Observations on the Heat Resistance of Some Ropy Milk Organisms

Abstract

Summary Ropy milk organisms were isolated from ( a ) an outbreak of ropiness in pasteurized milk, ( b ) butter which showed ropy moisture droplets at the surface and which was made from pasteurized cream and ( c ) ropy whey obtained from pasteurized milk set for the manufacture of soft cheese. All proved to be Bacterium viscosum. In heat resistance trials the organisms failed to survive 61.1°C. for 3 minutes; these trials involved primarily the exposure of organisms added to milk after growing for varying periods in diluted milk or on agar slopes, although in trial 1 young and old milk cultures were used direct. Three cultures of Aerobacter aerogenes that produced ropiness in milk were studied for their heat resistance and found to vary greatly in this character. With culture F. organisms from an old diluted milk or agar slope culture survived 62.8°C. for 10 minutes but those from a young milk culture did not; with culture G organisms from either a young or old diluted milk or agar slope culture failed to survive 61.1°C. for 3 minutes; with culture H organisms from an old diluted milk or agar slope culture regularly resisted 62.8°C. for 10 minutes and in some instances for 20 minutes although organisms from young cultures failed to resist these exposures. In two trials with culture H organisms from old cultures on agar slopes were more resistant than those in diluted milk. The results indicate that in some instances ropiness in pasteurized milk or products made from it is due to contamination following the heating and that this possibility should be considered along with heat resistant causative organisms.