT LOOKS LIKE Metschnikowia pulcherrima forms ovoid to ellipsoidal cells (2-12 µm x 2-10 µm). It reproduces asexually by multilateral budding and sexually through the for­ mation of elongated asci containing two needle-shaped spores. The spore formation is preceded by differentiation of

M. pulcherrima is least influenced by must nitrogen composition compared to S. cerevisiae and other non- Saccharomyces yeasts frequently encountered in wine fermentation. CURRENT AND PREVIOUS NAMES Metschnikowia pulcherrima has pre­ viously been designated under various names based on classical yeast taxonomy methods. The names include Candida pulcherrima (the anamorph), Eutorula pulcherrima , Monilia castellanii , Torula pulcherrima , Torulopsis pulcherrima , Torulopsis burgeffiana , Torulopsis dattila , Rhodotorula pulcherrima , Saccharomyces pulcherrimus and Cryptococcus castellanii (Vicente et al ., 2020) .

WINETECH TECHNICAL YEARBOOK 2020 113