Growth responses to environmental stress factors by osmophilic yeasts isolated from honey

Yeasts are continuously exposed to series of changes within their natural habitat in the environment. Their growth and proliferation are readily affected by certain factors in their environment. This study was designed to isolate osmophilic yeast from honey and to evaluate the effect of some stress factors such as temperature, pH, NaCl, heavy metals and preservatives on the growth of yeast. The honey samples for this study were collected from Ogige market in Nsukka. The yeasts were isolated, identified and characterized using standard microbiological techniques while the growth of the isolates at different concentration of stress factors were determined using spectrophotometer. The osmophilic yeasts identified as Candida sp., Debaryomyces sp., Rhodotorula sp. and Zygosaccharomyces sp. were isolated from honey samples. These isolates displayed varying degrees of responses to different stress factors. There was no significant difference (P≥0.05) in the growth of the isolates at the different concentrations of NaCl. The isolates had the highest growth at 0% NaCl and least growth at 20% NaCl. The growth of the isolates was also affected by increase in concentration of heavy metals. In the absence of HgCl2 and AgN03, the isolates showed very high growth while there was no apparent growth at 2.5% HgCl2 and AgNO3. The isolates also showed the highest growth at 0%

benzoic and sorbic acids and least growth at 1.0% and 1.5% benzoic acid and sorbic acid, respectively. All the isolates showed the highest growth at temperature of 30oC after 96 h of incubation. The optimum pH for the growth of Debaryomyces sp. was 4.0 while that ofRhodotorula sp., Zygosaccharomyces sp. andCandida sp. was 5.0. There was significant difference (p≤0.05) in the growth of isolates at different exposure times to uv with higher growth recorded at 0 mins and the least observable colonies were obtained at 60 mins. The results showed that yeasts can tolerate different stress conditions and also indicated that sorbic and benzoic acids can be useful in the preservation of food products whereas increase in exposure time to uv radiation can be a good method of inhibiting growth of yeast.