[0001] The present invention is concerned with non-bitter olive oil with a high content
of antioxidants and with processes for preparing such olive oil.
STATE OF THE ART
[0002] Olive oil is appreciated as a salad oil and as a frying oil for its delicious taste,
not only in the traditional olive oil consuming countries, the Mediterranean area,
but to an increasing extent also in Western Europe and the USA.
[0003] Traditionally, olive oil is prepared by harvesting the olive fruits and subjecting
these to malaxation: crushing and kneading the olives with water so that a mash is
obtained containing the whole content of the olive fruit including an aqueous phase.
The mash contains also enzymes, including oil hydrolysing lipases, which originate
from the olive fruits.
In order to avoid undesired olive oil hydrolysis the aqueous phase with all olive
enzymes is separated from the oil phase e.g. by decanting, as soon as possible, anyway
within one hour.
[0004] Many olive oils after separation of the aqueous phase still possess a strong bitter
taste. Bitter olive oil often can be made fit for consumption by washing the crude
oil with water which reduces the bitter taste to an acceptable level. This usual washing
treatment employs a centrifuge for a quick final water separation. The contact time
between water and oil therefore is very short, usually less than 30 seconds, practically
only 10 seconds.
[0005] The bitter taste of olive oil is caused by so-called polyphenols, compounds which
are native to olive oil. Those olive oil derived polyphenols comprise relatively apolar,
oil soluble phenolic compounds as well as relatively polar, water soluble phenolic
compounds. In the context of the present invention both groups are denoted as polyphenols.
The bitter taste is caused mainly by the more apolar polyphenols. Apolar polyphenols
comprise the compounds oleuropein, ligstroside and their aglycons. These are the main
bitter polyphenols (Grasas y Aceites, 46, 1995, 299-303). The less bitter ones are
polar polyphenols and comprise tyrosol, hydroxytyrosol, caffeic acid and vanillic
acid. These water soluble polyphenols, which largely have the same beneficial properties
(nutritional, oil stability) as the oil soluble ones, have a much lesser contribution
to the bitter taste. The average polyphenols content of freshly pressed olive oil
ranges from 100 - 300 ppm. The content depends on type of olives and on the time of
harvesting. Up to now an olive oil which contains more than 300 ppm polyphenols is
too bitter for consumption.
[0006] Polyphenols are potent antioxidants. Recently it was found that they play a role
in enhancing the oxidation stability of olive oil (Nutr. Metab. Cardiovasc. Dis. (1995)
5:, 306-314). Moreover it has been discovered that a diet comprising food containing
a substantial amount of olive oil polyphenols helps to prevent the harmful oxidation
of low-density lipoproteins (LDL) in the blood. In a population with a high polyphenol
intake the incidence of coronary heart disease is relatively low. Therefore the presence
of polyphenols in an edible oil, including olive oil, and preferably at high levels,
is much desirable. However, when the polyphenols content in olive oil raises over
300 ppm, the oil becomes unacceptably bitter, which correlates with a (K225) bitter
index value over 0.3.
[0007] The water washings used for reducing bitterness unfortunately also remove a substantial
part of the valuable polyphenols from the olive oil. Co-pending non-prepublished patent
application EP 849 353 describes a process for an olive oil which contains 300 - 1000
ppm of polyphenols, while the bitter index K225 is not higher than 0.3 or, alternatively,
containing 200 - 300 ppm of polyphenols, while the bitter index is not higher than
0.2. The native polyphenols are hydrolyzed employing a time consuming enzymatic process
at a pH (5) which is optimal for enzyme activity.
The aim of the present invention is to provide an improved process for obtaining non-bitter
olive oils with a high content of polyphenols.
STATEMENT OF INVENTION
[0008] We have found a new method for converting the olive oil derived oil soluble polyphenols
into less bitter water soluble ones.
[0009] The present invention enables the reduction of bitterness not by usual removing bitter
compounds from the olive oil, but predominantly by transforming them into compounds
with reduced bitterness.
The process according to the present invention starts with olive oil which possesses
a relatively high content of bitter polyphenols. The process comprises the following
steps: preparing an emulsion of olive oil with an aqueous phase, exposing the olive
oil to the dispersed aqueous phase for at least 30 seconds and removing the aqueous
phase from the olive oil, characterized in that the emulsified aqueous phase is maintained
at a pH of at least 5.5, preferably of at least 6, more preferably of at least 7,
still more preferably of at least 8 for at least 30 seconds of the exposure time.
DETAILS OF THE INVENTION
[0010] Hydrolysis at a pH > 5.5 has been found to be essential for the conversion of bitter
fat soluble polyphenols into less bitter water soluble polyphenols within a proper
period of time. At said pH such conversion has been found to occur after at least
30 seconds exposure of the olive oil to an aqueous phase. The pH may not remain constant
during the process. During exposure it may sink temporarily below the required minimum
value of 5.5. A minimum exposure time of 30 seconds, preferably 2 minutes, more preferably
5 minutes to a pH of at least 5.5 is required for debittering. The full effect is
attained after still longer exposure times, preferably at least 30 minutes. Generally,
after 120 minutes no further debittering effect is observed. Preferably, the aqueous
phase consists of water, which contains reactive ions which are able to sustain the
maintenance of the proper pH.
The presence of fatty acids originating from partial hydrolysis of the olive oil,
has a pH lowering effect. The resulting pH is too low for suitable polyphenols hydrolysis.
It has been found that use of common hard drinking water provides the proper conditions
for obtaining the found debittering effect. The presence of acid-binding reactive
ions is believed first to constitute a pH > 5.5, and then to maintain it for a time
which is long enough to hydrolyse the bitter polyphenols into less bitter polyphenols.
[0011] Such acid-binding ions are found in hard drinking water which is produced from natural
resources, particularly from surface water and from water pumped from deep ground
layers. Such ions are e.g. one or more of the ions in the group constituted by carbonate,
sulphate, magnesium and calcium. Maintenance of the required pH can be supported further
by the presence of a proper buffer substance such as a (pH 8) citric acid/phosphate
buffer.
[0012] Polyphenols originating from olive oil will migrate to the emulsion's aqueous phase.
This aqueous phase may be separated by settling and decantation or by centrifugation,
but preferably the aqueous phase is removed from the olive oil by evaporating the
water under reduced pressure (vacuum drying). The polyphenols and possibly other ingredients
in the aqueous phase phase are forced to get dispersed in the olive oil and then cause
some turbidity.
More preferably, the oil/aqueous phase emulsion is filtered by a membrane using established
membrane filtration technology such as known for the production of drinking water.
The pores of the membrane allow passage of the water molecules, but not of the larger
polyphenol molecules which stay in the olive oil. Use of membrane filtration also
retains desirable olive oil volatiles, which otherwise would be evaporated when using
the previously mentioned vacuum drying technique.
[0013] When an aqueous phase is selected which contains already a large amount of water
soluble polyphenols, less migration of water soluble polyphenols from the olive oil
into the aqueous phase will occur.
[0014] While normally olive oil is substantially depleted with polyphenols as an effect
of the usual washing treatment, said embodiments of the invented process retain polyphenols,
but remove bitterness by converting the bitter principle rather than by removing its
substance.
[0015] The ratio of aqueous phase and olive oil in the emulsion preferably is 1 : 10 to
4 : 1, more preferably 1 : 4 to 2 : 1. Large amounts of water are not functional,
and even not desired, because removal of water at the end needs energy and time. The
best results are obtained when the emulsion has a continuous oil phase.
[0016] Obviously, it is vital that the mixture of aqueous phase and oil is vigorously stirred
to ensure close contact of both phases. For obtaining a fine emulsion a powerful stirring
device is required which is able to dissipate 0.1 - 5 kW, preferably 0.5 - 2 kW of
energy for each 1000 kg of olive oil. Table I shows the effect of stirring on debittering.
TABLE I
Energy dissipation in 1000 kg olive oil (kW) |
Debittering attained after minutes |
0.025 |
60 |
0.25 |
45 |
1 |
5 |
[0017] For said energy dissipation a stirrer with a relatively large diameter and a relatively
low stirring rate is suitable, e.g. a paddle type stirrer or a ribbon stirrer, preferably
a stirrer with baffles.
[0018] For the debittering process an emulsion temperature of about 25°C is most advantageous.
[0019] With the present invention an olive oil can be prepared with polyphenol contents
as high as 300 - 1000 ppm, while nevertheless the bitter index K225 is not higher
than 0.3 and as high as 200 - 300 ppm, while the bitter index is not higher than 0.2.
[0020] An olive oil which has been debittered according to the present invention can be
recognized by its polyphenols composition as detected by high performance liquid chromatography
(HPLC). The bitter polyphenols show up at a retention time of 40 - 52 minutes. Before
treatment substantial peaks are found in that part of the HPLC spectrum. After the
treeatment those peaks are still visible but are much reduced. At the same time the
peaks for water soluble polyphenols, which show up at 7 - 20 minutes retention time,
have increased. The decrease and increase respectively of said peaks is characteristic
for olive oil as an effect of the treatment according to the invention.
[0021] Consequently a further effect of the invented process is that the olive oil's ratio
of apolar polyphenols and polar polyphenols shifts to more beneficial lower molar
ratios, preferably being below 3 (see Table II).
Obviously, absolute ratios depend on the initial ratio in untreated olive oil, which
may vary widely.
[0022] The invention therefore provides an olive oil which is characterized in that it contains
at least 200 ppm of polyphenols, while the weight ratio of apolar polyphenols and
polar polyphenols is less than 3, preferably less than 2, more preferably less than
1.
[0023] A surprising additional benefit of the found process is that the treated olive oil
may exhibit unexpected flavour improvements. Native Andalusian olive oil, for example,
may show flavour defects. These defects have disappeared fully or to a large extent
after the treatment of the invention using common hard drinking water.
[0024] The invention is illustrated by the following example:
Example
[0025] Two volumes of extra virgin Andalusian olive oil and one volume of regular hard drinking
water obtained from the local (city of Vlaardingen) tap water supply and having a
pH of 8 were thoroughly mixed at a temperature of 25°C using a mixing device which
dissipated 1 kW of energy per ton of oil. After 30 minutes the headspace pressure
was reduced to 10 mbar, the water was evaporated from the mixture and the oil became
dry.
A tasting panel assessed the oil on bitterness before and after the treatment using
a scale ranging from 0 (excellent) to 4 (unacceptable bitterness). The oil was rated
4 before the treatment and 0 thereafter.
The content of polyphenols was measured by standard High Performance Liquid Chromatography
(HPLC). See Table II.
TABLE II
POLYPHENOLS CONTENT by HPLC |
BEFORE TREATMENT |
AFTER TREATMENT |
polar polyphenols mmol/kg |
0.54 |
1.47 |
apolar polyphenols mmol/kg |
1.67 |
0.5 |
total polyphenols mmol/kg |
2.21 |
1.97 |
ratio apolar/polar polyph. |
3.1 |
0.3 |
total polyphenols (ppm) |
665 |
365 |
[0026] The drop in ppm values is relatively large, and is ascribed to the hydrolysed polyphenols
being lighter molecules.
[0027] The values for K270 (0.18) and free fatty acids (0.45) showed that the treated oil
still complied with requirements for extra virgin olive oil.
Comparison example
[0028] The preceding example was repeated, but using instead of hard drinking water demineralised
water, having a pH of 6.5. At the onset of emulsion formation the pH of the aqueous
phase immediately and definitely dropped below 5.5. Even after one hour of stirring,
the tasting panel noticed no significant bitterness reduction.
1. A process for treating olive oil which comprises the following steps:
- preparing an emulsion of olive oil with an aqueous phase,
- exposing the olive oil to the dispersed aqueous phase for at least 30 seconds and
- removing the aqueous phase from the olive oil, characterized in that the emulsified
aqueous phase is maintained at a pH of at least 5.5, more preferably at least 7, for
at least 30 seconds of the exposure time.
2. A process according to claim 1, characterized in that the aqueous phase consists of
water, which contains reactive ions which are able to neutralize substances which
have a pH lowering activity.
3. A process according to one of the previous claims, characterized in that said pH level
is maintained for at least 2 minutes, preferably for at least 5 minutes, more preferably
for at least 30 minutes.
4. A process according to one of the previous claims, characterized in that the aqueous
phase does not contain a substantial amount of debittering enzyme.
5. A process according to one of the previous claims, characterized in that the aqueous
phase to be added contains olive oil derived polyphenols.
6. A process according to one of the previous claims, characterized in that the ratio
of aqueous phase and olive oil in the emulsion is 1 : 10 to 4 : 1, preferably 1 :
4 to 2 : 1.
7. A process according to one of the previous claims, characterized in that the aqueous
phase is removed by evaporating the water under reduced pressure conditions.
8. A process according to one of the previous claims, characterized in that the aqueous
phase is removed by membrane filtration.
9. A process according to one of the previous claims, characterized in that the aqueous
phase substantially consists of tap water.