ABSTRACT

Effect of acetic acid bacteria (Acetobacter aceti) on yeast growth and metabolism during fermentation of soursop juice was studied. Preliminary studies were carried out to identify the endogenous species of yeast responsible for natural fermentation of soursop juice and the effect of exogenous acetic acid and pH on growth and tolerance rate of the different yeast isolates. Soursop juice was inoculated with the different yeast isolates Saccharomyces cerevisiae (SC), Hansenula anomala (HA), Canadida tropicalis (CA) and a brewery strain Saccharomyces pastorianus (SP) with and without Acetabacter aceti (AAB). The growth rate of yeast, alcohol production, total titratable acidity, carbon (IV) oxide production and sugar assimilation were monitored for 7 days. Results showed a reduction in maximal cell concentration with samples treated with Acetobacter aceti. The inhibition of growth rate was higher on non- Saccharomyces yeasts (HA) and (CA) while the brewery strain (SP) indicated a higher tolerance to Acetobacter aceti in the fermenting juice. Addition of 0.5m1 v/v of exogenous acetic acid resulted in a drop in pH from 6.8 to 4.1 and caused a reduction in cell count while the addition of 1 .0m1 v/v acetic acid resulted in a drop of pH from 6.8 to 3.7 and showed complete inhibition of growth of all the yeast isolates. When the medium was adjusted by iN hydrochloric acid to pH 4.1 and 3.7 all the yeast isolates showed the same growth as the control (pH 6.8). However, treatments without Acetobacter aceti yielded higher alcohol level (5.09% v/v for SC) while treatment with Acetobacter aceti yielded lower alcohol (2.2 1% v/v for SP + CA + AAB) on the 7th day of fermentation. Acidity level with all treatments with Acetobacter aceti (2.13g acetic acid/i for SC + HA + AAB) was the highest while treatments without the bacteria showed lower acidity levels (1.11 g acetic acidll for HA). The higher acidity level of samples treated with Acetobacter aceti led to corresponding drop in when compared to sample not treated with the bacteria. However, sugar assimilation by yeast was significantly different (P 0.05) for treatments with Acetobacter aceti (CA + AAB with 40.24g/l) than treatments without Acetobacter aceti which utilized sugar much higher (0.75g/l for SP + HA) after the 7th day of fermentation.