Quantification of Chinese yam processing methods based on pyrolysis characteristics and its relation to Maillard reaction

Chinese yam (Shanyao in Chinese, SY) as one of the representatives for Chinese medicines can be used as both of medicine and food with rich nutritional and medicinal value. Most of Chinese herbal medicines need to be processed prior to be used in clinical practice. SY was divided into Maoshanyao (Hairy Shanyao, MSY) and Guangshanyao (Smooth Shanyao, GSY) based on different processing methods at the place of origin, and it also could be processed as stir-fried SY and bran stir-fried SY to meet the different clinical use. Moreover, during the processing of Chinese herbal medicines, more complicated Maillard reaction occurs compared to food processing. Therefore, the objective of this research is to quantify the firepower of SY processing, and combined this with the relevant parameters of Maillard reaction. The MSY and GSY produced in Shanxi and Henan Provinces were chosen as the research objects. By using thermal analysis technology, we first established the correlation between pyrolysis and processing of SY and its mixtures. We also quantified the firepower of Shaoyao processing, and combined this with the relevant parameters of Maillard reaction (pH value, amino acid, and 5-HMF) and the changes in medicinal ingredients (allantoin). The SY was mainly fried with moderate-fire (190 °C−200 °C), and the starting temperatures of different SY–ingredient mixtures were (176.3 ± 5.33) °C for (honey) bran, and (205.9 ± 8.05) °C for rice. The upper limits of processing temperature were (289.9 ± 6.47) °C for (honey) bran and (298.9 ± 1.15) °C for rice. The cooking time was (10.80 ± 1.76) min for soil stir-fry, (10.31 ± 1.06) min for bran stir-fry, and (8.43 ± 0.68) min for rice stir-fry. Moreover, the pH values and the content of 5-HMF were increased (P < 0.001), while the content of glycine was decreased significantly (P < 0.001) after processing. The results verified and quantified the firepower of traditional processing of SY, and also provided scientific reference for other studies related to SY processing.