Organic acid and biomass production of propionic acid bacteria grown in lactose and lactate-based broths

The valorization of food production waste into new products is an actual topic
nowadays. Five strains of propionic acid bacteria (PAB) (Acidipropionibacterium
acidipropionici, Propionibacterium cyclohexanicum, Propionibacterium freudenreichii, Acidipropionibacterium jensenii and Acidipropionibacterium thoenii) were investigated for their ability to produce organic acids and biomass in laboratory-scale fermentation. The production of propionic acid and acetic acid was considerably influenced by the added carbon source (Na lactate or lactose), but did not differ between PAB strains (p ≥ 0.05). The concentrations of propionic acid and acetic acid produced in the lactate broths were 7.7–8.6 g L−1 and 3.5–3.9 g L−1, respectively. Lactose broth fermentation resulted in a significant pH reduction, and propionic and acetic acids’ outcomes were significantly lower (p < 0.05) – about two times less – which was most likely due to acid-induced inhibition. The results also indicate that lactose consumption, substrate conversion efficiency as well as biomass production are strain-dependent. Stirring reduced propionic acid outcome. Biomass production in lactose-based broths was significantly higher (p < 0.05) than in lactate-based broths. Neutralization of the environmental acidity during acid production could be a suitable solution to the abovementioned inhibition problem, as well as to achieve a higher concentration of acids or their salts with respect to the stability of the fermentation product (for example, to prevent the evaporation of undissociated volatile acids). Since lactose-rich dairy waste is a significant threat to the environment, its conversion into new products is of great interest, and the knowledge of this study can help to model PAB metabolite and biomass recovery by valorizing dairy waste through an environmentally friendly process.