Solid residue (kernels, peel films, parts of pulp) from the pressing of the olive paste. It represents 30 - 50% of the processed olives.
Its composition is:
Oil: 5 - 10%
Water: 25 - 30%
Solid fraction of which 30% is core: 60 - 75%
The acidity of the oil extracted from the pomace ranges from 15 - 80%; high acidity should be avoided because it complicates the rectification which becomes more expensive. The acidity increases with the stop at the oil mill due to the hydrolysis due to the lipase, active in the presence of H2O and the auto-oxidation due to the contact with the air and to the remarkable surface development. Antifermentatives like NaHSO3 (2 - 3%) are not recommended because they cause drawbacks.
The best conservation is obtained by drying in special rotary ovens, generally fed with pomace; with these, however, anhydrous pomace must never be obtained because cell membranes would be denatured which, forming an impermeable layer, would not allow the extraction of the oil. The optimal humidity value is 7% and in industrial practice it is controlled by listening to the noise produced by a handful of pomace tightly in hand. If the humidity is higher, the lipase can intervene with an exothermic reaction which can also cause fires due to the self-combustion of the product when parked before extraction.
Whisk extraction process (now disappeared).
It is a physical - mechanical procedure by which the regrind pomace is thrown into a tub of water. The light parts (sticks) rise to the surface, the stones go to the bottom and are discharged. By means of a stirrer (whisk), the separation of the oil which is freed from the nozzles for washing in successive cascade tanks is favored. Just over half of the oil is recovered.
A good solvent suitable for this purpose should have the following characteristics:
1) be easily transportable and storable;
2) not be toxic, nor be able to transfer odors and flavors to oil;
3) not be flammable;
4) be highly selective, ie not to extract gum resins, mucilage and other substances that complicate refining;
5) be of low latent heat of evaporation.
CS2 carbon disulfide is well stored, it is enough to cover it with a layer of water that prevents evaporation. It is however highly flammable, non-selective (extracts waxes, resins and mucilage), leaves unpleasant smells and flavors (releases S and H2S). It has now fallen into disuse.
Carbon tetrachloride (CCl4) would have the suitable characteristics, but it is too expensive. In some old plants, trichlorethylene is still used, but when hot it attacks metals, it is also not very selective and gives a very light green oil.
Currently the best solvent is the technical n - hexane which has a boiling point at 70 ° C and composition: 60% hexane + aromatic and olefinic hydrocarbons. It has a very low latent heat of evaporation and is very selective.
The dried pomace is regrinded or laminated, that is, passed through a rolling mill which reduces it into very thin, very disaggregated sheets. With the extraction techniques, 0.5 - 1% of oil remains in the pomace.
In 1 there is the pomace; in 2 bubbles the solvent with the extracted oil; in 3 (refrigerant) the vapors are condensed; 4 deposit of the solvent. The solvent enters 1, extracts the oil and passes into 2, from here it evaporates and condenses into 3, resuming the cycle. Once extraction is complete, the solvent condensed in 4 is diverted. Thus in 1 the exhausted (exhausted) pomace is found, in 4 the condensed solvent, in 2 oil. The cycle is discharged and resumed.
CONTINUOUS SYSTEM: IMMERSION SYSTEM
a) The solvent enters S1 goes to the other tower and leaves S2 loaded with oil.
b) The pomace does the opposite way: they enter the first immersion extractor from M, mix with the solvent that arrives in countercurrent, partially releasing the oil, fall to the bottom where they find a doser and then an auger that transports them to the second extractor P .
c) Here a bucket elevator brings them up and since they come into contact with the pure solvent they release the residual oil and are discharged outside. Since during the fall in the first extractor the kernels fall to the bottom, but the lighter parts remain suspended, filtration is essential.
CONTINUOUS SYSTEM: BY PERCOLATION
a) The pomace is loaded by a dispenser onto a perforated cup conveyor belt which follows a closed circuit path inside a container.
b) From above it rains the solvent which, percolating through the pomace, dissolves the oil and falls with it into a tank below.
c) The plant is divided into two sectors: when the pomace crosses the first one, it is sprayed with recycled solvent. This is further enriched with oil and goes to the distiller from the collection tank.
d) Entering the second zone, it is sprayed with pure solvent, which completely exhausts it; the diluted solution that is collected constitutes the solvent to be recycled.
e) The extraction runs out in one turn, at the end of which each cup is discharged outside and immediately after refilled, without interruption. The enrichment of the solvent in A has the purpose of sending to the distiller a solution enriched to the maximum in order to reduce operating costs.