ARTICLE
The olive fruit
The fruit contains about 50% water, 20% oil, 20% carbohydrates (pectic,
cellulosic and hemicellulosic substances), organic acids, pigments, phenolic
compounds and minerals.
Most of the oil, 96-98%, is found in the mesocarp (flesh) and epicarp
(skin); the remaining amount is located in the endocarp (stone). In the
plant-cell, the major part of the oil is located in vacuoles ("free oil"),
a minor part ("bound oil") is found in the cytoplasm in a dispersed form
[1]. This oil is bound to various substances with emulsifying properties,
like lipoproteins, and is hardly accessible in the extraction and separation
process and therefore lost with the waste. Those substances can also affect
the separation properties of the free oil and make processing difficult,
with a negative impact on yield and plant-capacity.
Processing of olives
The process can be divided into milling (stone mills, hammer mills),
mashing (malaxation) and separation of the obtained phases (presses, decanters,
percolators, centrifuges). Industrial enzymes are applied in the malaxation
process. This step has two purposes, a physical one, the coalescence,
where very small oil droplets may join together to form droplets of 30
microns and more in order to enable an efficient phase separation; but
also a biochemical one, where the fruit-own enzymatic system may react
on the paste.
Olives, like all living organisms have a complex system of so-called
endogenous enzymes. Amongst others, pectic, cellulosic and hemicellulosic
enzymes have been detected [2]. The amount of those enzymes varies with
fruit variety and fruit ripening. Those enzymes, the same mechanism is
found in all fruits, are responsible for the softening during ripening
as they are involved in the changes in the cell wall structure by a partial
degradation of the carbohydrates which make up the cell wall. The endogenous
enzymes are set free during the milling process where the plant cells
are mechanically disrupted. The action of those enzymes results in emulsion-breaking
and in changes of the rheological properties of the paste, giving a higher
yield and a better phase separation. Not always, the amount of endogenous
enzymes is sufficient in order to ensure optimal results. The addition
of industrial enzymes (a mixture of pectinases, hemicellulases and cellulases
mainly), of the same type as the naturally occurring endogenous enzymes
to the malaxation speeds up the process [3]. The bound oil is released
due to the degradation of the emulsifying substances resulting in a higher
overall yield and, due to a more complete degradation of the cell-material;
valuable substances from the fruit are set free to a higher extend, such
as antioxidants and taste and flavour determining compounds. Due to the
lipophobic character of the enzymes used in this process, no detectable
traces are left in the oil and the enzymes do not have any side activity
which could have a negative impact on oil quality.
The benefits of industrial enzymes in the
process
The use of those processing aids can give the following advantages;
to which extent, depends on fruit and process equipment:
*Improved storage stability of the oil due to the increased amount of
antioxidants
*Improved taste and flavour due to a more complete extraction
*Yield increase due to the release of the bound oil and more effective
breaking of the emulsion
*Increase in plant capacity due to changes of the rheological properties
of the paste
*Reduced amounts of oil in the waste phases
*Better filterability of the oil containing less impurities
In the following, some results of the various studies performed are
presented. All the trials have been done on an industrial scale and the
enzyme used was OLIVEX®, a preparation from Novo Nordisk Ferment Ltd.
(Switzerland).
Practical applications
The enzyme preparation is diluted with about five parts of water and
added either batchwise into the stone-mill or, in continuous systems,
by means of a dosing-pump into the hammer-mill or the malaxer. The dosage
applied is 200 ml of undiluted product per ton of olives.
Figure 1 shows an example
of a mass-balance obtained with a 3-phase continuous line (addition of
700 litres of water per ton of malaxed paste for the decanter passage).
Normally, the increase in oil yield using enzymes is mainly due to the
reduced oil content of the pomace.
The influence on plant capacity when a traditional press system is used
is shown in figure 2.
The paste which has been extracted with enzymes shows a faster release
of the oil over time and gives a higher total yield. This allows e.g.
to shorten the press-time by 50% still obtaining the same total yield
as obtained without using enzymes, thus doubling the plant capacity. In
addition, already during the piling of the mats, the increased amount
of free flowing oil allows to load up to 10% more mats per pile.
In continuous systems the decanter throughput can be increased, up to
40% increase is possible.
The malaxation time and temperature have an influence on the total yield,
the longer the reaction time and the higher the temperature (this of course
within the tolerable parameters), the higher the yield increase will be.
This effect has however its limitations and is more expressed when relatively
short malaxation times are used, e.g. working with stone-mills.
The effect of the enzymatic extraction in a combined percolation/decanter
system is shown in figure 3.
It is of special interest to note the drastical yield increase in the
percolation system, due to the deemulsifying action of the enzymes.
In general, the by-products of the extraction process show the following
characteristics:
*The cake shows a lower oil content (and a reduced humidity in 3-phase
separation systems).
*The vegetable water contains less oil and the total solids are reduced.
Due to the enzymatic degradation of carbohydrates into fermentable sugars,
the biodegradability is eased.
Figure 4 gives an example
about the composition of the solid waste (pomace). The waste water too
shows a reduction of the oil content when enzymes are used; typically,
the content in dispersed solids is reduced too.
The influence on the development of free acidity during storage of the
oil is illustrated in figure 5.
In addition to this example it can be stated that the use of enzymes in
the extraction process with regard to oil quality results in [4]:
*a significantly higher content of phenolic (polyphenols and alpha-diphenols)
compounds and therefore a higher oxidative stability;
*sensory scores in panel test are slightly higher;
*the quality indices are slightly higher, mainly as a consequence of
above results;
*whereas the other chemical characteristics of the oils do not show
significant changes.
The use of enzymes in olive oil extraction obviously offers certain
advantages (especially with olives which are known of being "difficult
to process"). The results obtained may be more or less expressed as many
parameters can influence the process, olive-variety, ripeness, extraction-system
and conditions, operating-conditions.
CONCLUSION Olive
oil belongs to the group of food with the longest tradition in human nutrition
(besides many other applications), like bread, beer, cheese and wine; and
enzymes have always been an important part of the production methods. Whereas
those traditional industries transformed into highly industrialised processes,
making use of the knowledge about industrial enzymes, olive oil industry
only slowly underwent changes; nevertheless, over the past decades, considerable
progresses on the equipment sector have been achieved. Enzymes represent
one contribution towards modern technology with a high quality level. REFERENCES
1. MONTEDORO GF (1987). Impiego di preparati enzimatici e drenanti
nell`estrazione meccanica di oli di oliva. La rivista delle sostanze
grasse - Vol. LXIV - Ottobre : 415-21.
2. HEREDIA A, GUILLEN R, JIMENEZ A, FERNANDEZ-BOLANOS J (1993).
Activity of glycosidases during development and ripening of olive fruit.
Z Lebensm unters Forsch, 196:147-51.
3. ALBA-MENDOZA J (1992). Avances recientes en la extraccion del
aceite de oliva. Aceite de oliva, Mira Editores, 50007 Zaragoza (Spain)
: 31-43.
4. RANALLI A, SERRAIOCCO A (1995). Effect induced by a pectolytical
adjuvant in olive oil extraction by the present technological systems.
Pluriannual research results. La rivista italiana delle sostanze grasse
- Vol. LXXII - Agosto : 355-64.
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