degradation starch

14Stability and Degradation of Polysaccharides

Valdir SoldiFederal University of Santa Catarina, Florianopolis, SC, Brazil

I. INTRODUCTION

The interest in degradation of polysaccharides is stronglyassociated with a variety of applications in the food, paper,pharmaceutical, cosmetic, textile, and oil industries. Morespecifically, polysaccharides have been used in drug deliv-ery, medical devices, oil exploration, water-based paints,emulsions, dispersions, wastewater treatment, etc. Besidesthe dependence on their chemical structure, these functionsare also dependent on a controlled molecular size. Forexample, the potential biomedical and food applicationsfor lower molecular weight chitosan encouraged the de-velopment of viable processes for controlling the degrada-tion of this polysaccharide [1,2]. Through decreasing themolecular weight of chitosan (and other polysaccharides),it is possible to control properties like viscosity, solubility,and biological activity. In a similar manner, the water-soluble polysaccharide, guar, needs to be depolymerizedfor use in applications such as food and oil production[3,4]. The primary degradation products from the thermaldegradation of polysaccharides are also of great interest.For example, monomeric anhydrosugars have beenobtained from the pyrolysis of polysaccharides. By thisprocess, levoglucosan was obtained from cellulose [5,6]and starch [7], and 2-acetamido-1,6-anhydro-2-deoxy-h-D-glucopyranose from chitin [5,6]. Most of the applicationsfor cellulose depend on enzymatic modification or otherstructural changes. This behavior is in general associatedwith a total absence of solubility either in water or inaqueous alkaline solutions due to the existence of intra-and intermolecular hydrogen bonds in solid-phase cellu-lose. To solve this problem, it is necessary to activatecellulose by swelling and degradation processes (mechan-ical or thermal treatment). In another example, cellulose iseasily dissolved in aqueous solution (9% NaOH) aftermodification with cellulase (enzyme) and treatment withsodium hydroxide [8].

The degradation of polysaccharides has been analyzedby chemical, enzymatic, thermal, mechanical (ultrasonic),and radiative processes, which are generally dependent onthe structure, conformation, and applicability of the poly-saccharides, reactive agents, etc. For example, chemicaldegradation reactions depend on the nature of initiation(thermal, high energy, UV radiation, etc.) and reactiveagents such as oxygen and hydrogen ions. Depending onthe structure, conformation, and reactive agents, differentchemical bonds of the polymer can be attacked. In dextran,for example, the bonds near the ends of a polymer chain aremore reactive than those at the center [9]. In cellulose, thecleavage of the glycosidic bond is catalyzed either by H3O

+

(acid hydrolysis) or by enzyme (cellulase) [10]. At the sametime, the acid hydrolysis in chitosan breaks preferentiallythe ether linkages in the macromolecule increasing thepolymer reactivity [11]. For chitosan, nitrous acid attacksthe amine groups but not the N-acetyl moieties, andsubsequently cleaves the h-glycosidic linkages [12]. Xan-than is able to retain a high molecular weight (106 g mol�1)when submitted to depolymerizing conditions such as acidsor free radicals [13,14]. This is because the double-helicalstructure of xanthan is stabilized by noncovalent bondsbetween adjacent chains. In contrast to other polysaccha-rides, in xanthan, the cellulosic regions are the primary sitesthat suffer the attack by cellulases (enzymatic degradation)[15,16].

The application of ultrasonic degradation in the lab-oratory and the food industry (not discussed in detail in thischapter) has received more attention in recent decadesmainly to depolymerize macromolecules, prepare emul-sions, and disrupt biological cells. For polysaccharides,the preliminary studies were related to cellulose derivatives[17], starches [18,19], dextrans [20], chitosans [21], andxanthans [22]. The main effect observed in these macro-molecules was a reduction in solution viscosity or gelstrength due to a decrease in molecular weight [20,21,23].

395

Recently, Lii et al. [24] studied the degradation kinetics ofagarose and carrageenans by ultrasound, observing thatfor solutions with 0.5 wt.% polysaccharide, the degrada-tion rate decreases linearly with the reduced viscosity. Thisresult indicated that the ultrasonic degradation of agaroseand carrageenans follows a first-order kinetics, and isdependent on molecular size (decrease in viscosity). Thedegradation mechanism has been attributed to cavitation(mechanical effect); that is, the formation and collapse ofmicroscopic vapor bubbles generated by the strong soundwaves [25].

The aim of this chapter is to analyze the stability anddegradation of polysaccharides considering three maingeneral topics: (1) chemical (acid and alkaline) degrada-tion in aqueous solution, (2) processes of biodegradation,and (3) thermal stability (thermal degradation) of poly-saccharides. The first topic was chosen because alkalinedegradation was the first process studied concerning thestability of mono- and polysaccharides. Although mostwork related to chemical degradation deals with celluloseand cellulose derivatives, it is important to point out theeffects of temperature, concentration, and mechanism ofdegradation (reaction products). In biodegradation, dif-ferent aspects are considered such as the hydrolysis pro-cess, effect on the molecular weight distribution, and effectof pH. The thermal stability was analyzed in terms of thekinetic parameters of degradation, viscosity (intrinsic vis-cosity), temperature, depolymerization, and mechanism ofdegradation. To better understand the mechanism ofdegradation, a section with the main products associatedwith the degradation of polysaccharides was included inthis chapter.

II. CHEMICAL DEGRADATION

In this section, different polysaccharides were analyzed interms of acid and alkaline degradation processes. Forexample, the depolymerization of N-succinyl-chitosan so-dium salt was studied in an aqueous solution of hydro-chloric acid by Kato et al. [26] in terms of the molecularweight and viscosity behavior. The acid treatment wasperformed by stirring a solution of N-succinyl-chitosan in7.5 M aq HCl at room temperature or 3.3 M aq HCl at40jC. In the depolymerization process, two degradedproducts were characterized in terms of the molecularweight by the authors, using size-exclusion chromatogra-phy-multiangle light scattering (SEC-MALS). Studies uti-lizing 1H NMR and elemental analysis indicated that thedegraded products retained the fundamental structure ofthe N-succinyl-chitosan. The molecular weights (Mw) forthe two degraded products were 70,000 and 28,000 g cm3,very low values in comparison to the N-succinyl-chitosanfor which Mw = 310,000 g cm3. The viscosity behavior[reduced viscosity (gred)] was analyzed as a function of theconcentration in salt solution at 37jC (Fig. 1A). As can beobserved, the viscosity of N-succinyl-chitosan, despite itshigher values, increased with the concentration. However,for the depolymerized products, the viscosity values were

lower and a slight reduction was observed; that is, practi-cally independent of the concentration. At the same time,the intrinsic viscosity, obtained from the intercept of thestraight lines of Fig. 1A, increased linearly with the molec-ular weight (Fig. 1B). The behavior in terms of viscosityand molecular weight, associated with the analysis of thecomposition of the degraded products indicated that thechain scission of the N-succinyl-chitosan probably oc-curred through the breakdown of the ether linkages inthe macromolecule, as was observed for chitosan [11].

Basedow et al. [9] studied the depolymerization reac-tion of dextran at 80jC in 0.12 N sulfuric acid. During thereaction, the molecular weight distribution was determinedby the gel permeation chromatography (GPC) method.Different samples with initial concentrations of dextran inthe range of 0.25–2% were used. Considering the results, afirst-order reaction with respect to dextran concentrationwas suggested by the authors. The average molecularweight (Mw) changed from 117,000 to 3650 Da in thereaction time range of 0–450 min and the rate constant(k) was 6.30 � 10�6 sec�1.

Changes in the molecular weight were also analyzed bythe size exclusion chromatography-refractive index meth-od (SEC-RI) considering the acid hydrolysis of (n-, L-, E-)

Figure 1 Reduced viscosity (gred) vs. concentration (A) andlog intrinsic viscosity (log [g]) vs. log molecular weight (logMW) (B). In A, 5 represents undegraded N-succinyl-chitosanand o, 4 represent two degraded samples of N-succinyl-chitosan. (From Ref. 26.)

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carrageenans, dextran sulfate, and heparin [27]. The desul-fation and depolymerization processes were studied at pH2 (0.008 M LiCl + 0.012 M HCl buffer) at 35jC and 55jC.After hydrolysis, the results showed that for carrageenansand heparin, the sulfation was unaffected by the hydrolysisconditions. However, for dextran sulfate, the desulfationincreased linearly with the hydrolysis time especially at55jC. The difference in terms of the observed behavior wasattributed to the conformation flexibility of the polymerchain. The average molecular mass (hMmi) measured as afunction of hydrolysis time is shown in Fig. 2. A significantdecrease in hMmi was observed for the carrageenans anddextran sulfate (Fig. 2a–d) in comparison with the theo-retical values determined assuming that desulfation was theonly mechanism for molecular mass loss. However, a veryslight effect was observed by the authors for heparin (Fig.2e). The comparative analysis with the theoretical valuesfor dextran sulfate (Fig. 2d) indicated that besides thesulfate loss, a chain scission occurred, significantly decreas-ing the average molecular mass. From Fig. 2 and using Eq.(1) [28,29] and the Arrhenius equation [Eq. (2)], the authorsdetermined the kinetic parameters (activation energy) forthe carrageenans and dextran sulfate. In Eq. (1), hMm

(t)iand hMm

(0)i are the molecular mass at time t and time zero,respectively

1

MðtÞm

D E ¼1

Mð0Þm

D E þkt

mð1Þ

ln k ¼ ln A� Ea=RT ð2Þ

where k is the first-order rate constant and m is themolecular weight of the repeating unit. In the Arrheniusequation [(Eq. (2)], Ea is the apparent activation energy, R

is the gas constant, A is the pre-exponential factor, and T isthe temperature. The activation energy can be obtainedthrough the slope of the ln k vs. 1/RT plot. The activationenergy values were in the range of 100–120 kJ mol�1 exceptfor L-carrageenan, which was 162 kJ mol�1. The higherstability for L-carrageenan was attributed to the steric effectcaused by the sulfate group in the polymer structure. Underthe above hydrolysis conditions, only the dextran sulfatewas sensitive to the process of desulfation. However, adepolymerization process was characterized by the authorsfor the carrageenans and dextran sulfate.

The cleavage of the glycosidic bond of cellulose iscatalyzed by H3O

+ (acid hydrolysis), producing glucose asthe main reaction product (yield > 95%) and, in general,the mechanism of degradation included the formation ofcationic intermediates that are responsible for the rateconstant. The degradation of cellulose fibers was analyzedby Phillip [10] in HCl (100jC) and 1 N H2SO4 (80jC). Inboth mediums, a significant decrease in the degree ofpolymerization was observed. The mechanism of degrada-tion includes a selective attack on the more unstableglycosidic bonds (less-ordered regions) by the H3O

+ ions.As described by the author, the acid hydrolysis affects alsothe fibrillar structure of the cellulose fibers.

In the degradation of xanthan in dilute acid (pH 1–4)at 80jC and at high (500 mM) and low (10 mM) ionicstrengths, the viscosity was described by a single exponen-tial-decay constant (the viscosity decreased with the hy-drolysis time). At the same time, the stability increased withthe ionic strength [30].

The thermochemical degradation of starch and cellu-lose to organic acids was studied by Krochta et al. [31–33] inalkaline solution. Experimentally, the reactions were per-formed at different temperatures (range of 180–240jC) and

Figure 2 Average molecular mass as a function of hydrolysis time at (.) 35jC and (o)55jC, for (a) n-carrageenan, (b) L-carrageenan, (c) E-carrageenan, (d) dextran sulfate, and (e) heparin. Theoretical molecular mass as a function of hydrolysis timebased on the desulfation effect is plotted as (–) at 35jC and (- - -) at 55jC. (From Ref. 27.)

Stability and Degradation of Polysaccharides 397

products (organic acids) were determined by high-perform-ance liquid (HPLC) and gas (GC) chromatography. Forboth polysaccharides, the degradation was described as asecond-order kinetics by Eq. (3)

lnCa

Cp

¼ lnMþ Cp0ðM�NÞkat ð3Þ

where Ca and Cp are the alkali and polysaccharide con-centrations, respectively; M is the ratio between the initialconcentrations (Ca/Cp); N is the relation ((Ca0)/(Ca) / (Cp0

)/(Cp)); ka is the rate constant, and t is the reaction time. Therate constant (ka) determined by Eq. (3) increased from0.0026 L mol�1 min�1 (180jC) to 0.3733 L mol�1 min�1

(240jC) for starch and from 0.0034 L mol�1 min�1

(180jC) to 0.6000 L mol�1 min�1 (240jC) for cellulose.A significant and similar effect due to the temperature, wasobserved for both systems. The activation energy deter-mined using the Arrhenius equation [Eq. (2)] was 165.10 kJmol�1 for the degradation of starch and cellulose. Asimilar value (162 kJ mol�1) was determined by Karlssonand Singh [27] for the degradation of L-carrageenan underacidic conditions.

The stability of starch, cellulose, and rice straw invarious NaOH solutions was compared by Krochta et al.[33]. The conversion of starch to water-soluble products(degradation) in 4% NaOH occurred just by heating to240jC. For cellulose and rice straw, the degradationoccurred in 40 min at 280jC, suggesting a higher stabilityin comparison to the starch. Less time was required for thedegradation when 6% NaOH was used. By HPLC, the au-thors identified seven organic acids resulting from thealkaline degradation of 10% (w/w) starch, cellulose, ricestraw, and glucose in 6% (w/w) NaOH at 280jC for 10 min.For both systems, the yields of lactic and formic acids weremore significant. For example, the starch degradationmainly produced 19.8% lactic acid, 13.0% formic acid,and 5.7% glycolic acid. Other organic acids such as aceticacid, 2-hydroxybutyric acid, 2-hydroxyisobutyric acid, and2-hydroxyvaleric acid were formed with ca. 2% content.Similar percentages were determined for cellulose;however, the percentages determined for the glucose weretotally different. For example, for glucose the amount oflactic acid produced was 36.1%, for formic acid 5.4%, andfor 2-hydroxybutyric and 2-hydroxyisobutyric acids ca.1%. In general, for both systems, the amount of organicacids produced corresponds to the NaOH equivalents.Recently, Knill and Kennedy [34] discussed the cellulosedegradation under alkaline conditions in terms of themechanism and physical aspects, which affect the degrada-tion reaction and products. The predominant mechanismsuggested by the authors under alkaline conditions and attemperatures <170jC, based on the literature [35–37], isthe formation of D-glucoisosaccharinic acids, as in thealkaline degradation of 4-O-methyl-D-glucose.

The different processes of polysaccharides depolymer-ization by acid hydrolysis discussed above, were in general,described in terms of effects on the viscosity and the averagemolecular weight. The extent of the effect depends on the

experimental conditions, polysaccharide structure, confor-mation, and other characteristics. The predominant mech-anism for the chain scission was the breakdown of theglycosidic bonds in the macromolecule, a well-known mech-anism, which occurs in a wide range of polysaccharides.

III. BIODEGRADATION PROCESSES

Biodegradation was defined as an event that take place inthe natural environment and in living organisms [38,39].Biodegradation, as well as other processes of degradation,is dependent on the chemical structure of the polymers (orbiopolymers) because it is fundamental how easily they canbe attacked by microorganisms. The hydrolysis process isalso dependent on the local order of the macromolecule.Unordered macromolecules are more susceptible to hydrol-ysis (acid hydrolysis). Complex macromolecules, such aslignin, which is a natural polymer, are less degradable thansynthetic polymers like aliphatic polyesters, confirmingthat the molecular composition and architecture determinethe accessibility of a polymer to degradation by micro-organisms. As described by Ratajska and Boryniec [40], themechanism of biodegradation by microorganisms primar-ily occurs through an attack on the surface of the biode-gradable fragments. By removing the fragments, theporosity of the material increases, allowing the permeationof water with the microorganisms that may cause mechan-ical stress facilitating the process of degradation by theenzymes produced.

The processes of biodegradation described in theliterature are, in general, associated with the hydrolysiscaused by added enzymes or those produced by the micro-organisms, and can be followed by changes in molecularweight, viscosity, and morphology. For example, in Fig.3A,B, the molecular weight distribution for the biodegra-dation process of pure viscose (cellulosic) fibers and mix-ture with 30% of cellulose carbamate, respectively, areshown under conditions of controlled temperature andhumidity of the air and soil [40]. For both systems, asignificant decrease in the molecular weight was observedafter the enzymatic degradation. The biodegradation pro-cess was also analyzed by the authors in terms of the degreeof polymerization (DP) observing that for pure viscose, itdecreased from 268 (untreated) to 53 after 6 months incontact with the microorganisms. For the viscose with 5%(figure not shown) and 30% of cellulose carbamate, theranges of DP variation under the same conditions were278–48 and 159–55, respectively.

The treatment of pectin extracted by autoclave fromsugar beet pulp (ABP), with enzymes to modify theirphysicochemical properties, was studied by Ooterweld etal. [41]. The experimental procedure included the dissolu-tion of the samples in 0.04 sodium acetate buffer containing0.01% of NaN3 (pH 5.0) to a final concentration of 5 mg/mL. The enzymes arabinofuranosidase B (AF), endo-arabinanase plus arabinofuranosidase (EA + AF), rham-nogalacturonase plus rhamnogalacturonan acetyl esterase(Rgase + RGAE), polygalactorunase plus pectin methyl

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esterase (PG + PE), were added to a final concentration of1 Ag protein/mL. Incubations were carried out at 20jC for42 hr. The digests were analyzed in terms of the molecularweight (Mw) at several stages of the incubation by high-performance size-exclusion chromatography (HPSEC).During the enzyme treatment, other parameters such asthe intrinsic viscosity ([g]w) and radius of gyration (Rgw

) ofthe polysaccharides were monitored. The behavior ob-served for the above parameters is shown in Fig. 4A–C.The Mw decreased from 271 to 112 kDa when ABP wastreated with RGase + RGAE. This effect was associatedwith the hydrolysis of the rhamnogalacturonan backbonepresent in the ABP. For the same enzymes, no significantchanges were observed in the intrinsic viscosity ([g]w) andradius of gyration (Rgw

). By adding EA + AF to ABP, theMw decreased to 207 kDa but a small change occurred inthe ([g]w) and (Rgw

). However, by adding PG + PE, theintrinsic viscosity ([g]w) and radius of gyration (Rgw

)showed a significant decrease (Fig. 4B,C) and the Mw

decreased to 5 kDa. A greater effect on the enzyme deg-

radation was observed by the authors when polygalactor-unase plus pectin methyl esterase (PG + PE) was used,suggesting that changes in the physicochemical propertiesdepend on the chemical structure of the ABP. Theseconclusions seem consistent if we consider that the struc-ture of ABP consists mainly of homogalacturonans and theenzyme used was polygalactorunase.

The effect on the molecular weight and structure of N-acetylated chitosan by hemicellulase was studied by Qin etal. [42]. For a sample with a degree of deacetylation of 73.2,the Mw decreased from 659,000 to 4200 kDa when thechitosan was submitted to degradation by hemicellulase for4 hr at 50jC and pH 5.5. The effect observed in the Mw wasaccompanied by an increase in the water solubility anddecrease in the thermal stability. For the authors, theresults suggested that the enzymatic hydrolysis was endo-action and mainly occurred in a random fashion.

Figure 3 Molecular weight distribution of (A) 100% viscosefibers and (B) viscose fibers containing 30% of cellulosecarbamate: (a) untreated, (b) treated for 3 months, (c) treatedfor 6 months. (From Ref. 40.)

Figure 4 Effect of enzymatic modification of sugar beetpulp, ABP, on (A) Mw, (B) [g]w, and (C) Rgw

: (�) EA + AF;(z) Rgase + RGAE; (.) PG + PE. (From Ref. 41.)


The biodegradation of the ethyl(hydroxyethyl cellu-lose) (EHEC) by endoglucanases was studied by Richard-son et al. [43]. Ethyl(hydroxyethyl cellulose) is a nonionic,water-soluble cellulose derivative produced by introduc-tion of ethyl and ethylene oxide groups to the hydroxylgroups of the cellulose backbone. In general, the endoglu-canases, which are enzymes for the cellulose hydrolysis,cleave the internal (1–4)-h-D-glucosidic linkages in thecellulose chain, producing smaller fragments of mono-and oligosaccharides [44]. From the literature, is knownthat endoglucanases hydrolyze unmodified cellulose tolow-molar-mass products such as glucose, cellobiose, andcellotriose [45]. For EHEC, the authors used two types ofendoglucanase from Trichoderma reesei (Cel7B, Mw

f

50,000 Da and Cel12A, Mwf 24,500 Da). The hydrolysis

products were analyzed by size-exclusion chromatographywith multiangle light scattering and refractive index detec-tion (SEC-MALS-RI). The molar mass distribution fortwo samples of EHEC with degrees of substitution (DS;average number of hydroxyl groups in the monomer unitsubstituted with ethyl) 0.9 (E481-02506) and 0.7 (EHMO),are shown in Fig. 5A and B, respectively. For the EHECwith DSethyl = 0.7 (EHM0), the molar weight of thehydrolysis products after being treated with Cel7B andCel12A were 83,000 and 102,000 g mol�1, respectively,which are values significantly lower than those for theintact EHM0 (Mw = 359,000 g mol�1). However, a higher

degradation was observed for the sample with DSethyl =0.9 (E481-02506) (Fig. 5B). The Mw decreased from772,000 (intact sample) to 57,000 and 67,000 g mol�1, forproducts obtained by hydrolysis with the same enzymes,respectively. The action of endoglucanase on caboxy-methylcellulose (CMC) also decreased the molecularweight when samples with DS 0.6 and 1.2 were submittedto enzyme degradation (0.2% solution) for 92 hr at 45jC[46]. For the sample with DS 0.6, the molecular weightdecreased from 249,000 to 10,700 g mol�1. The significantdecrease observed by the authors in the above value, incomparison to the sample with DS 1.2 in which the Mw

changed from 248,000 to 72,000 g mol�1, indicated that theendoglucanase accessed more easily the sample with lowDS. However, for EHEC, the greater effect was observedfor the sample with DSethyl 0.9, suggesting that the access ofthe enzyme to the internal (1–4)-h-D-glucosidic linkageswas more favorable. These opposite effects indicated that amore heterogeneous distribution of substituents occurredin the sample with DSethyl 0.9.

The effects on the viscosity were also described asbeing associated with enzymatic degradation processes.For example, the enzymatic degradation of caboxymethyl-cellulose (CMC) by endoglucanase significantly decreasedthe intrinsic viscosity [46]. For samples with DS 0.6 and 1.2,the viscosity decreased from 750 to 8.4 mL g�1 and from690 to 108 mL g�1, respectively. As was pointed out above,

Figure 5 Molar mass distribution of (A) intact E481-02506, E481-02506 hydrolyzed by Cel7B, and E481-02506 hydrolyzed byCel12A; (B) intact EHMO, EHMO hydrolyzed by Cel7B, and EHMO hydrolyzed by Cel12A. (From Ref. 43.)

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the viscosity values indicate that the endoglucanaseaccessed more easily the sample with a low degree ofsubstitution. Chitosan and substituted chitosans (methylpyrrolidinone chitosan, N-carboxymethyl chitosan) alsoshowed a rapid decrease in viscosity when samples weresubmitted to degradation in solutions with wheat germlipase [47].

IV. THERMAL DEPOLYMERIZATION

As was pointed out in the above sections, the depolymer-ization processes of polysaccharides are normally studiedin terms of hydrodynamic properties such as intrinsicviscosity ([g]) and average molecular weight (Mw). Ingeneral, both properties decreased during thermal, hydro-lytic, and oxidative processes and are affected by pH,heating temperature range (reaction temperature), process-ing time, mechanical shear, solvent quality, polysaccharide

concentration, and experimental conditions. For structur-ally related series of polysaccharides, the chemical reac-tions of degradation are associated with changes inenthalpy and entropy, which can be described by kineticparameters such as Ea (activation energy) and A (pre-exponential factor) derived from the Arrhenius equation[(Eq. 2)]. Depolymerization processes are commonly de-scribed by equations that consider that the chain scission ofthe glycosidic linkage of polysaccharides follows a pseudofirst-order reaction [48,49]. The rate constant will be relatedto the molecular weight through Eq. (1), which indicatesthat the inverse of the molecular weight increased linearlywith the depolymerization time and, in consequence, therate constant (k) can be obtained from the slope of thecorresponding plot.

The degradation rate (k) can also be described in termsof the intrinsic viscosity by Eq. (4), which was derivedcombining Eq. (1) and the Mark–Houwink–Sakuradaequation ([g] = KM a), where K and a are constants for a

Figure 6 Apparent viscosity of 1% solutions of thermally degraded chitosan chlorides with degree of acetylation (*) 0.02, (5)0.16, and (.) 0.35 vs. degradation time at (A) 60jC, (B) 80jC, (C) 105jC, and (D) 120jC. The degradation time for A and B isgiven in days, while for C and D it is given in hours. (From Ref. 48.)


given system. In Eq. (4), [g]t and [g]0 are the viscosities attime t and 0, respectively

1

g½ �1=at

�1

g½ �1=a0

¼k

MmK1=a

� �

t ð4Þ

For processes such as thermal- or acid-catalyzed degrada-tion, the rate constant can be obtained by plotting thedifferences between the inverses of the viscosities vs. thedegradation time. For a given system, the degradation ratesat different temperatures can be used to determine thekinetic parameters through the Arrhenius equation [Eq.(2)].

Polysaccharides such as dextran [50], chitosan [11],chitosan chloride [48], N-succinyl chitosan [26], agarose[24,49], and n-carrageenan [24,49], are examples in whichthe thermal depolymerization was followed by measuringthe apparent and intrinsic viscosity. For example, theviscosity behavior of 1% solutions of thermally degradedchitosan chloride [48] at four temperatures and threeacetylation degrees is shown in Fig. 6. An exponentialdecrease was initially observed for all the systems studied

independently of the temperature or acetylation degree.With the extended period of time, the viscosity decreases ata lower rate as a consequence of the chain scission. At thesame time, a more accentuated decreased was observedwith the increase in temperature and degree of acetylation.The degradation rates were determined by the authorsfrom the slopes of the D1/[g](1/a) vs. t plots (Fig. 7),considering Eq. (4). The values increased with both acety-lation degree and temperature. For example, at 120jC, kincreased from 120 � 10�6 to 1450 � 10�6 hr�1 when theacetylation degree increased from 0.02 to 0.35, respec-tively. At the same time, for an acetylation degree of 0.35,k increased from 3.5 � 10�6 to 1450 � 10�6 hr�1 whenthe temperature increased from 60jC to 120jC. There-fore the authors observed that regardless of the temper-ature, the degradation rates of the chitosan chloride withan acetylation degree of 0.35 was about ten times greaterthan those determined for a chitosan with an acetylationdegree of 0.02, indicated a dependence on the chemicalcomposition. On the other hand, the results also indicatedthat the effect on the viscosity was more significant as thetemperature increased.

Figure 7 Time course of thermal degradation of chitosan chlorides with degree of acetylation (*) 0.02, (5) 0.16, and (.) 0.35 at(A) 60jC, (B) 80jC, (C) 105jC, and (D) 120jC. The degradation time for A and B is given in days, while for C and D it is given inhours. (From Ref. 48.)

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Food polysaccharides such as agarose and n-carra-geenan were studied by Lai et al. [49] in terms of the kineticsof the depolymerization process. As observed in Fig. 8, theintrinsic viscosity decreased linearly with time, showing acertain dependence on temperature within a range of 75–95jC. From the slope of the plots using Eq. (4) (Fig. 9), theauthors determined the rate constants for the depolymer-ization processes, observing that the values increased bytwo- to threefold for a temperature increment of 10jC. Therate constants were in the range of 0.2–1.6 � 10�4 and 0.2–1.3 � 10�6 sec�1 for agarose and n-carrageenan, respec-tively, within the temperature range of 75–95jC. Appar-ently, the access to the glycosidic linkage was morefavorable for agarose.

The thermal degradation of cellulose was studied byPhillip [10] within the temperature range 100–200jC con-sidering the effect on the zero-order rate constant. For thedegradation reactions of cellulose in N2 + O2 and N2 +

H2O, the rate constant values were in the range of 0.6–40 �10�5 DP�1 hr�1 when the temperature changed from100jC to 200jC. However, in comparison to the degrada-tion reaction in N2 atmosphere, the above rate constantswere up two to eight orders of magnitude higher. Underthese conditions, the chain scission produced differentamounts and species of volatile low-molecular compoundsand of char. When only air was used, the mechanism ofthermal degradation of cellulose was described as havingthree stages, which included dehydroxylation (preheatingand preliminary drying for removing absorbed water),carbonization, and oxidative degradation [51]. Dependingon the heating rate, conjugate stages occurred. In theprocess of the thermal degradation of cellulose and chitinin supercritical acetone, a very slight evolution of gases was

Figure 8 Plots of intrinsic viscosity, [g], against heating timefor agarose (A) and n-carrageenan (B) depolymerized at 75–95jC. (From Ref. 49.)

Figure 9 Time dependencies on viscosity considering Eq. (4)for agarose (A) and n-carrageenan (B) depolymerized at 75–95jC. (From Ref. 49.)


observed [6]. The authors assumed that approximately98% of the cellulose is in fact liquefied.

The intrinsic viscosity and average molecular weight ofhigh methoxy pectin was evaluated at various temperatures(20–60jC) [52]. The results presented in Fig. 10A,B, forviscosity and Mw, respectively, indicated a similar behav-ior; that is, a decrease with temperature. For viscosity, theobserved effect must be associated with the molecularbreakdown of the chain and a conformational change toa more compact structure. The decrease in molecularweight was apparently due to the a h-elimination mecha-nism suggested by the authors.

The thermal stability of natural polymers, such as,sodium hyaluronate, xanthan, and methylcellulose in ni-trogen atmosphere were compared considering the kineticand thermogravimetric parameters [53]. The results indi-cated a higher thermal stability for methylcellulose, whichis a neutral polysaccharide. This conclusion was supportedby both the maximum degradation temperature and acti-vation energy. Similar behavior was described by Khomu-tov at al. [54] in studies of thermooxidative degradation ofanionic polysaccharides. They concluded that the thermalstability increased with a decrease in the charge of themacromolecule.

In the processes of thermal degradation, the kineticparameters (activation energy and pre-exponential factor)are important to analyze the reaction mechanism andthermal stability of different macromolecules. In Table 1,the activation energy and pre-exponential factor associatedwith different processes of degradation for a series ofpolysaccharides were compared. In general, and regardlessof the degradation conditions, the activation energy changewith the mass loss fraction in polysaccharides, also indi-cating changes in the reaction mechanism. For example, inthe thermal degradation of sodium hyaluronate, xanthan,and methylcellulose in nitrogen atmosphere, the activationenergy determined by the Ozawa method [55,56] changedfrom ca. 100 to 170 kJ mol�1 [53]. For the above-mentionedpolysaccharides, the activation energy in Table 1 representsaverage values. Variation in the activation energy with themass loss fraction was also observed for chitosan and amercaptan derivative of chitosan in both nitrogen and airatmospheres [57]. For these systems, the activation energyvaried from ca. 100 to 250 kJ mol�1. Another interestingaspect relating to the values in Table 1 concerns thecomparison between processes of thermal degradation thatoccurred in air (oxidative) and nitrogen atmosphere. Cel-lulose and starch showed similar activation energy values(58.5 and 50.0 kJ mol�1, respectively) when the thermaldegradation process occurred in air [58]. In nitrogen atmo-sphere, the activation energies were 242 and 474 kJ mol�1

for cellulose and starch, respectively. The extremely highvalue observed for starch in nitrogen was not expected if weconsider the similarity in the chemical structures of bothpolysaccharides. However, the authors explained it by thesmall mass loss (carbonaceous residue) observed for cellu-lose at temperatures above 400jC.

The thermogravimetric curves for sodium alginate inair showed two main stages of degradation with Tmax

(temperature of maximum degradation rate) at 240jCand 380jC [59]. The activation energy determined by theBroido method [60] were 171.4 and 174.4 kJ mol�1 for theabove two stages of degradation, respectively. These valueswere about three times higher than those observed forstarch and cellulose under the same conditions (air), indi-cating a high dependence on the structure, and probably onthe conformation, of the polysaccharides. For copolymersof sodium alginate with acrylonitrile, methyl acrylate, ethylacrylate, and methyl methacrylate, the activation energiesdecreased to values in the range 60–90 kJ mol�1 in the firststage (Tmax = 240jC) and increased up to ca. 240 kJ mol�1

in the second stage.The activation energy values presented in Table 1

clearly show some differences in magnitude when differentdegradation processes are used. For example, in the chem-ical degradation of chitosan, values in the range 80–100 kJmol�1 were determined. When chitosan was degraded by athermal process, values of 181 kJ mol�1 (in nitrogen) and160 kJ mol�1 (in air) were obtained. The significant differ-ence was apparently associated to the easier access of thechemical agents (acid) to the glycosidic bonds. Anotherinteresting point is the similarity in the activation energyobserved for most of the polysaccharides in Table 1,

Figure 10 Effect of increased temperature on the intrinsicviscosity, [g] (A) and average molecular weight, Mw (B) for ahigh-methoxy pectin in standard phosphate chloride buffer(pH = 6.8, I = 0.1 M). (From Ref. 52.)

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submitted to thermal degradation in nitrogen atmosphere.The values varied from ca. 100 to 150 kJ mol�1, suggestingthat the mechanism of degradation was, generally, byrandom scission of the chain. This conclusion was consis-tent with the large number of different products (residueand volatile products) detected in the degradation of poly-saccharides which is partially discussed in the next section.The activation energy values of cellulose (242 kJ mol�1)and starch (474 kJ mol�1) determined by thermal degra-dation in nitrogen atmosphere were in apparent discrep-ancy with most of the polysaccharides in Table 1.

V. PRODUCTS OF DEGRADATION

Products of degradation in polysaccharides have beendetermined mainly in systems submitted to thermal andpyrolytic degradation or chemical degradation at hightemperatures [33,51,53,66–75]. In this section, some exam-ples concerning these processes are analyzed.

Cellulose has been one of the most extensively studiedpolysaccharides not only in terms of the processes ofdegradation, but also in terms of the reaction products ofdegradation and pyrolysis under different conditions. Forexample, the amount of water evolved from cellulose

heated isothermally at 250jC was determined throughthermogravimetry-Fourier transform infrared (TG-FTIR)techniques (Fig. 11A) [51]. The water is produced in a greatamount in comparison with other products as shown inFig. 11A. Another significant effect was observed whencellulose was heated to 170jC in air (oxygen) (Fig. 11B)[51]. The water evolution was observed to be much higherin air than in nitrogen, because under these conditions,hydroperoxide groups are formed, which can dissociate togive hydroxyl radicals. The water is formed from hydroxylradicals by hydrogen abstraction. A detailed mechanism ofcellulose degradation was described by Scheirs et al. [51]considering the water evolution from cellulose after heat-ing to different temperatures. Up to ca. 300jC, a dehydra-tion process occurred that corresponded to the evolution ofphysical and chemical water. As suggested by the authors,the evolution of chemical water occurs by intramolecularelimination (from carbons 2 and 3 of the monomeric unit)with the formation of anhydrocellulose (enol or ketoforms). At the same temperature, cross-linking by etherlinkages due to the intermolecular elimination between thehydroxyl groups may occur. The degradation at temper-atures higher than 300jC induced elimination of waterfrom the carbon 6 forming a vinylene group. Other reac-tions such as ring rearrangement, which apparently oc-

Table 1 Activation Energy and Pre-Exponential Factor for Polysaccharide Depolymerization

Polysaccharides E (kJ mol�1) A (min�1) Degradation process Reference

Cellulose 58.5 (air) 3.4 � 1016 Thermal 5883.7 (air) 3.4 � 109 Thermal 61

242.(N2) 6.3 � 1020 Thermal 58189–199.(N2) — Thermal 62

D-glucose 174. (N2) — Thermal 62Cornstarch 50. (air) — Thermal 58

474.(N2) 2.0 � 1042 Thermal 58Sodium alginate 171.4 (air) — Thermal 59Xanthan 130 — Thermal 53Sodium hyaluronate 135 — Thermal 53Methycellulose 140 — Thermal 53Chitosan 87.1 — Nitrous acid 12

130 — Acid hydrolysis 6392 — Alkaline hydrolysis 64

181.(N2) 1.6 � 1014 Thermal 57160.(Air) 4.8 � 1012 Thermal 57

Chitosan chloride 109 — Thermal 48Chitin 124 — Acid hydrolysis 65n-Carrageenan 105 5.3 � 1012 Hydrolyze, pH 2 27

113 2.5 � 1013 Hydrolyze, pH 2 28126 4.6 � 1015 Hydrolyze, pH 2 29120 3.2 � 1016 Hydrolyze, pH 2 1497.2 7.0 � 107 Water, pH 7 49

L-Carrageenan 162 1.3 � 1021 Hydrolyze, pH 2 27133 7.4 � 1017 Hydrolyze, pH 2 14

E-Carrageenan 116 1.9 � 1013 Hydrolyze, pH 2 27Dextran sulfate 103 9.7 � 1010 Hydrolyze, pH 2 27Agarose 103 7.8 � 1010 Water, pH 7 49


curred after the initial water elimination, lead to furtherwater loss producing furanic species.

Richards [66] used gas chromatography and 1H NMRtechniques to identify the pyrolysis products of filter paper(cellulose) under nitrogen at 350jC. Glycolaldehyde, for-mic acid, 1-hydroxypropan-2-one, acetic acid, and ethyleneglycol were identified as the main reaction products. Gly-colaldehyde was the major product of the pyrolysis withyields up to 9.2%. As described in the literature [67], theformation of glycolaldehyde includes two initial steps: (1)the formation of levoglucosan from cellulose and (2)glucose from levoglucosan. Reaction products, such asformic, acetic, glycolic, and lactic acids (in a total yield of30% based on cellulose), were formed by alkaline degra-dation of cellulose [33]. In the presence of 10% sodiumchloride, the yield of glycolaldehyde decreased to 4.8%, theyield of 1-hydroxypropan-2-one (4.2%) increased at thesame time, suggesting that in the presence of salt, lactone

was preferentially formed. These results were supported byprevious studies related to the pyrolysis of curdlan a(1!3)-h-D-glucan in which the presence of sodium chlo-ride favored lactone formation [68]. The degradation ofglucose in alkaline media [aqueous solution of Ca(OH)2] at100jC produced a complex mixture with more than 50compounds (saccharinic acids) [69].

Recently, Chen et al. [70] analyzed the volatile com-pounds generated by the thermal degradation of gluco-samine and N-acetylglucosamine, which are the monomerforms of chitosan and chitin, respectively. Gas chromatog-raphy and gas chromatography/mass spectrometry tech-niques were used to identify the main degradation productsafter maintaining the samples at 200jC for 30 min. Thedegradation of N-acetylglucosamine produced mainly 3-acetoamido-5-acetylfuran and 2-acetylfuran. According tothe authors, the formation mechanism for the above com-pounds basically included successive dehydration stepsthat occurred when the monomeric unit was maintainedat 200jC. Another 13 compounds were identified in thesame degradation reaction, which to include furans, pyr-idines, pyrroles, and pyrazine derivatives. Studies related tothe thermal degradation of glucosamine were previouslyreported by Chen and Ho [71]. They identified a series offuryl-substituted pyrazines.

The glucan pyrolysis produced one-, two-, and three-carbon compounds: glycolaldehyde, acetol (hydroxipropa-none), acetic and formic acids [72]. The pyrolytic behaviorfor all the glucans studied was the same [73]. This behaviorsuggests that the nature of the glycosidic linkages in a givenglucan has little effect on major reaction pathways.

The FTIR analysis of the residual products of thethermal degradation of sodium hyaluronate was describedby Villetti et al. [53]. The analysis at 280jC (the tempera-ture of maximum degradation rate) indicated that thebands associated with exocyclic groups (1150, 1079, and1042 cm�1) and NH stretching (1320 cm�1) disappearedcharacterizing the cleavage (scission) of the glycosidiclinkage in the main chain. With a temperature increase(>400jC), the scission of strong links in the backboneoccurred.

Radlein et al. [74] considered the existence of twodecomposition pathways in the pyrolysis of cellulose. Attemperatures lower than 300jC (slow decomposition), thepyrolysis reaction produced mainly char and gases, con-firming previous results described by Shafizadeh [75], whichsuggested that in the range 150–190jC, a reduction in thedegree of polymerization, elimination of water, formationof carbonyl, carboxyl and hydroperoxide groups, andevolution of carbon mono- and dioxide occurred. Practi-cally the same products were identified by TG-FTIR (Fig.11A) from cellulose heated isothermally to 250jC [51]. Asecond pathway was related to the pyrolysis at temper-atures higher than 300jC (fast decomposition). At temper-atures close to 500jC, for example, the main products ofdegradation were hydroxyacetaldehyde and levoglucosanwith yields up to 15.3% and 38.4%, respectively, dependingon the cellulose source and temperature. Other prod-

Figure 11 Water evolution from (A) cellulose as measuredby TG-FTIR technique. (B) Heating cellulose at 170jC inoxygen and nitrogen. (From Ref. 51.)

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ucts such as acetic acid, formic acid, and acetol wereidentified by the authors with yields in the range of 5–10%.

VI. CONCLUSIONS

In this chapter, three different polysaccharide degradationprocesses were analyzed in terms of effects on viscosity andthe average molecular weight. The processes clearly indi-cate a dependence on experimental conditions (pH, tem-perature, reactive agents) and, on the structure andconformation of the polysaccharides. The effects on vis-cosity and average molecular weight were quite similar ifwe consider that both decreased with the extent of degra-dation (treated time). In chemical and thermal degradationprocesses, the predominant mechanism for the chain scis-sion is the breakdown of the glycosidic bonds in themacromolecule, which occurs in a wide range of polysac-charides. In biodegradation, the local order of the macro-molecule seems to be important. As initially only one bondis hydrolyzed by enzymatic attack, unordered macromole-cules are more susceptible to hydrolysis. Secondary cleav-ages occur on one of the fragments produced by the initialscission. Enzymatic degradation does not follow any of thetypical modes (random, gaussian) of chain scission. Dif-ferently to the biodegradation, the thermal degradation ofpolysaccharides occurs by random scission of the chain asindicated by the activation energy values that are higherthan 100 kJ mol�1 for most of the polysaccharides. Simi-larly, the degradation of guar, agarose, and carrageenansby ultrasound occurs also by random scission of glycosidiclinkages [23–25]. In the degradation of cellulose and starchin the presence of air, the activation energy has been foundto be lower than 100 kJ mol�1 (58.5 and 50.0 kJ mol�1,respectively), indicating a lower thermal stability for bothpolysaccharides. In terms of the products of degradationresulting from thermal processes, polysaccharides producemainly acids and other specific compounds depending onthe structure and conformation of the macromolecules andexperimental conditions. For example, glycolaldehyde isformed in great yields from cellulose degradation. Asglycolaldehyde is formed from the glucose monomeric unit,two initial steps occur: the formation of levoglucosan fromcellulose and of glucose from levoglucosan.

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