Subcritical water extraction scale-up from laboratory to pilot system for red algae residue valorization

Abstract

Gelidium sesquipedale solid residue after industrial agar extraction contains high amounts of proteins with all essential amino acids and carbohydrates such as glucans, galactans or arabinans [1]. Therefore, although it is generally discarded, its reincorporation in the industry as a value-added product brings an interesting solution. Traditional methods used for bioactive compounds extraction from different raw materials present numerous drawbacks, namely, time-consuming, costly to dispose of used products and harmful to environment and human health [2]. Among green technologies, subcritical water extraction (SWE) stands out as a great alternative to traditional extraction processes. SWE consists of using hot pressurized water above its boiling point, 100 °C, and below its critical point, 374 °C, which causes many of the properties of water to change, such as density or dielectric constant [3,4]. Water dielectric constant, which is related to its polarity, decreases with increasing temperature, similar to organic solvents, at 200 °C. As a result, through the dielectric constant modulation with temperature, SWE is able to selectively extract polar or non-polar compounds [5].

In order to assess the feasibility of industrial-scale subcritical solvent extraction, a pilot- scale process must first be tested. Generally, the design of the industrial SWE equipment

is preceded by the study of laboratory- and pilot-scale systems. Hence, although in many cases the pilot-scale study stage is eliminated, the scaling-up process would be much more efficient by incorporating the pilot-scale study to obtain quality data and determination of scale-up factor [6]. Therefore, the main goal of this research was to prove the feasibility of industrial-scale subcritical water system through scaling-up from lab to pilot system., by the comparison of lab- and pilot-scale subcritical water performance. For this, many analytical methods were applied for the comparison of the extraction yield of the two systems; such as, polysaccharide fraction identification and quantification, total protein content and free amino acids determination, and total polyphenol content (TPC) and antioxidant activity. Galactose was mainly recovered as oligomer fraction with maximum yields of 71.4 (36 minutes) and 74.5 % (45 minutes) for pilot and lab-scale, respectively (Figure 1a). Lower yields were determined for glucans, with maximum yields of 9.5 % for both systems, in which more than 6 % was extracted in the first minutes. Similar extraction curves and yields were determined for protein fraction with final extraction yields close to 40 % (Figure 1b), while free amino acids content was higher in laboratory scale. The greatest extraction yield was accounted for the smallest amino acids, such as glycine, alanine and aspartic acid, whereas polar amino acids such as glutamic acid and lysine were reduced, although lysine was not detected in pilot system. Differences in total polyphenolic compounds (TPC) extraction were observed for both systems. Increasing TPC content with time was determined for lab-scale system, while in pilot system a plateau phase was observed after 36 minutes of extraction. SWE has been proven to be an efficient technology for bioactive compounds recovery such as carbohydrates, protein and amino acids from algae residue. Scaling up of subcritical water system from laboratory to pilot scale resulted in good and reproducible

results. Therefore, feasibility of industrial-scale subcritical water system through scaling- up from lab to pilot system has been showed.