At last! A paper reporting the effects of real olive mill based filtration on extra virgin olive oil composition. To date all research on the effects of filtration have been conducted using small lab based simulations.
Bakhouche et al. (2014) Monitoring the moisture reduction and status of bioactive compounds in extra-virgin olive oil over the industrial filtration process. Food Control. 40, 292-299. http://dx.doi.org/10.1016/j.foodcont.2013.12.012
Filtration is performed to produce perfectly clear oil that most consumers desire. But what are the consequences of filtering?
45 tonnes of two extra virgin olive oils were filtered through ‘cakes’ of Vitacel_L90 (30 kg, composed of 100% cellulose) and Filtracel_EFC-950 (60 kg, composed of 70% cellulose, 30% lignin) and the change in moisture and polyphenol content was monitored throughout the during the filtration process.
- Filtering reduced the moisture content by between 20 and 36% depending on how ‘clogged’ the filter was at the time of sampling, and the initial moisture content of the oil.
- (illogically) filtration increased total polyphenols. However most polyphenol types were reduced following filtration particularly the small simple phenolics hydroxytyrosol and tyrosol.
The reduction in moisture content can be attributed to the water binding ability of polysaccharides (of which cellulose is one).
The observation that total polyphenols increased following filtration is ‘bizarre’. Filtration substrates do not contain polyphenols*, so logically, the filter material can either let polyphenols pass through resulting in no change, or adsorb them, which would result in a reduction in polyphenol content. The authors attribute the unexpected increase in polyphenols following filtration to limitations in how the dominant polyphenol class (the secoiridoids) are extracted from the oil prior to being quantified. That is, the illogical increase in polyphenols after filtration is an artifact of the laboratory measurement process.
The other polyphenol classes generally decreased due to filtration. As the filter material became more used, there was a trend for the difference between filtered and non-filtered samples to be reduced. This suggests that the filtration material was absorbing phenolics (because as the filter material has a finite number of binding sites, so as it becomes more used it is less effective in retaining phenolics – so the pass through).
So what can we take out of the study.
1) Filtration reduces the moisture content of extra virgin olive oil. Practically this is a positive effect as lower moisture means decreased chemical breakdown (hydrolysis) of the fat molecules that make up extra virgin olive oil that lead to free fatty acids (acidity). Lower oil acidity is associated with greater oxidative stability aka shelf life, and higher smoke points.
2) The effect of filtration on polyphenol content of extra virgin olive oil is still a ‘bit murky’, however there is some evidence that filtration does reduce polyphenols or at least some types of polyphenols.
* The increase in polyphenols was caused mainly by increases in secoiridoid compounds which are found in olive fruit. This rules out the possibility that the filter material inadvertently contained some form of phenolic contaminant which was extracted into the oil during filtration.