A method for producing a preservative salt includes feeding a biodegradable feedstock to a mixed consortium of microorganisms or microbiomes to produce a fermentation effluent comprising a carboxylic acid mixture; recovering the carboxylic acid mixture from the fermentation effluent; separating an acetic acid and propionic acid fraction from the carboxylic acid mixture; and reacting the acetic acid and the propionic acid fraction with a base to produce the preservative salt comprising an acetic acid and propionic acid salt. The carboxylic acids have greater than 80% bio-based carbon content.