Xylitol is a five-carbon sugar alcohol of high value added as a sweetener for high power, anticariogenic properties and insulin metabolism independent that guarantee its application in food and pharmaceutical industries. The power as a sweetener is similar to sucrose, and higher than ordinary polyols in addition to reduce caloric value, can be tolerated by diabetics. It is an anticariogenic and cariostatic compound that is not metabolized by microorganisms of the oral microbiota; thus, it is used in the manufactures of sweets. Can be used clinically for the prevention of otitis media because it inhibits the growth and adhesion of Pneumococcus spp and Haemophilus influenzae in nasopharynx cells, and it has skin smoothing properties [9, 10, 11, 12, 13, 14]. Owing to all these characteristics, xylitol is a feedstock of particular interest to the food, odontological and pharmaceutical industries.

Xylose is a sugar widely distributed in nature. Plants and fruits contain relatively low concentration, and extraction from natural sources is usually not profitable [10]. Nowadays, xylitol is derived industrially via a chemical process from hydrolyzates of lignocellulosic wastes by either chemical reduction or microbial fermentation [9]. However, due to a requirement for several chemical purification steps, such a process is very expensive. Therefore, this conversion could be alternatively performed by bacteria, filamentous fungi, yeasts or purified enzymes from these microorganisms which are capable of reducing xylose to xylitol as a first step in D-xylose metabolism [15]. Nevertheless, to make this process exploitable and economical at an industrial level, the bioconversion must be rapid, offer high yield, employ an alternative and cheap culture media and allow for results comparable to those of the present technology.

Lignocelluloses are the most abundant organic mass in the biosphere, which accounts for approximately 50% of the biomass. In nature the annual production of biomass is estimated to 10 to 50 x 109 tons [16] Their major components, cellulose, hemicellulose and lignin, vary with plant species. The pentose fraction, composed of D-xylose (usually not less than 95%) and L-arabinose is much higher in hardwoods (19 to 33%) than in softwoods (10 to 12%)[17]

Hemicellulose is a branched polymer, which is composed of both linear and branched heteropolymers of D-xylose, L-arabinose, D-mannose, D-glucose, D-galactose and D — glucuronic acid with a high content of xylans, that consist essentially in p-1,4 links with branching variables; due to it heterogeneous structure and low degree of polymerization, it is easily hydrolyzed to xylose [15, 16]. Xylan accounts for 11-35% (dry weight basis) of lignocellulosic materials such as hardwoods and agricultural residues, such as sugarcane bagasse [14, 18], rice straw [19], and soy hulls [20, 21] which are xylan-rich substrates and have been satisfactorily used as alternative media for xylitol production through different treatments [22] and cultivation conditions [23], aimed at increasing process yield and productivity.

D-xilose, also can be converted into a range of substances of industrial interest such as fuels and solvents (ethanol, butanol, 2,3-butanediol, acetone and 2-propanol), alditols (xylitol and glycerol) and organic acids (latic, acetic and butyric acid). It can also be used as a substrate for production of glucose isomerase [14].

For this process of xylitol production, pure xylose is necessary. The process starts with the production of xylose from xylan after acid-catalysed hydrolysis from hard-wood; however, the chemical process requires several purification steps, because only pure xylose can be used for chemical reduction. Therefore, overall xylitol yield is relatively low (50 — 60 %) from the total xylan content of the wood hemicelluloses [24, 11, 15].

Furthermore, the choice of cultivation and/or conversion system is another crucial point for the success of this bioprocess. Different bench-scale cultivation systems were investigated, utilizing batch, fed-batch and continuous processes [15]. Another important factors which affect the xylitol production is the quantity of inocula, substrate, media, temperature, pH and aeration [17, 25]

On the other hand, the biotechnological procedures are based on the utilization of microorganisms and/or enzymes. These procedures are interesting because they do not require a pure xylose solution as is the case when xylitol is produced by the chemical pathway. The bioconversion process would hold more promises of both hexoses and pentose sugars from lignocellulosic materials. The promising yeast species include the generous Candida, Pichia, Debaryomyces and Pachysolen [9, 26, 14, 16] by NADPH-dependent xylose reductase, enzyme which can ferment hemicelluloses hydrolysate from woody plant materials (Figure 3).

The first step in the metabolism of D-xylose is the transport of the sugar across the cell membrane. Once inside the yeast cell, D-xylose is reduced to xylitol by either NADH — or NADPH-dependent xylose reductase. Xylitol is either secreted from the cell or oxidized to xylulose by NAD — or NADP-dependent xylitol dehydrogenase. The first two reactions are considered to be limiting in D-xylose fermentation. The phosphorylation of xylulose to xylulose 5-phosphate is catalyzed by xylulokinase, which is a prerequisite for its utilization by the central catabolic pathways [23, 17, 16].

In most studies on xylitol production by fermentative processes, xylose of analytical grade is commonly the major substrate. The main problem in the fermentation of these hydrolysates is the presence of toxic compounds released from the lignocellulosic structure during the hydrolytic process, as well as those originated from the sugar degradation, which inhibit the microbial growth and the fermentative activity of the yeasts. In this way, several approaches have been assayed to minimize this effect. According to Silva et al, 1998 the maximum xylitol production (54 g/L) occurs when the hydrolysate is first treated with CaO until reaching pH 8.4 and then treated with H3PO4 until the pH decreases to 6.0. Thus, pH is an important factor to take into account for the xylitol fermentation. Its effect is related to the acetic acid concentration in the hydrolysate, which concentration, if it is higher than 3.0 g/L, can inhibit the yeasts capability to convert xylose into xylitol [27]. Nonionized acetic acid, which is found in the medium at pH < 7.0, has been found to be the main inhibitor compound in yeast metabolism [28]

The hemmicellulosic hydrolysates from agroforest residues can be efficiently utilized in fermentative processes for xylitol production after an initial treatment designed to remove or reduce the compounds known to be toxic to cell metabolism. This technology is still in its research and development stage, but the results attained points that it may be feasible to