On Line Monitoring The Aerobic Fermentation Process Of Black Tea Based On Optical Sensor Technology

Fermentation is the most important processing step of the production of black tea,and its essence is a series of physiological and biochemical reactions based on tea polyphenols,following with the significant changes on the surface of leaf surface and the generation of unique characteristic qualities which includes color,fragrance and flavor.Significantly,it is also the critical factor to determine the quality of black tea.At present,most of the enterprises engaged in black tea processing still rely on experienced tea makers who adopt the method of ’looking and then smelling’ to determine whether the fermentation degree is appropriate.However,this method is susceptible to environmental factors,subjective,has low efficiency and lacks automation and continuity during black tea production process.This therefore makes it difficult to ensure the stability of black tea quality.Sequel,this study developed an on-line optical sensor device for monitoring aerobic fermentation process of black tea.The main research contents and conclusions are as follows:1.Design of on-line monitoring system for aerobic fermentation process of black tea.On the basis of the existing black tea aerobic fermentation equipment,two sets of on-line monitoring equipment which consisted of hardware and software systems,were designed and used to monitor the changes of leaf surface color and internal tea polyphenol content during the aerobic fermentation process separately.Hardware mainly included SD1220 and integrated near infrared spectrometer,light source,integrating sphere,optical fiber,sample cup,bluetooth module and the host computer.Among them,the effective wavelength of SD1220 and integrated near infrared spectrometer were in the range of 300-1000 nm and 900-1700 nm respectively;light source was halogen lamp;integrating sphere was reflective;the material of straight optical fiber was quartz;the sample cup was round with no cover,and had 1mm thick quartz bottom;the bluetooth module models included HC-06 and CC2564MODN;and an android smart phone was employed as the host computer.The software mainly included bluetooth communication,spectral data processing and result display and client interface design module.Among them,the bluetooth communication module mainly included the signal locator of the spectrometer equipment and connector of the communication;the spectral data processing and the result display module mainly included the pre-processing,the calculation of the L*a*b values or the tea polyphenol content and the display model judgment result;the client interface design module mainly included some operation button and graphical display interface for connecting bluetooth,acquisition of the spectrum,computed outcomes and save display,and other functions.The two sets of on-line monitoring system were used for the collection of visible and near infrared spectra during the process of black tea aerobic fermentation and the calculation of leaf L*a*b values and tea polyphenol content.2.Study on the on-line monitoring of the chromaticity of leaf during the aerobic fermentation of black tea.The visible diffuse reflectance spectroscopy was used to monitor the color of the leaves outside during the aerobic fermentation of black tea,and the fermentation degree of the leaves were judged in real time according to the change in the L*a*b values.Firstly,the original reflection spectrum of the fermented tea samples were processed and fitted to obtain the whole wavelength values between 380 ~ 780 nm.According to the colorimetric standards of China(GB / T 3979-2008),the XYZ tristimulus values were calculated by the equal wavelength interval method,and converted into L*a*b values.Secondly,the color measurement accuracy of the instrument was verified.Three different levels of black tea,obtained from Hangzhou Tea Research Institute,were used as the standard samples;and the developed system and standard colorimeter were measure them at the same time.The correlation coefficients of regression equation of L*a*b values were higher than 0.9,which showed high accuracy and stability of the developed instrument.Then,a moderate color evaluation model was constructed.A total of 60 samples from the appropriate time node of the fermentation process was determined by the sensory evaluation group were judged as good,and then calculated L*a*b values were used established the discriminant model.Finally,the feasibility of the application of this model for on-line monitoring of the degree of aerobic fermentation of black tea was verified.The correct judgment rate was 85% by using 60 kinds of black tea with different fermentation degree as the verification sample based on the monitoring system and the sensory panel review results.The results showed that it is possible to judge the degree of oxygenation of black tea by online monitoring based on leaf color values.3.Study on the on-line monitoring of tea polyphenol content during the aerobic fermentation of black tea.The tea polyphenol content in the leaves of black tea during the aerobic fermentation was monitored by near infrared spectroscopy,and the fermentation degree of the leaves was evaluated in real time according to the change of the values.Firstly,the quantitative prediction model of the total amount of tea polyphenols during the process of oxygenation was established by processing the original spectrum.The reflectance spectra of the samples were collected at different time points,and the total amount of tea polyphenols was determined according to Chinese standards,second alternative method(GB / T 3979-2008).The SNV algorithm was used to preprocess the raw spectrum,and then the variable selection methods,including the full band interval,synergy interval(SI),genetic algorithm(GA),successive projections algorithm(SPA)and ant colony algorithm(ACO),were combined with PLS respectively to select variables and build the model.The results showed that compared with the full spectrum model,the other four types of variable screening methods effectively improved the model performance,and the prediction model obtained by ACO-PLS algorithm had the best result and the highest stability.The effective variables of the model were 19,Rp = 0.8317,RMSEP = 1.15.Therefore,ACO-PLS algorithm was selected as the best prediction model of tea polyphenol content.Secondly,the model of appropriate fermentation of tea polyphenols content was established.A total of 60 samples were collected under the appropriate fermentation time node identified by the sensory evaluation team and the national standard was used to determine the total amount of tea polyphenols,then the criterion was established.Finally,the model was validated for the on-line monitoring of the feasibility of the black tea oxygenation process.The correct discrimination rate of 81.67% was obtained by testing 60 samples of black tea leaves at different fermentation levels using the monitoring system and the manual review result.The results showed that it is feasible to evaluate the degree of tea polyphenols based on the change of tea polyphenols content.