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Preventing the further degradation of monomeric or oligomeric sugar into by-product during biomass conversion is one of the challenges for fermentable sugar production. In this study, the performance of subcritical water (SCW) and enzymatic hydrolysis of coconut husk toward reducing sugar production was investigated using a severity factor (SF) approach. Furthermore, the optimal condition of SCW was optimized using response surface methodology (RSM), where the composition changes of lignocellulose and sugar yield as responses. From the results, at low SF of SCW, sugar yield escalated as increasing SF value. In the enzymatic hydrolysis process, the effect of SCW pressure is a significant factor enhancing sugar yield. A maximum total sugar yield was attained on the mild SF condition of 2.86. From this work, it was known that the SF approach is sufficient parameter to evaluate the SCW and enzymatic hydrolysis of coconut husk. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Coconut husk (CCH) as an abundant agricultural waste in Indonesia has the potential to be utilized for sugar production, which is the intermediate product of biofuel. In this study, subcritical water (SCW) assisted by nitrogen (N2) was developed to enhance the enzymatic hydrolysis of CCH. SCW process was optimized by varying the operation condition: the pressure of 60-100 bar, the temperature of 150-190 °C, and the time of 20-60 min. The SCW-treated solid was subsequently hydrolyzed by utilizing a mixture of commercial cellulase and xylanase enzymes. The result shows that the optimum total sugar yield was obtained under the mild condition of SCW treatment, resulting in the sugar of 15.67 % and 10.31 % gained after SCW and enzymatic hydrolysis process, respectively. SEM and FTIR analysis of SCW-treated solid exhibited the deformation of lignin and solubilization of cellulose and hemicellulose, while XRD and TGA revealed an increase of the amount of crystalline part in the solid residue. The use of N2 in SCW treatment combined with enzymatic hydrolysis in this study suggested that the method can be considered economically for biofuel production from CCH waste in commercial scale. Copyright © 2020 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Subcritical water is one method of hydrolysis that can convert coconut husk to produce reducing sugars. However, this method has the disadvantage of producing derivative products such as furfural and phenolic compounds that act as inhibitors. One effective method is the addition of additives to the subcritical water process. The purpose of this study was to determine the effect of adding additives to subcritical water processes and optimizing the operating conditions on the production of reducing sugars. The analysis of reducing sugar was conducted by the dinitrosalicylic acid (DNS) method. Variables used in this study were time, temperature, pressure, water volume, pH, and several types of additives. Plackett-Burman was used for screening significant factors for the production of reducing sugars. The three most affecting factors were further investigated to find out the optimum point using Response Surface Methodology (RSM). The optimum point for subcritical water pretreatment operating conditions was the addition of sodium dodecyl sulfate (SDS) of 0.24 grams, reaction time for 80 minutes, and pH 11 yielding a reducing sugar yield of 22.7%, energy use of 291.3 kJ/g with desirability of 85%. Furfural content of all liquids after pretreatment can be neglected (

The role of three kinds of surfactant (by means of PEG, Tween 80, and SDS) on subcritical water (SCW) hydrolysis of coconut husk towards the reducing sugar production was studied comprehensively. The addition of Tween gave a significant escalation of sugar yield below the cloud point (around 130 °C). The simultaneous hydrophobic and hydrophilic interaction between lignin and SDS drove the highest delignification and solubilization of monomeric sugar during SCW process. On the contrary, adding PEG showed an adverse effect on the subcritical condition. The best scenario of surfactant addition producing higher sugar production was by the addition on SCW instead of enzymatic hydrolysis. The combination of SCW assisted by SDS and enzymatic hydrolysis generated the highest sugar yield and minimized the degradation compound and energy consumption, resulting in favorable fermentable sugar for subsequent biofuel process.

The objective of this work is to develop an integrated green process of subcritical water (SCW), enzymatic hydrolysis and fermentation of coconut husk (CCH) to biohydrogen. The maximum sugar yield was obtained at mild severity factor. This was confirmed by the degradation of hemicellulose, cellulose and lignin. The tendency of the changing of sugar yield as a result of increasing severity factor was opposite to the tendency of pH change. It was found that CO2 gave a different tendency of severity factor compared to N2 as the pressurizing gas. The result of SEM analysis confirmed the structural changes during SCW pretreatment. This study integrated three steps all of which are green processes which ensured an environmentally friendly process to produce a clean biohydrogen.

Coconut husk wastes are abundantly available in Indonesia. It has a potential to be used into alternative renewable energy sources such as hydrogen using enzymatic hydrolysis followed by a fermentation process. Unfortunately, enzymatic hydrolysis is hampered by the complex structure of lignocellulose, so the cellulose component is hard to degrade. In this study, Combined Subcritical Water (SCW) and enzymatic hydrolysis are applied to enhance fermentable, thereby reducing production of sugar from coconut husk. There were two steps in this study, the first step was coconut husk pretreated by SCW in batch reactor at 80 bar and 150-200°C for 60 minutes reaction time. Secondly, solid fraction from the results of SCW was hydrolyzed using the mixture of pure cellulose and xylanase enzymes. Analysis was conducted on untreated and SCW-treated by gravimetric assay, liquid fraction after SCW and solid fraction after enzymatic hydrolysis using DNS assay. The maximum yield of reducing sugar (including xylose, arabinos...

Coconut husk hydrolysis using subcritical water (SCW) with N2 as pressurizing gas was carried out in a batch reactor at temperature of 150 °C-190 °C, at pressure of 60-100 bar and the reaction time from 20-60 min. The effect of SCW pretreatment could reduce lignin and produced reducing sugars. The highest TRS (total reducing sugar) yield was 0,4904 at 170 °C, 80 bar, and the reaction time of 20 min. Reducing sugars concentration increased when temperatures rised from 150 to 170 °C. However, reducing sugar concentration decreased at temperature above 170 °C. The decrease of reducing sugar was due to degradation of reducing sugars into its degradation products. A first-order reaction model was determined to describe the reaction rate constant of the formation of reducing sugar and its degradation products. Kinetic parameters were obtained by fitting the proposed equation with the experimental data. The reaction rate constant increased when temperature increased from 150 to 170 °C, but then decreased when the temperature increased above 170 °C. Activation energy was also determined based on the Arrhenius equation.