Diversity within yeast involved in spontaneous fermentation of pito

A survey was conducted in ten (10) “Dagarti pito” production sites located in nine towns within eight administrative regions of Ghana to investigate diversity within yeast varieties involved in the spontaneous fermentation of Dagarti pito. Samples of dry yeast were obtained from commercial Dagarti pito producers from Tamale and Nyankpala (Northern Region); Ayigya- Kumasi and Monaco-Kumasi (Ashanti Region); Accra (Greater- Accra Region); Cape Coast (Central Region); Takoradi (Western Region); Sunyani (Brong Ahafo Region); Ho (Volta Region) and Suhum (Eastern Region). For purposes of comparison, dry yeast was also sampled from three dolo production sites in Ouagadougou, Burkina Faso. Yeast populations ranged between 106 and 108 cfu g-1. Twenty-five yeast isolates from each site were characterized phenotypically by colony and cell morphology as well as carbohydrate assimilation profiling, using the API ID 32 C Kit (Biomerieux SA, Marcy L’Etoile, France). Ninety-nine percent (247) of the isolates showed colony and cell morphologies typical of S. cerevisiae. Of these, 72 % (179) had fifty-three carbohydrate assimilation profiles similar to S. cerevisiae (according to Vaughan-Martini and Martini, 1998) and were subsequently identified as such while 28 % (68) which had four carbohydrate assimilation profiles atypical of S. cerevisiae or any other member of the sensu stricto complex could not be identified in API galleries. Two isolates (1%) which had colony and cell morphologies atypical of S. cerevisiae, and a broad-spectrum assimilation profile, were identified as Candida kefyr. Genotyping of five randomly selected isolates from each site was carried out using the Polymerase Chain Reaction (PCR) to amplify the region spanning the two intergenic transcribed spacers (ITS) and the 5.8S ribosomal gene (ITS1-5.8S rDNA-ITS2), followed by restriction analysis (ITS-PCR+RFLP) of the product, as well as Pulsed Field Gel Electrophoresis (PFGE). The genetic analyses indicated that all of them belonged to S. cerevisiae, notwithstanding the phenotypic differences. The mitochondrial cytochrome-c oxidase II gene (COX 2) of four isolates representing the four chromosome profile groupings that emerged after PFGE, were then sequenced to confirm their close relatedness to S. cerevisiae, particularly type strain CBS1171. Two isolates randomly selected from each of the ten production sites, (one with a broad carbohydrate assimilation spectrum and the other with a narrow carbohydrate assimilation spectrum) and assessed for technological properties showed different patterns of growth and flocculation without much change in pH during fermentation, and most of them produced pito having sensory attributes which compared favorably with commercially produced pito. Pito produced with each of ten out of the twenty yeast strains from Ghana used for the earlier investigations and three from Burkina Faso was analyzed by headspace, for its aroma constituents. All ten Ghanaian isolates could form aromatic compounds representing the alcohols, esters, and ketones which are among reported typical flavor compounds of conventional beer. This study has demonstrated diversity within S. cerevisiae strains involved in fermentation of pito wort. These strains possess desirable technological properties, including sufficient growth during fermentation and efficient hydrolysis of sugars for biomass enhancement. They also demonstrated fermentation activities, particularly, ethanol production, formation of aroma compounds and metabolites, which impart appropriate sensory attributes to pito.

Source: KNUST