Microbial Diversity And Quality Analysis Of Pickled Wax Gourd

Pickled processing of fresh vegetable is an important method for food preservation. In general,5-10%salt solution is used to form a high osmosis pressure condition to inhibit the growth ofharmful microbes. Meanwhile the salt tolerance microorganisms (mainly lactic acid bacteria)growth rapidly and produce acid which can preserve vegetables. The pickled processing oftraditional vegetables is involved in complex interaction between microorganisms and a series ofbiochemical changes. It results in a unique color, smell and taste. Therefore an in-depth study to theflora of the pickled vegetables can help to understand the mechanism of its flavor formation. In thisstudy the microbial communities and its predominant species in the fermentation process ofpickled wax gourd were investigated by using16S rDNA clone library technology combined withreal-time polymerase chain reaction (RT-PCR) method. Meanwhile, the changes of its productquality were also measured.For the pickled processing with cooked wax gourd, the results of16S rDNA clone libraryshowed that, on the beginning of pickling process, the species of Acinetobacter in the picklingsystem is the predominant bacteria in microbial community. After5days of fermentation, thepredominant microbes have changed to the Weissella sp., and on the10thday followed byLactobacillus. In the later phase of the process, Weissella paramesenteroide becomes the mainmicrobial species. But there is a bit variation between the results obtained by RT-PCR and16SrDNA clone library. The quantitative analysis of microbial population showed that Weissella sp.has been the predominant species from the initial phase to the end of fermentation, its populationreached4.72×106cfu/mL on the beginning of pickling process and maintained at above the level of106cfu/mL during the process and there is little change with Acinetobacter in the pickling process(except for10days), the number remained around107cfu/mL. For the pickled processing with rawwax gourd, the results showed that, on the beginning of pickling process, the species ofLactococcus lactis are the predominant bacteria in the pickling system, its population reached5.06×107cfu/mL. After15days of fermentation, the predominant microbes have changed to bePediococcus, its population maintained at above the level of106cfu/mL until the process finished.In the later phase of the process, Virgibacillus spp. becomes the main microbial species, thenumber of Virgibacillus spp. remained around107cfu/mL. Quality analysis study of the pickled processing with cooked wax gourd has shown that, thepH value decreased rapidly and reached the minimum value of3.33on the10thday, then itincreased slightly and finally stabilized at around3.85. Salinity continued to rise until reached tothe maximum value8.3, then it decreased slightly and stabilized at around8.1. The content ofnitrite gradually increased to50.85mg/kg in the first five days, and then decreased rapidly andstabilized at about3.55mg/kg. The content of lactic acid in the pickled juice increased through thepickling process with cooked wax gourd, finally reached the level of0.327g/L. The content ofcitric acid and acetic acid was at a low level at the beginning of fermentation, and then increasedrapidly. It tends to be at a relatively stable level on the day5. On the pickled processing with rawwax gourd, the pH value decreased in the early period, reached3.5on the day30th, and thenincreased slightly in the late stage and maintained at about3.7. Salinity concentration is about2.99at the beginning of the process. A lot of salt was added on the next step, therefore the salinityincreased to9.2rapidly on day15th, and then stabilized at about7.0. The concentration of nitritedeclined rapidly at the beginning and then stabilized at about0.5mg/kg on day30th. The content oflactic acid, acetic acid and succinic acid in the pickled juice increased at the beginning of thepickling process and reached to the highest level at day30, and then begins to decrease. Thecontent of oxalic acid, malic acid, ascorbic acid and citric acid increased with the pickle conducted.