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Chemical Engineering and Processing 45 (2006) 1041–1046

Xylitol crystallization from culture media fermented by yeasts

Fabio Coelho Sampaio a, Flavia M. Lopes Passos a, Frederico J. Vieira Passos b,Danilo De Faveri c,∗, Patrizia Perego c, Attilio Converti c

a Department of Microbiology, Instituto de Biotecnologia Aplicada a Agropecuaria, Federal University of Vicosa,Av. P. H. Rolfs s/n, 36570-000 Vicosa, Minas Gerais, Brazil

b Department of Food Technology, Federal University of Vicosa,Av. P. H. Rolfs s/n, 36570-000 Vicosa, Minas Gerais, Brazil

c Department of Chemical and Process Engineering “G.B. Bonino”, Genoa University,Via Opera Pia 15, I-16145 Genoa, Italy

Received 9 February 2006; accepted 17 March 2006Available online 9 May 2006

bstract

Among sugar substitutes, an important role is played by xylitol, an aliphatic pentitol provided with some interesting properties whichake it a high value product for pharmaceutical, odontological and food industries. Its production by biotechnological methods is based

n fermentation of agro-industrial residues and could potentially compete with the traditional chemical way. However, crystallization is anmportant stage of xylitol production, since in many respects it determines the yield and quality of the target product. In the present workests were made in order to determine the best conditions to clarify fermented media, which were then subjected to isothermal crystal-ization in the presence or in the absence of residual xylose and varying xylitol concentration (675 ≤ Po

Xyt ≤ 911 g l−1) as well as coolingemperature (−10 ≤ Tc ≤ 15 ◦C). Besides, the kinetics of xylitol crystallization from fermented solutions was investigated as function of thenitial solution supersaturation and cooling temperature. The effect of the presence of residual xylose on the rate of crystallization was

lso evaluated. The best clarifying treatment was found to be 20 g l−1 activated charcoal at room temperature for 1 h. The study of xylitolrystallization revealed the positive effect of the presence of residual xylose, which ensured a 1.6-fold increase in the crystallization yieldfrom 0.27 to 0.42).

2006 Elsevier B.V. All rights reserved.

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eywords: Xylitol; Fermented media; Crystallization; Kinetics; Xylose

. Introduction

The recovery and purification of a fermentation product is aundamental step in several industrial processes, which dependsn the nature of the product that can be biomass, an intracellularr an extracellular compound. When recovering a product withomplex chemical structure that requires high purity for com-ercialization, its recovery and purification often imply steps

haracterized by costs even higher than the very production pro-

ess [1].

Since phase diagrams and kinetics are not usually availableor each specific system, the recovery-purification procedure

∗ Corresponding author. Tel.: +390 10 353 2584; fax: +390 10 353 2586.E-mail address: danilo.defaveri@unige.it (D. De Faveri).

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255-2701/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2006.03.012

as to be experimentally optimized. Crystallization is often thenal step for the obtaining of highly purified products [2–4].hen performed at low temperature, it allows minimizing the

hermal degradation of compounds sensitive to heat; besides,ts unit operating cost is particularly low if compared with otherecovery techniques, because of the use of very high productoncentrations [1].

The polyol sweeteners producing industry has registered arowing demand for the consumption of sugar-free and low heat-alue products. Among these, xylitol is nowadays produced byn expensive catalytic hydrogenation of d-xylose coming fromcid hydrolysis of lignocellulosics [5]. In this chemical process,

fter removal of the catalyst by filtration, the hydrogenated solu-ions are processed to recover xylitol by crystallization. How-ver, when non-purified solutions are employed, polyols otherhan xylitol must be preliminarily removed by ionic exchange

1042 F.C. Sampaio et al. / Chemical Engineering

chromatography using cationic resins [6]. Xylitol is then crystal-lized from aqueous solutions separated by the above fractioningprocess.

The microbial production of xylitol has been investigatingsince several years as a possible alternative process, utiliz-ing semi-synthetic (semi-defined) media [7–10] or detoxifiedlignocellulosic hydrolyzates [11–14]. Promising results havebeen recently obtained with different microorganisms [15–17],mainly yeasts belonging to the genera Candida [18–21] andDebaryomyces [22–24]. However, xylitol crystallization fromfermented lignocellulosic hydrolyzates proved to be a verydifficult task [25–27] owing to the presence either of severalimpurities coming from the hydrolysis of the lignocellulosicmatrix (sugars, metallic ions, phenolic compounds, furfural,hydroxymethylfurfural, etc.) or nutrients added for mediumpreparation (amino acids, peptides, proteins and inorganic salts)[11,17,26,28,29].

Gurgel et al. [25], who used cationic and anionic exchangeresins to purify xylitol coming from the fermentation of sug-arcane bagasse hydrolyzate, observed a xylitol loss by about46–57%. Although the preliminary treatment with 200 g l−1

activated charcoal at 80 ◦C and pH 6.0 for 60 min effectivelydecolorized the fermented hydrolyzate, about 20% of xylitolwas lost by adsorption. Besides, after concentration of the broth,the solution resulted to be colored and viscous, thus making thecrystallization difficult and too long (1 week at −15 ◦C).

De Faveri et al. [30], using response surface methodologyand xylitol–xylose synthetic solutions (582 ≤ Po

Xyt ≤ 730 g l−1)at −10 ≤ Tc ≤ 0 ◦C identified the operating conditions (Po

Xyt =728 g l−1 and Tc = −6.0 ◦C) under which purity degree (0.97)and xylitol crystallization yield (0.54) were simultaneously opti-mized. The model also predicted that, under the conditionsensuring a purity degree close to 100% to meet the industrialstandards (Po

Xyt = 583 g l−1 and Tc = −2.4 ◦C), the crystalliza-tion yield would be unsatisfactory (0.25).

Membrane separation was proposed as an alternative methodfor the recovery of xylitol from fermentation broths, because ithas the potential for energy savings and higher purity [26]. A10,000 nominal molecular weight cutoff polysulfone membranewas found to be the most effective for the separation and recoveryof xylitol. The membrane allowed 82.2–90.3% of xylitol in thefermentation broth to pass through, while retaining 49.2–53.6%of certain impurities such as oligopeptides and peptides. Oncecollected and crystallized the resulting permeate, crystals withpurity up to 90.3% were obtained. However, due to impuritiesin the solution, crystallization was slow and required 14 days at8 ◦C.

To minimize the interference of the above substances onxylitol crystallization, the present study was focused on syn-thetic broths fermented by a new yeast strain of Debaryomyceshansenii, which recently proved to be a very efficient xylitolproducer [10]. The results of crystallization tests carried outusing different activated charcoal concentrations for prelimi-nary broth purification, crystallization temperatures and startingxylitol concentrations, either in the presence of residual xyloseor not, were utilized to get additional information on xylitolrecovery by crystallization.

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and Processing 45 (2006) 1041–1046

. Materials and methods

.1. Treatment with activated charcoal

Batch tests of xylitol crystallization were performed on twoifferent synthetic media fermented by Debaryomyces hanseniiFV-170. Medium (A) contained, before fermentation, 5 g l−1

eptone, 3 g l−1 yeast extract, 3 g l−1 malt extract and different d-ylose concentrations, whereas medium (B) 0.62 g l−1 KH2PO4,.0 g l−1 K2HPO4, 1.0 g l−1 (NH4)2SO4 and 1.1 g l−1 MgSO4,.0 g l−1 yeast extract, pH 6.0, supplemented with different d-ylose concentrations.

After cell removal by centrifugation at 8000 × g, the fer-ented broths were micro-filtered through Millipore mem-

ranes with 0.2 �m pore diameter. Samples of the filtrate werenalyzed, according to the methodologies described below, foreterminations of the initial concentrations of xylitol and xyloses well as the contents of amino acidic impurities and total pro-eins. The pH of the broths before the treatment was alreadylose to 6.0.

After micro-filtration, aliquots of the filtrate (50 ml) wereransferred into 100 ml-flasks for the treatment with variableoncentrations of activated charcoal (Carlo Erba Reagenti,odano-MI, Italy) (5–200 g l−1) at two different temperatures

25 and 50 ◦C) inside a climatic hood, model 810 (Asal, Cer-usco S.N.-MI, Italy). After magnetic agitation for 1 h, theixture was micro-filtered again and the same analytical deter-inations were performed as described above.

.2. Crystallization tests

After treatment with activated charcoal, fermented mediaere concentrated in rotavapor, model LABOROTA 4000 (Hei-olph Instruments, Schwabach, Germany) at 30 ◦C up to thechievement of the selected concentrations. Aliquots of theoncentrated solutions (25 ml) were transferred to 50 ml Fal-on tubes which were submerged for about 5 min into anthylene glycol bath of a cryostat, model F25-MP (Julabo,eelbach, Germany) thermostatted at the selected temperature−10 ≤ Tc ≤ 15 ◦C). After this period, finely ground commer-ial xylitol (1.0 g l−1) was added to favor nucleation of crystals.fter completion of crystallization, evidenced by almost con-

tant values of xylitol concentration in the supernatant, therecipitated crystals were removed and dried at 30 ◦C. Littleortions of crystals were dissolved in water to determine theirontents of xylitol, xylose, amino acidic impurities and totalroteins.

.3. Analytical procedures

The concentrations of xylitol and xylose either in the fer-ented broths or in the solutions obtained by crystal dissolution

n water were determined at 50 ◦C by HPLC, model HP 1100

Hewlett Packard, Palo Alto, CA), equipped with an RI detec-or, model HP 1047A, and a 300 mm × 7.8 mm C610H columnSupelco, Bellefonte, PA). A 0.01N H2SO4 solution was useds a mobile phase at a flow rate of 0.5 ml min−1.

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F.C. Sampaio et al. / Chemical Engine

The amino acidic impurities were determined measuring theptical density (OD) at 280 nm using an UV–vis spectropho-ometer, series 634 (Varian, Palo Alto, CA), while the clarifica-ion effect was quantified by absorbance at 540 nm.

The total protein content of samples was determined accord-ng to Lowry [31] using a protein assay kit, 500-0112 (Bio-ad Laboratories, Hercules, CA); a standard curve was madeith bovine serum albumine at five different concentrations in

he range 0.022–0.710 g l−1 and OD measurements were deter-ined at 750 nm with the same spectrophotometer as above.All the tests were performed in triplicate and the results

xpressed as mean values. Standard deviations never exceeded%, therefore no statistical analysis was considered to be nec-ssary.

. Results and discussion

.1. Treatment with activated charcoal

Table 1 lists the results of tests at pH 6.0 performed usingoncentrations of activated charcoal in the range 5–200 g l−1

nd two different temperature, namely 25 and 50 ◦C. Such a dis-oloring treatment was suggested by preliminary control testserformed without any treatment that provided very dark solu-ions and dirty crystals after medium concentration and crystal-ization, respectively. The control solutions showed particularlyigh values of OD at 280 nm (4.45) because of the large presencef impurities, like amino acids, peptides, proteins, and nucleiccids.

The treatment at 25 ◦C led to a progressive fall in OD either at80 or 540 nm with increasing charcoal concentration (CC), the

atter parameter becoming negligible for CC ≥ 50 g l−1, whilehe former reached a minimum value of 0.080 at CC = 50 g l−1

nd then slightly increased. Such an observation was the likelyesult of some interference during the OD measurements, as

able 1esults of treatment of fermented broths with activated charcoal under differentonditions

reatment CC (g l−1)a OD540 OD280 TP (g l−1)b Xylitol (g l−1)

ontrolc – 0.062 4.45 1.80 60.4

emperature 25 ◦C1A 5 0.013 0.553 0.68 59.02A 10 0.012 0.370 0.60 60.93A 20 0.013 0.265 0.56 61.54A 50 0.000 0.080 0.40 54.45A 100 0.000 0.090 0.31 49.96A 200 0.000 0.123 0.24 37.9

emperature 50 ◦C1B 5 0.000 0.792 0.64 61.12B 10 0.000 0.312 0.49 61.63B 20 0.000 0.374 0.50 58.14B 50 0.000 0.202 0.41 51.25B 100 0.000 0.123 0.48 51.06B 200 0.000 0.135 0.34 41.4

a Activated charcoal concentration.b Total protein level.c Values detected before the treatment.

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and Processing 45 (2006) 1041–1046 1043

uggested by the progressive and rather regular decrease in totalrotein level (TP) from 1.80 to 0.24 g l−1. It should be noticedhat xylitol concentration in the medium kept almost constant59.0–61.5 g l−1) with increasing charcoal concentration up toC = 20 g l−1 and then progressively decreased, likely due toxcess adsorption power of carbon. No more than 63% of thenitial xylitol concentration present in the control kept in the

edium when using CC = 200 g l−1.The same experiments were then repeated at 50 ◦C in order

o point out a possible effect of temperature on the removalf contaminants. OD measurements at 540 nm were practicallyegligible even after the treatment with the lowest carbon con-entration, which means that the amount of activated carbonecessary to clarify the media was less at higher tempera-ure. These results are in good agreement with those reportedy Gurgel et al. [25], who worked with fermented sugarcaneagasse hydrolyzate and proposed that the adsorbent was chem-cally linked to the colored substances.

Measurements at 280 nm were higher, on an average basis,han those at the lower temperature, although following a similarehavior, which suggests that an increased temperature made thedsorption process less efficient, with particular concern to com-ounds characterized by high absorbance at such a wavelength,uch as nucleic acids and mainly aromatic amino acids. Thereatment with activated carbon likely removed these substancesy physical adsorption, and a temperature increase resulted inesorption owing to the molecular vibration intensification andhe tendency of physical interactions to break. Finally, total pro-ein content did not show any regular trend; the values collectedt CC up to 20 g l−1 were in fact slightly lower and those atC > 20 g l−1 slightly higher than those observed at 25 ◦C, hence

uggesting a scanty dependence of their adsorption on tempera-ure. Similarly to 25 ◦C, an appreciable loss of xylitol took placet high charcoal levels, as the likely result of xylitol adsorption byhis material. However, the concentration variations with respecto the values obtained at 25 ◦C were only 1.1–8.8%, thus high-ighting a little effect of temperature on this parameter as well.

On the basis of these results taken together we can con-lude that the decolorization of the fermented media can beerformed with satisfactory efficiency at room temperature andC = 20 g l−1. Although more effective removal of protein mate-

ial was obtained at higher levels, this activated carbon con-entration can be considered to be a reasonable compromiseetween the opposite economic requirements of obtaining thelearest and purest solution as possible with the lowest amountf adsorbent. Under these conditions we were in fact able toemove nearly 79% and 94% of the contaminants responsibleor OD at 540 and 280 nm, respectively, and 69% of total pro-eins, while xylitol recovery was almost complete.

.2. Isothermal crystallization of xylitol in the absence ofylose

The first set of crystallization tests was performed onompletely fermented media (A), i.e. in the absence of residualylose, and concentrating them to xylitol concentrationsPo

Xyt) ranging from 675 to 911 g l−1. Besides, a relatively

1044 F.C. Sampaio et al. / Chemical Engineering and Processing 45 (2006) 1041–1046

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Table 3Results collected at the end of xylitol crystallization tests performed at differenttemperatures from fermented broths containing 687 ± 27 g l−1 xylitol in theabsence of xylose

Tc (◦C)a −10 −5 0SXyt (g l−1)b 532 602 627CXyt (%)c 87.6 91.5 95.4CI (%)d 11.79 7.89 3.83CTP (%)e 0.61 0.61 0.77YC

f 0.23 0.12 0.09

a Crystallization temperature.b Residual xylitol concentration in the supernatant after crystallization.c Xylitol content of crystals.

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Table 4 lists the results of additional crystallization tests per-formed at different temperatures, ranging from −10 to 15 ◦C, onconcentrated broths (A) containing 168 ± 8.0 g l−1 xylose (Po )

ig. 1. Time behaviour of the actual (PXyt) to starting (PoXyt) xylitol concen-

rations ratio during crystallization tests at different xylitol concentrations: (�)11 g l−1; (�) 782 g l−1; (�) 675 g l−1.

ow temperature (−10 ◦C) was selected to ensure effectiverystallization.

The data on xylitol crystallization are shown in Fig. 1 inhe form of typical kinetic curves, from which it is evidenthat an increase in Po

Xyt accelerated remarkably xylitol sepa-

ation into the solid phase. In particular, at 911 g l−1 xylitolrystallization took place 14–15 times more quickly than at82 and 675 g l−1, being the process almost complete after onlyh, whereas the other two tests lasted 28–30 h, respectively.esides, the results listed in Table 2 show that an increasedylitol concentration favored the crystallization, being able toemove increasing amounts of this pentitol, while the xylitol con-ent of crystals was always very high (96.0% ≤ CXyt ≤ 97.8%)nd did not vary so much. But the most interesting finding was,esides the low contents of impurity (1.20% ≤ CI ≤ 3.23%) androteins (0.77% ≤ CTP ≤ 1.0%), the marked raise (from 0.27 to.46) of the crystallization yield (YC), expressed as the massatio of recovered xylitol to starting xylitol.

Further tests were then carried out at fixed xylitol concen-ration (Po

Xyt = 687 ± 27 g l−1) but using three different val-

es of temperature, namely −10, −5 and 0 ◦C, in order tonvestigate the effect of this parameter on xylitol crystalliza-ion. As expected, the residual xylitol concentration in the

edium increased (from 532 to 627 g l−1) with temperature

able 2esults of xylitol crystallization performed at −10 ◦C from fermented brothsontaining different xylitol concentrations in the absence of xylose

oXyt(g l−1)a 675 782 911

Xyt (g l−1)b 490 492 494

Xyt (%)c 97.7 96.0 97.8

I (%)d 1.44 3.23 1.20

TP (%)e 0.86 0.77 1.0

Cf 0.27 0.37 0.46

a Xylitol concentration in the concentrated broth before crystallization.b Residual xylitol concentration in the supernatant after crystallization.c Xylitol content of crystals.d Impurities content of crystals.e Protein content of crystals.f Crystallization yield.

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d Impurities content of crystals.e Protein content of crystals.f Crystallization yield.

Table 3) because of the increased solubility threshold. There-ore, although the xylitol content of crystals also increased from7.6% to 95.4%, and the total content of impurities complemen-arily decreased, a temperature raise led to unacceptable fall ofhe crystallization yield from 0.23 to 0.09. Besides, it shoulde taken into consideration that this set of tests was carried outsing fermented broths (B) having different composition withespect to those (A) utilized for tests at variable Po

Xyt; therefore,he results obtained with these media at −10 ◦C and similar con-entration levels (675 g l−1 for A and 687 g l−1 for B) were quiteifferent, with particular concern to xylitol, protein and impu-ities content of crystals (Tables 2 and 3). Nevertheless, theseesults seem to demonstrate that the lowest the temperature, theost effective the xylitol crystallization in term of crystalliza-

ion yield, while the crystals purity degree showed a completelypposite behavior.

.3. Isothermal crystallization of xylitol in the presence ofesidual xylose

Xyl

nd 690 ± 27.3 g l−1 xylitol, i.e. a ratio of about 1/4. An increase

able 4esults of xylitol crystallization performed at different temperatures

−10 ≤ Tc ≤ 15 ◦C) from a fermented broth containing 168 ± 8.0 g l−1 xylosend 690 ± 27.3 g l−1 xylitol

c (◦C)a −10 −5 0 5 10 15

Xyt (g l−1)b 400 421 427 450 531 554

Xyl (%)c 2.79 3.62 3.42 3.73 4.23 0.00

Xyt (%)d 85.3 90.0 92.7 93.1 93.7 99.2

I (%)e 10.97 4.98 2.08 1.97 0.87 0.18

TP (%)f 0.94 1.4 1.8 1.2 1.2 0.62

Cg 0.42 0.39 0.38 0.35 0.23 0.20

a Crystallization temperature.b Residual xylitol concentration in the supernatant after crystallization.c Xylose content of crystals.d Xylitol content of crystals.e Impurities content of crystals.f Protein content of crystals.g Crystallization yield.

ering and Processing 45 (2006) 1041–1046 1045

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mented solutions as a function of crystallization time (�t) under different con-ditions: (�) Tc = −10 ◦C, Po

Xyt = 782 g l−1; (�) Tc = −10 ◦C, PoXyt = 675 g l−1;

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F.C. Sampaio et al. / Chemical Engine

n crystallization temperature led to a dramatic raise in the timeeeded to complete crystal precipitation, which passed from 25 ht −10 ◦C to 196 h at 15 ◦C. Besides, as previously observed27], xylose concentration in the medium increased with respecto the starting value (20% on an average basis) owing to volumeeduction consequent to xylitol precipitation. Xylitol concentra-ion in the medium after crystallization (SXyt) showed a regularncrease with temperature from 400 up to 554 g l−1 because ofhe increased solubility threshold, thus resulting in a decrease ofhe crystallization yield from 0.42 to 0.20.

No less than 85% of the recovered crystals were made up ofhe desired polyol at all the tested temperatures, the rest con-isting of xylose (0.00% ≤ CXyl ≤ 4.23%), proteins (0.62% ≤TP ≤ 1.8%) and other impurities (0.18% ≤ CI ≤ 10.97%). Inarticular, crystal purity increased with temperature and becamelmost total at 15 ◦C (CXyt = 99.2%), whereas xylose con-ent reached a negligible value. These experimental data takenogether permit to conclude that a temperature increase led toimultaneous increases in the time necessary to complete crystal-ization and the crystal purity (CXyt), as well as marked decreasesn impurities content of crystals and crystallization yield.

Finally, comparison of the results of the test performed at10 ◦C with those obtained in the absence of xylose at the

ame temperature and similar xylitol concentration (PoXyt =

75 g l−1) (Table 2) demonstrates that the simultaneous pres-nce of xylose decreased the xylitol content of crystals (from7.7% to 85.3%), but ensured a 1.6-fold increase in the crystal-ization yield (from 0.27 to 0.42).

.4. Crystallization modeling

Xylitol crystallization from fermented solutions was satis-actorily described by an expression derived from the chemicalinetics equations for heterogeneous reactions [32]:

t = A − 1

K1log

(PXyt

P∗Xyt

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= 1

K1log

(Po

Xyt

P∗Xyt

)(2)

here �t is the crystallization time (min), K1 the rate constantf xylitol crystal growth (min−1), and P∗

Xyt the xylitol satura-ion concentration at a given temperature. The suitability of thesequations to describe xylitol crystallization from fermented solu-ions is confirmed by the linear dependence (0.90 ≤ r2 ≤ 0.96)f log(PXyt/P

∗Xyt) on the crystallization time depicted in Fig. 2.

As the crystallization temperature was increased, for exam-le from −10 to −5 ◦C, the xylitol crystallization rate (K1)f solutions containing 690 g l−1 xylitol and 168 g l−1 xyloseecreased from 3.2 to 1.4 × 10−4 min−1, i.e. by a factor of

pproximately 2.3. In addition, the crystallization rate slightlyncreased with starting xylitol concentration: for example, K1rew from 1.3 × 10−4 to 1.4 × 10−4 min−1 when Po

Xyt increased

rom 675 to 782 g l−1 at constant temperature (Tc = −10 ◦C).

spct

�) Tc = −10 ◦C, PoXyt = 690 g l−1, Po

Xyl = 168 g l−1; (�) Tc = −5 ◦C, PoXyt =

90 g l−1, PoXyl = 168 g l−1.

Moreover, the analysis of the xylitol crystallization rate per-itted us to put in evidence the positive effect of operating in

he presence of residual xylose. The crystallization behaviort −10 ◦C in the absence of xylose in the fermented solutionontaining 782 g l−1 xylitol was in fact kinetically coincidentith that observed at −5 ◦C in the fermented solution contain-

ng 690 g l−1 xylitol and 168 g l−1 residual xylose, being K1 foroth tests 1.4 × 10−4 min−1. Therefore, the presence of resid-al xylose allowed working at higher crystallization temperaturend lower Po

Xyt, thus making the process more profitable becausef reduced heating and cooling costs.

. Conclusions

The results obtained by different treatments with activatedharcoal showed that the use of 20 g l−1 charcoal at 25 ◦C forh is an efficient method to clarify fermented media. Such a

reatment was in fact considered to ensure the best compromiseetween the opposite economic requirements of obtaining thelearest and purest solution as possible with the lowest amountf adsorbent, being able to remove nearly 79% and 94% of theonitored contaminants and 69% of total proteins, while xyli-

ol recovery was almost complete. The crystallization techniquehowed good performance and demonstrated that: (1) an increasen xylitol concentration (Po

Xyt) accelerated remarkably xylitoleparation into the solid phase (by a factors 14–15), (2) the low-st the temperature, the most effective the xylitol crystallizationn term of crystallization yield, while the crystals purity degreehowed a completely opposite behavior, and (3) the simultane-us presence of residual xylose reduced the xylitol content ofrystals (from 97.7% to 85.3%), but ensured a 1.6-fold increase

tudy of xylitol crystallization revealed the positive effect of theresence of residual xylose that enhanced the rate constant ofrystal growth, thus permitting to operate at higher crystalliza-ion temperature and lower Po

Xyt.

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[31] O.H. Lowry, N.J. Rosebrough, A.L. Farr, R.J. Randall, Protein measure-ment with the folin phenol reagent, J. Biol. Chem. 193 (1951) 265–275.

046 F.C. Sampaio et al. / Chemical Engine

ppendix A. Nomenclature

C activated charcoal concentration (g l−1)I impurities content of crystals (%)TP protein content of crystals (%)Xyl xylose content of crystals (%)Xyt xylitol content of crystals (%)1 rate constant of xylitol crystal growth (min−1)Xyt actual xylitol concentration (g l−1)∗Xyt xylitol saturation concentration at a given temperature

(g l−1)oXyt xylitol concentration in the concentrated broth before

crystallization (g l−1)oXyl xylose concentration in the concentrated broth before

crystallization (g l−1)Xyt residual xylitol concentration in the supernatant after

crystallization (g l−1)t crystallization time (min)P total protein level (g l−1)c crystallization temperature (◦C)C crystallization yield

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[7] M.F.S. Barbosa, M.B. Medeiros, I.M. Mancilha, H. Schneider, H. Lee,Screening of yeasts for production of xylitol from d-xylose and some factorswhich affect xylitol yield in Candida guilliermondii, J. Ind. Microbiol. 3(1988) 241–251.

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