Air drying of fucus vesiculosus seaweed and its effect on phytochemical characteristics of aqueous extracts

Abstract

The purpose of this study was to determine the effect of air drying at four different temperatures (35, 50, 60 and 75ºC) on drying kinetics at different configurations (deep-bed and thin layer) and properties of dried Fucus vesiculosus seaweed and the phytochemical constituents of its aqueous extracts. Water adsorption and desorption isotherms were also determined at four different temperatures (5, 25, 45 and 65ºC). Different models (GAB, BET, Caurie, Halsey and Oswin) were tested for water sorption isotherms modelling. Halsey model showed the best fit for all water sorption isotherms. In the case of drying kinetics modelling for deep-bed configuration, bibliographic models were tested (Newton, Henderson-Pabis, Logarithmic, Two-Term, Weibull, Page and Modified Page). Page and modified Page models were selected as the most adequate under statistical analysis criteria. Thin layer dyring kinetics of F. vesiculosus showed a constant rate period and falling rate period at all drying temperatures tested. Critical moisture content increased with drying temperature. For post-critical period, Fick’s second law equations were successfully applied and allowed the determination of effective coefficients of diffusion of water, which increased with drying temperature (from 95·10-12 up to 260·10-12 m2/s). This water effective diffusivities were estimated considering the shrinkage of samples. The shrinkage analysis indicated that real shrinkage was larger than ideal shrinkage (evaluated by means of water loss). Effect of different load densities (from 1.25 up to 14.88 kg/m2) was studied at 75ºC and a linear correlation between load density and drying time was achieved. Dried seaweed at 75ºC exhibited the largest colour differences. Seaweed powders previously dried at 35, 60 and 75ºC exhibited an appreciable yellow-tone coloration, whereas system dried at 50ºC presented a greenish coloration. F. vesiculosus extracts were obtained performing aqueous ultrasound assisted extractions. Two key extraction parameters (from 20 to 40 (g/g) liquid/solid ratio and contact time from 4 to 20 minutes) on the extraction yield of three bioactive compounds (polyphenols, carbohydrates and alginates) were studied for the system dried at 35ºC. The effect of drying temperature on antioxidant activity and the yield of bioactive compounds was studied by means of extractions performed at optimized L/S ratio of 30 (g/g) and contact times of 4 minutes for all dried systems. Maximum phenolic content (1738 mg PHL/100 gdw) and antioxidant activity (measured by means of DPPH radical scavenging activity, 57%) were obtained for the system dried at 35ºC and declined significantly with drying temperature. Higher drying temperatures had a positive effect on alginate extraction yield (2069 mg GLU/100 gdw). Total phenolic content of extracts from different particle size fractions of dried F. vesiculosus powder was determined. Maximum phenolic content was attained at intermediate fractions 80-200 μm (2272 GLU/100 gdw).