YU Jing ,LU Kuan ,SUN Jinyuan ,XIE Wancui, ,SONG Lin ,and CHE Hongxia,

1) Shandong Provincial Key Laboratory of Biochemical Engineering, College of Marine Science and Biological Engineering,Qingdao University of Science and Technology, Qingdao 266042,China

2) Beijing Key Laboratory of Flavor Chemistry,Beijing Technology and Business University (BTBU), Beijing 100048,China

Abstract To study the changes of volatile compounds and antioxidant activity of fast fermented shrimp head paste during the fermentation processing,Penaeus vannamei boone head was used as the raw material,and compound starter was added for fermentation.During the 14 days of fermentation,the changes of free amino acids,volatile flavor compounds and main components were determined by SPME-GC-MS and electronic nose.The oxidation-resistance changes of water extract at different fermentation stages were evaluated.The results showed that the total free amino acids increased from original 2320 to 5640 mg (100 mL)-1.SPME-GCMS analysis found that aldehydes,pyrazine compounds increased from 10.80%,1.94% to 24.35%,13.63% respectively during the fermentation process.The HO· scavenging ability of shrimp head paste increased from 52% to 86%.Our results showed that the shrimp paste produced from shrimp head fermentation could obtain good flavor and possess certain antioxidant activity of aquatic fermented condiment.

Key words shrimp head paste;antioxidant activity;volatile constituents;electronic nose;SPME-GC-MS;ATP-related compound

1 Introduction

Fermentation plays an important role in food markets.Unique flavors,aromas and texture characteristics make fermented foods represent,on an average,one-third of total food consumption (Tamanget al.,2017).Increasing studies have verified that the nutritional and functional properties of a food can be greatly enhanced due to the protein and lipid degradation by autolytic and bacterial enzymes during the fermentation process (Kumaret al.,2018;Zhuet al.,2019).Fermented food products are,therefore,good sources of peptides and amino acids.Many peptides released from food proteins exhibit biological activities,such as antimicrobial properties,blood-pressure lowering effects,cholesterol-lowering ability,antithrombotic and antioxidative activities (Faithonget al.,2010;Kleekayaiet al.,2015a).

Vast quantities of solid waste are generated during the shrimp processing,such as shrimp head and shell,which will cause environmental problems.Many studies have reported that shrimp by-products are a source of proteins,chitin,pigments,and flavor compounds (Kandraet al.,2012).However,the waste is almost completely discarded,except that only a little is processed as an animal feed supplement.Therefore,appropriate technologies should be applied to convert shrimp head into valuable products.Maoet al.(2013) indicate that bio-deproteinization of shrimp head byB.licheniformisOPL-007 can increase its antioxidant activity,and shrimp head has the potential application in the production of functional foods.Fermented shrimp products are popular in Asian countries and used as important food items or condiments.Traditional shrimp paste is a kind of seafood condiment produced with naturally fermented shrimp.It is of practical significance to realize the high-value utilization of shrimp heads through fermentation of shrimp paste.

Previous studies have showed that shrimp paste exhibited antioxidant activity and health benefits after fermentation.Halderet al.(2013) found that hydrolyzed shrimp shell had the capability of preventing fat oxidation in mice,and also found that the hydrolyzate had the function of lowering cholesterol and promoting the growth of beneficial microbes.Kleekayaiet al.(2015a) demonstrated that traditional fermented shrimp products were potential sources of bioactive peptides with ACE inhibition and antioxidant activity.Faithonget al.(2010) found that the water-soluble fraction from Kapi exhibited high antioxidant activity,indicating fermented shrimp and krill products could be used as a potential source of nutrients and natural antioxidants.

However,few studies have focused on the changes in antioxidant activity during the fermentation process,especially those contributing to flavor as well as bioactivities during processing and fermentation of shrimp head paste.This study therefore aimed to analyze the changes of volatile compounds,free amino acids,the content of flavor nucleotides,and antioxidative activities during the fermentation of shrimp head paste.

2 Materials and Methods

2.1 Starter Cultures Preparation

In our previous studies,strains ofPichia gilliermondii,Lactobacillus planticola,Aspergillus Nigerwere isolated and identified during spontaneous fermentation of shrimp paste (Xieet al.,2018).These strains contributed to the volatile flavor and antioxidant activity of shrimp head paste.The strains were preserved in sterilized glycerol (40%) and stored at -80℃.The purity ofLactobacillus planticolawas detected by the de Man Rogosa Sharpe (MRS) Agar (Oxoid),Pichia gilliermondiiandAspergillus Nigerwere detected by the Malt Extract Agar (MEA) (Oxoid).TheA.Nigerspore collected from plate cultures was aseptically harvested and washed with sterilized water,then the spore was resuspended in sterilized water.TheL.planticolacolonies were cultured in MRS Broth (Oxoid) at 30℃ for 24– 48 h;P.gilliermondiicolonies were cultured in Malt Extract Broth (Oxoid) at 30℃ for 24– 48 h.Subsequently,the cells for each organism were centrifuged at 10000gfor 5 min at room temperature and then the pellets were resuspended and washed with sterilized water.Finally,the inoculation concentrations of these three strains were approximately 8 log (cfu mL-1).

2.2 Shrimp Head Paste Preparation

Shrimp head paste was prepared according to the method we established (Xieet al.,2018).Briefly,500 g freshPenaeus vannameishrimp heads were purchased from Yanjiashan Market,Qingdao.They were washed by water,and then totally 90 g of salt was added in 3 times.Subsequently,the mixture was put into a fermenter (1.5 L) and autoclaved at 65℃ for 30 min.Then the compound starter cells(P.gilliermondii:L.planticola:Aspergillus niger=1:5:3 by volume) were inoculated into the sterilized shrimp heads(2% v/w).The samples of inoculated shrimp heads were incubated aseptically at 50℃ for 14 d.Stirring was performed every day during the fermentation process,and samples were obtained every 2 days during fermentation (0,2,4,6,8,10,12 and 14 d),then stored at -80℃ until further analysis were undertaken.Samples of 0,8,14 d were named of K1,K5,K8,respectively.

2.3 Determination of Antioxidant Activity

2.3.1 Extraction of soluble fraction of the shrimp head paste

Shrimp head paste (2 g) was mixed with distilled water and the mixture was homogenized at 10000gfor 2 min using a homogenizer.The homogenate was stirred at room temperature for 30 min,then centrifuged at 4000 r min-1for 10 min at room temperature using TG16-WS centrifuge(LuXiangyi,Shanghai,China) (Faithonget al.,2010).The supernatant was collected for the determination of antioxidative activity.

2.3.2 DPPH radical scavenging activity

2,2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activity was measured according to the method of Broncanoet al.(2012).Briefly,2 mL of 0.2 mmol L-1DPPH in 95% ethanol was added to the sample (2 mL).The mixture was mixed vigorously and allowed to stand at room temperature in dark for 30 min.The absorbance of the resulting solution was measured at 517 nm using a UV-3200PC spectro-photometer (Jinpeng,Shanghai,China).Results were calculated according to the following formula:

Scavenging activity(%) =［A0-A1］/A0×100%,

whereA1is the absorbance of the sample andA0is the absorbance of the control.

2.3.3 HO· scavenging activity

HO· scavenging activity was determined by Hydroxyl assay as described by Denevet al.(2010).Briefly,the sample (1.5 mL) was mixed with 0.3 mL of 6 mmol L-1FeSO4,1.5 mL of 6 mmol L-1salicylic acid and 1.5 mL of distilled water,and then 0.3 mL of 6 mmol L-1H2O2was added to start reaction.The mixture was mixed vigorously and kept at 37℃ for 30 min.The absorbance of the reaction mixture was measured at 510 nm using a UV-3200PC spectrophotometer (Shanghai,China).

2.3.4 Reducing power assay

The reducing power of shrimp head paste was determined according to the method of Limsuwanmanee with a slight modification (Limsuwanmaneeet al.,2014).Briefly,1 mL of sample was mixed with 0.2 mol L-1sodium phosphate buffer (1 mL,pH 6.6) and potassium ferricyanide (1 mL,1 g (100 mL)-1).The reaction mixture was incubated at 50℃ for 20 min,followed by the addition of TCA (1 mL,10 g (100 mL)-1).The mixture was then centrifuged at 750gfor 10 min.The obtained supernatant (1 mL) was added to 1 mL of distilled water and 200 μL of ferric chloride(FeCl3) (0.1 g (100 mL)-1).The blank was prepared in the same manner as the experimental samples,except that potassium ferricyanide (1 g (100 mL)-1) was replaced with distilled water.The absorbance of the reaction mixture was measured at 700 nm using a UV-3200PC spectrophotometer (Shanghai,China).

2.4 Chemical Analysis

2.4.1 Determination of free amino acids

The free amino acid contents in the shrimp pastes were estimated with the 80% ethanol extracts.Ethanol extracts were concentrated and dried in a rotary evaporator (Tegent Technology Co.,Ltd.).The dried extracts were then dissolved with an appropriate amount of lithium citrate buffer (pH 3.0) and filtered through a Millipore filter (0.22 μm).The amino acid content was determined using Hitachi L8900 automatic analyzer.

2.4.2 Changes in ATP-related compounds

The content of flavor nucleotides in the samples were determined using HPLC according to the method of Jinap with a slight modification (Jinapet al.,2010).The sample(5 g) was completely homogenized with 10% pre-cooled perchloric acid (20 mL),and then ultrasoniced for 5 min in cold water,followed by centrifuged at 8000gfor 20 min at 4 ℃.Then the supernatant was collected,the pellet was re-extracted in the same manner,and the supernatants were combined.The pH of the mixture was adjusted to 6.5– 6.8.Then the sample was left to stand at 4℃ for 30 min,and centrifuged at 8000gat 4℃ for 10 min to precipitate potassium perchlorate.The supernatants were filtered (Millex-LG 0.22 μm) and subjected to analysis using HPLC (Hitachi L2130,Hitachi koki Co.,Ltd.,Tokyo,Japan) equipped with the column Shiseido C 18 SG (4.6 mm × 150 mm).The condition included mobile phase A:20 mmol L-1KH2PO4and K2HPO4(1:1,pH=6.5).The conditions included mobile phase B:methanol,isocratic elution,flow rate:1 mL min-1,temperature:28℃,and injection volume:10 μL.The absorbance at 260 nm was monitored with a detector (Hitachi L7420,Hitachi koki Co.,Ltd.,Tokyo,Japan).The content of nucleotides of samples during shrimp paste fermentation were expressed as mg (100 g)-1.

2.5 Electronic Nose Analysis

The odor characteristics of shrimp head paste were analyzed using the electronic nose system (PEN3,Airsense Analytics GmbH,Germany) (E-nose).The system consists of a sampling apparatus,a detector unit containing an array of ten different metal oxide sensors,and pattern recognition software (Win-Muster) for data recording and analysis.The sensor array includes 10 metal oxide gas sensors,which can detect olfactory cross-sensitive information.The response characteristics of each sensor were shown as follows:W1C (aromatic compounds);W5S (nitrogen oxide);W3C (ammonia and aromatic compounds);W6S (hydrogen);W5C (olefin and aromatic compounds);W1S (hydrocarbons);W1W (hydrogen sulphide);W2S (alcohols and partially aromatic compounds);W2W (aromatic compounds and organic sulphides);W3S (alkanes (methane,etc.)).

Briefly,the sample of shrimp paste was placed in an airtight 25 mL vial at 25℃ ± 1℃ for about 20 min.The volatile gases were replaced by clean air which was supplied through a second hollow needle with charcoal filter.The measurement time was 150 s and a standby procedure,up to 500 s,was applied to flush the chamber with clean air until the sensor signals returned to baseline.All experiments were repeated three times.

2.6 SPME-GC-MS Analysis

2.6.1 Sample pretreatment and SPME conditions

The SPME manual device equipped with a 50/30 mm DVB/CAR/PDMS fiber (Supelco,Bellfonte,PA,USA) was used for the extraction of volatile compounds in shrimp paste samples.Sample (2 g) was placed into a 15 mL vial,and then equilibrated at 40℃ for 10 min before extracted with a DVB/CAR/PDMS fiber for 40 min at the same temperature (Fanet al.,2017).

2.6.2 GC-MS analysis

Mass spectral ionization was set at 230℃ .The mass spectrometer was operated in the electron ionization mode at a voltage of 70 eV.The helium was used as the carrier gas with a flow rate of 0.7 mL min-1.A 0.75 mm liner was used and the analysis was performed in the splitless mode.The temperature of injector was 250 ℃.The column was held at 40℃ for 5 min,then increased to 250 ℃ at a rate of 2.5℃min-1,and finally held at 250℃ for 10 min.Compounds detected by GC-MS analysis were identified by comparing mass spectra with published data obtained under similar conditions,as well as by comparing their mass spectra with the MS library of Wiley 7.0 and Nist 05.

2.7 Statistical Analysis

One-way ANOVA Tukey test,Pearson’s correlation coefficients andPvalues were calculated with SPSS software version 19 (SPSS,Illinois,USA).Other data processing is applied to software such as SIMCA 14.1 (UMETRICS),Excel and Origin 9.0.The data are presented as mean ± SD (standard deviation).The difference is significant whenP<0.05.

3 Results and Discussion

3.1 Changes in Antioxidant Activities

To verify the antioxidant activity of the shrimp head paste during the fermentation,DPPH free radical scavenging ability,hydroxyl free radical scavenging ability,and reducing power were investigated.Siddhuraju and Becker (2007)founded that extraction media affected the antioxidative activity of shrimp paste.Binsanet al.(2018) compared the antioxidative activity of the shrimp paste extracted with distilled water/ethanol mixture and ethanol,respectively.Distilled water exhibited the highest efficacy in extracting the antioxidants.

As shown in Fig.1,DPPH free radical scavenging ability increased rapidly up to 8 d,followed by a moderate growth until 14 d.The change of DPPH was similar to that of Faithonget al.(2010).Interestingly,the HO· scavenging ability increased from 52% to 86%,and declined after 12 d.As for the reducing power,a moderate growth was observed till 12 d,and a significant decrease followed (P<0.05).Low molecular weight peptides and amino acids have been reported to possess antioxidant activity (Kleekayaiet al.,2015b).Protein hydrolysis occurs in the fermentation process of shrimp head paste,accumulating a large amount of low molecular peptides or amino acids,which can increase DPPH radical scavenging activity and reducing power.At the end of fermentation,further degradation or aggregation of low molecular peptides or amino acids may restrain antioxidant activity.So does the carbonyl ammonia reaction.

Fig.1 Changes in antioxidant capacity of water soluble fractions of shrimp head paste during fermentation.

3.2 Changes in Free Amino Acid During Fermentation

Amino acids are important flavor substance and flavor precursor in aquatic products.The taste function mainly depends on flavor characteristics,such as umami,bitter,sweet,threshold,content and the interaction with other compounds(Bermudezet al.,2014).The results of amino acid content in the shrimp head paste are shown in Table 1.After 14 d of fermentation,most essential amino acids increased significantly compared with the raw material (0 d).Interestingly,the total amino acid content of fast fermented shrimp head paste exhibited a significant steady increasing trend during the fermentation process,from initial 2320 to 5640 mg (100 mL)-1(P<0.05).

Table 1 The changes of free amino acid in fast fermentation shrimp head paste during fermentation

Amino acids usually contribute a sour,bitter or sweet taste.Those containing a sulphur atom have a sulphury note(Shallenberger,1993).Aspartic acid and glutamic acid contribute to sour taste,but their sodium salts are monosodium glutamate-like components.Glycine and alanine have a pleasant sweet taste,and they are widely presented in large quantity in seafood,such as snow crab,clam and scallop.Houet al.(2011) demonstrated that the flavor amino acid content in scallop paste increased obviously compared with raw material.Compared with the original products,the sweet amino acids of shrimp head paste increased 2.9 times,and bitter taste of amino acids improved 2.6 times.Aspartic acid,belonging to tasty amino acids,changed significantly from initial 54 to 376 mg (100 mL)-1during fermentation,and increased by 6.9 times.Glutamic acid increased from 140 to 550 mg (100 mL)-1,a 4.0-fold increase.Alanine increased from 276 to 595 mg (100 mL)-1,up to 2.2 times.The changes of taste amino acids contribute to the freshness and fermented flavor formation of shrimp head paste.

3.3 Changes in Nucleotide Contents

Changes in ATP-related compounds during fermentation of shrimp head paste are shown in Fig.2.Inosine monophosphate (IMP) and disodium guanosine 5’-monophosphate (GMP) are intense flavor-enhancers for the umami taste,which are much stronger than monosodium glutamate (MSG).The taste profile contributed by adenosine monophosphate (AMP) depends on its concentration (Zhanget al.,2019).

Fig.2 Changes in nucleotide contents during fermentation.

Previous studies have demonstrated that ATP in shrimp could be rapidly converted into AMP and IMP after the death of shrimp,particularly during transportation (Xieet al.,2018).In the present study,the content of AMP and IMP in dry samples (shrimp head) were 50.39 and 47.33 mg(100 g)-1,respectively,while the content of GMP was only 10.28 mg (100 g)-1.When shrimp were subjected to salting overnight,followed by sun-drying,a sharp decrease in AMP was noted with coincidental increase in IMP (P<0.05),suggesting a rapid conversion of AMP to IMP in shrimp head after fermentation.The IMP content reached its maximum value on the second day of fermentation (52.79 mg(100 g)-1),and then began to decrease.The continued decline of AMP and IMP may contribute to the formation of other ATP-related compounds,such as inosine and hypoxanthine,which play the role in taste or flavor of many marine products,especially for umami or sweet-taste.Changes in ATP-related compounds in some fermented products have been reported.Kim and Lee (2008) demonstrated that ATP,ADP and IMP were not detected in soy sauce during fermentation process.Pongsetkulet al.(2016) found that ATP was not detected in raw material,whereas adenosine diphosphate (ADP) and adenosine monophosphate (AMP)disappeared after salting and drying during the fermentation of traditional salted shrimp paste of Thailand.

3.4 Analysis of Shrimp Head Paste Volatile Flavor in the Process of Fermentation

The changes of volatile compounds in shrimp head paste during fermentation are shown in Fig.3.The radar graph automatically generated by the e-nose system showed the responses of the 10 sensor to the samples.According to Fig.3A,the response of W1S and W1W sensors increased sharply in the early stage of fermentation (0– 8 d),indicating that inorganic sulfides and a large amount of organic compounds were produced.The values of the W2S and W2W sensors represented an increase in the amount of aromatic compounds.In the late stage of fermentation (8– 16 d),only the response of W1W sensor increased significantly,indicating that the amount of some sulfur compounds were still increasing.These results revealed that the main volatile compounds of shrimp paste were mainly contributed by the volatile compounds of W1S (hydrocarbons),W1W(hydrogen sulphide),W2S (alcohols and partially aromatic compounds),and W2W (aromatic compounds and organic sulphides).

Fig.3 Radar chart (A) and principal component analysis (PCA) plot of fermentation shrimp head paste in different stages (B).

Principal component analysis (PCA) is a multivariate statistical analysis technique,which can represent many complex and difficult-to-find variables in the original samples by determining a few principal component factors.The regularity and difference among samples can be evaluated according to the contribution rate of principal component factors in different samples (Bounouaet al.,2012).The PCA of volatile compounds in the fermentation of shrimp head paste is shown in Fig.3B.It showed the distribution map for the first two principal components determined by PCA,which contributed 81.3% and 15.0%,respectively.A visualization of the data was also obtained.These components were thought to show the different fermentation processes during the mature of shrimp head paste.The PCA results clearly showed that three different fermentation periods were well distinguished in the distribution map,indicating the difference of volatile compounds during the shrimp paste fermentation.

3.5 Changes in Volatile Compounds

SPME-GC-MS was used to analyze the volatile compounds of shrimp head paste during the fermentation.The data provided by GC-MS analysis of shrimp head pastes are listed in Table 2.Totally 60 types of main volatile compounds were identified,including aldehydes (6),ketones(9),alcohols (13),pyrazines (12),ethers (2),esters (3),hydrocarbons (9) and others (5).Previous studies have shown that the formation of volatile compounds were associated with the degradation of proteins,lipolysis,lipid oxidation,Maillard reaction,interaction between Maillard reaction products with lipid-oxidized products and other reactions(Kleekayaiet al.,2015b;Rahayuet al.,2017).

The aldehydes produced by the oxidation of unsaturated fatty acids have a low threshold,so they play an important role in the flavor characteristics of fermented aquatic products.Aldehydes are the most prominent volatiles produced during lipid oxidation.Six aldehydes were found during the fermentation of shrimp head paste.Nonanal,the main oxidation product of oleic acid,was decreased after being salted,which contributed to the specific odor of shrimp paste.Benzaldehyde,possesed a nutty,fruity and pleasant aroma hyacinth fragrance and other unique aromas,was found as a major aldehyde in the mature shrimp head paste (Pongsetkulet al.,2017).Branched-chain aldehydes are important flavor compounds in fermented foods.3-Methylbutyraldehyde and 2-methylbutyraldehyde have a strong malt-like or chocolate-like aroma.These substances are mainly produced by the Ehrlich pathway of microorganisms and the biosynthesis pathway of branched chain amino acids.

Ketone compounds are formed by oxidation or thermal degradation of unsaturated fatty acids,which are considered to enhance the flavor quality associated with fruit aroma and flower fragrance.Changes in ketones were also found during the fermentation of shrimp head paste.Pentanal,neryl acetone,3-undecanone decreased during processing.However,some ketones were generated during fermentation and still remained in the final product,which included 2-butanone,methyllavender ketone,3,5-octadien-2-one.Generally,both aldehydes and ketones were more likely generated from lipid oxidation during fermentation(Takeungwongtrakulet al.,2012)

Various alcohols are found in shrimp head fermentation.Alcohols are known as the secondary products generated by the decomposition of hydroperoxides of fatty acid.Some alcohols,including ethanol,1-pentanol and 2-ethyl-hexanol were gradually decreased during the fermentation of fresh shrimp head.1-Octen-3-ol is an important contributor to offflavor due to its low odor threshold.It is formed from oxidation of arachidonic acid by lipoxygenase.Overall,alcohols found in shrimp head paste were quite low in abundance.They might not have a paramount impact on shrimp head paste flavor because of their high flavor thresholds.

Pyrazines,generatedviaMaillard and pyrolysis reactions through Strecker degradations from various nitrogen sources such as amino acids in heat-processed foods,have been shown to contribute to nutty,roasted and toasted characteristics in roasted shrimp (Giriet al.,2010;Kleekayaiet al.,2015b).Pyrazine derivatives were the major compounds found in shrimp head paste (Table 2).In general,the abundance of these compounds increased throughout the production,especially as the fermentation time increased.Five pyrazine derivatives were observed in shrimp head and were still found throughout the fermentation,which included 2,5-dimethyl-pyrazine,2,3,5-trimethyl-pyrazine,3-ethyl-2,5-dimethyl-pyrazine,etc.Pyrazines,with low flavor thresholds,have been known to have both positive and negative impacts on the flavor quality of cooked foods.However,Pongsetkulet al.(2017) found that pyrazine derivatives seem to give desirable odor or flavor to several fermented seafoods (Pongsetkulet al.,2017).

Table 2 Volatile compounds in different stages of fast fermentation shrimp head paste

(continued)

Esters are important compounds that contribute to the distinct flavor of fermented shrimp head paste,and are generally originated from esterification of short-chain fatty acids with alcohols,contributing to fruity aroma (Maggiet al.,2010).After 14 days,three types of ester compounds were detected in shrimp head paste.

Totally nine hydrocarbons were found during the fermentation of shrimp head paste.However,due to low abundance,these compounds seemed not to be the major contributor to flavor characteristics of shrimp head paste.

Overall,changes in volatile compounds were observed during shrimp head paste fermentation.Aldehydes,ethers,as well as pyrazines were drastically increased.The decomposition of protein and lipids during the fermentation process might be associated with flavor or odor of shrimp head paste.

The types and the contents of volatile aroma compounds during the fermentation process are shown in Fig.4.The volatile components of various fermented seafood are almost the same,mainly including alcohols,aldehydes,ketones,acids,esters and aromatic.After fermentation,the content of aldehydes and pyrazines increased significantly.Aldehyde compounds increased from 10.8% to 24.35% (P<0.05),and pyrazine compounds increased from 1.94% to 13.63% (P<0.05),which were the main aroma substances of fermentation flavor.Amine compounds decreased from 54.91% to 40.12%.Ketone compounds,alcohols,and hydrocarbons showed a slight increase trend.

Fig.4 Types and contents of volatile flavor compounds in different stages of fast fermented shrimp head paste (*means P < 0.05).

4 Conclusions

In the present study,shrimp head paste was produced with short production cycle and low salt content by adding microbial fermentation.The taste amino acids and volatile flavor compounds of the product increased significantly,the flavor increased at the same time.The research showed that modern fermentation method could improve the value of shrimp by-products.It also proved that fermented shrimp heads possess DPPH radical scavenging ability,reducing power,and hydroxyl radical scavenging ability.Therefore,shrimp head fermentation has potential application value in the functional food.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos.32072348 and 31 671825),and the Open Project Program of Beijing Key Laboratory of Flavor Chemistry (No.SPFW2019YB01).