Enzymatic extraction of sustainable protein sources from sheep wool and spent yeast for optically pure l-lactic acid production

Sheep wool and spent yeast are abundant by-products in Ireland, with wool largely derived from hill and meat-type flocks yielding coarse, low-value fibres, and spent yeast generated in large quantities by the brewing and distilling industry. Both are protein-rich and represent underutilised resources with potential as alternative protein sources for microbial fermentation. In this study, enzymatic hydrolysis of sheep wool and spent yeast was optimised using a Design of Experiments (DoE) approach, evaluating substrate load, enzyme dosage, and hydrolysis time to maximise extract and protein yields. Results showed that optimal conditions differed for each substrate: sheep wool achieved highest extract and protein yields under the lowest tested parameters, whereas spent yeast protein yield was maximised at intermediate hydrolysis times and enzyme dosages, indicating potential protein denaturation at prolonged hydrolysis. When used as protein sources in lactic acid fermentation, sheep wool hydrolysate supported complete glucose consumption and higher lactic acid production than spent yeast hydrolysate. These findings demonstrate that sheep wool hydrolysate, previously untested in lactic acid fermentation, can serve as a sustainable and effective protein supplement. The study highlights the potential of these by-products to replace conventional protein sources, offering a circular, value-added approach to bioprocessing in Ireland.