# Understanding Olive Oil Stability Using Filtration and High Hydrostatic Pressure
## 3 Discussion  
According to the literature data [21; 28; 29], the applied filtration and HHP treatments were able to create olive oil samples with different microbial contamination, water content and water activity levels (Table 1). Therefore, this work achieved its aim of creating olive oil samples with different susceptibilities to microbial, enzymatic and non-enzymatic transformation phenomena [30]. The CON oil samples were highly susceptible to all the above phenomena, since they had a high level of microbial contamination, water content and water activity. Indeed, water activity values \(>0.6\) A\({}_{\text{w}}\) potentially make foods more prone to transformation phenomena [31]. The HHP oil samples were highly susceptible to enzymatic and non-enzymatic phenomena only, since they had no microbial contamination, but a high water content and level of water activity. The FIL and F-HHP oil samples were not very susceptible to any of the above phenomena, since they had no microbial contamination, a low water content and low water activity (\(<0.6\) A\({}_{\text{w}}\)). The evolution of the measured EU legal chemical limits during storage showed that neither enzymatic hydrolysis by lipases nor radical oxidation of the triacylglycerlycerols occurred on any of the olive oil samples. The potential lipases from microorganisms [15] were not active, since the acidity value did not change in the CON samples. The potential endogenous lipases were not active either, since the acidity value did not change in the HHP samples. The relatively short storage time may explain the above behaviour; [PERSON] et al. [16] observed a hydrolysis of triacylglycerlecrools in unfiltered oil samples, but they were working under accelerated storage conditions at 40 \({}^{\circ}\)C in the dark. No effect of water content or water activity was evidenced on the rate of radical oxidation of the triacylglycerols. The relatively short storage time may explain the above phenomenon, but contradictory literature data have suggested that water has a protective effect against oxidation [14; 32] or that the rate of lipid oxidation is lowest at a water activity of 0.2-0.4 Aw [33]. However, [PERSON] et al. [34] also showed no significant effect between the oxidation levels of filtered and unfiltered olive oil. Instead, the veiled oil samples were affected by significant changes of sensory attributes, and volatile organic and phenolic compound contents, which can be explained by the experimental data as an effect of either a microbial contamination or a high level of water activity. Only the CON oil samples (i.e., with a high level of microbial contamination, water content and water activity) had a \"fusty\" defect during storage (Table 2) and an increasing content of microbial metabolite compounds during storage (Table 3). These behaviours can be considered congruent, related to each other and in line with some literature data [15; 35]; Figure 1 clearly shows that the removal of microorganisms by filtration and HHP treatments prevented the formation of volatile organic compounds, which were responsible for the \"fusty\" defect. Only the CON oil samples had a rancidity defect, which increased during storage (Table 2). Therefore, microbial activity may be related to the formation of the above sensory defect; this phenomenon, even though not well studied, has already been reported by both [PERSON] et al. [24] and [PERSON] and [PERSON] [15], who linked the rapid appearance of the \"rancid\" defect with olive oil samples contaminated by yeasts. In this way, the appearance of the rancidity defect without a significant radical oxidation of the triacylglycerlecrools in all the olive oil samples (as reported above in the text) may be explained.