荔枝果皮乙醇提取液抑菌及抗氧化作用的研究

劉文群,韓 偉,聶永建,楊 光

南昌大學食品科學與技術國家重點實驗室,南昌 330047

Introduction

Litchi(L itchi chinesisSonn.),a member of the Sapindaceae family,originated in china and is famous as a subtropical evergreen fruit tree in south of china.Unfortunately,in present litchi processing industry,all litchi pericarp are dealt as rubbish.It makes huge waste of resource.And it is“environment-unfriendly”to throw this natural resuorse into the rubbish bin.As a matter of fact,litchipericarp could be used as an important resource for trea tment,such as curing diarrhea,flooding, eczema and so on[1].And many different litchi pericarp extracts can inhibit the mycelium growth of some plant pathogen,asAspergillus niger,Colletotrichum m usae[2]. By the TLC and chemical color reaction,there are phenolic compounds and flavonoids in litchi pericarp extracts[3].

At present,a lot of research work about litchi pericarp involved in the browning phenomena and how to inhibit it[4].Very few reports concern with the wide-spectrum antimicrobial activity and free radical scavenging activity of litchi pericarp.The study in this paper will deal with the above-mentioned problem in order to provide some basal data to promote the exploitability of litchi pericarp.

Materials andM ethods

Materials and reagents

Litchiwas purchased from local super market;1,1-diphenyl-2-picrydrazl(DPPH)was purchased from Sigma;All other chemicalswere homemade and analytical grade.

The tested microbes:Escherichia coli,Proteus vulgaris, Staphylococus aureus,Aspergillum niger,Penicillium sp..Theywere isolated and provided by the Lab ofMicrobiology of Nanchang University.Culture medium: potato glucose medium(PDA);beef-protein medium.

M ethods

Litchi pericarp extracting

The pericarp was peel off from the fresh litchi,60℃drying,grinding.This crushing pericarp was extracted by ethanol for 24 h,filtered,and prepared several content class,namely 40,20,10,5 and 2.5 mg/mL by corresponding solvents(Fig.1).

Fig.1 L itchi pericarp extracts(Left:95%ethanol extract ing;Right:sterile water extracting)

Inhibition of m icrobial grow th

Freshly grown cultures of the tested microbes were made up to suspensions(106-107 mL-1),the latter, 0.2 mL,were coated to beef-protein medium(PDA) plate.For growth inhibition test as shown Fig.2,the activity of the tested microbeswas deter mined by observing inhibition zones,after incubation at 37℃(28℃) for 24 h to 48 h[5].

DPPH free radical scavenging test

The scavenging ofDPPH by litchi pericarp extractswas analyzed by a modification ofmethod utilized by Chung HLet al[6,9].1ml of litchi pericarp and 1ml of freshly prepared DPPH solution(0.2 mM in ethanol)were mixed and allowed to react for 30 min.Blank samples contained deionized water.The scavenged DPPH was then monitored by measuring the decrease in absorbance at 517 nm.The scavenging abilitywas defined as follows:[1-A517sample/A517blank]＊100%.

Hydroxy radical scavenging test

The hydroxy radical(OH·)scavenging activity was analyzed by a modification of method utilized by Liu JW et al[7].The sampling procedure used a 50 mM Tris-HCL(PH7.4)as the solvent.0.5 mL of 5 mM phenanthroline and 0.5 mL of 7.5 mM FeSO4solution were placed into a test tube.After shaken up immediately,2.5 mL sample solution and 0.5 mL of 1%H2O2were added to the mixture.Afterwater-bath for 60 min at 37℃,the scavenging rate was monitored in absorbance at 536 nm.

Results and D iscussion

Ant im icrobial activity of the litchi pericarp extracts The 95%ethanol extracts of litchi pericarp showed the antimicrobial activity againstEscherichia coli,Proteus vulgaris and Penicillium sp.,with inhibiting diameters from 8.6 to 18.7 mm,but no antimicrobial activitywas detected againstStaphylococus aureusandAspergillum niger(Table 1).In particular,the antifungal effect of litchi pericarp extracts was stronger than the bacteriostasis.At the same time,Escherichia coliandPenicilliumsp.were the best sensitive to 5 mg/mL extract,insteadProteus vulgarishad connected to the content of extract,this finding implies thatmaybe there are different principles of inhibition between 2 classmicroorganism.

Inhibition effect against Penicillium sp.of the concentration of ethanol and extractBecause antifungal activity of litchi pericarp extracts is considerable,and this Penicilliumsp.is one of the important factors affecting shelf life of citrus produced in Jiangxi Province,we explored the effect factors to inhibit against Penicillium sp.in detail(Table 2).

In the exper iment changing the concentration of ethanol (0%,25%,50%,75%,95%,100%),it was found that antifungal activityof the extractsof high concentration solvent was stronger than that of low concentration’s.Whether the effective components inhibiting against Penicillium sp.are related to the solvent polarity or solubility,wasworth further studying.In addition,after the contrast data on the concentration of ethanol and content of extract were analyzed,we found that inhibition zone was the biggest at 5 mg/mL 95%ethanol extract of litchi pericarp,with diameter up to 18.7 mm.

Table 1 ant im icrob ial effect of 95%ethanol extract of litchi pericarp

Table 2 ant im icrob ial effect of extracts of different concentration and content

Fig.2 Ant im icrob ial effect of ethanol extracts of litchi pericarp

Scavenging of 1,1-diphenyl-2-picrydrazl(DPPH) free radical

The results of scavenging ofDPPH by 95%ethanol extracts of litchi pericarp are showed in Fig.3.The scavenging of DPPH was monitored by measuring absorbance decrease at 517 nm.As shown in Fig.3,the extracts of all tested content demonstrated high levels of scavenging ability.W ith the increase of extracting content,DPPH scavenging rate rised at first and lowered later;once the content of litchi pericarp cl imbed above 20 mg/mL,the scavenging rate reached a peak,namely 96.1%.

Fig.3 DPPH scavenging ability of the 95%ethanol extracts of litchi pericarp

Scavenging of hydroxy radical

The hydroxy radical scavenging activity of the test sam-ple is shown in Fig.4.Five extracts,including 2.5,5, 10,20,40 mg/mL,had antioxidative activity of different degree,shaped an“S”curve similarly.The scavenging rates climbed,with the increasing content of extracts,by 1.7%,12.9%,78.6%,97.5%and 100% respectively.

Fig.4 OH· scavenging activity of the 95%ethanol extracts of litchi pericarp

D iscussion

In this study,we deter mined that the ethanol extracts of litchi pericarp inhibited growth of some familiar microbes,especially againstPenicillium sp.,which let us have more thoughts about exploring litchi pericarp.According to the statistics[8],the grossweight of rot fruits exceed 80,000,000 t annually in our country.Short shelf life has been one of the important factors affecting fruits suppliesworldwide.As is known,Jiangxi Province is the main manufacturing location of oranges;therefore,exploring natural antistaling agent againstPenicilliumsp.,such as litchi pericarp,is necessary and urgently needed.In the succedent tests about scavenging DPPH radical and OH·,the considerable scavenging rates indicate the strong antioxidative activity of the extracts of litchi pericarp,which suggested litchi pericarp become the cheap natural antioxidative product possibly.However,as the prel iminary study to this resource, further research will be needed to determine the inhibition mechanis m,the chemical composition of the effective components and how to obtain it a lot.

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