Impurity and cost considerations for nutrient supplementation of whey permeate fermentations to produce lactic acid for biodegradable plastics

A potential application for whey permeate is to ferment it to lactic acid for use in the manufacture of biodegradable plastics. High purity lactic acid is required, thus the separation of impurities after fermentation is a major process cost. For batch fermentations, whey permeate requires nutrient supplementation, which can add to impurity levels at the end of fermentation. This work presents the results of whey permeate fermentations using L. casei, supplemented with yeast extract, whey protein hydrolysate, and malt combing nuts (MCN). There is an emphasis on investigating how little supplement needs to be added while still achieving high lactose conversion and lactic acid yield in a reasonable fermentation time. This was around 0.4% w/w for yeast extract, 5% for the hydrolysate (10% whey protein solution) and 5% for MCN. At these levels of supplement addition, the majority of the supplement is actually not utilised and just contributes to a major increase in the concentration of impurities at the end of the fermentation with a corresponding increase in separation cost, in addition to raw material cost. The ash content of regular whey permeate is another significant source of impurities that need to be separated to produce high purity lactic acid. Fermentations were performed to investigate the effect of low ash content on fermentation performance. Fermentations performed in supplemented demineralised whey permeate, with at least 70% ash reduction, showed no negative effect on fermentation performance, however fermentations conducted in a supplemented lactose solution did show a slowdown in fermentation performance. This is most likely due to mineral deficiency which should not pose a problem as minerals are usually micronutrients, and addition of very small amounts are required to restore fermentation performance.