Bahari, Alireza (2012)
Ph.D. thesis, University of Birmingham.
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Spent cider lees(yeast), a by-product of the cider industry, was subjected to subcritical water in a batch process over a temperature range of 100 °C to 300 °C. The efficacy of subcritical water to convert the waste biomass into a mixture of valuable compounds was investigated by studying formed products in the extracts.
At milder temperatures (up to 150°C), yeast intracellular contents such as proteins were released. Also, phenolic compounds which were previously adsorbed on cell walls during fermentation start to release, which was shown for
the first time, leading to an increase in the antioxidant activity of the extracts (maximum 80g ascorbic acid equivalent in 100 ml). The release of phenolics was compared to organic solvent extraction and the same level of extraction was observed in subcritical water treatment.
At higher temperatures, polymers of the yeast cell wall started to solubilise producing monosaccharides which later were converted to hydroxymethyl furfural (HMF). Kinetics of HMF synthesis were determined using first order kinetics suggesting that milder conditions are required for HMF production when compared to the same process using
cellulose as substrate. Results support the potentials of the studied feedstock as a substrate for numerous bio-based chemicals.
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