Proximate composition and the production of fermentable sugars, levulinic acid, and HMF from Gracilaria fisheri and Gracilaria tenuistipitata cultivated in earthen ponds

Abstract

The red seaweeds are generally known to have a high content of polysaccharides and low content of lignin. They can be used as a bioethanol feedstock and to produce biochemicals. This study was conducted to examine the pretreatment conditions to improve the production of fermentable sugars and by-products from Gracilaria fisheri and Gracilaria. tenuistipitata. The algal materials were gathered from earthen pond cultivation. The pretreatment was conducted at different concentrations of H2SO4 (0.2–1 M) and time (30–150 min) at 95 °C. The proximate composition and contents of glucose, galactose, levulinic acid, and 5-hydroxymethylfurfural (5-HMF) were analyzed. Our results showed high carbohydrate content of 63.01 ± 0.47 g carbohydrate (100 g TS)−1 for G. fisheri and 59.07 ± 0.43 g carbohydrate (100 g TS)−1 for G. tenuistipitata. The optimal pretreatment with 1 M of H2SO4 at 95 °C for 150 min resulted in high concentrations of sugars in G. fisheri (7.86 g L−1 glucose, 8.37 g L−1 galactose) compared to G. tenuistipitata (3.15 g L−1 glucose, 5.75 g L−1 galactose). The pretreatment of the algae resulted in concentrations of 5-HMF for G. fisheri and G. tenuistipitata of 1.55 and 1.42 g L−1, respectively. The levulinic acid concentration was 3.66 g L−1 for G. fisheri and 6.12 g L−1 for G. tenuistipitata. Gracilaria fisheri was more susceptible to the sulfuric acid hydrolysis compared to G. tenuistipitata. Our study revealed that the acid hydrolysis of G. fisheri and G. tenuistipitata can improve the yield of sugars to produce bioethanol feedstocks.