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Siti Aminah Mohd Hassan
Optimization of Sodium Hydroxide pretreatment and enzymatic saccharification of spent rubberwood sawdust for glucose production
2014
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Siti Aminah Mohd Hassan
Large quantities of oyster mushroom spent medium (MSM) were disposed of as agricultural waste to the environment and thereby constituting environmental pollution. Economic reuse of the waste mushroom medium to produce glucose might be a viable solution instead of disposal. Therefore, the study was conducted to improve the added value of mushroom spent medium as an alternative glucose source to the current expensive sugar crops and starchy products. Firstly, the chemical characteristics, surface morphology, and suitability of mushroom spent medium in comparison to rubber sawdust (RSD) and growth medium (GM) were examined in order to investigate its potential as a new glucose feedstock. The composition analysis proved that decreases in lignocellulosic contents occurred after cultivation of Pleurotus sajor caju. The amount of lignin, hemicelluloses, and cellulose in MSM showed lower values than those in RSD and GM which were 22.40, 27.93, and 27.97% respectively. The surface morphology of MSM appeared to be rough and broken and traces of hyphen on the fibre surface were observed as a result from the mushroom cultivation process. RSD and FM had an even and smooth flat surface, indicating a rigid and highly ordered surface structure. Preliminary study on effectiveness and feasibility of the three pretreatment techniques (autoclaving at 121°C, heating in water bath, soaking at room temperature) with different NaOH concentration on RSD, GM, and MSM revealed the best result of 30.13 g glucose/100g dry substrate and 33.50% of hydrolysis weight decrease obtained from MSM, which had been treated in water bath at 90°C for 2 h. The scanning electron microscope (SEM) images of the corresponding MSM hydrolysate showed severe disruptions of biomass structure, irregular cracks, and pores. One factor at a time (OFAT) method was applied to screen the range of parameters in NaOH pretreatment via heating in water bath and enzymatic saccharification. With known parameters’ range, the Response surface methodology (RSM) based on Box-Behnken Design (BBD) was adopted to optimize the conditions of NaOH pretreatment via heating in water bath method and enzymatic saccharification of MSM. The optimum conditions of MSM pretreatment at substrate loading of 5.0% (w/v) were found to be NaOH concentration of 2.63 M, reaction temperature of 92.26°C, and treatment time of 112.92 min with maximum glucose yield of 34.55 g/100g dry substrate after 48 h of enzymatic saccharification at constant enzyme loading of 67 FPU/g dry substrate and substrate loading of 1.0% (w/v). The MSM hydrolysate obtained under optimal NaOH pretreatment conditions were further used to optimize enzymatic saccharification conditions at constant substrate loading of 1.0% (w/v). Under optimized conditions (agitation rate of 150.74 rpm, enzyme loading of 94.92 FPU/g substrate, and hydrolysis time of 56.89 h), a maximum glucose yield of 71.21 g/100 g dry substrate was achieved. The Analysis of Variance (ANOVA) test revealed that the model and all independent parameters were considered statistically significant at 95% for both optimization studies using the second order polynomial equation. The model validation showed a good agreement between experimental results and the predicted responses. Therefore the models could be successfully used to identify the effective combinations of the three different factors in both optimization studies for predicting the glucose yield from MSM.
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