Débora Danielle Virgínio da Silva

Improvement of biotechnological xylitol production by glucose during cultive of Candida guilliermondii in sugarcane bagasse hydrolysate

The effect of glucose on xylose-to-xylitol bioconversion by Candida guilliermondii was examined by adding it to sugarcane bagasse hydrolysate medium to obtain different glucose:xylose ratios (1:25, 1:12, 1:5 and 1:2.5). Under experimental conditions, increasing glucose:xylose ratio improved the assimilation of the xylose present in the hydrolysate by yeast, resulting in biomass increase, and in the formation of xylitol and glycerol/ethanol by-products. Maximum values of xylitol yield (0.59 g g-1) and volumetric productivity (0.53 g l-1.h-1) were obtained with glucose:xylose ratio of 1:5, resulting in the higher conversion efficiency (64.3%).

Inhibitory effect of acetic acid on bioconversion of xylose in xylitol by Candida guilliermondii in sugarcane bagasse hydrolysate

Sugarcane bagasse hydrolysate (initial acetic acid concentration = 3.5g/L), was used as a fermentation medium for conversion of xylose into xylitol by the yeast Candida guilliermondii FTI 20037. Acetic acid (2.0g/L) was added to the medium at different times of fermentation, with the aim of evaluating its effects on the bioconversion process. The addition of acetic acid to the medium after 12h of fermentation resulted in the strongest inhibition of the yeast metabolism. In this case, the xylose consumption and cell growth were, respectively, 23.22 and 11.24% lower than when acid was added to the medium at the beginning of fermentation. As a consequence of the inhibitory effect, lower values of the xylitol yield (0.39g/g) and productivity (0.22g/L.h) were observed, corresponding to a reduction of 36 and 48%, respectively, in relation to the values obtained with the addition of acetic acid after other fermentation times. The results obtained allowed to conclude that, under the experimental conditions employed in this work, the inhibitory effect of acetic acid on the xylose-xylitol bioconversion depends on the fermentation time when this acid was added, and not only on its concentration in the medium.

Evaluation of hexose and pentose in pre-cultivation of Candida guilliermondii on the key enzymes for xylitol production in sugarcane hemicellulosic hydrolysate

The evaluation of hexose and pentose in pre-cultivation of Candida guilliermondii FTI 20037 yeast on xylose reductase (XR) and xylitol dehydrogenase (XDH) enzymes activities was performed during fermentation in sugarcane bagasse hemicellulosic hydrolysate. The xylitol production was evaluated by using cells previously growth in 30.0 gl(-1) xylose, 30.0 gl(-1) glucose and in both sugars mixture (30.0 gl(-1) xylose and 2.0 gl(-1) glucose). The vacuum evaporated hydrolysate (80 gl(-1)) was detoxificated by ion exchange resin (A-860S; A500PS and C-150-Purolite®). The total phenolic compounds and acetic acid were 93.0 and 64.9%, respectively, removed by the resin hydrolysate treatment. All experiments were carried out in Erlenmeyer flasks at 200 rpm, 30°C. The maximum XR (0.618 Umg (Prot) (-1)) and XDH (0.783 Umg (Prot) (-1)) enzymes activities was obtained using inoculum previously growth in both sugars mixture. The highest cell concentration (10.6 gl(-1)) was obtained with inoculum pre-cultivated in the glucose. However, the xylitol yield and xylitol volumetric productivity were favored using the xylose as carbon source. In this case, it was observed maximum xylose (81%) and acetic acid (100%) consumption. It is very important to point out that maximum enzymatic activities were obtained when the mixture of sugars was used as carbon source of inoculum, while the highest fermentative parameters were obtained when xylose was used.

New cultive medium for bioconversion of C5 fraction from sugarcane bagasse using rice bran extract

The use of hemicellulosic hydrolysates in bioprocesses requires supplementation as to ensure the best fermentative performance of microorganisms. However, in light of conflicting data in the literature, it is necessary to establish an inexpensive and applicable medium for the development of bioprocesses. This paper evaluates the fermentative performance of Scheffersomyces (Pichia) stipitis and Candida guilliermondii growth in sugarcane bagasse hemicellulosic hydrolysate supplemented with different nitrogen sources including rice bran extract, an important by-product of agroindustry and source of vitamins and amino acids. Experiments were carried out with hydrolysate supplemented with rice bran extract and (NH4)2SO4; peptone and yeast extract; (NH4)2SO4, peptone and yeast extract and non-supplemented hydrolysate as a control. S. stipitis produced only ethanol, while C. guilliermondii produced xylitol as the main product and ethanol as by-product. Maximum ethanol production by S. stipitis was observed when sugarcane bagasse hemicellulosic hydrolysate was supplemented with (NH4)2SO4, peptone and yeast extract. Differently, the maximum xylitol formation by C. guilliermondii was obtained by employing hydrolysate supplemented with (NH4)2SO4 and rice bran extract. Together, these findings indicate that: a) for both yeasts (NH4)2SO4 was required as an inorganic nitrogen source to supplement sugarcane bagasse hydrolysate; b) for S. stipitis, sugarcane hemicellulosic hydrolysate must be supplemented with peptone and yeast extract as organic nitrogen source; and: c) for C. guilliermondii, it must be supplemented with rice bran extract. The present study designed a fermentation medium employing hemicellulosic hydrolysate and provides a basis for studies about value-added products as ethanol and xylitol from lignocellulosic materials.

Biotechnological Production of Xylitol from Biomass

Xylitol is a polyol of interest to food, dental and pharmaceutical industries because of its favorable characteristics such as sweetening capacity, insulin-independent metabolism effects and its lack of carcinogenic properties. It is usually produced by chemical processes that are expensive due to their high energy consumption and many purification steps. Biotechnological routes are promising because they can be carried out using mild conditions and have the possibility of using hydrolysates from renewable sources as raw materials without the need of extensive purification of xylose before the fermentation step. Different lignocellulosic materials have been studied as alternative raw materials in the fermentative process for xylitol production. However, the structure of lignocellulose is recalcitrant and a pretreatment step is necessary to release monomeric sugars that does not form compounds toxic to microorganisms. Another challenge for xylitol production by fermentation is the identification of efficient microorganisms for converting the pentose sugars present in hemicellulosic hydrolysates. Different strategies have also been investigated, aiming to optimize the biotechnological way, such as use of different configurations of bioreactors, process options and downstream steps. This chapter will explore biotechnological xylitol production from the selection and preparation of the raw material to fermentative process conditions, downstream strategies and future perspectives. These topics will be discussed to offer readers a better understanding of biotechnological routes to xylitol as well as their potential and future prospects.

Biomass Pretreatment With Oxalic Acid for Value-Added Products

Monomeric sugars released from carbohydrate fractions of lignocellulosic materials can be substrates for industrial fermentative processes in order to obtain bioproducts with potential economic and social interest. When diluted acid solutions are used to pretreat biomass, the result is a solid fraction in which cellulose is more accessible to hydrolytic enzymes and a liquid fraction enriched in sugars (mainly pentoses) from hemicellulose. Pretreatment with dilute oxalic acid is a promising approach, resulting in high efficiency in hemicellulose hydrolysis, which generates a lower quantity of compounds toxic to microbial metabolism. In addition, once this acid is organic, it can be recovered by usual techniques and reused in a pretreatment step. Within this context, this chapter presents the use of oxalic acid pretreatment for different biomasses, including the structural changes that occur after using this method as well as applications of the obtained solid and liquid fraction in fermentative processes.