| Rhubarb(Dahuang in Chinese),belongs to the family Polygonaceae,genus Rheum,has been employed as a traditional Chinese medicinal material for a long time.Rheum officinale,R.palmatum,R.tanguticum,R.rhaponticum,R.ribes,R.franzenbachii,and R.rhabarbarum have been reported to possess many pharmacological active substances that are used to treat diseases.The petioles of R.rhabarbarum,R.ribes,R.rhaponticum are well-known vegetables in some regions of the world.Edible parts of these rhubarb can be processed as salad,jam,or jellies in the food industry.Three rhubarb,Rheum officinale,R.palmatum,and R.tanguticum,have been included in all previous versions and the latest edition in 2020 of Chinese Pharmacopeia.As same as R.officinale and R.palmatum,the underground parts(roots and rhizomes)of R.tanguticum have been primarily used as a medicinal part of this plant.The overground parts of R.tanguticum,stem(the inflorescence stem accurately),leaves,petioles,and flowers,are less utilized and usually discarded.The petioles of R.tanguticum have been reported to contain protein,crude fat,crude fiber,18 amino acids,and some mineral elements.In the production area of R.tanguticum in Sichuan Province,local people have the habit of eating the tender stems and petioles directly or making them as processed foods such as rhubarb-honey and rhubarb-pickle.Based on this folk application and active ingredients that may be present in stems,the present study was conducted to analyze the content of nutritional components and active secondary metabolism components,and evaluate the antimicrobial activity and antioxidant activity with prepared solvent extracts.The main aim of this study was to explore the material basis that the inflorescence stem can be eaten safely as food and possess potential bioactivity on antibacterial,antifungal,and antioxidant.Through the evaluation of antibacterial,antifungal,and antioxidant activity of stem extracts,the study can lay some important foundation for the development and utilization of the inflorescence stem which is the rich by-product resource of R.tanguticum production.The main research contents,used methods,and results of this study are listed as follows:(1)Content determination of chemical components in the inflorescence stem of R.tanguticumThe total protein,crude fat,crude fiber,vitamin C,ash,and moisture contents were analyzed using the methods published by the Association of Official Analytical Chemistry(AOAC).The results showed that the inflorescence stem of R.tanguticum contained a higher content in crude fiber content(89.14 %)and vitamin C(16.96 %).The content of total protein and crude fat was 8.49 % and 1.34 % respectively.The contents of free and total anthraquinone in the sample were analyzed by High-Performance Liquid Chromatography.The analysis results showed the total anthraquinone content was 0.694 mg/g which was consist of 0.160 mg/g aloe-emodin,0.351mg/g emodin and 0.183 mg/g chrysophanol and the free anthraquinone content was 0.240mg/g which was consist of 0.021 mg/g aloe-emodin,0.182 mg/g emodin and 0.024 mg/g chrysophanol and 0.013 mg/g emodin methyl ether.Rhein acid was absent in both free and total anthraquinone contents.The extract also showed a higher value of total phenolic content(4.138 mg/g gallic acid equivalent)as compared to the total flavonoid content(0.022mg/g).(2)Antioxidant activity analysis with different solvent extractsThe crude extracts of seven solvents(water,55%,75%,and 99.5% ethanol and methanol)were prepared by using the Soxhlet extraction and antioxidant activity was analyzed using the DPPH radical scavenging activity method for selecting the desired solvent.The results showed that the yield of crude extracts prepared with different solvents was from 7.6% to12.3%.A significant difference in DPPH radical scavenging activity(RSA)was found among different solvent extracts.99.5% methanol extract showed the highest DPPH RSA with 81.4% followed by 75% methanol with 68.5% and 99.5% ethanol with 52%,and the lowest RSA was found in water extract only with 21%.(3)Establishment of a suitable level for extraction conditions in single factor experimentThe crude yield and the antioxidant activity of crude extracts were further analyzed by performing a single factor experiment using an ultrasonic extractor to determine the effect of extract time,temperature,and power on crude extract yield and DPPH RSA.The results showed the highest DPPH RSA in three factors was recorded at 1 h,50 ℃,and 53 KHz respectively.However,there was no significant difference of DPPH RSA among the choosing extraction power(0,35,53 KHz)and temperature(50,55,and 60℃).However,there was a significant difference of DPPH RSA between 1 h and 2 h(p<0.05)but no significant difference between 1 h and 3 h,and between 2 h and 3 h.(4)Orthogonal array test to select optimal factors/conditionsAn orthogonal array test was performed to optimize the selected conditions(power,time,and temperature)based on the single factor experiment.It was revealed that the extraction time had a greater impact on both the antioxidant activity and the crude yield than extract temperature and ultrasonic power.The optimized extraction conditions were at a temperature of 55℃ with ultrasonic power 53 KHz for 3 hours.(5)Determination of DPPH RSA of the extract prepared with optimized conditionsThe DPPH RSA of the extract prepared with optimized conditions was detected at different concentrations.The results showed that the DPPH RSA of extract had a concentration-dependent effect.The highest activity(91.96%)of the 99.5% methanolic extracted sample was recorded at 2.5 mg/m L and the lowest activity,69.82%,was recorded at 0.5 mg/m L.(6)Antimicrobial activity evaluation of extractAntibacterial activity of stem extract against five bacteria(Escherichia coli,Staphylococcus aureus,Xanthomonas campestris,Xanthomonas oryzae,and Pseudomonas solanacarum)was tested by using the paper disc diffusion method and agar well diffusion method.Antifungal activity of stem extract against phytopathogenic fungi,Sclerotinia sclerotiorum,was tested by using the mycelium growth rate method.At 5 mg/m L concentration tested,petroleum ether extract and n-hexane extract showed a larger zone of inhibition against all the tested bacteria than the other four used solvent extracts(water,methanol,ethyl acetate,and ethanol)in the paper disk method.Methanol and ethanol extract only showed a small inhibition zone against Escherichia coli with 7.6 and 7.3 mm respectively.With the agar well diffusion method,none of the extracts showed an inhibition effect against the tested bacteria.All five solvents(acetone,petroleum ether,ethyl acetate,n-hexane,and ethanol)extracts at 0.5 mg/m L concentration tested showed very weak antifungal activity against Sclerotinia sclerotiorum,in which the inhibitory rate of ethanol and ethyl acetate extracts was only 16.05% and 14.97% respectively.In summary,the results of this study confirm that the inflorescence stem of R.tanguticum contains some rich nutritional components and bioactive secondary metabolism components that are beneficial for human health as edible parts or processed products.The results of bioactivity evaluation in this study also showed that the inflorescence stem of R.tanguticum could be developed natural antioxidant and antibacterial agent as a potential resource.This may possibly provide a feasible solution for non-medicinal parts utilization. |
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