# Understanding Olive Oil Stability Using Filtration and High Hydrostatic Pressure
## 2 Results
### Quality Evolution During Storage
#### 2.2.2 Volatile Organic Compounds  
The volatile organic compound content of the oil samples was studied as the three following groups of compounds, in relation to their assumed role in oil sensory quality: (i) compounds with five and six carbon atoms, which are usually associated with the lipoxygenase (LOX) pathway and, consequently, with the \"fruity\" and \"green\" positive sensory attributes [25]; (ii) microbial metabolite compounds, which are usually associated with negative sensory attributes such as \"fusty\", \"muddy\", \"vinegary\" and \"mouldy\" defects [2; 15; 17]; (iii) compounds with seven, eight and nine carbon atoms, which are usually associated with the \"rancid\" negative sensory attribute [2; 3]. The list of the above measured compounds is presented as Supplementary Material in Table S2. The experimental data were processed as the sum of the three above groups of volatile organic compounds, except for the (\(E\))-2-hexenal compound, which was certainly associated in the literature with the \"fruity\" and \"green\" positive sensory attributes (Table 3).  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c c} \hline \hline
**Storage Time (MonMonths)** & \multicolumn{3}{c}{**F-HIP**} & \multicolumn{3}{c}{**FIL**} & \multicolumn{3}{c}{**HIP**} & \multicolumn{3}{c}{**CON**} & \multicolumn{3}{c}{**PF**} & \multicolumn{3}{c}{**HP**} & \multicolumn{3}{c}{**PP**} & \multicolumn{3}{c}{**HP**} & \multicolumn{3}{c}{**PT**} & \multicolumn{3}{c}{**PF**} & \multicolumn{3}{c}{**PF**} & \multicolumn{3}{c}{**PF**} & \multicolumn{3}{c}{**PF**} & \multicolumn{3}{c}{**PF**} \\ \hline
0.5 & 1 & 6 & 0.5 & 1 & 6 & 0.5 & 1 & 6 & 0.5 & 1 & 6 & 0.5 & 1 & 6 & 0.5 & 1  
Data from all of the oil samples showed that C\({}_{6}\) compounds from linolenic acid were the most abundant, in agreement with the literature [25; 26], demonstrating that the LOX pathway had a preferential action on linolenic acid. (_E_)-2-Hexenal was consistently the most abundant compound (i.e., from 85% to 92% of the sum of C\({}_{6}\) compounds).  
The sum of the LOX compound content with five carbon atoms was not significantly related to filtration, HHP treatment or storage time, while sum of the LOX compound content with six carbon atoms significantly decreased during storage (_p_ < 0.05). The (_E_)-2-hexenal content also showed a significant change as a function of filtration and storage time. The filtered oil (FIL and F-HHP samples) had a significantly (_p_ < 0.01) higher content of (_E_)-2-hexenal than the veiled oil (CON and HHP samples). Furthermore, in all of the olive oil samples the (_E_)-2-hexenal content significantly decreased during storage (_p_ < 0.001).  
The sum of the microbial metabolite compound content was significantly influenced by the interaction between the filtration and HHP treatments (_p_ < 0.05). Low contents of the above compounds were measured both in the filtered oil (FIL and F-HHP samples) and in the veiled oil treated with HHP (HHP samples), while the veiled oil (CON samples) had the highest content of microbial metabolite compounds.  
The sum of the C\({}_{7}\), C\({}_{8}\), C\({}_{9}\) and C\({}_{10}\) compounds (i.e., the \"rancid\" compounds) showed significant differences as a result of filtration (_p_ < 0.01), HHP treatment (_p_ < 0.01) and interaction between filtration and HHP treatment (_p_ < 0.01). After 15 days of storage, a higher content of the \"rancid\" compounds was measured in the veiled oil (CON samples) than in both the filtered oil (FIL and F-HHP samples) and the veiled oil treated with HHP (HHP samples). The above difference between the oil samples was lost after 1 and 6 months of storage.