Elsevier

Renewable Energy

Volume 141, October 2019, Pages 728-735
Renewable Energy

Third generation bioethanol from invasive macroalgae Sargassum muticum using autohydrolysis pretreatment as first step of a biorefinery

https://doi.org/10.1016/j.renene.2019.03.083Get rights and content

Highlights

  • Autohydrolysis is a suitable pretreatment for bioethanol production from Sargassum.

  • Almost 100% of hexoses are recovered in the optimum conditions.

  • Glucan to glucose conversion above 90% was achieved in enzymatic hydrolysis assays.

  • SSF without nutrients reached 80–95% theoretical ethanol yield in less than 10 h.

Abstract

Sargassum muticum, an invasive macroalgae in Europe, was employed as material for third generation bioethanol production. As a first step, autohydrolysis was chosen as an eco-friendly pretreatment, seeking for a high enzymatic susceptibility of the solid phase and high content of hexoses as glucose, galactose and mannose, in both liquid and solid phases, which can be subsequently transformed in ethanol via fermentation. Besides, the search of a minimum consumption of energy in the pretreatment is also a key challenge in bioethanol production.

At optimum conditions of autohydrolysis pretreatment, more than 90% of the glucan was recovered in the solid phase (while the other 10% appeared as glucooligosaccharides and glucose in the liquid phase). In the enzymatic hydrolysis carried out with the solid phases, glucan to glucose conversions of 94 and 89% were obtained, with the solid mixed with water and the whole slurry, respectively.

Moreover, the whole slurry experiments, where all hexoses present in the raw material (glucose, galactose and mannose) from the solid and the liquid phases are fermented, allows to reach maximum ethanol yields of 80% (14.10 g of ethanol/L) referred to the theoretical yield, in a short time.

Introduction

In the last century, fossil resources were the main feedstock used to produce fuels, chemicals and all kinds of materials. Human dependence, especially of fossil fuels, have started to become a worldwide problem in the last decades. Consequently, new and sustainable energies have become a high-potential alternative, and biorefinery has stood out as an interesting way to produce them [1,2].

For this purpose, sustainable feedstock has gained a lot of attention due to its low cost [3]. Seaweeds have become very popular recently, having fast growing rates, huge biomass yields and the advantage that no land is needed for cultivation. In addition, the high carbohydrate content allows them to be a suitable feedstock for the production of biofuels such as ethanol, hydrogen or butanol [4,5].

The seaweed Sargassum muticum was employed as raw material due to its potential in industrial applications and production of chemicals [6]. It is a brown alga originate in Japan, and mainly present in the European Atlantic waters (from the south of Portugal to the south coast of Norway) and West Coast of America, where is considered invasive. Invasive macroalgae are considered main menace to oceanic native species and resources all over the world, so many strategies are being evaluated in order to control their proliferation. However, in Europe, Sargassum muticum has a high biomass production and a high physiological tolerance towards dryness, salinity, temperature and sun exposure which made it greatly competitive, even displacing other seaweeds [[7], [8], [9]] so its exploitation could be an interesting solution. As far as we know, few works of bioethanol production from Sargassum have been released [[10], [11], [12], [13]], but none about Sargassum muticum.

The first step to take advantage of this seaweed would be to pretreat it, and autohydrolysis is a suitable way. It consists on heating a mixture of the raw material with water at high temperature, hence the reaction is only catalyzed by hydronium ions and organic acids generated consequently, like acetic acid, without the addition of any other compound. Therefore, autohydrolysis is an eco-friendly pretreatment, which allows to obtain a solid phase with high enzymatic susceptibility and a liquid phase rich in oligosaccharides [14].

Saccharomyces cerevisiae has become the most employed microorganism to obtain bioethanol [15] from hexoses as glucose and galactose [16,17]. Thus, it is desirable to make a whole slurry fermentation, where both liquid and solid fractions are employed simultaneously, without the need to use stages of separation or detoxification. Besides, the maximum quantity of sugars can be fermented and the cost can be reduced by avoiding the separation process and/or the washing of the solid fraction [18]. However, laboratory yeast strains are not suitable for fermenting cellulosic hydrolysates containing inhibitors [19], which can be found in the liquid phase. The first solution could be to remove the inhibitors, but it would increase the cost of the process, so the employment of genetically modified or robust yeast strains isolated from industrial environments, which have shown superiority fermentation performance in presence of inhibitor compounds, can be an interesting alternative to chemical detoxification processes [20].

In this work, an invasive seaweed (Sargassum muticum) has been explored as alternative renewable resource for the production of bioethanol, following the scheme displayed in Fig. 1. An environmentally friendly treatment, using water as reaction media was used for the fractionation of Sargassum muticum. Soluble oligosaccharides and insoluble polysaccharides were identified and quantified as function of treatment of severity. Moreover, enzymatic saccharification and fermentation of whole slurry derived from the treatment was also evaluated using two industrial and one laboratory Saccharomyces cerevisiae strains.

Section snippets

Raw material

The raw material employed in this study was the macroalgae Sargassum muticum (Sm), collected in Praia da Mourisca (Pontevedra, NW Spain) in August 2016. Sm was firstly frozen until use, process that is usually employed with algae [21,22] and do not alter significantly the composition and behavior of raw material [23]. Afterwards, Sm was cleaned, washed with tap water, chopped until size of particle smaller than 8 mm, air dried and stored in plastic bags.

Analysis of the raw material

Samples from the lot were milled to a

Autohydrolysis pretreatment

Non-isothermal autohydrolysis was chosen as a suitable green pretreatment for Sargassum muticum, where no other compounds but water are added to the raw material, making this pretreatment an environmentally friendly first step of macroalgae processing.

Experiments were carried out in a wide range of mild severities, 1.69–3.06 (corresponding to TMAX between 130 and 180 °C) to study the whole process, beginning with low severities (with high solid yields but minimum fractionation of the raw

Conclusions

Sargassum muticum is a suitable material for third generation bioethanol production, due to its hexose content (15.29% of the raw material on a dry basis) and high protein content (10.55%).

Non-isothermal autohydrolysis is an effective pretreatment to the fractionation of Sargassum muticum. Glucan can be almost totally retained in the solid phase, while other compounds can be almost totally or partially solubilized (as xylan and galactan + mannan, respectively). Optimal conditions lead to high

Acknowledgments

Authors are grateful to spanish Ministry of Economy and Competitiveness (research project “Multistage projects for the integral benefit of macroalgae and vegetable biomass” with reference CTM2015-68503) and to Xunta de Galicia (Galician Competitive Research Group GRC 2017/62 and to the CITACA Strategic Partnership), these three programs partially funded by FEDER of European Union; to the Portuguese Foundation for Science and Technology (reference UID/BIO/04469/2019 unit), and to European

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