## 1 Introduction  
Cow milk has a recognized role in human diet reflected in its inclusion in the most recognized international health organization's dietary recommendation model [1]. There is some consensus that milk is on average composed of 87% water, 4-5% lactose, 3% protein, 3-4% fat, 0.8% minerals, and 0.1% vitamins [2]. Meanwhile, determined by the contents of milk fat, cow milk is classified as whole milk and skim milk [3]. The nutritional value of cow milk is evaluated by its nutritional richness in high biological-value proteins, several B complex vitamins and minerals such as calcium [4; 5], while milk fat is mainly in charge of the flavor and sensory feel and unique in making a direct contribution to sensory aroma and flavor perception [6; 7; 8; 9]. It is reported that fat is not only responsible for many of milk's sensory attributes and flavor, but also affects appearance, texture, and palatability [10; 11]. Several studies have carried out that sensory properties of milk vary with fat content, and it is clear that milk fat plays a role in appearance and texture attributes of fluid milk [12; 13; 14; 15; 16]. However, studies have shown that a high fat diet increases the fat in serum and liver, which causes intestinal dysfunction, intestinal barrier function damage and an increase of circulating lipopolysaccharide [17]. Besides, dietary recommendations began changing in the 1980s, and the reason was based on a research linking fat consumption to cardiovascular disease and obesity [18]. Whole milk consumption has shown a declining trend since 1970, and more consumers have shifted from whole milk to skim milk [19]. Therefore, a low-fat diet became an overarching ideology, promoted by physicians, governments and food industries, but the flavor of skim milk is inferior compared to whole milk. Flavor changes caused by the lack of fat are the reason why many general consumers are still unwilling to accept skim milk. Hence, improving the flavor of skim milk is a main focus of food scientists.  
The flavor of skim milk should be improved by processing but not by adding ingredients. New and effective methods of whole milk processing have been developed, including high pressure [20], ultrasound [21] and preheating treatments [22]. The advantages and disadvantages of each of these methods are listed in Table 1. Preheating treatment is always a necessary step in the processing of dairy products, and its different intensity has significant effects on the milk, such as heat-induced denaturation of milk protein [23; 24], cross-linking of whey protein and casein [25], fat oxidation [26], deterioration of flavor at ultrahigh temperature [27]. For this reason, we attempted to preheat at a slight temperature softly prior to defatting to obtain a favorable impact on the flavor of skim milk.  
Various methods have been popularized for food analysis, including milk, such as gas chromatography-mass spectrometry (GC-MS), electronic tongue (e-tongue) and electronic nose (e-nose). E-nose are a focus of applications within the food quality monitoring field [30], especially in the detection of adulteration of edible oils [31], and food classification based on shelf-life [32; 33]. The operating principles and configuration of e-nose have been reported by previous studies [34; 35].  
The aim of this study was to evaluate the flavor and taste changes after skim milk processing by preheating at different temperatures and for different times prior to defatting. For this purpose, a sensory evaluation analysis was performed on samples subjected to different treatments, followed by e-tongue analysis supplemented with principal component analysis (PCA) and cluster analysis (CA). The volatile components were isolated by headspace solid-phase micro-extraction (HS-SPME, PDMS/DVB fiber) and further analyzed by GC-MS.