STABILITY OF OILS HEATED BY MICROWAVE: UV - SPECTROPHOTOMETRIC EVALUATION

ESTABILIDADE DE ÓLEOS REFINADOS AQUECIDOS POR MICROONDAS: AVALIAÇÃO ESPECTROFOTOMÉTRICA NA FAIXA DO UV

STABILITY OF OILS HEATED BY MICROWAVE: UV — SPECTROPHOTOMETRIC EVALUATION¹

Thais M. F. S. VIEIRA², Marisa A. B. REGITANO-D’ARCE^2,*

SUMMARY

The effects of microwave heating on the oxidative stability of refined canola, corn and soybean oils were determined by absorptivity in the UV spectrum and by chemical analysis (peroxide and acid values). Samples were heated in a microwave oven (800 W, 2,450 MHz) for 0 to 36 min. Microwave heating produced oxidative degradation in the three oils. Absorptivity at 232 and 270 nm increased gradually with an increase in microwave exposure time (0-36 min) for canola, corn and soybean oils. Values of absorptivity at 232 nm increased from 4.812, 3.568 and 4.183 to 10.579, 12.874 and 15.950 after 36 min of heating canola, corn and soybean oil, respectively. The absorptivity at 232nm, due to the formation of conjugated dienes, was a good index for measuring the degradation of microwaved samples. UV scanning (220 - 320 nm) detected alterations in the spectrum of microwaved samples. Acid value also increased within 36 min of heating for all oils. Peroxide value showed a significant difference (P<0.05) in the initial stage of heating (0-6 min) for all oils. After this period it could not be correlated with absorptivity at 232 nm, due to the instability of hydroperoxides at high temperatures.

Keywords: canola oil, corn oil, soybean oil, microwave, stability, UV - spectrophotometry, oxidation.

RESUMO

ESTABILIDADE DE ÓLEOS REFINADOS AQUECIDOS POR MICROONDAS: AVALIAÇÃO ESPECTROFOTOMÉTRICA NA FAIXA DO UV: O efeito do aquecimento por microondas sobre a estabilidade oxidativa de óleos refinados de canola, milho e soja foi determinado através de análises físicas (absortividade na faixa do UV) e químicas (índice de peróxido e de acidez). Amostras de óleo foram aquecidas em um forno de microondas (800 W, 2450 MHz) por períodos de 0 a 36 min. O aquecimento por microondas promoveu a degradação oxidativa em todos os óleos. Os valores de absortividade em 232 nm e em 270 nm apresentaram acréscimos crescentes com o aumento do período de exposição às microondas, indicando a ocorrência do processo oxidativo. A absortividade em 232 nm aumentou de 4,812, 3,568 e 4,183 para 10,579, 12,874 e 15,950 depois de 36 min de aquecimento nos óleos de canola, milho e soja, respectivamente. Devido à formação de dienos conjugados, compostos da oxidação, a absortividade em 232 nm mostrou-se um bom índice para o acompanhamento do efeito do aquecimento por microondas sobre a estabilidade dos três óleos. Através do espectro traçado na faixa de 220 a 320 nm para todas as amostras testadas, foi possível acompanhar as alterações ocorridas. O índice de acidez também sofreu aumentos até os 36 minutos de aquecimento em todos os óleos. O índice de peróxido sofreu aumentos significativos no estágio inicial de aquecimento (0 - 6 min) em todos os óleos, mas a partir daí as alterações não puderam ser relacionadas aos valores obtidos para a absortividade em 232 nm, devido à instabilidade dos hidroperóxidos às altas temperaturas.

Palavras-chave: óleo de canola, óleo de milho, óleo de soja, microondas, estabilidade, espectrofotometria no UV, oxidação.

1 - INTRODUCTION

Microwave mechanism of heating is different from the conventional heating. The heating by microwave is produced by interaction of an electromagnetic field with chemical constituents of foods, due to molecular friction and excitation. The application of microwave energy reduces speed and time when it is compared with other methods of heating. Microwave heating is affected by the characteristics of the equipment (power and frequency) and the food (mass, density, initial temperature, size, form, moisture, physical state, chemical composition, electric conductivity, thermal conductivity and specific heat) [6, 8, 14]. Food with high moisture has high dielectric constant and dielectric loss, so its heating is faster. Foods that present low electric polarity, as lipids, also present low dielectric loss factors. However, the specific heat for lipids is low and they are heated quickly [4, 5, 8, 12].

Only a few studies about the chemical and physical changes in lipids heated by microwave explain the experimental conditions used (like number and way of distribution of samples submitted to the microwave heating, power used) and most of the researches have been conducted to investigate how the nutritional properties of food treated in a microwave oven were affected. Concerning vegetable oils and fatty foods, works can be found on the occurrence of fatty acid isomerization and vitamin loss after microwaving.

Thermal reaction involving long chain fatty acids esters, that usually require many hours of heating or stirring at room temperature, can be accelerated by the use of microwave oven [11] and the effect of microwave treatment on fatty acid composition of fatty foods (chicken fat, beef tallow, bacon fat, fish oils and peanut oil) did not yield any alterations in the fatty acid composition nor isomerization on the unsaturated fatty acids [13, 19].

However, after 8-10 min of microwave exposure, tocopherols levels of linseed, olive and palm oil decreased substantially. The occurrence of oxidation was greater in linseed oil, the most unsaturated of them [21]. Some works indicate that peroxide value of microwave heated oils increases with increase of microwave heating time [17, 20]. Palm, olive, corn and soybean oils showed increasing peroxide values during microwave heating (0-20 min); linseed oil (initial peroxide value of 6,5 meq kg¹) showed a maximum value at 12 min (24 meq kg^-1), followed by a reduction [21].

The initial rate of hydroperoxide formation exceed its decomposition rate during fats and oils oxidation, but this is reverted in the following stages. The method for hydroperoxide quantification is the determination of peroxide value [18]. However, hydroperoxides are intermediate products in the formation of carbonyls and hydroxy compounds. An accelerated test at 100ºC showed that the peroxide value passes through a maximum and is very sensitive to temperature changes [9].

Oxidation of polyunsaturated fatty acids is accompanied by an increase of absorption in the ultraviolet range. Lipids containing dienes or polyenes show a shift in their double bond position during oxidation due to isomerization and conjugation formation. Conjugated dienes exhibit an intense absorption at 234 nm; similarly conjugated trienes absorb at 268 nm. Conjugated dienes and peroxide value were well correlated during oxidation of canola and soybean oil [18] and of Brazil nut crude oil [15] in accelerated tests (oven test at 65ºC). The conjugated dienes method is faster than peroxide value determination and simpler, it does not depend on chemical reactions or color development and it requires smaller oil volumes [9, 18].

UV absorptivity changes were observed in sunflower and olive oils heated by microwave during 120 min at 170ºC, indicating oxidation occurrence [2]. Reactive free radicals can be formed due to microwave energy exposure. The first step of lipid peroxidation is the abstraction of a hydrogen atom from the active methylene group to form the free radical and this reaction can be accelerated by microwaves [7].

This study aimed to analyze chemical and physical alterations that occur during microwave heating of refined-bleached-deodorized canola, corn and soybean oils, under controlled conditions (same number and size of samples heated at each period). UV-spectrophotometric analysis were performed.

2 — MATERIALS AND METHODS

2.1 – Oils

Commercial refined, bleached and deodorized canola, corn and soybean oils were obtained from industrial plants. They contained no added antioxidants and showed Wijs iodine values of 116,15; 116,18 e 129,56, respectively.

2.2 – Microwave heating

Three samples (20 g) of oil for each treatment were placed in 50 mL beakers and covered with PVC film. 12 samples were placed at equal distances on a 25 cm diameter circumference on a rotary plate of the microwave oven (Sanyo, model EM - 804 T GR, 800 W effective power 800 W, 2.450 MHz frequency) and heated for 0, 2, 4, 6, 8, 10, 12, 16, 20, 24, 28, 32 and 36 min. Individual essays were conducted for each oil. Oil temperature was determined after every microwave exposure period with a K-termocouple (Hanna instruments, model HI 93530). Oil samples were cooled at room temperature to 23 ± 1ºC for the analysis.

2.3 – Analytical procedures

Absorptivity at 232 nm and 270 nm and UV-scans (220-320 nm) were determined following the analytical methods described by IUPAC (1979) method II.D.23 [10]. Oil samples were dissolved in isooctane. A Shimadzu UV 1203 spectrophotometer, with the software "Personal Spectroscopy 1.1", was used to determine absorptivity at UV spectrum.

Peroxide and acid values were determined according to AOCS (1983) [3], methods Cd 8-53 e Ca 5a-40, respectively.

2.4 – Statistical analysis

Statistical analysis were conducted in a randomized complete model and Tukey test was applied to exposure time average values. Tukey test was applied for each oil, at 5% of significance level. Sanest software was used.

3 — RESULTS AND DISCUSSION

Heating by microwave promotes oxidation in canola, corn and soybean oil and UV-absorptivity was a good analytical index to measure oxidative alterations. Figure 1 presents temperature development during microwave heating with time. Changes in temperature were similar for canola, corn and soybean oils.

FIGURE 1.
Microwave exposure time (min) and oil temperature (ºC).

Some products of oxidation process show maximum absorption at specific wavelengths. Monitoring oil samples alterations in the UV spectrum furnishes a good indication of the oxidation. Primary compounds of oxidation exhibit maximum values of absorptivity at 220-234 nm. At 265 nm, secondary products show absorption (Table 1).

TABLE 1.
Maximum absorption of main cromophore groups of oxidation products.

Product	UV (maximum nm)	E%
monoene	190	10000
diene	220-230	10000
triene	265-270	10000
tetraene	310-320	10000
ketonic aldehyde	265-280	10-100
ketonic aldehyde a , b ethylene	220-250 310-330	10000 50-100
a -diketone	280	20
a -ketoaldehyde	282	19
b -diketone	274	1500
a -ketonic acid	210	400

Rovellini, Cortesi; Fedeli (1997) [16]

The results of absorptivity at 232 (Figure 2) indicate that microwave heating produced lipid oxidation. Tukey test applied to averages of exposure time demonstrated that there were differences between absorptivities at 232 nm and absorptivities at 270 nm in the three oils at 5% of significance.

FIGURE 2.
Absorptivity at 232 nm of canola ^a, corn ^b and soybean ^c oils heated in microwave oven (^a CV = 4,822%, ^b CV = 5,758%, ^c CV = 10,817%)

Absorptivity at 232 nm of canola, corn and soybean oils showed no significant changes until 12 min of microwave exposure; however values increased gradually after 12 min, due, perhaps to the larger formation of conjugated dienes.

Similarly, values of absorptivity at 270 nm increased significantly after 4 min of heating, in accordance with the results by Albi et al. [2]. Molecular friction during microwave heating could promote the formation of trienes and unsaturated ketones or aldehydes, secondary products of oxidation that show absorption at 270 nm (Table 2). UV-scannings of oil samples heated by microwave are presented in Figure 3.

TABLE 2.
Absorptivity at 270 nm of canola, corn and soybeans oils heated by microwave.

Microwave exposure		Oils
time (min)	Canola 1	Corn 2	Soybean 3
0	0,762 g	1,587 f	3,324 f
2	0,808 g	1,626 f	3,379 f
4	0,735 g	1,673 ef	3,388 f
6	0,958 f	1,860 de	3,506 ef
8	1,028 ef	1,970 d	3,630 de
10	1,112 de	2,067 d	3,702 cd
12	1,163 de	2,096 d	3,784 bcd
16	1,210 d	2,340 c	3,935 ab
20	1,445 c	2,527 bc	3,875 abc
24	1,514 bc	2,665 ab	3,934 ab
28	1,642 b	2,699 ab	3,973 ab
32	1,799 a	2,891 a	3,982 a
36	1,885 a	2,833 a	3,894 ab

Means within the same column sharing a common letter are not significantly different at p<0.05 ¹ CV = 3,290% ²CV = 3,298% ³ CV = 1,744%

FIGURE 3. Spectra of absorptivity at 220-320 nm of canola, corn and soybean oils heated by microwave for different periods of time.

Acid value (% free fatty acid as oleic acid) showed little, but significantly increases during microwave exposure time, indicating hydrolytic alteration under heating in the three oils (Table 3).

TABLE 3.
Acid value (% FFA as oleic acid) of canola, corn and soybean oils heated by microwave.

Microwave exposure		Oils
time (min)	Canola 1	Corn 2	Soybean 3
0	0,075 c	0,077 d	0,075 f
2	0,080 c	0,075 d	0,079 f
4	0,080 c	0,101 c	0,079 f
6	0,075 c	0,104 c	0,079 f
8	0,080 c	0,104 c	0,102 e
10	0,080 c	0,127 b	0,102 e
12	0,080 c	0,127 b	0,107 e
16	0,080 c	0,127 b	0,132 d
20	0,080 c	0,129 b	0,132 d
24	0,101 b	0,132 b	0,157 c
28	0,106 b	0,132 b	0,157 c
32	0,156 a	0,154 a	0,208 b
36	0,156 a	0,154 a	0,233 a

Means within the same column sharing a common letter are not significantly different at p<0.05¹ CV = 2,405% ² CV = 2,230% ³ CV = 2,193%

Peroxide value showed significant alterations during microwave heating, but the values did not increase gradually or constantly. Figure 4 presents peroxide value alterations during microwave heating. All samples showed maximum peroxide values at 4 and 6 min of heating. Canola oil showed a decrease in peroxide value after 6 min of heating, followed by an increase (to 5,74 meq kg^-1 ) at 20 min and stabilization until 36 min.

FIGURE 4.
Peroxide value (meq kg^-1) of canola ^a, corn ^b and soybean ^c oils heated in microwave oven. (^a CV = 13,909%, ^b CV = 21,871%, ^c CV = 8,394%).

Corn and soybean oils showed maximum peroxide values at 4 and 6 min which decreased after this period, but the reduction observed in corn oil did not occur in soybean oil at the same rate. Corn oil peroxide value at 36 min was not statistically different from the initial value. Albi et al. [1] reported a small increase in peroxide value in olive and sunflower oil heated in the microwave oven for 120 min (170ºC), in contrast with the results by Yoshida et al. [20] who observed increasing canola, soybean and palm oils peroxide values with microwave heating times. Rancidity is related to peroxide value increase. However, hydroperoxides are unstable under heating at high temperatures. Microwave heating promotes rapid transformation to secondary products which contributes to the off-flavors. Monitoring peroxide value during microwave heating showed to be a questionable analytical procedure.

4 — CONCLUSIONS

Heating of canola, corn and soybean oils by microwave accelerated lipid oxidation. UV-spectrophotometric analysis demonstrated the occurrence of oxidative degradation by the increase in absorptivity at 232 nm.

Hydroperoxides instability at high temperatures was attributed to the uneven changes in peroxide value during microwave heating and to the increase of absorptivity at 270 nm, due to the formation of secondary products.

The three oils showed different behaviors according to its fatty acid composition (unsaturation level).

Microwave heating may be employed to compare the oxidative stability of different oils like the oven test.

5 — REFERENCES

1. ALBI, T.; LANZÓN, A.; GUINDA, A.; LEÓN, M; PÉREZ-CAMINO, M.C. Microwave and conventional heating effects on thermoxidative degradation of edible fats. Journal of Agricultural Food Chemistry. v.45, n.10, p.3795-3798, 1997.

2. ALBI, T.; LANZÓN, A.; GUINDA, A.; PÉREZ-CAMINO, M.C.; LEÓN, M. Microwave and conventional heating effects on some physical and chemical parameters of edible fats. Journal of Agricultural Food Chemistry. v.45, n.8, p.3000-3003, 1997.

3. AMERICAN OIL CHEMISTS' SOCIETY. Official and tentative methods. 3. Champaign, 1983.

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7. FARAG, R.S.; HEWEDI, F.M.; ABU-RAIIA, S.H.; EL-BAROTY, G.S. Comparative study on deterioration of oils by microwave and conventional heating. Journal of Food Protection, v.55, n.9, p. 722-727, 1992.

8. FELLOWS, P. Heating by irradiated energy. In: FELLOWS, P. Food processing technology - principles and practice. Cambridge: VCH, 1989. Cap. D-17, p.343-356.

9. GRAY, J.I. Measurement of lipid oxidation: a review. Journal of the American Oil Chemists’ Society, v.55, n.6, p.539-546, 1978.

10. INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY. Standard methods for the analysis of oils, fats and derivatives. 6. 1979.

11. LIE KEN JIE, M.S.F.; YAN-KIT, C. The use of a microwave oven in the chemical transformation of long chain fatty acid esters. Lipids, v.23, n.4, p.367-369, 1988.

12. LOUPY, A. Les micro-ondes: leurs applications potencielles dans la chimie des corps gras. Oléagineux corps gras lipides. v.1, n.1, p.62-68, 1994.

13. MAI, J.; TSAI, C.H.; ARMBRUSTER, G.; CHU, P.; KINSELLA, J.E. Effects of microwave cooking on food fatty acids: no evidence of chemical alteration or isomerization. Journal of Food Science, v.45, n.6, p.1753-1755, 1980.

14. MUDGETT, R.E.; WANG, D.I.C.; GOLDBLITH, S.A. Prediction of dielectric properties in oil-water and alcohol-water mixtures at 3,000 MHz, 25ºC based on pure component properties. Journal of Food Science, v.39, n.3, p.632-635, 1974.

15. REGITANO-d’ARCE, M.A.B. & VIEIRA, T.M.F.S. Storability of Brazil nut (Bertholletia excelsa) crude oil. In: WORLD CONFERENCE ON OILSEED AND EDIBLE OILS PROCESSING’S: EMERGING TECHNOLOGIES, CURRENT PRACTICES, QUALITY CONTROL, TECHNOLOGY TRANSFER AND ENVIRONMENTAL ISSUES, Istanbul, 1996. Proceedings. AOCS, 1996. p.174-176.

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17. RUIZ-LOPEZ, M.D.; ARTACHO, R.; FERNANDEZ PINEDA, M.A.; LOPEZ GARCIA de la SERRANA, H.; LOPEZ MARTINEZ, M.C. Stability of a -tocopherol in virgin olive oil during microwave heating. Lebensmittel-Wissenschaft & Technology, v.28, n.6, p.644-646, 1995.

18. SHAHIDI, F. Stability of fats and oils. In: Latin American Congress and EXHIBIT on Fats and Oils Processings, 6., Campinas, 1995. Proceedings. Campinas: Sociedade Brasileira de Óleos e Gorduras, 1995. p.47-54.

19. THOMAS, L.H.; SGOUTAS, S.A.; SGOUTAS, D.S.; HEARN, J.A. Stability of polyunsaturated fatty acids after microwave cooking of fish. Journal of Food Science, v.52, n.5, p.1430-1431, 1987.

20. YOSHIDA, H.; KAJIMOTO, G.; EMURA, S. Antioxidant effects of d -tocopherols at different concentrations in oils during microwave heating. Journal of the American Oil Chemists’ Society, v.70, n.10, p.989-995, 1993.

21. YOSHIDA, H.; NOBUHISA, H.; KAJIMOTO, G. Microwave energy effects on quality of some seed oils. Journal of Food Science, v.55, n.5, p.1412-1416, 1990.

6 — ACKNOWLEDGMENTS

The authors want to acknowledge the financial support received from FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for the conduction of the research.

² Departamento de Ciência e Tecnologia Agroindustrial - ESALQ/USP. Cx. Postal 9, CEP 13418-900 Piracicaba - SP. e-mail: [e-mail]

* A quem a correspondência deve ser endereçada.

¹

Recebido para publicação em 05/07/98. Aceito para publicação em 13/11/98.

Publication Dates

• Publication in this collection
  24 May 1999
• Date of issue
  Oct 1998

History

• Received
  05 July 1998
• Accepted
  13 Nov 1998