food and bioproducts processing 9 0 ( 2 0 1 2 ) 180–186

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Food and Bioproducts Processing

j ourna l ho me p age: www.elsev ier .com/ locate / fbp

Extraction of dietary fiber from Citrus junos peel withsubcritical water

Masahiro Tanakaa,c, Arata Takamizua,b, Munehiro Hoshinob, Mitsuru Sasakia,Motonobu Gotoa,∗

a Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japanb ASCII Co., Ltd., 2349 Tabara, Kawasaki-mati Tagawa-Gun, Fukuoka 827-0004, Japanc Maruboshi Vinegar Co., Ltd., 2425 Tabara, Kawasaki-mati Tagawa-Gun, Fukuoka 827-0004, Japan

a b s t r a c t

The juice processing by-product of Citrus junos is a high potential source of valuable compounds such as essential

oils and a high amount of dietary fiber, consisting of pectin, hemicellulose, and cellulose. The residues obtained from

supercritical CO2 extraction of C. junos peel was used as a starting material for hydrothermal treatment to separate

pectin and hemicellulose. The experimental apparatus used was a semi-continuous flow extractor. Treatment con-

ditions were in the temperature range of 160–320 ◦C and water flow rates of 2.1, 3.5, and 7.0 mL/min under a pressure

of 20 MPa. Approximately 78% of the pectin was contained in the fraction collected at 160 ◦C at each flow rate. Most of

the hemicellulose was separated from cellulose up until the fraction obtained at 200 ◦C. The proportion of cellulose

in the residue obtained after hydrothermal treatment at 200 ◦C reached about 80%. Moreover, the characteristics

of recovered cellulose were expected to exhibit greater crystallinity and lower impurity than that of the raw mate-

rial based on the results of scanning electron microscopy (SEM), attenuated total reflectance Fourier transmission

infrared (ATR-FTIR), and thermogravimetric-differential thermal analyses (TG-DTA).

© 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Citrus junos; Pectin; Hemicellulose; Cellulose; Hydrothermal

can be controlled by pressure and temperature. Bobleter

1. Introduction

Biomass has been considered an alternative source forfuels, chemicals, and materials conventionally derived frompetroleum. Research on bioethanol production from ligno-cellulosic plant materials has been in progress over the lastdecade. Due to the depletion of non-renewable fuels and anincrease in greenhouse gas emissions, the development ofalternative non-fossil transportation fuel has become a pri-ority.

Cellulosic biomass must be hydrolyzed to monosaccha-rides for bioconversion to bioethanol. However, it is difficultto digest most types of biomass to cellulose because thecomplex structure between hemicellulose and lignin inhibitscellulose-catalyzed enzymatic hydrolysis (Converse et al.,1989). Therefore, the typical hydrolysis of cellulosic biomass

requires pretreatment processes to remove hemicellulose and

∗ Corresponding author. Tel.: +81 96 342 3664; fax: +81 96 342 3679.E-mail address: mgoto@kumamoto-u.ac.jp (M. Goto).Received 12 February 2010; Received in revised form 16 February 2011

0960-3085/$ – see front matter © 2011 The Institution of Chemical Engidoi:10.1016/j.fbp.2011.03.005

lignin (Lynd et al., 1999). Dilute sulfuric acid pretreatmenthas been studied extensively (Tompson and Grethlein, 1979;Hinman et al., 1992). However, the use of dilute sulfuric acidpretreatment involves additional costs for neutralizing chem-icals, as well as the handling and disposal of large amounts ofgypsum (Hinman et al., 1992).

Sub-critical water pretreatment is an alternative methodthat can be used for removal of hemicellulose and lignin.Sub-critical water has unique properties: the hydrogen bondbetween water molecules weakens with increasing temper-ature, and the dielectric constant can be changed greatly.Additionally, the ion product of water (Kw) increases dramat-ically as the temperature rises to around 270 ◦C (Marshalland Franck, 1981). Due to these properties, solubility andreactive selectivity for polar substances in sub-critical water

; Accepted 21 March 2011

et al. pioneered subcritical water pretreatment and demon-

neers. Published by Elsevier B.V. All rights reserved.

food and bioproducts processing 9 0 ( 2 0 1 2 ) 180–186 181

swhsb2

pfiIyfhdYoapc

setpbeSatrsTnAi

2

2

Utad1meetca

mhCtaPKdff

trated that more digestible cellulose was obtained from birchood (Hormeyer et al., 1988). Subsequently, many researchersave attempted to fractionate hemicellulose and lignin usingub-critical water pretreatment for various lignocellulosiciomasses (Allen et al., 1996; Laser et al., 2002; Liu and Wyman,003, 2005; Kumagai et al., 2007; Kristensen, 2008).

The development of transportation has increased citrusroduction and processing in countries with a suitable climateor citrus culture (Braddock, 1999). Therefore, it has becomemportant to deal with the increasing juice processing residue.n particular, the juice processing waste of Citrus junos, calleduzu in Japanese, has high potential. The yuzu peel accountsor approximately 50% of the whole fruit. It contains valuableydrocarbons and carbohydrates, such as essential oils andietary fiber, consisting of pectin, hemicellulose, and cellulose.uzu essential oil is very expensive due to its characteristicdor, while pectin accounts for 20–30% of dried citrus peel,nd so it is the major source of pectin products. Especially,ectin has been conventionally extracted and separated usinghemicals such as acid or chelating agent from citrus fruit.

We succeeded in effectively extracting essential oils withupercritical carbon dioxide (SC-CO2) from yuzu peel (Royt al., 2007). In a second step, we applied sub-critical water tohe residue of SC-CO2 extraction and successfully separatedectin (Ueno et al., 2008). In this work, as a final step in citrusy-product treatment from the viewpoint of a biomass refin-ry, we investigated the composition rate of cellulose fromC-CO2 extraction residue using subcritical water treatmentt a temperature range of 160–320 ◦C. The effects of tempera-ure under hydrothermal conditions on the composition of theesidues and citrus peel cell wall disruption, composition, andurface properties were also investigated by SEM observation.he effect of hydrothermal treatment temperature on compo-ent composition of the residues was investigated by TG-DTA.TR-FTIR spectroscopy was used as an analytical tool to qual-

tatively determine the chemical change in the material.

. Experiments

.1. Materials and chemicals

sing yuzu flavedo as a model of juice processing by-product,he residue from supercritical CO2 extraction of essential oilst 60 ◦C and 20 MPa was obtained. The residue was freezeried, ground, and sieved. Flavedo with a particle size of70–450 �m was obtained as the raw material for hydrother-al treatment. This pretreatment of the raw material may

liminate the mass transfer limitation during the reactivextraction process, which helps in understanding the dissolu-ion behavior of materials in subcritical water. The elementaryomposition of the sample was C (41.00 wt.%), H (6.28 wt.%)nd N (1.48 wt.%).

Chemical reagents used for component composition of rawaterial were as follows: acetic acid, acetone, ethanol, n-

exane, sodium chlorite, and sodium hydroxide (Wako Purehem. Ind., Ltd., Osaka, Japan). Reagents used for quan-

itative analysis of pectin were as follows: d-galacturoniccid, sodium chloride, sulfuric acid, acetic acid (Wakoure Chem. Ind., Ltd.) and 3,5-dimethylphenol (Tokyo Kaseiogyo Co., Ltd., Tokyo, Japan). Reagents used as stan-ard compounds obtained under hydrothermal treatment

or HPLC analysis were as follows: d-(+)-arabinose, d-(−)-ructose, d-(+)-glucose, d-(+)-xylose (Wako Pure Chem. Ind.,

Ltd.), d-(+)-cellobiose (Nacalaitesque, Inc., Kyoto, Japan), 5-hydroxymethyl-2-furfural (5-HMF) (Sigma–Aldrich Japan Inc.Tokyo, Japan) and 2-furfural (Tokyo Kasei Kogyo Co., Ltd.).Reagents for FTIR and TG-DTA analysis were as follows: cellu-lose microcrystalline (Merck KGaA, Darmstadt, Germany) andxylan from beech wood, >90% xylose residue (Sigma–AldrichJapan Inc.).

2.2. Component composition of raw material

Component composition of the raw material was analyzedaccording to the modified Wise method (Wise and Ratliff,1947). Approximately 2 g of raw material was defatted withethanol and n-hexane using a Soxhlet extractor. The defat-ted and dried material was suspended in 150 mL of ultrapurewater, and 0.2 mL of acetic acid and 1 g of sodium chlorite werethen added at 80 ◦C for 1 h until the insoluble solid turnedwhite. Then, the insoluble solid was filtered with a glass fil-ter and rinsed repeatedly with ultrapure water and acetone toobtain the dried holocellulose component. Holocellulose wassoaked in 25 mL of 17.5% sodium hydroxide solution at roomtemperature for 30 min, and 25 mL of ultrapure water was thenadded to the solution. The insoluble solid was filtered witha glass filter and rinsed repeatedly with ultrapure water and10% of acetic acid to obtain dried �-cellulose. The rinse liq-uid was collected and diluted with ultrapure water up to avolume of 800 mL. Then the solution was mixed with 40 mLof 30% acetic acid and heated until the solution turned col-orless. The insoluble solid was recovered by glass filter andrinsed repeatedly with ultrapure water. The solid obtainedwas mainly �-cellulose. The amount of hemicellulose wascalculated by subtracting the amount of �- and �-celluloseand pectin from holocellulose. The amount of pectin wasestimated as the amount of branched chain sugars and galac-turonic acid. The resulting component composition (on a dryweight basis of raw material) was: fat (7.99 wt.%), cellulose(29.04 wt.%), pectin (32.21 wt.%), hemicellulose (11.70 wt.%),and others (19.06 wt.%).

2.3. Sub-critical water treatment

The semi-continuous flow extractor is described in our pre-vious paper (Ueno et al., 2008). The maximum operatingconditions of the apparatus are 450 ◦C and 45 MPa. A 0.5 g por-tion of raw material was charged in the extractor (SUS 316;8.7 mm i.d. × 118 mm length, 7.0 mL volume), and the extrac-tor was capped with gasket filters (average pore size, 20 �m)to prevent sample particles from flowing out. Distilled waterwas degassed and continuously delivered through a heatingcoil, placed in a thermostat air oven, into the extractor bya high-pressure pump (PU-980; Jasco Co., Ltd., Tokyo, Japan)at water flow rates of 2.1, 3.5, and 7.0 mL/min. The pressurein the extractor was controlled by a back-pressure regulator(HBP-450; Akico Co., Ltd., Tokyo, Japan) at 20 MPa. The reac-tion temperature was monitored by thermocouples at the inletand outlet of the extractor. Prior to extraction, degassed water(70 mL) was delivered to the extractor at room temperatureto remove the air and water-soluble substances. The initialfraction was collected at room temperature. Then, the reac-tion temperature was raised in a stepwise fashion from roomtemperature to the desired temperatures of 160, 200, 240, 280,and 320 ◦C. About 2 min was required until the reactor tem-

perature reached the desired temperatures at each water flowrate. The hydrothermal treatment solutions were fractionated,

182 food and bioproducts processing 9 0 ( 2 0 1 2 ) 180–186

0

20

40

60

80

100

320280240200160RTTemperature [ºC]

TOC

Yie

ld [%

]

2.1ml/min3.5ml/min7.0ml/min

Fig. 1 – Effect of water flow rate on sample TOC yield.

and 5 fractions (70 mL) were collected at each temperaturecondition. The fractionated solutions were assayed for theamounts of total organic carbon (TOC), soluble sugars andits dehydration products. The solid residues were weightedafter freeze-drying and analyzed for component composition,surface properties, and chemical changes.

2.4. Total organic carbon analysis

The solutions fractionated under each temperature and waterflow rate condition were diluted 5-fold for TOC analysis.The samples (25 �L) were measured with a TOC analyzer(TOC-5050A, Shimadzu Co., Ltd., Kyoto, Japan). The TOCyield of the aqueous phase was defined by the followingequation:

TOC yield

= TOC of fraction [mg]Raw material (dry basis) [mg] × Element ratio of C

× 100

2.5. Quantitative analysis of saccharides and theirdehydration products

The acidic polysaccharide of pectin was quantitatively ana-lyzed, in terms of its galacturonic acid content, according tothe dimethylphenol method (Scott, 1936). The fractionatedsamples and hemicellulosic solutions obtained by Wise’s step(125 �L) were mixed with 5% NaCl (125 �L), and 2 mL of H2SO4

was subsequently added to the mixture in an ice bath. Themixture was thoroughly mixed and placed in a 70 ◦C waterbath for 10 min. 3,5-Dimethylphenol–acetic acid reagent (0.1%,100 �L) was added and stirred at room temperature for 10 min.Absorbance of the mixture was measured at 400 and 450 nm.Absorbance of galacturonic acid and its methyl ester were cal-culated as the difference between the absorbances at 400 nmsubtracted from absorbance at 450 nm.

Identification and quantitative analysis of neutral sugarsand their hydrothermal dehydration products were conductedusing a Sugar-SH1011 column (8.0 × 300 mm, Showa DenkoK.K. Tokyo, Japan) maintained at 60 ◦C. The mobile phase was3 mM perchloric acid solution at a flow rate of 1.0 mL/min andthe coloring reagent was bromothymol blue (BTB) solution atflow rate of 0.5 mL/min. They were mixed immediately at theoutlet of the column, which was monitored by a refractiveindex detector (RI-2031; Jasco Co., Ltd.).

2.6. Surface and chemical properties of the residues

An 8 nm coating of Au was applied to surface of the residues,obtained by hydrothermal treatment, for SEM analysis (JSM-5310LM, JEOL, Ltd., Tokyo, Japan). The residues (1.0–2.0 mg)were placed in an aluminium sample holder and heated from40 to 500 ◦C at a heating rate of 5 ◦C/min in a flowing atmo-sphere of nitrogen using TG-DTA (TG/DTA EXSTAR6000 series,SII NanoTechnology, Inc., Chiba, Japan). The solid residueswere further ground using liquid nitrogen for ATR-FTIR anal-ysis (Spectrum One, PerkinElmer Japan Co., Ltd., Kanagawa,

Japan). The samples were characterized in the near IR region(wave numbers: 4000-650 cm−1).

3. Results and discussion

3.1. Analysis of liquid fractions

3.1.1. TOC analysisThe amounts of TOC in the fractions obtained under each tem-perature and water flow rate condition are shown in Fig. 1.The first fraction was collected at room temperature in orderto remove water soluble compounds and contained 17.4% ofcarbon in the raw material, which was essentially composedof sugars, organic acids, and pigments. The amount of TOCwas dramatically increased for the 160 ◦C fraction becausepectin started to actively separate from the cell wall in thistemperature range (Ueno et al., 2008). In fact, approximately78% of total pectin was contained in the 160 ◦C fraction ateach flow rate, whereas pectin was not detected in fractionsobtained at higher temperatures. It was thought that excessivehydrothermal decomposition of pectin occurred simultane-ously while being separated from the cell wall. The TOCgradually increased to over 70% of raw material at each waterflow rate, with stepwise increases in the treatment temper-ature, from room temperature (RT) to 320 ◦C. Moreover, theTOC was higher for lower water flow rates, due to longer expo-sure to the reaction condition. At the flow rate of 2.1 mL/ min,the elution time should be 3.33 times longer than that of7.0 mL/min for each fraction of 70 mL. It was likely that thedissolution of raw material progressed more rapidly at highertemperatures and longer times.

3.1.2. HPLC analysis of neutral sugars and theirdehydrated productsThe products, obtained under various hydrothermal condi-tions, were analyzed for neutral sugars and their dehydratedproducts to investigate the behavior of hemicellulose separa-tion from the cell wall and cellulose decomposition. Arabinoseand xylose, which are the main components of pectin andhemicellulose present in citrus peel (Manabe, 2001), wereselected as standard substances to trace the separation tem-perature of pectin and hemicellulose. Other selected standardsubstances have been previously confirmed through experi-ments on the cellulose decomposition pathway in subcriticaland supercritical water (Kabyemela et al., 1997; Sasaki et al.,1998; Adschiri and Arai, 2001). As an example, the HPLC-RI chromatogram for the fraction obtained at 240 ◦C and aflow rate of 2.1 mL/min is shown in Fig. 2. The products,such as xylose and arabinose, derived from hemicellulose

were detected. Reaction products from cellulose degradationwere also detected. d-Glucose and d-cellobiose are hydroly-

food and bioproducts processing 9 0 ( 2 0 1 2 ) 180–186 183

6258545046423834302622181410Retention time [min]

RI r

espo

nse a b

c

d e

fg

Fig. 2 – HPLC-RI chromatogram of hydrothermal decomposition products fractionated at 240 ◦C and 2.1 mL/min. (a)d-Cellobiose (13.8 min), (b) d-glucose (15.9 min), (c) d-xylose (16.7 min), (d) d-fructose (16.8 min), (e) d-arabinose (17.8 min), (f)5

sodsun1cs

Fa

-HMF (41.2 min), (g) 2-furfural (60.5 min).

is products. d-Fructose can be obtained by the isomerizationf d-glucose. 5-HMF and 2-furfural are dehydration productserived from d-fructose and d-glucose, respectively. Fig. 3hows the product composition of liquid samples obtainednder various temperature and flow rate conditions. Arabi-ose and xylose were detected in the temperature range of60–240 ◦C. This indicates that the hemicellulosic polysac-haride was separated from cellulose, and further hydrolysis

ubsequently occurred. It is thought that the removal of hemi-

ig. 3 – Effects of temperature and water flow rate on themount of sugars and their dehydration products.

cellulose was almost complete at treatment temperatures upto 240 ◦C. On the other hand, glucose was detected at tempera-tures higher than 200 ◦C. The amount of dehydrated productsobtained from glucose and fructose tended to increase withtreatment time, with the exception of the treatment at 320 ◦C.At the treatment condition of 320 ◦C and 2.1 mL/min, theamount of residual solids was minimal and insufficient forfurther reactions.

3.2. Analysis of solid residues

3.2.1. Component composition of residuesFig. 4 shows the recovery of residues and the compositionrate and yield of cellulose in solid residue at a flow rate of7.0 mL/min. The residue component composition was ana-lyzed in the same way as that of the raw material. The amountof residues decreased with increasing treatment temperature,which corresponded to its TOC yield (Fig. 1). The yield ofcellulose decreased gradually with increasing treatment tem-perature. This result can be attributed to two reasons: (1) thedehydration and degradation of glucose derived from cellulosehydrolysis, and (2) the passing out of oligomers of glucose andcellulose fragments which are insoluble in cold water whereasthey are soluble in high temperature water. We observed pre-cipitated material in collected sample which were soluble inhigh temperature water and some of the solid precipitationflowed out through the gasket filter (average pore size, 20 �m).

Fragmentation was notably observed at 200 ◦C after the pectin

0

20

40

60

80

100

Rawmaterial

160 o 200C o 240C o 280C o 320C oC

Com

posi

tion

rate

of c

ellu

lose

[%]

0

20

40

60

80

100

Yield of cellulose [%

]

Composition rate of celluloseYield of cellulosewt % of residue

Fig. 4 – Residue recovery and the composition rate andyield of cellulose obtained after hydrothermal treatment ata water flow rate of 7.0 mL/min.

184 food and bioproducts processing 9 0 ( 2 0 1 2 ) 180–186

Fig. 5 – Photographs (left group) and SEM micrographs (right group) of the starting material and its hydrothermal treated

a

c

d

e

f

g

b

h

650850105012501450165018502050225024502650285030503250345036503850

Tran

smitt

ance

Wave numbers [cm-1 ]

aa

c

d

e

f

g

b

h

Fig. 6 – IR spectra of cellulose, xylan, raw material, andresidues obtained after hydrothermal treatment. (a)Cellulose, (b) xylan, (c) raw material, (d) 160 ◦C, (e) 200 ◦C, (f)240 ◦C, (g) 280 ◦C, (h) 320 ◦C.

residues obtained at a flow rate of 7.0 mL/min.

separation. The fragmentation seems to be associated withthe removal of the pectin because pectin plays a role as abonding material that unites cell walls by hydrogen bonding(Henglein, 1958). At 160 ◦C, the composition rate of cellulosein solid residue increased dramatically, due to the removal ofthe pectin and hemicellulose fragments. The highest cellu-lose composition rate was obtained at 200 ◦C, achieving levelsof up to 80%. The concentration of cellulose decreased attemperatures greater than 200 ◦C, whereas the componentcomposition of unknown compounds was higher. Sebilla andFuertes (2009) reported on the generation mechanism of insol-uble hydrochar from cellulose under hydrothermal conditions.Similarly, the conversion of cellulose to water insoluble struc-tural polymers might occur.

3.2.2. Surface properties of residues obtained afterhydrothermal treatmentFig. 5 shows the effect of treatment temperature onresidue appearance using microscopic observation. Theappearance of the material changed with treatment tem-perature: light yellow particle (raw material) → yellow crispfilm (160 ◦C) → light yellow cottony puff (200 ◦C) → lightgray cottony puff (240 ◦C) → dark gray carbonic oily puff(280 ◦C) → black oily burnt residue (320 ◦C), as shown in the leftgroup of photographs. The residue obtained after hydrother-mal treatment at 200 and 240 ◦C visually appeared to containa high content of cellulose. Similar findings were obtainedfrom the SEM micrographs (right group). The cell wall struc-ture can be clearly seen in the residue obtained at 200 ◦C. Itis likely that other various components contained in the rawmaterial were effectively removed. However, further decom-position and conversion from cellulose to unknown structuralsubstances seemed to be enhanced with increased treatmenttemperature.

3.3. Chemical composition of residues

ATR-FTIR spectroscopy was used as an analytical tool to quan-titatively determine the chemical changes in the surface ofhydrothermally treated yuzu peel to complement and aid inthe interpretation of microscopic investigations. Fig. 6 showsthe FTIR spectra of untreated, hydrothermally treated yuzu

peel, and cellulose as a control material (all spectra were opti-mized by applying the ATR correction). Among the absorption

spectra, except at 320 ◦C, the presence of an oxygen groupis suggested by the region from 3100 to 3600 cm−1 due toO–H stretching vibration in hydroxyl and carboxyl groups(Bilba and Ouensanga, 1996). Additionally, the absorbanceintensity in the 950–1200 cm−1 range, which constitutes thefingerprint region for each polysaccharide (Takamine et al.,2007), vanishes in the 320 ◦C spectra. This result, along withthe SEM image (Fig. 5), indicates that the residue obtainedafter hydrothermal treatment at 320 ◦C consisted not of cel-lulose but its conversion products. Sebilla and Fuertes (2009)suggested that this weakened intensity was attributable todehydration during hydrothermal carbonization of cellulose.The intensity ratio of the spectra at approximately 2850 and2920 cm−1 was due to CH2 bond-stretching, which indicatesthe presence of aliphatics, such as wax, which increased withhigher treatment temperatures. In fact, it is known that thecitrus peel surface is composed of a cuticular layer, whosesurface is coated by a layer of thick wax to guard against tran-spiration of water and invasion from the outside. From theabove result, it was deduced that the proportion of aliphatics

was higher with increased temperature due to hydrothermal

food and bioproducts processing 9 0 ( 2 0 1 2 ) 180–186 185

Fig. 7 – DTG curves of cellulose, xylan, raw material, andresidue obtained after hydrothermal treatment. (a)Cellulose, (b) xylan, (c) raw material, (d) 160 ◦C, (e) 200 ◦C, (f)240 ◦C, (g) 280 ◦C, (h) 320 ◦C.

dt

hcfapmoApHt(rtthtt1ltt

4

DwCts1teXahdt

egradation of the raw material starting at the side oppositeo the wax layer.

TG-DTA was carried out to investigate the effect ofydrothermal treatment temperature on residue componentomposition. As shown in Fig. 7, the DTG curves obtainedor various solid samples were in agreement with the FTIRnalysis. The DTG curve for the raw material (c) showed twoeaks. The first is thought to be made up of water-soluble lowolecular mass components, such as monosaccharides and

rganic acids, while the second corresponds to cellulose (a).t the treatment temperature higher than 200 ◦C, the decom-osition behavior of hemicellulose (b) was scarcely detected.owever, maximum degradation rates obtained at treatment

emperatures higher than 160 ◦C gradually shifted from 330 ◦Cobtained from cellulose DTG curve) to a higher temperatureegion with increased treatment temperature. It is assumedhat this shift was caused by the formation of high order struc-ural cellulose and the removal of other components, such asemicellulose and lignin. Others have reported on the rela-ionship between the crystallinity of cellulose material and itshermal decomposition behavior in nitrogen (Yang and Kokot,996; Ouajai and Shanks, 2005). Their data indicated that aarger crystalline structure required a higher decompositionemperature. Thus, cellulose crystallinity was increased withhe treatment temperature in this experiment.

. Conclusion

ietary fibers, such as pectin, hemicellulose and cellulose,ere effectively separated from the residue obtained by SC-O2 extraction of Citrus junos peel by applying hydrothermal

reatment at a temperature range of 160–320 ◦C using aemi-continuous flow extractor. In the fraction collected at60 ◦C, the amount of TOC dramatically increased as it con-ained most of the pectin. Solubilization of raw material wasnhanced by the increased treatment temperature and time.ylose and arabinose were detected in the fractions obtainedt 160 and 200 ◦C, whereas the solid residue obtained afterydrothermal treatment at temperatures greater than 200 ◦C

id not contain hemicellulose. Moreover, the cell wall struc-ure was described for the residue obtained at 200 ◦C in the

SEM micrographs. The highest cellulose composition rate, upto 80%, was obtained at the 200 ◦C treatment. However, itcontained certain aliphatics derived from the cuticular layer.From the perspective of biomass refining, this work providesan important sequential method for the recovery of valuablecompounds from citrus fruit waste using an environmentallyfriendly technique.

Acknowledgment

This work was supported by the Kumamoto University GlobalCOE Program “Global Initiative Center for Pulsed Power Engi-neering”.

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