The germicidal effects of free and combined sulphur dioxide

Yeasts have been exposed to SO 2 in nutrient solutions with two concentrations of dextrose; chosen so as to induce the same rate of growth in the yeast, but to combine with quite different proportions of the SO 2 added. It was found that the behaviour of the yeast was related to the concentration of “free” (i.e. iodinetitratable) SO 2 , and not to the amounts “ combined “ or added. In another experiment, a correlation was demonstrated between the rate of growth of yeasts in concentrated orange juice and the concentration of free SO 2 , but not with the combined or total SO 2 ; in this experiment however, larger quantities of free SO 2 were involved than in the first. It is concluded that in relation to the preservative action of SO 2 the quantities added and going into combination are irrelevant, such action being exerted only through the proportion remaining “free.” It is nevertheless clear that there are present in fruit juices factors which reduce the effectiveness even of the “free” SO 2 .