# Filtration Scheduling: Quality Changes in Freshly Produced Virgin Olive Oil
## 3 Results and Discussion
### Volatile Compound Contents
#### 3.3.2 Unpleasant Volatile Compound Contents  
Unpleasant volatile compounds were found in all samples, and their kinetics were measured during storage time (Figure 4).  
Figure 3: Mean contents of the LOX compounds for the C5 branch in veiled (blue squares) and filtered (red circles) olive oil samples during storage.  
A statistically significant interaction between filtration and storage time (\(p\)-value \(<\) 0.05) was found for the isobutanol, 2-methyl-butanal, and 3-methyl-butanal compounds. These compounds are related to microbial amino acid metabolism and are derived from valine, isoleucine, and leucine, respectively [45]. Their contents increased during storage in the veiled samples. In the filtered samples, the initial content was low and did not increase during storage. The same significant interaction between filtration and storage time was observed for two volatile phenol compounds--namely, phenol-2-methoxy and phenol-4-ethyl-2-methoxy. According to the literature, the former is related to the metabolic activity of several yeasts [46], while the latter is usually related to Brettanomyces contamination [47]. Since cinnamic acid is the precursor of phenol-4-ethyl-2-methoxy, which is an olive oil biophenol [48], the high experimental microbial content in the veiled oil samples could explain both its presence and its increase with storage time.  
Similarly, a significant interaction between filtration and storage time was observed for the following five volatile compounds: 2-heptanone, 5-hepten-2-one-6-methyl, E-2-octenal, 2-octenal, and 2-nonanone. In this case, the contents were initially the same in both the filtered and veiled samples, but after a few days, the contents increased in the veiled samples. The detection threshold was exceeded after a storage time ranging from three days (for 2-octanol) to 15 days (for E-2-octenal and 5-hepten-2-one-6-methyl).  
According to data reported in the literature [39; 49] the different behaviours of the 10 volatile compounds in the veiled and filtered samples could be caused by both microbial contamination and factors promoting microbial activity, such as oil turbidity, water and solid contents, and water activity. The high turbidity and high water activity (in the veiled samples) that promoted volatile compound formations were consistent with the high microbial contents; on the other hand, low turbidity and low water activity (in the filtered samples) were consistent with an undetectable experimental microbial count, and the volatile compounds remained constant during storage.  
All 10 volatile compounds are unpleasant in an olive oil and are frequently related to the fusty and other sensory defects. For example, 2-heptanone, 2-nonanone, and 5-hepten-2-one-6-methyl have been related to the \"mustiness-humidity\" defect and 2-octanol to the \"earthy\" and \"mustiness-humidity\" defects [2; 8]. The fusty attribute perceived by the panel test after five days in veiled samples is consistent with the experimental volatile compound kinetics (Figure 4).  
Figure 4: Mean contents of unpleasant volatile organic compounds in veiled (blue squares) and filtered (red circles) olive oil samples during storage.  
Other volatile compounds, typically related to the rancid defect, have also been observed [50; 51; 52]. A significant main effect of filtration has been found for 2,4-heptadienal, E-2-hepteneal, 2,4-nonadienal, nonanal, decanal, and E-2-decenal. In our experiment, their contents slightly increased as an immediate effect of filtration (Figure 5), but we found no significant interaction between the filtration and storage time. It seems that, as reported in the literature [19; 27; 53; 54; 55] filtration resulted in little oxidation of the oil samples in our experiment. The panel test supports the hypothesis of limited auto-oxidation, as no rancid defect was perceived. However, [PERSON] et al. (2020) [11] argue that one month of storage is insufficient to be able to observe the development of the rancid defect.