GrasasYaceites 1-57. pdf

4. OLIVE PASTE PREPARATION
Olive paste preparation can be separated into
two phases, fruit milling and paste kneading. The
fruit milling has the main objective of breaking the
plant tissues in order to liberate the oil drops
contained in the mesocarp cells.
In the press system stone mills are generally
used, the cylindrical or truncated conical shaped
stones rotate on a granite base. Nowadays,
although this system shows some advantages such
as the ‘torn effect’, emulsions are avoided and there
is no metal contamination, they are not used
because of their low capacity and the large amount
of space needed.
At present the crushing is performed with metal
mills, especially hammer mills equipped with a
single or double sieve. Disc mills are used as the
finishers of fruit crushing. This step is very
important to avoid the emulsions produced by a non
optimal milling level, water addition in this step or
even the use of small size sieves. On the other
hand, the crushing level is essential to obtain good
process yields. The milling should be adapted to the
fruit characteristics using smaller sieves for unripe
olives or for cultivars with hard consistence pulp
whereas for ripe fruits the milling grade can be
higher.
To obtain quality oil, metal traces should be
avoided in the olive paste because of their negative
effects on oil color and flavor, reducing its oxidative
stability by their catalysts activity in the oxidation
process. In order to reduce this problem the mills
are made using inert material as stainless steel
although a complete elimination is difficult.
The malaxation of the olive paste is used to
group the oil drops liberated during the fruit milling,
giving a continuous oily phase that can be
separated later. The oily phase is separated by
mechanical malaxation that improved the drop
coalescence into larger drops and breaks the
oil/water emulsions. The olive paste kneading can
be improved by heating since it reduces the oil
viscosity helping the oil drops to group whereas the
enzymatic activity into the paste is increased. This
operation is performed in a thermo-beater formed
by one or more malaxation containers where
inclined blades or spiral shaped parts stir the paste
giving a shearing effect. Depending on the rotation
26
GRASAS Y ACEITES
, 57 (1), 
ENERO
-
MARZO
, 25-31, 2006, 
ISSN
: 0017-3495
MARINO UCEDA, ANTONIO JIMÉNEZ AND GABRIEL BELTRÁN

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axis location the thermo-beaters can be classified
in horizontal and vertical mixers although based on
technical and economical reasons horizontal mixers
are more often used.
This step is essential to obtaining optimal oil
yields especially when hammer mills are used since
emulsions often appear and they can be broken by
an efficient malaxation. Two variables can be
regulated in the paste mixing to obtain good quality
oil and oil yield, kneading time and temperature.
From experimental results, on laboratory and
industry scales, it can be concluded that a minimum
kneading time is needed to obtain a reasonable
process yield although the kneading time interacts
with temperature. This minimum time may be
established between 60 and 90 minutes. When
malaxation temperature is fixed and three different
times are compared (50, 75 and 105 min), the
pomace oil content is lower for 75min whereas no
differences were obtained for 105 min. In
experiments at varying temperatures, significant
differences were observed at 18ºC, showing the
lowest values for 90 min. This trend was found for
higher temperatures (30 and 40ºC) although the
differences were not significant. Furthermore, a
long kneading time produces a decrease in oil
phenol content and the related parameters such as
oxidative stability and bitterness. Similar results
have been described by Solinas et al 1978.
The kneading temperature has a great influence
on the process yield since the oil droplets are
grouped due to a reduction in the oil viscosity.
However for excessive heating undesirable effects
can be observed: loss of aromatic compounds
responsible for oil flavor and fragrance and
accelerates its oxidative process.
In an experiment performed in the experimental
oil mill of IFAPA during three crop years (2000, 2001
and 2003) it has been observed that as kneading
temperatures increased the oils had a more intense
green color because of the higher chlorophyll
content and higher phenol and orthodiphenol
contents and therefore, they were more bitter and
unbalanced showing a decrease in oil flavor due to
a volatile loss.
Now the use of inert atmosphere in the mixer is
being studied preliminarily. Inert gas is used, mainly
nitrogen, to control the oxidation process and
enzymatic activities presents in the olive paste
during its malaxation in order to obtain an
equilibrium between the oil characteristics and the
process yield. Both, oil quality and process yield
show antagonism that should be solved. For this
reason, for high quality oils the malaxation should
be performed at low temperatures for a sufficient
time although these conditions can produce some
difficulties during the oil extraction reducing the
process efficiency. ‘Difficult pastes’ appears even
when kneading conditions are aimed to obtain the
maximum process yields.
In general, the solution to ‘difficult pastes’
proposed by some industrial mills was the fruit
storage but it produces lower oil quality or increasing
the malaxation temperature that has no effect and
negatively affects the oil quality. Another method,
more interesting and efficient, is to reduce the
process capacity of the oil mill but it increases the
production costs and the fruit storage period. A
technological approach to this problem has been the
application of technological coadjuvants. As a result
of these studies Spanish regulation authorizes the
use of natural micronized talc (hydrated magnesium
silicate). The addition of micronized talc to difficult
pastes improves the paste structure, reducing
emulsions. Visually, its use can be distinguished by
a higher free oil amount in the mixer, clean mixer
blades, a reduction in emulsions and clearer oils in
the outlet of horizontal centrifuge. The dose ranges
between 0.5 and 2% on fruit dry weight, depending
on the paste difficulty.
From the analytical point of view, the correct
application of micronized talc reduces the by-
products in oil content and therefore, improves the
process yield. As described, for three way systems
micronized talc increases the process yield by a
decrease in the oil content of the waste water
(alpechín) and higher oil content in the pomace.
Talc overdose, depending on the paste
characteristics, can reduce the process yield since
the higher pomace oil content does not compensate
for the oil content of the waste water. Therefore the
optimal use of talc and its dosification should be
monitored checking the alpechín and pomace oil
contents and performing quantitative balances. The
use of automatic dispenser is the only accurate way
to control the talc dose.
For the two way continuous system, the
micronized talc has shown high efficiency for
difficult pastes since the oil content on the dry
weight of pomace was reduced significantly and the
process yield was greater.
Micronized talc, does no affect the oil
composition and sensory characteristics
significantly although it has shown higher phenol
content and slightly more bitter and pungent flavor.