Moderate amount of lecithin improving rheological properties of low-methoxyl apple pectin gel

Abstract: Pectins were traditionally applied in food industry as well as other industries such as pharmaceutical industry as typical gelling, emulsifying, thickening and stabilizing agents. Lecithin, a highly compatible lipid and an important constituent of biological membranes, is applied to the pectin gel as a colon-specific carrier in recent years. In order to investigate the gel properties of pectin, the effects of soybean lecithin on the rheological properties of low-methoxyl apple pectin gels were studied in this paper. The weight average molecular weight of pectin obtained by the size exclusion chromatograph method was 1.27×105, the number average molecular weight was 3.77×104, and thus the molecular weight distribution index was 3.36. In the presence of calcium ion, adding different proportions of lecithin to low-methoxyl apple pectin, the storage modulus (G') and loss modulus (G") were recorded through the observation during the gelation process of the pectin, when the plate was cooled from 80 to 26℃ at a certain rate with the rheological technology. Then the structure development rate (SDR) analysis method was adopted to explore the effects that the presence of lecithin brought to the gelation system in the gelation process of pectin. The results showed that the addition of lecithin had significant effects on SDR and G'. With a low amount of lecithin (<0.4%) added into the pectin gel, both SDR and G' had no considerable changes in comparison with the low-methoxyl pectin calcium gel without lecithin; with a higher amount (0.4%), a significant increase in SDR and G' suggested a faster gel development rate and a stronger strength of gel than the sample without lecithin, which indicated that adding lecithin could promote the formation of the pectin gel. However, with a much higher amount (>0.4%), SDR and G' tended to decrease. The average structure developing rate and the end storage modulus showed the same tendency as above; when adding 0.4% lecithin, the average structure developing rate increased to 4.2 Pa/s and the end storage modulus increased to 2950 Pa; both of them were significantly higher than the sample without lecithin (P<0.05). Loss tangent curves of all gel samples showed the same trend that the loss tangent began to decrease while the temperature dropped. But when adding 0.4% lecithin, the loss tangent of pectin calcium gel was the lowest, which indicated the best elasticity. Then a non-isothermal kinetic model, which described the storage modulus change during the gelation induced by cooling dispersions, was adopted for analysis. Pectin gelation could be described by a two-step process corresponding to 2 different temperature ranges. Adding suitable amount of lecithin (0.4%) resulted in more obvious difference between the pectin gelation processes of the 2 temperature districts as well as the values of activation energy, which were 290.6 kJ/mol for high temperature district and 67.1 kJ/mol for low temperature district. Finally, after the vacuum freeze drying treatment, the scanning electron microscopy showed that the three-dimensional network structure of pectin calcium gels added with 0.4% lecithin was characterized by greater uniformity and compactness. The research results will provide a theoretical basis and reference for the application of the system of lecithin pectin gel.