FT-IR to study the plasticization by glycerol and water of a high methoxyl-pectin edible film developed for carrying L-(+)-ascorbic acid

PÉREZ C.D., DE’NOBILI M.D., FLORES S.K., GERSCHENSON L.N., ROJAS A.M.

Córdoba, Córdoba

Congreso; III Congreso Internacional de Ciencia y Tecnología de los Alimentos; 2009

Abstrac An edible film based on high methoxyl (HM) pectin was developed to support L-(+)-ascorbic acid (AA) for natural antioxidant protection of foods. Glycerol was added as plasticizer, which acts by interfering polymer-polymer interactions, producing flexible materials. At the same time, presence of water after film storage at constant relative humidity (RH: 33.3, 57.7 or 75.2%) and 25ºC, contributed to plasticization. FT-IR was used to investigate glycerol and water interactions with the polymeric network. Films were made through casting technology. A film forming solution was obtained by dissolution of HM pectin in water at high shear rates, followed by glycerol, potassium sorbate (antimicrobial) and AA. The hot solution was poured onto polystyrene plates, dried, and film samples were equilibrated at one of the mentioned RH. Films made only with HM pectin, either with or without glycerol, were also considered after equilibration at 0% or 33.3% RH. FT-IR spectra were recorded on a Nicolet 8700 (USA) spectrometer equipped with a diamond attenuated total reflection (ATR) device, by recording reflectance (%) through 64 scans, between 4500 and 525 cm-1. Typical wavenumbers of bands corresponding to pectin polysaccharide were obtained as reported in the literature, like that attributed to C=O stretching of the esterified carboxyl group at 1749 cm-1 and the broader band manifested, at 33.3 or 57.7%-RH, by the carbonyl stretching of the carboxylic (non-esterified) group at 1615-1650 cm-1, which was coherent with HM pectin. Fingerprint patterns (1200 to 900-850 cm-1) developed the characteristic peaks of the polygalacturonic backbone at 1020 and 1105 cm-1, mainly due to C-H stretching in the carbohydrate backbone: 1145, 1105, 1014 and 952 cm-1. From all results obtained, it was suggested that stable hydrogen bonds may be formed between glycerol plasticizer and O atom in the C-O covalent bond of pectin polymers, showed by the band broadening at 1020-1022 cm-1 as well as by the band shift of the OH-stretch from » 3363 cm-1 to » 3320 cm-1. Small but characteristic peaks showed by pectin-alone-films at 1052 and 1085 cm-1 were masked by broadening of band at 1020 cm-1, when glycerol or other compounds of the edible film system were also present in the formation of the pectin polymeric network. At the same time, it was hypothesized that water coming from storage RH, penetrated the network through interaction with glycerol, which must be previously H-bonded with polymer chains. In the present work, water may prefer to interact with other water molecules when it is in the vicinity of the apolar methyl groups of the HM pectin, which would lead to a lower hydration time (t) of the pectin methyl groups. However, as water finds glycerol molecules next to pectin polymers, it would easily hydrate glycerol, which probably presents high t. As a conclusion, it is suggested that water seemed to interact directly and positively with film pectin macromolecules, at least, at the carboxylic groups, and negatively through the methyl groups, leading the latter effect to a higher frequency of water-glycerol interaction in the vicinity of apolar groups.