Abstract: A simultaneous sequential anaerobic fermentation process for the in vitro production of propionic and acetic acids is disclosed. The process comprises employing an obligatory two-component co-culture which maintains a relatively constant ratio of species populations over multiple passages. A first co-culture component is a Lactobacillus or Streptococcus which homofermentatively converts the hexose to lactic acid. A second microorganism in the co-culture is a Veillonella which is metabolically incapable of assimillating the hexose and converts the lactic acid product to propionic and acetic acids. The co-culture is inoculated into a nutrient growth feedstock such as whole whey or a clarified dairy whey lactose permeate which contains a metabolizable source of a hexose such as glucose, lactose or sucrose.