Effect of Different Drying Processes on Active Ingredients of Rheum officinale Baill.

Yiming DING, Tong SHANG, Yue SHI, Boyang PENG, Ting ZHAO,2, Yuan ZHANG,2, Li LI, Shengli WEI,2*

1. School of Chinese Pharmacy, Beijing University of Chinese Medicine, Beijing 102488, China; 2. Engineering Research Center of Good Agricultural Practice for Chinese Crude Drugs, Ministry of Education, Beijing 100102, China; 3. Beijing Chinese Medicine Hospital Affiliated to Capital Medical University, Beijing 100010, China; 4. Beijing Institute of Chinese Medicine, Beijing 100035, China

Abstract [Objectives] To determine the drying process of different functional rhubarbs. [Methods] The contents of tannins, bianthrone, free anthraquinones and combined anthraquinones in medicinal rhubarb were determined by HPLC. The results were analyzed by two-way analysis of variance. [Results] Under different drying conditions, the contents of tannins, bianthrone, free anthraquinones and combined anthraquinones in medicinal rhubarb were significantly different. Taking the tannins content as the index, the optimum drying condition of the rhizome is 1 cm thick, dried at 60 ℃, the root is cut 1 cm thick, and dried at 30 ℃; the bianthrone content is used as an indicator, the rhizome is optimally dried. The condition is to cut 1 cm thick, dry at 60 ℃, the root is cut to 5 cm thick, and dried at 40 ℃; the free anthraquinones content is used as an index, and the optimum drying condition of the rhizome is 3 cm thick and dried at 50 ℃. The root is cut to a thickness of 3 cm and dried at 30 ℃. The combine anthraquinones content is used as an indicator. The optimum drying conditions for the rhizome are 5 cm thick, dried at 40 ℃, and the root is cut to 5 cm thick and dried at 70 ℃. [Conclusions] Different functional components of rhubarb have different directional processing methods. The drying process can reduce the drying temperature or increase the thickness of the slice, and the directional processing of the diarrhea-type rhubarb can be processed. The drying process can be carried out by increasing the drying temperature or reducing the thickness of the slice directed processing of heat-clearing and purging-fire rhubarb.

Key words Rheum officinale Baill., Drying process, Active ingredient

1 Introduction

Rhubarb (Radix et Rhizoma Rhei) refers to dried rhizome or root ofRheumpalmatumL.,RheumtanguticumMaxim. ex Balf andRheumofficinaleBaill.[1]. It is currently widely used in the fields of medicine and pharmaceutical chemicals. Rhubarb is a commonly used bulk Chinese medicinal material, and is also a exported medicinal herb well-known in China and abroad[2-3], with high economic value. It can be developed into an industry for poverty alleviation[4]. The preliminary research has found that based on the content and proportion of active ingredients, rhubarb can be divided into different functional types,i.e., based on the content of anthraquinones, bianthrone and other components, rhubarb is divided into purgative, heat-clearing, blood-cooling and detoxifying types, thus providing reference for the oriented cultivation, processing and rational use of rhubarb[5-8].

Processing in producing area is an important link in the production and quality formation of medicinal materials[9]. Rhubarb is thick and difficult to dry naturally, and its drying requires a long time. In the processing process, it is easy to be spongy, moldy, deteriorated, discolored,etc[10], causing the loss of active ingredients. The 2015 edition of thePharmacopoeiaofthePeople’sRepublicofChina(hereinafter referred to asChinesePharmacopoeia) stipulates that rhubarb should be cut into petals or segments, and dried in strings or dried directly. At present, there have been reports on the effect of drying process on the quality of rhubarb. Hu Huijuanetal.[11]found that when dried at 50 ℃, the content of sennoside A inR.tanguticumMaxim. ex Balf was the lowest. Tang Wenwenetal.[12]concluded that the optimal drying temperature for rhubarb was 45 ℃. The study results of Fang Qingweietal.[13]show that the content of combined anthraquinones inR.officinaleBaill. was the highest when it was dried at 60 ℃ for 15 h. The above-mentioned research lacks the distinction of the medicinal parts and the investigation of the slice thickness, which is an important factor for the processing in production area, and lacks a certain degree of systematicness.

In this study, taking the contents of tannins, bianthrone, free anthraquinones and combined anthraquinones as evaluation indices, the effect of medicinal part, slice thickness and drying temperature onRheumofficinaleBaill. was investigated, in order to explore the optimal drying process for rhubarb with different function, and lay the theoretical and technical foundation for the oriented drying of rhubarb decoction pieces.

2 Materials and methods

2.1 Materials

2.1.1Instruments and equipment. The main instruments and equipment used were high-performance liquid chromatograph (1200, Agilent, USA), electronic analytical balance (accurate to 0.000 1, ME104, METTLER TOLEDO, Switzerland), electronic analytical balance (accurate to 0.000 01, BT 25S, Sartorius, PRC), ultrasonic cleaner (KQ5200D, Kunshan Ultrasonic Instrument Co., Ltd., PRC), electric heating constant temperature water bath (DK-98-IIA, Tianjin Taisite Instrument Co., Ltd., PRC), rotary evaporator (RE52CS, Shanghai Yarong Biochemical Instrument Factory, PRC), high-speed universal pulverizer (FW100, Tianjin Taisite Instrument Co., Ltd., PRC) and electric heating constant temperature drying oven (101-A , Tianjin Tongli Xinda Instrument Factory, PRC).

2.1.2Reagents and drugs. Gallic acid standard (lot: Y19M8C36143), catechin standard (lot: P20O7F23087), sennoside A standard (lot: P01S8F42887), sennoside B standard (lot: Y23M8Y17128), rhein standard (lot: T30A8F42628), chrysophanol standard (lot: T23A8F42188), emodin standard (lot: C18F8Q29652), aloe-emodin standard (lot: T28D6F8264) and physcion standard (lot: T26A8F34784) were purchased from Shanghai Yuanye Biotechnology Co., Ltd., and their purity was all ≥ 98%. Methanol and acetonitrile were of chromatographic grade (Fisher). Other reagents were of analytical grade. The water used was Watsons distilled water.

2.1.3Medicinal material. Fresh rhubarb was sampled from three-year-oldR.officinaleBaill. cultivated in Pingwu, Sichuan from March to April. It was identified by Professor Wei Shengli of Beijing University of Chinese Medicine. In order to eliminate individual differences and maintain the uniformity of the samples, 15R.officinaleBaill. plants with uniform growth were selected. Based on the center of the cross section ofR.officinaleBaill, the rhizome or root was cut longitudinally into 5 pieces, 1, 3 and 5 cm long in random. The slices were mixed and then dried at 30, 40, 50, 60 and 70 ℃, respectively (Table 1).

2.2 Methods

2.2.1Content determination. The content of combined anthraquinones was determined referring toChinesePharmacopoeia, and the contents of free anthraquinones, bianthrone and tannins were determined with the methods developed previously[14], and the determination results were analyzed using two-way analysis of variance.

The content of combined anthraquinones with purgative effect referred to the sum of the contents of rhein, emodin, physcion, aloe-emodin and chrysophanol. The content of bianthrone referred to the sum of the contents of sennoside A and sennoside B. The content of free anthraquinones with heat-clearing effect referred to the sum of the contents of rhein, emodin, physcion, aloe-emodin and chrysophanol. The content of tannins with blood-cooling and detoxifying effect referred to the sum of the contents of gallic acid and catechin[4].

2.2.2Data analysis. SPSS 24.0 statistical software was used for data analysis. The means of each index were subjected to significant test using two-way analysis of variance. When the differences were significant, the contribution of drying temperature and slice thickness to the variation of each functional component was determined by comparing the partial Eta square (ηP2). When 0.2 ≤ηP2< 0.5, it indicates a low degree of influence; when 0.5 ≤ηP2< 0.8, it indicates a moderate influence; and whenηP2≥ 0.8, it indicates a high degree of influence.

Table 1 Dried samples and processing methods

3 Results and analysis

3.1 Effect of drying temperature, slice thickness and their interaction on active ingredients in rhizome ofRheumofficinaleBaill.The mass fractions of the 4 functional components in rhizome ofR.officinaleBaill. dried by different processes (Table 2) were subjected to an interactive two-way analysis of variance with drying temperature and slice thickness as the control variables (Table 3). The results show that the contents of tannins, bianthrone, free anthraquinones and combined anthraquinones in rhizome ofR.officinaleBaill. were significantly different under different drying temperatures, different slice thicknesses and their interaction (P<0.01), indicating that drying temperature, slice thickness and their interaction had significant effects on the contents of tannins, bianthrone, free anthraquinones, and combined anthraquinones in the rhizome ofR.officinaleBaill.

Table 2 Mass fractions of four active ingredients in rhizome and root of Rheum officinale Baill. dried by different processes

The contents of tannins, bianthrone, free anthraquinones and combined anthraquinones in rhizome ofR.officinaleBaill. that had different slice thickness and was dried at different temperature were compared. The results show that the change trends of tannins, bianthrone, free anthraquinones, and combined anthraquinones contents were not obvious, which may be related to the greater interaction between drying temperature and slice thickness. The content of tannins was highest in sample 4 (60 ℃, 1 cm) (10.43 mg/g) and lowest in sample 2 (70 ℃, 3 cm) (6.96 mg/g). Taking the content of tannins as the index, the optimal drying process for rhizome ofR.officinaleBaill. was as follows: drying temperature of 60 ℃ and slice thickness of 1 cm. The content of bianthrone was highest in sample 4 (60 ℃, 1 cm) (10.66 mg/g) and lowest in sample 2 (70 ℃, 3 cm) (5.39 mg/g). From the perspective of content of bianthrone, the optimal drying process for the rhizome ofR.officinaleBaill. was drying temperature of 60 ℃ and slice thickness of 1 cm. Sample 8 (50 ℃, 3 cm) had the highest (2.57 mg/g) and sample 7 (50 ℃, 1 cm) had the lowest free anthraquinones content (1.18 mg/g). Taking the content of free anthraquinones as the index, the optimal drying process for rhizome ofR.officinaleBaill. was as follows: drying temperature of 50 ℃ and slice thickness of 3 cm. Combined anthraquinones had the highest content (14.91 mg/g) in sample 12 (40 ℃, 5 cm) and the lowest content (8.10 mg/g) in sample 2 (70 ℃, 3 cm). Considering the content of combined anthraquinones, the most suitable drying process for rhizome ofR.officinaleBaill. was drying temperature of 40 ℃ and slice thickness of 5 cm.

Table 3 Effect of drying temperature and slice thickness on the contents of the four active ingredients in rhizome of Rheum officinale Baill. n=3)

3.2 Effect of drying temperature, slice thickness and their interaction on active ingredients in root ofRheumofficinaleBaill.The mass fractions of the 4 functional components in root ofR.officinaleBaill. dried by different processes (Table 2) were subjected to an interactive two-way analysis of variance with drying temperature and slice thickness as control variables (Table 4). The results show that there were significant differences in the contents of tannins, bianthrone, free anthraquinones and combined anthraquinones in root ofR.officinaleBaill. under different drying temperature, different slice thickness and their interaction (P<0.01), indicating that drying temperature, slice thickness and their interaction had significant effect on the contents of tannins, bianthrone, free anthraquinones and combined anthraquinones in root ofR.officinaleBaill.

Table 4 Effect of drying temperature and slice thickness on the contents of the four active ingredients in root of Rheum officinale Baill. n=3)

4 Conclusions and discussion

The effect of drying temperature and slice thickness on the four functional components ofR.officinaleBaill. is different. In terms of the contents of tannins, bianthrone and free anthraquinones, the interaction between drying temperature and slice thickness was the main factor influencing the rhizome and root ofR.officinaleBaill. From the perspective of content of combined anthraquinones, drying temperature showed the greatest effect on the rhizome and slice thickness showed the greatest effect on the root ofR.officinaleBaill.

Rhubarb is a typical Chinese herbal medicine with multiple functions. The active ingredients related to its purgative effect are mainly bianthrone and combined anthraquinones. Bianthrone has stronger purgative effect, and combined anthraquinones have weak purgative effect. Sennoside A is the active ingredient with the strongest purgative effect[15]. At the slice thickness of 3 cm, the content of bianthrone in rhubarb showed a decreasing trend as the drying temperature rose. This may be due to the fact that bianthrone and combined anthraquinones are prone to degradation during high temperature heating[16-17]. The results of this experiment are also in line with the traditional understanding that "decoction is prone to weakening the purgative effect of raw rhubarb"[18]. Under the high-temperature drying condition, the contents of combined anthraquinones in the rhizome and root ofR.officinaleBaill. were relatively low, while the contents of free anthraquinones were relatively high. This might be related to the easy degradation of combined anthraquinones into free anthraquinones during high temperature heating[19]. In this study, the change trend of tannins content was not obvious. This might be related to the large increase in gallic acid content and the decrease in catechin content during high temperature heating[20]. The effect of different drying processes on the tannins in rhubarb still needs further study.

In summary, under the process with lowered drying temperature and increased slice thickness, the purgative effect of rhubarb is enhanced; and under the process with increased drying temperature and reduced slice thickness, the heat-clearing effect of rhubarb is enhanced, thereby providing guidance for the oriented production of rhubarb and providing a theoretical basis for the oriented processing and clinical application of other precision medicinal materials.