# Dietary Compounds Influencing the Sensorial, Volatile and Phytochemical Properties of Bovine Milk
## 2 Results and Discussion
### Volatile Analyses (Feed, Raw and Pasteurized Milk)
The same compounds were present in the CLV  
Figure 2: (**a**) 3D pl ot demonstrating the effect of storage time (days) on the volatile profile of the raw milk grass (rGRS), raw milk grass/clover (rCLV) and raw milk total mixed ration (rTMR) milk samples; pink—day three, light blue—day nine and dark blue—day 14; (**b**) 3D plot demonstrating the effect of feeding system (grass (GRS), grass/clover (CLV) and total mixed ration (TMR)) on the volatile profile of the rGRS, rCLV and rTMR milk samples; pink—GRS, light blue—TMR and dark blue—CLV; (**c**) 3D plot demonstrating the effect of pasteurization on the volatile profile of the r and p GRS, CLV and TMR milk samples. Grey—rGRS grass, pink—pGRS, orange—rCLV, dark blue—pCLV, light blue—pasteurized TMR and green—raw TMR. p = pasteurized, r = raw. feed samples and the corresponding r milk samples excluding cyclohexanone, \(\alpha\)-pinene, and methyl isobutyl ketone. Acetyl valaryl was the only compound present in CLV feed and corresponding r mills that was not in GRS samples. The majority of the same compounds were present in TMR feed and corresponding r milk, excluding octanal, pentanal, 2-hexanone, cyclohexanone, 1-pentanol, and dimethyl sulfone. The following three compounds, 2-butanone, 1-hexanol, and ethylbenzene, were identified in TMR feed samples, but not in GRS or CLV feed samples. Figure 3 demonstrates the correlation of the volatile compounds to the r and p milk samples. Fifty-five compounds were identified in the p milk samples at day three, nine of which varied significantly. Further, 2-Methyl-1-butanol was significantly higher in pTMR samples, and has been linked to a malty, microbial-induced off-flavor related to the poor refrigeration of milk [19]. Further, 1-Pentanol (fermented, bready, yeasts, fusel) was significantly correlated with pCLV samples, and as previously mentioned, 1-pentanol is derived from pentanal [1], and its concentrations were linked to this aldehyde, which was also greater in CLV \(>\) GRS \(>\) TMR. Reportedly, 2-Butanone originates from the cows' feeding system [19], specifically from carbohydrate metabolism which could explain why levels of this compound were highest in pTMR samples. The level of 3-Hexen-2-one (nutty, blue-cheese, plastic) was higher in GRS and CLV milk samples compared to TMR milk samples. It is likely that 3-hexen-2-one is derived from the aerobic oxidation of linoleic or linolenic acid (C18:2 and C18:3) [72]. Dimethyl sulfone was highest in the pGRS milk samples. Heptanal and nonanal were significantly correlated with r and p TMR milk samples and their presence has previously been  
Figure 3: Principal component analysis (PCA) Biplot of raw (r) and pasteurized (p) grass (GRS), clover (CLV) and total mixed ration (TMR) milk samples and the top 60 volatile compounds contributing to the differences between the samples, identified by HS-SPME GCMS. Color gradient; low = white, mid = blue and high = red, midpoint set at 1.0.  
reported in milk [73]. Both compounds are transferred from feed, but are also products of lipid oxidation [1]. Further, 3-Hydroxy-2,2,4-trimethylpentyl-ester-2-methyl-propanoic acid (sour, bitter, herb) was more closely correlated with GRS milk samples. It has been identified in numerous plant species [74; 75; 76] and as an odorant of some hardwood species [77]. In TMR milk, 2-Methyl-propanoic acid (Isobutyric acid) has previously been reported [1]. It has a characteristic sweet-like odor and is a plant metabolite produced from the intermediary hepatic and microbial metabolism of the amino acids valine and leucine [78; 79]. However, conflicting results exist on whether 2-methyl-propanoic acid is transferred from feed to milk as [PERSON], et al. [80] reported that no carryover was evident in the milk of cows supplemented with 170 mg/kg/day of the acid for 10 days due to the rapid metabolism of dairy cattle. It is also possible that the compound entered the milk through the inhalation pathway.