Bioethanol is considered as the most widely used renewable and environmental friendly fuel produced by the fermentation of plant material. In this study, pomegranate peels were used as substrate for ethanol production. The peels were hydrolyzed by three different alkalis i.e. NaOH, KOH, and Ca(OH)2. The variable factors for hydrolysis were alkali concentration (X1), hydrolysis temperature (X2) and hydrolysis time (X3). These factors were optimized by central composite design (CCD) using response surface methodology (RSM) at three factorial level. The observed responses were reducing sugars, Total carbohydrates, weight loss, extractives, hemicellulose, lignin and cellulose contents.
In NaOH hydrolysate, the observed optimum values for reducing sugars, total carbohydrates, weight loss, extractives, hemicellulose, lignin and cellulose contents were 15.82+0.02, 38.1+2.5, 78.42+0.16, 5.02+0.35, 45.01+0.78, 11.99+0.43 and 38+1.33 % respectively at 0.5% alkali concentration, 80ºC for 45.61 minutes. The experimental values were found to be more than the predicted values for total carbohydrates, hemicellulose and lignin and less than the predicted values for reducing sugars, extractives, weight loss and cellulose. The model was non- significant for all other responses with F value less 4 and R2values varied from 0.3-0.7. On the basic of results 15.82% reducing sugar were recorded with 78.42% weight loss.
In KOH hydrolysate, the optimum values were 10.44+0.37, 32.82+0.24, 74.78+0.31, 1.83+0.5, 16.67+2.14, 5.84+0.63, 75.67+1.78 % for reducing sugars, Total carbohydrates, weight loss, extractives, hemicellulose, lignin and cellulose contents at 0.4% alkali concentration, 80ºC for 90 minutes. The experimental values for reducing
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sugar, lignin and cellulose were less than the predicted values while total carbohydrate, extractive weight loss showed more experimental values than the predicted values. The model was significant for reducing sugars, total carbohydrates, hemicellulose, and weight loss and not significant for lignin, cellulose and extractives with F value less than 4 and R2 valued varied from 0.5-0.7. On the basis of results 10.44% reducing sugar were recorded with 74.78% weight loss.
In Ca(OH)2 hydrolysate, the optimum values were 7.59+0.57, 26.25+0.43, 60.56+0.44, 2.5+0.5, 33.33+0.61, 4.5+0.79, 59.67+1 for reducing sugars, Total carbohydrates, weight loss, extractives, hemicellulose, lignin and cellulose contents at 0.21% alkali concentration, 80ºC for 90 minutes. The experimental values for all responses were less than the predicted values except hemicellulose and cellulose contents. The model was significant for all responses except extractives, hemicellulose and cellulose with F value less than 4 and R2 valued varied from 0.5-0.7. On the basis of results 7.59% reducing sugar were recorded with 60.56% weight loss. In the present study the detoxification was done with 2.5% charcoal treatment for improving the quality of fermentation. The reduction in phenol contents can be seen by investigating untreated hydrolysate and treated with sodium hydroxide, potassium hydroxide and calcium hydroxide. 29.41% phenolics were reduced in calcium hydroxide hydrolysate. After detoxification and neutralization, fermentation was carried out using yeast strains Metschnikowia sp. Y31 and Metschnikowia cibodasenesis Y34. While Saccharomyces cerevisiae K7 was used as standard yeast in fermentation experiment.
The reducing sugars released after NaOH, KOH and Ca(OH)2 hydrolysis, yielded 3.63±0.02, 2.71±0.05 and 1.81±0.01 % ethanol respectively by the fermentation of
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Metschnikowia sp. Y31 on 3rd day of incubation and they yielded 2.83±0.02, 2.1±0.03 and 1.55±0.01 % ethanol correspondingly by the fermentation of Metschnikowia cibodasensi Y34 also on 3rd day of incubation. While with Saccharomyces cerevisiae K7, they yield 2.46±0.01, 2.24±0.03 and 2.03±0.02 % ethanol on day 1 of incubation. The maximum yield of ethanol i.e. 3.63±0.02 %, was obtained by the fermentation of Metschnikowia sp. Y31 on 3rd day of incubation