Dark fermentation of sheep cheese whey: biochemicals and biofuels production as a function of fermentation time and pH

Cheese whey (CW) is the main by-product of the cheese making process and is composed mainly by lactose, proteins, lipids and mineral salts. The environmental impacts and other potential negative effects linked to the traditional management strategies of CW are no longer considered sustainable, therefore alternatives need to be explored. Dark fermentation (DF) may be a promising approach for CW valorization. Though the issue has been already addressed by several studies with particular emphasis on biohydrogen production, less attention has been paid to the possibility of recovering also other valuable products. Optimizing the effects of operating pH and fermentation time is a strategy worth to be studied in order to obtain specific biochemicals and/or biofuels from CW. In this framework, batch DF tests were performed under mesophilic conditions on sheep CW, without inoculum addition, adopting different operating pHs, and relating type and production yields of the observed gaseous and liquid byproducts to the duration of the process. CW fermentation evolved over time according to two steps; the first phase was characterised by lactose conversion to lactic acid, whilst during the second one lactic acid was degraded to soluble and gaseous products such as short chain fatty acids (mainly acetate, butyrate and propionate) and hydrogen. The adopted operating pHs affected the production kinetics and yields, as well as metabolic pathways. In particular, setting the operating pH to 6 proved to be optimal in terms of both lactic acid or hydrogen production yields.