A general view of the FT-IR spectra of pectin-linoleate (compounds 1a,b) pectin-oleate (compounds 2a,b) and pectin-palmitate (compounds 3a,b) are shown in Figure 1, Figure 2, Figure 3. For comparison purposes a spectrum of the native pectin is also reported in each figure [20].

The obtained spectral data were analyzed by comparing the following characteristic regions: O–H stretching (3100–3600 cm⁻¹); C–H stretching (2800–3000 cm⁻¹); carboxylic group stretching region (1200–1800 cm⁻¹) [21,22,23] and the “fingerprint” region (under 2000 cm⁻¹) which reflects the monosaccharide composition of the analyzed pectin [24,25].

The infrared analysis of the modified pectin samples revealed that the main changes occurred in those regions accounting for the chemical modifications introduced into the polysaccharide. In major details, a decrease of the O–H stretching band (in the 3500–3200 cm⁻¹ range) is related to the decrease of the hydroxyl pectin groups upon reaction with the fatty acid anhydrides. On the other hand, an increase in the C–H stretching region (2931–2934 cm⁻¹ for pectin-linoleate; 2926 cm⁻¹ for pectin-oleate; 2919–2926 cm⁻¹ for pectin-palmitate) and the appearance of well defined band (2854 cm⁻¹ for pectin-linoleate; 2849–2855 for pectin-oleate; 2847–2850 cm⁻¹ for pectin-palmitate) was also observed.

The region featuring the carboxylic groups (1750–1350 cm⁻¹) is the more interesting. Concerning the FT-IR spectrum of the pectin, it shows three peaks at 1747, 1649, 1452 cm⁻¹ in the carboxylic groups region assigned to the C=O stretching of methylated carboxyl groups, and to anti-symmetric and symmetric stretching modes of COO⁻, respectively [20]. For each sample the appearance of a new band in the C=O ester stretching region, which overlaps with the pectin methyl ester band, accounted for the esterification of the fatty acid employed (1700–1705 cm⁻¹ for pectin-linoleate; 1695–1700 cm⁻¹ for pectin-oleate; 1698–1705 cm⁻¹ for pectin-palmitate) [13]. Also the band in the COO⁻ antisymmetric stretching region (1620–1630 cm⁻¹) was partially overlapped. Moreover, for each synthesized pectin derivative an increased intensity of the band at 1410–1433 cm⁻¹ was observed. In this regard, it is worth remembering that this region does not account, in a quantitative manner, for the pectin free carboxylic groups [26].

Other changes are registered in the C–O stretching region at 1300–1100 cm⁻¹; in particular new bands at 1207 cm⁻¹ and 1142–1133 cm⁻¹ were observed in the spectrum of each sample. Concerning the “fingerprint region” (1100–1000 cm⁻¹), no significant changes were observed for any analyzed sample. The 850–600 cm⁻¹ region contains new bands which are related to the introduced fatty acid chains [27,28].

All of the samples present a characteristic three-step thermal degradation, typical of pectin [29,30]. In pure pectin, the first step, occurring at about 80 °C, corresponds to the water loss and can be evaluated as 12% 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 about 60% of mass loss, is primarily derived from pyrolytic 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 modified samples of pectin with linoleic (1b), oleic (2b), and palmitic (3b) acids (Figure 4), it can be observed that the first weight loss due to the adsorbed water is consistently 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 higher values for all the three modified pectins. The water evaporation step is greatly reduced in these samples, indicating that a very small quantity of water is absorbed. 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. The third stage, due to oxidative reactions is postponed, particularly for linoleic residues, as expected for the oxygen scavenging effect of the double bonds.

Apple peel pectin was purchased from Fluka (Buchs, Switzerland). It is a powder sample with high molecular weight (30,000–100,000 g/mol) and a high degree of esterification (70–75%) on a dry basis. The fatty acids and all solvents were purchased from Sigma-Aldrich (Buchs, Switzerland).

Reactions were carried out in a domestic 900 W (2450 MHz) microwave oven (DeLonghi, Treviso, Italy). The FT-IR spectra were recorded on a Perkin-Elmer spectrometer, model Vertex 70. Samples were ground into a fine powder using an agate mortar before being compressed into KBr discs. The characteristic IR transmission spectra peaks were recorded at a resolution of 4 cm⁻¹ over the 600–4000 cm⁻¹ wavenumber region. Thermal analysis (TGA) was carried out in air atmosphere with a Mettler TC-10 Thermobalance (Novate Milanese, Italy) from room temperature to 1000 °C at a heating rate of 10 °C/min on 10 mg samples in duplicate.

Using an agate mortar, pectin (30 mg) was manually milled with the appropriate fatty acid anhydride (30 mg) in the presence of K₂CO₃ (0.1 equiv.) to obtain the different pectin-derived materials. Samples were irradiated with microwaves (900 W) for one or two cycles of 3 min each. After cooling to room temperature, the final crude product was washed with ethyl acetate. 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 6000–8000) for one day in Milli-Q water and finally lyophilized to give the desired product. All modified pectin samples were analyzed by FT-IR spectroscopy. The bands relevant for the structural organization are: pectin-linoleate (1): FT-IR (cm⁻¹): 3434 ν (O–H), 2930 and 2854 ν (C–H), 1732 ν (C=O methyl ester), 1700 ν (C=O fatty acid ester), 1629 ν_as (COO⁻), 1436 ν_as (COO⁻), 1207 and 1133 ν (C–O); pectin-oleate (2): FT-IR (cm⁻¹): 3437 ν (O–H), 2926 and 2855 ν (C–H), 1750 ν (C=O methyl ester), 1695 ν (C=O fatty acid ester), 1621 ν_as (COO⁻), 1410 ν_as (COO⁻), 1207 and 1142 ν (C–O); and pectin-palmitate (3): FT-IR (cm⁻¹): 3440 ν (O–H), 2919 and 2847 ν (C–H), 1750 ν (C=O methyl ester), 1698 ν (C=O fatty acid ester), 1629 ν_as (COO⁻), 1433 ν_as (COO⁻), 1207 and 1142 ν (C–O).