Asia Pacific J Clin Nutr (1996) 5: 105-107
Asia Pacific J Clin Nutr (1996) 5: 105-107
content of olive oil is reduced in extraction and refining
Analysis of phenolic content of
three grades of olive and ten seed oils
Colquhoun DM1 MBBS FRACP, Hicks BJ1 RN BHMS, Reed AW2 Mphil(Sc)
- The Wesley and Greenscopes Hospitals,
Brisbane, Queensland, Australia;
- Department of Primary Industries,
Brisbane, Queensland, Australia
Three grades of olive oil and ten vegetable cooking
oils were analysed for their phenolic content. It was hypothesised
that as olive oil passed through the chemical extraction process,
polyphenols would also be removed, thus reducing the antioxidant
properties of olive oil. Other commonly used edible vegetable oils
were analysed for comparative reasons. Extra virgin olive oil was
found to have the greatest amount (48 mg/gram of oil) of polyphenols, when compared with other olive or
vegetable oils. No polyphenols were detected in sunflower, walnut,
peanut or almond oils. All other oils tested had a polyphenolic
content between 2 and 10 mg/gram of oil. The results of
the study confirms the above hypothesis that the phenolic content
of olive oil is reduced by chemical extraction and refining.
The Mediterranean diet, though it can be very high
in fat, is associated with a low incidence of coronary heart disease
(CHD)1,2. In the Seven Countries Study, it was found that
in Crete 40% of the energy came from fat, yet the incidence of CHD
was approximately one-thirtieth the incidence of Finland which had
a similar level of fat intake3. The fat intake in Crete
was predominantly from olive oil, whereas the fat in Finland was predominantly
from animal sources. Olive oil is rich in the monounsaturated fatty
acid, oleic. In contrast, animal fat is rich in saturates such as
palmitic, myristic and lauric acids.
The saturates from animal sources increase serum cholesterol,
where unsaturates tend to lower serum cholesterol. An elevated serum
cholesterol or more precisely an elevated low density lipoprotein
(LDL) is the primary cause of atherosclerosis4.
A diet rich in monounsaturated fat has similar efficacy
to a diet rich in polyunsaturated fat with regard to lowering serum
LDL. A diet rich in polyunsaturated fat may induce a decrease in high
density lipoproteins (HDL), whereas monounsaturated fat maintains
or increases HDL5,6.
Native LDL is poorly atherogenic, at least experimentally,
unless it is modified. In vivo, the modification process appears
to be predominantly oxi 1000 dation7. A diet rich in monounsaturated
fat from olive oil has been found to inhibit oxidation of the LDL8.
However, Scaccini et al have found that the antioxidant effect is
not fully explained by the monounsaturated fatty acid content nor
by the vitamin E and carotenoids which are in olive oil9.
Olive oil has other components apart from fatty acids,
which effect taste, colour and may have significant biological actions.
In the "polar fraction" of olive oil there are more than
a hundred different compounds, a significant proportion being the
phenolic compounds; polyphenols and tocophenols. Phenols are a large
class of compounds seen in nature and are molecules with an hydroxyl
group attached to a benzene ring. There are over 4000 different phenols
found in plants. In olive oil the term "polyphenols" is
used by convention because not all of them are polyhydroxy derivatives.
There are at least ten polyphenols which are reported to occur frequently
in olive oil. (Figure 1).
Figure 1. Common phenolic compounds in virgin
The polyphenolic content of olive oils affects the
stability of the oil and there is a strong antioxidant effect according
to the amount in the oil10. They also have a significant
effect on flavour.
Polyphenols also inhibit oxidation (lag phase) of
LDL and may stimulate antioxidant enzymes such as catalase11,12.
Phenolic compounds may inhibit eicosanoid metabolism, have antiplatelet
effects and may increase HDL13. Antifungal and anticarcinogenic
properties have also been reported. The phenols; Butylated hydroxyanisole
(BHA) and butylated hydroxytoluene (BHT) which have significant anti-oxidant
properties inhibit experimental atherosclerosis. Other phenolic compounds
have been shown to have a vasodilator action and phenols may inhibit
ischaemic and reperfusion arrhythmias14. Recently, the
Zutphen Elderly Study was reported to show flavonoid intake significantly
inversely associated with mortality from CHD (p=0.015)15.
The relative risk of CHD mortality in the highest versus the lowest
tertile of flavonoid intake was 0.42 (95% CI 0.20-0.88). In olive
oil 3,4-dihydroxyphenyl-ethanol (hydroxytyrosol) probably is the most
important phenol responsible for the highest oxidation resistance
of the oil10. In addition, hydroxytyrosol purified from
olive oil has been shown to inhibit oxidation of LDL in vitrol6.
|Table 1. Polyphenolic contents
of olive oils and other seed oils
Total Polyphenols (mg/g oil)
|Olive - extra
|Olive - extra
|Olive - cold pressed
Aussie Mate, Aust.
Meadow Lea, Aust.
Top Cook, NZ
Meadow Lea, Aust.
Expeller pressed, Hain USA
The oils were commercially produced. The polar fraction of the
oils was extracted by a methanol-water mixture. This was evaporated
in a rotary evaporator, the residual was dissolved in ethanol and
spectrophotometric determination was made according to the method
Results and discussion
There was a significant difference in polyphenolic
content in the three grades of the olive oil (Table 1). With refining,
the polyphenolic content decreased significantly and there was also
fewer polyphenols in the nut and seed oils. The extra virgin olive
oil had significantly more polyphenols than the other oils and four
of the other oils had no detectable phenols. Clearly, the phenolic
content in commercially available edible oils varies considerably,
wi 1000 th the highest content being in extra virgin olive oil in
this study. The amount found in our study is considerably less than
in the Papadopoulos study10, and this is not explained.
The amount of polyphenol in oil depends upon the conditions in which
the oil is extracted from the fruit. For example, a continuous centrifugal
system of extraction may reduce the amount of phenols in the oil compared
to classic pressing. Also in the refining process longer mixing times
lower phenolic content of oils11. In contrast higher temperatures
increase phenolic content. The nut and seed oils analysed in our study
had little or negligible polyphenolic content and this may relate
to the extraction process rather than the presence in the nut itself.
Seed oil extraction almost invariably involves the
use of hexane as an organic solvent extractor. Probably this is responsible
for the low or nondetectable levels of polyphenols in these oils.
When hexane extraction is used to extract oil from the olive pomace
the resultant oil is also low in polyphenols. Further refining by
bleaching and decolouration may also contribute to polyphenol loss.
Along with polyphenols, other significant compounds may be lost with
The high polyphenolic content in extra virgin olive
oil may be of clinical relevance in a diet rich in monounsaturated
fat. These compounds appear to have favourable health effects, and
their levels may be a surrogate for other important biologically active
constituents in the polar fraction of oils. More research is needed
to establish the polyphenolic content and individual phenols in various
edible oils and foods. These s 1000 o-called "minor constituents"
of food and oils may have a considerable influence on health and disease,
and in particular, atherosclerosis.
of olive oil is reduced in extraction and refining
Colquhoun DM, Hicks BJ,
Asia Pacific Journal
of Clinical Nutrition (1996) Volume 5, Number 2:105-107
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