# Comparison of Automated Extraction Techniques for Volatile Analysis of Whole Milk Powder
## 4 Conclusions  
The evaluation of WMP by these four extraction techniques has highlighted the extent of VOCs present, which consisted mainly of ketones, aldehydes, lactones, and alcohols with lower numbers of esters, benzenes, phenols, furans, terpenes, sulphur compounds, and one acid. The overall difference in selectivity between the extraction techniques also highlights the need for multiple extraction techniques in order to obtain as true a representation of the complete volatile profile as possible. This is a simple fact, but often forgotten in volatile research of dairy and other foods. In relation to the four techniques, DI-HiSorb, HS-HiSorb, TD, and HS-SPME, the impact of GC column polarity was far greater than the impact of salting out under the conditions evaluated. It would appear that, unless specifically required to target a VOC (or specific VOCs) using a polar GC column, significantly more VOC information can be attained than using a non-polar GC column. As stated, the impact of salting out was minimal, but did vary depending upon the extraction technique, GC column polarity, and in relation to individual VOCs. Overall, the greatest number of VOCswere extracted by DI-HiSorb using the non-polar GC column, and slightly more without salting out. However, even though the numbers of VOCs extracted by DI-HiSorb was considerably reduced using the polar GC column, the overall abundance of VOCs was higher than achieved with the non-polar GC column. A key element as to why the overall abundances and numbers of VOCs were generally higher with DI-HiSorb, as opposed to the other techniques, was the ability of DI-HiSorb to extract large quantities and volumes of lactones. Only TD failed to extract any lactones in these WMP samples. As HS-HiSorb has the same sorptive PDMS phase as DI-HiSorb, the different conditions between DI and HS was a key factor influencing the effectiveness of each of these techniques in extracting lactones and other VOCs. It appears that it was more difficult to extract some VOCs using HS than DI, possibly due to their affinity with sample components adversely impacting their phase transition from a liquid to the gas phase during HS analysis, likely exacerbated by higher molecular weight VOCs. It is possible that the importance of lactones in many dairy products may have been underestimated due to the widespread use of HS-SPME DVB/CAR/PDMS and CAR/PDMS phases, where the CAR component may exclude some higher molecular weight lactones [29]. Differences in the apparent capacities of the phases associated with the different techniques did not have as much of an impact on VOC extraction as the difference between DI and HS. Differences between dynamic HS (TD) and static HS (HS-HiSorb and HS-SPME) techniques also did not significantly influence VOC extraction in terms of numbers and abundance. The reproducibility of most of the techniques, as assessed by the average relative percentage deviation, were similar, apart from HS-HiSorb, independent of salting out using the polar GC column, which was much diminished. However, reproducibility was very much VOC-dependent and also influenced by salting out and GC column polarity. Thus, in this study, differences between the techniques were impacted more by the choice of DI or HS, phase composition, and GC column polarity than phase capacity or salting out.  
The conceptualization of this study was undertaken by K.N.K.; methodology by D.T.M.; formal analysis and the investigation by Z.C.; software by D.T.M. and Z.C.; resources by K.N.K.; writing--original draft preparation by Z.C.; writing--review and editing by K.N.K.; supervision by M.G.O., J.F.K., S.M. and K.N.K.; funding acquisition, K.N.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Teagasc, grant number 0003-Cross Cultural Sensory Perception of Selected Irish Dairy Products. [PERSON] was the recipient of a Teagasc Walsh Scholarship.  
Not applicable.  
The authors declare no conflict of interest.