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ARISTOTLE UNIVERSITY OF THESSALONIKI

FACULTY OF SCIENCE

DEPARTMENT OF CHEMISTRY

Zografia Nystazou

Postgraduate work

Study of The Effects of Frying of Sliced Potato on

Virgin Olive Oil

Under the supervision of theAssociate Professor G. Blekas

Laboratory of Chemistry and Food Technology

Thessaloniki 2011


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OLIVE OIL

"Olive oil" refers to the oil which is obtained from the flesh of the olive of the European variety (Olea

europea) exclusively by mechanical means and by physical methods or processes at temperatures that will not

cause degradation of the oil.

The olive oil

can be consumed immediately after its reseipt without any pretreatment. In this form, it retains its important

components (flavor-aroma).

The olive oil

has been used since the ancient times as a nutrient constituent, as a medicine or a drug carrier and as a

cosmetic.

A century ago it was found that the addition of some olive oil in the meal helped in the reduction of the

concentration of gastric juices,in the reduction of the indigestion and in the reduction of pain by the taking

olive oil with orange juice.


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As virgin is characterized the olive oil, which is

extracted from the flesh of the olive solely by

mechanical or other physical means under

conditions particularly thermal conditions, that

do not lead to changes, without being subjectedto others treatments except for washing with

water, the transfusion the centrifugation and the

filtration.

The extra virgin olive oil is used as it is for edible

purposes and has a perfect taste and aroma.

Its acidity can reach 0,8 g/100 g, expressed as oleic

acid.


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Recommendation virgin olive oil fatty acids

To the triacylglycerols of the olive oil, which are the main lipid fraction, predominantly

are found the following fatty acids:

Fatty acids (%)

oleic (C18: 1) 55 to 83palmitic (C16: 0), 7.5 to 20

linoleic (C18: 2), 3.5 to 21

stearic (C18: 0), 0.5 to 5

palmitoleic (C16: 1) 0.3 to 3.5

linolenic (C18: 3).


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Components of virgin olive oil with antioxidant

The polar phenolic compounds of virgin olive oil are the main antioxidants. When the levels in

phenolic compounds exceed 300 mg/kg of oil, the oil may have a bitter taste. However, the high

content in polyphenols seems that it is advantageous for the life of the oil, and there is good correlation

between the stability and the total content of polyphenols. Amongst various phenolic compounds which

were tested for their contribution to stability, the hydroxy-tyrosol and caffeic acid were the most potent

antioxidants. In extra virgin olive oil the phenols are calculated in 232 mg / kg.

Some other antioxidants of the olive oil is squalene, alpha-tocopherol and D5-avenasterol.


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Use virgin olive oil asfrying medium different raw

materials

Virgin olive oil shows remarkable resistance to household deep frying of chopped potato or to other

use in frying temperatures. In comparison to other vegetable oils, olive oil has a significantly lower

rate of change, as was found by measurements of viscosity, total polar components and the loss of

tocopherols. This increased stability to thermal oxidation is why this oil can be used in repeated

frying before exceeding the limit of discharge , which is about 27% total polar components. The high resistance to oxidative decomposition is due to its high content of monounsaturated fatty

acids and to the presence of minor components with strong antioxidant activity.


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In conclusion, olive oil is excellent for applications involving high temperatures. Specifically,

it is rich in monounsaturated and sufficiently low in polyunsaturated fatty acids, particularly linolenic acid,

it contains no trans unsaturated fatty acids and

it is quite low in saturated fatty acids.

Finally, the olive oil has a sufficiently low freezing point that allows easy absorption from the food being fried,in result the ready-to-eat product contains a small quantity of it.


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The methods for controlling the quality of the olive oil

GC (Gas chromatography, GC) is a chromatographic technique

which is used among others to assess the composition of fats into

fatty acids for more than 50 years.

The fatty acid composition of edible oils mainly is used to assessauthenticity, but also to monitor changes occurring during heating of

oils.

Prerequisite to assess the composition of an oil in fatty acids is the

convertion of its triacylglycerols in methyl esters of its fatty acids.

1. Gas chromatography


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The determination of the anisidine number is based on the

reaction of p-anisidine with volatile and non-volatile

aldehydes, which are products of degradation of mainly

bound triacylglycerols of fatty acids. The reaction results in

the formation of Schiff bases which are yellow and have a

maximum absorbance at 350 nm.

During heating of an oil at high temperature, such as frying,

the number anisidine is strongly increased. This increase has

been associated mainly with the formation of secondaryoxidation products.

2.Number anisidine


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The 2,2-diphenyl-1-picrylhydrazyl (DPPH) is a stable radical, which has been used to assess the ability

of phenolic compounds which bind free radicals. The higher this is, the stronger the antioxidant activity of

the phenolic compound.

The DPPH radical does not dimerize and exist in the monomeric form both in the solid state as dissolved

in an organic solvent. Also easily accepts unpaired electrons or hydrogen atoms and hardly oxidized. The

solutions of the root are colored, relatively stable, and its concentration in them can be determined by

recording the optical absorption in the visible light (515 nm). The optical absorbance of its solutions are

stable and independent of the value of the pH, if this is between 5 and 6.5.

3. Estimation of free radical scavenging ability of olive oil samples

with the DPPH assay


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4. Polar component

The polar components are products of different reactions including oxidation, hydrolysis and

polymerization. Some of these are free fatty acids, aldehydes, di-and monoglycerides and

polymerized triglycerides. Generally, polar compounds have been blamed for delay in growth,

increase in liver and kidney weight problems and problems in the function of enzymes in animal

experiments. Also, some polar compounds are responsible for toxicity and may affect the

absorption of triglycerides.

The calculation of the polar components is the best indicator for assessing the quality of an oil, as

it refers to all the degradation products of the original triglycerides which are present in the oil.

The polymerised triglycerides are the largest group of polar compounds. In most European

countries, the limits for disposal and replacement of cooking oil range from 25 to 27% total polar

components and 10-16% polymerized triglycerides.


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5. Infrared spectrometry by transformation Fourier (FTIR)

The decomposition of the frying oils leads to changes:

at the level of free fatty acids,

the color of used oil or

the polarity of the oil.

Infrared spectroscopy, namely the transformation infrared spectroscopy with Fourier (FTIR)

is a method:

less expensive

best in performance and

easier to use.

Generally it has been used for determining

the content in cis and trans double bonds,

the iodine value of the saponification number,

the peroxide value, the anisidine number,

the content of free fatty acids of the oil samples

and the fat and the composition of the food.


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EXPERIMENTAL

Extra virgin olive oil ELAIS SA.

The frying was held at 150oC and 180oC.

The ratio of the weight of raw material to the volume of oil: 1/7 and 1/35.

Number of successive fryings 10, 20, 30 and 40.

Thus 16 samples of olive oil were obtained , 4 for each discrete case of 40 consecutive fryings,

whose duration was 4 hours.

1. Samples

10-1/7-150 10-1/7-180 10-1/35-150 10-1/35-180

20-1/7-150 20-1/7-180 20-1/35-150 20-1/35-180

30-1/7-150 30-1/7-180 30-1/35-150 30-1/35-180

40-1/7-150 40-1/7180 40-1/35-150 40-1/35-180


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1. There has been convertion of the acylglycerol into methyl esters of fatty acids according to the

standard method (IUPAC, 1987). Then a GC analysis of the methyl esters of fatty acids was

carried out and from the diagrams of the GC the ratios of peak areas of the two methyl esters of

palmitic acid and linoleic acid were calculated .

2. Also the number anisidine was determined in accordance to the standard method (IUPAC, 1987)

and by using the relationship:

p-A.V. = 25 x (1.2 As - Ab) / m.

where As is the absorbance of the solution of the oil after the reaction with the solution of p-

anisidine,

Ab is the absorbance of the solution of the oil prior to reaction with the solution of p-anisidine

and m is the mass of the oil in g.

2. Methods


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3. The antioxidant capacity with the binding test of the root of DPPH was estimated and calculated

according to the equation:

RSA% = [(A0 - A30) / A0] * 100.

4. Also the polar components were determined with a financial micromethod and by the

relationship

cp= [(Ws - Wn) / Ws] * 100

where cp : Percentage of polar constituents

ws: The sample weight in mg and

wn: The weight of non-polar substances in mg.

5. Finally the changes in the composition were estimated based on the FTIR spectras measuring the

absorbances initially for the estimation of the effect of cooking on cis isomers of the oil in the

region 3003 cm-1, whereas for the evaluation of the effect of frying in trans isomers in the

region 968 cm-1, corresponding to the bending vibrations of the trans double bond-HC = CH-.


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RESULTS

The increase in the ratio was more intense in the case of exposure to high temperature (180o

C).

More intense change in the ratio EP / EL was observed when the quantity of the raw material

which is fried in the same amount of oil was higher (weight ratio of the potato to the volume of

oil equal to 1/7) irrespectively to the temperature.

Changes in the ratio of palmitic acid tolinoleic acid

Number of fryings


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The increase in temperature from 150oC to 180oC for a constant weight ratio of potatoes to volume

oil had the effect of increasing the number of anisidine.

The increase of the number of anisidine was not observed during the growth of ratio "potato

weight to volume oil" while keeping the temperature constant.

The anisidine number of the oil that was not used in frying was equal to 2.5.

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50

AnV

1:7 150

1:7 180

1:35 150

1:35 180


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There is a rapid reduction of free radical binding capacity after only the 10th frying compared to

olive oil which is not used in frying (RSA=44%). While at 40th frying the binding capacity of

free radicals does not exceed 3%. This suggests rapid destruction of natural antioxidants of

virgin olive oil during heating and thus drastical reduction of the the nutritional value.


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What can be observed is that even after 40 fryings the percentage of polar components does not

exceed 24-26%, which is a limit for disposal and replacement of the oil in most European

countries. The higher rate that was found was 19.6%.

The content of unused oil in total polar components was equal to 2.1%.

0

5

10

15

20

25

0 10 20 30 40 50

TPC%

1:7 150

1:7 180

1:35 150

1:35 180


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Changes in FTIR absorption spectra

The differences that showed the samples of olive oil after frying regardless of frying

conditions were as to the intensity of all their absorptions very small.

The results of the reactions occurring are the reduction of the absorption due to components

with cis double bonds in their molecule and the increase of the absorption which is due to

components with trans double bonds in their molecule. For this reason there was a separate

identification of absorption in the range of 3003 cm-1 due to stretching vibrations of cis

double bonds, and of the absorption in the region of 968 cm-1 due to vibrations of bending of

trans double bonds.


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Changes ofabsorption in the region of 968 cm-1 The samples that were taken after the 40th frying showed more intensive absorption than the

samples that were taken after the 10th , the 20th and the 30th frying, except for the sample 1/7-

150.

The other samples that were taken after the 20th frying showed more intensive absorption than

the samples that were taken after the 10th and the 30th frying, in contrast to the expectations.

It is also noteworthy that the samples that were taken after 10 and 30 fryings showed an

absorption with nearly equal intensity.


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CONCLUSIONS FUTURE WORK-

The main conclusions from all the experiments are:

The most critical parameter for the extent of the changes, which were observed in the composition of

the olive oil, was the temperature.

The olive oil even after the 40th successive frying (refers to a total duration of successive frying

equals to 4 h) could be reused as found in the proportions of polar components.

The olive oil sample after the 10 th successive frying had lost the ability to bind free radicals at a rate

higher than 60%. This loss indicates that the nutritional benefits of extra virgin olive oil are lost andonly the thermal stability remains.


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It would be interesting to research for other absorptions in the infrared spectra of used virgin olive

oil for frying chopped potato, which allow correlation of their intensity with the content in whole or

individual polar components or sub-classes of products of hydrolysis, oxidation and / or

polymerization.

In the future the changes in the concentration of both alpha-tocopherol and the total polar phenolic

compounds, and other minor components of virgin olive oil, could be also studied in the use of this

oil as mediumof heat transfer in the continuous frying of chopped potatoes for an hour the most.


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Thank you very much

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