Enzymatic hydrolysis of bovine hair wastes to keratin-based added bio-products

Liming represents one of the most environmentally impactful operations of the leather tanning process. Besides hydrated lime - Ca(OH)2, this treatment is traditionally carried out with Na2S, to break the disulfide bonds of the pristine substrate, by this way preparing the raw hide for next tanning operations. However, use of such sulfide(s) leads to negative environmental and health impacts, due to emissions of harmful hydrogen sulfide gases, thus requiring the development of more sustainable and safer approaches. For this purpose, enzymatic path with proteolytic enzymes is an appropriate choice, enabling the reduction of the organic loading and avoiding/reducing the sulphide load in the corresponding process wastewaters. Anyway, proteolytic enzymes can lead to relevant undesired collagen damage, so this biochemical treatment requires appropriate optimization. Moreover, these milder dehairing treatments generate a new solid waste, e.g. “hair”, which could be further converted into keratin-based products (for cosmetic, retanning and food uses) by enzymatic and/or chemical hydrolysis. The ENZYCAL project “New processes of bovine hair removal and its valorisation by means of enzymatic hydrolysis” aims 1) to optimise the enzymatic dehairing, proposed as a replacement for traditional liming treatment, and 2) to convert the obtained stream by enzymatic hydrolysis to keratin-rich hydrolysates. Focusing on the 2nd goal, 13 keratinases were tested towards the hydrolysis of decalcined hair, working at their corresponding optimal pHs, keeping constant the temperature (35°C), the solid/liquid weight ratio (1/20 wt/wt) and the hair/enzyme loading (10 wt%). The yield in the posthydrolysis residue (wt%) and the corresponding percent nitrogen yield (Ninitial – Nfinal /Ninitial x 100) were both considered as main responses of interest for screening and optimization purposes. At the typical pH value of the tested enzymes (pH=9), the yields in solid residue were between 55 and 81 wt%, whilst the corresponding nitrogen yields were between 37.1 and 3.7 %. After having identified the most active enzymes, the reaction parameters were further optimized. Working at pH=9 and 40 °C, with a hair/enzyme loading of 7.5 wt% and a solid/liquid ratio of 1/20 wt/wt, a minimum yield of posthydrolysis residue (55.1 wt%) and a maximum nitrogen yield of 40.1 %, were ascertained, which is a really interesting achievement for such a biochemical approach. Starting from these promising results, this reaction will be scaled-up, to favor the process development/intensification.