## 3 Discussion
### Comparison of Results with Prior Analyses of Sheep Milk Volatiles  
Despite the differences in milk supply and processing, the results of this study agreed with previous literature. The four techniques, SAFE, SPME, HSSE, and SBSE, detected between 20 and 48 compounds, well in-line with other studies that identified between 22 and 67 volatiles [8; 10]. The range of compound classes was also in agreement with previous studies on fresh milk and included alcohols, aldehydes, alkanes, carboxylic acids, ketones, lactones, sulphur compounds, nitrogen compounds, and terpenes. [4; 6; 8; 9; 10; 11]. The broader range of compounds identified by [PERSON] et al. can largely be explained by the esters, of which 13 were identified in the raw milk [4]. Esters were not found in the spray-dried New Zealand sheep milk by any of the techniques. [PERSON] and [PERSON] suggested that the heat load applied to milk by pasteurization may decrease the levels of esters in milk [32]. Studies of pasteurized and unpasteurized milk for cheese-making have also noted a decrease in esters when pasteurized milk was used [33; 34]. In addition to other sample differences, the high thermal load involved in pasteurization and subsequent spray-drying the sheep milk may therefore account for some of the differences in the compounds detected between this study and the ones by [PERSON] et al. [4; 5; 6].  
Another key difference between this study and previous examinations of sheep milk was the abundance of lactones in the New Zealand sheep milk. In the context of the processing, an increased concentration of lactones is not unexpected. [PERSON] et al. noted that spray-drying could impact the volatile flavour compounds of sheep milk and that it led to increased levels of volatile branched chain fatty acids. It has also been found in past studies that heating fluid milk, in this case by both pasteurization and spray-drying, could lead to the formation of lactones [35]. The propensity of SBSE to extract lactones makes it attractive for the isolation of volatiles from dairy products. \(\delta\)-decalactone and \(\delta\)-dodecalactone have been shown to be present in a wide range of dairy products, from milk, non-fat dry milk, and butter to blue cheese [11; 21; 36]. Lactones have been hypothesized to be important in the volatile profile of sheep milk and their odour-activity and importance has been demonstrated in other dairy matrices [4; 6; 11; 24]. However, the extraction of lactones from milk is inconsistent in the literature, especially when SPME is employed as the isolation technique. While variation in sample origin or processing could explain some of the variability of lactones, it is also possible that lactones are simply not well recovered by SPME or other headspace techniques due to the lower volatility of the lactones. As suggested by [PERSON] et al., it is possible that their importance in sheep milk has been underestimated in the past and this is especially plausible in sheep milk, where the lactones are more likely to partition into the greater lipid phase (5.5-9.27%) compared to cow milk (3.1-5.5%) [4; 37]. [PERSON] et al. also demonstrated the potential of SBSE to extract lactones from human milk and proposed it as a useful tool for the analysis of milk [24].