In small scale fermentation systems, the maximum growth rate for yeast is attained at a zinc concentration of 0-2 ppm. This is used as a base for a suggested assay for zinc availability. The availability of certain elements to the yeast cell can be regulated by certain organic compounds which will act as chelators. Some of these compounds may have the effect of rendering elements unavailable, whereas others may act as messenger molecules and speed up the transport of elements to the yeast cell. The described fermentation assay may be used for the screening of organic compounds for their chelating ability.