Standardizing process for xylitol production from corn cobs using Candida tropicalis

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Date
2023
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DIVISION OF MICROBIOLOGY ICAR-INDIAN AGRICULTURAL RESEARCH INSTITUTE NEW DELHI-110012
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ABSTRACT Crop residues are valuable sources of lignocellulosic biomass that will continue to be available as long as food production for humanity continues. Maize, a cereal crop that serves as a staple food in many parts of the world and is cultivated in numerous countries, holds great importance. With its high genetic potential and low water requirement, global annual maize production (dry grain) is estimated at 1,137 million tonnes, surpassing both rice and wheat by about 50% (FAOStat 2021). The Government of India is also promoting hybrid maize to meet 20% blending of ethanol to cut carbon emission by 30 % by 2025. This results in a surge in production generating substantial amounts of corn cob waste, which can be used as feedstock for production of various valuable products like xylitol, a compound with a rapidly growing market. Traditional chemical methods of xylitol production are expensive and require significant energy inputs. However, a biobased, cost-effective, and environmentally friendly approach utilises biocatalysts (microorganisms or enzymes), fermenting the xylose rich corn cob hydrolysates to xylitol. Thus the current study was aimed at standardisation of the process for production of xylitol. Corn cobs from five selected high-yielding hybrid varieties (PJHM-1, AH-4271, AH-8181, AH-4142, and AH-4158) were studied and were found to contain cellulose and hemicellulose content 34-38% and ~32-36% respectively. To extract hemicellulose and obtain xylose, the corn cobs were subjected to acid hydrolysis with 1% H2SO4 at 10-20% substrate loading, at temperatures 110 °C and 121 °C for 30 and 20 minutes yielded xylose concentration in acid prehydrolysates ranging from 25.56 – 102.26 g L-. Amongst the five varieties, AH-4271 yielded the highest xylose concentration of 102.26 g/L when subjected to acid pre-treatment at.Further Statistical Optimisation of acid hydrolysis of AH-4271 corn cobs was done using Response Surface Methodology. The highest yield percentage was achieved at a loading of 20% with 2% acid concentration, at 121 °C, 15 psi for 20 minutes. Four native non-conventional pentose fermenting yeast strains (Candida tropicalis Y6, Pichia stipitis, Kodamaea ohmeri, Rhodotorula glutinis) were tested for growth and xylose utilisation potential. All four strains showed xylanase activity when grown on agar containing xylan and stained with Congo red. C. tropicalis Y6, P. stipitis, and K. ohmeri demonstrated the best growth and efficiency in utilising xylose. Consequently, C. tropicalis Y6 and K. ohmeri were selected for further tested for growth and xylose utilisation potential on acid prehydrolysate medium supplemented with mineral salts. Candida tropicalis Y6 exhibited the 65 highest growth and xylose utilisation, making it the preferred choice for xylitol production. On synthetic medium it achieved a maximum conversion efficiency (53%) of xylose to xylitol at 24 hours, with a production of 6.71 g/L of xylitol. When C. tropicalis Y6 was cultured on corn cob hydrolysate, it produced 0.41 g/L of xylitol along with 0.74 g/L of ethanol under standard conditions. However, its growth and xylose conversion was severely affected in acid hydrolysates possibly due to presence of inhibitors. The noticeable feature was the high level of ethanol produced in acid prehydrolysate fermentation indicating the expression levels of XR and XDH enzymes under conditions of study. Overall, this study highlights the potential C. tropicalis Y6, for co-production of xylitol and ethanol, making it a promising candidate for application in biorefinery. 66
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