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How long should olives be conserved before pressing them?

Hours? Days? The answer is apparently simple: as short a time as possible. Some factors can affect the ageing of the fruits

The cultivation of olives is turning more and more into a part time activity. It is more and more complicated to fine harvesters, because of the ever stricter controls, of the low wages, and of the age of the active workers. The mechanization and the work for third parties are not common yet. The shakers have been spreading, but the daily production for small harvests is often insufficient to guarantee the minimal quantity to press, so that the olives are often stocked for some time. Also in mid sized firms the olives are conserved, in order to lose some humidity and save on the costs of olive pressing.

For how long? In which conditions?
The research in this field already answered these questions several years ago, and the recommendation is to conserve the olives in holed, 20 kg capacity cases, for not longer than two days. This advice is still valid but more and more often ignored, with the effect of dilating the times of transportation to the press.

Hence, it is useful to summarize the data and results that the scientific world published on this issue.
When the fruit is harvested, biochemical phenomena begin under the impulse of enzymatic complexes, that make the fruit softer and softer, make it lose water, break the vacuoles, permit the installation of bacteria and fungi, whose populations exponentially grow just during the first two days of stocking, thus generating fermentation processes that bring to disagreeable smells and tastes (organoleptic defects).

For instance, a study by Prof. Di Giovacchino showed that, just two days into the stocking period, the content in ethanol is four times bigger (from 30 up to 125 ppm), then stabilizing on such values. The acetic acid concentration is three times as large just four days after the harvest (from 0.2 up to 0.6 ppm). The propionic acid is measurable only after six days into the stocking phase (0.9 ppm).

The deterioration in the olives quality due to the stocking can be better shown looking at other chemical values, such as acidity, peroxides, and spectral-photometrics values. In particular, the acidity grows beyond the value of 0.8, which is the limit for the definition of an extra virgin oil, after one week of conservation in aired cases, with a temperature of approximately 20 C; the number of peroxides increases of 30% in one week, but if the olives are healthy and the pressing is correctly carried out, should remain below the value of 20. The acidity is not the only parameter sensitive to a prolonged stocking. Poliphenols decrease exponentially too, halving their content after the first week of stocking, and further decreasing to a third of their original content after the second week.

The stocking temperature greatly affects the occurrence of rots. For temperatures around 5 C, no rot is noticed after 7 days from the harvest. For temperatures around 12 C, the effects are very different, with rots occurring in 70% of the cases, and a 30% reduction in the consistency of the fruits.

Some researches also pointed out that fruits from different harvests and types of olives can react to the stocking in different ways. Prof. Montedoro found that the Leccino olive reacts better than the Moraiolo olives, while Prof. Di Giovacchino has shown that the Leccino displays a better response than the Dritta and Nebbio olives.

Obviously, the riper the fruit, that is the less consistent, the more apparent the effects of a prlonged stocking will be.

In conclusion, the recommendation of pressing the olives within two days from their harves is definitely reasonable and should be followed. Anyway, waiting for more than for days should be absolutely avoided. The temperature of stocking is one of the more important factors: it is recommended to stock olives in a fresh place, not a little aired cellar, where moulds would end up altering the chemical and organoleptic profile of the oil.

by Alberto Grimelli
07 december 2009, Technical Area > Olive & Oil