## 3 Results and Discussion  
### Milk  
Organic and conventional milk samples were analyzed using different equilibrating and sampling conditions (Table 2). All samples underwent an equilibration step at 40 \(\,\)C for 1, 5, or 30 min; one sample (Experiment 4) was held at 21 \(\,\)C for 90 min prior to this step and another (Experiment 2) was held at 21 \(\,\)C for 60 min following this step. The volatiles were adsorbed onto the SPME fiber at 40 \(\,\)C for 30 or 60 min, and some samples (Experiments 6 and 7) were equilibrated at 21 \(\,\)C for various times before injection. A comparison of the results is shown in Table 2. The total relative abundances increased with time and temperature. Almost all of the volatile organic milk compounds exhibited a higher relative abundance than the conventional milk compounds, indicating that the organic milk, which came from cows eating pasture plants, might be more flavorful. The relative abundance of hexanoic, octanoic, nonanoic, and decanoic acids increased with time at 40 \(\,\)C and were highest when the 90-min equilibration at 21 \(\,\)C was added. Fatty acids in milk arise from incomplete triacylglycerglycerol synthesis in the cow, microbial action, and lipase activity [11]. Indigenous lipases and bacteria apparently cleaved triacylglycerglycerides during the period before injection. The methyl ketones 2-heptanone and 2-nonanone and the aldehydes nonanal, pentanal (in the organic milk only), and 2-methylbutanal and heptanal (in the conventional milk only) tended to increase with equilibration time, often from undetectable levels. Aldehydes and ketones are lipid oxidation products from breakdown of fatty acids [12]. Methyl ketones with 7-11 carbon atoms have been found by others to increase in concentration in milk with \begin{table}
\begin{tabular}{c c c c c c c} \hline Sample & \begin{tabular}{c} **Flow rate** \\ **(mL/min)** \\ \end{tabular} & \begin{tabular}{c} **Initial oven** \\ **temperature (\(\,\)C)** \\ \end{tabular} & \begin{tabular}{c} **Hold time** \\ **(min)** \\ \end{tabular} & \begin{tabular}{c} **Ramp speed** \\ **(\(\,\)C/min)** \\ \end{tabular} & \
\begin{tabular}{c} **Final oven** \\ **temperature (\(\,\)C)** \\ \end{tabular} &
\begin{tabular}{c} **Hold time** \\ **(min)** \\ \end{tabular} \\ \hline Milk & 0.6 & 40 & 2 & 10 & 200 & 5 \\ Cheese & 1.0 & 40 & 10 & 5 & 225 & 5 \\ WPC & 1.0 & 40 & 3 & 10 & 250 & 5 \\ \hline \end{tabular}
\end{table} 
Table 1: Carrier gas and oven temperature programming for milk, Queso Fresco cheese, and whey protein concentrate (WPC). heating intensity [13]. Methyl propionate (in the conventional milk only) and 1-pentanol (in the organic milk only) may have arisen from microbial activity [14]. Dimethyl sulfide and dimethyl sulfone, from thermal decomposition of cysteine and methionine [14], were found in the organic milk samples that were treated for \(>\)1 h. \(\alpha\)-Pinene, present only in the organic milk, is a terpenoid found in pasture plants [15]. Small peaks corresponding to naphthalene, its saturated analog decalin, and several other naphthalene derivatives were seen in the organic milk from 10.2 to 12.2 min (these numbers were not included). The presence of polycyclic aromatic hydrocarbons in milk has been attributed to pollution from sources not necessarily close to the farm [16]. The number of compounds detected in these milk samples was less than the number found by others in raw milk. For instance, [PERSON] _et al_. [14] identified 33 compounds in raw milk from conventionally- and pasture-fed cows. Pasteurization inactivates nearly all indigenous enzymes and microbes in milk, resulting in greatly reduced compound formation between milking and analysis. [PERSON] and [PERSON] examined sterilized milk samples subjected to pasteurization above 120 \(\,\)C and found fewer than 12 compounds [17].