The Impact Of Douchi Starter Culture On The Extractive Components, Nutritive Value And Bioactivc Compounds Of Fermented Silver Carp Fish Paste

Silver carp (Hypophthalmichthys molitrix) has attracted much attention in recent years due to itsfast growth rate, a low feed demand, and resistance to diseases. Silver carp is intensively grown inChina, other Asian countries and U.S. To control over-population thereby reducing its threat tonative plankivorous species, research incentives and initiatives have been proposed toward marketdevelopment of silver carp. Processing silver carp to produce value-added products for humanconsumption is considered to be an attractive alternative.Douchi, a fermented soybean product, is a traditional food flavoring ingredient commonly usedin China, Japan, and other southeastern Asian countries, it is also used in many Chinese cuisines inthe U.S due its rich nutritional and medicinal compounds. Douchi find its practical applications inmany of fermented food products such as miso where it serves as starter culture, producing varietiesof enzymes including proteases, amylases and lipases which are suitable in fish fermentation.To expand the applications of douchi, numbers of experiments were conducted to test thehypothesis that douchi cultures could serve as a potential starter for enhancing the quality attributesof fermented silver carp meat and accelerate the fermentation processes. In experiment1, an active,pre-fermented douchi culture was incorporated into a fish paste to aid in the fish fermentation (30days) and facilitates biochemical production of extractive flavor components (CulF). In experiment2, a fully fermented (30days) douchi was added to a fish paste and the mixture was fermented for30days (PSF). In experiment3, a fish paste without the douchi culture was fermented for30d(ConF). Total extracted free amino acids increased by68.0,68.6, and78.8%(P <0.05) from theirinitial levels to2930,2422, and1573mg/mL after30days of fermentation for CulF, PSF, andConF fish pastes, respectively, of which, glutamic acid, aspartic acid, alanine, lysine, and leucinewere the major amino acids (>100mg/mL). The concentrations of both formaldehyde-reactivenitrogen and ammonia nitrogen extractives increased significantly (P <0.05) during fermentation,following the order of CulF> PSF> ConF. Low amounts of biogenic amines (<25ppm) wereproduced in all samples. Sensory panel evaluation showed that CulF fish pastes had desirable aromaand taste. The douchi-inoculated fermentation could be a novel technique for expanding theutilization, consumption, and the economic values of silver carp meats.Furthermore, silver carp fish pastes inoculated with a douchi starter culture (CulF) and withoutstarter culture (ConF) were fermented, and the consequent protein degradation and flavorcompound production were investigated. The protease activity in CulF, higher (P <0.05) than inConF, increased3-fold after15days of fermentation. The extractive fraction was largely comprisedof low-molecular-weight (<1.3kDa) peptides. SDS–PAGE analysis revealed complete degradationof myosin heavy chain in the extract after1and5days for CulF and ConF, respectively. CulF had ahigher (P <0.05) content of free amino acids and TCA–soluble peptides than ConF. Enzymeinhibitor assays showed that the fermentation system was dominated by acidic and serine proteases.Ethanol, silanediol, pyrazine, phenol, and formic acid were prevalent volatile compounds in CulF,while butanol, butanoic acid and acetic acid were abundant in ConF. For its strong capability to synthesize distinct taste and aroma compounds, douchi–inoculated fermentation was considered tobe superior to natural fermentation in the production of savory fish pastes.Free radical scavenging activity is an important mechanism for controlling peroxidation of lipids.Silver carp fish pastes inoculated with douchi starter culture (CulF) or without douchi starter culture(ConF) were further fermented, and the liquid extracts tested towards scavenging of2,20-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS),1,1-diphenyl-2-picrylhydrazyl (DPPH), andmetal chelating ability. The extract was also tested for inhibition of lipid oxidation in linolenicmodel system. The results indicated that CulF was superior to scavenging over50%of ABTS,DPPH at0.2mg/mL protein. The ferrous ions (Fe2+) chelating ability, showed that CulF wasstronger (P <0.05) than ConF. Radical scavenging capacity was fermentation dependent and itincreased with time except for DPPH. A strong DPPH scavenging ability was registered on day15for CulF. Although CulF exhibited good antioxidative capacity, it was significantly (P <0.05) lesspotent than douchi. The inhibition of lipid oxidation showed that both CulF and ConF couldstabilize radicals to minimize over80%lipid damage. The free fatty acid assay showed that CulFpossessed significantly higher (P <0.05) amounts of EPA and DHA than ConF after30dfermentation, inferring the advantage of douchi inclusion as starter culture.Since low molecular weight (MW) peptides of liquid extracts are known to play parts inantioxidantive activity of protein-derived antioxidants, further investigation was carried up to theliquid extracts (LE) of fish paste inoculated with douchi starter culture (CulF). The fractionationassay based on molecular weight revealed eight fractions which were able to scavenge the ABTSradicals. The scavenge capacity decrease with increases of molecular weights. Fraction7exhibiteda strong antioxidant activity among others. These low MW peptides also were considered to playsignificant role as reactants for flavor development.The microbial analysis found that fish pastes were largely dominated by lactic acid bacteriawhich played a supporting role in lowering pH and flavor developments. The gram positive bacteria(micrococci) populated the fermented fish pastes. Furthermore, the study found that CulF containedsubstantial amount of peroxide value (16.8meq/kg of fish), and total volatile base (TVB-N) of111.9mg/100g, however paint-like smell was not detected into the product.Therefore, the findings from the present study indicated that douchi as a novel starter can be usedto produce fermented silver carp fish pastes with excellent flavor and consumer acceptability.Fermentation time was reduced to30days. The results may be applicable to other fish species toproduce similar fermented products.