Yeast Lab Report

2. Materials and methods

2.1.Yeast cells and culture growth conditions

Yeast cells S. cerevisiae VIN 13 strain (commercial dry yeast) used for laboratory experiments were provided by Anchor Yeast (South Africa). Yeast cells were grown in a defined medium containing (per liter of deionized water): 100 g D-glucose, 1 g K2HPO4, 1 g K2H2PO4, 0.2 g ZnSO4, 0.2 g MgSO4, 2 g yeast extract and 2 g NH4SO4. All the media components were purchased from the Sigma Chemical Company (St Louis, MO, USA).

2.2.Yeast cell preconditioning and inoculum preparation

1 g dry weight of yeast was resuspended in 100 mL of deionized water in an Erlenmeyer flask of 250 mL volume, at 30–35°C, for 30 min with NaCl 6% w/v. Inoculum for experimental fermentations was prepared

1mL of this solution was dissolved with 1mL of 10% v/v methylene blue solution and left for 10 min. 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

Monitoring of fermentation kinetics

OD at 600nm was determined spectrometrically (phasmatophotometer mini 1240, SHIMADZU, China) and was used for the monitoring of fermentation kinetics and the discrimination of exponential, lag and death phase of cells growth during the fermentation process 12b.

2.8. Determination of cells dry weight

The sediment of yeast cells was frozen (-80oC/24 h), lyophilized (-50oC /vacuum/24 h) using a Thermo Fischer (USA) drying digital unit. The weight of each yeast cells freeze dried sample was measured gravimetrically with an analytical balance (Kern, Kern and Sohn GmbH, Germany) with an accuracy of four decimals.

2.9. Determination of β-glucan concentration (%)

The determinations of total yeast β-glucan concentration in lyophilized yeast biomass were performed with the use of Enzymatic K-EBHLG Yeast Beta-Glucan Assay Kit (Megazyme,