Physicochemical and gel properties of tangerine peel pectins in microwave extraction processing

Abstract: Because tangerines' peels have a large amount of pectin, tangerines' peels are ideal materials to extract pectin. In order to increase the utilization rate of tangerines' peels and improve the yield and quality of pectin from tangerines' peels, 5 types of tangerines' peels were adopted as raw materials to extract pectin by the microwave-assisted method, which were Rough-peel mandarin, Lu tangerine, Sha Tang tangerine, Kumquat tangerine, and Gong tangerine. The physicochemical properties and gel properties of pectin were studied, including pectin yield, galacturonic acid content, esterification degree, methoxy content, gel hardness and viscosity. Galacturonic acid content represents its purity and esterification degree or methoxy content decides pectin's type that contains high ester pectin and low ester pectin. The correlation between physicochemical properties and gel properties was studied and the main factors were determined by the principal component analysis. Finally, by the response surface central composite method, the optimization of microwave extraction parameters was performed, which contained microwave power, microwave time, solid-to-liquid ratio and pH value. The results showed that 5 kinds of tangerines' peel pectin was all high ester pectin because the degree of esterification was greater than 50% and the methoxy content was over 7%, and the physicochemical properties and gel properties had significant difference (P<0.05). Pectin yield of Lu tangerine and its galacturonic acid were the highest, 12.36% and 78.61%, respectively; their physicochemical properties and gel properties showed significant correlation (P<0.05), and what was more, pectin yield, methoxy content and degree of esterification had significant positive correlation with gel hardness (P<0.05). Gong tangerine had the largest gel hardness and viscosity and Lu orange was ranked in the middle. Principal component analysis indicated that the methoxy content and pectin yield were the main influencing factors in the principal component 1 and 2 and the methoxy content determined pectin type, and the factor loadings of methoxy content and pectin yield were 0.4554 and 0.6324, respectively. Therefore, pectin type and yield played a decisive role in the choice of tangerines' peels. Further, as the yields gradually increased, its effect on the microwave-assisted extraction processing was more significant and the transformation of soluble pectin was more complete. Lu tangerine was very appropriate as the material to extract pectin because of its highest yield and ideal gel properties. By the response surface method, the optimization results showed that the microwave extraction parameters were the solid-to-liquid ratio of 1:26 g/g, the pH value of 1.30, the microwave power of 440 W, and the microwave time of 62 s, and the optimal pectin yield was 16.00%. Therefore, the microwave-assisted extraction method could improve the ability of pectin to escape from tangerines' peels. The increase in microwave power within a certain range was conducive to cell tissue damage and pectin dissolution, but the power over the range led to the degradation of pectin. In conclusion, this study provides theoretical support for the extraction and application of pectin from tangerines' peels.