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
Abstract
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
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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.
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