# Comparison of Automated Extraction Techniques for Volatile Analysis of Whole Milk Powder
## 3 Results and Discussion  
### Comparison of Volatile Compounds Extracted from Whole Milk Powder by Each Technique  
A summary of all the VOCs identified by each technique, with and without salting out, in terms of percent of abundance, including standard deviations for each VOC, are provided in Table 2a (results using a non-polar GC column) and Table 2b (results using a polar GC column). In total, the number of individual VOCs identified in these samples across all four extraction techniques, with and without salting out and with both GC column polarities, was 85 (Table 3). This is considerably more VOCs than previously found in WMP, which, albeit, only used a single extraction technique [20; 21]. Twenty-five VOCs were identified using salting out with SBSE (PDMS) using a non-polar GC column [21] and ten VOCs by HS-SPME (DVB/CAR/PDMS) with salting out using a non-polar GC column [20]. The 85 VOCs identified in this study consisted of 20 ketones, 18 aldehydes, 11 lactones, 11 alcohols, 7 esters, 6 benzene/phenols, 5 furans, 4 terpen, 2 sulphur compounds, and 1 acid. Most VOCs were identified using the non-polar GC column (80) as opposed to the polar GC column (54) across all extraction techniques, independent of salting out (Table 3). A previous study comparing four volatile extraction techniques on natto (a fermented food) also found considerably more volatiles using a non-polar GC column than a polar GC column; 70 compared to 47 VOCs, with 40 VOCs recovered by both column polarities [22]. In this study, 30 (ethanol, 1-butanol, 1-octanol, (Z)-4-heptenal, (E)-2-octenal, (E)-2-nonenal, (E)-2-decenal and undecanal, benzeneacetaldehyde, benzyl alcohol, diacetyl, 2-hexanone, 2-octanone, 3-octanone, 2-tridecanone, 2-pentadecanone, 2-heptadecanone, \(\gamma\)-crotonolactone, \(\delta\)-caprolactone, \(\delta\)-nonalactone, \(\gamma\)-dodeclactone, \(\delta\)-undecalactone, \(\delta\)-tridecalactone, \(z\)-dairylactone, longifolene, methyl hexanoate, methyl pyruvate, 2-methylfuran, 2-pentyl furan, and acetic acid) VOCs were extracted, independent of salting out, using the non-polar GC column in comparison to the polar GC column across all four extraction techniques (Table 2a,b). In contrast, only six (1-3-pentanol, 1-nonanol, 2,3-pentanedone, \(\delta\)-caprolactone, butyl acetate, and 2-ethyl furan) VOCs were extracted across all four extraction techniques, independent of salting out, on the polar GC column, but not on the non-polar GC column (Table 2a,b). Therefore, the VOCs were present in the extract(s) in each case, but did not interact with the particular GC column phase in order to be identified. This further highlights the significance of GC column polarity in volatile extraction/identification by GC-MS.  
More VOCs were extracted and identified across all four techniques with salting out (75) and without salting out (72) with the non-polar GC column, than with salting out (48) and without salting (45) on the polar GC column (Table 4). Therefore, the impact of salting out was much less than the impact of column polarity in relation to the number of VOCs extracted. In general, salting out modifies the ionic strength of the sample solution with the aim of improving the extraction of polar VOCs, but may adversely impact the extraction of non-polar VOCs [23]. However, in practice the impact of salting out in relation to polar and non-polar VOCs is often unclear as many additional factors relating to the composition of the sample and the parameters of the specific extraction technique may also influence the extraction [23].