Monitoring The Fermentation Process Of Pickled Radishes Based On Colorimetric Sensors

As a representative of traditional fermented foods in China,pickles are deeply loved by people for their sour and delicious flavor.Traditionally,sensory assessment,chemical,and instrumental methods can be used to judge and analyze the maturity of pickles(p H,total acid,etc.)and harmful substances(nitrite,biogenic amines,etc.)produced during the fermentation process.However,traditional testing methods require specialized personnel,cumbersome operations,or expensive equipment.In addition,most products are susceptible to contamination due to the need for actual sampling.These problems seriously affect the control of the fermentation process of pickles.As an emerging detection method,the colorimetric sensor array(CSA)has been used in the detection field due to its advantages of simple operation,low price,and visual results.In this study,pickled radish,a typical representative of Sichuan pickles,was selected as the carrier.The change of the volatile and non-volatile substances in pickled radish during the fermentation process was monitored by the gas-sensitive CSA and the liquid-sensitive CSA.The CSA was modified to increase sensitivity by introducing nanomaterials and fluorescence detection methods.This study provides a theoretical basis for the CSA to apply to the fermentation of pickles.First,the natural fermentation process of pickled radish was monitored by the gas-sensitive CSA and the liquid-sensitive CSA in this study.A 3×3 gas-sensitive or liquid-sensitive CSA was prepared using eight p H-adjusted,sulfur-derived indicators as color-sensitive materials and hydrophobic polyvinylidene fluoride(PVDF)membranes or96-well plates as solid supports.The results of biochemical indices of fermented radish showed that:the p H value dropped below 4.0 on the 10th day of fermentation;the total acid content exceeded 0.5 g/kg to reach maturity on the 3rd day of fermentation;the lactic acid bacteria count was closer to the total plate count at the later stage of fermentation,that is,lactic acid bacteria was the dominant bacteria;the nitrite content was more than 20 mg/kg on the 2nd and 3rd days of fermentation,which exceeded the national standard limit.The CSA was applied to monitor the fermentation process of pickled radish,and it was found that the CSA had good discrimination for naturally fermented radish at different fermentation times.Compared with the principal component analysis(PCA)method,linear discriminant analysis(LDA)can better distinguish the CSA data.The PCA results showed that 8 sensor points(S1,S3-S9)had a certain contribution to the discrimination of volatile and non-volatile substances in pickled radish,while the S2 sensor point only played a positive role in the discrimination of volatile substances.The LDA results showed that the accuracy of back-validation was 100%for both the gas-sensitive and liquid-sensitive sensors,and the accuracy of cross-validation was 94.4%and 95.6%,respectively.The partial least squares regression(PLSR)method was used to fit the CSA data with the biochemical indices data.The PLSR results showed that the determination coefficient of the prediction set(Rp~2)was more than 0.86 for both the gas sensor and the liquid sensor,indicating a good correlation.After that,the inoculated fermentation process of pickled radish was monitored by the CSA in this study.Through the screening of growth characteristics and acid production characteristics,Lactobacillus Brevis and Lactobacillus Plantarum were selected as fermentation strains,and the strain ratio of Lactobacillus Brevis to Lactobacillus Plantarum was 1:3.The biochemical indices results of fermented radish showed that:the p H value dropped below 3.5 on the third day of fermentation;the total acid content was greater than 0.5g/kg to reach maturity on the 1st day of fermentation;the lactic acid bacteria count increased sharply on the 1st to 3rd days of fermentation;the nitrite content was far lower than the national standard limit.The results of inoculated fermentation of radish showed that the added lactic acid bacteria quickly became the dominant bacteria during the fermentation process.In addition,the added lactic acid bacteria had a strong acid-producing ability,and they could inhibit and reduce the production of nitrite and other harmful substances.The LDA results showed that the accuracy of back-validation was 100%for both the gas-sensitive and liquid-sensitive sensors,and the accuracy of cross-validation was 96.8%and 98.4%,respectively.The PLSR results showed that Rp~2was more than 0.75 for the gas sensor and Rp~2was more than 0.87 for the liquid sensor,indicating a good correlation.Then,a nano-based colorimetric gas-sensitive sensor and a nano-based fluorescent liquid-sensitive sensor were fabricated and applied to monitor the inoculated fermentation process of pickled radish in this study.The nanomaterials and fluorescent detection were used to improve the sensor response,aiming to achieve a detection effect with the fewest types of indicators or the smallest array.A 4×4 gas-sensing CSA was prepared using four original,p H-adjusted,sulfur-derived,and nanomaterial-modified indicators as color-sensitive materials and hydrophobic PVDF membranes as solid support.In order to further expand the detection method,a 2×2 liquid-sensitive CSA was prepared using two fluorescent dyes and two fluorescent nanomaterials as color-sensitive materials and hydrophilic PVDF membranes as solid support.The nano-based sensor was used to monitor the inoculated fermentation process of pickled radish in this study.The LDA results showed that the accuracy of back-validation and cross-validation was 100%for both the gas-sensitive and liquid-sensitive sensors.The PLSR results showed that Rp~2was more than 0.76 for the gas sensor and Rp~2was more than0.74 for the liquid sensor,indicating a good correlation.In conclusion,the gas-sensitive and liquid-sensitive sensors as well as the nano-based gas-sensitive colorimetric sensor and the nano-based liquid-sensitive fluorescent sensor prepared in this study can effectively monitor the fermentation process of pickled radish.