Microbial spoilage of dried prunes: II. Studies of the osmophilic nature of spoilage organisms

Eleven strains of yeast, representing species most frequently isolated from spoiled dried prunes, and 124 strains of molds isolated from the same source were studied for their osmophilic character. Strains of Saccharomyces rouxii, S. mellis, and Torulopsis stellata were able to ferment in a medium containing 70 per cent soluble solids, but failed to grow in two weeks in a medium containing 75 per cent soluble solids. Strains of T. magnolieae and S. rosei were able to ferment in 65 per cent soluble solids but not in 70 per cent. The growth rate of Saccharomyces rouxii was suppressed considerably when soluble solids were increased from 40 to 60 per cent, was very slow at 65 per cent, and not detectable at 70 per cent soluble solids. Colony diameters of the mold strains were measured daily for up to seven days, on solid media containing 40, 50, and 60 per cent soluble solids. Strains of Aspergillus glaucus showed pronounced osmophilic characteristics whereas those of A. niger and Penicillium spp. were some-what less osmophilic. Strains in the genera Alternaria, Monilia, and Chaetomella showed no osmophilic characteristics. The single strain of Mucor grew luxuriantly on 40 per cent soluble solids, barely grew on 50 per cent, and not at all on 60 per cent. The effect of increasing the soluble-solids content in a medium on the lag phase and the growth rate was least for strains of Aspergillus glaucus, confirming the definite osmophilic nature of this group of molds, and greatest for strains of Penicillium. A. niger strains were affected in an intermediate fashion on the same media.