The impact of sample preparation of the macroalgae Laminaria digitata on the production of the biofuels bioethanol and biomethane

Washing macroalgae is a 'standard' initial pre-treatment step, reported in a number of papers on biofuel production from macroalgae. Washing removes particulate matter; however, in this study, we show that washing may also have an adverse effect on the water-soluble carbohydrate contents present in the macroalgae, potentially reducing the quantity of the biofuel produced. This has major implications if macroalgae are to be considered as a feedstock for biofuel and platform chemicals. We compared washed and unwashed material from summer and winter harvests which were subsequently dried by three methods as follows: (1) immediate oven drying, (2) freezing, then oven drying and (3) freeze-drying. The proportions of the water-soluble carbohydrates were assessed and a decrease of up to 49.3 % seen in the laminarin content of those washed. Oven drying also resulted in some degradation of the laminarin with approximately 10x increase in glucose concentrations compared to freeze-drying. When this material was used as a substrate for biofuel production, unwashed Laminaria digitata generated a higher concentration of ethanol in all the differently dried summer samples and two thirds of winter samples, suggesting that not washing is advantageous unless other factors are involved, e.g. a large quantity of particulate material is present on the macroalgae. In contrast, washed samples used in an anaerobic digestion study gave higher methane yields in two thirds of the drying treatments than the unwashed, possibly due to lower salt-tolerance in the anaerobic microbial consortia, so a pre-washing step could be considered more suitable for this conversion route.