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1000 Asia Pacific J Clin Nutr (1996) 5: 105-107

Asia Pacific J Clin Nutr (1996) 5: 105-107

Phenolic 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)

  1. The Wesley and Greenscopes Hospitals, Brisbane, Queensland, Australia;
  2. 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.


Introduction

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 olive oil.

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
Oil

Description

Total Polyphenols (mg/g oil)

Olive - extra virgin

Giralda, Spain

48

Olive - extra light

Giralda, Spain

11

Olive - cold pressed

Bertolli, Italy

10

Macadamia

Aussie Mate, Aust.

9

Avocado

Australia

6

Sesame

Proteco, Australia

6

Canola

Meadow Lea, Aust.

4

Soya

Top Cook, NZ

4

Grapeseed

Azalea, Italy

2

Sunflower

Meadow Lea, Aust.

0

Peanut

Chefol, Australia

0

Walnut

Rougie, France

0

Almond

Expeller pressed, Hain USA

0

Methods

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 of Gutfinger17.

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 refining.

Conclusion

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.


Phenolic content of olive oil is reduced in extraction and refining

Colquhoun DM, Hicks BJ, Reed AW

Asia Pacific Journal of Clinical Nutrition (1996) Volume 5, Number 2:105-107


References

  1. Ferro-Luzzi A, Sette S. The Mediterranean Diet: An attempt to define its present and past composition. Eur J Clin Nutr 1989; 43(Suppl 2), 13-29.
  2. Keys A, Menotti A, Aravanis C, et al. The Seven Countries Study. 2,289 deaths in 15 years. Prev Med 1984: 13:141-154.
  3. Keys A (ed) Coronary heart disease in the Seven Countries Study. The Diet. Circulation 1970; 41 (Suppl 1): 1; 162-183.
  4. La Rosa JC, Hunninghake D, Bush D et al. AHA Medical/Scientific Statement. The Cholesterol Facts. Special Report. Circulation. 1990; 81:1721-1733.
  5. Kaufmann N. Dietary Fat and Blood Lipid Levels: The role of monounsaturated fatty acids . Nutr Metab Cardiovasc Dis 1 99 1; 1: 104- 108.
  6. Mensink R, Katan M. Effect of Dietary Fatty Acids on Serum Lipids and Lipoproteins. A Meta-analysis of 27 Trials. Arterioscler Thromb 1992; 12:911919.
  7. Steinberg D, Parthasarathy S, Carew T, et al. Beyond cholesterol: Modifications of low-density lipoprotein that increase its atherogenicity. N Engl J Med 1989;320:915-934.
  8. Parthasarathy S, Khoo J, Miller E, et al. Low Density Lipoprotein Rich in Oleic Acid is Protected Against Oxidative Modification: Implications for Dietary Prevention of Atherosclerosis. Proc Natl Acad Sci USA 1990; 87:3894-3898.
  9. Scaccini G, Nardini M, D’Aquino M, et al. Effect of Dietary Oils on Lipid Peroxidation and on Antioxidant Parameters of Rat Plasma and Lipoprotein Fractions. J Lipid Res 1992; 33:627-633.
  10. Papadopoulos G, Boskou D. Antioxidant Effect of Natural Phenols on Olive Oil. J Am Oil Chem Soc 1991; 68:669-671.
  11. de Whalley C, Rankin S, Robin J, et al. Flavonoids Inhibit the Oxidative Modification of Low Density Lipoproteins by Macrophages. Biochemical Pharmacology 1991; 42: 1673-1681.
  12. Khan S, Katiyar S, Agarwal R and Mukhtar H. Enhancement of Antioxidant and Phase II Enzymes by Oral Feeding of Green Tea Polyphenols in Drinking Water to SKH-1 Hairless Mice: Possible Role in Cancer Chemoprevention. Cancer Research 1992; 52:4050-4052.
  13. Laughton M, Evans P, Moroney A, et al . Inhibition of Mammalian 5 Lipoxygenase and Cyclo-oxygenase by Flavonoids and Phenolic Dietary Additives: Relationship to Antioxidant Activity and to Iron Ion-reducing ability. Biochemical Pharmacology 1991; 42: 1673- 1681.
  14. Vasilets L, Mokh V, and Plekhanova 1. Antiarrhythmic and Vasodilator Action of the Antioxidant Phenosan During Acute Ischaemia and Reperfusion: Translated from Byulleten "Ekspenmental" noi Biologii i Meditsiny 1988; 106:554-557.
  15. Hertog M, 58a Feskens E, Hollman P, et al. Dietary antioxidant flavonoids and risk of coronary heart disease: the Zutphen Elderly Study. Lancet 1993; 342: 1007-1011.
  16. Roma P, Galli C, Catopano AL. Protection of low-density lipoprotein from oxidation by 3,4- dihydroxyphenylethanol. Lancet 1994; 343:1296-1297.
  17. Gutfinger T. Polyphenol in Olive Oils: J Am Oil Chem Soc. 1981; 58:966-968.


Copyright 1996 [Asia Pacific Journal of Clinical Nutrition]. All rights reserved.
Revised: January 19, 1999 .

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