Polymalic acid fermentation by Aureobasidium pullulans for malic acid production from soybean hull and soy molasses: Fermentation kinetics and economic analysis

•Malic acid production via PMA fermentation by Aureobasidium pullulans was studied.•High PMA production was obtained from soybean hull hydrolysate and soy molasses.•Soy molasses is the cheaper feedstock without needing prior enzymatic hydrolysis.•Malic acid can be produced from soy molasses at $1.10/kg for a 5000 MT plant.•This offers an economical process for industrial production of biobased malic acid. Polymalic acid (PMA) production by Aureobasidium pullulans ZX-10 from soybean hull hydrolysate supplemented with corn steep liquor (CSL) gave a malic acid yield of ∼0.4g/g at a productivity of ∼0.5g/L·h. ZX-10 can also ferment soy molasses, converting all carbohydrates including the raffinose family oligosaccharides to PMA, giving a high titer (71.9g/L) and yield (0.69g/g) at a productivity of 0.29g/L·h in fed-batch fermentation under nitrogen limitation. A higher productivity of 0.64g/L·h was obtained in repeated batch fermentation with cell recycle and CSL supplementation. Cost analysis for a 5000 MT plant shows that malic acid can be produced at $1.10/kg from soy molasses, $1.37/kg from corn, and $1.74/kg from soybean hull. At the market price of $1.75/kg, malic acid production from soy molasses via PMA fermentation offers an economically competitive process for industrial production of bio-based malic acid.