Cell viability was determined using the Thoma haemocytometer as follows: 1 mL of sample medium was taken and diluted in 9 mL of deionised water. The sample of 1 mL of this solution was dissolved with 1 mL of 10% v/v methylene blue solution and left for 10 minutes. Aliquots of 1µL were placed on the haemocytometer by using a Pasteur pipette. The haemocytometer was then microscopically observed by an optical microscope (Olympus model CHK2-F-GS microscope). Yeast cell viability was calculated and expressed as follow: Viability (%) = a/n x 100.