Microalgae Biorefinery: Optimization of Starch Recovery for Bioplastic Production

Abstract

Microalgae are a promising source of starch with the potential to reduce land and water footprints as compared to terrestrial plants. However, so far, there are no specific downstream processes to recover microalgal starch. In this work, the development of a lab-scale biorefinery to produce microalgal starch is described. Guidelines are provided on how to set up microalgae cultivation, attaining until 37% starch content and 0.48 g L–1 d–1 starch productivity in 500 mL lab-scale photobioreactors. Cell lysis by ultrasonication in water and ethanol was studied, obtaining better disruption rates at a lower temperature (≈30 °C) in water. The refinery of the lysate was studied by comparing the conventional Percoll protocol with more potentially scalable methods: aqueous two-phase systems (ATPSs) and ethanol extraction. Ethanol allowed attainment of the best results, separating quantitatively lipids, with reduced pigment degradation, by ensuring higher starch recovery (91%) and starch content (57%) in the refined pellet. Finally, refined starch was used to produce a plastic film that showed mechanical properties comparable to those obtained by using corn starch. This study provides preliminary evidence that microalgal starch could replace conventional starch sources in the biobased industry, possibly with reduced environmental impacts.

From raw microalgae to bioplastics: conversion ofChlorella vulgarisstarch granules into thermoplastic starch

“…The starch granules exhibited a mean diameter of 1.5 µm, as determined by laser granulometry ( Figure 4A ). Although this size was relatively small compared to plant starch, it is typical of green microalgae (Di Caprio et al, 2023). Interestingly, the microalgal starch contained 13.0 % amylose ( Figure 4B ), a relatively low content for storage-type starch usually obtained under nitrogen-deprived conditions (Findinier et al, 2019).…”

“…Current starch extraction methods from microalgae are typically conducted at laboratory scale. The different protocols either exhibit purity or extraction yield below industrial standards (Di Caprio et al, 2023;Gifuni, 2017), involve complex process (Suarez Ruiz, Baca, et al, 2020), or require expensive products (Delrue et al, 1992). One objective of our work was to adapt the expensive yet simple, efficient protocol described by Delrue et al (1992) and developed in (Vonlanthen, Dauvillée, & Purton, 2015) to the pilot scale by changing the cell Cell disruption occurred similarly with French press and HPH at the same pressure (Figure S1).…”

“…For robust microalgal cells, such as Chlorella vulgaris, disruption is commonly achieved through chemical hydrolysis, enzymatic treatment, or mechanical stress (Brányiková et al, 2011;Gerken, Donohoe, & Knoshaug, 2013;Yap, Crawford, Dagastine, Scales, Martin, & 2016). Next, the separation phase entails the separation of starch granules from the other components present in the cell lysate (Di Caprio, Amenta, Francolini, Altimari, & Pagnanelli, 2023;Gifuni, Olivieri, Krauss, D'Errico, Pollio, & Marzocchella, 2017;Suarez Ruiz, Baca, van den Broek, van den Berget, Wijffels, & Eppink, 2020). It relies on the high density of starch granules, which allows for an easy recovery by centrifugation.…”

“…This degree of purity might not be required for applications such as bioplastics. Other purification approaches have been tested, with acetone or ethanol extraction, and aqueous two-phase systems (Di Caprio et al, 2023;Gifuni et al, 2017;Suarez Ruiz, Baca, et al, 2020;Suarez Ruiz, Kwaijtaal, Peinado, van den Berg, Wijffels, & Eppink, 2020). Yet, these methods are quite complex and fail to achieve both high purity and recovery yield.…”

“…Shear and temperature are controlled with a screw extruder, typically in the range of 100-150 °C. The possibility of producing TPS from microalgal starch has already been demonstrated, using the solvent casting technique to produce thin TPS films (Di Caprio et al, 2023). Still, the solvent casting technique is a laboratory-scale method, and the properties of microalgal TPS produced with industrially relevant processes, such as plasticization in twin-screw extruder and injection molding, remains to be evaluated.…”