Water relations and metabolism of propionate in two yeasts from hay

Many yeasts and bacteria were isolated from moist hays (>30% water content) treated with up to 3% propionic acid‐based preservatives. Predominant yeasts were Candida guilliermondii var. guilliermondii and Hyphopichia burtonii. Growth of both species was decreased more than 50% in liquid medium containing 54 mmol/l ammonium propionate but some still occurred in 135 mmol/l propionate. Both metabolized between 80 and 85% of 27 mmol/l ammonium propionate in 1% malt broth within four weeks at 25°C. Growth on solid malt extract agar containing ammonium propionate was decreased by decreasing the water availability (water activity, a w ) in the medium. Growth rates were slightly greater when glycerol rather than NaCl was used to alter a w in the range 0.995 to 0.93. At both 0.995 and 0.95 a w optimum growth was at pH 6. The significance of these findings with regard to the preservation of moist hay is discussed.