| Vinegar is not only a popular condiment but also a kind of traditional fermented food in China. As an important link in vinegar processing, acetic fermentation of vinegar greatly determines the final favor of the finished vinegar product. The volatile organic compounds(VOC) in the acetic fermentation process of vinegar is a process with coexisting VOC-producing substances whose major materials transit from ethanol to acids. The rapid detection of the VOC which is the primary index to reflect the quality in the fermentation process is an important path to realize the intelligent detection of the quality of vinegar. This study put forward a method of adopting color sensor to detect the VOC in the fermentation process, made an efficient sensor to detect VOC and developed relevant detection system through the study of the mechanism of the combination between color sensor and VOC, the main research work is as follows:(1) Analysis on the volatile gases in the acetic acid fermentation. The headspace solid phase micro-extraction and gas chromatography- mass spectrometry were combined to detect the volatile gases of fermented grain samples with different fermentation days(1st, 3rd, 5th, 7th, 9th, 11 th, 13 th, 15 th, 17 th and 19 th day of acetic acid fermentation). A total of 50 kinds of volatile gases were detected during the fermentation process of acetic acid, mainly including ethanols, acids, esters,aldehydes, ketones and heterocyclic compounds. The research showed: the content of ethanol, acetic acid and ethyl acetate undergoes significant changes during solid-state fermentation, for example, the content of ethanol was from 10.79 declined to 0.15 along with the fermentation.(2) Construction of the portable color sensor system and image signal processing.This research built a set of experiment platforms based on color sensor arrays, and and compiled a software of signal acquisition and analysis for the detection and analysis of VOC during the vinegar fermentation process. This research optimized the hardware of the system, including the image acquisition unit, photo source and the reaction chamber; handled the digital image signal that acquire from the color sensor array, including the selection of filtering method and range of Region of Interest(ROI)of the image. As a result, it could effectively extract characteristic values of imagesand characterize information about flavors of vinegar substrate during the acetic acid fermentation process. Finally, evaluate the stability of the portable color sensor system,it turns out that the olfactory visualization system is stable and reliable, and it can be used for the effective characterization of VOC information in the acetic acid fermentation process.(3) Analysis on the function mechanism of color-sensitive materials and ethanol.In order to clarify the detection mechanism of porphyrin materials on ethanol, the research took the binding energy before and after the reaction between ethanol and color-sensitive materials obtained from the calculation of quantum chemistry, energy level difference, changes of charge distribution, dipole moments, distances of central metal ions deviated from the porphyrin molecular plane, peak displacement obtained from the ultraviolet-visible spectrum titration and equilibrium constants of coordination numbers and coordination reactions as the input variables, and regarded the differences in images before and after the reaction as the output variables to respectively build correlation models between input variables and output variables.The results show that, after the combination of porphyrin and VOC, the center metal will could be stretched, which caused the metal deviate from its original plane, the greater distance between the plane and metal, the bigger difference of reaction in general, and the change also reflected in the electric charge changes of 4 N atom on the plane of porphyrin molecules and center metal atom, these changes present displacement of Soret band in the spectrum. In addition, the dipole moment will be changed before and after the reaction between porphyrin molecular and VOC, these changes also has a great effect on the reaction difference, the bigger the dipole moment is, the stronger the polarity will be, and the difference value between the porphyrin and ethanol will be lager. What’s more, the HOMO-LUMO and HOMO-1-LUMO+1 orbital energy difference of porphyrin also has a certain influence with the reaction value, the smaller the orbital energy difference is, the easier electron transition will be. Finally, synthesized the reaction mechanism analysis and the experimental results, MnTPP and ZnTPP were selected for further research.(4) Modification of color sensors. According to the principles of self-assembly,respectively self-assembled dimethyl acetamide solutions with MnTPP(5, 10, 15,20-Tetraphenyl-21 H, 23H-porphine manganese) and ZnTPP(5, 10, 15,20-Tetraphenyl-21 H, 23H-porphine zinc) in the neutral condition for one day through the combination of PEG-600 and the sedimentation method and obtained N-MnTPP and N-ZnTPP nanoparticles. The nanoparticles were characterized with the ultraviolet-visible spectrum. The results showed that both N-MnTPP and N-ZnTPP Soret bands experienced fracturing. Moreover, N-MnTPP represented a blue shift in comparison with MnTPP; N-ZnTPP represented a red shift in comparison with ZnTPP.Thus, it was inferred that N-MnTPP was primarily H- type polymer formed under the strong π-π accumulation between porphine rings while N-ZnTPP was mainly J-typed polymer formed under the joint effects of π-π interactions and hydrogen bonds. The research utilized scanning electron microscope and transmission electron microscope to characterize the appearances of N-MnTPP and N-ZnTPP: N-MnTPP is rod-like;N-ZnTPP particles are square pieces with a certain thickness. Four color sensors of N-MnTPP, N-ZnTPP and original MnTPP, ZnTPP porphyrin solutions were respectively in contact with the ethanol volatile gases with different volume concentrations(5%-60%) for 5 minutes; the colors of color-sensitive materials before and after the reaction were obtained by 3CCD cameras. The results showed that the sensitivity of N-MnTPP and N-ZnTPP to ethanol was more greatly enhanced than that of MnTPP and ZnTPP. Though the original ZnTPP didn’t have an obvious color change(the G component difference of reaction was less than 3) after exposed to ethanol with the concentration of 40%, there was a significant effect with 5% ethanol(the G component difference of reaction was 29) after nanocrystallized; what’s more,N-ZnTPP could restrain the interference of acetic acid effectively. Finally, the optimal conditions of self-assembly were obtained by Genetic Algorithm(GA): pH=12; the surfactant is PEG-600; the self-assembly lasts for one day. Under these conditions, the formed N-ZnTPP nanoparticles were spherical, with good uniformity.(5) Application of the color sensor array technology in acetic acid fermentation.The homemade portable color sensor system was adopted to track the acetic acid fermentation process of Zhenjiang Vinegar. Firstly, the volatile organic compounds in acetic acid fermentation were characterized by the color sensor arrays made of nine kinds of porphyrins and three kinds of pH indicators(neutral red, bromocresol green,Nile red). The changing trends of acetic acid fermentation were rendered via principal components analysis. Based on the linear discriminant analysis, the researchrespectively established identification models of fermented grain samples in the 19 days of the fermentation process. The cross-validation identification rate was 80.53%(153/190). Furthermore, nine kinds of N-ZnTPP solutions which nanocrystallized under different conditions and three kinds of pH indicators(neutral red, bromocresol green, Nile red) were made into the color sensor array, and the colorimetric sensor array was combined with GC-MS to conduct quantitative analysis on the characteristic VOC(ethanol) of vinegar culture with different fermentation days: take the three color components(RGB) of 12 kinds of color-sensitive materials(a total of36 characteristic values) as the characteristic variables and combine them with errors to BP-ANN models, thereby quickly detecting the ethanol content during the acetic acid fermentation process, the correlation coefficient related to ethanolic strength detected by the combination of smell visualization technology and BP-ANN models in acetic acid fermentation was 0.9578 and the root mean square error of prediction(RMSEP) was 1.2204. The research results showed that the color sensor array can effectively detect the ethanol content during the acetic acid fermentation process.This research provides an objective, stable and quantifiable method to characterize and analyze the VOC in the acetic acid fermentation, and this method could be used for qualitative and quantitative monitor and detection of the vinegar grains quality in the vinegar fermentation process, and it has direct and realistic significance to meet the consumers’ demand for food quality and safety. |
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