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酸水解玉米秸秆发酵生产木糖醇马伟超;李一婧;念亚男【摘要】利用不同浓度稀硫酸处理玉米秸秆,然后吸附脱色,用热带假丝酵母进行液体发酵培养,72 h后,4％硫酸脱色处理组发酵液中木糖醇含量最高,达10.507 6mg/100mL;比较各组发酵液中木糖醇转化率,脱色处理组比未脱色处理组高,且以4％脱色处理组最高,木糖醇转化率可达46.59％；比较木糖醇生成速率,仍以4％硫酸脱色处理组最高,达到1.37 mg/L·h.%The com stalk was hydrolyzed by different concentrations of sulfuric acid, and partial decolorized, then the hydrolyzate was fermented by Candida tropicalis in liquid culture. The results showed that the highest concentration of xylitol reached 10.507 6 mg/l00mL after 72 h fermentation in hydrolyzate treated by 4% sulfuric acid treatment and partial decolorized. By comparison of the conversion rate of xylose in each fermentation broth, a higher rate was determined in decolorized hydrolyzate in comparison with that of undecolorized hydrolyzate, especially in hydrolyzate treated by 4% sulfuric acid, which reached 46.59%. The generation rate of xylitol was also the highest in fermentation of decolorized hydrolyzate treated by 4% sulfuric acid, which reached 1.37 mg/L·h.【期刊名称】《湖南农业科学》【年(卷),期】2011(000)021【总页数】3页(P76-78)【关键词】木糖醇;玉米秸秆;酸处理【作者】马伟超;李一婧;念亚男【作者单位】天水师范学院生命科学与化学学院,甘肃天水741001;天水师范学院生命科学与化学学院,甘肃天水741001;天水师范学院生命科学与化学学院,甘肃天水741001【正文语种】中文【中图分类】TQ223木糖醇（xylitol）的化学名称为l，2，3，4，5-戊五醇，是木糖代谢的中间产物，甜度与蔗糖相当，可替代蔗糖作为甜味剂，且具有良好的抗龋齿性能，故在食品、医药工业具有广泛的应用价值[1]。
弱碱性亚硫酸盐预处理甘蔗渣过程中脱木素规律付时雨;李坤;罗小林;陈成聪;梁日喜;詹怀宇【摘要】木素障碍是影响木质纤维素水解转化的主要原因,弱碱性亚硫酸盐条件既能降解木素,又能减少碳水化合物脱水降解.本文研究了甘蔗渣在弱碱性亚硫酸盐条件下的脱木素行为,建立以H因子和亚硫酸钠浓度为变量的蔗渣脱木素的经验模型, 并优化出蔗渣的亚硫酸钠预处理最佳工艺条件为:亚硫酸钠浓度为20%(质量),温度为160℃,保温时间为30 min.【期刊名称】《化工学报》【年(卷),期】2010(061)006【总页数】5页(P1523-1527)【关键词】蔗渣;亚硫酸盐;预处理;脱木素【作者】付时雨;李坤;罗小林;陈成聪;梁日喜;詹怀宇【作者单位】华南理工大学制浆造纸工程国家重点实验室,广东,广州,510640;华南理工大学制浆造纸工程国家重点实验室,广东,广州,510640;华南理工大学制浆造纸工程国家重点实验室,广东,广州,510640;华南理工大学制浆造纸工程国家重点实验室,广东,广州,510640;华南理工大学制浆造纸工程国家重点实验室,广东,广州,510640;华南理工大学制浆造纸工程国家重点实验室,广东,广州,510640【正文语种】中文【中图分类】TQ352.62Abstract：Lignin in biomass defenses the hydrolysis of polysaccharides by microbes or enzymes.Destruction and removal of lignin can greatly facilitate conversion of biomass.Pretreatment with sulfite in weak alkaline condition is an effective and environmentally friendly way to degrade lignin in biomass without toxic substance produced.The optimum condition for sulfite treatment is as following：sulfite20%(mass),temperature 160℃and reaction time 30 min.Key words：bagasse;sulfite;pretreatment;delignification能源短缺问题是目前人类面临的重大问题,据联合国统计,世界石油储量只能维持到2035年,到2060年天然气也将消耗殆尽[1]。
Bioconversion of D-xylose to xylitol by Debaryomyces hanseniiUFV-170:Product formation versus growthFa´bio C.Sampaio a,Hila´rio C.Mantovani a,Frederico Jose´Vieira Passos b,Ce´lia Alencar de Moraes a,Attilio Converti c,*,Fla´via M.Lopes Passos aa Department of Microbiology,Instituto de Biotecnologia Aplicada a`Agropecua´ria,Federal University of Vic¸osa,Av.P.H.Rolfs s/n,36570-000Vic¸osa,Minas Gerais,Brazilb Department of Food Technolgy,Federal University of Vic¸osa,Av.P.H.Rolfs s/n,36570-000Vic¸osa,Minas Gerais,Brazilc Department of Chemical and Process Engineering,Genoa University,Via Opera Pia15,16145Genoa,ItalyReceived3October2004;received in revised form17February2005;accepted28March2005AbstractThe relationship between D-xylose-to-xylitol conversion and cell growth was investigated in the yeast Debaryomyces hansenii UFV-170.A first phase of rapid growth(average specific growth rate,m,of0.24hÀ1)dissociated from xylitol accumulation was followed by a second phase during which growth was associated with low xylitol formation(D P=1.27g lÀ1)and a third phase characterized by the largest D-xylose consumption(Xyl c=21.4g lÀ1)and xylitol production(D P=14.4g lÀ1)but exhibiting very slow growth.Xylitol accumulation in the medium was then associated with a decelerated growth rather than being completely dissociated from it.During the bioconversions without a nitrogen source performed at very high inoculum(X o=24.0g lÀ1),non-growth conditions were able to address most of consumed D-xylose (86%)to xylitol formation,thus ensuring the best xylitol production performance(Q P=4.09g lÀ1hÀ1,q P=0.17g gÀ1hÀ1and Y P/S=0.78g gÀ1).Xylitol formation was simultaneously influenced by the physiological state of the culture and the concentration of biomass.After four operations of cell recycling in the absence of the nitrogen source,long-term viability of the system but unstable xylitol formation was observed.#2005Elsevier Ltd.All rights reserved.Keywords:Debaryomyces hansenii;D-Xylose consumption;Xylitol formation;Cell growth;Carbon material balances1.IntroductionXylitol is afive-carbon sugar polyalcohol that has been attracting the interest of food,odontological and pharma-ceutical industries.It possesses a sweetening power comparable to that of sucrose and a solution heat of À145J gÀ1.Because of the absence of carbonyl groups,it does not participate in Maillard-type reactions[1–3].Xylitol can be utilized for the prevention of caries,being able to inhibit the growth of bacteria responsible for tooth enamel [3–5],and of some pathologies,such as medium–acute otitis [6,7]and experimental osteoporosis[8].Besides,since its metabolism is not dependent on insulin,it can also be used by diabetics[5].At present,xylitol is industrially obtained by a chemical process from hydrolyzates of lignocellulosic wastes. However,due to a requirement for several purification steps,such a process is very expensive[3].Therefore,this conversion could be alternatively performed by bacteria[9],filamentous fungi[10,11],yeasts[12–15]or purified enzymes from these microorganisms[16].Nevertheless, to make this process exploitable at an industrial level,the bioconversion must be rapid,offer high yield,employ alternative and cheap culture media and allow for results comparable to those of the present technology.Hydrolyzates from different lignocellulosic residues, among which sugarcane bagasse[17–19]and rice straw [20,21],have been satisfactorily used as alternative media. These hydrolyzates have been subject to different treatments [18–23]and cultivation conditions[24–27],aimed at increasing process yield and productivity./locate/procbioProcess Biochemistry40(2005)3600–3606 *Corresponding author.Tel.:+390103532593;fax:+390103532586.E-mail address:converti@unige.it(A.Converti).1359-5113/$–see front matter#2005Elsevier Ltd.All rights reserved.doi:10.1016/j.procbio.2005.03.039The 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[28,29],fed-batch[15,30]and continuous processes[31,32].The continuous process is advantageous at the industrial level when immobilized cells of a microorganism are utilized to allow their catalytic activity to be continuously exploited,with consequent increase in volumetric productivity.However,a number of preliminary studies are needed to optimize an immobilized-cell system for xylitol production.Thefirst point to be dealt with is the relationship existing between xylitol and biomass forma-tions,i.e.it is necessary to establish whether or not xylitol formation is associated with yeast growth.In an effective continuous immobilized-cell system,xylitol formation is in fact expected not to depend on growth,not only to minimize the carbon source loss,but also to make the immobilized-cell mass able to work as a resting cell‘‘biocatalyst’’.On the basis of these considerations,the relationship between D-xylose-to-xylitol conversion and cell growth has been investigated in the yeast Debaryomyces hansenii UFV-170with the aim of dissociating these metabolic activities, as afirst effort for future development of an effective immobilized-cell system for continuous xylitol bioproduc-tion.For this purpose,simple carbon material balances were applied to the experimental data of batch runs by this yeast, based on the general knowledge available in the literature on D-xylose metabolism in pentose-fermenting yeasts.2.Materials and methods2.1.Microorganism and maintenanceD.hansenii UFV-170is a new strain selected as a promising xylitol producer[33]and was utilized in this work.Stock cultures were maintained atÀ808C on YPD (10g lÀ1yeast extract,20g lÀ1peptone and20g lÀ1D-glucose)containing40%glycerol.Before each experiment, cells were transferred and grown for48h at308C on Petri plates containing YPD medium supplemented with15g lÀ1 agar.2.2.Cultivation media and inoculum preparationBioconversions were carried in a medium containing 3.4g lÀ1KH2PO4,6.8g lÀ1K2HPO4,1.0g lÀ1(NH4)2SO4, 1.1g lÀ1MgSO4and5.0g lÀ1yeast extract(pH6.0)and about20or50g lÀ1D-xylose(according to circumstances).A medium containing only water and50g lÀ1D-xylose was also utilized for a reference test.Solutions of D-xylose,yeast extract,MgSO4and the rest of salts were sterilized separately by autoclaving at1218C for20min.For pre-cultivations, cells from plates were transferred to125-ml Erlenmeyerflasks containing50ml of the above medium.Theflasks were maintained at308C under agitation in an orbital shaker at 200rpm.After20–24h of incubation,cells were collected by centrifugation at48C(4000Âg for5min),then washed twice with sterile water and used for the inoculum.2.3.Shakenflask experimentsCells were inoculated into125-ml Erlenmeyerflasks containing50ml of the same medium used for pre-cultivations and incubated for24h at308C and200rpm. The starting inoculum level was0.05–0.06g lÀ1cell dry weight for these fermentations and3.0,13and24g lÀ1for the reference tests in water plus50g lÀ1D-xylose.Final tests of biomass recycling were carried out for15h under the same conditions but using very high biomass level(12g lÀ1) in water plus50g lÀ1D-xylose.Biomass from a preceding fermentation was recovered by centrifugation at48C (4000Âg for5min)and reused as inoculum for the subsequent one.2.4.Analytical methodsAliquots of the culture were centrifuged at8000Âg (5min)and the supernatants used for determination of D-xylose and xylitol with a model1050HPLC System (Hewlett-Packard,Palo Alto,CA),equipped with a refractive index detector(HP1047A)and a Bio-Rad Aminex HPX-87H(300mmÂ7.8mm)column.A0.01N H2SO4solution was used as mobile phase at aflow rate of 0.7ml minÀ1.The optical density(OD600)was related to cell concentration,X(g dry weight lÀ1),through a calibra-tion curve(OD600=1.6717X).Cells were diluted with sterile water(serial dilution1:50)to determine the colony forming units per unit volume(CFU mlÀ1).Aliquots of 50m l of the last two dilutions(8Â10À6and1.6Â10À7) were spread in duplicate onto plates containing YPD-agar and incubated at308C for24–48h.2.5.Carbon material balancesThe microaerobic behaviour of D.hansenii UFV-170was studied through simple carbon material balances,using the batch experimental data of xylitol,D-xylose and biomass concentrations.The metabolic scheme for this isolate is based on the observation that no ethanol was formed during the bioconversion.This is consistent with the presence of only NADPH-dependent xylose reductase activity[12],as well as with the general knowledge about the metabolism of D.hansenii[25–27].It consists of the following equations with stoichiometric coefficients expressed as C-mol: (a)Biomass growth:1:095CH2Oþ0:200NH3!CH1:79O0:50N0:20þ0:095H2þ0:095CO2þ0:405H2O(1)F.C.Sampaio et al./Process Biochemistry40(2005)3600–36063601(b)Xylitol formation from D -xylose:CH 2O þ0:100H 2O !0:900CH 2:4O þ0:100CO 2þ0:020‘‘H 2’’(2)(c)Catabolic reaction:CH 2O þH 2O !CO 2þ2‘‘H 2’’(3)where ‘‘H 2’’stands for reducing equivalents in the form of NADH or any other equivalent form of metabolic reducer.3.Results and discussion3.1.Bioconversions in complete mediumFig.1shows the xylitol production profiles compared to the growth curves of D.hansenii UFV-170in cultivation media containing about 20or 50g l À1D -xylose.Three different phases can be observed in both cases (phases 1–3).In the first phase the growth appeared to be dissociated from xylitol accumulation and the cells grew rapidly without releasing the desired product (Table 1).At S o ffi50g l À1,only 8%of D -xylose consumed during this phase (Xyl c =3.78g l À1)was utilized for biomass production (m =0.24h À1)while the rest (92%)being addressed to energy production through the catabolic reaction (respira-tion)(Table 2).During the second phase,rapid growth associated with low xylitol accumulation took place(D P =1.27g l À1)and led to volumetric productivity (Q P ),specific productivity (q P )and yield of xylitol on consumedD -xylose (Y P/S )of 0.20g l À1h À1,0.16g g À1h À1and 0.34g g À1,respectively (Table 1).An appreciable fraction (37%)of consumed D -xylose (Xyl c =3.77g l À1)was utilized to produce xylitol,while the largest fraction was employed for growth and only 1%by the catabolic reaction (Table 2).The latest phase was characterized by a physiological state of the cell responsible for the largestD -xylose consumption (Xyl c =21.4g l À1)and xylitol pro-duction (D P =14.4g l À1),whereas biomass showed the typical slow growth of the late exponential or decelerated growth phase (Table 1).In particular,biomass grew linearly with time in this phase according to the equation X =m t +a ,by which a very low average specific growth rate was estimated (m =0.07h À1).As a consequence,more than 74%of consumed D -xylose was addressed to xylitol production,25%to growth and only 0.5%to the catabolic reaction (Table 2),leading to the highest values of the kinetic parameters of xylitol formation (Q P =1.22g l À1h À1,q P =0.13g g À1h À1and Y P/S =0.58g g À1)(Table 1).The same phases were observed for the bioconversion carried out at about 20g l À1starting D -xylose,which showed a similar D -xylose consumption profile,the only appreciable differ-ence being the existence of a stationary growth phase after complete D -xylose depletion.The results of Table 1also suggest that the specific growth rate of both bioconversions quickly reached maximum values,whereas the specific xylitol productivity (q P )was negligible;the former parameter then progressively decreased while the latter increased,thus evidencing that xylitol accumulation in the medium was associated to a decelerated growth rather than completely dissociated from it.The same approach was utilized by Ramsay et al.in determining the growth-associated feature of propionic acid production by Propionibacterium acidipropionici [34].Unfortunately,it is not possible to compare the results of this work with those reported for D -xylose-to-xylitolF .C.Sampaio et al./Process Biochemistry 40(2005)3600–36063602Table 1Experimental data and kinetic parameters referred to Debaryomyces hansenii UFV-170cultivation performed at 200rpm,S o =49.7Æ0.5g l À1and X o =0.05–0.06g l À1Phase Xyl c (g l À1)D P (g l À1)m (h À1)Q P (g l À1h À1)q P (g g À1h À1)Y P/S (g g À1)1 3.7800.240002 3.77 1.270.230.200.160.34321.414.40.071.220.130.58Table 2Fractions of D -xylose consumed by different metabolic activities during the cultivation of Debaryomyces hansenii UFV-170performed at 200rpm,S o =49.7Æ0.5g l À1and X o =0.05–0.06g l À1Phase Xylitol Growth Catabolic reaction (%)(g l À1)(%)(g l À1)(%)(g l À1)10080.3192 3.47237 1.3962 2.3310.05374.515.8255.430.50.11bioconversion under similar conditions,because most of the research work in this field was carried-out at relatively high inoculum levels,aimed at maximizing volumetric produc-tivity rather than xylitol yield [35].3.2.Bioconversions without the nitrogen sourceConsidering the above results,we can deduce that xylitol formation could be linked to the physiological state of the cell in the decelerated-growth phase,to the achievement of a minimum amount of biomass or to both.To shed light on this question,cells were then cultivated under conditions of negligible growth (absence of the nitrogen source)on 47.3Æ0.3g l À1D -xylose dissolved in water and using three different levels of biomass pre-cultivated for 24h,namely 3.0(T2),13(T3)and 24g l À1(T4).The results of these bioconversions are compared in Fig.2to those obtained on the complete medium at X o =3.0g l À1(T1).As expected,under conditions of no nitrogen source limitation (T1),biomass grew (12.7g l À1)consuming 99%of starting D -xylose after 24h of cultivation and xylitol accumulated in the medium up to 29.3g l À1(Table 3).The metabolic material balance presented in Table 4highlights that 64%of consumed D -xylose was utilized by the system for xylitol production,34%for the growth and only 2%by the catabolic reaction,providing Q P =1.22g l À1h À1,q P =0.13g g À1h À1and Y P/S =0.58g g À1(Table 3).The bioconversion T2exhibited after 12h a residual biomass growth (D X =1.99g l À1),due likely to cell pre-cultivation in complete medium and/or to nitrogen release by dead biomass,a very low xylitol production (3.53g l À1)and a D -xylose consumption of only 9.47g l À1.More thanF .C.Sampaio et al./Process Biochemistry 40(2005)3600–36063603Fig.2.Time course of Debaryomyces hansenii UFV-170cultivations performed at 200rpm and S o =47.3Æ0.3g l À1.T1=complete medium,X o =3.0g l À1;T2=H 2O +D -xylose,X o =3.0g l À1;T3=H 2O +D -xylose,X o =13g l À1;T4=H 2O +D -xylose,X o =24g l À1;(*)ln X (g l À1);(~)xylitol concentration (g l À1);(&)D -xylose concentration (g l À1).Table 3Experimental data and kinetic parameters referred to Debaryomyces hansenii UFV-170cultivation performed at 200rpm and S o =47.3Æ0.3g l À1Treatment Time (h)Xyl c (%)P max (g l À1)D X (g l À1)Q P (g l À1h À1)q P (g g À1h À1)Y P/S (g g À1)T1a 249929.312.7 1.220.130.58T2b 1222 3.53 1.990.290.070.37T3c 148123.00.49 1.650.130.62T4d88632.74.090.170.78a Medium A,X o =3.0g l À1.b H 2O +D -xylose,X o =3.0g l À1.c H 2O +D -xylose,X o =13g l À1.dH 2O +D -xylose,X o =24g l À1.40%of consumed D -xylose was utilized for xylitol production,28%for growth and 31%by the catabolic reaction,thus leading to the worst xylitol production kinetics (Q P =0.29g l À1h À1,q P =0.07g g À1h À1and Y P/S =0.37g g À1).Although the above residual growth was also observed after 14h when the bioconversion was performed at the intermediate biomass level (T3),it was much less significant (D X =0.49g l À1),almost 68%of consumed D -xylose being utilized for xylitol production (P max =23.0g l À1),30%by the catabolic reaction and only 2%for growth,and led to Q P =1.65g l À1h À1,q P =0.13g g À1h À1and Y P/S =0.62g g À1.The residual biomass growth was negligible at the highest starting biomass level (T4),thereby resulting in the highest values of xylitol formation (32.7g l À1)and percentage of D -xyloseconsumption (86%).These non-growth conditions were in fact able to address most of consumed D -xylose (86%)to xylitol formation as well as to provide the best xylitol formation kinetics (Q P =4.09g l À1h À1,q P =0.17g g À1h À1and Y P/S =0.78g g À1)(Table 3),whereas only 14%was utilized for energy production through the catabolic reaction (Table 4).The same D.hansenii strain cultivated in complete medium with S o ffi100g l À1exhibited,under semi-aerobic conditions,P max =76.6g l À1,corresponding to a comparable xylitol yield (Y P/S =0.73g g À1),while the volumetric productivity was about one order of magnitude lower (Q P =0.37g l À1h À1)due to much lower inoculum level (X o ffi1.4g l À1)[36].These results on the whole suggest that a very high biomass level (24g l À1)could have caused the dissolved oxygen level in the medium to decrease to the semi-aerobic conditions favouring xylitol formation.On the contrary,at X o =3.0g l À1,the absence of nitrogen in the medium prevented biomass growth and,consequently,the dissolved oxygen likely kept at aerobic levels.On the basis of these considerations,it is likely that xylitol formation could have been simultaneously influenced by the physiological state of the culture and the biomass concentration,to which the oxygenation level is notoriously linked.3.3.Bioconversions with cell recyclingCell recycle has been reported to increase productivity and yields of fermentation.For example,long-term cellF .C.Sampaio et al./Process Biochemistry 40(2005)3600–36063604Table 4Fractions of D -xylose consumed by different metabolic activities during cultivations of Debaryomyces hansenii UFV-170performed at 200rpm and S o =47.3Æ0.3g l À1Treatment Xylitol Growth Catabolic reaction (%)(g l À1)(%)(g l À1)(%)(g l À1)T1a 6432.23416.92 1.01T2b 41 3.8728 2.6631 2.94T3c 6825.320.663011.3T4d8635.8145.93a Medium A,X o =3.0g l À1.b H 2O +D -xylose,X o =3.0g l À1.c H 2O +D -xylose,X o =13g l À1.dH 2O +D -xyose,X o =24g l À1.Fig.3.Time course of Debaryomyces hansenii UFV-170cultivations performed at 200rpm and S o =52.1Æ0.3g l À1.R1–R4=successive recycling of cells after 15h from the preceding bioconversion;(*)ln (CFU ml À1);(~)xylitol concentration (g l À1);(&)D -xylose concentration (g l À1).recycle(by14successive operations)of Candida tropicalis in a chemically defined medium using urea as a nitrogen source increased xylitol productivity by140%(Q P= 5.4g lÀ1hÀ1)and yield by7%(Y P/S=0.81g gÀ1)with respect to batch fermentation[37].Therefore,on the basis of the promising results obtained in the present study without nitrogen source,the long-term viability of the proposed biosystem has been checked through additional experiments carried out,under the same stress nutritional conditions,by using biomass(X o=12–23g lÀ1)successively recycled from the preceding run.The results of these tests(Fig.3)demonstrated that cell viability remained constant(average concentration of 5.5Â108CFU mlÀ1)during all the experiments after 15h of cultivation,while the percentage of consumed D-xylose and the concentration of produced xylitol progres-sively decreased,passing from thefirst bioconversion(R1) to the last(R4)recycling,from78to18%and from30.3to 3.62g lÀ1,respectively(Table5);as a result of the progressively longer time needed to get total D-xylose consumption and maximum xylitol production,a similar behaviour was observed for the kinetic parameters (Q P=2.02–0.24g lÀ1hÀ1,q P=0.11–0.01g gÀ1hÀ1and Y P/S=0.75–0.40g gÀ1),which suggests that complete absence of nitrogen does not allow maintaining conditions of stable xylitol formation.Mendes-Ferreira et al.[38], working with Saccharomyces cerevisiae in a defined medium with D-glucose as the only carbon and energy source,observed that initial nitrogen concentrations in the range16.5–805mg lÀ1had no effect on the specific growth rate.However,fermentation rate and time required to complete fermentation were strongly dependent on this nutritional requirement.Therefore,considering the satisfactory long-term viabi-lity of the system observed in this work,nitrogen-limited conditions in continuous operation can be suggested,instead of the complete absence of the nitrogen source,to obtain information on D-xylose metabolism in D.hansenii UFV-170and,above all,to set up a stable process for xylitol production.Some nitrogen availability is in fact expected to be fundamental for protein turnover in the proposed resting cell system,taking in account its significance not only for D-xylose transport and metabolism but also for cell home-ostasis.AcknowledgementThe authors wish to thank the Brazilian agency,CNPq, for thefinancial support.References[1]Russo JR.Xylitol:anti-carie sweetener?Food Eng1977;79:37–40.[2]Emodi A.Xylitol:its properties and food applications.Food Technol1978;32:20–32.[3]Hyvo¨nen L,Koivistoinen P,V oirol F.Food technological evaluation ofxylitol.In:Chichester CO,Mrak EM,Stewart G,editors.Advances in food research,vol.28.New York:Academic Press;1982.p.373–403.[4]Ma¨kinen KK.Xylitol and oral health.In:Chichester CO,Mrak EM,Stewart G,editors.Advances in food research,vol.25.New York: Academic Press;1979.p.373–403.[5]Yilikari R.Metabolic and nutritional aspects of xylitol.In:ChichesterCO,Mrak EM,Stewart G,editors.Advances in food research,vol.25.New York:Academic Press;1979.p.159–80.[6]Uhari M,Kontiokari T,Koskela M,Niemela M.Xylitol chewing gumin prevention of acute otitis media:double blind randomised trial.Int J Pediatr Otorhinolaryngol1996;40:217.[7]Uhari M,Tapiainen T,Kontiokari T.Xylitol in preventing acute otitismedia.Vaccine2000;19:S144–7.[8]Mattila PT,Svanberg MJ,Jamsa T,Knuuttila MLE.Improved bonebiomechanical properties in xylitol-fed aged rats.Metabolism 2002;51:92–6.[9]Rangaswamy S,Aglevor FA.Screening of facultative anaerobicbacteria utilizing D-xylose for xylitol production.Appl Microbiol Biotechnol2002;60:88–93.[10]Chiang C,Knight SG.Metabolism of D-xylose by moulds.Nature1960;188:79–81.[11]Dahija JS.Xylitol production by Petromyces albertensis grown onmedium containing D-xylose.Can J Microbiol1991;37:14–8. [12]Barbosa MFS,Medeiros MB,Mancilha IM,Schneider H,Lee H.Screening of yeasts for production of xylitol from D-xylose and some factors which affect xylitol yield in Candida guilliermondii.J Ind Microbiol1988;3:241–51.[13]Rodrigues DCG,Silva SS,Felipe ing response-surfacemethodology to evaluate xylitol production by Candida guilliermondii by fed-batch process with exponential feeding rate.J Biotechnol 1998;62:73–7.[14]Rodrigues DCG,Silva SS,Prata AMR,Felipe MGA.Biotechnologicalproduction of xylitol from agroindustrial residues.Appl Biochem Biotechnol1998;70–72:869–75.[15]Ikeuchi T,Azuma M,Kato J,Ooshima H.Screening of microorgan-isms for xylitol production and fermentation behavior in high con-centrations of xylose.Biomass Bioenerg1999;16:333–9.[16]Rosa SMA,Felipe MGA,Silva SS,Vitolo M.Xylose reductaseproduction by Candida guilliermondii.Appl Biochem Biotechnol 1998;70–72:127–34.[17]Chen L,Gong C.Fermentation of bagasse hemicellulose hydrolysateto xylitol by a hydrolysate-acclimatized yeast.J Food Sci 1985;50:226–8.[18]Domı´nguez JM,Gong GS,Tsao GT.Pretreatment of sugar canebagasse hemicellulose hydrolysate for xylitol production by yeasts.Appl Biochem Biotechnol1996;57–58:49–56.[19]Gurgel PV,Furlan SA,Martinez SER,Mancilha IM.Evaluation ofsugarcane bagasse acid hydrolyzate treatments for xylitol production.Braz J Chem Eng1998;15:309–12.[20]Roberto IC,Mancilha IM,Felipe MGA,Souza CA,Sato S,CastroHF.Evaluation of rice straw hemicellulose hydrolysate in the production of xylitol by Candida guilliermondii.Biotechnol Lett 1994;16:1211–6.F.C.Sampaio et al./Process Biochemistry40(2005)3600–36063605 Table5Experimental data and kinetic parameters of Debaryomyces hansenii UFV-170cultivations performed at200rpm and S o=52.1Æ0.3g lÀ1usingbiomass recycled from the preceding bioconversion(X o=12–23g lÀ1)Recycling Xyl c(%)P max(g lÀ1)Q P(g lÀ1hÀ1)q P(g gÀ1hÀ1)Y P/S(g gÀ1)R17830.3 2.020.110.75 R24411.10.850.040.56 R329 6.610.480.020.47 R418 3.620.240.010.40[21]Roberto IC,Sato S,Mancilha IM.Effect of inoculum level on xylitolproduction from rice straw hemicellulose hydrolysate by Candida guilliermondii.J Ind Microbiol1996;16:348–50.[22]Frazer FR,McCaskey TA.Wood hydrolyzate treatments for improvedfermentation of wood sugars to2,3-butanediol.Biomass1989;18: 31–42.[23]Alves LA,Felipe MGA,Silva JBAE,Prata AMR.Pretreatment ofsugarcane bagasse hemicellulose hydrolysate for xylitol production by Candida guilliermondii.Appl Biochem Biotechnol1998;70–72: 89–97.[24]Silva SS,Felipe MGA,Mancilha IM.Factors that affect the biosynth-esis of xylitol by xylose-fermenting yeasts.A review.Appl Biochem Biotechnol1998;70–72:331–9.[25]Converti A,Domı´nguez JM.Influence of temperature and pH onxylitol production from xylose by Debaryomyces hansenii.Biotechnol Bioeng2001;75:39–45.[26]Converti A,Perego P,Sordi A,Torre P.Effect of starting xyloseconcentration on the microaerobic metabolism of Debaryomyces hansenii.The use of carbon material balances.Appl Biochem Bio-technol2002;101:15–29.[27]Rivas B,Torre P,Domı´nguez JM,Perego P,Converti A,Parajo´JC.Carbon material and bioenergetic balances of xylitol production from corncobs by Debaryomyces hansenii.Biotechnol Prog2003;19: 706–13.[28]Domı´nguez JM,Gong CS,Tsao GT.Production of xylitol from D-xylose by Debaryomyces hansenii.Appl Biochem Biotechnol 1997;63–65:117–27.[29]Gı´rio FM,Amaro C,Azinheira H,Pelica F,Amaral-Callac¸o MT.Polyols production during single and mixed substrate fermentations in Debaryomyces hansenii.Bioresour Technol2000;71:245–51.[30]Furlan SA,Dupuy MLD,Strehaiano P.Xylitol production inrepeated fed batch cultivation.World J Microbiol Biotechnol 1997;13:591–2.[31]Cruz JM,Domı´nguez JM,Domı´nguez H,Parajo´JC.Xylitol produc-tion from barley bran hydrolysates by continuous fermentation with Debaryomyces hansenii.Biotechnol Lett2000;22:1895–8.[32]Tavares JM,Duarte LC,Amaral-Collac¸o MT,Gı´rio FM.The influenceof hexoses addition on the fermentation of D-xylose in Debaryomyces hansenii under continuous cultivation.Enzyme Microb Technol 2000;26:743–7.[33]Sampaio FC,Chaves-Alves VM,Converti A,Passos FML,CoelhoJLC.Influence of cultivation conditions on the D-xylose-to-xylitol bioconversion by the new isolate Debaryomyces hansenii UFV-170.Bioresour Technol,submitted for publication.[34]Ramsay JA,Aly Hassan M-C,Ramsay BA.Biological conversion ofhemicellulose to propionic acid.Enzyme Microb Technol1998;22: 292–5.[35]Parajo´JC,Domı´nguez H,Domı´nguez JM.Biotechnological produc-tion of xylitol.Part2.Operation in culture media made with com-mercial sugars.Bioresour Technol1998;65:203–12.[36]Sampaio FC,Torre P,Passos FML,Perego P,Passos FJV,Converti A.Xylose metabolism in Debaryomyces hansenii UFV-170.Effect of the specific oxygen uptake rate.Biotechnol Prog2004;20:1641–50. [37]Kim T-B,Lee Y-J,Kim P,Oh D-K.Increased xylitol production rateduring long-term cell recycle fermentation of Candida tropicalis.Biotechnol Lett2004;26:623–7.[38]Mendes-Ferreira A,Mendes-Faia A,Lea˜o C.Growth and fermentationpatterns of Saccharomyces cerevisiae under different ammonium concentrations and its implications in winemaking industry.J Appl Microbiol2004;97:540–5.F.C.Sampaio et al./Process Biochemistry40(2005)3600–3606 3606。
carbohydratesThe food scientist has a many-sided interest in carbohydrates. He is concerned with their amounts in various foods, availability (nutritional and economic), methods of extraction and analysis, commercial forms and purity, nutritional valve, physiological effects, and functional properties in foods. Understanding their functional properties in processed foods requires not only knowledge of the physical and chemical properties of isolated carbohydrates, but also knowledge of the reactions and interactions that occur in situs between carbohydrates and other food constituents and the effects of these changes upon food quality and acceptance. This is a tall order for knowledge. Because processing affects both nutritional and esthetic values of food, knowledge of the changes that carbohydrates undergo during milling, cooking, dehydration, freezing, and storage is especially important.Students are advised to study the fundamental chemistry underlying useful carbohydrates properties Of service will be an understanding of the association of polar molecules through hydrogen bonding, ionic effects, substituent effects, chelation with inorganic ions, complexing with lipids and proteins, and decomposition reaction. This background will provide an understanding of properties that affect the texture and acceptance of processed foods (e.g., solubility, hygroscopicity, diffusion, osmosis, viscosity, plastity, and flavor production), properties that enable the formation or high quality pastries, gels, coatings, confections, and reconstitutable dehydrated and frozen foods.Ability to predict what changes in functional properties are likely to ensue from incorporating various types of carbohydrates into processed foods is a practical goal of the food scientist.Such forecasting requires either a wealth of experience with trial-and-error methods or a deep knowledge of carbohydrate properties as related to structure—perhaps both. However, scientific knowledge of cause and effect is highly respected when it shortens industrial development timeSource, Types, and TerminologyThe layman‟s conception of carbohydrates generally involves only the sugars and starches of foods—those that generate calories and fat. The food chemist knows many other types that are ingested.Because most people enjoy the sweetness of sugars and the mouth feel of cooked starches, they become familiar by association with table sugar (sucrose), invert sugar‟s hydrolyzed sucrose, corn syrup sugars (D-glucose and maltose), milk sugar (lactose), and the more starchy foods. These carbohydrates are nutritionally available; i .e., they are digested (hydrolyzed to component monosaccharides) and utilized by the human body。
Esterification of free fatty acids in waste cooking oils(WCO):Role of ion-exchange resinsNalan O¨zbay,Nuray Oktar *,N.Alper Tapan Faculty of Engineering and Architecture,Department of Chemical Engineering,Gazi University,06570Maltepe,Ankara,TurkeyReceived 3October 2007;received in revised form 17December 2007;accepted 19December 2007Available online 15January 2008AbstractAlthough WCO plays a crucial role for the economical production of biodiesel,free fatty acid (FFA)level in the nature of WCO cause saponification problems during transesterification.Acidic ion-exchange resins can be used to decrease WCO free fatty acid level.In this study,activities of resins (Amberlyst-15(A-15),Amberlyst-35(A-35),Amberlyst-16(A-16)and Dowex HCR-W2)in direct FFA ester-ification were examined in the temperature range of 50–60°C and the effect of catalyst amount (1–2wt%)on FFA conversion was also analyzed.FFA conversion increased with increasing reaction temperature and catalyst amount.Order of catalytic activities was found as A-15>A-35>A-16>Dowex HCR-W2.This was related to the size of average pore diameters and magnitude of BET surface area.Ó2007Elsevier Ltd.All rights reserved.Keywords:Biodiesel;Esterification;Ion-exchange resins1.IntroductionSince fossil fuels increase greenhouse gas emissions and cause global warming,the use of alternative resources like biofuels are more pronounced everyday.For example,European Community has decided to replace at least 5.75%of the yearly consumed fossil fuels by biofuels,by the year 2010.The use of these biofuels does not contribute to the growth of greenhouse gases (2003/30/EC directive)[1].As an alternative to fossil fuels,biofuels must be techni-cally feasible,economically competitive,environmentally acceptable,and readily available [2].Regarding these prop-erties,biodiesel has been widely recognized as a promising biofuel.On the other hand,biodiesel is usually more expen-sive than petroleum-based diesel fuel when it is produced from vegetable oils or animal fats [3].Biodiesel can be produced by transesterification,whereby oil reacts with low molecular weight alcohols [4].Generally,alkaline catalysts (like sodium hydroxide),are used with low FFA content oils to catalyze transesterifica-tion reaction.Biodiesel can also be produced from waste cooking oils (WCO)[5–14];it is being tested for diesel engine perfor-mance [15,16]and has similar performance with biodiesel produced from fresh vegetable oils.In spite of its low cost,WCO pretreatment is still a problem [17]due to its high free fatty acid tely,Canakci and Van Gerpen developed a two step pretreatment reaction that reduces the acid level of the high FFA feedstocks to <1%[18].Transesterification of WCO feedstocks with FFAs (lar-ger than 0.5%)cause an undesired saponification reaction.In order to eliminate this problem,alternative acid catalysts can be employed [19].When strong liquid acid catalysts (sulfuric acid,hydrofluoric acid,and p -toluenesul-fonic acid)are used,they are less sensitive to FFA conver-sion during transesterification.In addition,the rate of reaction is slow and higher reaction temperatures are required.On the other hand,if solid acid catalysts like0016-2361/$-see front matter Ó2007Elsevier Ltd.All rights reserved.doi:10.1016/j.fuel.2007.12.010*Corresponding author.Tel.:+9031223174002556;fax:+903122308434.E-mail address:nurayoktar@.tr (N.Oktar).www.fuelfiAvailable online at Fuel 87(2008)1789–1798ion-exchange resins,zeolites and superacids like sulphated zirconia and niobium acid[20]are used,they could prevent corrosion[21],their separation is easy,and also high FFA conversions can be achieved[22].Ion-exchange resins are commonly used for esterifica-tion[23–26]and transesterification reactions[27].They have a gel type structure of microspheres that form a mac-roporous polymer[28].Conventional ion-exchange resins are composed of copolymers of divinylbenzene(DVB),sty-rene and sulfonic acid groups[being the active site (Brønsted acidity)].Polymer structure of resin is character-ized by the content of the crosslinking component(DVB), which determines surface area and pore size distribution of the resin[29].The aim of this study is to compare activities of different strong acidic ion-exchange resins like Amberlyst-15, Amberlyst-35,Amberlyst-16and Dowex HCR-W2for FFA esterification in WCO and examine the effect of cata-lyst amount,reaction temperature and reaction time on the esterification.2.Experimental2.1.MaterialsThree WCO batches were obtained from university caf-eterias in ANKARA andfiltered to remove impurities.The acidities of the feedstocks were0.47,0.42and0.41wt%. Analytic grade methanol was purchased from Merck Chemicals for esterification mercial strong acidic ion-exchange resins(A-15(Merck),A-35(Rohm and Haas),A-16(Fluka)and Dowex HCR-W2(Merck))NomenclatureA acidity(g OA/g oil)C NaOH concentration of NaOH(mol/L)V NaOH volume of solution consumed during titration (mL)MW OA molecular weight of oleic acid(g/mol) m sample weight of sample(g)Table1Characteristics of ion-exchange resinsProperty A-15A-35A-16DowexHCR-W2 Structure MR MR MR GMatrix Styrene–divinyl-benzene Styrene–divinyl-benzeneStyrene–divinyl-benzeneStyrene–divinyl-benzeneForm H+H+H+H+ Crosslinking level H H L L Exchange capacity(meq/g)5.2d 5.2c 4.8c 4.8e Surface area(m2/g)53c50c30c–Porosity(%)33a29a25b–Average porediameter(nm)30c30c25c–Particle diameter (mm)0.74f0.15–0.25g0.38–0.45g>1.2(%2)e<0.42h(%1)eMR:macroreticular structure,G:gel type polymer,H:high,L:low. a[27].b[29].c[30].d[31].e[32].f[33].g[34].h[35].Surface area Porosity (%)Average porediameter (nm)Ion exchangecapacity(meq/g)Particlediameter(nmx10)A-15A-35A-16Dowex HCR-W2m2/g1.Properties of ion exchange resins.050Reaction Time (min)Acidity(wt%)Comparison of different acidity test(at60°C2wt%catalyst amount).method,ITM:indirect titration Method.1790N.O¨zbay et al./Fuel87(2008)1789–1798were used as solid acidic catalysts.Amberlyst and Dowex HCR-W2resins(in wet form)were dried in an oven for 12h after washing with methanol at110°C and105°C, respectively.Physical properties of ion-exchange resin cat-alysts are summarized in Table1.It is clearly seen in Fig.1that A-15has superior physical properties with respect to others.2.2.FFA esterificationEsterification reactions were performed in a three-necked batch reactor(total volume1000ml)equipped with a reflux condenser to avoid alcohol vaporization.The three-necked reactor was immersed in a constant temperature water bath equipped with a temperature controller.A plate typeTable2Comparison of strong acid ion-exchange resins for FFA esterification at50and60°CT(°C)Catalystamount(wt%)X FFA(%)A-15A-35A-16Dowex HCR-W2 50125.423.022.120.8232.630.629.924.260137.236.534.130.4245.741.940.134.7Reaction time=3h.X FFA:conversion of FFAs in WCO.N.O¨zbay et al./Fuel87(2008)1789–17981791magnetic stirrer was used to control the stirring rate of the reaction mixture.In order to eliminate external mass trans-fer effects,the stirring speed was adjusted to1500rpm which was much higher than values indicated in the litera-ture[36].Esterification reactions were carried out in the temperature range of50–60°C.Resin catalysts(1and 2wt%)and20vol.%methanol were used in the reaction mixture[37].Sampling were done manually at reaction times of3,5,10min and every20min after that.In our esterification experiments,after each sampling of the reaction mixture(10g)the new FFA concentration value was evaluated based on the remaining part of the reaction mixture.2.3.Titration analysisReactant(WCO)and reaction mixture were analyzed by standard titration method for the evaluation of free fatty acid level[1].Before titration,a certain amount of sample ($10g)was dissolved in diethyl ether(with a purity of 99.9%(Merck))and ethanol mixture(with a purity of 99.9%(Merck)).Phenolphthalein indicator was used to determine pH change during esterification reaction.Aque-ous solution of0.01N NaOH were used as a titrant.Acid-ity(A)was determined according to the equation given below:A¼C NaOH V NaOH MW OAm sample1000100ðwt%Þð1ÞIn Eq.(1)the subscript‘‘OA”refers to oleic acid which is a type of FFA.The conversion of FFA can be determined from the equation given below:FFA conversionð%Þ¼A iÀA tA iÂ100ð2ÞIn Eq.(2)the subscript‘‘i”refers to initial acidity level and‘‘t”refers to the acidity at certain reaction time.In addition to the standard analysis method,indirect titration method in aqueous-alcohol media(described else-where[38])was also used(with Dowex HCR-W2(60°C, 2wt%))in order to double check the validity of acidity lev-els during reaction.1792N.O¨zbay et al./Fuel87(2008)1789–17982.4.Swelling experimentsBefore analyzing the effect of swelling on activities of catalysts,the swelling capacities were determined as described below:Amberlysts(A-15,A-35,A-16)and Dowex HCR-W2 catalysts were dried in an oven at110°C and105°C, respectively.Then0.5cm3of treated resin was placed into a measured tube.Methanol was added into the tube and tempered at50–60°C in temperature controlled water bath for2h.The change of the ion-exchange resin volume was recorded.Percentage of swelling at room temperature was calculated according to the following equation:Swellingð%Þ¼swollen volumeðcm3Þinitial volumeðcmÞ3Â100ð3Þ3.Results and discussionIn this part of the study,the effect of reaction parame-ters(the amount and type of acidic ion exchange resin and reaction temperatures)on FFA conversion were exam-ined and to see the reproducibility of the results FFA acid-ity during reaction was confirmed by two different test methods.And also the relation between some physical properties like crosslinking level and swelling capacity was confirmed.3.1.Validation of two different WCO acidity test methodsThe FFA acidity of WCO(during esterification with Dowex HCR-W2(60°C,2wt%)acidic resin)was con-firmed by indirect titration method in aqueous-alcohol media[38]and standard titration method[1].The agree-ment between these two test methods can be seen in Fig.2.3.2.Swelling capacity of ion-exchange resinsThe degree of crosslinking determines the rigidity of the resin and the extent of swelling in the presence of a polar component.Resins with lower crosslinking have higher swelling capacity[30].Fig.3indicates the swelling capacity of resins at50and60°C in pure methanol.The literatureN.O¨zbay et al./Fuel87(2008)1789–17981793data for crosslinking level in Table1and the order of swell-ing capacities in Fig.3confirms the relation given above.parison of different ion-exchange resinsIt is generally known that lower crosslinking levels will cause higher swelling of ion-exchange resins[39].Besides in spite of lower swelling capacity,if the ion-exchange resin has high pore diameter it will let FFA molecules enter into the inner surface of the catalyst and will increase esterifica-tion rate[39].In Table1,the structural characteristics of A-15,A-35,A-16,Dowex HCR-W2are given.Although A-15 has the highest crosslinking level and the lowest swelling capacity as expected(Fig.3.)since it has a high pore diam-eter and superior physical properties(surface area)than others,it would exhibit highest FFA conversions as seen in Table2.Finally it can be said that,high average pore diameters(with high BET surface area)has more dominant effect than low crosslinking level in esterification reaction. FFA conversion for different catalysts are shown in Fig.4.It is seen from thisfigure the catalytic activities of resins can be arranged as A-15>A-35>A-16>Dowex HCR-W2.As described above,this order of catalytic activ-ities is related with the size of average pore diameters and magnitude of BET surface area(Table1).3.4.The effect of catalyst amountAt constant reaction temperature,acidity(increase in FFA conversion)decreased with increasing catalyst amount.In the case of non-catalyzed reaction,almost no esterification was observed(Figs.5and6).In the previous studies[5,6,13,40,41]similar behaviour was also reported.3.5.The effect of temperatureWhen esterification reactions were carried out at tem-peratures less than the boiling point of methanol(50–60°C)and at atmospheric pressure,the FFA conversions were46%maximum.The advantage of this process may be that lower operating temperatures and pressures are required unlike previous works[42,43].The effect of temperature on the FFA conversion at dif-ferent catalyst amounts(1and2wt%)is presented in Figs.1794N.O¨zbay et al./Fuel87(2008)1789–17987and8.Because of the increase in reaction rate and equi-librium constant for an endothermic reaction,it was seen that FFA levels decreased with increasing reaction temper-ature.This behaviour was independent of the type and amount of catalyst[5,40].3.6.Effect of catalyst type on reaction rateThe rate of change of FFA conversion can be seen in Figs.9and10,it was seen that the reaction rate drops to zero after40min for all type of catalysts.In the literature, the FFA esterification reaction is completed in15min for homogenous catalyst(H2SO4)[44]and100min for Relite CFS(acidic ion exchange polymeric resin)[41].3.7.Maximum performance level comparison with conventional catalystAll the experiments were conducted with an initial FFA concentration of$2.54mol/L.After the esterification reac-tion FFA conversions were observed as45%maximum at 60°C for2wt%A-15catalyst.In the literature,Sendziki-ene et al.[44]have achieved nearly$50%conversion for 1%homogenous acidic catalyst(H2SO4)at50°C,so it is clearly seen that A-15heterogenous catalyst can take place of conventional homogenous catalyst.Since the perfor-mance levels of the two types of catalysts mentioned above are comparable,it is believed that they will show resem-blance at higher FFA levels.3.8.The role of internal diffusion on reaction rateIn order to understand the effect of internal diffusion,an additional run was performed onfinely powdered(<6l m) A-15,A-16,A-35,Dowex HCR-W2catalysts(2wt%)at 60°C reaction temperature.In Fig.11it can be seen that there is no significant difference(<10%)in FFA conver-sions between powdered and particle catalysts.In addition, as we have mentioned before(in Fig.1)A-15has better physical properties(particle diameter,surface area etc.) than the other type of resins which may decrease the inter-nal mass transfer resistance.N.O¨zbay et al./Fuel87(2008)1789–179817951796N.O¨zbay et al./Fuel87(2008)1789–17984.ConclusionsHighest FFA conversion(45.7%)was obtained over strong acidic macroreticular ion-exchange resin A-15at 60°C with2wt%catalyst amount.FFA conversion increased with increasing temperature and increasing cata-lyst amount.All resin catalysts were active for esterifica-tion.Superiority of physical properties of resins may be a dominant factor for higher catalytic parable performance level of A-15catalyst with conventional cata-lyst makes it the best probable candidate among A-15, A-16,A-35,Dowex HCR-W2catalysts for FFA esterifimercial strong acidic ion exchange resins can be used for esterification of FFA in WCO. 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