# Effect of Preheating Treatment before Defatting on the Flavor Quality of Skim Milk
## 2 Results and Discussion
### E-Nose Response to Milk Aroma  
The complexity of the milk matrix can affect the release of aroma in the milk. The difference in the aroma of skim milk was difficult to distinguish by sensory evaluation personnel. Therefore, based on the results of the previous study [20], E-nose was used to further analyze the skim milk's aroma. The milk aroma was detected by E-nose with typical response signals from ten sensors. The data were used for the principal component analysis (PCA). PCA is a way to identify data patterns and can reduce the dimensionality of initial data and highlight differences and similarities of analyzed samples [26]. The PCA plot with E-nose is shown in Figure 2. The results showed that the total contribution variance of PC1 and PC2 was over 99%. There was a distinct separation among the RSM, PSM, and USM. The first principal component factor allowed the separation of the RSM, the PSM in the positive region, and the USM in the negative region, which indicated that the RSM and PSM had significantly different aromas compared with the USM. The second factor separated the RSM with the PSM and USM, which meant that the RSM had differences in the aroma with the PSM and USM. Overall, the RSM had significant differences in the milk aroma with the PSM and USM. Preheating treatment of milk causes the formation of volatile compounds from milk proteins, carbohydrates, and lipids, as well as some other compounds, which can affect the aroma of skim milk. [PERSON] et al. indicated that heat treatment can induce lipid modification, notably oxidation, which affects aroma and flavor [16]. Figure 1: Cluster analysis (CA) of the sensory evaluation for three different skim milk samples. RSM, skim milk obtained by centrifugation of raw milk; PSM, skim milk obtained by centrifugation of raw milk after preheating treatment at 85 \({}^{\circ}\)C, 15 s; USM, skim milk obtained by centrifugation of raw milk after preheating treatment at 137–141 \({}^{\circ}\)C, 4 s.  
Analysis of the Volatiles in the RSM, PSM, and USM by Headspace Solid-Phase Microextraction (HS-SPME)-GC-MS  
As can be seen from Figure 3a, the polydimethylsiloxane/divinylbenzene (PDMS/DVB) presented the highest sensitivity for volatile compounds of the milk. It could absorb nine types of volatiles, and the total concentrations of volatile compounds extracted were significantly higher than the other fibers. Therefore, the PDMS/DVB fiber was used to extract the volatile compounds in the milk samples. The volatile compounds were isolated from three kinds of skim milk samples using SPME-GC-MS. The difference in the total ion currency (TIC) profile of the volatiles from the skim milk samples are shown in Figure 3b. The type and content of flavor compounds of the RSM, PSM, and USM were analyzed (Table 2). Based on the qualitative and quantitative analysis of GC-MS, a total of 14 volatile  
Figure 3: GC-MS analysis of volatile compounds from the milk samples: (**a**) Comparison of amounts and types of volatile compounds from milk extracted by four types of solid-phase microextraction (SPME) fiber (100 \(\upmu\)m polydimethylsiloxane (PDMS), 50/30 \(\upmu\)m DVB/CAR/PDMS, 65 \(\upmu\)m PDMS/divinylbenzene (PDMS/DVB), and 75 \(\upmu\)m carboxen/PDMS (CAR/PDMS); all from Supelco Inc., Bellefonte, PA, USA). Means within a group followed by different letters (a–c) are different (\(p<0.05\)). (**b**) Total ion currency (TIC) profile of the volatiles from three kinds of skim milk samples. Figure 2: Principal component analysis (PCA) plot for electronic nose data of three kinds of skim milk samples. A, RSM; B, PSM; C, USM. compounds was found in the three skim milk samples, which were identified in previous reports [19; 27]. As shown in Table 2, it was observed that the types of volatile compounds had significant differences among the three samples. Numerically, 7 volatile compounds were identified in the RSM and PSM, and 10 volatile compounds were extracted from the USM.