Effects of storage period and heat treatment on phenolic compound composition in dried Citrus peels (Chenpi) and discrimination of Chenpi with different storage periods through targeted metabolomic study using HPLC-DAD analysis

Dried Citrus peels, known as Chenpi in Chinese medicine, are a traditional medicine for the treatment of indigestion and inflammatory syndromes. In this study, we evaluated the effect of storage periods (1-year vs. 3-year) and heat treatment (90 min vs. 3 h at 120 °C) on the total phenolic content (TPC) and bioactivity (anti-oxidant activity) of Chenpi. It was found that the long-term stored Chenpi had a higher TPC and superior 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity levels compared to the regular stored Chenpi, and that heat treatment increased both TPC and bioactivity. Subsequently, we developed and validated a high performance liquid chromatography with diode-array detection (HPLC-DAD) method to determine individual phenolic acids. Eleven phenolic compounds were determined in different Chenpi samples. Concentrations of total phenolic compounds were higher in long-term stored Chenpi and heat treatment raised the levels of those in regular stored Chenpi. In further study, a targeted metabolomic approach was applied to discriminate Chenpi with different storage periods. Two different phenolic acid fractions (free and ester) from the regular and long-term stored Chenpi were analyzed using the developed HPLC-DAD and the data were used in principal component analysis (PCA) on the HPLC-DAD peak areas of the 11 phenolic acids. Two principal components (PC1 and PC2) accounted for 87.1% of the variation between the regular and long-term stored Chenpi. In a two-dimensional plot of PC1 and PC2, the samples divided into four groups: two storage periods and two fractions.