The interrelated factors that influence the effectiveness of SO2 as a preservative against yeast were studied and the correct quantitative relationships of SO2 molecular species determined. Widely differing pK values for SO2 were found in the literature, compared with experimental data, and one set of values was selected. Undissociated H2SO3 is the only effective form of SO2 against yeast and can be calculated from measurement of free SO2 and pH, and the correct dissociation constants. Duration of contact, pH, concentration of SO2 and yeast, and binding of SO2 all influence the preservative action of SO2. Lower total SO2 concentrations can be used for food preservation by optimum control of these factors.
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