## 3 Results and Discussion
### Cheese  
Higher analysis temperatures may be used for QF since this variety does not melt. Equilibrating and extracting at 60 \(\,\mathrm{\SIUnitSymbolCelsius}\) produced better resolution of peaks than at 50 \(\,\mathrm{\SIUnitSymbolCelsius}\), so a holding period of 60 \(\,\mathrm{\SIUnitSymbolCelsius}\) for 10 min was used. When samples of a QF frozen for 42 weeks were extracted onto the fiber at 60 \(\,\mathrm{\SIUnitSymbolCelsius}\) for 60 min, the relative abundance of most of the volatile compounds was lower than when the extraction lasted 30 min (Table 4). The longer time may have caused the fiber to become saturated and lose volatiles, so the shorter time was optimal. Table 5 shows the volatile compounds in the QF cheeses stored for up to 8 weeks. Because of variations between the many cheeses analyzed, categories of relative abundance are given instead of averages. Many of the compounds present in the milk samples were also detected in the cheese. The acids and ketones increased with storage as did nonanal and decanal, which continued to form from lipolysis of longer-chain fatty acids. In contrast, the other aldehydes decreased to undetectable levels. The shorter-chain aldehydes are relatively transient compounds in cheese, breaking down into alcohols \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**Retention time (min)**} & \multicolumn{4}{c}{**Time at 21 \(\,\mathrm{\SIUnitSymbolCelsius}\)**} \\ \cline{3-6}  & & **2.50** & **5.67** & **7.33** & **9.00** \\ \cline{3-6}  & & **Relative abundance (thousands)** \\ \hline
1-Pentanol & 4.9 & 22 & 40 & 40 & 40 \\ Butanoic acid & 5.1 & 30 & 50 & 40 & 40 \\ Dimethyl sulfone & 7.5 & 15 & 20 & 20 & 20 \\ Hexanoic acid & 8.8 & 400 & 450 & 450 & 450 \\ Octanal & 9.1 & 18 & 20 & 30 & 35 \\ Nonanal & 10.8 & 25 & 25 & 120 & 140 \\ Octanoic acid & 11.9 & 400 & 560 & 550 & 550 \\ Nonanoic acid & 13.2 & 22 & 60 & 65 & 60 \\ Decanoic acid & 14.6 & 120 & 300 & 310 & 280 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Relative abundance of volatile compounds in milk and variations with equilibration time. and acids [19]. Aldehydes were absent from the 42-week cheese, and five different alcohols were present. Nonalactone was not observed in the milk but is a common volatile compound in cheese. Furaneol and esters were first detected after 4 weeks storage. Furans such as furaneol, which has an IUPAC nomenclature of 4-hydroxy-2,5-dimethyl-3-furanone, are formed in dairy products by some Lactobacilli strains [20]. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**Retention time (min)**} & \multicolumn{2}{c}{**Extraction time (min)**} \\ \cline{3-4}  & & **30** & **60** \\ \cline{3-4}  & & **Relative abundance (thousands)** \\ \hline
2-Pentanone & 3.1 & 920 & 620 \\
2-Pentanol & 3.3 & 1400 & 700 \\
3-Methyl-1-butanol & 4.1 & 500 & 260 \\
2-Hexanone & 5.7 & 100 & 100 \\ Ethyl butanoate & 6.3 & 420 & 200 \\
1-Hexanol & 10.7 & 150 & 175 \\
2-Heptanone & 12.2 & 3200 & 3200 \\
2-Heptanol & 13.0 & 2400 & 1900 \\
1-Heptanol & 16.5 & 40 & 40 \\ Octanoic acid & 24.8 & 500 & 360 \\ Ethyl octanoate & 25.0 & 375 & 200 \\ Furanone & 26.7 & 55 & 0 \\ Nonanoic acid & 27.1 & 44 & 0 \\
2-Undecanone & 27.8 & 1160 & 1120 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Relative abundance of volatile compounds in Queso Fresco when extracted at 60 °C for 30 or 60 min. \begin{table}
\begin{tabular}{l c c c} \hline \hline  & \multicolumn{3}{c}{**Storage time (week)**} \\ \cline{2-3}  & **1** & **4** & **8** \\ \hline _Acids_ & & & \\ Acetic & ND & M & M \\ Butanoic & ND & ND & M \\ Octanoic & M & M & VH \\ Nonanoic & ND & ND & M \\ Decanoic & M & M & H \\ Dodecanoic & M & L & M \\ \hline _Aldehydes_ & & & \\ Pentanal & M & M & ND \\ Hexanal & M & M & ND \\ Heptanal & L & L & ND \\ Nonanal & M & VH & VH \\ Decanal & L & L & M \\ \hline \hline \end{tabular}
\end{table}