Study On The Effect Of Suan Yu Property Of Fermented Freshwater Fish

Suan yu, which is a traditional fish product in China that maintains stable and retainsnutritional substances during storage without fishy odor and taste, can be eaten directly withsoftened fish bone spur. Therefore, freshwater fish can be potentially processed properly withconsiderably elevated product yield. However, this traditional product suffers from longfermentation cycle, uneven salt distribution, unstable product quality, random preparation anddifficultly NSlable fermentation condition. Therefore, the product has not been industrializedin large-scale manufacture. In our study, autochthonous microfloras were isolated from thetraditional Suan yu as starter cultures that were the most appropriate strains owing to theirtechnological and antimicrobial characteristics, armed with which will develop high qualityfermented fish that tastes uniquely and suits industrial production combining modern food andfermentation technology. This study is dedicated to understanding the changes of microbialphase and characteristics of selected starter cultures as well as chemical and microbialproperties, the origin of volatile flavor and the safety of Suan yu. A solid theoretical basis wasprovided using modern food fermentation technology to overcome the bottleneck, aiming topromote the development of freshwater fish processing industry eventually.The chemical and microbial properties of Suan yu, a sanitary Chinese traditional low-saltfermented whole fish product which is highly nutritional, particularly flavored and long-termstorable, were studied. Six brands of Suan yu from different locations in China were selected.The characteristics of this traditional fermented fish, raw material and other spontaneouslyfermented fish differed significantly. The counts of Enterobacteria and Pseudomonads in thesix brands were below the detection limits. In addition, the products contained high proteinand low moisture contents ranging from17.2%to22.9%and52.9%to58.1%, respectively.The pH values ranged between4.27and5.18, and lactic acid was the major organic acid. Thesodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS-PAGE) analysis revealsthe proteolysis of myosin heavy chain and actin chain. Moreover, lowly concentrated biogenicamine (BA) and thiobarbituric acid (TBARS) were found in all samples. The characteristicsof the six Suan yu brands were all influenced by raw materials, ingredients, and fermentationprocesses and conditions.The variations of microbial phase and the distribution of dominant microorganism werestudied at different fermentation stages in naturally fermented Suan yu. Furthermore, thecharacteristics of21strains of Lactobacillus plantarum,17strains of Pediococcuspentosaceus,19strains of Staphylococcus xylosus, and13strains of Saccharomycescerevisiae were investigated. To select the most suitable candidates to be used as the startercultures for fermented fish products, the lactic acid bacteria (LAB) and Staphylococcus xylosus were screened for the acidifying rate, the amino-biogenic ability, and the proteolysis,lipolysis and antimicrobial activities. The Saccharomyces cerevisiae were screened for gasrelease rate, ester production ability, salt tolerance, acid tolerance and etc. The counts of LABand yeast gradually increased during fermentation and ripening to8log cfu/g and6log cfu/grespectively at the end of fermentation. The number of Staphylococci gradually increasedduring the first fermentation stage and then slowly decreased until the end of ripening (4logcfu/g). The counts of Enterobacteria, Pseudomonas and aerobic spore-formers whichdecreased during fermentation were lower than the detection limits ultimately. Most of theLAB isolated from Suan yu were Lactobacillus plantarum and Pediococcus pentosaceus,followed by leuconostoc and food lactobacillus, as well as sporadic enterococcus andlactobacillus casei. The predominant Staphylococcus isolated from Suan yu comprisedStaphylococcus saprophyticus and Staphylococcus xylosus. The yeast mainly consisted ofSaccharomyces cerevisiae and Hansenula anomala. The predominant microorganism in earlyfermentation included leuconostoc and staphylococcus aureus, and those detectedsubsequently included Lactobacillus plantarum, Pediococcus pentosaceus, staphylococcusxylose, Staphylococcus saprophyticus, Saccharomyces cerevisiae and Hansenula anomala.Enterobacteria and other spoilage organisms were mainly concentrated in the thresholdfermentation stage. The counts of Enterobacteria and other spoilage organisms were below thedetection limits with the growth of LAB and the decrease of pH during the later fermentation.All of these organisms were dynamically related in Suan yu during fermentation and ripening.Major LAB dramatically reduced the pH to lower than4.5at each temperature, which thussuppressed the growth of Listeria monocytogenes, Staphylococcus aureus and Escherichiacoli. Ten L. plantarum strains and four Pediococcus pentosaceus produced bacteriocins. Theagar plate assay and SDS-PAGE suggest that five L. plantarum strains mildly proteolyzedmyofibrillar proteins. None of LAB showed any lipolytic activity.13and11Staphylococcusxylosus strains were of intense proteolytic and lipase activities, respectively. Besides, LABwere mostly decarboxylase-negative, but the strain Lp-10was of L-tyrosine decarboxylaseactivity.4Staphylococcus xylosus strains exhibited decarboxylase activity, most of whichtargeted tyrosine. All the Saccharomyces cerevisiae were ester-production capable andprotease-active. Ss-3and Ss-8tolerated10%salt. Hence, Lp-15, Lp-21, Pp-15, Sx-8, Ss-3andSs-8displayed the optimal technological properties out of the isolated strains, which arefeasible to be applied as starter cultures in fermented fish products manufacture.Three groups of single starter cultures (Lactobacillus plantarum15, Lactobacillusplantarum21,8and Pediococcus pentosaceus15) and three groups of mixed starter cultures(S1: Lactobacillus plantarum15, Staphylococcus xylosus8and Saccharomyces cerevisiae8;S2: Lactobacillus plantarum21, Staphylococcus xylosus8and Saccharomyces cerevisiae3;S3: Pediococcus pentosaceus15; Staphylococcus xylosus8and Saccharomyces cerevisiae8), isolated from Suan yu, were inoculated to produce the traditional fermented fish. Duringfermentation and ripening, the samples inoculated with different starter cultures weresubjected to rapid-growing LAB and yeast, declined pH, slow-growing spoilage bacteria andpathogens, and low TBARS accumulation. However, Staphylococcus grew more rapidly atthe beginning of fermentation. Staphylococcus and yeast inoculated mixed starter culturesalso grew more quickly initially. The TA values of the samples with single starter cultureswere higher than those with mixed starter cultures. The total free amino acids (FAA) of thesamples with starter cultures were higher compared to those of the NS (p<0.05). The proteinswere severely hydrolyzed during fermentation (SDS-PAGE). Moreover, the sensoryevaluation indicates the inoculator fermented fish was more acceptable than the NS was.Nevertheless, the samples inoculated with single starter cultures tasted slightly more acid thanthose inoculated with mixed starter cultures did. Therefore, the results suggest that beinginoculated with mixed starter cultures shortened the lag time that fermentation was initiatedand improved the quality of Suan yu. The alterations of the volatile flavor compounds in alltreated fermented Suan yu were identified by a GC/MS system during ripening, including NSbatch, sample with starter cultures batch and those with starter cultures and spices batch. Theresults show that the53,84and101volatile flavor compounds were detected in those samples,respectively. These compounds in Suan yu were clustered in acids, aldehydes, ketones,hydrocarbons, esters, alcohols and furans. Besides, the contents of terpenoids, one type ofhydrogens, were higher in the batch spices than those in other batches. The levels of estersand alcohols were higher in the batch spices and starter cultures compared to those in the NS.The main volatile flavor compounds included hexanal, acetic acid, ethanol,3-hydroxy-2-butanone,2,3-butanedione,3-methyl butyl aldehyde, ethyl acetate, ethylcaproate and benzaldehyde during ripening. The obviously increased quantities of acetic acidand ethanol during processing led to the distinct flavor. We infer that Strecker degradation,lipids hydrolysis oxidation, carbohydrate catabolism and spices mainly accounted for thegeneration of volatile compounds during ripening. Therefore, the most appropriate strainsisolated from the traditional Suan yu were used as the starter cultures, the flavor of Suan yuwas adjusted for public acceptance, and a solid theoretical basis was provided for the possibleindustrialization production. The effects of mixed starter cultures on the suppression of BAproduced during ripening were investigated. The results show that the mixed starter culturesrapidly decreased pH, and the accumulation of TVB-N were reduced compared to that of theNS. Moreover, the growth of contaminant microorganisms in the sample with mixed startercultures was inhibited. In addition, the contents of tyramine, cadaverine and putrescine in thesamples inoculated with starter cultures (S1and S3) were also restricted. The levels ofputrescine in the samples with S2were similar to those in the NS. Histamine and sperminewere not produced in any batch. Furthermore, pH and FAA and BA productions were not directly correlated. The study suggests that the samples inoculated with S1and S3inhibitedBA more significantly compared to those with S2did, thereby securing the product better.Thus, the quality of Suan yu can be substantially improved using the mixed starter culturesisolated from traditional Suan yu. The results herein are able to assist the commercializationof Suan yu theoretically.