| One of characteristics of TCM is that their properties and bioactivities were altered bythe processing in Chinese materia medica (Chinese herbology). Rehmannia (Dihuang) is oneof the herb, which mostly present this characteristic, although its mechanism remainsunknown. From appearance, fresh rehmannia is light yellow and cooked rehmannia turnsto dark brown color or total black, which indicated a browning reaction. The color changecould be caused by the Maillard reaction, which is a chemical browning reaction between thenucleophilic amino groups of amino acid, peptides, proteins and the carbonyl group of thereducing sugars. Maillard reaction occurs during the heating processing of rehmannia andproduces a key Maillard reaction intermediates-5-hydroxymethyl-furfural (5-HMF), whichhas been used as quality control indicator of rehmannia.By analyzing the amino acid composition of FR, RR and CR, respectively, the resultshowed that there was no significant difference on total sugar content among FR, RR and CR,while on the reducing sugar content, CR (36.33%) was much higher than RR (7.97%) andFR (5.37%). Along with FR change to CR, a large number of generated reducing sugarcontribute to Maillard reaction.Analysing sugar content of fresh rehmannia (FR), RR and CR, respectively, the resultshowed that alkaline amino acids content significantly reduced, especially the content of freearginine (FR0.442, RR0.063, CR0.006), and total lysine content (FR0.219RR0.119, CR0.029). This phenomenon often occurs in processing, such as Isatis tinctoria. It waspresumed that alkaline amino acids, mainly free-arginine and reducing sugar occurredMaillard reaction, producing black color low molecular weight products. These Maillardreaction products combined with proteins or peptides by lysine residues, and forwardnon-enzymatic glycosylation modification.Keeping FR at100℃and60℃respectively, observe its reducing sugar content, freeprotein content, free arginine content,5-HMF content and Rehmannia extract420nmabsorbance changes. The results showed that FR kept in100℃, its reducing sugar contenincreased form less than10%to the maximum (47.4%) in day1, and then gradually reduced to close to20%(day7). At60℃, its content increased from the initial6%(day1) to about22%(day7). And rehmannia invertase showed the greatest activity (188.49U) at100℃,while only20.00U,10.00U at60℃,40℃respectively. On free protein content change,FR kept in100℃, its content increased gradually to the first5days to reach maximum, andthen gradually reduced. At60℃, the value fluctuated, remained at2%. On free argininecontent changes, FR kept in100℃, its content sharp declined in the1st day, and can not bedetected in day3. At60℃, the rate of decrease was more slowly, also can not be detected inday6. On5-HMF content changes, FR kept in100℃, its content sharp increased from0.02to0.321in day1, and then stabilized at0.408. At60℃, its content increased more slowly,from the1st day of the0.021gradually increased to the7thday od0.386. On the change of420nm absorbance, FR kept in100℃, the value gradually increased from0.1(day1) to0.7(day3), and then gradually reduced. At60℃, its absorbance value fluctuated among0.15.These results confirmed that Maillard reaction occurred during the processing.Maillard reaction products with its unique molecular structure, show a strongantioxidant activity. Keeping FR at100℃and60℃respectively, observe its antioxidantactivity by DPPH spectrophotometric assay. The results showed that FR kept in100℃, theanti-oxidation activity in the first two days to reach a maximum value of94.7%, and thendecreased gradually to the first7days of65.4%, close to the performance of the activity ofCR; at60℃, the value fluctuated between30%and50%.The presented work here keen to provide more detailed evident for rehmannia qualitycontrol standard and optimize the processing conditions. |
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