Rapidly detecting total acid distribution of vinegar culture based on hyperspectral imaging technology

Abstract: In China and Southeast Asian countries, the solid-state fermentation (SSF) process is maintained empirically, especially in fed-batch fermentation by layers. In this study, the feasibility of determination of the total acid content (TAC), pH value and moisture content of Zhenjiang balsamic vinegar during SSF process were investigated. Hyperspectral imaging technology (HSIT) was combined with an appropriate multivariate analysis method. A synergy interval partial leastsquare (siPLS) was used to select the efficient spectral subintervals and wavelengths by k-fold cross-validation during the development of model. The performance of the final model was evaluated by use of the root mean square error of cross-validation (RMSECV) and correlation coefficient (Rc) for the calibration set, and verified by use of the root mean square error of prediction (RMSEP) and correlation coefficient (Rp) for the validation set. The changes of TAC, pH value and moisture were obtained by normal standard methods. TAC was constantly increased during fermentation process. The change of pH value was from 3.6 to 3.99, which was appropriate for the growth of the main microbes and can inhibit the growth of other unnecessary bacteria in the fermentation process. The moisture was increased in all stages during SSF process. Besides, the experimental results showed that the optimum siPLS model for TAC was achieved by use of 7 PLS factors, when 4 spectral subintervals were selected by siPLS. The predicted precision of the best model obtained was: RMSECV = 0.625, Rc = 0.8316, RMSEP = 0.773, and Rp = 0.7965. The pH value was achieved using siPLS with 6 PLS factors. The predicted precision of the best model obtained was: RMSECV = 0.0465, Rc = 0.9455, RMSEP=0.0482, and Rp = 0.9321. Besides, the moisture content was achieved using siPLS with 4 PLS factors. The predicted precision of the best model obtained was: RMSECV = 0.2104, Rc = 0.8503, RMSEP = 0.2459, and Rp = 0.8277. Finally, the superior performance of the siPLS model was demonstrated by comparison with two other PLS models. The overall results indicated that HSIT can be successfully used for the measurement of TAC, pH value and moisture content in SSF, and the use of siPLS algorithm is the best means of calibration for modelling. Besides, in order to get the distribution map of TAC, principal component analysis (PCA) and stepwise linear regression (SMLR) were used to calculate the TAC in each pixel points. The distribution map of TAC in vinegar culture was obtained. The homogeneity of SSF also could be determined by hyperspectral imaging.