ARTICLE pubs.acs.org/Biomac
Physical and Water Sorption Properties of Chemically Modified Pectin with an Environmentally Friendly Process Luca Monfregola,† Valeria Bugatti,‡ Pietro Amodeo,§ Stefania De Luca,*,† and Vittoria Vittoria*,‡ †
Institute of Biostructures and Bioimages, National Research Council, 80138 Naples, Italy Department of Industrial Engineering University of Salerno, 84084 Fisciano (Sa), Italy § Institute of Biomolecular Chemistry, National Research Council, 80078 Pozzuoli (NA), Italy ‡
ABSTRACT: A synthetic process was developed to modify pectin samples under solvent free conditions, obtaining pectin at increasing concentration of palmitic, oleic, and linoleic acids. The weight loss of the modified powders showed a degradation path very similar to the pure pectin, indicating that the pristine structure was preserved after the chemical modification. A decreasing mass of evaporating water on increasing the fatty acid concentration, in particular for the palmitic acid modification, indicated a reduced water sorption by the modified powders. Differential scanning calorimetry confirmed the thermogravimetric results and in addition indicated the crystallization of the lateral chains in the case of palmitic-acid-modified pectins. This result was confirmed by X-ray diffractograms of the palmitic acid samples, indicating the main crystallization of the form C, although possible orientation phenomena can be inferred. The sorption curves of either the pristine pectin or the modified samples showed a dual sorption behavior. The sorption curves were interpreted by the BET and GAB equations, both giving very similar results. Palmitic acid modification was very effective in reducing all sorption parameters, whereas in the case of oleic and linoleic acids, only at high concentrations was the hydrophobic influence detected.
’ INTRODUCTION The increase in consumer demand as well as environmental legislation, pointing to reduce the use of synthetic-oil-derived materials, has recently spurred a big research effort to explore natural materials. Indeed, recyclability and biodegradability are nowadays considered to be the most important issues for the introduction of new products. One of the most promising approaches to overcome the safety and environmental problems is the use of renewable resources for obtaining biodegradable polymers useful for various applications in medical, pharmaceutical, agriculture, drug release, and packaging fields. A variety of renewable biopolymers have been investigated for the development of biodegradable materials, helping reduce waste disposal problems, although they are not currently meant to replace traditional synthetic materials totally. Biopolymers, such as polysaccharides, proteins, and their composites, which can be obtained from agriculture byproduct, and natural resources, have been studied.112 The use of renewable polymers depends on several features, like availability, optical quality, barrier properties to water, O2, and CO2, but principally mechanical properties and structure resistance to water. Among natural polymers, pectins, being a secondary product of fruit juice, sunflower oil, and sugar manufacture, are a very good candidate for eco-friendly biodegradable materials.13 Pectins are mainly present in the primary cell wall and in the middle lamella of plants. They constitute ∼40% (dry matter basis) of the cell wall of fruits and vegetables and are complex mixtures of r 2011 American Chemical Society
polysaccharides composed of a galacturonic acid backbone (homogalacturonan or so-called smooth regions) of which variable proportions can be methyl-esterified.14 Poor water resistance and low mechanical properties, however, are limiting factors for the use as materials manufactured only from natural polymers. For this reason, several strategies have been tested to enhance water resistance and barrier properties of natural polymers such as polymer cross-linking,1518 production of multilayered films,19,20 and dispersion of inorganic impermeable particles.2125 Another more recent approach consists of chemically modifying natural polymers to modulate their properties.2629 One of the major problems related to the preparation of new materials concerns the amount of solvent needed, particularly when the time for its large-scale production comes. In general, industrial chemical processes take into account the recovery, reuse, and recycling of the solvent. It has been estimated that the energy required to recover and recycling the solvent is ∼50% of the total used in chemical processes.3033 We have employed our recently published synthetic strategy to modify chemically pectins under solvent-free conditions, limiting the number of chemicals and reaction steps involved, and, as a consequence, their environmental impact.34 In particular, we performed the esterification of the pectin OH groups by using Received: March 17, 2011 Revised: May 9, 2011 Published: May 10, 2011 2311
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Scheme 1. Synthetic Strategy of Pectin Functionalization
several fatty acid anhydrides. Palmitic acid, oleic acid, and linoleic acid were chosen to functionalize the polysaccharide; modified pectins were obtained according to Scheme 1. For each chosen fatty acid, different amounts of the corresponding anhydride were employed to react with the pectin to obtain compounds characterized by different percentages of fatty acid substitution. We obtained modified pectin samples by using renewable resource via a new process. The new materials are characterized by the presence of a hydrophobic part that can enlarge the application field of pectins, improving their water resistance. In this Article, we present a study of the structural and physical properties of the new materials to evaluate the influence of the chain saturation of the fatty acids (saturated, one, and two double bonds for palmitic, oleic, and linoleic acids, respectively) and the extent of modification able to modulate the pectin properties for water sorption.
’ EXPERIMENTAL SECTION Materials. The apple peel Pectin was purchased from Fluka. It is a powder sample with high molecular weight (30 000100 000 g/mol) and a high degree of esterification (7075%) on a dry basis. The fatty acids and all solvents were purchased from Sigma-Aldrich. Synthesis of Modified Pectin Samples. By using an agate mortar, 30 mg of pectin was manually milled with 10, 20, and 40 mg of the appropriate fatty acid anhydride in the presence of K2CO3 (0.1 equiv) to obtain the different pectin-derived materials characterized by an increasing substitution percentage. The reaction mixture obtained was heated in oil bath at 160 °C for 1525 min. After cooling at room temperature, the final crude product was washed with dicloromethane. The obtained solid was dissolved in water, and 0.5 N HCl was titrated into the final solution until a neutral pH was reached. This solution was then dialyzed (membrane cut off 60008000) for 1 day in Milli-Q water and finally lyophilized to give the desired product.
Table 1. Substitution Percentages Calculated for Each Pectin Derivativea fatty acid
substitution (%)
1a
palmitic
1.8
2a
palmitic
9
3a
palmitic
90
1b 2b
oleic oleic
3b
oleic
1c
linoleic
2c
linoleic
3c
linoleic
3.5 9.3 35 3.5 9.3 35
a
In the sample codes, the letters a, b, and c refer to palmitic, oleic, and linoleic substitution, respectively, and the numbers 1, 2, and 3 refer to an increasing percentage of substitution.
The substitution percentages for each obtained compound were calculated by nuclear magnetic resonance (NMR) analysis, according to the previously published procedure34 and are reported in Table 1. All modified pectin samples were analyzed by FT-IR spectroscopy.34,35 The bands relevant for the structural organization are: pectinpalmiteate (1a3a): FT-IR (cm1): 3450 ν(OH), 2934 and 2852 ν(CH), 1747 ν(CdO ester), 1624 νas(COO), 1444 νas(COO), 1149 ν(COC)glycosidicbond,ring; pectinoleate (1b3b): FT-IR (cm1): 3437 ν(OH), 2924 and 2857 ν(CH), 1739 ν(CdO ester), 1636 νas(COO), 1442 νas(COO), 1146 ν(COC)glycosidicbond,ring; and pectinlinoleate (1c3c). FT-IR (cm1): 3434 ν(OH), 2927 and 2855 ν(CH), 1739 ν(CdO ester), 1639 νas(COO), 1445 νas(COO), 1146 ν(COC)glycosidicbond,ring. Film Preparation. We prepared films of pristine and modified pectins by dissolving the powders (400 mg) in 40 mL of distilled water and stirring at 70 °C for 30 min. The resulting solutions were cast in plastic Petri dishes (diameter 6 cm), dried under vacuum for 3 days at room temperature, and stored in vacuum. 2312
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Figure 1. FT-IR spectra of the pectinpalmitate samples: 1.8 (1a), 9 (2a), and 90% (3a) percentage of substitution.
Methods. The FT-IR spectra were recorded on a Jasco spectrometer. Samples were ground into a fine powder using an agate mortar before being compressed into NaCl discs. The characteristic peaks of IR transmission spectra were recorded at a resolution of 4 cm1 over a wavenumber region of 6004000 cm1. Thermal analysis (TGA) was carried out in air atmosphere with a Mettler TC-10 thermobalance from room temperature to 1000 °C at a heating rate of 10 °C/min on 10 mg samples in duplicate. Differential scanning calorimetry (DSC) data were obtained with a Mettler DSC-30 under a N2 atmosphere at a heating rate of 10 °C/min between room temperature and 400 °C on 10 mg samples in duplicate. X-ray powder diffraction measurements (WAXD) were performed with a Bruker diffractometer (equipped with a continuous scan attachment and a proportional counter) with Ni-filtered Cu KR radiation (λ = 1.54050 Å). Sorption experiments were performed on the dry films using a conventional McBain spring balance system, which consists of a glass water-jacketed chamber serviced by a high vacuum line for sample degassing and vapor removal. Inside the chamber, samples were suspended from a helical quartz spring supplied by Ruska Industries (Houston, TX) having a spring constant of 1.90 cm/mg. The temperature was controlled to 30 ( 0.1 °C by a constant temperature water bath. Samples were exposed to the water vapor at fixed pressures, p, giving different water activities aw = p/po, where po is the saturation water pressure at the experimental temperature. The spring position was recorded as a function of time using a cathetometer. The spring position data were converted to mass uptake data using the spring constant, and the process was followed to a constant value of sorption for at least 24 h.
’ RESULTS AND DISCUSSION Infrared Analysis. In Figure 1, the FT-IR spectra of the obtained pectinpalmitate samples characterized by different percentages of substitution values are shown, as an example. A careful analysis of the FT-IR spectra (Figure 1) for each pectinpalmitate derivative reveals main changes in those regions particularly related to the structure and composition of the inserted modifications. In major details, an increase in the CH stretching region (29402960 cm1) and the
Figure 2. TGA curves of the samples: (A) pectinpalmitate derivatives at different percentage of substitution (1a for 1.8%, 2a for 9%, 3a for 90%) and pectin; (B) pectinoleate derivatives at different percentage of substitution (1b for 3.5%, 2b for 9.3%, 3b for 35%) and pectin; (C) pectinlinoleate derivatives at different percentage of substitution (1c for 3.5%, 2c for 9.3%, 3c for 35%) and pectin.
appearance of a new band at 2855 cm1 was also observed; indeed, the CH stretching band intensity increases with the percentage of substitution values calculated for each compound.34 The region featuring the state of carboxylic groups (1750 1350 cm1) is also interesting. In particular, the relative intensities of the CdO esterified band (1750 cm1) and of COO symmetric stretching band (16001650 cm1) may be correlated to the degree of methylation (DM)34 that for each compound is slightly changed. Thermogravimetric Properties. Loss of weight of the powder samples in air was performed to investigate possible changes 2313
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Figure 3. Weight loss of water as a function of the fatty acid substitution for pectinpalmitate, pectinoleate, and pectinlinoleate samples. The point at 0% fatty acid substitution corresponds to the pristine pectin weight loss of 10.2%.
of thermal degradation of pectin modified with fatty acids. As a matter of fact, the thermal degradation of dry pectin is a rather complex subject, depending on composition, molecular parameters, and chemical modification.31,32 All of these parameters influence the drying procedures and the dissolving behavior, too, determining as a consequence the physical properties. In Figure 2 is shown the loss of weight of the pristine and modified pectin samples (1a3a, 1b3b, 1c3c) as a function of temperature. All of the samples present a characteristic three-step thermal degradation, typical of pectin.31,32 In the pure pectin, the first step, occurring at ∼80 °C, corresponds to the water loss and can be evaluated as 10% of the initial mass; then, it is followed by the second step, between 200 and 400 °C. In this temperature range, it has been reported that the degradation, covering ∼60% of mass loss, is primarily derived from pyrolitic decomposition. It consists in a primary and secondary decarboxylation involving the acid side group and a carbon in the ring. The third step between 500 and 700 °C corresponds to the oxidation region. In the samples of pectinpalmitate (1a3a) (Figure 2A), it is observed that the first weight loss due to the adsorbed water is somewhat lower compared with that shown by the pristine pectin, giving a first indication that the presence of the fatty acid chain drastically reduces the adsorbed water under the same storage conditions as the pure pectin. Moreover, the chemical modification produces an increase in the evaporation temperature, going from 80 °C for pristine pectin to 97 °C for 1a and 115 °C for 2a, respectively. The step of water evaporation is very much reduced in 3a, indicating that a very small quantity of water is absorbed by this sample. The second degradation stage is very similar for either the pure or the modified pectin samples, although a slightly lower mass loss and a lower midpoint temperature are observed for the modified samples. Also, the third stage is very similar for all samples. At variance, during the degradation of 3a, the second step is dominant, whereas the first and the third are almost absent, indicating the prevalence of the decarboxylation reactions due to the high percentage of palmitic acid substitution. Figure 2B,C shows the loss of weight of pectinoleate samples (1b3b) and pectinlinoleate samples (1c3c), respectively, as a function of temperature. As in the previous case, the most important difference is observed in the first step because of the water evaporation. Also, in the case of pectin modified with oleic
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Figure 4. Degradation temperature as a function of the fatty acid substitution for pectinpalmitate, pectinoleate, and pectinlinoleate samples. The point at 0% fatty acid substitution corresponds to the pristine pectin degradation temperature of 248 °C.
and linoleic acids, the weight loss is lower and decreases on increasing the fatty acid substitution percentage. In Figure 3, the weight loss of water as a function of the substitution for each of the modified pectin samples with palmitic, oleic, and linoleic acids is shown. The point at 0% fatty acid substitution corresponds to the weight loss of 10.2% of the pristine pectin. A very steep decrease in water present is observed for all samples, particularly in the case of pectinpalmitate for low substitution values, whereas a slower decrease is observed for higher substitution values. Concerning the pectinoleate samples, a less effective decrease in the water content is observed, halving the quantity of adsorbed water for the higher substitution value. The degradation temperature in the second step slightly decreases for all modified pectin samples, as shown in Figure 4, where the degradation temperature is reported as a function of the fatty acid substitution. The decrease in the main degradation temperature has been already reported in the case of pectin chemical modification.30 The reduced values tend to level off at higher concentrations. Thermal Properties. In Figure 5, the DSC curves of pristine pectin, pectinpalmitate (1a3a) (5A), pectinoleate (1b3b) (5B), and pectinlinoleate (1c3c) (5C) are reported. The pristine pectin shows an endothermic peak around 100 °C due to water evaporation and an exothermic peak around 240 °C due to the main degradation step, as already reported in the TGA results and in the literature.31 The modified pectin samples show the same peaks, although with different intensity and temperature, depending on the fatty acid substitution, as already shown by the previous TGA curves. The DSC confirms all previous results of the water content and thermal degradation. As matter of the fact the evaporation water enthalpy decreases on increasing the fatty acid substitution. Interestingly, in the pectinpalmitate samples, a new peak appears around 63 °C (Figure. 5A). It is very sharp, with an enthalpic content increasing on increasing the fatty acid substitution. The peak temperature strictly corresponds to the palmitic acid melting temperature; therefore, we interpreted this peak as the melting of palmitic chains linked to the pectin. In Figure 6, the melting enthalpy ΔHm (J/g) is reported as a function of the palmitic acid substitution value. 2314
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Figure 7. XRPD patterns of pectin and pectinpalmitate samples at different percentage of substitution (1a for 1.8%, 2a for 9%, 3a for 90%).
Figure 5. DSC curves of the samples: (A) pectinpalmitate derivatives at different percentage of substitution (1a for 1.8%, 2a for 9%, 3a for 90%) and pectin; (B) pectinoleate derivatives at different percentage of substitution (1b for 3.5%, 2b for 9.3%, 3b for 35%) and pectin; and (C) pectinlinoleate derivatives (1c for 3.5%, 2c for 9.3%, 3c for 35%) and pectin.
Figure 6. Melting enthalpy (ΔHm) (J/g) as a function of the acid palmitic substitution for the pectinpalmitate samples.
To ascertain this suggestion, we performed the WAXD of the pectinpalmitate samples at different substitution percentages (Figure 7). The diffractogram of the pristine pectin shows two distinct “humps” at 12.9 and 22° of 2θ, corresponding to those observed in the literature.36 In addition to the pectin peaks, the pectin palmiteate samples show very crystalline peaks attributable to crystalline palmitic acid, which crystallizes as lateral chains on the pectin backbone. Palmitic acid shows a very complex polymorphic behavior, and seven crystal modification were identified.37 In the present case, the most stable form C is prevalently present, showing the peaks at 2.5, 5, 7.5, and 12.5° of 2θ, although small traces of other polymorphs are present; also, some indication of chain orientation can be inferred. This worth-noting result is under further investigation and will be presented in a forthcoming paper. It is worth recalling that oleic and linoileic acid are liquid at room temperature (their melting temperatures are 16 and 5 °C, respectively); therefore, the eventual crystallization could not be detected for these acids. Also, in this case, further investigation is in progress at lower temperatures to put in evidence the crystallization in their case. Water Solubility and Sorption Properties. The solubility in liquid water of the modified pectin samples containing different fatty acids chains at different substitution values, obtained as powder, was investigated with the aim to obtain films by casting from aqueous solution. In the case of pectinpalmitate, only the sample with the lower substitution percentage (1a) was water-soluble, and a film could be obtained. It is worth recalling that this is the only sample with palmitic acid for which crystallization of the fatty acid is very scarce (ΔHm = 1.9 J/g). In the other cases (9 and 90% of substitution), the fatty acid chains are well-crystallized, thus impeding total pectin dissolution in water. In the case of pectinoleate and pectinlinoleate, all powders were water-soluble, and films by casting were obtained. All samples, obtained by casting from water as smooth films with a thickness of ∼0.01 cm, were dried and exposed to water vapor at low activity, between 0.2 and 0.6. It is worth recalling that the morphology of the cast films is very different from that of the powders on which we determined the loss of water by TGA; therefore, different results can be expected. As a matter of the fact, morphology, microstructure of multiphase systems, and compatibility of the constituting components are expected to play a very important role in determining the water transport phenomena. In our systems, a hydrophilic 2315
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Figure 9. Equilibrium sorption at activity aw = 0.6 as a function of fatty acid substitution for pectinpalmitate (1a), pectinoleate, and pectinlinoleate samples.
Figure 8. Equilibrium concentration of water vapor for the samples: (A) pectinpalmitate with 1.8% of substitution (1a) and pectin, (B) pectinoleate derivatives and pectin, and (C) pectinlinoleate derivatives and pectin.
part, the pectin backbone, is linked to a hydrophobic moiety (fatty acid chains), determining transport phenomena strongly depending on the distribution of the different phases and on the free volume between them. Water sorption properties of pectins have been investigated in recent years for samples of different origin,38 samples differently blended39 and composite with inorganic particles.2325 The attempt to correlate water sorption properties and molecular structure for pectin extracted under different conditions did not result in a straightforward answer because of the complexity
of the process. It is influenced by multiple factors, such as chemical composition, molecular weight, crystallinity, thermodynamic state of the amorphous component, Tg, free volume, and many others not easily recognizable and measurable. With the purpose of possible applications, the comparison between pristine pectin and composite or chemically modified pectin samples is easier because it can directly indicate the improvement of sorption properties or its contrary. In Figure 8AC, we show the equilibrium concentration, Ceq dry basis (g/100 g) of water vapor as a function of the water activity, aw = P/Po, for the pristine pectin and the modified pectin samples with palmitic, oleic, and linoleic acid, respectively. The sorption curve of the pure pectin follows the classical dual sorption behavior:40 at low activity (up to aw = 0.2), a rapid increase in vapor concentration, followed by a linear dependence indicates that besides the normal dissolution process the sorption of the polar solvent occurs on preferential sites, in which the molecules are adsorbed, immobilized, or both. It is generally assumed that these specific sites on the matrix have a finite capacity. They can be points in which a hydrogen bond is formed or frozen voids are formed in the structure. When the preferential sites are occupied, the isotherm becomes linear because of the normal vapor dissolution. At higher activities, the presence of water molecules determines the plasticization of the matrix, and we observe a transition in the curve, followed by an exponential increase in vapor concentration. At activity a = 1, the pectin is dissolved in the water. The sorption curve of 1a (pectinpalmitate 1.8% of substitution) shows the same behavior as the pristine pectin, that is, a rapid increase at aw = 0.2, a linear range, and a more than linear increase at activities >0.4. All values of equilibrium concentration for each activity are much lower than those of the pristine pectin, indicating the effectiveness of introducing a hydrophobic molecule into the backbone. The sorption curves for pectinoleate and pectinlinoleate show the same dual sorption curves, too. However, in these last two cases, the sorption values are consistently higher than the pristine pectin for samples 1b2b and 1c2c (pectinoleate and pectinlinoleate with 3.5 and 9.3% of substitution). Concerning the sample 3b, it reaches values of sorption equal to pectin at activity aw = 0.6, whereas the sample 3c reaches lower values for each water activity. The behavior is compared in Figure 9, where the equilibrium sorption at activity aw = 0.6 is reported as a 2316
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Table 2. Parameters Derived from eqs 1 and 2a BET
GAB
Vmb gr/100gr
R
Cg
Vmg gr/100gr
k
pectin 137
0.040
0.99
69.5
0.042
0.98
0.99
1a
0.007
0.98
13.2
0.007
1
0.98 0.98
Cb
13.2
R
1b
140
0.114
0.99 134
0.119
0.57
2b
139.9
0.075
0.99
29.4
0.103
0.54
0.99
3b
1.1
0.043
0.98
1.1
0.043
1
0.98
3.1
0.040
0.98
3.1
0.043
0.95
0.97
0.0005
0.97 300
0.0006
0.9
0.97
2c /3c
600
a
Figure 10. Interpretation of the experimental water content as a function of water activity, according to the models of eq 2
function of the kind and the substitution value of the employed fatty acids. The decreasing equilibrium sorption of palmitic acid is noteworthy compared with the other cases, where a higher substitution percentage of fatty acid is needed to reduce the water content. Hydrophobic behavior and free volume are the two effects mainly influencing the water sorption. Whereas in the case of palmitic acid the first is prevalent, in the oleic and linoleic acids, at low substitution level the free volume effect prevails on the level of sorption. The more rigid chains of oleic and linoleic acids create much more free volume, contrasting and limiting the hydrophobic effect of aliphatic chains, allowing the water molecules to cluster into the structure. Only for higher substitution values does the hydrophobic effect show its increasing influence and we observe a decreasing water sorption. The experimental data were interpreted by the models following the BET and GAB equations.4042 The BET model is widely believed to give the best fit to the data at water activity aw of up to 0.6. It is based on the equation ðcb 1Þ 3 aw aw 1 ¼ þ Vmb 3 cb ½ð1 aw Þ 3 m Vmb 3 cb
ð1Þ
The equation used in the GAB model is the following ðcg 1Þ 3 aw aw 1 þ ¼ Vmg 3 cg 3 k ½ð1 k 3 aw Þ 3 m Vmg 3 cg
ð2Þ
In the previous equations, m is the dry basis moisture content, aw is the water activity, Vmb and Vmg are the monolayer moisture content, cb and cg are the surface heat constant, and k is the GAB correction factor. In Figure 10, we show the results of interpreting the experimental data with the proposed models, and in Table 2, the BET and GAB constants are presented with the respective correlation coefficients, R, obtained after approximation of the experimental data using eqs 1 and 2. We can notice a good agreement of the two models, giving very similar values for the monolayer content of water, that is, Vmb and Vmg. Moreover, the values for the pristine pectin are very similar to those reported in literature.4042 The large effect of palmitic acid modification in reducing the water sorption, even for low substitution values (compound 1a, 1.8% of substitution), is worth noting.
In the sample codes, the letters a, b, and c, refer to palmitic, oleic, and linoleic substitution, respectively, and the numbers 1, 2, and 3 refer to an increasing percentage of substitution.
Oleic and linoleic acid modifications do not decrease the sorption properties at low substitution value; on the contrary, sorption increases very much for low substitutions (1b, 2b, and 1c) and afterward begins decreasing (2c, 3b). The very different influence of the three fatty acid chains on the water sorption can be explained by considering opposite effects: hydrophobicity of the introduced modifiers, possibility of water molecules to form hydrogen bonds with the pectin, and free volume. The different phases formed by oleic and linoleic acids, linked to the pectin backbone, both reduce the possibility of hydrogen bonds and create a large free volume, also considering that oleic and linoleic acids are liquid at room temperature. Only when the hydrophobic molecule concentration overcomes a critical value does the sorption start decreasing very rapidly. At variance, in the case of palmitic acid, already at 1.8% of substitution, we observe a strong decrease of sorption. It is worth recalling that this acid, linked to the pectin backbone, is ordered in a crystalline structure that on the one hand reduces the free volume and on the other hand shields the backbone thus reducing the penetration of the water molecules.
’ CONCLUSIONS We have prepared chemically modified pectin samples under solvent free conditions, obtaining samples at increasing substitution percentages of fatty acids with an environmentally friendly process. Palmitic, oleic, and linoleic substitution were compared to enlighten the influence of chain saturation on the investigated physical properties. The characterization was performed on the one hand to ascertain that the pristine structure was preserved and on the other hand to detect the improvement of properties, in particular, the water sorption. The weight loss of the modified powders showed a degradation path very similar to the pristine pectin, indicating that the pristine structure was preserved after the chemical modification. A decreasing mass of evaporating water on increasing the fatty acid substitution, in particular, for the palmitic acid modification, indicated a reduced water sorption by the modified powders. DSC indicated the crystallization of the lateral chains in the case of pectinpalmitate samples. This result was confirmed by X-ray diffractograms of the palmitic acid samples, showing the main crystallization of the form C, although possible orientation phenomena could be inferred. 2317
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Biomacromolecules The sorption curves of either the pristine pectin or the modified samples showed a dual sorption behavior. At low activity (up to aw = 0.2), a rapid increase in invapor concentration, followed by a linear dependence, indicates that besides the normal dissolution process the sorption of the polar solvent occurs on preferential sites, in which the molecules are adsorbed, immobilized, or both. These sites were interpreted as points in which the water molecules can bind the pectin through hydrogen bonds or frozen voids in which the water molecules can be immobilized. It was found that the rigid chains of oleic and linoleic acids create much free volume not allowing a decrease in water sorption, despite their hydrophobicity. The sorption curves were interpreted by the BET and GAB equations, both giving very similar results. Palmitic modification was very effective in reducing the sorption parameters, whereas in the case of oleic and linoleic acids, only at high concentrations was the hydrophobic influence detected.
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ39-089-964114. Fax: þ39-089-964057. E-mail: vvittoria @unisa.it (V.V.). Tel:þ39-081-2534514. Fax: þ39-081-2536642. E-mail:
[email protected] (S.D.).
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