THERMAL STABILITY OF SOME COMMERCIAL NATURAL AND SYNTHETIC ANTIOXIDANTS AND THEIR MIXTURES

SAMAH SAID MAHMOUD ALLAM' and HANI MOUSTAFA ALY MOHAMEDZ

'Oils and Fat Research Department Food Technology Research Institute

Agricultural Research Center Giza, Egypt

2Department of Food Science Faculty of Agriculture Minia University

El-Minia, Egypt

Received for Publication May 5,2002 Accepted for Publication September 4, 2002

ABSTRACT

The thermal stability of some commercially available antioxidants alone or in binary or ternary mixtures was investigated in order to find the optimal combination of antioxidants for food processing. The synthetic antioxidants used were butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG) and tertiary-butyhydroquinone (TBHQ). Other antioxidants such as ascorbyl palmitate (AP), mixed tocopherols (TOCO) and monoacylglycerol citrate (MGC) were also employed. The effectiveness of these antioxidants and their combinations was assessed using the Rancimat test by measuring the induction period for the oxidation of sunflower oil after heating at 180C for I h and comparing it to the oxidation kinetics of the oil without added antioxidants. All antioxidants and their binary or ternary mixtures showed diferent degrees of thermal instability. TBHQ individually, among all the examined antioxidants, showed the highest thermal stability. On the other hand, A P as an antioxidant in sunflower oil exhibited low stability during heating. The thermal stability of AP could be enhanced by the addtion of BHA or BHT in binary mixtures at a ratio of 1:3 (w/w). In addition, the ternary mixture of AP, TOCO and MGC (65:25:10) in sunflower oil also showed a higher stability to thermal inactivation. However, the ternary mixture containing 0.13% AP, 0.05% TBHQ and 0.02% MGCprovided the optimal protection during thennaI treatment. Although a combination of 0.13% BHT 0.05% AP and 0.02% MG was very effective synergistically at room temperature, it showed a higher susceptibility to thermal inactivation. It was

'Corresponding author: samahallam@yahoo.com

Journal of Food Lipids 9 (2002) 277- 293. AN Rights Reserved. 'Copyright 2002 by Food 6; Nutrition Press, Inc.. Trurnbull, Connecticut 277

278 S.S. MAHMOUD ALLAM and H.M. ALY MOHAMED

concluded that the interaction between antioxidants could lead to a negative or positive synergism during the high-temperature treatment depending on the type and concentration of individual components in the mixture. Synthetic antioxidants such as TBHQ, BHA and BHT could be added at relatively low levels to retard loss of natural antioxidants during heating.

INTRODUCTION

Many lipids are labile during high-temperature treatment in the presence of air. Under these conditions, acceleration of both thermal and oxidative decomposition reactions occur (Sebedio et al. 1987). Several authors have demonstrated the harmful effects on fats and oils during excessive heating. Formation of compounds such as enzyme inhibitors that possess antinutritional properties (Ruiz-Gutierrez and Muiana 1992), and accelerated loss of antioxidant vitamins, such as vitamin E (Sheely et al. 1994) may be encountered. Moreover, oxidized lipids enhance peroxidation of membrane macromolecules (Hayam et al. 1997), thus contributing to mutagenicity (Hageman et al. 1988), genotxicity (Kitts 1996) and angiotoxicity (Addis 1986). These cellular aberrations are llkely implicated in linking oxidized oils to growth retardation (Saito et al. 1981), colon carcinogenesis (Yang et al. 1998) and reproductive disorders (Isong et al. 1997).

Protection of oils against thermal and oxidative deterioration has been achieved by the adhtion of various synthetic antioxidants, such as butylated hydroxytoluene (BHT) (Peled et al. 1975; Augustin and Berry 1983; Dobarganes et al. 1986), butylated hydroxyanisole (BHA) (Augustin and Berry 1983; Dobarganes et al. 1986), propyl gallate (PG) (Boers 1998) and tertiary-butylhydroquinone (TBHQ) (Warner et al. 1985; Sanhueza et al. 2000).

The effectiveness of antioxidants varies depending on the food and conditions of processing and storage (Coppen 1989). Most antioxidants become less effective during the high-temperature treatment, and also some may be lost through volatilization (Augustin and Berry 1983; Dobarganes et al. 1986; Tyagi and Vasishtha 1996). Volatiles resulting from antioxidants may play an important role on the stability of oils during thermal processing. Compounds with higher polarity have lower volatility regardless of their complicated interaction with lipids. Consequently, BHT had the highest volatility due to its lower polarity, which is consistent with the observation of Freeman et al. (1973). Studies regarding breakdown of phenolic antioxidants and interaction between secondary degradation products of oils and antioxidants at high temperatures are scarce. Decomposition products of TBHQ under frying temperatures in a model system were identified and characterized by Kim and Pratt (1 990). Tert-butylbenzoquinone (TBBQ) was identified as the primary and major oxidation product of TBHQ. Interconversion between TBHQ and TBBQ plays an important role in antioxidant effectiveness and the carry-through effect of TBHQ.

THERMAL STABILITY OF ANTIOXIDANTS 219

Increasing concern over the safety of chemical additives in foods has given rise to an interest in replacing synthetic antioxidants with natural antioxidants. There are relatively few reports concerning the stabilizing effect of natural compounds on fats subjected to elevated temperatures, e.g. a-tocopherol (Kajirnoto et al. 1985), ascorbyl palmitate, a semi-natural antioxidant, (Gwo et al. 1985), sterols (Sims et al. 1972; Boskou and Morton 1976; Gordon and Magos 1983), rosemary and sage (Houlian and Ho 1985), flavonoids (Das and Pereira 1990) and oat extract (Tian and White 1994). Among these natural additives, tocopherols exhibited a superior permanence and resistance to volatility compared with synthetic antioxidants such as BHA and BHT in long-term Gying tests and during storage (Dougherty 1993).

Antioxidants are usually combined to take advantage of their different properties (Coulter 1988). The use of synergistic mixtures of antioxidants allows a reduction in the concentration of each substrate and also increases the antioxidative effectiveness as compared with the activity of each compound alone (Kikugawa et al. 1990). Composites of antioxidants are known to exhibit synergistic effects for increasing oil stability. AP in combination with PG and thiodipropionic acid (0.0 1% of each) rendered extended protection to safflower, sunflower, peanut and corn oils (Cort 1974). The antioxidant activity of TBHQ and of its mixtures with 0.005-0.020% BHA and BHT on refined corn oil and virgin olive oil heated at 75 C was studied by Thomopoulos and Grigoropoulou (1982). The results showd that, at equal concentration, TBHQ was more efficient than its mixtures with BHA and BHT. The mixtures TBHQ + BHA were more efficient than TBHQ + BHT at equal concentration, though BHT alone was more efficient than BHA. Kiritsakis et al. (1983) reported that combinations of BHA and BHT with TBHQ displayed better stabilizing effect on virgin unrefined olive oil under accelerated conditions (oven test). Yanishlieva and Marinova (1988) observed that the best antioxidative effect to sunflower oil against autoxidation was experienced using a mixture of 0.05% ascorbyl palmitate and 1% lecithin, which produced a stabilization factor of 2.5. Aoyama et al. (1993) found that mixed tocopherols combined with other antioxidants had enhanced antioxidative activity; this was most effective when mixed tocopherols were combined with TBHQ, followed by AP and PG. On the other hand, Che-Man el al. (1999) observed no pronounced synergistic effect between TBHQ and a-tocopherol.

A synergistic effect was also demonstrated between TBHQ and BHA used in combination in lard (Dougherty 1993). Kejian (1994) reported that the addition of 0.10% natural tocopherols, 0.02% ascorbic acid, 0.02% citric acid, and 0.20% soybean lecithin to menhaden oil increased its induction period from 0.90 to 25.93 h, as determined by the Rancimat method. In addition, a mixture of 0.02% ascorbic acid, 0.02% citric acid, and 0.20% soybean lecithin doubled the induction time of soybean oil. Morais et al. (1 994) examined the efficiencies of several antioxidants alone or in combination in retarding the development of rancidity in fish oil. They

280 S.S. MAHMOUD ALLAM and H.M. ALY MOHAMED

demonstrated that a mixture of 0.02% TBHQ, 0.02% BHA and 0.05% rosemary extract only reduced the peroxide value by 10% compared to approximately 50 and 45% by 0.02% TBHQ or 0.05% rosemary extract alone. Gordon and Kourimska (1 995) assessed the effect of AP (0.02%) on tocopherol loss from the oil during deep fat frying. They found that the addition of AP increased the stability of the tocopherols to a considerable extent. Moreover, AF' was found to act as a powerful synergist with tocopherols during storage of oils. Ap also showed some synergistic effects with PG, but not with BHA or BHT during deep fat frying of potato chips in peanut oil (Satyanarayana et al. 2000).

However, information to establish relationships between the composition of antioxidant mixtures and thermal stability remains fragmentary and dispersed. The present study was carried out in order to: (1) clarify the effects of the recommended synthetic (BHA, BHT, PG and TBHQ) and natural/semi-natural antioxidants (mixed tocopherols, ascorbyl palmitate and monoacylglycerol citrate) on the oxidative and thermal stability of sunflower oil during high-temperature treatment (1 80C for 1 h) in air; (2) investigate the thermal resistance of certain binary blends and ternary mixtures of natural and synthetic antioxidants; and (3) develop new, effective antioxidant mixture for enhancing the stability of sunflower oil during storage and high-processing temperatures.

The maximum usage level of single or multiple antioxidants approved by the FDA is 200 ppm or 0.02%, based on the weight of fat or oil (Buford 1994). In the present study we have chosen conditions that are not typical in food processing systems by using high concentrations of antioxidants (0.2%) to simplify the effect of the interaction of these commercial antioxidants.

MATERIALS AND METHODS

Materials

Refined, bleached and deodorized (RBD) sunflower oil free from additives was obtained from Savola Egypt Company (Ramadan City, Egypt). BHA, BHT, TBHQ and PG were supplied by Eastman Chemical Co. AP, TOCO and MGC were kindly donated by Daminco Inc. (Denmark). All other chemicals used in this study were of analytical grade.

Antioxidant Combinations

To better understand the effect of the interaction of synthetic and natural antioxidants and to determine where the maximum sunflower oil thermal stability is located two experimental designs were considered.

THERMAL STABILITY OF ANTIOXIDANTS 281

Design A- Binary mixtures of some synthetic and natural antioxidants:

1- 75% BHA+25% MGC 2- 75% BHT+25% MGC

4- 75% AP+25% BHA 3- 75% AP+25% MGC

5- 75% AP+25% BHT

Design B-Ternary mixtures of some synthetic and natural antioxidants:

1- 65% BHA+25% AP+10% MGC 2- 65% BHT+25% AP+lO% MGC 3- 65% AP+25% TOCO+IO% MGC 4- 65% AP+25% TOCO+IO% PG 5- 65% AP+25% TBHQ+IO% MGC

Heating Procedure

Various antioxidants or their combinations were added to sunflower oil at a level of 0.2% (w/w). Oil samples (100 g) were heated continuously in 250 mL pyrex beakers on a Coming hot plate (model PC-35 1) at 180 f 5C for 1 h. At the end of thermal treatment, samples were set aside to cool and then stored in a freezer (-20C) for up to two weeks until analyzed. The experiments were performed in triplicates.

Analytical Methods

The AOCS method (1989) was used for determination of peroxide value (method Cd 8-53). Conjugated dienes (CD), and conjugated trienes (CT) were determined using IUPAC (1987) method of analyses (method 2.505). The contents of CD and CT were expressed as absorptivities of the 1% oils in 2,2,4- trimethylpentane at 232 and 268 nm, respectively.

Gas Liquid Chromatographic Analysis of Fatty Acids

The fatty acid methyl esters were prepared using benzenelmethanollconcen- trated sulfuric acid (10:86:4, v/v/v) and methylation was camed out for one hour at 80-90C according to Stahl (1967). The composition of fatty acids were established by gas liquid chromatography using a PYE Unicam model PV 4550 gas chromatograph fitted with flame ionization detector, the column (1.5 m x 4 mm) packed with diatomite C (100-120 mesh) and coated with 10% polyethylene glycol adipate (PEGA). The oven temperature was programmed at 8Clmin from 70C to 190C then isothermally at this temperature for 20 min and nitrogen flow rate was

282 S . S . MAHMOUD ALLAM and H.M. ALY MOHAMED

30 mL/min. Detector and injector temperatures, hydrogen and air flow rates and chart speed were 300C, 250C, 33 mL/min, 330 ml/min and 2 cdrnin, re- spectively. The fatty acids were identified according to retention times using an authentic mixture of fatty acids chromatographed under the same conditions. Methyl heptadecanoate was used as an internal standard.

Determination of Oxidative Stability Using Rancirnat Method

The protective effect of several antioxidants on sunflower oil was tested by the Rancimat method, in duplicate, using Metrohm Rancimat 679 (Metrohm Ltd., Herisau, Switzerland) as described by Taskins et al. (1999). Each oil (5.0 f 0.05 g) was accurately weighed into each of the six reaction vessels, and the following procedure was carried out. The Metrohm Rancimat was switched on until the temperature of the oil batch reached 1OOC. Then 60 mL of distilled water were placed into each of the six conductivity cells, and the air flow rate was set at 20 L h-’. The temperature was checked to ensure that it had a constant value. The air supply was connected to the tubes containing the oil samples and recording the output. The determination continued automatically until the conductivity reached the maximum value and the induction period (IP) was read.

Results were calculated from the ratio of the IP (expressed in hours) obtained for the antioxidant assayed after the thermal treatment (IPJ and the IP obtained before the thermal treatment (Pi), and expressed as percentage of inactivation according to Sanhueza et al. (2000), expressed as inactivation%= ( l-IPfiPi) x 100.

Statistical Analysis

Statistical analysis was performed using statistical analysis systems (SAS 1987) software package. Difference between the mean values was determined by Duncan’s multiple range test.

RESULTS AND DISCUSSION

Chemical Properties of RBD Sunflower Oil

RBD sunflower oil used in this study as a substrate of oxidation, had good initial qualities, i.e. having iodine value of 139.86 iodine/100 g oil, free fatty acid of 0.08% (as oleic acid) and peroxide value of 0.73 meqkg oil (Table 1). The fatty acid composition showed that sunflower oil had a reasonably high content of the essential linoleic (cis-9, cis-1Zoctadecadienoic) acid (73.28%). The fatty acid composition was similar to that reported by Warner ef al. (1 989).

THERMAL STABILITY OF ANTIOXIDANTS 283

Effect of Single Antioxidants on the Oxidative and Thermal Stabilities of Sunflower Oil

Table 2 shows the IPf obtained for each antioxidant after the thermal treatment compared to the IPi obtained for the same antioxidant in its untreated form, when assayed according to the Rancimat test. All individual antioxidats significantly (P< 0.05) increased IPi of the oil except MGC. Kiritsakis et al. (1983) found that MGC used alone did not affect oxidative stability of the oil. The most efficient antioxidants were PG and TBHQ, which increased IPi by 6.7 and 6.5 times, respectively, compared with the control (no antioxidant). Natural compounds were less antioxidative than synthetic antioxidants, although effectiveness of TOCO was close to that of BHA. The low activity of the AP in the Rancimat test is due to the rapid flow of oxygen, since Ap acts as an antioxidant by removing oxygen and hence is most active under conditions where oxygen supply is limited (Gordon 1990).

TABLE 1. ANALYTICAL DATA OF RBD SUNFLOWER OIL'

Parmeters Sunflower Oil ~~

Free Fatty Acids % (as Oleic acid) 0.08

Peroxide Value (rneq.1 kg oil)

Iodine value (g iodine/100 g oil)

Fatty acid profile (wt %)

0.73

139.86

C 16:O 6.67

C 18:O 4.10

c 18:l 15.95

c 18:2 73.28 'Mean of duplicate determinations

After application of thermal treatment, differences were observed for the IP,of each antioxidantwhen compared to its respective IPi. TBHQ exhibited the highest thermal stability and showed the least inactivation. This means that TBHQ acts as an mhibitor of the chain radical processes even at high-temperatures. This c o n f i i published data with a wide range of lipid substrates (Dougherty 1993; Gordon and Kourimska 1995; Sanhueza et al. 2000). However, PG had a moderate thermal stability. Dougherty (1993) reported that PG is usually lost under conditions of coolung or deep fiying. On the other hand, BHT and BHA were thermally unstable

284 S.S. MAHMOUD ALLAM and H.M. ALY MOHAMED

at 180C for 1 h and this probably reflects the volatility of these compounds. In this respect, Kim and Pratt (1992) have reported ineffectiveness of BHA and BHT under frying conditions.

The most pronounced effect among natural compounds is observed with mixed tocopherols (TOCO). AP and MGC show great susceptibility to thermal inactivation and exhibited a negative effect as antioxidants under test conditions. The same order of effectiveness (P < 0.05) of antioxidants was observed by conjugated dienes and trienes values (Table 3).

TABLE 2. EFFECT OF THERMAL TREATMENT ON THE INDUCTION PERIOD (IP) OF RBD

SUNFLOWER OIL ASSESSED THROUGH THE RANCIMAT TEST, AFTER THE ADDITION OF SEVERAL SINGLE ANTIOXIDANTS'

Antioxidant IPi IPf

Control 12.40 8.17d

BHA 19.89' 7.84'

BHT 21.03b 7.58d

PG 81.56' 26.42

TBHQ 80.41 a 76.81

AP 15.0ge 4.78'

TOCO 16.8Sd 10.03'

MGC 12.2' 3 S O r 'Means within each column with different superscripts are significantly different (P < 0.05). IPi : induction period (h) obtained before the thermal treatment IPf : induction period after the thermal treatment

Effect of Some Binary Mixtures of Antioxidants on the Oxidative and Thermal Stability of Sunflower Oil

The results of the effect of some binary mixtures of synthetic and natural antioxidants assayed according the Ranchat test are given in Table 4. It is obvious that IP, was significantly improved (K0.05) for all binary mixtures tested. Furthermore, MGC (0.05%) significantly extended the IPf of oils containing 0.15% BHA, BHT or AP, unldce the behavior of the oil containing individual components. It is also interesting to note that BHA and MGC display a strong synergy as a binary mixture.

In addition, the results clearly demonstrate that AP is positively synergized by 0.05% of BHA or BHT and had improved thermal stability. However, the

THERMAL STABILITY OF ANTIOXIDANTS 285

combination of 0.05% MGC and 0.15% AF' was not as effective as the addition of BHA or BHT. This could encourage the trend of using a minor portion of synthetic antioxidants for protection of natural antioxidants. Formation of conjugated dienes and trienes during heating followed the same pattern of change in the IP (Table 5).

TABLE 3. EFFECT OF SOME SEVERAL SINGLE ANTIOXIDANTS ON CONJUGATED DIENE AND

TRIENE VALUES OF THERMALLY TREATED RBD SUNFLOWER OIL'

Antioxidant - UV (at 232 nm) UV (at 270 nm) Unheated Heated Unheated Heated

Control 1.009" 1.942 0.096 " 0.631 BHA 1.020 " 1.817d 0.094 " 0.468 BHT 1.014" 1.816d 0.091 " 0.485 PG 1.007 a 1.529 ' 0.095 " 0.187e

TBHQ 1.005 " 1.126' 0.092 " 0.126' AP 1.018" 1.940 0.094 a 0.670 "

TOCO 1.013" 1.916' 0.092 " 0.582' MGC 1.020 " 1.981 a 0.090 " 0.689 "

'Means within each column with different superscripts are significantly different (P < 0.05).

TABLE 4. EFFECT OF THERMAL TREATMENT ON THE INDUCTION PERIOD (IP) OF RBD

SUNFLOWER OIL ASSESSED THROUGH THE RANCIMAT TEST, AFTER THE ADDITION OF SEVERAL BINARY MIXTURES OF ANTIOXIDANTS'

Antioxidant IPi IPf

75 % BHA + 25 % MGC 23.96' 18.03'

75 7'0 BHT + 25 Yo MGC 19.52 14.49b

75 % AP + 25 Yo MGC 13.74d 9.02

75 % AP + 25 % BHA 18.12' 12.90'

75 % AP + 25 % BHT 17.59' 11.8Id I Means within each column with different superscripts are significantly different (P C 0.05).

286 S.S. MAHMOUD ALLAM and H.M. ALY MOHAMED

TABLE 5. EFFECT OF SOME SEVERAL BINARY MIXTURES OF ANTIOXIDANTS ON CONJUGATED

DlENE AND TRIENE VALUES OF THERMALLY TREATED RBD SUNFLOWER OIL'

Antioxidant blend UV (at 232 nm) UV (at 270 nm)

Unheated heated Unheated heated

75 % BHA + 25 Yo MGC 1 , 0 ~ 2 a 1.549 0.097 a 0.333 75 % BHT + 25 % MGC 1,019a 1.619' 0.089 a 0.592 a

75 % AP + 25 % MGC 1,008 a 1.705 a 0.095 a 0.586 a

75%AP+25%BHA 1.012a 1.638 ' 0.092 a 0.460 75 % AP + 25 % BHT 1.014a 1.657 0.090 a 0.559

'Means within each column with different superscripts are significantly different (P < 0.05).

Effect of Some Ternary Mixtures of Antioxidants on the Oxidative and Thermal Stabilities of Sunflower Oil

The effectiveness of some ternary blends of antioxidants on the stability of sunflower oil is presented in Table 6. The sample containing 0.13% BHT, 0.05% AP and 0.02% MGC acted as prooxidant in the heated sample as it significantly shortened IP, compared to that of the control. Therefore, thermal decomposition products of AP and MGC when formed in an oil sample may act as promotor of lipid oxidation reactions.

When 0.05% TOCO was added, AP showed strong synergistic antioxidative effect. The ternary mixture involved can be regarded as an extension of the binary system APMGC in which a synergistic effect was observed (Table 4). AP, which acts as an oxygen scavenger, enhances the formation of carbon linked tocopherol dimers whch have been observed amongst the products of tocopherol oxidation (Fujitani and Ando 1977). These dimers are expected to retain the antioxidant activity of the compound. The addition of 0.02% PG instead of MGC significantly improved the thermal stability of this mixture. Satyanarayana et al. (2000) found that AP had higher synergistic effects with PG than with BHA or BHT.

No significant differences (P > 0.05) were observed on the IP, of the antioxidant composites of BHA plus AP + MGC and that of AP plus TOCO+MGC. Furthermore, a combination of 0.13% AP, 0.05% TBHQ and 0.02% MGC produced an optimal IP, indicating that this additive formula was superior for use in the stabilization of RBD sunflower oil against thermal autoxidation. Th~s mixture also had the lowest conjugated dienes and trienes levels (Table 7). The protective effect of adding TBHQ is well documented (Aoyama et al. 1993; Morais et al. 1994).

THERMAL STABILITY OF ANTIOXIDANTS 287

TABLE 6. EFFECT OF THERMAL TREATMENT ON THE INDUCTION PERIOD (IP) OF RBD

SUNFLOWER OIL ASSESSED THROUGH THE RANCIMAT TEST, AFTER THE ADDITION OF SEVERAL TERNARY MIXTURES OF ANTIOXIDANTS'

Antioxidant Blend IPi IPf

65 % BHA+ 25 %AP + 10 % MGC

65 % BHT + 25 %AP + 10 % MGC

65 % AP + 25 % TOCO + 10 % MGC

65 % AP + 25 % TOCO + 10 % PG

65 % AP + 25 % TBHQ + 10 % MGC

36.77b 10.81 '

21.64 * 5.62 *

18.28 11.45

26.23 22.48

46.05 a 31.13 a

'Means within each column with different superscripts are significantly different (P< 0.05).

TABLE 7.

EFFECT OF SOME SEVERAL TERNARY MIXTURES OF ANTIOXIDANTS ON CONJUGATED DIENE AND TFUENE VALUES OF THERMALLY TREATED RED

SUNFLOWER OIL'

Antioxidant blend UV (at 232 nm) UV (at 270 run) Unheated Heated Unheated Heated

65 % BHA + 25 % AP + 10 % MGC 1.01 1 a 1.703 b 0.097 a o.607 b

65 % BHT + 25 % AF' + 10 % MGC 1,007 a 1,981 a o.090 a o,688 a

65 % AP + 25 % TOCO + 10 % MGC a c o,091 a o,578 F

65 % AP + 25 % TOCO + 10 % PG 1.012' 1.313d 0.094' 0.229d

6 5 % A P + 2 5 % T B H Q + I O % M G C 1.008a 1.214' 0.089a 0.131'

'Means within each column with different superscripts are significantly different (P < 0.05)

Improving the Thermal Resistance of BHA

BHA is commercially produced as a mixture of 2- and 3- isomers of tertiary- butyl-4-methoxyphenol (Dziezak 1986). Because of the tertiary-butyl group ortho to the hydroxyl group, the tertiary butyl group interferes with the antioxidant activity of the phenolic group (Dziezak 1986). This structural steric hindrance may also be responsible for the low thermal stability observed for thls antioxidant. Therefore, BHA should be combined with other antioxidants. Figure 1 illustrates the IP,values and the calculated inactivation % of BHA throughout the study on both binary and ternary systems of antioxidant mixtures. The oil sample

288 S.S. MAHMOUD ALLAM and H.M. ALY MOHAMED

containing 0.15% BHA and 0.05% MGC had the longest IP,( 18.03 h) and showed only 24.75% thermal inactivation. On the other hand, AP when added at 0.05% level of oil containing 0.13% BHA and 0.02% MGC led to a decrease in the IPf of sunflower oil and exhibited 70.60% inactivation.

Improving the Thermal Resistance of AP

Little is known about the efficacy of AP in combination with other antioxidants during thermal processing (Berger 1975; Hamilton et al. 1998; Satyanarayana et al. 2000). In this study, antioxidative properties of AP alone and in combination with other known antioxidants were investigated during thermal processing. The synergistic action of different combinations under Rancimat test conditions is shown in Fig. 2. All combinations had significantly increased IPf values compared to that of AP and therefore enhanced its retention during the thermal treatment. The highest IPfwas recorded for the ternary mixture of 0.13% AP, 0.05% TBHQ and 0.02% MGC. The results also suggest that AP has a synergistic action with TOCO as described previously (Pongracz 1973). Moreover, in oil sample containing 0.13% AP, 0.05% TOCO and 0.02% PG the lowest thermal inactivation was observed. It is therefore proposed that the effectiveness of this ternary system arises through the ability of PG to interact with free radicals and in particular with ascorbyl radicals. This, combined with the strong antioxidant effect of tocopherol, results in a high degree of protection against thermal deterioration. In this respect, Berger (1975) showd that 0.2% of a mixture containing tocopherol, ascorbyl palmitate and lecithin had a slightly greater protective effect in various oils heated to 150C than 0.02% BHT, while this mixture plus small amounts of propyl gallate gave the best protection to all oils tested.

CONCLUSIONS

Food processing operations require antioxidants that survive high temperatures experienced during baking, coolung or frying and provide protection to finished products. In the present study, relative heat stabilities of natural antioxidants (AP, TOCO and MGC) at 180C for 1 h were compared with those of synthetic antioxidants (BHA, BHT, TBHQ and PG). TBHQ was the most powerful antioxidant for increasing thermal stability of sunflower oil. However, other antioxidants become less effective during the high-temperature treatment and some may be lost due to volatilization. Treated oils with antioxidant mixtures showed improved oxidative and thermal stability than those treated with individual antioxidants. The highest relative stability alone or in mixtures was seen with TBHQ followed by PG and TOCO. MGC had no antioxidant activity

THERMAL STABILITY OF ANTIOXIDANTS 289

A C

Antioxidant FIG. I . ENHANCEMENT THE THERMAL STABILITY OF SUNFLOWER OIL BY ADDING

SEVERAL ANTIOXIDANT MIXTURES BASED ON BHA

Where A: BHA

B: 75% BHA+25% MGC

C: 65% BHA+25% AP+IO% MGC

35

30

25

20

15

10

5

0

80 70

60

50 3 0

40 '2 30 2

u 20 2 - 10

0

c U A B C D E F G

Antioxidant

FIG. 2. ENHANCEMENT THE THERMAL STABILITY OF SUNFLOWER BY OIL ADDING SEVERAL ANTIOXIDANT MIXTURES BASED ON AP

Where A : AP B: 75% AP+ 25% MGC

C: 75% AP+ 25% BHA

D: 75% AP+ 25% BHT

E: 65% AP+ 25% TOCO+ 10% MGC F: 65% AP+ 25% TOCO+ 10% PG G: 65% AP+ 25% TBHQ+ 10% MGC

290 S.S. MAHMOUD ALLAM and H.M. ALY MOHAMED

of its own but enhanced the stability of synthetic phenolic antioxidants such as BHA and BHT. However, MGC had a lower synergistic effect on oil stability when combined with AP than with BHA or BHT. The combination of tocopherol resulted in a synergistic antioxidant effect for oils, fats and foods in the mixture composed of 0.13% AP, 0.5% TOCO and 0.02% MGC. However, this mixture was still not very suitable for preventing thermal deterioration. The replacement of MGC with PG in this ternary mixture provided a stable composition for deep frying. Based on these results, the optimum antioxidant mixture was achieved by the combination of 0.13% AP, 0.05% TBHQ and 0.02% MGC. This mixture could extend the shelf life of sunflower oil and protects it fiom the decomposition that could occur during thermal treatments.

REFERENCES

ADDIS, P.B. 1986. Occurrence of lipid oxidation products in foods. J. Food Chem. Toxicol. 24, 1021-1030.

AOCS. 1989. Official Methods and Recommended Practices of the American Oil Chemists’ Society, 4th Ed., American Oil Chemists’ Society, ChampaignJL.

AOYAMA, M., KANEMATSU, H., TSUKAMOTO, M., TOKAIRIN, S. and MWA, I. 1993. Effect of several antioxidants on the stability of fish oil. J. Jpn. Oil Chem. SOC. 42,680-684.

AUGUSTIN, M.A. and BERRY, S.K. 1983. Efficiency of antioxidants BHA and BHT in palm oil during heating and frying. J. Am. Oil Chem. SOC. 60, 1520- 1523.

BERGER, K.G. 1975. Catalysis and inhibition of oxidation processes. 11. The use of antioxidants in foods. Chem. Ind. 5, 194-199.

BOERS, J.F. 1998. Fresh fat to fry. Food Ingr. Anal. Intern. Sept., 34-36,38,40- 41.

BOSKOU, D. and MORTON, I.D. 1976. Effects of plant sterols on the rate of deterioration of heated oils. J. Sci. Food Agric. 27, 928-932.

BUFORD, R.J. 1994. A review of natural and synthetic food antioxidants. Presented at the Southern California Food Industry Conference, January 24, Chapman, Orange, Calif.

CHE-MAN, Y.B., JIALONG, L. and LIU, J.L. 1999. The effects of TBHQ and alpha-tocopherol on quality characteristics of refined-bleached and deodorized palm olein (RBDPO) during deep-fat frying. J. Food Lipids 6, 1 17- 129.

COPPEN, P.P. 1989. Rancidity in Foods, 2nd Ed., (J.C. Allen and R.J. Hamilton, eds.) p. 83, Elseiver Applied Science, London.

CORT, W.M. 1974. Antioxidant activity of tocopherols, ascorbyl palrmtate and ascorbic acid and their mode of action. J. Am. Oil Chem. SOC. 51, 321-325.

THERMAL STABILITY OF ANTIOXIDANTS 29 1

COULTER, R.B. 1988. Extending shelf life by using traditional phenolic antioxidants. Cereal Foods World 33,207-2 10.

DAS, N.P. and PEREIRA, T.A. 1990. Effects of flavonoids on thermal autoxidation of palm oil structure-activity relationships. J. Am. Oil Chem. SOC. 67,255-259.

DOBARGANES, M.C., PEREZ-CAMINO, M.C. and GUTIERREZ-ROSALES, F. 1986. Accion protectora de 10s antioxidantes BHT y BHA en grasas calentadas a elevada temperatura. Grasas y Aceitas 37, 262-266.

DOUGHERTY, M.E. 1993. Effectiveness of natural antioxidants compared to synthetic antioxidants. Intern. Food Ingr. 3,27-32.

DZIEZAK, J.D. 1986. Antioxidants: The ultimate answer to oxidation. Food Technol. 40,94-102.

FREEMAN, I.P., PADLEY, F.H. and SHEPPARD, W.L. 1973.5. Am. Oil Chem. SOC. 50, 101.

FUJITANI, T. and ANDO, H. 1977. Oxidative dimerisation of tocopherols during the course of thermal oxidation of saturated and unsaturated triglycerides. J. Jpn. Oil Chem. SOC. 26,337-342.

GORDON, M.H. 1990. The mechanism of antioxidant action in vitro. In Food Antioxidants, (B. J. F. Hudson, ed.) pp. 1-8, Elsevier Science, London.

GORDON, M.H. and KOURIMSKA, L. 1995. Effect of antioxidants on losses of tocopherols during deep-fat frying. Food Chem. 5 2 , 175-177.

GORDON, M.H. and MAGOS, P. 1983. The effect of sterols on the oxidation of edible oils. Food Chem. 10, 141-147.

GWO, Y.Y., FLICKJR, G.J. and DUPW, H.P. 1985. Effect of ascorbyl pah ta t e on the quality of frying fats for deep frying operations. J. Am. Oil Chem. SOC.

HAGEMAN, G., JSIKKEN, R., TENHOOR, F. and KLEINJANS, J. 1988. Assessment of mutagenic activity of repeatedly used deep fat frying fats. Mut. Res. 204, 593-596.

HAMILTON, R. J., KALU, C., MCNEILL, G.P., PADLEY, F.B. and PIERCE, J.H. 1998. Effects of tocopherols, ascorbyl palmitate, and lecithin on autoxidation of fish oil. J. Am. Oil Chem. SOC. 75,813-822.

HAYAM, I., COGAN, U. and MOKADY, S. 1997. Enhanced peroxidation of proteins of erythrocyte membrane and of muscle tissue by dietary oxidized oil. J. Biosci. Biotechnol. Biochem. 61, 101 1-1012.

HOULIAN, C.M. and HO, C.T. 1985. Natural Antioxidants. In Flavor Chemistry of Fats'and Oils, (D. Min and T. Smouse, eds.) pp. 117-143, American Oil Chemists' Society, Champaign, IL.

ISONG, E.U., EBONG, P.E., IFON, E.T., UMOH, I.B. and EKA, O.U. 1997. Thermooxidized palm oil induces reproductive toxicity in health and malnourished rats. Plants Foods Human Nutr. SZ, 159-166.

62, 1666-1671.

292 S.S. MAHMOUD ALLAM and H.M. ALY MOHAMED

nrPAC 1987. Standard Methods for the Analysis of Oils, Fats and Derivatives, 7th Ed., (C. Paquot and A. Haufenne, eds.) International Union Pure Appl. Chem., Blackwell Scientific Pub., Oxford.

KAJIMOTO, G., YOSHIIDA, H. and SHIBANARA, H. 1985. A role of tochopherol on the heat stability of vegetable oils. Nippon Eiyo Shokuryo Gakaishi 38,301-307.

KEJIAN, W. 1994. Autoxidation and stabilization of polyunsaturated oils. Dissertation Abstracts Intern. 54, 3922.

KIKUGAWA, K., KUNUGI, A. and KURECHI, T. 1990. In Food Antioxidants, (B.J. F. Hudson, ed.) p. 85, Elsevier Science Publishers, London.

KIM, C.M. and PRATT, D.E. 1990. Degradation products of 2-tert- butylhydroquinone at frying temperature. J. Food Sci. 55,847350,853.

KIM, M.C. and PRATT', D.E. 1992. Thermal degradation of phenolic antioxidants. In Phenolic Compounds in Foods and Their Effects on Health, (M.T. Huang, C.T. Ho and C.Y. Lee, eds.) pp. 200-218, ACS symposium ser., New York.

KIRITSAKIS, A.K., STINE, C.M. and DUGAN JR., L.R. 1983. Effect of selected antioxidants on the stability of virgin olive oil. J. Am. Oil Chem. SOC. 60, 1286- 1290.

IUTTS, D.D. 1996. Toxicity and safety of fats and oils. In Bailey's Industrial Oil and Fat Products, 5th Ed.,Vol. 1, (Y. H Hui, ed.) pp. 215-280, Wiley- Interscience, New York.

MORAIS, C. DE, CARVALHO, J.B. DE, TAVARES, M., GONCALVES, L.A.G. and CAVALETTI, R.N. 1994. Evaluation of antioxidant efficiencies in fish oils from mechanically deboned curimbata (Prochilodus scrofa) using the Rancimat method. Rev. Inst. Adolfo Lutz 54, 5-10.

PELED, M., GUTFINGER, T. and LETAN, A. 1975. Effect of water and BHT on stability of cottonseed oil during frying. J. Sci. Food Agric. 26, 1655-1 666.

PONGRACZ, G. 1973. Antioxidant mixtures for use in foods. Intern. J. Vit. NU@. Res. 43, 517-525.

RUIZ-GUTIERREZ, V. and MURIANA, F.J.G. 1992. Effect of ingestion of thermally oxidized frying oil on desaturase activities and fluidity in rat-liver microsomes. J. Nub. Biochem. 3,75-79.

SAITO,Y., OHISHI, N. and YAGI, K. 1981. Protective effect of riboflavin on suppression of growth caused by oxidized oil. J. Nutr. Sci. Vitaminol. 27,17-21.

SANHUEZA, J., NIETO, S. and VALENZUELA, A. 2000. Thermal stability of some commercial synthetic antioxidants. J. Am. Oil Chem. SOC. 77,933-936.

SAS. 1987. SASISTAT Guide for personal computers, Version 6 ed., SAS Institute, Cary, NC.

SATYANARAYANA, A., GIRIDHAR, N., JOSHI, G.J. and RAO, D.G. 2000. Ascorbyl palmitate as an antioxidant for deep fat fiying of potato chips in peanut oil. J. Food Lipids 7, 1-10.

THERMAL STABILITY OF ANTIOXIDANTS 293

SEBEDIO, J.L., PREVOST, J. and GRANDGIRARD, A. 1987. Heat treatment of vegetable oils. 11. GC-MS and GC-FTIR Spectra of some isolated cyclic fatty acid monomers. J. Am. Oil Chem. SOC. 64, 1324-1333.

SHEELY, P.J.A., MOURISSEY, P.A. and HYUN, A. 1994. Consumption of thermally- ox ibed frying sunflower oil by chcks reduces-tocopherol status and increases susceptibility of tissues to lipid oxidation. Br. J. Nutr. 71, 53-65.

SIMS, R.J., FIORITI, J.A. and KANUK, N.J. 1972. Sterol additives as polymerization mhibitors for frying oils. J. Am. Oil Chem. SOC. 49, 298-301.

STAHL, E. 1967. Thin-layer Chromatography. A Laboratory Handbook, p. 359. Springer Verlag, Berlin.

TASKINS, J., LALAS, S., GERGIS, V., DOURTOGLOU, V. and SPILIOTIS, V. 1999. Characterization of Moringa oleifera seed oil of Kenya. J. Agric. Food Chem. 47,4495-4499.

THOMOPOULOS, C.D. and GRIGOROPOULOU, H.P. 1982. Antioxidant activity at 75 "C of BHA, BHT, TBHQ and their mixtures on a refrned corn oil and a virgin olive oil. Revue-Francaise-des-Corps-Gras 29, 329-33 1.

TIAN, L.L. and WHITE, P.J. 1994. Antipolymerization activity of oat extract in soybean and cottonseed oils under frying conditions. J. Am. Oil Chem. SOC. 73,

TYAGI, V.K. and VASISHTHA 1996. Changes in the characteristics and comparison of the oils during deep-fat frying. J. Am. Oil Chem. SOC. 73,499- 506.

WARNER, K., FRANKEL, E.N. and MOUTS, T.L. 1989. Flavor and oxidative stability of soybean, sunflower and low erucic acid rapeseed oils. J. Am. Oil Chem. SOC. 66,558-564.

WARNER, K., MOUNTS, T.L. and KWOLEK, W.F. 1985. Effects of antioxidants, methyl silicone and hydrogenation on room odor of soybean cooking oils. J. Am. Oil Chem. SOC. 62, 1483-1486.

YANG, C.M., KENDALL, C.W.C., STAMP, D., MEDLINE, A., ARCHER, M.C. and BRUCE, W.R. 1998. Thermally oxidized dietary fat on colon carcinogenesis in Rodents. Nutr. Cancer 30,69-73.

YANISHLIEVA, N. and MARINOVA, E. 1988. Stability of sunflower oil against autoxidation. Khranitelna Promishlenost 37(6), 17-18.

499-506.