# Evaluation of Yogurt Quality during Storage by Fluorescence Spectroscopy
## 1 Introduction  
Yogurt, as one kind of dairy products, has attracted more and more attention in recent years, especially since various brands of yogurt now contain special strains of \"probiotics\" that can help boost the immune system and promote a healthy digestive tract. Yogurt provides not only rich nutrient elements from milk but also varieties of vitamins produced in the process of bacterial fermentation of milk, which can contribute to health care and prevent diseases [1]. However, when exposed to ambient air, yogurt is vulnerable to microbial contamination and, thus, spoiled. In addition, with the fast development of the yogurt industry, various types of yogurt emerge in the market, followed by the difference in yogurt quality and flavor. Therefore, the accurate identification of different yogurt species and the quantitative evaluation of yogurt quality become very important for the development of the dairy industry.  
The traditional detection methods for yogurt quality include biological and chemical methods, such as the standard plate colony counting method for bacterial plate counts and chromatography-massspectrometry for protein structure analysis [2; 3; 4; 5]. However, these methods are time-consuming, labor-intensive, cumbersome, and destructive. Optical spectroscopy techniques have been widely used to study dairy products due to their advantages of being rapid, having high sensitivity, and being non-invasive. Near-infrared spectroscopy (NIR) technique allows rapid and accurate determination of typical chemical structures presenting in nutrients, such as C-H, N-H, and O-H, due to their characteristic absorption spectrum in the near-infrared range (750-2500 nm) caused by the molecular transition [6; 7; 8; 9; 10]. Based on visible/NIR spectroscopy, principal component analysis (PCA) and artificial neural network (ANN) has been used for the discrimination of five kinds of yogurt, as well as for the determination of the sugar content and acidity of yogurt [11; 12]. Ultraviolet (UV)-visible spectroscopy is a rapid and alternative technique that provides chemical information, which has been used to monitor the stability of yogurt samples stored at 4 \({}^{\circ}\)C up to 49 days [13]. Laser-induced fluorescence (LIF) spectroscopy is a well-known noninvasive method for highly sensitive and selective analysis of molecules, which has been used for the qualitative estimation of proteins in milk at different milking times and for the freshness detection of milk, as well as for studies on deterioration of fresh milk [14; 15; 16]. Spatially resolved diffuse reflectance spectroscopy has been used to non-invasively evaluate the fat content in milk and yogurt by measuring the reduced scattering and absorption properties [17]. Although several studies have been conducted, most of which were focused on the quantitative analysis of nutritional components in milk, to our knowledge, few studies have been developed to address the changes of the fluorescence signal and the physico-chemical parameters (such as pH value and viscosity) during yogurt deterioration.  
The basic aim of this work was to explore the potential for fast and accurate evaluation of yogurt quality by measuring the fluorescence spectra, pH value, and viscosity during storage. A total of five typical brands of yogurt were investigated. Linear discrimination analysis (LDA) was first attempted to distinguish different yogurt species. To quantify the fluorescence spectra related to porphyrin and chlorin compounds, a calculating method of the average rate of change (ARC) was proposed. Subsequently, the time-dependent ARC, pH value, and viscosity in the process of yogurt storage were analyzed. In the end, multiple linear regression (MLR) models were built to evaluate the relationship between the fluorescence signal with the physico-chemical parameters pH value and viscosity.