Production and characterization of bioethanol derived from cashew apple juice for use in internal combustion engine

Abstract

Progressive global dilemmas such as greenhouse gas emissions and depleting crude oil supply have sparked an exponential interest in the use of bioethanol in the motor-fuel industry. To date, bioethanol production from cereal grains and lignocellulose feedstock has been extensively researched but the use of agro-industrial waste products for bioethanol production also has potential. One such product is cashew apples. Cashew apples are regarded as waste after cashew nut harvesting and are an attractive feedstock for bioethanol due to the lack of commercial usage. The present work, therefore aimed to investigate the characteristics of cashew apple juice extracted from cashew apples grown in South Africa and to evaluate the concentration of bioethanol obtainable from cashew apple juice by fermentation using Saccharomyces cerevisiae yeast strains NRRL Y2084 and Vin13. Furthermore, this research aimed to show the compatibility of ethanol/petrol blends and determine the change in fuel level of a generator fuelled with ethanol/petrol blends. During the February harvest season, a mixed variety of cashew apples collected from the KwaNgwanase region revealed the following characteristics: specific gravity of 1.050 and pH of 4.52 by direct measurement; total sugars: 100g/L by HPLC; total minerals: 15.89ppm by iii AAS; condensed tannins: 55.34mg/L by the Vanillin-HCl assay; proteins: 1.78g/L by the Coomassie Blue assay; Vitamin C: 112mg/100mL by the Iodine Titration. Post characterization, the cashew apple juice was fermented in a BIOSTAT® Bplus fermentor. The fermentation conditions were as follows: pH=4.50, agitation=150rpm, temperature=30ºC (Y2084) and 20ºC (Vin13), oxygen saturation=0% or 50%. During fermentation samples were analysed for sugar consumption, ethanol production and glycerol production by HPLC and yeast viability by plate counts. HPLC revealed that glucose and fructose are the predominant sugars of CAJ and utilized by both strains during fermentation. The maximum ethanol concentration achieved by NRRL Y2084 was 65.00g/L and oxygen saturation affected the fermentation time. The maximum ethanol concentration achieved by Vin13 was 68.00g/L at 50% oxygen, whilst at 0% oxygen; 31.00g/L of ethanol. Both yeast strains produced a higher glycerol concentration at 0% oxygen and yeast viability counts were higher at 50% oxygen. Post fermentation, the product was distilled by simple distillation using a rotary evaporator and 75% of bioethanol was recovered. For the blending experiments, concentrations of E4.4, E5 and E10 ethanol/petrol were prepared using absolute ethanol and SASOL 95 unleaded petrol grade. The blends were analysed by NMR Spectroscopy using the Bruker Ultrashield™ 500 PLUS to determine the 1H and 13C chemical pattern of ethanol/petrol blends. Qualitative analyses revealed ethanol/petrol blends possess structural compounds consistent with neat petrol and ethanol. These blends were used to fuel a spark ignition generator coupled with a graduated dipstick to monitor the fuel level. From the results, the fuel consumption using neat petrol and the E5 blend were similar. For iv the E10 blend fuel consumption was slower, whereas when the generator was run using the E4.4 blend, the engine abruptly switched off without complete consumption of the fuel. The fermentation experiments performed under defined conditions in a controlled system showed the suitability of cashew apple juice as a substrate for bioethanol production in conjunction with S. cerevisiae yeast strains. Ethanol concentration ranged from 31.00g/L to 68.00g/L, with a higher concentration and shorter fermentation time achieved during aerobic fermentation. From the blending of ethanol with commercial petrol, qualitative analysis confirmed the structure of each individual component is unchanged in a blend and the addition of ethanol to a specific grade of petrol is compatible. Generator analysis revealed the E10 blend decreased the fuel consumption, thus the addition of ethanol to fuel is advantageous.