Volatile components,total phenolic compounds,and antioxidant capacities of worm-infected Gomphidius rutilus

Libin Sun,Wei He,Guang Xin,Pengju Cai,Yin Zhang,Zhiyong Zhang,Yunyun Wei,Bingxin Sun,Xiaowen Wen

College of Food Science,Shenyang Agricultural University,Shenyang 110866,China

ABSTRACT This study evaluated the effects of worm infection on the volatile components,total phenolic compounds,and antioxidant capacities of Gomphidius rutilus.G.rutilus without worms(GW),G.rutilus infected by a small amount of worms (GS; infected area ＜50%),and G.rutilus infected by a large amount of worms(GL;infected area＞50%)were investigated.The volatile components of G.rutilus were analyzed by simultaneous distillation-extraction (SDE) and headspace solid-phase microextraction (HS-SPME) using gas chromatography-mass spectrometry (GC-MS).A total of 17 and 19 types of volatile compounds were detected,including ketones,alcohols,benzene,alkenes,aldehydes,esters,acids,and alkanes.Alcohols comprised the most abundant compound in GL,GS,and GW.The relative content of 1-octen-3-ol was the highest in all mushrooms.The concentration of eight-carbon (C8) compounds relative to the total volatile compounds varied widely,ranging from 40%(GW)to 64.34%(GS)and 84.42%(GS)and to 91.59%(GL),respectively,among the three samples.The antioxidant capability and the total phenolic contents of G.rutilus were evaluated in this study.The highest total phenolic content (TPC) of 192.23 mg GAE/g was found in GL,which differed significantly(P ＜0.05)from the latter two samples,whereas the lowest value of 156.11 mg GAE/g was found in GW.ABTS radical cation scavenging activity,FRAP ferric reducing antioxidant capacity(FRAP)radical scavenging activity,and oxygen radical absorbance capacity(ORAC)were investigated to screen the antioxidant properties of extracts.The contents of total phenolic compounds and their antioxidant capacities in vitro showed significant correlations (P ＜0.01).Among the three types of samples,the phenolic compounds of GL exhibited the highest antioxidant capacity,showing the values of 0.089 mM TE/g for ABTS,0.949 mM Fe2+ E/g for FRAP,and 1.952 M TE/g for ORAC.However,regarding the total antioxidant capacity,GS exhibited the highest antioxidant capacity,showing the values of 0.002648 mM TE/g for ABTS,0.004437 mM Fe2+ E/g for FRAP,and 0.256 μM TE/g for ORAC.In conclusion,HS-SPME was more suitable for the extraction of volatile aroma components from G.rutilus.GL had the most abundant aroma components.GL had the highest TPC and antioxidant capacity compared with those of GS and GW,whereas GS showed the opposite results.Interestingly,GS was found to have the highest total antioxidant capacity in vitro.Based on these measured indicators,worm infection had no negative effect on the quality of G.rutilus.Therefore,worm-infected G.rutilus can also be consumed by humans.

Keywords:Gomphidius rutilus Worm infection Volatile components Total phenolic Antioxidant capacity

1.Introduction

Mushrooms are widely found in nature,and it has been estimated that there are about 140,000 mushroom species on earth,of which only 10% of them may be found [1].The majority of them are edible and have been used as food and medicine throughout the world [2].Due to their unique flavor and taste,mushrooms are commonly consumed as raw materials,condiments,and highgrade natural substances worldwide,especially in some oriental countries[3].Edible fungus has become an important component of human diet because ofits high nutritional value,which can be attributed to its high content of proteins,fatty acids,vitamins,and minerals but low content of calories and fat[4].In addition,mushrooms contain a variety of bioactive molecules such as phenolic compounds,organic acids,terpenes,polysaccharides,and steroids[5,6].Edible mushroom has been considered as a novel source of dietary fiber and beneficial to human health [7].Mushroom and its processed products have been widely accepted as health nutraceuticals [8-10],pharmaceuticals [11-13],and cosmeceuticals[14].

The characteristic flavor of mushrooms has been considered to be due to the presence of volatile and nonvolatile components.The aroma of mushrooms is attributed to fruiting bodies,which have been claimed to produce pleasant,odorous substances[15].Extensive evidence has demonstrated that terpenes,aromatic alcohols,aldehydes,ketones,eight-carbon compounds,and their derivatives are responsible for the characteristic flavor of mushrooms [16].It is known that linoleic acid is the precursor of 1-octen-3-ol,which is considered as an alcohol present in fungi,with a unique earthy taste and sweetness[17].The unique taste of mushroom has been ascribed to the nonvolatile components,including free amino acids and 5′-nucleotides[18].

Phenolic compounds are one of the most important groups of secondary metabolites that are derivatives of the pentose phosphate,shikimate,and phenyl propanol pathways.Based on their structure,which possesses an aromatic ring bearing one or more hydroxyl groups,phenolic compounds can range from simple molecules to complex high molecular weight polymers [19].It has been reported that the phenols present in mushroom fruit bodies significantly contribute to the antioxidant activity [4].The mechanism of their antioxidant activity involves several methods.Especially,they can act as reductants by donating electrons and protect cells against damage by neutralizing reactive oxygen species and chelating those elements(e.g.,Fe,Cu)that can generate reactive oxygen species[20].

Gomphidius rutilusis a traditional Chinese wild edible and medicinal fungus,belonging to the subphylum Basidiomycotina.It is often found beneath pine trees.The mushroom is widely distributed in China,including Liaoning,Jilin,Heilongjiang,Hebei,Shanxi,Hunan,Sichuan,and Tibet [21].The present study evaluated the polysaccharides fromG.rutilusand their antioxidant activities and immune activities in vitro [22-24].A novel fungal immunomodulatory protein fromG.rutiluswas identified,cloned,and analyzed [25].Similar to the majority of ectomycorrhizal fungi,the phenomenon by which the fruiting bodies of this fungus are infected with worms was often observed.However,G.rutilusinfected with worms did not exhibit any poisonous effect.Instead,worm infection might be related to the volatile components,total phenolic compounds,and antioxidant capacities of the mushroom—a question that remains unexplored.We had recently demonstrated that worm infection affects the texture profile and the nutritional and flavor components ofG.rutilus[26].The aim of the present study was to examine the effect of the degree of worm infection on the volatile components,total phenolic compounds,and antioxidant capacities.

2.Materials and methods

2.1.Samples and sample preparation

Fresh fruiting bodies ofG.rutilusmushrooms were collected from a local producer in Chaoyang,Liaoning Province,China.The harvested mushrooms were expediently transferred to the Food Analysis Laboratory at Shenyang Agricultural University on the second day of collection.All the wild mushrooms were identically selected in terms of the degree of worm infection.The samples were identified as follows:G.rutiluswithout worms(GW),G.rutiluswith a small amount of worms (GS; infected area ＜50%),andG.rutiluswith a large amount of worms (GL; infected area ＞50%).Subsequently,the samples were kept in a freezer at-40°C.All mushrooms from the same types were homogenized for analysis to minimize variability among individuals.

2.2.Extraction of volatile aroma components

2.2.1.Simultaneous distillation and extraction(SDE)

SDE analysis was performed as described by Du etc [27].with few modifications.A total of 500 g of fresh mushrooms was immersed in a flask containing 1000 mL of distilled water,and 40 mL of dichloromethane was added as an organic solvent in another flask.The sample was left for 15 min to maximize the enzymatic production of the flavor compounds,as reported by Hong et al.[28].Both flasks were placed in a Likens-Nickerson apparatus and heated up to their boiling points.When the two flasks started to reflux,the distillation extraction was continued for 2 h to allow the collection of the volatile components in the organic solvent.After the procedure,the extract was collected at room temperature.The flavor extract was dried overnight over sodium sulfate(anhydrous)and approximately concentrated to 2 mL at 35°C in a water bath.The extract was stored at-4°C temporarily before analysis.

2.2.2.Headspace solid-phase microextraction(HS-SPME)

HS-SPME was performed according to the method described by Ouzouni,Koller,Badeka,and Riganakos[29],with slight modifications.Approximately 20 g of freshmushroom sampleand3 gofNaCl were homogenized.The homogenate was filtered through Whatman No.1 filter paper.The filtrate was placed in a 15-mL glass vial and sealed with a plastic screwed cap equipped with a Tefloncoated,needle-pierceable septum (Supelco,Bellefonte,PA,USA).The SPME fiber was coated with 50/30 μm DVB/CAR/PDMS on polydimethylsiloxane.The vial was placed in a 40°C water bath and equilibrated for 10 min.After equilibration,the needle of the SPME holder was inserted into the vial through the septum and the fiber was exposed to the headspace of the sample for 40 min to adsorb the volatile components.

2.2.3.Gas chromatography-mass spectrometry analysis

GC-MS analysis was performed as described by Du etc.[27],with some modifications.The volatile compounds obtained by SPME and SDE were analyzed by GC-MS using an Agilent 5975C mass selective detector coupled to an Agilent 7890 A GC (Agilent,Santa Clara,CA),equipped with an HP-5MS capillary column(60 m×250 μm×0.50 μm).Helium was used as the carrier gas,and the flow rate was 1.0 mL min-1.The flow rate of the helium carrier gas was 1.0 mL min-1.The injector temperature was 250°C in the splitless mode.The GC oven temperature gradient was maintained as follows:the initial temperature of the column was 40°C(held for 2 min),increased to 180°C(held for 2 min)at 3°C min-1,and finally increased to 260°C(held for 1 min)at 10°C min-1.The transfer line temperature was 280°C.The ion source temperature was 230°C,and the MS was scanned at 70 eV over 50-450 mass range.

The compounds were tentatively identified using the NIST11.L mass spectra library.Each compound was further confirmed by comparing its mass spectra,linear retention index,and retention times with those obtained for the reference standards.

2.3.Extraction of phenolic compounds from mushrooms

Based on the previous experiments,the response surface method was used to optimize solvent extraction for the extraction of polyphenols fromG.rutilus.The optimal processing parameters were as follows:ethanol concentration 69%,the ratio of solvent and material 20 mL/g,extraction time 1.5 h,extraction temperature 70°C,and extraction times 3.Crude polyphenol solutions were dynamically adsorbed and desorbed by an AB-8 macroporous resin column[30].The eluted solution was collected and concentrated in a rotary evaporator at 40°C to remove the ethanol and then freezedried.The purified products were weighed and dissolved in distilled water to prepare sample solutions.

2.4.Determination of total phenolic content(TPC)

The TPC of the material was measured using the Folin-Ciocalteu colorimetric method as described by Wu,Guan,Yan,Lei,Xu,and Wang [31],with few modifications.Briefly,the extract (0.2 mL)used for the antioxidant activity assay was mixed with 0.5 mL of Folin-Ciocalteu reagent(Beijing Dingguo Changsheng Biotechnology,Beijing,China)and incubated for 5 min at 25°C.Then,1.5 mL of 20% sodium carbonate was added.Absorbances (at 764 nm)were recorded for the mixtures after 15 min ofincubation at 75°C.The TPC was expressed as gallic acid equivalents (mg GAE/g sample) in accordance to the standard calibration curve of gallic acid(y=0.1195x+0.0029,R2=0.9999).

2.5.Antioxidant capacity

2.5.1.ABTS assay

The ABTS free radical scavenging activity was determined following the protocols described by Z?otek,Michalak-Majewska,and Szymanowska [32],with few modifications.The absorbance was measured in an MRX microplate reader(Mutiskan Mk3)at 414 nm after 6 min of reaction in a spectrophotometer set at room temperature;distilled water was used as blank.The ABTS was expressed as Trolox equivalents(mM TE/g sample)in accordance to the standard calibration curve of Trolox,with a linear range from 0 to 0.8 mM(y1=-1.6753x1+1.4382,R12=0.9993)and from 0 to 1.5 mM(y2=-0.8383x2+1.3646,R22=0.9996).

2.5.2.FRAP assay

FRAP was determined using a colorimetric method as described by Islam,Yu,and Xu[5],with some modifications.The absorbance was measured in an MRX microplate reader (Mutiskan Mk3) at 593 nm using distilled water to replace the sample as blank.The FRAP value was expressed as Trolox equivalents (TE) in mM/ g,according to the standard calibration curve of Fe2+with a linear range from 0 to 1.5 mM (y1=0.3818x1+ 0.0908,R12=0.9992;y2=0.2681x2+0.0623,R22=0.9997).

2.5.3.ORAC assay

The ORAC assay was carried out according to the procedure established by Xi,Mu,and Sun [30],with slight modification.All the samples and reagents used in this experiment were dissolved and diluted with phosphate buffer (0.075 M,pH 7.4).Briefly,25-μL sample solutions were added to 150-μL staining solution in a clear 96-well microplate and incubated at 37°C for 30 min.Then,25 μL of oxidation working solution was rapidly added to the wells.After vigorous shaking,the microplate was placed in the multifunctional citation i5 reader (BioTek,America).The system was set in the fluorescence mode.The excitation and emission filter wavelengths were set at 485 and 535 nm,respectively,and the detection temperature was set at 37°C.The ORAC value was expressed as Trolox equivalents (μM TE/g sample) in accordance to the standard calibration curve of Trolox,with a linear range from 0 to 1 μM(y1=97804x1+165505,R12=0.9971;y2=39.787x2+10.463,R22=0.9998).

2.6.Statistical analysis

The results were presented as mean±standard deviation.Data analysis was carried out using Microsoft Office Excel 2010.Significant differences were analyzed by Duncan’s multiple range tests using the SPSS (SPSS 20.0,Chicago,IL,USA) statistical analysis software.Data with P ＜0.05 and P ＜0.01 were considered to be statistically significant.

3.Results

3.1.Fraction components extracted by SDE

The fractions of the mushrooms (Table1) [33]were compared to examine the effect of worm infection on the quality and quantity of fractions.A total of 17 compounds were detected,including ketones,terpenes,alcohols,acids,and esters.A total of 12 compounds were detected in GL,13 in GS,and 8 in GW.The major compounds identified in the fresh samples of the species were 1-octen-3-ol (38.36%-63.07%),n-hexadecanoic acid(8.47%-21.17%),9,12-octadecadienoic acid (Z,Z) (2.86%-17.12%),and acetoin (3.41%-9.29%).The total contents of C8 compounds were 40%,55.85%,and 64.34%(GW,GL,and GS,respectively).The fractions obtained in this experiment were partially high molecular weight compounds,such as linoleic acid,palmitic acid,geranyl linalool,and farnesyl acetone.

Few differences were observed among the samples.Nerolidol 1 (0.97%) was found only in GW but not in GS and GL.Some of the compounds were present only in GS,which included geranyl linalool (0.65%),3,7,11-trimethyl-2,6,10-dodecatrien-1-ol 2,6,10-dodecatrien-1-ol,3,7,11-trimethyl-(0.33%),and Octyl trichloroacetate trichloro-,and octyl ester(2.46%),2,3-Diazabicyclo[2.2.2]oct-2-ene (0.72%),9-hexadecenoic acid (0.41%),and nerolidol (0.7%)were identified in GL.

3.2.Volatile components extracted by HS-SPME

A total of 19 volatile compounds were unequivocally identified and are listed in Supplementary Table2[33,34].A total of 8 alcohols,3 aldehydes,2 esters,2 olefins,2 ketones,1 benzene,and 1 alkanes were identified.Most of these compounds have been widely reported in the literature.A total of 15 compounds were detected in GL,9 in GS,and 12 in GW.The contents of alcohols in the three samples were 86.86%(GL),76.36%(GS),and 84.98%(GW),respectively.The contents of ketones varied from 4.91%(GW)to 11.12%(GS).The total amount of aldehydes ranged from 0.91%(GW)to 1.33%(GL).The compounds detected were primarily small molecular weight compounds.

1-Octen-3-ol content was the highest among the three samples,ranging from 74.4% (GS) to 81.07% (GL),followed by 3-octanone.The contents of C8 compounds ranged from 84.42%(GS)to 91.59%(GL),which contributed to the flavor of fresh mushroom.

3.3.Total phenolic content(TPC)of mushrooms

The total phenolic contents (expressed as mg GAE/g) of the three mushroom samples are presented in Fig.1.A relatively higher content of phenolic compounds was recorded in GL (192.232±1.742 mg GAE/g),whereas the lowest content was recorded in GS (156.109±4.526 mg GAE/g) (315.244±3.084 mg GAE/g).

Table1 Relative concentrations(%)of fraction compounds of G.rutilus extracted by SDE.

Table2 Relative concentrations(%)of volatile compounds of G.rutilus extracted by HS-SPME.

3.4.Antioxidant capacity

3.4.1.ABTS radical cation scavenging activity

The results of ABTS radical scavenging activity of the three mushrooms are presented in Fig.2.GL(0.089±0.001 mM TE/g)and GW(0.084±0.002 mM TE/g)exhibited a relatively higher ABTS radical scavenging activity,whereas the lowest activity was found in GS(0.052±0.002 mM TE/g).

3.4.2.FRAP radical scavenging activity

FRAP(expressed in mM Fe2+E/g)results are presented in Fig.3.A relatively higher FRAP radical scavenging activity was found in GL (0.949±0.078 mM Fe2+E/g) and GW (0.883±0.027 mM Fe2+E/g),whereas the least antioxidant capacity was detected in GS(0.494±0.049 mM Fe2+E/g).

3.4.3.Oxygen radical absorbance capacity

The ORAC values(expressed in M TE/g)of the three samples are presented in Fig.4.Relatively higher ORAC values were detected in GL(1.952±0.544 M TE/g)and GW(1.927±0.506 M TE/g),whereas the least ORAC value was found in GS(1.462±0.411 M TE/g).

3.4.4.Antioxidant capacities of mushrooms

Three methods were used to measure the total antioxidant capacity of mushrooms.As shown in Fig.5,GS exhibited the highest ABTS (0.002648±0.0002 mM TE/g) activity among the mushrooms evaluated.This was followed by GL with a value of 0.001673±0.0003 mM TE/g and GW with a value of 0.002308±0.0002 mM TE/g.

Fig.1.The TPC in three kinds of samples(mg GAE/g sample).

Fig.2.ABTS radical cation scavenging activity of 3 samples.

Fig.3.FRAP radical scavenging activity of 3 samples.

Fig.4.ORAC radical absorbance capacity of 3 samples.

Fig.5.ABTS radical cation scavenging activity of G.rutilus.

Fig.6.FRAP radical scavenging activity of G.rutilus.

Fig.7.ORAC radical absorbance capacity of G.rutilus.

As shown in Fig.6,relatively higher FRAP values were recorded in GS(0.004437±0.0002 mM TE/g)and GL(0.003611±0.0001 mM TE/g),whereas the least antioxidant capacity was found in GW(0.002373±0.0001 mM TE/g).

Regarding ORAC (Fig.7),GS showed the highest antioxidant capacity with a value of 0.256±0.001 μM TE/g,followed by GL(0.237±0.003 μM TE/g)and GW(0.249±0.006 μM TE/g).

4.Discussion

4.1.Volatile compounds

4.1.1.Fraction components extracted by SDE

The content of C8 compounds relative to the total volatile components varied widely,ranging from 40%to 64.34%among the three samples.The content of 1-octen-3-ol in GS was more than 1.5 times that in GW.Nerolidol,one of the sesquiterpenes,has been reported to be associated with the distinctive cedar odor [35].Previous research has demonstrated that linoleic acid,a constituent of mushrooms,is a common precursor of C8 compounds,utilized by certain enzymes as the starting substrate[36].There is a view that linoleic acid and enzymes are distributed in different compartments in the cell.When the tissue is damaged,they can contact and react with each other,resulting in the generation of 1-octene-3-ol and other substances[37].Combet et al.[38]have shown that the formation of C8 compounds in mushroom tissues is proportional to the damage suffered during sample preparation.The characteristic flavor of the mushrooms may be attributed to C8 volatile compounds common to the majority of mushroom species,with differences arising due to their concentrations and particular compounds characteristic of specific strains [29].The major volatile compounds are predominantly eight-carbon-containing compounds,namely 1-octen-3-ol,3-octanol and 3-octanone,straight-chain sulfur compounds,such as dimethyl disulfide,dimethyl trisulfide,and cyclic sulfur compounds [39].However,in this study,no sulfur compounds were found.This phenomenon may be explained by the fact that sulfur compounds are highly unstable and readily decomposed at higher temperatures.

4.1.2.Volatile compounds extracted by HS-SPME

The modern solid-phase microextraction (SPME) technique is more efficient,sensitive,and highly reproducible compared with earlier technologies (steam distillation or simultaneous extraction)[40].The volatile components isolated by HS-SPME from the mushroom extracts may be formed upon tissue disruption.C8 compounds were the primary volatile compounds,including 1-octen-3-ol,3-octanol,and 3-octanone.This result was found to be consistent with other reports on the aroma components of edible fungi[41,42].

Some compounds are formed by the metabolism of polyunsaturated fatty acids through the lipoxygenase pathway,which is known to be responsible for the generation of the “green” aroma notes [43].The relative abundance of alcohols varied between 76.36%and 86.86%(GL and GS,respectively)of the total peak areas of the volatile components identified.Phenylethylene and(2Z)-2-octene-1-ol are formed from the combination reaction between unsaturated alkane radicals or between unsaturated alkane radicals and hydrogen radicals[44].Benzaldehyde,a compound associated with bitter almond sensorial notes,was found to be present in a much higher amount in GL(1.33%)[45].It has been reported that C8 compounds and other alcohols could be degraded during heat treatment.This may explain the lower content of 1-octen-3-ol in the extracts isolated using SDE,in which high temperature conditions were applied.The occurrence of alkanes,which were identified only in the extracts isolated by HS-SPME,could be due to the cracking reaction of alkoxyl radicals[46].Scalone reported that pyrazine compounds were generated at high temperatures [47].However,pyrazine compounds were not detected in the present study.

The volatile composition of wild mushrooms can be influenced by several factors such as the species,the part of the mushroom,their maturity,and the agroecosystem where they are grown[45].Eight-carbon compounds play an important role in attracting flies and mosquitoes,which are likely to help mushrooms to disseminate spores and in reproduction,known as mushroom-insect mutualistic interactions[48].

In this study,two methods were applied to extract aroma compounds from the mushrooms,and the results were detected by GC-MS.There were differences in the types and contents of compounds in the extracts isolated by the different methods.The content of C8 compounds extracted by SDE was less than that extracted by HS-SPME.The conditions of a higher temperature and a longer time of SDE may cause the oxidation of volatile compounds.The appearance of fatty acids in the fractions may be due to the extraction methods and conditions.The degree of the effects of worm infection on the C8 compounds of mushrooms extracted by HS-SPME was as follows:GL ＞GW ＞GS.The complex relationship between insects and aroma components of edible fungi may be an adaptation in mushroom growth.

4.2.Phenolic compounds

The highest TPC value of 192.23±1.74 mg GAE/g was recorded in GL,which was significantly different(P ＜0.05)from the latter two samples,whereas the lowest TPC value of 156.11±4.53 mg GAE/g was found in GW.Islam et al.[5]have reported that the TPC of 43 mushroom species ranged from 26.21(Stone ear)to 0.19 mg GAE/g(Tuckahoe).Gursoy et al.[49]found that the TPC of seven mushroom species ranged from 25.38±0.70 mg GAE/g (M.conica) to 12.36±1.21 mg GAE/g(M.deliciosa).This finding indicates that the mushroom is an important source of phenolic compounds.To the best of our knowledge,the TPC ofG.rutilushas not been evaluated yet.The Folin-Ciocalteu assay is highly sensitive for monohydric phenols,polyphenols,flavonoids,and tannins.However,the presence of some easily oxidizable substances,including sugars and amino acids,could cause interference[50].

4.3.Antioxidant capacities of phenolic compounds

4.3.1.ABTS radical cation scavenging activity

The ABTS radical cation scavenging activity of GL was the highest,followed those of GW and GS in a descending order.The ABTS radical cation scavenging activity was significantly different between GL and GS and between GS and GW (P ＜0.05).However,there were no significant differences between GL and GW(P ＞0.05).Islam et al.[5]found that the ABTS radical cation scavenging activity among the mushroom species ranged from 2.25±0.12 μM TE/g(Tremella) to 109.19±5.06 μM TE/g (Stone ear).Several factors could contribute to the observed higher values;perhaps,the phenolic compounds ofG.rutiluscontain more hydroxyl groups that could donate electrons.

4.3.2.FRAP radical scavenging activity

GL and GW showed significantly higher FRAP antioxidant activity than GS(P ＜0.05).However,there was no significant difference between GL and GW in this context (P ＞0.05).Islam et al.[5]reported that the FRAP of 43 edible mushrooms commonly consumed in China ranged from 0.09 to 39.98 mM Fe2+E/100 g.The antioxidants reduce the yellow ferric form ofiron to the blue ferrous form by donating an electron.The high reducing power might have been due to the high TPC that broke the free radical chain by donating an electron to stabilize and terminate radical chain reactions.

4.3.3.Oxygen radical absorbance capacity

GL exhibited the strongest superoxide ORAC value,which was significantly higher than that of GS(P ＜0.05).However,there was no significant difference in the ORAC values between GL and GW(P ＞0.05).Smolskait ˙e et al.[51]reported that the ORAC values of extracts varied from 104.0 (X.chrysenteron) to 461.9 μM TE/g (P.schweinitzii).

The TPC values were in agreement with the antioxidant capacity values.Furthermore,highly strong positive and significant(P ＜0.05)correlations were found between TPC and the three assays according to the Pearson’s correlation tests.The FRAP,ABTS,and ORAC values strongly correlated with the TPC values (r=0.866,0.861,and 0.818,respectively,P ＜0.01).These strong correlations confirm the antioxidant potential of the phenolic compounds present inG.rutilus.

4.4.Antioxidant capacities of mushrooms

In all the tests,the antioxidant capacity was in the following order:GS ＞GL ＞GW.GS exhibited the highest antioxidant activity(P ＜0.05),and GW showed the lowest antioxidant activity(P ＜0.05)in the FRAP and ABTS assays.However,no significant difference was detected between GS and GL in the ORAC assay.

The reaction mechanism of antioxidants to different radicals or oxidant sources is different.Therefore,a single assay cannot accurately reflect the mechanism in a complex system.Hence,three assays were used to assess the antioxidant activity of the mushroom extracts in the present study.The results showed the antioxidant capacity in the following order:GS ＞GL ＞GW.The differences may be influenced by several factors.Polysaccharides,a biologically active substance present in mushrooms,are also known to contribute to the antioxidant activity [52,53]In addition,the antioxidant capacities of mushrooms are generally related to low-molecular-weight compounds,particularly the phenolic compounds [54].Polar solvents are most frequently used to extract antioxidants from plant parts containing polyphenolics;however,lipophilic compounds existing in some plant-origin materials may interfere with the antioxidant capacity,e.g.,tocopherols,carotenoids,and terpenoids.In addition,solvents with different polarities may provide comprehensive information regarding their antioxidant potential[51].

Several reasons could be considered to explain the differences between these assays.The reaction mechanisms,media,pH,and the structure of the antioxidant compounds may be the primary factors.

5.Conclusions

HS-SPME was more suitable for extracting volatile aroma components in mushrooms,and GL had the most abundant aroma components.Based on the abovementioned measured indicators,worm infection had no negative effect on the quality ofG.rutilus.Furthermore,GL showed a higher TPC and antioxidant capacity than GW.G.rutilusinfected by worms exhibited higher total antioxidant capacity than GW.

Funding

This work was supported financially by Edible mushroom resources exploitation and the key technology development in efficient processing,“National Key R&D Program of China” [Project No.2018YFD0400200]and Liaoning Province,Shenyang Agricultural University,high-end talent introduction fund project [grant number SYAU20160003].

Conflicts ofinterest

There are no conflicts ofinterest to declare.