# Evaluation of Yogurt Quality during Storage by Fluorescence Spectroscopy
## 3 Results and Discussion
### The Quality Evaluation  
Each sample was regularly measured over a period of one week to monitor the deterioration process of yogurt samples, measurements were performed at a regular time every day since the porphyrins and chlorins naturally existing in yogurt both can be acted as photosensizers, which are highly sensitive to light and easy to suffer from light-induced degradation. By selecting the fluorescence spectral band in the wavelength range of 600 to 650 nm, where five narrow shape emission peaks  
Figure 2: (**a**) Linear discriminant analysis for five different brands of yogurt samples (A, B, C, D, and E) based on the first two discriminant functions; (**b**) Bar plot of LDA discrimination model for the predication set from five different kinds of yogurt. attributed to the porphyrins and chlorins are presented, the normalized fluorescence spectra of five brands of yogurt are shown in Figure 3A-E. During the successive measurements from one to seven days, these emission peaks exhibit major changes due to the photodegradation of the porphyrins and chlorins. Therefore, these fluorescence peaks can be used as indicators to estimate the changes of the porphyrins and chlorins during yogurt deterioration. To extract the spectroscopic information related to time-dependent deterioration process of yogurt, a calculating method of the average rate of change (ARC) was proposed, which can be used to evaluate the relative change of fluorescence intensity in a given spectral range. The formula for ARC can be expressed by:  
\[K_{\lambda_{1}/\lambda_{2}}=\frac{\Delta I}{\Delta\lambda}=\frac{I_{\lambda_{1 }}-I_{\lambda_{2}}}{\lambda_{1}-\lambda_{2}} \tag{5}\]  
where \(I_{\lambda_{1}}\) and \(I_{\lambda_{2}}\) are the fluorescence intensity at the wavelength of \(\lambda_{1}\) and \(\lambda_{2}\). Considering the fluorescence change related to porphyrin and chlorin compounds in the wavelength range of 600 to 750 nm, \(K_{622/633}=\frac{I_{622}-I_{\lambda_{3}}}{622-633}\) was defined for estimating the change of fluorescence peak at 622 nm and 633 nm, \(K_{661/672}=\frac{661-672}{661-672}\) was defined for evaluating the peak change at 661 nm and 672 nm. The time evolution of calculated \(K_{622/633}\) and \(K_{661/672}\) for these five types of yogurt samples are displayed in Figure 4. A decreasing tendency of \(K_{622/633}\) and an increasing tendency of \(K_{661/672}\) are clearly found during the whole measurements of seven days. As suggested by [PERSON] [22], the peak at 633 nm could be attributed to protoporphyrin, while the double peak at 661 nm and 672 nm might mostly be due to the chlorophyll residues. Therefore, the decrease of \(K_{622/633}\) indicates the photodegradation of protophyrin, and the increase of \(K_{661/672}\) represents the photodegradation of chlorophyll residues. The results show that the characteristic fluorescence intensity is highly sensitive to the changes of yogurt composition during the deterioration process, and therefore the fluorescence  
Figure 3: Normalized fluorescence spectra of each kind of yogurt during the measurements over one week. peaks of photodegradation of protophyrin could be used as indicators to quantitatively estimate the quality of yogurt during storage. For comparison purposes, the physico-chemical parameters pH value and viscosity of yogurt samples were also measured during the deteriorating process. As shown in Figure 5, a decreasing tendency was found both for pH value and viscosity during a period of 7 days, while individually, each type of yogurt sample has a slight difference. The reduction of pH in yogurt could be mostly caused by the production of lactic acid as a result of the action of the starter bacteria, which was actually apparent in the case of room temperature storage. The pronounced viscosity-decreasing behavior might be attributed to the protein denaturation and the destruction of protein colloid structure in the room temperature. Therefore, the reduction of pH value and viscosity determinate the quality of yogurt.