Subcritical water mediated hydrolysis of cider spent yeast: Kinetics of HMF synthesis from a waste microbial biomass

• Subcritical water can fully depolymerise cider spent yeast to mixture of monosaccharides at 225 °C.
• Kinetics of the yeast conversion to monosaccharides and HMF were determined.
• Compared to cellulose, yeast cells are easier to breakdown into monosaccharides.
• Waste yeast can be a feasible source of biomass for thermochemical transformation.

Cider lees, a microbial by-product of the cider industry which is primarily comprised yeast cells was subjected to subcritical water mediated hydrolysis using a batch tubular reactor over a temperature range of 175–275 °C. The efficacy of subcritical water to hydrolyse the polysaccharides and mannoproteins in the yeast's cell wall to monosaccharides and for the glucose and mannose to be dehydrated to 5-hydroxymethylfurfural (HMF) was investigated in a non-catalysed reaction using a consecutive first order kinetic model. Results suggest that the cider yeast cell wall, which is backboned by milder bonds, was decomposed at lower temperatures when compared to cellulose hydrolysis under the same conditions. Glucose and mannose formation was observed at temperatures as low as 175 °C and their conversion to HMF at 225 °C resulted the yield of more than 12%, based on the crude feedstock. Reaction rates for other decomposition routes were also calculated and discussed using the developed model.