# Effect of Preheating Treatment before Defatting on the Flavor Quality of Skim Milk
## 2 Results and Discussion
### E-Nose Response to Milk Aroma
In terms of concentration, the total concentration of volatiles in the USM was almost ten times higher than that of the RSM and PSM. Moreover, a heat map was also created to show the difference in volatile compositions among the three kinds of skim milk samples (Figure 4). The USM sample contained more volatiles in higher concentrations, whereas the PSM and RSM sample had less volatiles in higher concentration. On the other hand, the samples were grouped into two clusters. The RSM and PSM were classified as a cluster and the USM was divided into a separate cluster, which was similar to the CA result of the sensory evaluation. Our previous study addressed a pretreatment at low temperature [20]. Compared with the low temperatures (30-60 \({}^{\circ}\)C), we found in this study that the concentrations of the key flavor compounds (2-heptanone, 2-nonanone, octanal, hexanoic acid, etc.) were significantly higher in the skim milk obtained under the high-temperature (137-141 \({}^{\circ}\)C) treatment. Specifically, the total concentration of the volatiles in the USM (137-141 \({}^{\circ}\)C, 4 s) was almost two times higher than that of the W50/T30 (skim milk obtained by preheating at 50 \({}^{\circ}\)C, 30 min). Ketones, as a producer of cream and sweetness in the dairy products, were significantly higher in the USM (almost 30 times) than that of the W50/T30. However, the total concentrations of acids in the USM were significantly lower than that of the W50/T30 (50 \({}^{\circ}\)C, 30 min). It could be caused by the acid compound (as precursors for methyl ketones, alcohols, and esters [28]), which was produced by lipid oxidation and further induced to produce ketone compounds at high temperatures (137-141 \({}^{\circ}\)C, 4 s). The aldehydes, ketone compounds, and organic acids were the main products of the Maillard reaction [29; 30]. Ketones were the important flavor compounds in the dairy products, which offered a unique flavor and low perception threshold. It could produce an odor of cream and sweetness in the milk samples [17; 31]. Alkane compounds could have derived from the automatic oxidation of free fatty acids in milk. Their odor thresholds were high and they had a weak odor in the milk [32]. In this study, ketone and alkane compounds were found almost exclusively in the USM. Therefore, it was deduced that in the presence of fat, the preheating treatment process at 137-141 \({}^{\circ}\)C might be more prone to the Maillard reaction, the product of which remains in the skim milk sample. In general, the acids from the hydrolysis of triacylglycerlycerols (TAG) were the key flavor components in the milk [19; 33]. Hexanoic acid and octanoic acid had oily and waxy aromas, and they were precursors for other flavor compounds, such as methyl ketones, alcohols, and esters [28]. As shown in Table 2, the total concentrations of acids in the USM were about five times higher than those of the RSM and PSM. It could be attributed to the preheating treatment of raw milk before the fat was removed, because studies had reported that the concentrations of lipid-oxidized flavor compounds (hexanal, 2-heptanone, etc.) were very low in the UHT-treated skim milk where fat was removed [12]. In addition, in the presence of fat, the native structure of the MFGM could be damaged by heat treatment, especially under UHT temperatures [14]. The rupture of MFGM could cause the formation of free fat, which was hydrolyzed to produce large amounts of short chain fatty acids at UHT temperatures. These changes could also affect the aroma and flavor of skim milk. Aldehydes of the milk were derived from fatty acid metabolism, transamination of amino acids, or Strecker degradation. The hexanal and nonanal were common aldehydes in the milk and had a grassy taste [18]. The total concentrations of aldehydes were not significantly different between the PSM and USM. Additionally, naphthalene and 2-methylnaphthalene were only extracted in the RSM. These compounds were probably transferred from forages [34; 35].