Recovery of proteins and free amino acids from Gelidium Sesquipedale alga residue by subcritical water extraction (SWE)

Abstract

Gelidium sesquipedale is a red alga that provides the best raw material to obtain the highest quality agar in the spanish agar industry [1]. Industrial process generates a residue that is usually discarded; however, this by-product still contains important amounts of different valuable compounds, such as structural carbohydrates, bioactive compounds and proteins. To valorize by-product supplied by Hispanagar company (Burgos), subcritical water extraction (SWE) is a promising green technology since water presents unique properties as solvent [2]. This work is focused on the valorization of the protein fraction to produce free amino acids and to extract and hydrolyze protein into small peptides. A semi-continuous reactor has been used to perform the extraction, and protein fraction hydrolysis was studied at different temperatures (125, 140, 155, 170, 185, 200ºC), times, and solvent flow rate (2 and 6 mL/min). Total protein content in alga residue was 20.11±1.53% in which it was determined free amino acids, among which VAL, LEU, ILE, PHE, LYS and HIS are majority (Table 1), being total essential amino acids found 10.50±0.16%, whereas non-essential amino acids like GLY, PRO and GLU+GLN represent 4.57±0.06% Protein extraction grows with increasing temperature at constant flow, reaching a maximum at 200ºC. Moreover, when flow and temperature are increased, protein recovery shows the highest and fastest extraction because of its less residence time, what makes able to get a marked extraction yield improvement (Fig. 1). Amino acids extraction follows a similar trend than proteins: an increasing extraction up to a maximum at 185ºC when flow rate is 2 ml/min, but lower than 6 ml/min. Greatest extraction was found for ALA, GLY, SER and the mixture of ASN+ASP; also for MET, TYR and GLU+GLN mixture, with lower but remarkable yield (Table 1). At constant flow rate, maximum extraction is reached at 185ºC for all amino acids determined, decreasing at 200ºC. Nevertheless, flow rate increasing makes 200ºC extraction much higher, about all for GLU+GLN mixture, LYS, HIS, TYR, PRO, LEU and MET (Fig. 2). This agrees with other studies carried out with fish protein and amino acids standard [3,4]. Moreover, it has been noted than amino acids extraction presents a similar behaviour according to its properties: basic amino acids extraction remains practically constant, whereas than neutral and acid, hydrophobic and sulfur amino acids extraction increase when temperature raises. SWE is a useful technique to extract bioactive compounds. Parameters as temperature or solvent flow rate have much influence on the protein and amino acids extraction yield. All of this makes SWE an interesting alternative to conventional treatments.