In order to obtain excellent quality extra virgin olive oil, extreme care must be taken during every single phase of the transformation process which concludes with olive oil production.

Let’s take a look at which chemical parameters must be monitored during each phase, to improve oil quality, along with some of the most common analysis methods.


Harvesting is not actually a “processing” phase, however it is an extremely important production process phase.

During harvesting, decisions are made which will influence the quality of oil produced. The first important decision is when to harvest.

On the one hand, harvesting too early may result in a lower yield, on the other hand, harvesting too late results in lower polyphenol content.

Deciding on how long to wait between harvesting and pressing also requires careful consideration, especially with typically warmer winter temperatures, as acidity and peroxide values can rise sharply in the space of a few days, even in healthy fruit.

Obviously these effects are more pronounced on compromised fruit, for example due to flies.

In such cases it is essential that olives are shipped to the mill as quickly as possible, to ensure acceptable acidity values within statutory limits.


Leaf stripping, washing and pressing

Pressing techniques have improved over time, to prevent heating during the process. In any case these phases generally tend not to have an excessive bearing on product quality.


Kneading is a delicate phase as over a more or less prologued period of time, the paste is exposed to oxygen in the air.

The correct management of kneading is very important because it enables a correct balance between yield and quality. Indeed excessively lengthy kneading may improve yield, but it also results in excessive paste oxidation.

It also exposes the oil to excessive contact with amurca, resulting in polyphenols being “washed away” and an overall depletion of extra virgin olive oil quality.

During this phase it is also important to keep the temperature under control as excessive temperatures may accelerate oxidative processes, with a consequent negative effect on the peroxide value.

During this delicate phase, a few oil mills use nitrogen atmosphere to prevent contact with the air and minimise oxidation.

Separation and Extraction

After kneading, the paste is placed in a decanter, or a horizontal centrifugal separator, which separates the solid part, the pomace, from the liquid part, consisting of water and oil.

Pomace is fed though a screw into special storage receptacles, whereas the liquid part is sent to the separator, a vertical centrifuge, which separates water from the oil.

Compared to the pressure system used in the past, the centrifugation system has the advantage of ensuring continuity of processing, however it also presents a disadvantage: an increased chance of polyphenol loss in amurca.

For this reason, over the last few years centrifuges which requiring the addition of little or no water have become popular as they minimise the probability of reduced polyphenol concentration. It is essential to preserve natural  polyphenol content during the transformation process, for an excellent quality extra virgin olive oil, rich in flavour.

Separation and extraction can also be carried out using alternative methods such as percolation or sinolea, where oil is separated by selective filtration at low temperatures to prevent water’s wash-out effect.

Polyphenol analysis can also provide oil mill directors with useful information so that they can act to amend parameters and increase the quality of oil produced.

The acidity of an olive oil must also be measured. Indeed acid concentration is the oldest system used to control and classify oils, however alone it is not sufficient for defining total product quality.


Technically, storage is not a phase of the oil extraction process, however it considerably influences quality over time.
Most people are aware that oil should be stored in clean receptacles away from light.
Correctly stored oil may maintain its organoleptic and chemical characteristics for several months.
Incorrect oil storage may result in a rapid rise of peroxide values.
Checking these parameters during storage may be extremely useful.

Analysis methods

In order to obtain an optimal extra virgin oil, it is essential for the transformation process to be improved, by continuously monitoring all stages and carrying out different chemical analyses. Let’s take a look at the most common methods.

Acidity and peroxides

Acidity and peroxide value analyses are carried out in a laboratory using the official method: titration.

This time-consuming method requires a qualified operator capable of using all necessary equipment and handling all reagents used.

Titration also requires specific precautions, such as the use of a extractor hood and relative mandatory protection devices for the handling of inflammable and/or cancerous substances. Therefore this method for determining acidity and peroxide levels in oils requires a chemical laboratory.

Sometimes this method is also used at oil mills, where it is carried out using acidimeters which simplify the procedure, despite yielding less accurate results, characterised by reduced sensitivity.


Polyphenols can also be detected using complex devices, such as the high performance liquid chromatograph (HPLC), in a long and laborious method involving chemical reagents, including dangerous ones, laboratory equipment and qualified personnel.

A simple method to control the transformation process at the oil mill: CDR OxiTester

The CDR OxiTester is a valid tool for monitoring the transformation process and controlling extra virgin olive oil quality. In just a few minutes it detects acidity, peroxides, polyphenols and K270, without requiring expert laboratory personnel, complex instruments or equipped laboratories.