Elsevier

Bioresource Technology

Volume 97, Issue 15, October 2006, Pages 1974-1978
Bioresource Technology

Short Communication
Xylitol production from corn fiber and sugarcane bagasse hydrolysates by Candida tropicalis

https://doi.org/10.1016/j.biortech.2005.08.015Get rights and content

Abstract

A natural isolate, Candida tropicalis was tested for xylitol production from corn fiber and sugarcane bagasse hydrolysates. Fermentation of corn fiber and sugarcane bagasse hydrolysate showed xylose uptake and xylitol production, though these were very low, even after hydrolysate neutralization and treatments with activated charcoal and ion exchange resins. Initial xylitol production was found to be 0.43 g/g and 0.45 g/g of xylose utilised with corn fiber and sugarcane bagasse hydrolysate respectively. One of the critical factors for low xylitol production was the presence of inhibitors in these hydrolysates. To simulate influence of hemicellulosic sugar composition on xylitol yield, three different combinations of mixed sugar control experiments, without the presence of any inhibitors, have been performed and the strain produced 0.63 g/g, 0.68 g/g and 0.72 g/g of xylose respectively. To improve yeast growth and xylitol production with these hydrolysates, which contain inhibitors, the cells were adapted by sub culturing in the hydrolysate containing medium for 25 cycles. After adaptation the organism produced more xylitol 0.58 g/g and 0.65 g/g of xylose with corn fiber hydrolysate and sugarcane bagasse hydrolysate respectively.

Introduction

Xylitol is one of the most expensive polyol sweeteners and has specific health claims in the world market. It is suitable for diabetes, recommended for oral health and paranteral nutrition (Makinen, 2000, Sreenivas Rao et al., 2004). On an industrial scale, xylitol is currently produced through chemical reduction of xylose derived from birchwood chips and sugarcane baggase hemicellulose hydrolysate. It is relatively expensive by about $7 kg−1 (Leathers, 2003) comparatively with other natural sweeteners. The chemical process is expensive because of the high temperature and pressure required for hydrogenation of xylose. Further more, extensive steps for separation and purification add to the cost. Availability and cost of production are the obstacles impending the increased use of xylitol. Biotechnological production of xylitol could be of economic interest and attractive, as crude hemicellulosic hydrolysates can be used as potential substrates, instead of pure xylose, to reduce the cost of production.

Hemicellulose is a plant cell wall polysaccharide and third most abundant polymer in nature. In some plants, it comprises upto 40% of the total dry material. Hemicelluloses are short branched chain heteropolysacharides of mixed hexosans and pentosans that are easily hydrolyzed. The most common form of hemicellulose is xylose polymer (xylan). As hemicellulose is abundant in nature and renewable, extensive research has been undertaken to convert hemicellulose derived carbohydrates, particularly xylose, into useful products. Hydrolysis of hemicellulose, yields glucose, d-xylose, l-arabinose and other minor sugars.

During acid hydrolysis, xylose is degraded rapidly to furfural and other condensation byproducts. These degradation products are inhibitory to microorganisms. The inhibitory effect of different compounds like furfural, 5-hydroxymethyl furfural (HMF), acetate, hydroxybenzaldehyde (HBA), siringaldedyde (SGA) and vanillin on yeast growth is well documented- (Tsao et al., 1999).

Various approaches are being considered to remove fermentation inhibitors or minimize their formation, CaO treatment (Silva et al., 1998), use of activated charcoal (Pandey et al., 2000), overliming (Dien et al., 2000), ion exchange resins (Mancilha and Karim, 2003), Ca(OH)2 (Dien et al., 2000), solvent extraction (Cruz et al., 1999), intracellular acidification (Lohmeier-vogel et al., 1998), yeast strain variation (Martin and Jönsson, 2003), laccase enzyme (Cassland and Jönsson, 1999), recombinant strains (Dien et al., 1997) and adaptation of the microbial strains (Rivas et al., 2003) have been reported.

Many studies have been conducted utilizing the hemicellulose portion of agricultural residues like Eucalyptus grandis (Silva et al., 1998), rice straw (Roberto et al., 1994), aspen wood hemicellulosic hydrolysate (Preziosi-Belloy et al., 2000), barley bran (Cruz et al., 2000), hybrid polar wood chips (Domínguez et al., 1997) and corn cobs (Rivas et al., 2003) for xylitol production. In the present investigation two substrates, corn fiber and sugarcane bagasse have been selected.

Corn fiber represents a renewable resource that is available in significant quantities from the corn wet milling industries (Doner et al., 2001). Corn fiber contains about 70% fermentable sugars (Dien et al., 1999). Corn fiber is rich in pentose sugars (Saha, 2003) in the form of arabinoxylan (hemicellulose). Because of its relatively low commercial value and increasing supply, considerable interest exists in finding new, value-added uses for this material (Dien et al., 1999). The low cost high carbohydrate content of corn fiber makes it an attractive feed stock for conversion to xylitol. About 3.4 × 106 dry tons of corn fiber are produced each year in the United States alone (Dien et al., 1999), and about 203.6 Tg of dry corn stover are globally available (Tsao et al., 1999) for d-xylose and xylitol production. Nearly half of all fuel ethanol production in the USA employs a wet-milling process utilizing more than 300 million bushels of corn. Every bushel of maize processed for sweetener, oil or ethanol generate nearly 7 kg of protein and fiber rich residues.

Many research efforts were made to achieve the production of value added products from sugarcane bagasse (Pandey et al., 2000). The annual global production of dry cut sugarcane is about 328 Tg and Asia (44%) is the primary production region (Kim and Dale, 2004). Each ton of milled sugarcane gives 180–280 kg of bagasse residues (Pessoa et al., 1997). The acid hydrolysate from bagasse contains xylose as the main component (Toit et al., 1984). When sugarcane bagasse hemicellulose hydrolysate was evaluated for cultivation of 18 species of different yeasts, Candida sps. emerged as the best performer (Pessoa et al., 1997).

Though these two substrates have been studied for the production of different products using microorganisms, there are very few reports available with these substrates for xylitol production. Therefore in the present study corn fiber and sugarcane bagasse hydrolyastes have been tested for the economical production of xylitol using C. tropicalis.

Section snippets

Preparation of corn fiber hydrolysate

Wet corn fiber was obtained from Gayatri Starchkem Limited. Corn fiber was dried at 55 °C for 24 h in a oven and ground in a mixer and passed through a 28 mesh screen. The ground corn fiber was mixed with 1% (v/v) H2SO4 at a ratio of 1 g of biomass to 5.0 ml of acid solution and autoclaved for 1 h. After cooling, the liquid was separated from the solids using cheese cloth. Corn fiber hydrolysate was prepared as previously described (Saha and Bothast, 1999). Hydrolysate was neutralized using Ca(OH)2,

Results and discussion

Corn fiber hydrolysate and sugarcane bagasse hydrolysate were neutralized with Ca(OH)2 and treated with activated charcoal and ion exchange resins to reduce inhibitors and conductivity of the hydrolysate. Corn fiber has four-fold dissolved solids and more conductivity than sugarcane bagasse hydrolysate. In both cases conductivity was reduced up to 500 μΩ/cm by ion exchange resins.

Sugars derived from corn fiber and sugarcane bagasse and other hemicellulose material can be converted to xylitol

Conclusion

To produce xylitol by yeasts, in a cost-effective manner the fermentation of xylose from hemicellulosic material is important. The potential of corn fiber and sugarcane bagasse hemicellulosic hydrolyzates as substrates for xylitol production by natural isolate and adapted strains of C. tropicalis was demonstrated.

Acknowledgements

We are grateful to Department of Biotechnology, Government of India and Gayatri Starchkem Ltd., (Corn Wet-Milling and Sweetener Industry) Hyderabad, AP, India for providing partial financial assistance and to Y. Bhaskar Reddy, GSL and K. Badhraiah, Swaroop Technologies, Secunderabad for technical support to complete this work.

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