# Effect of Preheating Treatment before Defatting on the Flavor Quality of Skim Milk
## 2 Results and Discussion  
### Sensory Descriptive Analysis (DA)  
The sensory profiles of three different samples are described in Table 1. The ANOVA analysis for the individual descriptors indicated that 6 of the 12 descriptors showed significant differences among the products. Specifically, it was observed that the sensory score of 7 descriptors (overall liking, milk aroma, cooked/sulfurons, caramelized, milk flavor, sweet taste, and residual mouthfeel) was increased with the increase of preheating temperature, whereas the score of fishy and salty was reduced gradually. It was reported that raw milk was characterized by a low but distinct salty taste and by grassy and mothball flavors [21]. It showed that preheating treatment could remove the raw milk's bad flavor. Compared with the PSM (skim milk obtained by preheating at 85 \({}^{\circ}\)C, 15 s), RSM (skim milk without preheating) and W50/T30 (skim milk obtained by preheating at 50 \({}^{\circ}\)C, 30 min) [20], the intensity of overall liking, milk aroma, caramelized, milk flavor, and residual mouthfeel was closer to the maximum of the hedonic scale in skim milk after preheating treatment at 137-141 \({}^{\circ}\)C. cooked/sulfurons and sweet taste intensity increased marginally at a higher temperature. The potential sources of these flavors were often attributed to Maillard and lipolysis reaction products [22]. In addition, lipidoxidation may also produce \"oxidized\" flavors caused by the formation of ketones and aldehydes [23]. It was consistent with the result of the GC-MS analysis. The aldehydes and ketones were increased significantly (\(p<0.05\)) in the preheating treatment at 137-141 \({}^{\circ}\)C (Table 2). There was no difference in the intensity of other sensory attributes (e.g., umami, sour, and astringency) under different conditions (\(p>0.05\)). It was demonstrated that these sensory attributes may not give rise to flavor changes of samples under different heat treatment conditions. Sensory descriptive analysis was the most effective means of evaluating the milk's flavor quality. It can truly reflect the wishes of consumers. Compared with the sensory evaluation analysis of the previous study [20], the sensory descriptive analysis was further discussed by cluster analysis (CA). CA was widely used to identify groups of similar characteristics [24]. In this study, cluster analysis was used to show sensory attribute differences between the three samples. As shown in Figure 1, the differences were present in the sensory characteristics of the three samples. Among them, the RSM and PSM were classified as a cluster and the USM (skim milk obtained by preheating at 137-141 \({}^{\circ}\)C, 4 s) was divided into a separate cluster. The cluster analysis clarified the fact that the sensory characteristics of skim milk were not changed significantly at pre-pasteurization temperature, whereas the preheating treatment at 137-141 \({}^{\circ}\)C could cause a great change in terms of the milk's sensory characteristics. This change may be due to lipid oxidation and Maillard reaction caused by the ultra-high temperature (UHT) preheating treatment in the presence of fat [25]. Meanwhile, the analysis showed that, compared with the sensory quality of preheating treatment at 137-141 \({}^{\circ}\)C, it did not change much in skim milk obtained by preheating treatment at low temperatures.