# Effect of Whey Protein Changes on Milk Flavor and Sensory Characteristics During Heating
## 1 Introduction  
The quality of milk is crucially affected by volatile organic compounds (VOCs), which impact consumer preference [1]. Heat treatment is an essential process during the production of liquid milk and most dairy products. While it effectively controls microorganisms, reduces enzyme activity, and prolongs the shelf-life of milk, heat treatment also affects its flavor and sensory characteristics [2]. Flavor intensity perception is determined by the release of certain compounds from the food matrix and their effect on the sensory system. However, since the chemical composition and structure of milk are complex, the different affinities for flavor compounds influence flavor compound release and perception [3]. Milk contains various VOCs, including alcohols, aldehydes, acids, esters, ketones, sulfur compounds, and other compounds, which mainly result from milk degradation. For example, lipid oxidation and thermal degradation can produce aldehydes or ketones, while the thermal reactions of amino acid side chains produce H\({}_{2}\)S and other sulfur-containing compounds. Furthermore, heatingreducing sugar (lactose) in the presence of amino acids, peptides, or proteins may induce the Maillard reaction, which influences the VOC levels [4]. Current research on the flavor of heat-treated milk mainly focuses on the effect of heat treatment on milk flavor, the generation of off-flavors [2,5], and the influence of the Maillard reaction during processing [6]. However, minimal studies are available regarding the interaction between milk proteins and flavor compounds. The interaction between proteins and flavor compounds depends on the protein (e.g., \(\beta\)-lactoglobulin (\(\beta\)-LG)) and VOC (e.g., aldehydes) properties, such as protein concentration and conformation and VOC structures [7]. In most cases, the protein and flavor compound interactions, such as hydrophobic interactions, hydrogen bonding, ionic bonding, and van der Waals forces, are reversible and can be used to reduce flavor loss during processing and re-release flavor components during consumption [8, 9, 10]. Some VOCs can irreversibly bind with protein side chains via covalent interactions, playing a vital role in eliminating off-flavors from foods [11]. Casein (CN) is thermally stable, and both casein micelles and caseinate display unstructured properties, while whey protein denaturation in milk is considered a good indicator of the degree of thermal damage [12,13]. whey protein is heat-sensitive and accounts for about 18-20% of the total milk protein [14]. When exposed to high temperatures, whey proteins undergo denaturation, coagulation, interaction with casein micelles, and Maillard Browning reactions, which change the chemical and nutritional properties of milk [15]. The degree of change depends on the heat treatment time and temperature. Previous research has found that aldehydes, sulfur molecules (especially thiols), and functional furans form covalent bonds with \(\beta\)-LG via Schiff bases, Michael additions, and disulfide bonding, which could cause flavor loss [16]. The methyl distribution in the pyrazine ring significantly affects its release from bovine serum albumin solution [10]. Therefore, understanding the release behavior of flavor compounds in milk proteins, as well as the strength and nature of the interactions between milk proteins and flavor compounds, is particularly important for improving the overall sensory characteristics and quality of milk. This study used milk collected from the same farm to assess different heat treatment durations (10 min and 30 min) and temperatures (60 \({}^{\circ}\)C, 75 \({}^{\circ}\)C, and 100 \({}^{\circ}\)C). The whey protein denaturation and secondary structure changes after different heat treatments were examined via sodium dodecyl sulfate-polyacrylamideamide gel electrophoresis (SDS-PAGE), high-performance liquid chromatography (HPLC), and circular dichroism analysis (CD).