| Low methoxyl pectin (LMP) is widely applied as thickener and emulsifier in food industry. Low methoxyl pectin forms gels in the presence of a bivalent cation (such as calcium) win a wide pH range regardless of sugar concentration. This property makes LMP useful for many applications in which high methoxyl pectin cannot be used, such as low calorie or dietetic foods. The main methods reported for the preparation of low methoxyl pectin from high methoxyl pectin are chemical or enzymatic methods. Chemical methods are accompanied by environmental pollution and depolymerization of pectin chains. Enzymatic de-esterification represents an attractive alternative to chemical de-esterification, for it is without pollution, but it is time-consuming. Therefore, there is a need to explore new methods to produce pectin with high quality and efficiency. In this study, a novel method of high hydrostatic pressure combined with enzymatic treatment (pectin methyl esterase) (E-HHP)was applied in pectin de-esterification. In order to find out the effect of enzymatic treatment assisted with high hydrostatic pressure on the properties of pectin and promote the industrial development of this technology, the physicochemical properties, molecular weight distribution and rheological characteristics of pectins de-esterified by this treatment were investigated and compared with conventional alkaline method (A-method) and enzymatic under atmospheric pressure (E-AP). The main results and conclusions are summarized as follows:(1) A single factor experiment and an orthogonal test were performed to optimize the de-esterification condition of E-HHP. The optimal conditions of E-HHP consisted of a pressure of 400 MPa, a temperature of 45 ℃ and a pressure-holding time of 15 min. Under the optimal conditions, the DE was decreased to 28.08 ± 1.39% without no significant difference compared to that of A-method and E-AP. The de-esterfication time is close to that of A-method, while only one quarter of that of E-AP.(2) There was no significant difference (p>0.05) in the galacturonic acid content of the pectins prepared by E-HHP, E-AP and A-method, while the apparent viscosity of pectins prepared by E-HHP was higher significantly than that by alkaline method (p<0.05). The viscous flow activation energy of HHP-pectin and E-pectin were lower than A-pectin, indicating lower temperature dependence and better processability. It was also showed that HHP-pectin had much higher viscosity and could induce a rapid and homogeneous gelation, leading to the formation of gel with better viscoelastic properties.(3) From the profile of molecular weight distribution and viscosity average molecular weight, E-HHP and E-AP showed no degradation action on the pectin molecular, whereas A-method hydrolyzed pectin significantly. Although, the chemical groups and structural composition were not influenced notably by different de-esterification methods, the monosaccharide composition of HHP-pectin differed from that of E-pectin significantly (p<0.05), HHP-pectin contained relatively lower amounts of neutral sugar side chains. It could be used to explain the difference of rheological and gelling properties in these three pectins.(4) Reaction rate of pectin methyl esterase catalyzed de-esterification was indeed elevated under high hydrostatic pressure. From the fluorescence spectrum and SDS-PAGE, pectin methyl esterase was degraded by high hydrostatic pressure. The effects of HHP on the physicochemical properties were further investigated and the results showed that no significant change happened in pectin conformation. |
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