Valorization of the industrial solid residue generated after agar extraction from Gelidium sesquipedale, by emerging technologies

Abstract

Increasing wastes as consequence of the growing population and higher quality of life, is one of the biggest problems that society faces with nowadays. The concept of biorefinery, in which residues are reincorporated into industrial processes to obtain base molecules, emerges as a solution. In this work, a complete valorization of the red alga Gelidium sesquipedale solid byproduct generated after industrial agar extraction (GBP) is proposed. Despite being usually discarded, it still contains high content of interesting molecules such as carbohydrates, proteins, amino acids, etc., that can be recovered. Traditional extraction methods have several drawbacks. They are usually time-consuming and have low selectivity and extraction yields. In addition, large volumes of organic solvents, which are dangerous, too expensive and can be harmful for environment and human health, are used. As an alternative, subcritical water treatment (SWT) is proposed in this work to recover the compounds of interest in the above mentioned Gelidium sesquipedale byproduct (GBP). Also, enzymatic assisted extraction (EAE) has been studied in order to compare SWT with other technologies. Different hydrolytic enzymes

(cellulase and protease) have been also used for this purpose. Finally, different pressure- driven membrane technologies have been evaluated for fractionation and concentration

of the subcritical water extracts, with the purpose of obtaining isolated and high-purity concentrates of the extracted bioactive compounds. The results obtained allow us to conclude that the extraction/hydrolysis taking place during SWT favor the recovery of several bioactive compounds from GBP (see Fig. 1), providing high yields through a complete byproduct valorization. SWT showed to be highly influenced by temperature, heating rate and residence time [1, 2]. SWT led to an efficient extraction/hydrolysis of the protein fraction of GBP. The best experimental

conditions in a semi-continuous fix- bed reactor were 200 °C and 6 mL/min

with nearly 70% of the solubilized protein content. The highest content of individual amino acids was obtained for small amino acids such as valine, alanine and glycine as well as aspartic

acid. Therefore, an increase in the non- polar selectivity was observed by

working at high severity factors. Positive and strong correlation was obtained between the phenolics recovered and the reducing capacity of the BP extracts. EAE has also been proven to be an efficient technology to valorize the GBP [3]. Longer times were needed and lower yields of biocompounds extraction/hydrolysis were observed for EAE in comparison with SWT. Ultrafiltration with tubular inorganic membranes has been proven to be a suitable separation technology to fractionate subcritical water extracts from macroalgae residue. The most influential parameter in separation process was the MWCO of the membrane. Further research about the functional properties of the concentrated and isolated biocompounds is needed in order to study their possible applications.