# Filtration Scheduling: Quality Changes in Freshly Produced Virgin Olive Oil
## 1 Introduction  
Olive oil is composed of triacylglycerols, which make up over 98% of the total weight and about 2% minor components--aliphatic and triterpene alcohols, hydrocarbons, sterols, non-glyceride esters, pigments, lipophilic and hydrophilic phenols, and volatile compounds [1]. Volatile compounds are a complex mix of aldehydes, alcohols, ketones, acids, hydrocarbons, and esters and are closely associated with both oil flavour and its positive and negative sensory attributes [2, 3]. Flavour is not only a key characteristic that affects consumer preferences [2, 4], but it is also a quality parameter in oil classification. In 1991, the European Commission laid down legal limits for many quality parameters, including sensory attributes [5]. The panel has to confirm the absence of sensory defects (median of defect equal to 0) and the presence of the positive, fruity attribute (fruity median greater than 0) in order to classify an olive oil as \"extra virgin\". On the other hand, when the median of the defects is between 0 and 3.5, and the fruity median is greater than 0, the olive oil is classified as \"virgin\"; when the median of the defects is greater than 3.5, the olive oil is classified as \"lampante\" [6]. Five sensory defects, in particular, are described in the official sensory evaluation methodology: fusty, muddy, mustiness-humidity, winey-vinegary, and rancid. It is well-known that the volatile compound content of olive oil is affected by several operating factors. The first group relate to \"in-field\" factors such as the environment, agronomy, genetics, timing,and type of harvesting; then, there are \"out-of-field\" factors, such as the transport and storage of fruit; operating conditions during extraction; and oil storage, packaging, and transport conditions [7; 8; 9; 10; 11]. The pleasant, fruits attribute of extra virgin olive oil (EVOO) is mainly related to lipoxygenase (LOX) pathway volatile compounds [12; 13]. Z-3-hexenal and E-2-hexenal compounds are described as having \"green leaves\" and \"green and sweet\" sensory notes, respectively. Their low odour thresholds mean that they are the most important volatile compounds in the LOX pathway [8; 12; 13; 14], along with several others that contribute to the fruits attribute. The rancid attribute is a widely studied sensory defect. It is due to lipid auto-oxidation molecules, generally heptane, E-2-heptenal, 2,4-heptational, 2-heptanol, nonanal, 2,4-nonadienal, and decanal volatile compounds [2; 8]. During oil storage, oxidation is promoted by factors such as light (photo-oxidation), temperature, and minor components such as metals and pigments. Furthermore, it is well-studied that olive oil antioxidant phenolic compounds (i.e., especially the secoiridoids) are able to slow down rancid defect formations [7; 15]. The remaining defects (fusty, muddy, mustness-humidity, and winey-vinegary) are commonly related to microbial spoilage [16]. Incorrect fruit storage conditions (long period of time, high relative humidity, and high temperature) and poor hygiene during oil extraction support microbial activity and growth. Various sensory defects have been related to different microorganism genera, with a predominance of yeasts and moulds [7; 16; 17]. Volatile compounds such as 2-methyl-butanal, 3-methyl-butanal, isobutanol, 6-methyl-5-hepten-2-one, 2-octanol, 2-heptanone, 2-nonanone, and many others have been described as having sensory notes of \"muddy sediment\", \"earthy\", \"mushroom\", \"oily\", \"winey\", and \"vinegar\" [2; 17; 18; 19]. Extra virgin olive oil producers aim to ensure that the quality characteristics of the extracted oil remain as stable as possible over time. Typical operations to achieve this objective include finishing centrifugation, oil clarification, and filtration; they transform the appearance of the oil from veiled to limpid, either individually or in combination.