# Effect of Preheating Treatment before Defatting on the Flavor Quality of Skim Milk
## 1 Introduction  
Milk is a valuable nutritious food, which contains proteins, fat, sugar, amino acids, minerals, vitamins, and other ingredients required by the human body. It has been considered one of the perfect foods in the human diet. Milk consumption is always listed as an important element in a healthy and balanced diet [1]. However, with the improvement of the living standard, people (especially obese people) have become more sensitive to foods that contain fat. Studies show that a high-fat diet increases the amount of fat in the serum and liver, leading to intestinal dysfunction and impaired intestinal barrier function [2]. Therefore, skim milk is often preferred by consumers due to the loss of fat. Additionally, some studies demonstrate that the intake of skim milk is considered more beneficial than whole milk [3]. Nevertheless, loss of fat can lead to skim milk's poor flavor. Previous studies have shown the influence of milk fat content on flavor release in dairy desserts [4, 5]. [PERSON] et al. pointed out that the consumer's preference for milk increased with increasing fat content [6]. Therefore, it is particularly important to improve the flavor of skim milk. Milk fat is known to have the most direct contribution to sensory aromas and flavor perception [7; 8]. Milk contains numerous lipids, such as triacylglycerols, diacylglycerols, saturated/polyunsaturated fatty acids, and phospholipids. Heat treatment, as an important step in the milk process, can induce lipid oxidation and produce free fatty acids, methyl ketones, and oxidized fatty acids, which are directly correlated with milk flavor [9; 10]. In heated (at or above 72 \({}^{\circ}\)C) mills, some lactones, methyl ketones, and aldehydes are formed from milk fat. Some carbonyl compounds that have also been identified in heated milk may be derived from milk fat [11]. Notably, [PERSON] et al. pointed out that in the presence of lipids, the concentrations of free fatty acids, methyl ketones, and oxidized fatty acids were directly related to the heat treatment temperature [9]. In addition, compared with ultra-pasteurized (UP)-treated (about 138 \({}^{\circ}\)C, 2 s) skim milk, UP-treated 2% fat milk had better sensory properties (overall aroma, sweetly aromatic, etc.) and a higher content of lipid-oxidized flavor compounds (hexanal, 2-heptanone, etc.) [12]. In general, in the presence of lipids, heat treatment (at or above 60 \({}^{\circ}\)C) can also promote the formation of milk fat globule membrane (MFGM) fragments or induce direct interactions with casein or serum proteins [13; 14]. Moreover, the interaction of these milk matrices due to thermal treatment can lead to the changes in taste and aroma of milk [10; 15]. This may be attributed to the fact that the lysine residues of proteins can react with the milk's reducing carbohydrates, resulting in the so-called non-enzymatic browning or Maillard reaction during heat treatment and inducing both protein and lipid modification [9; 16]. Taken together, these reactions affect the sensory properties (aroma, flavor, and appearance) of milk by heat treatment in the presence of fat. However, raw milk is usually directly centrifuged (or preheated at a lower temperature, about 50 \({}^{\circ}\)C) to remove fat, and then heat-treated, whereby skim milk is unable to obtain a good flavor brought about by fat. Notably, the level of most oxidized fatty acids and the concentration of methyl ketones (mainly 2-heptanone and 2-pentanone, the key flavor components in the milk [17]) were increased with temperature in the presence of fat [18]. Therefore, we assume that in the presence of fat, the milk is preheating treated (at a higher temperature) and then centrifuged to remove fat, which produces better flavored skim milk. Currently, reports on the improvement of skim milk flavor by changing the process parameters are very rare. In our previous study, we improved the skim milk flavor profiles by lipase-catalyzed reactions and preheating at a lower temperature [19; 20].