10Olive cleaningClaudio PeriUniversity of Milan, Milan, Italy

Abstract

Olive cleaning is carried out in two steps. In the first ‘separation’ step, particulateforeign materials are removed by sifting, vibrating screens and air blowing of leaves.In the second ‘washing’ step olives are shaken into a washing basin and finally rinsedwith clean water. Suggestions are given concerning the rinsing step, the daily plantcleaning, the frequency of water replacement and the control of water temperature.

10.1 Introduction

The aims of this operation are twofold: (i) removing leaves, as well as pieces ofwood, small branches, stones, damaged olives and other particulate material thatmay have been accidentally collected with the olives, and (ii) washing dust and soilfrom the olives.

Particulate material should be separated from the olives by mechanical and pneu-matic action, whereas removal of soil requires washing with water. The cleaningoperation is therefore carried out in two sections: the separation and the washingsections.

10.2 The separation section

Figure 10.1 presents the functional scheme of the separation section.The separation of olives from particulate foreign material is carried out in three

steps:

1. In the first step, the olives are fed into a rotating cylindrical sifter with chinksor holes larger that the size of the olives. Two actions take place: (i) a shearingaction causing the olives to be pulled off the small branches and (ii) separation

The Extra-Virgin Olive Oil Handbook, First Edition. Edited by Claudio Peri.© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

114 CH10 OLIVE CLEANING

The rotatingcylindrical

sifter

The air-blowingleaves

separator

W2

W1

W3

To the washingsection

The vibratingscreen

Figure 10.1 Olive cleaning: the separation section.

of the olives through the holes of the sifter, while larger material, such as smallbranches or stones, is retained inside the sifter and discharged as the first drywaste (W1 in Figure 10.1).

2. In the second step, the olives pass over a vibrating screen with openings smallerthan the olives. In this step, small particulates including soil, pebbles, brokenolives, pass through the screen and are discarded as the second dry waste (W2).

3. In the third step olives slide over an inclined screen while air is blown acrossthem. Particles that are lighter than the olives, leaves in particular, are sweptaway and finally discarded as the third dry waste (W3).

In some plants, the order of the three steps may be different, but the aim and theresult are the same. There is an increasing interest in carrying out the separationstep at the olive grove, as a pre-processing step. Such an arrangement allows the drywaste to remain in the field and do not accumulate at the olive mill site.

10.3 The washing section

Figure 10.2 presents the functional scheme of the washing section.The washing of the olives is carried out in two steps:

1. In the first step, olives are dumped into a washing basin where they are vigor-ously shaken by bubbling air. This action facilitates the wetting and removal ofdust and soil. The olives are pushed through the washing basin by the incoming

10.4 CONTROL POINTS 115

Olives from the separationsection

The airinlet

The washingbasin

The waterrecycling

The waterreservoir

The finalrinsing

W4

Figure 10.2 Add outlet W4 including the arrow.

flow of the recycled water and overflow with the water at the opposite end ofthe basin.

2. In the second step, the washed olives, still wet with dirty water, are transferredthrough a draining belt under sprays of clear potable water for the final rinsing.The clean wet olives are finally transferred by a belt elevator to the millingsection.

A reservoir underneath receives both the dirty water overflowing from the washingbasin and the final rinsing water coming from the draining belt. The reservoir actsas a sedimentation tank in which the heavier soil particles settle to the bottom, whilea centrifugal pump recycles the water back to the washing basin.

The addition of rinsing water (at a flow rate roughly 10% of the weight of theolives) causes a continuous overflow of dirty water from the sedimentation tank,which is discarded as waste water (W4).

10.4 Control points

The critical points of this operation are:

• water replacement and plant cleaning

• rinsing

• water temperature.

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Water replacement and plant cleaning

Recycling of the rinsing water causes partial replacement of the dirty water withrelatively clean water. This is not sufficient to guarantee an acceptable level of watercleanliness. A periodical (daily at least) total replacement of the dirty water withclean potable water is needed. At the same time, the entire washing section shouldbe cleaned: removal of sediment from the washing basin and the sedimentationtank, washing of the draining belt, the recycling pump and piping, and the sprayrinsing nozzles. The separation section must also be cleaned at a similar frequency,in particular the vibrating screens and the air blowing circuit.

The rinsing step

One of the lesser understood points is the essential role of the final rinsing of theolives. Observing the olives coming out of the washing basin, they appear to bewet and brilliant as if they were perfectly clean. In fact, they are wet with dirty waterand therefore far from being really clean. The dirty water often contains Fe (iron)and Cu (copper) ions, which are strong catalysts of oil oxidation. The purpose ofthe rinsing step is precisely to replace the dirty water wetting the olive surface withclean, uncontaminated water. This necessitates a suitable number of sprays over theentire mass of olives and on each single fruit while it is being transferred over thedraining belt.

The water temperature

In the northern hemisphere, olive harvesting takes place in the winter, mostly fromlate October to early December, when the atmospheric temperature is often lowerthan 10 ∘C. Tap water in this period also has a low temperature (12–14 ∘C). If wash-ing with cool water is carried out on even cooler olives, two problems may occur:lower effectiveness of the washing operation and, secondly, a low temperature atthe milling operation, resulting in a low temperature of the olive paste at the begin-ning of the malaxing operation. The optimal range of temperature in the malaxingoperation is 24–27 ∘C.

A good solution is to use water at a temperature of 20–24 ∘C for washing theolives. This allows more effective washing and better control of the temperature con-ditions during the following milling-malaxing-decanting sequence of the process.

Such a temperature can be easily obtained by circulating water at a suitabletemperature in a coil heat exchanger in the washing basin or in the decantation tank.