Natural occurrence and production of tenuazonic acid in wine grapes in Argentina.

A survey was carried out to determine natural occurrence of tenuazonic acid (TA) in healthy and rotten wine grapes samples from different varieties (n = 37) collected during 2016 vintage in the region of DOC San Rafael (Argentina). In addition, inoculation experiments with three Alternaria alternata strains in wine grapes were done to elucidate TA production and its major influencing factors. The 16.2% (6/37) of total wine grape samples showed TA contamination with 4% (1/25) of incidence in healthy samples (77 µg·kg-1) and 42% (5/12) in rotten samples (10-778 µg·kg-1). Malbec, Cabernet Sauvignon, and Syrah varieties showed TA contamination, whereas Bonarda, Ancelota, Torrontés, Semillón, and Chenin did not. During inoculation experiments in wine grapes, two of three strains were able to produce TA among the evaluated conditions and the highest TA production was observed at 15°C and 25°C after 24 days of incubation. Nutritional composition of grapes results appropriate for A. alternata infection and TA production and, together with the adequate field conditions, favors TA natural occurrence in wine grapes.

Isolation, identification and selection of antagonistic yeast against Alternaria alternata infection and tenuazonic acid production in wine grapes from Argentina.

Epiphytic isolates with yeast characteristics from grapes of the Malbec cultivar were obtained in order to find antagonists against Alternaria alternata. From a total of 111 isolates, 82% corresponded to the yeast-like organism Aureobasidium pullulans and the rest to the non-Saccharomyces yeasts Hanseniaspora uvarum (6.3%), Metschnikowia pulcherrima or spp. (5.4%), Cryptoccocus laurentti II (2.7%), Starmerella bacilaris or Candida zemplinina (2.7%) and Rhodotorula spp. (0.9%). The 22.4% (15 out of 67) of epiphytic yeasts and yeast-like organisms evaluated were able to reduce A. alternata infection from 0.0 to 4.4% when applied 2h previous to pathogen inoculation on wounds of grape berries. From these selected strains, 14 out of 15 strains completely prevented A. alternata infection (0.0%), which implies potential for field application. All Metschnikowia (pulcherrima or spp.), S. bacillaris and almost all H. uvarum evaluated strains showed antagonist capability against A. alternata. Meanwhile, none of the lesser nutritional requirement strains belonging to A. pullulans, Cr. laurenti II and Rhodotorula spp. did. All the yeasts with capacity to prevent A. alternata infection also reduced tenuazonic acid (TA) production by 81.2 to 99.8%, finding TA levels similar to negative controls. Therefore, the epiphytic yeasts selected are promising as biological control agents against Alternaria infection and toxin production in grapes for winemaking.

Characterization of wines produced by mixed culture of autochthonous yeasts and Oenococcus oeni from the northwest region of Argentina.

Two autochthonous yeasts from the northwest region of Argentina, Kloeckera apiculata mc1 and Saccharomyces cerevisiae mc2, were used as pure or mixed starter cultures in microvinification trials conducted in Malbec red must. Also, the effect of Oenococcus oeni X(2)L was evaluated. S. cerevisiae mc2 showed adequate growth and fermentative activity in single and composite fermentations, producing standard concentration of ethanol. The amount of esters was higher in fermentations conducted using mixed yeast starters. Independent of the timing of inoculation of O. oeni, this malolactic bacterium completely depleted malic acid. Sensory evaluation indicated that young wines fermented with mixed yeast cultures and sequential inoculation of O. oeni were preferred, achieving the highest scores for positive descriptors and they allowed better control of the sensory quality. Consequently, this study proposes inclusion of autochthonous K. apiculata mc1 as an adjunct culture to S. cerevisiae mc2 during Malbec must fermentation to improve the organoleptic properties of red wines. Furthermore, sequential inoculation of O. oeni X(2)L should be carried out after completion of the alcoholic fermentation to enhance sensory characteristics.