effectiveness of modified zeolites as adsorbent materials for frying oils

Research Article

Effectiveness of modified zeolites as adsorbent materialsfor frying oils

Alper Dulger and Emin Yılmaz

Faculty of Engineering, Department of Food Engineering, Canakkale Onsekiz Mart University,

Terzioglu Campus, Canakkale/Turkey

Natural zeolite was modified by four different techniques namely using Tween80, b-cyclodextrine and

olive mill waste water at RT and hydrochloric acid at 1108C, and then modified zeolites were used as

adsorbent materials in six consecutive days dough frying with refined hazelnut oil. Some parameters in

the oil samples of control and experimental groups were measured and the ranges were determined as

following; total polar materials (TPM)-chromatography (4.09–70.22%), free acidity (0.16–1.19%),

smoke point (221.50–184.008C), conjugated dienoic acids (0.36–2.19%), L value (35.65–52.15),

a� value (�0.18–5.98), b� value (�3.73–10.40), turbidity (0.70–10.40 NTU), viscosity (67.67–825.50

cP), and oil absorbed by the dough (5.10–8.85%). The results of this study have shown that both natural

zeolite and modified zeolites have different level of activities as frying oil adsorbent materials. The best

results were achieved with Tween80 modified zeolite for the TPMs, free acidity, instrumental color values,

smoke point, and conjugated dienoic acid measurements. Similarly better result for turbidity was with olive

mill waste water modified zeolite and for viscosity was with hydrochloric acid modified zeolite. It was

determined that adsorbent treatment did not affect fat absorption level of fried dough.

Practical applications: The results of this study have shown that natural zeolite modification with

different techniques can improve its adsorption capacity significantly. In this respect, utilization of

modified zeolite for frying oil recovery applications can be an industrially sound practice since natural

zeolite is a very cheap and easily found material.

Keywords: Adsorption / Frying / Modification / Oil mill waste water / Zeolite

Received: September 3, 2012 / Revised: January 9, 2013 / Accepted: January 25, 2013

DOI: 10.1002/ejlt.201200312

1 Introduction

Despite many negative aspects, food frying is still one of the

most preferred food preparation techniques worldwide [1, 2].

Frying is immersing and cooking foods in hot oil which serves

as the heat and mass transfer medium. During the process,

the presence of high temperature, oxygen, moisture, and

minerals leaching from the food cause a variety of reactions

(oil hydrolysis and oxidation, dimer and polymer formation,

Maillard reactions etc.) to deteriorate the oil, and reduce the

frying oil quality and healthiness properties of the fried

products. Therefore, some actions like regular cleaning

and maintenance of equipment, selection of good quality

frying oil, adjustment of proper frying conditions, addition

of some natural antioxidant extracts into the oil, and use of

some adsorbents for removing soluble degraded compounds

from the frying oil have been in practice [3, 4].

Regeneration and shelf-life extension of frying oils by

selective adsorption and removal of undesirable polar com-

pounds by adsorbent materials has been researched exten-

sively [1, 4–6]. Practically, the adsorbent material is mixed

with used frying oil for a while and then filtered. The treated

oil is put back into the fryer for subsequent use. This treat-

ment has been shown to maintain frying life of oil by con-

trolling the buildup of total polar materials (TPMs) and free

fatty acids. Also, in most cases the amount of absorbed fat by

the fried food is reduced and none sensory defect formed in the

food by this treatment. Many different adsorbent materials

including activated carbon, clay, aluminum hydroxides, char-

coal, celite, silica gel, silicondioxide, oyster shell, ceramic plate,

Correspondence: Dr. Emin Yılmaz, Faculty of Engineering, Department of

Food Engineering, Canakkale Onsekiz Mart University, Terzioglu Campus,

17020, Canakkale/Turkey

E-mail: [email protected]

Fax: þ90 286 2180541

Abbreviations: OMWW, Olive mill wastewater; TMP, total polar material

668 Eur. J. Lipid Sci. Technol. 2013, 115, 668–675

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ejlst.com

calcium oxide, and others have been researched for this

purpose. There are also commercial adsorbent products

like Magnesol1, BritesorbTM, Frypowder(Miroil), and

OilFreshTM around world markets [5–7].

Equal weighed portions of natural zeolite, diatomaceous

earth and lime were mixed and used as frying oil adsorbent,

and found effective in free acidity, viscosity, and color pro-

tection of the frying oils [7]. It was reported that when the

surface of natural zeolite was modified with a surfactant, its

potential for pollutants removal from water has enhanced

considerably [8]. Similarly, zeolite surface modification by

b-cyclodextrin for p-nitrophenol removal from water has

shown great potential [9]. Since in our previous study [7],

natural zeolite has shown some potential as frying oil

adsorbent material and it is a very cheap and common

material, it might have importance to study surface modified

zeolites in frying oil recovery.

The objectives of this study were to prepare modified

natural zeolites by four different techniques and compare

their effectiveness as frying oil adsorbent materials against

control group (no adsorbent treatment) under actual frying

conditions with refined hazelnut oil and dough frying to find

out the best performing modified zeolite as frying oil adsorb-

ent material.

2 Materials and methods

2.1 Materials

Refined hazelnut oil (Helvacızade Food Pharma &

Chemicals, Konya, Turkey) and natural zeolite with 50–

200 mm particle size (Rota Madencilik, Istanbul, Turkey)

were provided by the producers. Olive mill wastewater

(OMWW) was collected from Taris Cooperation Factory

located in Ezine, Canakkale (Turkey) during the production

season of 2009 and kept frozen until use. White wheat flour

(Kepez Flour, Turkey), instant yeast (Dr. Oetker) and salt

(Billur salt, Turkey) were bought from a local store. All

chemicals used for the analyses were of analytical grade

and bought from either Sigma Chemicals (St. Louis, US)

or Merck (Darmstadt, Germany).

2.2 Preparation of modified zeolites

Techniques used for natural zeolite surface modification for

water cleaning applications [8, 9] have inspired the modifi-

cations used in this study. Since the medium (polar versus

non-polar) in which the zeolite is used totally different,

modification techniques in this study were developed by

our pre-experiments. Hence, they are novel and can be

further developed by other researchers.

The OMWW used in this study was collected from sep-

arator exit. OMWW is chosen due to its very high antioxidant

capacity arising from the polyphenolic compounds. Hence,

it was free of residual olive oil, and it was filtered through

53 mm synthetic filter to remove suspended particles before

using in zeolite modification. Then OMWW and natural

zeolite were mixed by 1:2 weight ratio and stirred for 4 h

at RT. Finally, it was set for half hour to reach the equilibrium

and sedimentation. The slurry was filtered through regular

filter paper, washed twice with pure water, and dried in an

oven (Ecocell Drying Oven, MMM Medcenter, Germany) at

1108C until the moisture content was around 10% which

measured by MB45 moisture analyzer (Ohaus Corp.,

Switzerland). All modified zeolites were put into amber col-

ored and tightly capped glass bottle and kept in fridge until

the use. In order to prepare hydrochloric acid (HCI) modified

zeolite, 0.02 M HCI was mixed with natural zeolite in 1:2

weight ratio and mixed at 1108C for 4 h under the hood.

Then the slurry set, decanted, filtered through regular filter

paper and washed with pure water twice. Finally dried and

stored in the same way. For the cyclodextrine modification,

aqueous b-cyclodextrine solution of 0.006 M was prepared

and mixed with natural zeolite in 1:2 weight ratio at RT for

4 h. Then filtered, washed twice with pure water and dried in

the same way. Tween80 modified zeolite was prepared by

mixing 0.06 mM Tween80 solution with natural zeolite by

1:2 weight ratio at RT for 4 h, and then filtering, washing

and drying the matter in the same manner. Before using the

modified zeolites for frying oil adsorption treatment, the

moisture level was measured again, and if needed it was

adjusted by vacuum drying. It was decided not to dry the

zeolites further below 10% moisture level, since the structure

of material changes by cracking down and darkening.

2.3 Adsorption capacity measurement

Oleic acid adsorption capacities of the natural and modified

zeolites were measured according to the technique of

Taspınar and Ozgul-Yucel [10]. First, 0.05 M oleic acid

solution in hexane was prepared. Then each of the 0.5 g

adsorbent material was mixed with 25 mL of 0.05 M oleic

acid solution and mixed for 30 min at RT. After waiting

20 min, the mixture was filtered. 10 mL of this filtrate was

mixed with 10 mL neutralized (against phenolphthalein with

0.05 M NaOH) ethyl alcohol and titrated with 0.05 M

NaOH solution. The blank sample was 25 mL of the same

oleic acid solution that was not treated with the adsorbent,

was also titrated with the same base solution. For each

adsorbent, three measurements were completed. The adsorp-

tion capacity was then calculated with the given formulae

as mg oleic acid/g adsorbent. Finally, since the number was

very small, it was converted to mg oleic acid/g adsorbent

material and the results are shown in Fig. 1.

2.4 Dough frying

Sixty-two percent wheat flour, 37% water, and 0.5% each of

instant yeast, baking powder and a little salt were mixed and

prepared dough was fermented 30 min at RT after cutting

Eur. J. Lipid Sci. Technol. 2013, 115, 668–675 Modified zeolites for frying oils 669


into 35 g patties. Frying was performed with 2 L Fryers

(Fakir-Hausgerate Nista, Enz, Germany), for 6 consecutive

days for both control and treatment groups. At the beginning,

the fryers were filled with 2 L of fresh hazelnut oil and heated

to 1808C. During the day, the fryers were kept open for 5–

5.5 h. The fryers were stopped during the night, and restarted

in the next day without any oil replenishment. Each day in

each treatment group, two dough patties were fried for

10 min every half hour until a total of 20 patties were fried.

After each frying day, the oil remaining in the fryer was

weighted and around 10% w/w of the weight amount of

the adsorbent material was added into the oil and mixed

for 30 min. Then the slurry was vacuum filtered through

regular filter paper and clean oil was put back into the fryer.

Meantime 100 mL of treated oil samples were collected from

each fryer and stored frozen in amber colored and capped

glass bottles. The fried dough was put into bags, labeled, and

kept frozen until the analyses. All frying treatments were in

duplicate. All analyses listed below were done at least twice

within each duplicate of the treatment.

2.5 Chemical analyses

Free acidity values were determined by AOCS method Ca

5a-40 [11]. Conjugated dienoic acid content of the samples

was measured following AOCS Ti 1a-64 method [11] on a

UV Mini 1240 Spectrophotometer (Shimadzu Co, Japan).

Chromatographic TPMs analysis was accomplished follow-

ing AOCS method Cd 20-91 [11].

2.6 Physical analyses

Viscosity measurements of the oils were carried out by placing

7.5 mL of sample in a special sample holder, and direct

measuring centipoises (cP) with a Brookfield viscosimeter

(model DV II þ Pro with Rheocalc software, Brookfield

Eng. Lab., MA, US) equipped with LV-SC4-18 spindle at

258C. Turbidity values of the samples were measured

by Micro T100 Lab Turbidimeter (HF Scientific, US)

calibrated with 0.2, 10, and 1000 NTU calibration liquids

at 258C. Smoke points of the samples were measured follow-

ing AOCS method Cc 9a-48 [11]. Instrumental color of the

samples were measured by a Minolta CR-400 Chroma Meter

(Osaka, Japan) by immersing the probe of instrument into the

oil sample which put in a Petri dish on the white tile.

2.7 Measurement of absorbed fat

Total amount of absorbed fat by the fried dough samples was

measured with Soxhlet technique described in AOAC

method 920.39 [12].

2.8 Statistical analysis

Whole study was replicated two times and within each rep-

licate the sample analyses were done at least twice. The

statistical package program Minitab (ver. 13.1) [13] was used

to observe the mean differences among the treatment groups

by ANOVA and mean separation by Tukey’s test.

3 Results and Discussion

3.1 Fresh hazelnut oil

Some physico-chemical properties measured in the fresh

refined hazelnut oil are shown in Table 1.

It is important to have this knowledge to realize how oil

properties changes during frying. The manufacturer of the

Figure 1. Adsorption capacity values of the natural and modified zeolites used as frying oil adsorbent materials (the capital letters show the

statistical differences by Tukey’s test).

670 A. Dulger and E. Yılmaz Eur. J. Lipid Sci. Technol. 2013, 115, 668–675


hazelnut oil indicated its fatty acid composition as following:

myristic acid 0.03%, palmitic acid 5.35%, palmitoleic acid

0.17%, stearic acid 2.67%, oleic acid 82.04%, linoleic

acid 9.52%, linolenic acid 0.08%, arachidic acid 0.12%,

and behenic acid 0.02%. It is obvious that fresh hazelnut

oil is a very ideal one for frying as long as fatty acid compo-

sition and other properties are considered. It is very high in

oleic acid (82.04%) and low in free acidity and TPMs, and

ideal in color and appearance properties. Although not

measured here, hazelnut oil has a very pleasant nutty flavor

which in most cases liked in fried products.

3.2 Adsorption capacity

Free fatty acid adsorption capacities of the natural zeolite and

the modified zeolites were measured (mg oleic acid/g adsorb-

ent material) and the values are shown in Fig. 1.

Since hazelnut oil is used as the frying oil and its main fatty

acid is oleic acid, measuring adsorbent free fatty acid adsorp-

tion capacity with oleic acid is more meaningful. Of course,

adsorbent materials can adsorb other fatty acids as well as

some polar materials found in the frying oil. Treatment of

frying oils with adsorbent materials is based on the theory of

selective adsorption of these materials, as suggested [4–6, 14].

As can be observed, compared to natural zeolite, the adsorp-

tion capacity of Tween80 modified zeolite has enhanced 15

times. Similarly other modification techniques have also

yielded statistically significant enhancements in the oleic

acid adsorption capacity. Hence, it would be expected that

modified zeolites might perform better than natural zeolite

in the adsorbent treatment of frying oils.

3.3 Chemical properties

In order to test this hypothesis, 6 days actual dough frying

with hazelnut oil has been completed. Daily sampling and

analysis of the frying oils after adsorbent materials treatment

have been accomplished together with absorbed fat analysis

in the dough fried within. The results of TPMs measure-

ments by official chromatographic technique are shown in

Table 2. There was a control group (no adsorbent material

treatment) and five adsorbent treatment groups (one natural

zeolite and four modified zeolites).

The table shows both daily comparison of the results

within each treatment group (column wise comparison),

and comparison of the six different treatment group within

each frying day (row wise comparison). The same statistical

technique was used in all subsequent comparisons. TPM

values in all treatment groups have increased by the days

of frying, but all modified zeolite adsorbent treatment groups

were better than the control and natural zeolite treatment

group. The enhancement of TPM was lowest in the HCI and

Tween80 modified zeolites. These findings are in good agree-

ment with the general frying oil literature [14]. According to

the frying oil regulation in Turkey [15], the discard limit of

frying oil for TPM value is 25%. Under the stated conditions

of frying, the control oil passes the limit after second day,

while modified zeolites treated groups can stay up to 4 days of

frying as shown with chromatographic TPM analysis which

is the officially accepted measurement technique. The free

acidity (% oleic acid) and conjugated dienoic acid (%) values

of the samples are shown in Table 3.

Table 1. Physico-chemical properties (mean W SD) of the fresh

hazelnut oil used in the frying experiments

Property Value

Free acidity (%) 0.14 � 0.01

Total polar materials – chromatography (%) 2.69 � 0.01

Conjugated dienoic acid (%) 0.20 � 0.01

Viscosity (cP, 258C) 67.85 � 0.25

Turbidity (NTU) 0.97 � 0.03

L value 36.61 � 0.18

a� value 2.12 � 0.05

b� value �3.75 � 0.41

Table 2. Total polar materials values (Mean W SD) measured in the frying oil samples treated with natural and modified zeolites

Days of

frying Control group Natural zeolite

Modified zeolites

OMWWa) HCI Cyclodextrine Tween80

Total polar materials – chromatography (%)

1 12.13 � 0.03 Eb)ac) 10.73 � 1.04 Ea 6.80 � 0.06 Db 7.56 � 0.16 Eb 7.54 � 0.39 Eb 4.09 � 0.06 Ec

2 14.90 � 0.07 Eab 18.99 � 0.15 Da 11.83 � 0.94 Db 14.34 � 1.04 Dab 10.63 � 0.48 Eb 10.33 � 1.96 Db

3 29.01 � 1.03 Da 23.53 � 0.71 Db 18.38 � 1.15 Ccd 21.66 � 0.08 Cbc 16.05 � 0.41 Dd 12.80 � 0.64 Dd

4 42.42 � 0.51 Ca 33.03 � 0.19 Cb 32.55 � 0.28 Bb 24.11 � 1.09 Cc 23.04 � 0.34 Cc 21.91 � 1.07 Cc

5 55.09 � 1.45 Ba 50.45 � 1.00 Ba 38.12 � 1.80 Bb 33.87 � 0.39 Bb 36.45 � 0.89 Bb 31.97 � 0.75 Bb

6 67.06 � 1.75 Aab 70.22 � 1.48 Aa 58.17 � 1.15 Acd 44.60 � 0.78 Ae 61.58 � 1.30 Abc 51.72 � 1.47 Ade

a) Olive mill waste water.b) Capital letters shown in the same columns compare the frying days for each of the frying oil samples.c) Small letters shown in the same rows compare each oil samples per same frying days.



Table 3. Free acidity and conjugated dienoic acid values (mean W SD) measured in the frying oil samples treated with natural and modified

zeolites

Days of


Modified zeolites

OMWW HCI Cyclodextrine Tween80

Free acidity (% oleic acid)

1 0.25 � 0.01 Fb 0.25 � 0.01 Eb 0.33 � 0.01 Ea 0.20 � 0.01 Fc 0.16 � 0.01 Fd 0.17 � 0.01 Ed

2 0.36 � 0.01 Ea 0.24 � 0.01 Ec 0.34 � 0.01 Ea 0.30 � 0.01 Eb 0.21 � 0.01 Ec 0.25 � 0.01 Dc

3 0.48 � 0.01 Da 0.34 � 0.01 Dc 0.46 � 0.01 Da 0.39 � 0.01 Db 0.29 � 0.01 Dd 0.29 � 0.01 Cd

4 0.75 � 0.01 Ca 0.41 � 0.01 Cd 0.63 � 0.01 Cb 0.47 � 0.01 Cc 0.41 � 0.01 Cd 0.31 � 0.01 Ce

5 0.99 � 0.02 Ba 0.61 � 0.01 Bc 0.78 � 0.02 Bb 0.78 � 0.02 Bb 0.61 � 0.01 Bc 0.42 � 0.02 Bd

6 1.17 � 0.01 Aa 0.71 � 0.01 Ac 1.05 � 0.02 Ab 1.19 � 0.01 Aa 1.19 � 0.01 Aa 0.76 � 0.01 Ac

Conjugated dieonic acids (%)

1 0.36 � 0.01 Fb 0.41 � 0.01 Fab 0.40 � 0.01 Fab 0.42 � 0.01 Fa 0.37 � 0.01 Fb 0.39 � 0.01 Eab

2 0.79 � 0.01 Eb 0.75 � 0.01 Eb 0.59 � 0.01 Ecd 0.91 � 0.01 Ea 0.55 � 0.01 Ed 0.61 � 0.01 Dc

3 1.10 � 0.03 Da 1.08 � 0.01 Da 0.89 � 0.01 Db 1.02 � 0.02 Da 0.91 � 0.01 Db 0.80 � 0.02 Cc

4 1.47 � 0.03 Ca 1.23 � 0.01 Cd 1.23 � 0.01 Cd 1.31 � 0.02 Cbc 1.35 � 0.01 Cb 1.27 � 0.01 Bcd

5 1.75 � 0.01 Ba 1.76 � 0.01 Ba 1.28 � 0.02 Bd 1.62 � 0.01 Bb 1.44 � 0.01 Bc 1.30 � 0.01 Bd

6 2.19 � 0.03 Aa 1.99 � 0.02 Ab 1.83 � 0.01 Acd 1.87 � 0.02 Ac 1.76 � 0.01 Ad 1.63 � 0.01 Ae

Capital letters shown in the same columns compare the frying days for each of the frying oil samples.

Small letters shown in the same rows compare each oil samples per same frying days.

Table 4. Physical properties measured (mean W SD) in the frying oil samples treated with natural and modified zeolites

Days of frying Control group Natural zeolite

Modified zeolites


Viscosity (cP, 258C)

1 67.67 � 0.28 Db 70.60 � 1.65 Eb 72.42 � 0.81 Db 71.25 � 1.56 Eb 88.60 � 1.79 Da 85.20 � 0.64 Da

2 73.08 � 1.03 Dc 75.82 � 0.94 Dbc 78.17 � 0.35 CDb 76.72 � 1.05 Dbc 92.95 � 1.48 Da 89.62 � 0.69 Da

3 86.62 � 0.54 Db 86.30 � 1.53 Db 85.18 � 1.46 CDb 83.07 � 0.33 Db 103.20 � 2.70 Da 97.15 � 0.31 Da

4 113.93 � 1.30 Cb 103.55 � 2.49 Cc 99.10 � 0.64 Cc 97.32 � 1.13 Cc 130.75 � 4.27 Ca 115.05 � 0.72 Cb

5 226.90 � 4.88 Ba 157.88 � 1.29 Bc 131.67 � 3.79 Bde 118.08 � 1.87 Be 187.58 � 3.31 Bb 144.15 � 2.79 Bcd

6 825.50 � 13.50 Aa 386.78 � 6.04 Ac 247.20 � 10.70 Ad 185.28 � 5.68 Ae 471.38 � 6.32 Ab 282.75 � 9.21 Ad

Turbidity (NTU)

1 0.70 � 0.02 Eb 0.72 � 0.02 Db 0.73 � 0.01 Cb 1.00 � 0.03 Ea 1.07 � 0.07 Ca 1.09 � 0.10 Ca

2 0.79 � 0.02 Ed 0.83 � 0.02 Dd 0.69 � 0.01 CDe 1.31 � 0.02 DEa 1.16 � 0.01 Cb 1.01 � 0.04 Cc

3 1.03 � 0.03 Db 0.88 � 0.01 Dc 0.66 � 0.02 Dd 1.74 � 0.03 Da 1.09 � 0.06 Cb 1.00 � 0.03 Cbc

4 1.37 � 0.02 Cc 1.63 � 0.04 Cb 0.54 � 0.02 Ee 2.81 � 0.06 Ca 1.09 � 0.01 Cd 1.13 � 0.02 Cd

5 1.72 � 0.01 Bbc 1.91 � 0.05 Bb 0.95 � 0.01 Bd 4.22 � 0.05 Ba 1.56 � 0.05 Bc 1.62 � 0.15 Bbc

6 3.31 � 0.09 Ab 2.66 � 0.05 Ac 1.20 � 0.01 Ad 10.40 � 0.23 Aa 2.79 � 0.06 Ac 2.86 � 0.06 Abc

Smoke point (8C)

1 221.50 � 0.50 Acd 224.00 � 1.00 Abc 226.50 � 0.50 Ab 219.50 � 0.50 Ad 235.00 � 1.00 Aa 236.50 � 0.50 Aa

2 214.50 � 0.50 Bbc 219.00 � 3.00 ABb 211.50 � 0.50 Bbc 206.50 � 0.50 Bc 233.00 � 1.00 Aa 232.50 � 2.50 ABa

3 211.00 � 5.00 Bbc 214.50 � 2.50 ABb 204.00 � 1.00 CDc 204.00 � 1.00 Bc 230.50 � 1.50 Aa 230.50 � 2.50 ABa

4 208.50 � 0.50 Bbc 211.00 � 0.00 BCb 202.00 � 1.00 CDcd 196.00 � 0.50 Cd 224.50 � 2.50 Aa 227.00 � 2.00 ABa

5 195.50 � 2.00 Cb 202.50 � 0.50 CDb 195.50 � 4.00 Db 186.00 � 0.50 Dc 213.50 � 7.50 Ba 222.50 � 0.50 Ba

6 186.50 � 1.00 Db 191.50 � 2.50 Db 190.50 � 4.00 Db 184.00 � 0.50 Db 207.00 � 5.00 Ba 210.50 � 2.50 Ca





Natural zeolite was better in reducing free acidity than

the modified zeolites. Only Tween80 modified zeolite has

reduced free acidity more than the control group. Contrarily,

HCl and cyclodextrine modified zeolites had no effect on

controlling free acidity in the frying oil. Generally, in all

treatment groups free acidity has increased through time,

as expected, the level has never exceeded the 2% which is,

most of the time, suggested as the critical point [14] in terms

of degradation indication. Conjugated dienoic acids were

an important criteria for monitoring the quality and stability

of frying oils. It shows the level of primary oxidation products

in an oil sample [16]. According to previous findings [7],

conjugated dienoic acids (% CD) value is a better estimation

of oxidation in frying oils than peroxide value measurements.

All adsorbent treatment groups were better than the control

group for conjugated dienoic acid reduction (Table 3)

through 6 days of frying. The best oxidation protection

was, in fact, with Tween80 modified zeolite.

3.4 Physical properties

Some physical parameters, which can be important in the

frying oils, are measured and the results are shown in Table 4.

The viscosity of frying oils in all treatment groups has

increased slowly in the first four days, and then increased

sharply. Compared to control group, adsorbent treatments

had controlled the rate of viscosity enhancement. Especially,

HCl modified zeolite treatment was very effective in

reduction of oil viscosity. It was explained [17] that the

increase in oil viscosity during frying is due to the C–C

and C–O–C bridges formed between the fatty acids due

to high thermal energy yielding various sized polymers.

Turbidity values of the oil samples have increased through

the days of frying, as indicated in a previous study [18]. The

lowest value was in all days with OMWW modified zeolite. In

fact, it is thought that this zeolite may have an exchange

process to release some phenolics attached previously onto

it, into the oil and adsorb the polar materials formed during

frying process (polar–polar exchange). It is difficult to link

this hypothesis directly with the findings, but OMWW modi-

fied zeolite treated oil was always more luminous than the

others. Contrarily, HCI modified zeolite treated sample had

the highest turbidity. It was quite obvious that this modifi-

cation had caused the natural zeolite to crack into more fine

particles. It might be possible that these very small particles

may have passed through the filter pores to yield more turbid

oil. According to Turkish frying oil controlling codex [15], an

oil must have smoke point value above 1708C to be/continued

be used as frying oil. In all treatment groups, smoke point has

decreased steadily, as expected. But Tween80 modified

Table 5. Instrumental color values measured (mean W SD) in the frying oil samples treated with natural and modified zeolites

Days of


Modified zeolites


L value

1 43.57 � 0.64 Ab 44.23 � 0.50 Ab 43.86 � 0.09 Ab 43.98 � 0.26 Ab 51.86 � 0.25 Aa 51.67 � 0.13 Aa

2 43.64 � 0.42 Ab 44.12 � 0.11 Ab 43.68 � 0.71 Ab 43.34 � 0.10 Ab 50.75 � 0.28 Aa 51.30 � 0.26 ABa

3 43.15 � 0.57 Ab 43.80 � 0.62 Ab 43.21 � 0.69 ABb 42.52 � 0.13 Ab 51.53 � 0.23 Aa 52.15 � 0.31 Aa

4 42.29 � 0.21 Ab 42.95 � 0.72 Ab 42.04 � 1.40 ABb 36.62 � 0.34 Bc 50.49 � 0.32 Aa 51.12 � 0.40 ABa

5 39.99 � 0.08 Bb 42.10 � 0.37 Ab 40.18 � 0.99 BCb 36.07 � 0.48 Cc 48.68 � 0.24 Ba 50.33 � 0.18 Ba

6 36.68 � 0.68 Cb 38.03 � 0.36 Bb 38.14 � 0.82 Cb 35.65 � 0.56 Cb 43.93 � 0.75 Ca 46.30 � 0.18 Ca

a Value

1 0.53 � 0.02 Ca 0.26 � 0.01 Dc 0.34 � 0.02 Cc 0.36 � 0.02 Dbc 0.33 � 0.03 Cc 0.45 � 0.03 Cab

2 0.47 � 0.01 Cb 0.01 � 0.00 Ed 0.27 � 0.03 CDc 1.29 � 0.01 Ca 0.09 � 0.01 CDd 0.03 � 0.01 Dd

3 0.52 � 0.01 Cb �0.03 � 0.01 Ed 0.11 � 0.03 Dc 2.04 � 0.01 Ba �0.06 � 0.01 Dde �0.12 � 0.01 De

4 2.18 � 0.05 Bb 0.63 � 0.04 Cc 0.27 � 0.02 CDd 4.67 � 0.10 Aa 0.28 � 0.01 Cd �0.18 � 0.02 De

5 5.41 � 0.17 Aa 3.34 � 0.06 Bc 2.41 � 0.03 Bd 4.99 � 0.11 Ab 2.95 � 0.04 Bc 0.77 � 0.03 Be

6 2.49 � 0.02 Bc 5.45 � 0.09 Ab 5.15 � 0.07 Ab 2.12 � 0.03 Bd 5.98 � 0.12 Aa 5.93 � 0.11 Aa

b Value

1 0.69 � 0.04 Cb 1.15 � 0.08 Ca �0.54 � 0.06 De 0.38 � 0.04 Ccd 0.46 � 0.05 Cbc 0.17 � 0.01 Ed

2 2.22 � 0.06 Ba 2.33 � 0.03 Ba 1.05 � 0.03 Cc 0.99 � 0.02 Bc 1.63 � 0.11 Bb 1.96 � 0.06 CDa

3 3.52 � 0.05 Aa 2.41 � 0.03 Bb 1.62 � 0.05 Bc 1.49 � 0.06 Ac 1.60 � 0.07 Bc 2.27 � 0.07 Cb

4 3.65 � 0.13 Aab 3.16 � 0.04 Ab 2.31 � 0.12 Ac �0.59 � 0.06 Dd 3.96 � 0.19 Aa 3.48 � 0.03 Bab

5 2.04 � 0.09 Bd 2.82 � 0.08 Ac 1.82 � 0.03 Bd �3.73 � 0.14 Ee 4.24 � 0.12 Ab 5.30 � 0.24 Aa

6 0.36 � 0.04 Cb �1.08 � 0.04 EDd �0.81 � 0.05 Dd �3.56 � 0.21 Ee �0.22 � 0.01 Dc 1.61 � 0.04 Da


Small letters shown in the same rows compare each oil per the same frying days.



zeolite treated samples had kept smoke point as high as

possible. On the other hand, the lowest value was seen with

HCI modified zeolite treated samples (Table 4). Likely, after

six days of frying, the smoke point of hazelnut oil was still

high enough to be used as frying oil. In fact, data resulted

from this study indicates that hazelnut oil is a very good

candidate for continuous frying processes. The measured

instrumental color values of the frying oil samples are shown

in Table 5.

The value of L indicates level of luminosity or brightness,

and found higher in the cyclodextrine and Tween80 modified

zeolite treated groups. In fact, the value was lower than the

control group in the HCI modified zeolite treated group. This

might be caused from the same reason of the very fine zeolite

material particles dispersed in the oil. Clearly, dark color

forming chemicals have removed more successfully with

Tween80 modified zeolite. Hence, this modified zeolite

may find usage in crude oil discoloration process as well.

On the other hand, color components of a� and b� values have

showed little changes by frying time and treatment groups.

The value of a� has showed a steadily increase (development

of red color) in all treatment groups, but in the sixth day,

control and HCI modified zeolite treated groups have showed

a deviation. Similarly, the trend of changes in the b� value has

an increase in positive direction or increase in yellowness, but

the trend has changed sharply at the sixth day toward a

negative value, which indicated a more blue color develop-

ment. In a previous study [7], the color value changes in

sunflower and refined olive pomace oil was different than this

study, indicating that changes in color values might be very oil

type dependent.

3.5 Absorbed fat

The amount of fat absorbed by the fried dough in the six

treatment groups by six frying days is shown in Table 6.

As can be observed easily, there was no statistically sig-

nificant difference either between the treatment groups or the

days of frying. Hence, it would be said that adsorbent treat-

ment of hazelnut oil has not caused any differences in the

amount of oil absorption by dough frying. On the other hand,

in one study [7], it was found that, adsorbent treatment of

frying oil had reduced amount of oil absorption significantly.

Again, this difference can be due to the different level of oil

deterioration during that particular frying operation and

properties of the oils studied. On the other hand, it can be

said in general that the total amount of oil absorption in this

study was somewhat lower than similar ones [7] reported in

literature. This would be due to the properties of hazelnut oil

which should be further evaluated for this property.

4 Conclusion

The main objective of this study was to investigate the

effectiveness of some surface modified natural zeolites during

deep-fat frying process as adsorbent materials. Measurement

of adsorption capacity indicated that modifications were

effective in increasing oleic acid adsorption significantly.

The results of frying experiments have shown that both

natural zeolite and modified zeolites have different level of

activities as adsorbent materials. While the best results were

achieved with Tween80 modified zeolite for the TPMs, free

acidity, instrumental color values, smoke point, and conju-

gated dienoic acid measurements, the best results for turbid-

ity was with OMWW modified zeolite and the best result for

viscosity was with HCI modified zeolite. It was determined

that adsorbent treatment did not affect fat absorption level of

dough fried. It can be concluded that different strategies of

surface modifications of natural zeolite may further increase

its ability as frying adsorbent material. Especially, by using

the tools of nanotechnology, the level of porosity and surface

affinity of natural zeolite can be modified. This study has also

shown that hazelnut oil is highly stable oil for frying and may

yield less oil absorption.

This research was funded by the Scientific Researches Commission

of Canakkale Onsekiz Mart University (Fund no: BAP 2010/

152). The authors thank for the funding.

The authors have declared no conflict of interest.

Table 6. The amount of absorbed fat (mean W SD) by the dough fried in different adsorbent materials treated frying oils

Days of


Modified zeolites


Absorbed fat by fried dough (%)

1 6.09 � 0.07 Bc 5.10 � 0.06 Bd 7.63 � 0.02 Bb 7.59 � 0.10 Ab 8.85 � 0.05 Aa 6.21 � 0.22 Bc

2 6.72 � 0.18 Acd 6.09 � 0.02 Ad 8.47 � 0.02 Aa 7.96 � 0.16 Aab 7.26 � 0.32 Bbc 4.99 � 0.01 Ce

3 6.26 � 0.05 Abd 5.47 � 0.01 Be 7.44 � 0.01 Bb 7.46 � 0.09 Ab 8.19 � 0.02 ABa 7.17 � 0.03 Ac

4 5.12 � 0.03 Cd 5.35 � 0.16 Bcd 7.60 � 0.04 Ba 6.31 � 0.14 Bbc 7.43 � 0.37 Bab 5.21 � 0.29 Ccd

5 5.92 � 0.04 Bcd 5.50 � 0.02 Bd 7.63 � 0.08 Ba 6.78 � 0.20 Abb 6.05 � 0.04 Cc 5.71 � 0.02 BCcd





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effectiveness of modified zeolites as adsorbent materials for frying oils

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