Polysaccharide impregnation: a pretreatment method for improving scallop quality and flavor

2023-01-21 05:02MengwenSunHuiminLinChenZengShngguiDengAlessndrGuidi
食品科学与人类健康(英文) 2023年2期

Mengwen Sun,Huimin Lin, *,Chen Zeng,Shnggui Deng,Alessndr Guidi

a Zhejiang Provincial Key Laboratory of Health Risk Factors of Seafood,College of Food and Pharmacy,Zhejiang Ocean University,Zhoushan 316022,China

b Department of Veterinary Science,University of Pisa,Pisa 56124,Italy

Keywords:Pretreatment Amino acids ATP-related compounds Betaine Volatile component

ABSTRACT The quality and flavor of scallops changes during the fresh dry rehydration process.In order to control the changes after rehydration,a scallop pretreatment method was adopted.The pretreatment method involved impregnating fresh scallops with 0.2 g/100 mL konjac glucomannan (KGM),carrageenan (CA),and sodium alginate (SA) before the drying and rehydration process.After rehydration,the KGM treated samples had better water retention and the highest betaine content.The CA treated samples had a deepened yellow color and the highest content of flavoring amino acids (FAA) and inosine monophosphate (IMP).What’s more,a total of 77 volatile compounds were detected,with a maximum of 15 esters.The highest proportion in the four groups was eugenol,followed by n-hexadecanoic acid,methyl hexadecanoate etc.The SA group had no significant contribution in terms of quality and taste.The samples pretreated with KGM had the best quality,and those pretreated by CA had the best f lavor.

1.Introduction

Scallops are important economic marine resources in China.They are also widely desired low-fat and high-protein seafood [1].Fresh scallops are perishable and not suitable for long-term storage,so they are usually dried [2].Dried scallops are typically eaten after rehydration.In the past,the traditional process was to soak the scallop in salt water before drying,but this can cause the scallops to become cracked and even scattered after drying,which affects their appearance and economic value.In order to prevent these phenomena and enhance product quality,a pretreatment is undertaken.Marquez-Rios et al.[3]found that pretreating the Pacific Lion’s Paw scallop with citric acid could shorten its drying time and reduce the processing cost,and the finished product had good color.Shi et al.[4]found that the toughness of sodium alginate (SA) pretreated scallops was higher than that of uncoated scallops.Therefore,pretreatment is necessary to improve the quality,economic value and taste of scallops.

Nowadays,polysaccharides are often used to pretreat food materials.Polysaccharide can react with protein at high temperature,which makes the product show complex taste [5].In our previous studies,the pretreatment of scallops with xanthan gum and carrageenan (CA) made scallops more aromatic [6].Konjac glucomannan (KGM) is a kind of non-ionic water-soluble polysaccharide with high molecular weight and viscosity.It has been widely used as a food additive in the food industry due to its excellent gel properties [7].Liu et al.[8]found that the products of KGM and silver carp myof ibrin at different Maillard reaction stages had better solubility and thermal stability than the original protein.CA are sulphated linear polysaccharides ofD-galactose and 3,6-anhydro-Dgalactose extracted from certain red seaweeds of the Rhodophyceae class.They have been used extensively in the food industry as thickening,gelling and protein-suspending agents [9].In addition,CA has obvious biological activity.Studies have proved that it can adjust the structure of intestinal flora of obese mice induced by high fat diet,improve excessive weight gain caused by high fat diet,disorders of fat,blood lipid and glucose,and have a certain inhibitory effect on the occurrence of obesity [10].Sittichoke et al.[11]found that the addition of 50%κ-CA could improve the gelling property of fish gelatin gel by increasing the gel strength and melting temperature.SA is a linear anionic polysaccharide produced from brown algae.The SA-based bioactive seafood plastic wrap prepared by SA has good film-forming performance [12].Li et al.[13]found that a SA coating combined with high CO2pretreatment could delay water migration in pufferfish and reduce the relative content of fishy flavor compounds.

In this study,fresh scallops were pretreated with 0.2 g/100 mL KGM,CA,and SA solutions at the same concentration,and the quality and flavor of scallops were compared with those without pretreatment after drying and rehydration.Meanwhile,changes in the quality were studied by detecting the moisture content and microstructure of the rehydrated scallops,and changes in the flavor were studied by detecting the contents of amino acids,betaine,volatile components,ATP-related compounds.This study provides a reference for ensuring retention of good quality and flavor in the processing of scallops.

2.Materials and methods

2.1 Materials

Fresh scallops caught in June (Mimachlamys nobilis,the total weight with shell was 15 kg;the number of samples was 400-420)were purchased from a local market in Zhoushan,Zhejiang,China.They were refrigerated with ice at 0 °C and quickly brought back to the laboratory within 2 h.After removing the shells and internal organs,the adductor muscles were obtained,which were then washed with distilled water and packed separately.The test was conducted immediately.KGM (purity >95%) was purchased from Xian Zelang Siotech Co.,Ltd.,China.CA and SA (purity >99%) were purchased from Jiangsu Baiyao Biotechnology Co.,Ltd.,China.

2.2 Soakage,drying and rehydration procedure

Soakage: The adductor muscles were divided into 4 batches (about 100 per treatment): (1) soaked in 3 g/100 mL NaCl aqueous solution at 4 °C for 30 min (control);(2) soaked in mixed solution of 0.2% KGM and 3 g/100 mL NaCl at 4 °C for 30 min (KGM);(3) soaked in mixed solution of 0.2% CA and 3 g/100 mL NaCl at 4 °C for 30 min (CA);(4) soaked in mixed solution of 0.2% SA and 3 g/100 mL NaCl at 4 °C for 30 min (SA).

Drying: Adductor muscles were dried in a drying oven (Shanghai Yiheng Technology Co.,Ltd.,China) at 80 °C for 6 h.They were then placed in a vacuum packaging bag and stored at room temperature for 48 hours.

Rehydration: Dried scallops were soaked in purified water at 25 °C for 2 h for rehydration.

2.3 Measurement of moisture content and rehydration ratio (RR)

Moisture was determined with the atmospheric drying method (GB 5009.3-2016Determination of moisture in food) [14].

Rehydration is the process of revitalizing a dried product in water.Dry samples were placed in 100 mL distilled water in a beaker.After covering the surface of the sample with a piece of filter paper to absorb the excess absorbed water,the samples were placed on a plate.The weight of the sample was recorded,and the RR was calculated according to formula (1) [15]:

Where RR represents the rehydration ratio,Wrrepresents the mass of the sample after rehydration,andWdrepresents the mass of the dried sample.

2.4 Measurement of color

The method of Jafarpour et al.[16]was used.Briefly,the color of scallops was determined by a colorimeter (CM-5;Konica Minolta,Tokyo,Japan).The instrument was calibrated with a calibration whiteboard before the measurement.In this system,the degree of lightness,redness,greenness,yellowness,or blueness were represented byL*,+a*,-a*,+b*,-b*,respectively.Meanwhile,the total color difference value was evaluated by ΔE*,as ΔE*=[(ΔL*)2+(Δa*)2+(Δb*)2]1/2.The colorimetry was carried out in natural light.Each group contained 6 samples,and each sample was measured 3 times.The average value was taken.

2.5 Microstructure

The cross-section and vertical-section of scallops were viewed at ×500,×2 000,×5 000 magnification with a scanning electron microscope (SEM) (Hitachi S-4300SE,Hitachi Science System Ltd.,Japan).The samples were cut into 1 mm × 1 mm × 3 mm pieces,fixed in 2.5% glutaraldehyde solution,rinsed with phosphate buffer,dehydrated with gradient ethanol,dried at the critical point of carbon dioxide,coated with ion spraying gold,and observed using SEM.

2.6 Measurement of amino acids

The amino acids were determined by high performance liquid chromatography (HPLC) according to Chen et al.[17]with some modifications.The chromatographic conditions were as follows:Kromat C18column (250 mm × 4.6 mm,5 μm),column temperature:40 °C,mobile phase A: acetonitrile,mobile phase B: 0.03 mol/L acetate buffer (pH 5.2),detection wavelength: 254 nm,flow rate:1 mL/min,and injection volume: 10 μL.

2.7 Measurement of betaine

The method was referred to Chie et al.[18]with some modifications.An appropri-ate amount of the evenly mixed sample was put into a volumetric flask,and then de-ionized water was added to 10 mL for a constant volume.After mixing,the sample was filtered through 0.22 μm membrane and analyzed.HPLC conditions:chromatographic column: Agilent Zorbax NH2(4.6 mm × 150 mm,5 μm);mobile phase: 0.05 mol/L KH2PO4-K2HPO4buffer (pH 3.88);flow rate: 0.5 mL/min,with equal elution;column temperature:30 °C;detection wavelength: 205 nm;injection quantity: 20 μL.The taste activity value (TAV) was used to evaluate the taste of betaine.

2.8 Determination of ATP-related compounds

The method was referred to Li et al.[19]with some modifications.

HPLC conditions: chromatographic column: COSMOSIL 5C18-MS-II (4.6 mm × 250 mm,5 μm);mobile phase: 0.05 mol/L KH2PO4-K2HPO4buffer containing 2% methanol solution (pH 6.5);flow rate: 0.6 mL/min,with equal elution;column temperature:25 °C;detection wavelength: 254 nm;injection quantity: 20 μL.

2.9 Volatile components analysis

The method of Jo et al.[20]was used for GC-MS analysis.The conditions were determined as follows:

SPME conditions: The extraction head was inserted into the sample bottle for extraction at 80 °C for 30 min and then moved into the GC-MS combined sampler for desorption at 250 °C for 3 min.

GC conditions: HP-5MS column (30 m × 0.25 mm,0.25 μm);temperature programming: initial column temperature was 30 °C,maintained for 3 min,then increased to 250 °C at 6 °C/min and maintained for 5 min,then increased to 250 °C at 10 °C/min and maintained for 2 min;inlet temperature: 250 °C;air carrying capacity(He) flow rate: 1.0 mL/min.

MS conditions: electron ionization source;electron energy 70 eV;interface temperature 250 °C;ion source temperature 230 °C;mass scan range 40-400 U.

2.10 Statistical analysis

All experiments were carried out six times,with each sample repeated in triplicate.The results were expressed as the mean ±standard deviation.The results used the SPSS analytical software version 25.0 (SPSS Inc.,Chicago,IL,USA).Significance analysis was performed using a single factor variance ANOVA(LSD),with a significance level of 5%,whereP<0.05 means the difference is significant andP>0.05 means there is no significant difference.

3.Results and discussion

3.1 Moisture content and RR

As can be seen from Fig.1,after rehydration,the moisture content of scallops in the control,KGM,CA and SA groups was (76.06 ±2.17)%,(73.93 ± 1.06)%,(77.25 ± 1.34)% and (73.13 ± 2.51)%,respectively.The RR was 1.73 ± 0.12,1.83 ± 0.15,1.47 ± 0.07 and 1.73 ± 0.17,respectively.The moisture content of SA group was lower than the control group (P<0.05),the RR of CA groups was higher than those of the control group (P<0.05),and the RR of KGM groups was higher than control group (P>0.05).The moisture content of samples in KGM group was low but the RR was the highest.KGM can fix water molecules by hydrogen bond,molecular dipole,instantaneous dipole and other forces to gather into huge macromolecules that are difficult to move freely.At the same time,the composite gel space formed by KGM and fibrillary protein has stronger binding force on water and is more conducive to the retention of water in the composite gel [21].Xiong et al.[22]found that with increased amounts of KGM,the water retention of surimi gels was improved.SA is an excellent water retention agent because of its strong hydrophilicity [23].In the CA group,the moisture content was the highest,while the RR was the lowest (P<00.05).A reason for this might be that CA dissolved in water to form a hydrophilic gel,attached to the surface of the scallops,and solidified during drying.This sticky layer reduced the water absorption capacity of the scallops [24].Pietrasik [25]stated that addition ofκ-CA significantly decreased the percentage of water loss indicating that the hydrocolloid improved the water retention of meat gels.

Fig.1 Moisture content and RR of control,KGM,CA and SA groups.Different letters indicate significant difference (P <0.05).

3.2 Color

As can be seen from the Table 1,theL* and ∆Eof all samples had no significant differences (P>0.05).There was a significant difference in thea* between the CA and SA groups (P<0.05).Thea* of the samples from the CA group decreased,indicating that the color was greener than that of the untreated scallops,while thea* of the samples from the SA group increased,indicating that the color was redder.After pretreatment with CA,theb* of the sample was significantly different from that of the control,KGM and SA,and theb* was significantly reduced.These results were similar to those of Sinthusamran et al.[26]who found that yellowness of fish gelatin gel decreased with increased agar and CA concentrations.Pietrasik [25]found that after addingκ-CA,the gela* were higher and theL* andb* were lower.It occurred due to the Maillard reaction between the amino group of the protein in the adductor muscle and the carbonyl compound,which deepened the surface color of scallops to a deep yellow color and while also improving the quality of the scallops [27].

Table 1 Color parameter of control,KGM,CA and SA groups.

3.3 Microstructure

The scanning electron microscope (SEM) results of the four samples are shown in Fig.2 and Fig.3.From the longitudinal section(× 500),the control,KGM and CA groups all had different degrees of curl,and SA was relatively smooth.Under a double lens of× 2 000,it could be seen that the KGM group had more fracture and curl,the control group had slight fracture,and the SA group was the straightest group with almost no fracture.Under the × 5 000 mirror,the fractures of the four groups could be clearly seen to have different crimp degrees and could be sorted from strong to weak as KGM >CA >control >SA.The fractures are associated with the scallop drying process.During heating,the proteins gradually unfold and aggregate,eventually form a dense network [28].Due to the excellent gel structure between KGM and protein,the protein may have a certain resistance to water loss during the heating process,resulting in fracture.On the other hand,CA adheres to the surface of scallops and forms a gel film.During the drying process in the oven,there is a temperature difference between the inside and outside of scallops,resulting in uneven water loss inside and outside,resulting in large internal stress and fracture of different degrees [29].

Fig.2 SEM images of longitudinal section of the sample under different magnification.Lines a,b,and c represent the magnification of × 5 000,× 2 000 and× 500 respectively.

Fig.3 SEM images of the cross section of the sample under different magnification.Lines a,b,and c represent the magnification of × 5 000,× 2 000 and× 500 respectively.

From the cross-sectional view (× 500),the control group had a white boundary with many cracks.The KGM group had slight fractures.There were many cracks in the CA group.Meanwhile,there were almost no cracks in the tissue of the SA group.Under the microscope at × 2 000,tight tissue structures were observed in all four groups.Although obvious white gap boundaries could be seen in the control group,the gap did not affect the tightness of the tissue structure.In addition,the KGM group had a significant curl at the end.Under × 5 000 magnification,the end tissues of the four groups could be clearly seen,and the degree of crimp in the four groups was as follows: KGM >CA >control >SA.Proteins and KGM were interconnected to form a dense network structure,which could lock up water or other nutrients [30].

Thus,the sample processed by KGM had a relatively high degree of fracture and more curl,but it also had high tightness,which is due to the denaturation of proteins during the drying process of the scallops.Denaturation of proteins can establish protein-protein interactions,leading to protein aggregation [31].The SA group had the highest fiber structural integrity.

3.4 Measurement of amino acids

Amino acids are the basic units of proteins,and their composition and content are often used as quality indicators for fish and crustacean products.Amino acids are not only the source of seafood umami but also embody many complex taste characteristics,such as sweet and bitter tastes [20].Delicious amino acids including asparagine,glycine,alanine,arginine,glutamic acid,and proline have been identified as the active components of primitive scallops and the main free amino acids of boiled dried scallops [17].Glycine is closely related to the palatability of crustacean muscles and is the main amino acid affecting the taste of scallops [32].

The amino acid content in the four groups is shown in Table 2.The delicious amino acid content was higher in the CA group than in the control group,while that in the KGM and SA groups were lower than that in the control group.Amino acids mainly come from the decomposition of proteins by endogenous proteolytic enzymes.For scallops treated with CA,the presence of Na+affected the gel strength and attached to the surface of the scallops [9].During the oven drying process,a temperature difference between the inside and outside of the scallops was generated,the internal temperature was lower than the actual temperature.At this point,compared with the control group,the activities of endogenous enzymes such as scallop protease and peptidase in the CA group were higher,the proteins were decomposed and the content of amino acids was higher.

Table 2 Amino acids contents of control,KGM,CA and SA groups.

3.5 Measurement of betaine

Betaines are abundant in crustaceans,mollusks,aquatic animals and fish,and they are one of the main taste substances of these aquatic products,contributing to sweetness [17].It can be seen from Table 3 that the TAV of the 4 groups is greater than 1 [33],and the TAV of the KGM group is the largest (35.82),followed by the SA,CA,and control groups is 30.67,25.34 and 20.72,respectively.This indicates that the KGM,CA,and SA pretreatments of samples all contribute to scallop flavor,and KGM contributes the most.This may be due to the stable physicochemical properties and high temperature resistance of betaine [34].During the processing,the high temperature treatment destroyed the structure of the scallop,fully releasing the betaine [18].It has been shown that betaine,like free amino acids,can be eluted into cooking water during sample boiling [35].Therefore,the threedimensional network structure formed by KGM and protein may retain the betaine and elute it into the solution during rehydration.On the other hand,the release of betaines changed the structure of water and made the three-dimensional network structure formed by KGM and protein more stable,which makes the protein more thermodynamically favorable [36].

Table 3 Betaines content of control,KGM,CA and SA groups.

3.6 Determination of ATP-related compounds

In addition to amino acids,certain nucleotides also enhance the overall taste of seafood [19].Table 4 shows that the CA group had the highest detectable ATP content of the four groups at 96.8 mg/kg,while the KGM group had the least,which was not detected.The 5’-adenosine monophosphate (AMP) concentration in control group was the highest at 429.3 mg/kg,followed by the SA,CA,and KGM groups (251.1,203.2,and 124.1 mg/kg,respectively).The maximum inosine monophosphate (IMP) content was present in the CA group at 880.8 mg/kg,followed by the control,KGM,and SA groups (717.4,593.6,and 507.7 mg/kg,respectively),which were all higher than the threshold value [27],indicating that they contributed to the taste of the scallops.It may be that CA inhibits the degradation of ATP and other related compounds,delaying the decrease of IMP content and the accumulation of inosine (HxR) and hypoxanthine (Hx).This is related to the metabolic pathways of mollusks such as shellfish,which constantly degrade ATP and its related compounds.In general,the specific decomposition approach after the death of a shellfish is ATP→ADP→AMP→IMP→HxR→Hx [37].AMP is a good flavor enhancer in shellfish meat [38].IMP is an umami substance fortified by glutamate,which gives meat a sweet flavor and can exhibit a strong umami flavor,while its transformation into HxR and Hx leads to unpleasant bitterness [13,35].

Table 4 ATP-related compounds contents of control,KGM,CA and SA groups.

In general,the addition of CA increases the richness of the scallop’s flavor,improves the umami taste,and reduces the generation of bitter tastes to a certain extent.

3.7 Volatile component analysis

Table 5 shows the results of the volatile component analyses from the scallops in the control,KGM,CA,and SA groups.A total of 77 compounds were identified.Among them,esters (15) were the major class.Other compound classes included amines (7),lipids (8),acids (2),ketones (6),alkanes (6),aldehydes (7),aromatics (5),phenolic compounds (6),alcohols (5),alkaline (3),furans (3),naphthalenes (2),pyrazines (1),and pyridine (1).

Table 5 Volatile compounds contents of control,KGM,CA and SA groups.

Table 5 (Continued)

A total of 15 esters were detected in the four groups,among which methyl hexadecanoate was common in the four groups,accounting for 12.96% in KGM,followed by SA,CA,and control,accounting for 12.83%,8.57%,and 7.17%,respectively.Ceesay et al.[39]have also detected fatty acids and other substances in sea cucumbers,which they believe are derived from the environmental seafloor sediments on which the species depends.But for scallops,the fatty acids are more likely to come from their food algae.Butyl butyrate and carbamodithioic acid,N,N-diethyl-methyl ester were found in the control,KGM,and CA groups.Thiocyanic acid carbazol-3,6-diyl ester was found in the remaining three groups,except the CA group.Butyl butyrate is a low fatty acid ester that has a fruity aroma and is widely used as a flavor additive [40].Residual esters were unique to each of the four groups.For example,dodecanoic acid,methyl ester was unique to the control group,accounting for 5.65% .Glycerol tricaprylate was unique to the CA group,accounting for 3.80% .Methylp-tert-butylphenylacetate was specific to the SA group,accounting for 2.21% .Salaam et al.[41]found the presence of methyl dodecate in microalgae,and thus transmitted from biological chain to scallop.The reason why methyl dodecate was not found in the other three experimental groups may be that the three substances have some biochemical reactions with scallops during processing,producing other flavor substances.For example,KGM group had a high proportion of 2,4-ditert-butylphenol,while CA group had a high proportion of 3-methyindole and nonanol.

A total of 6 ketones were detected.Acetophenone was common in the KGM,CA and SA groups,accounting for 0.44%,0.39%,and 0.49%,respectively.The only ketone detected in the control group was 2-propen-1-one,1-(4-aminophenyl)-3-phenyl,which accounted for 3.18% and was unique to the control group.Other ketones detected in the CA group were 3-methyl-3-cyclohexen-1-one (0.84%)and 7-methoxy-2,3-diphenyl-4H-chromen-4-one (0.43%).In general,ketones are derived from lipid oxidation and degradation and Maillard reactions [42].Scallops are low in fat and therefore low in ketones.

A total of 8 lipids and 2 acid substances were detected,among which tetradecanoic acid andn-hexadecanoic acid were common in the control,KGM,and SA groups.n-Hexadecanoic acid accounted for 11.16%,9.46%,and 12.01%,respectively,in the three groups.Tetradecanoic acid accounted for 3.16%,1.76%,and 2.34%,respectively,in the three groups.In addition,pentadecanoic acid(0.62%) andn-exadecanoic acid (0.66%) were separately detected in the control group.Decanoic acid was detected alone in the KGM group (1.36%).In the CA group,docosanoic acid (1.07%) and benzenepropanoic acid (0.62%) were detected separately.Fumaric acid (0.44%),folic acid (2.28%),and 16-hydroxy-hexadecanoic acid(0.99%) were detected in the SA group.Zhang et al.[43]found that tetradecanoic acid was present in fried chicken breast meat.Similarly,Nikiforova et al.[44]determined the fatty acid content of raw fish,preserved fish and fermented fish,and the contents of myristic acid and palmitic acid were the highest.They thought fermentation would reduce fatty acid content,while pickling would do the opposite.

A total of 7 aldehydes were detected.Benzaldehyde was the common among the four groups,and the proportion of the total volatile components in the CA group was the highest (5.09%),followed by the control,KGM,and SA groups,which accounted for 1.36%,1.15%,and 0.42% respectively.Benzaldehyde,which has a pleasing aroma of almond,nut and fruit,is an important flavor ingredient in crab meat and the wild long-snout catfish [45].Isovaleraldehyde (1.47%) and (E)-2-heptenal (1.15%) were separately detected in the KGM group.In the CA group,4-(1-methylethyl)-benzaldehyde (1.03%) and acetaldehyde (0.53%) were detected separately.Different aldehydes have different odors,and C3C4 aldehydes have strong odors.The C5C9 aldehydes have the flavor of green incense,oil wax,and oil putty.C10C12 aldehydes have orange peel and lemon flavors [46].These aldehydes are usually produced by lipid degradation oxidation [2].Aubourg et al.[47]found that NaCl pretreatment can promote the oxidation of fish lipids,and CA can combine with Na+,thus producing more oxidation products benzaldehyde.

A total of 5 alcohols were detected.4-Heptanol,2-methyl was detected in the control and SA groups (0.92% and 1.14%,respectively).1-Tetrasaccharide (0.73%) was isolated from the KGM group.In the CA group,5-nonanol (3.95%) and menthol (1.11%)were detected separately.11-Heneicosanol (0.37%) was detected solely in the SA group.Alcohols are derived from lipid oxidation and degradation.Giri et al.[48]found that heptyl alcohols contribute to fragrance.However,the threshold value of alcohols is higher,except in very high concentrations,and generally has little effect on flavor.

Six phenolic compounds were detected.Eugenol is the compound with the highest proportion in the four sample groups.The control group,KGM group,CA group and SA group accounted for 42.54%,31.28%,36.30%,and 40.17%,respectively.Eugenol is the main flavor substance in the volatile oils of cloves,with an especially strong spicy,smoked meat-like aroma [49].Polowsky et al.[50]found that the presence of Na+could promote the release of eugenol in CA containing eugenol system.However,the experiment showed that it may not only be effective for CA gel,but also have promoting effect on other gels.5-Isopropyl-2-methylphenol was also detected simultaneously in the control group (0.87%),KGM group (0.69%),CA group (1.10%),and SA group (0.76%).Phenol compounds have a sweet smell [51].2,4-Bis(1,1-dimethylethyl)-phenol (4.44%) was detected in the KGM group.2-Methoxy-4-(1-propenyl)-phenol(0.51%) and 3,5-bis(1,1-dimethylethyl)-phenol (6.42%) were detected in the CA group.Phenolic compounds are generally synthesized from alcohols produced by fat oxidation and esters of free fatty acids,which give meat products a fruity flavor [51].Phenolic compounds may also be derived from food or the intestinal microbial fermentation of scallops [52].

Six hydrocarbons species were detected.In the control group,cyclotetradecane (0.41%),2,3-dimethylnonane (0.64%),and 1,4-octadiene (0.64%) were detected.Octadecane (0.44%) and cyclooctene (0.58%) were separately detected in the KGM group.In the SA group,1-methyl-2-methylene-cyclohexane (0.53%) and hexadecane (0.43%) were detected separately.Hydrocarbons are generally derived from lipid degradation [2].Most hydrocarbons have a sweet and fragrant odor,but the general threshold is high and,therefore,the contribution to odor is not large [20].

Five aromatics were detected.Benzothiazole was jointly detected in the control,CA,and SA groups,accounting for 0.40%,0.74%,and 1.86%,respectively.1,1’-Biphenyl,3,3’,4,4’-tetramethyl was unique to the KGM group,accounting for 1.48% .Naphthalene was detected in all four groups,with the control,KGM,CA and SA groups accounting for 1.41%,2.22%,1.02%,and 2.12%,respectively.Biphenyl (2),naphthalene (2),and two esters of phthalates are considered environmental pollutants.They were detected to varying degrees in all four groups.1,1’-Biphenyl has a pleasant,pungent,mild,geranium,and strangely complex odor [2].Naphthalene may be a product of microbial degradation of plant materials or environmental pollutants [2].Olefins are thought to be oxidized products of fatty acids [45].

Heterocyclic compounds were detected,including furan,pyridine,and thiazole.Furans are mainly produced by the thermal decomposition of amino acids,fat oxidation,and the Maillard reaction and often have a meat flavor [53].Indoles in the control,KGM,CA and SA groups accounted for 2.65%,3.09%,1.77%,and 3.14%,respectively.Among them,3-methyl-1H-Indole was unique to the CA group and accounted for 11.05% .The presence of indoles indicated that microorganisms contributed to the flavor of the products [2].Amines were also detected,with four in the control group,two in the KGM group,and two in the SA group.Amines generally have a putrid,fishy smell.This also indicates that the pretreatment combination produces a better taste than the control group to some extent.Other compounds,such as guanidine,are derived from protein degradation and may be related to added sugars and processing.

4.Conclusions

After pretreatment,drying and rehydration,the samples of KGM group had good water retention and high content of betaine.The samples of CA group had lower yellowness and highest content of flavor amino acids,and the highest content of IMP.In the volatile components,esters are the main compounds.The content of eugenol with bacon aroma was the highest in the four groups.Benzaldehyde and other aldehydes that contribute greatly to the flavor of the samples were also detected in the four groups,among which the proportion of CA group was the highest.In conclusion,in the analysis of influencing factors of sample quality and flavor,KGM treatment improved scallop quality on the basis of moisture retention,while CA treatment had a greater effect on scallop flavor and increased the influencing factors of scallop umami taste.CA is rich in biological activity.Scallops are high in protein and low in fat.The combination of the two will bring human nutrition and improve the quality and flavor of scallops.CA has the basic characteristics of soluble dietary fiber,and the CA degradedin vivocan form a soluble complex with blood fibrin.It is fermented by coliform bacteria into short chain fatty acids,which become the energy source of probiotics.Moreover,they may prevent the occurrence of obesity to a certain extent.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This study was funded by the National Key R&D Program of China (2020YFD0900900).