Evaluation of diluents and media for enumerating Zygosaccharomyces rouxii in blueberry syrup

Combinations of diluents and enumeration media were evaluated for their efficacy in enumerating Zygosaccharomyces rouxii in blueberry syrup (aw 0.818, 0.870 and 0.921). Diluents consisted of deionized water containing 0.1% peptone, 20%, 30%, 40% or 50% glucose, 50% glucose plus 0.05% Tween 80, and 12%, 18%, 26% or 35% glycerol, all calculated on a ww basis. Enumeration media were dichloran rose bengal chloramphenicol (DRBC) agar, tryptone glucose yeast extract (TGY) agar, dichloran 18% glycerol (DG18) agar, plate count agar containing 52% sucrose (PCA52S) and malt extract yeast extract 50% glucose (MY50G) agar. Two test strains of Z. rouxii grown in blueberry syrup for 7 or 14 days responded similarly to diluent/enumeration medium combinations. The use of 0.1% peptone diluent or DRBC agar in combination with other enumeration media or diluents was inferior for recovering Z. rouxii. As the aw of the blueberry syrup decreased, the sensitivity of Z. rouxii to diluent containing decreased concentrations of glucose increased. The use of glycerol as a solute in diluent did not result in a similar trend, indicating that the protective effect of glycerol against osmotic stress to Z. rouxii cells is greater than that of glucose at high aw and that cells adapted to low aw in blueberry syrup are more sensitive to high aw in diluents. The addition of 0.05% Tween 80 to 50% glucose diluent did not influence performance. Overall, diluents containing 50% glucose (aw 0.898), 18% glycerol (aw 0.956) or 26% glycerol (0.934), in combination with TGY agar (aw 0.982), resulted in the highest recovery of Z. rouxii from blueberry syrup at aw 0.818 to 0.921.