Thermal Acid Hydrolysis Pretreatment, Enzymatic Saccharification and Ethanol Fermentation from Red Seaweed, Gracilaria verrucosa

Abstract

In this study, the red seaweed Gracilaria verrucosa was used as a bioethanol producing biomass. G. verrocosa has a high content of easily degradable carbohydrates, making it a potential substrate for the production of liquid fuels [8]. The carbohydrates in G. verrucosa can be categorised according to their chemical structures: alginate, carrageenan, and agar. Carrageenan and agar, which are plentiful in the seaweed, can be used as a source of galactose and glucose. Various pretreatment techniques have been introduced to enhance the overall hydrolysis yield, and can be categorized into physical, chemical, biological, enzymatic or a combination of these [1]. Dilute acid hydrolysis is commonly used to prepare seaweed hydrolysates for enzymatic saccharification and fermentation for economic reasons [13]. However, thermal acid hydrolysis pretreatment for 3,6anhydrogalactose from G. verrucosa have produced 5hydroxymethylfurfural (HMF), an inhibitory compound for ethanol production [8]. One of the problems encountered in G. verrucosa fermentation has been high concentrations of NaCl due to its origin from sea water [3]. High-salt stress to yeasts is a significant impediment on the production of ethanol from seaweed hydrolysates. Salt stress in yeasts results in two phenomena: ion toxicity and osmotic stress [11]. Defense responses to salt stress are based on osmotic adjustments by osmolyte synthesis and cation transport systems for sodium exclusion [20]. The preferential utilization of glucose over non-glucose sugars by yeast often results in low overall ethanol production and yield. When yeast grows on a mixture of glucose and galactose, the glucose is metabolized first, whereas The seaweed, Gracilaria verrucosa, was fermented to produce bioethanol. Optimal pretreatment conditions were determined to be 12% (w/v) seaweed slurry and 270 mM sulfuric acid at 121C for 60 min. After thermal acid hydrolysis, enzymatic saccharification was carried out with 16 U/ml of mixed enzymes using Viscozyme L and Celluclast 1.5 L to G. verrucosa hydrolysates. A total monosaccharide concentration of 50.4 g/l, representing 84.2% conversion of 60 g/l total carbohydrate from 120 g dw/l G. verrucosa slurry was obtained by thermal acid hydrolysis and enzymatic saccharification. G. verrucosa hydrolysate was used as the substrate for ethanol production by separate hydrolysis and fermentation (SHF). Ethanol production by Candida lusitaniae ATCC 42720 acclimated to high-galactose concentrations was 22.0 g/l with ethanol yield (YEtOH) of 0.43. Acclimated yeast to high concentrations of specific sugar could utilize mixed sugars, resulting in higher ethanol yields in the seaweed hydrolysates medium.