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Effects of microwave heating on the quality characteristics and thermal properties of RBD palm olein
Innovative Food Science & Emerging Technologies 3 Ž2002. 157᎐163
Effects of microwave heating on the quality characteristics and thermal properties of RBD palm olein C.P. Tana , Y.B. Che Mana,U , S. Jinap b, M.S.A. Yusoff c a
Department of Food Technology, Faculty of Food Science and Biotechnology, Uni¨ ersiti Putra Malaysia, 43400 UPM Serdang, Selangor D.E., Malaysia b Department of Food Science, Faculty of Food Science and Biotechnology, Uni¨ ersiti Putra Malaysia, 43400 UPM Serdang, Selangor D.E., Malaysia c Malaysian Palm Oil Board, No. 6 Persiaran Institusi, Bandar Baru Bangi, 43000, Kajang, Selangor D.E., Malaysia
Abstract In the present work, the influence of microwave power Žlow-, medium- and high-power settings. and heating time on lipid deterioration produced during the microwave heating of RBD palm olein ŽRBDPOO . was evaluated. The changes in thermal profiles by differential scanning calorimetry ŽDSC. were studied in comparison to the changes in chemical parameters. The DSC method was based on the cooling and melting curves of oils at a scanning rate of 5 ⬚Crmin. The chemical evaluation of the oils was based on free fatty acid content, C18:2rC16:0 ratio, peroxide, iodine, and anisidine values. The DSC results were explained on the basis of the endo- or exothermic peak temperatures. A statistical comparative study was carried out on the DSC and chemical parameters. The results indicate that there is a good correlation between the DSC and chemical methods. Based on the results obtained, the DSC appears to be a useful instrumental method in monitoring the oxidation of microwave heated oils, and it may have the potential to replace the time- and chemical-consuming standard chemical methods. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Microwave heating; Differential scanning calorimetry ŽDSC.; Refined-bleached-deodorised palm olein; Oxidation; Chemical analyses; Quality characteristics Industrial rele¨ ance: Microwave heating has become an essential tool in home meal preparation and in the institutional food service sector. Few studies exist on the influence of microwave heating on the composition and quality of edible oils especially refined-bleached-deodorized palm olein ŽRBDPOo .. Consequently the objective of this work was to investigate the effect of microwave heating Žin combination with microwave power and heating time. on RBDPOo . Exposing RBDPOo to various microwave power settings and heating periods caused the formation of hydroperoxides and secondary oxidation products, hydrolysis of free fatty acids, decreased level of unsaturated fatty acids and iodine as well as changes in the differential scanning calorimeter cooling and melting parameters.
1. Introduction The application of microwave heating for both home meal preparation and institutional food service trade has increased because of its speed and convenience ŽMudgett, 1989; Yoshida & Takagi, 1997.. Today, microwave heating offers the food industry the opportunity to develop new food products with high nutritional U
Corresponding author. Tel.: q603-89486101, ext. 3468; fax: q603-89423552. E-mail address: [email protected] ŽY.B. Che Man..
and sensory quality, novel texture, food safety, and an extended shelf-life. In the food industry, microwave heating operations have been increasingly successful in baking, blanching, cooking, drying, pasteurisation, sterilisation and thawing of various food products ŽRosenberg & Bogl, ¨ 1987a,b.. It is important to recognise that the microwave heating characteristics of food products may vary considerably with the processing frequency and with the temperature, chemical composition, and physical state of the product ŽMudgett, 1989.. The chemical constituents of oils that degrade during microwave heating do so at rates that vary with heating
1466-8564r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 6 - 8 5 6 4 Ž 0 2 . 0 0 0 2 6 - 7
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C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163
temperature and time in a similar manner to many other type of different domestic processing methods Že.g. frying, steaming and roasting.. Suitable quality parameters can, therefore, be used as time᎐temperature integrators of quality deterioration of oils during microwave heating. Such quality parameters have been reviewed by many researchers ŽFristch, 1981; Robards, Kerr & Patsalides, 1988; Paul & Mittal, 1997.. However, many of these are chemical intensive methods. Also, the methods for measuring such components can be relatively complex and time-consuming, which could be a major drawback in industry applications. Instrumental methods involving a simpler technique, with a fast method for determining the change in oil quality is, therefore, required. Differential scanning calorimetry ŽDSC. has long been available as an instrumental method to the oils and fats researcher ŽCebula & Smith, 1992.. This technique is used for studying various heat-related phenomena in materials by monitoring associated changes in enthalpy. Recently, we used this technique to monitor oxidation process and determine total polar compounds in heated oils ŽTan & Che Man, 1999a,b.. Few studies concerning the influence of microwave on composition and quality of edible oils have been reported. Recently, the results of the effects of microwave treatments on the thermoxidative degradation and some physical and chemical parameters of edible oils were published in two separated papers ŽAlbi, Lanzon, & Leon, ´ Guinda, Perez-Camino ´ ´ 1997a; Albi, Lanzon, 1997b.. The ´ Guinda, Leon ´ & Perez-Camino, ´ effects of microwave and conventional heating on physical and chemical parameters of five edible oils and fats Žvirgin olive oil, olive oil, sunflower oil, high oleic sunflower oil and lard. were investigated by Albi and co-workers Ž1997a.. Meanwhile, the effects of microwave treatments on the thermoxidative degradation of five edible oils and fats Žsunflower oil, high oleic sunflower oil, virgin olive oil, olive oil, and lard. were determined by Albi et al. Ž1997b.. However, little has been reported about how microwave heating affects the quality of the refined-bleached-deodorised palm olein ŽRBDPOO .. Consequently, the primary objective of this research was to investigate the effect of microwave heating in combination with microwave power and heating time in terms of RBDPOO quality. A secondary objective was to investigate the correlation between DSC curve parameters and other standard chemical methods in microwave heated RBDPOO .
and was used in all experiments. All chemicals and solvents used were of analytical grade unless otherwise specified. 2.2. Microwa¨ e treatments A domestic microwave oven ŽNational, Model NN5656F, Matsushita Electric Industrial Co., Ltd., Osaka, Japan. was used in this study. In order to standardise the operation of the microwave oven for each heating test, some additional steps were done between tests. Between tests, the oven door was opened and an electrical fan was used to blow air into the oven cavity Žmax. 23 l. for 5 min to facilitate the cooling process. Thus, the temperature of the oven was reduced to approximately 30 " 2 ⬚C between tests. All oil samples were simultaneously exposed at a frequency of 2450 MHz. There were three settings of heating corresponding to low-, medium- and high-power, and when operated at high power, it provided approximately 900 W. Eighty grams of oil were divided into two 40-ml amber bottles Ž40 g each; 4 cm internal diameter., which were placed at equal distances on the turntable rotary plate of the microwave oven. The oil samples were heated for various periods Ž4, 8, 12, 16 and 20 min. at each power setting. The two samples were combined after microwave treatment and before analysis. The final oil temperatures at various heating times and power settings were measured by inserting a calibrated chromel᎐alumel thermocouple into the oil samples immediately after removal from the microwave oven. These temperature data are presented in Fig. 1. All oil samples were stored under nitrogen at y18 ⬚C for further analyses. Analysis of oil was carried out immediately after the heating experiments and completed within 2 weeks. 2.3. Standard chemical analyses The AOCS Official Methods were employed for the determination of free fatty acid ŽFFA. content, iodine ŽIV., anisidine ŽAnV. and peroxide ŽPV. values in the oil samples ŽAOCS, 1993.. The fatty acid compositions of the microwave-heated oil were analysed by gas chromatography ŽGC. after transesterification. GC analysis was performed as described previously for edible oils ŽTan & Che Man, 2000.. The ratios of C18:2rC16:0 were then calculated. 2.4. DSC analysis
2. Materials and methods 2.1. Materials RBDPOO was purchased from a local grocery store
A Perkin-Elmer differential scanning calorimeter, DSC-7 ŽPerkin-Elmer Corp., Norwalk, CT, USA. was used for the thermal analysis of oil samples. Purified nitrogen Ž99.999% purity. was the purge gas for the dry box and flowed at approximately 20 mlrmin. The DSC
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Fig. 1. Comparison of oil temperatures heated in microwave oven at three different power settings with different heating times.
instrument was calibrated with indium and n-dodecane. Samples of approximately 6᎐12 mg were weighed into aluminium pans to the nearest 0.1 mg, and the covers were hermetically sealed into place. An empty, hermetically sealed aluminium pan was used as reference. Prior to analysis of samples, the baseline was obtained with an empty, hermetically sealed aluminium pan. RBDPOO samples were subjected to the following temperature program: 80 ⬚C isotherm for 5 min, cooled at 5 ⬚Crmin to y80 ⬚C and held for 5 min. The same sample was then heated from y80 to 80 ⬚C at the same rate. The cooling curves of the RBDPOO samples revealed one major crystallisation peak with two lowtemperature shoulder peaks. The temperature of the peak Žshoulder peak with lower-temperature., Tc , was obtained by analysing the DSC cooling curve with the 7 SeriesrUNIX DSC software library ŽAnon, 1995.. While the melting curves showed one major endothermic peak, which corresponded in temperature to the melting of olein fraction ŽTan and Che Man, 2000. and a small endotherm region at lower temperature consisted of a plateau with a pair of shoulder peaks. We speculated that there was a chain rearrangement in the crystallites and thus a smaller fraction of RBDPOO melted at a low temperature. A trough Žexotherm. was observed in between the two shoulder peaks. The temperature of the trough Žexothermic peak., Tm , was obtained by analysing the DSC melting curve with the same software library ŽAnon, 1995.. 2.5. Statistical analysis All experiments andror measurements were duplicated. The data were analysed using the SASrSTAT release 6.08 program ŽSAS Institute, Cary, NC, USA.. To examine the influence of microwave heating, data
from the oils were analysed as a 3 = 5 factorial arrangement of treatments with microwave power Žlow, medium and high. and heating time Ž4, 8, 12, 16 and 20 min. as main effects using the ANOVA procedure ŽSAS, 1989.. Differences between microwave powers and between times of heating in oil quality variables were tested for significance using a one-way analysis of variance, whereas for interaction between the two main effects Žmicrowave power= heating time. a two-way analysis of variance was used. When variance analysis revealed a significant effect, differences between heating time were tested using Duncan’s multiple range test ŽDuncan, 1955.. The relationships between each of the DSC curve parameters and standard chemical methods were determined by Pearson’s correlation analysis with the SAS program. The coefficient of determination Ž R 2 . was calculated for the relationship between each of the oil temperature᎐time᎐DSC curve parameter combinations and standard chemical methods by using the SAS REG procedure ŽSAS, 1989..
3. Results and discussion The initial characteristics of unheated RBDPOO used in this study are shown in Table 1. In this study, PV, AnV, FFA, IV and C18:2rC16:0 ratios were employed to measure heated oil deterioration. The major chemical and DSC parameter changes affected by microwave power setting and heating time are given in Tables 2 and 3. Data regarding the chemical changes of the oils are in general agreement with those published in the literature ŽYoshida, Hirooka & Kajimoto, 1990; Yoshida & Kajimoto 1994.. Microwave power from low to high has been shown to greatly influence the quality of oils. In general, microwave heating had significant
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Table 1 Characteristics of unheated refined-bleached-deodorised palm olein used in experiments a Characteristics of the oil
Value
Peroxide value Žmeqrkg oil. Anisidine value Free fatty acid Ž%. Iodine value Žg of I2r100 g oil. C18:2rC16:0 ratio DSC lower-temperature shoulder peak, Tc Ž⬚C. DSC exothermic peak, Tm Ž⬚C.
2.12" 0.30 1.37" 0.04 0.09" 0.00 59.92" 0.02 0.28" 0.00 y54.49" 0.10 y34.58" 0.18
a
Each value in the table represents the mean " S.D. of four measurements from two replicates. Abbre¨ iations: C18:2rC16:0, ratio of linoleic acidrpalmitic acid.
Ž P- 0.05. effect on these quality parameters. As expected, oil samples heated at high-power setting showed significantly Ž P- 0.05. more heat abuse than oil samples heated at medium- and low-power settings. Highly significant Ž P- 0.05. increases or decreases for PV, AnV, FFA, IV, C18:2rC16:0, DSC peak temperatures Žboth Tc and Tm . were noticed for oil samples heated at high-power settings. Generally, the experimental results of the AnV and FFA showed an increase with heating time. In contrast, the IV, C18:2rC16:0, DSC peak temperatures Žboth Tc and Tm . decreased with the increasing time of heating. The variance analysis indicated that oil quality was significantly Ž P- 0.05. affected by both microwave power and heating time. In this study, the interaction between microwave power= heating time was significant Ž P- 0.05.. The changes in PV are given in Table 2. Edible oils with a PV of 7.5 meqrkg have been deemed as unacceptable from a sensory viewpoint by some investigators ŽRobards et al., 1988.. The PV values at each power setting were all below this critical value. At
low-power setting, the PV increased gradually. PV for the oil samples heated at medium- and high-power settings exhibited two Žboth at 4 and 20 min of heating. maxima. Another distinct observation clearly demonstrated that the formation of hydroperoxides was more pronounced at a low-power setting than at mediumand high-power settings. This resulted from rapid decomposition of hydroperoxides to secondary oxidation products at elevated temperatures. They are extremely unstable and decompose via fission, dehydration, and the formation of free radicals to form a variety of chemical products, such as alcohols, aldehydes, ketones, acids, dimers, trimers, polymers, and cyclic compounds. Because hydroperoxides are unstable and do not accumulate in heated oil, PV is not a good indicator of heated oil deterioration ŽFristch, 1981.. The AnV values of microwave-heated oils are shown in Table 2. RBDPOO was less susceptible to oxidation because of lower levels of highly unsaturated fatty acids Že.g. linoleic and linolenic acids.. Generally, the oil AnV increased slowly at the low-power setting and
Table 2 Changes in chemical characteristics of refined-bleached-deodorised palm olein during microwave heating a Power
Heating time Žmin.
Peroxide value Žmeqrkg oil.
Low
4 8 12 16 20
2.32" 0.06e 2.61" 0.07d 2.75 " 0.02c 3.05" 0.14b 3.20" 0.06a
Medium
4 8 12 16 20
High
4 8 12 16 20
Anisidine value
Free fatty acid Ž%.
Iodine value Žg of I2 r100 g oil.
C18:2r16:0
1.72" 0.03e 1.92" 0.02d 2.16" 0.03c 2.34" 0.09b 2.46" 0.03a
0.09" 0.00c 0.09" 0.00c 0.10" 0.01bc 0.11" 0.01ab 0.12" 0.01a
59.46" 0.10a 59.37" 0.06a 59.19" 0.30a 58.79" 0.17b 58.28" 0.09c
0.28" 0.00a 0.27" 0.00b 0.27" 0.00bc 0.26" 0.00cd 0.26" 0.00d
4.18" 0.06c 5.43" 0.16a 5.09" 0.05b 3.29" 0.11d 3.42" 0.13d
1.72" 0.05e 5.39" 0.10d 9.04" 0.06c 14.91" 0.02b 15.37" 0.06a
0.10" 0.01c 0.11" 0.01bc 0.11" 0.00b 0.12" 0.01a 0.13" 0.01a
59.60" 0.22a 59.39" 0.18ab 58.62" 0.12bc 58.41" 1.24cd 57.63" 0.08d
0.27" 0.00a 0.27" 0.00b 0.26" 0.00b 0.26" 0.00c 0.26" 0.00c
4.85" 0.31b 5.73" 0.04a 3.63" 0.04c 3.05" 0.02d 4.72" 0.14b
3.42" 0.06e 8.77" 0.02d 14.31" 0.11c 15.12" 0.15b 16.38" 0.21a
0.09" 0.00d 0.11" 0.00c 0.12" 0.01b 0.13" 0.00ab 0.14" 0.01a
59.36" 0.00a 59.14" 0.64a 57.97 " 0.71b 57.31" 0.65b 56.37" 0.59c
0.27" 0.00a 0.27" 0.01ab 0.26" 0.01bc 0.25" 0.01cd 0.24" 0.00d
a Each value in the table represents the mean " S.D. of four measurements from two replicates. For each heating power, means within each column with different superscripts are significantly Ž P- 0.05. different.
C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163 Table 3 Changes in characteristics of refined-bleached-deodorised palm olein during microwave heating a Power
Heating
DSC peak temperature Ž⬚C. Cooling curve, Tc
Melting curve, Tm
Low
4 8 12 16 20
y56.60" 0.32a y58.05" 0.13b y59.04" 0.17b y60.52" 0.18c y60.76" 0.05d
y35.20" 0.02a y35.58" 0.03b y35.89" 0.04c y36.31" 0.03d y36.60" 0.06e
Medium
4 8 12 16 20
y59.52 " 0.12a y60.32" 0.05b y60.41" 0.06c y60.80" 0.09d y61.38" 0.08d
y35.87" 0.06a y36.70" 0.06 y37.42" 0.04c y37.80" 0.05d y38.04" 0.10e
High
4 8 12 16 20
y59.94" 0.06a y60.76" 0.05b y60.96" 0.28b y61.74" 0.10c y62.26" 0.08d
y36.38" 0.08a y36.89" 0.04b y37.57" 0.14c y38.18" 0.13d y39.31" 0.12e
Abbre¨ iation: Tc , temperature of the lower-temperature shoulder peak; Tm , temperature of trough Žexothermic peak.. a Each value in the table represents the mean " S.D. of four measurements from two replicates. For each heating power, means within each column with different superscripts are significantly Ž P0.05. different.
more rapidly at the medium- and high-power settings. As compared to PV, the AnV is a more reliable and meaningful test, because it measures the accumulation of secondary oxidation products Žaldehyde compounds.. Changes in the FFA of oil samples during microwave heating are shown in Table 2. Although the FFA content is an index of hydrolytic rancidity, it was nevertheless measured, as free acids contribute to the development of off-flavours and off-odours in the oil. There were no significant differences Ž P) 0.05. in the FFA contents until 8 min of heating at a low-power setting. The FFA of the oil samples increased slightly with longer heating time, particularly those heated at a high-power setting. These unexpected results showed that the FFA contents increased significantly Ž P- 0.05. due to production of fatty acids by microwave energy. The IV for microwave heated oils gradually decreased with increasing heating power settings and times ŽTable 2.. The reductions in IV were highest for oil samples heated at the high-power setting. A decrease in IV is consistent with the decreasing number of double bonds in the oil as it becomes oxidised. Thus, the reduction in IV during heating is often taken as a measure of oil deterioration ŽFristch, 1981.. The analysis is analogous to that for the reduction of unsaturated fatty acids, but gives more weight to the higher unsaturated oil samples. The changes in IV over 20 min of microwave heating were 1.18, 1.97 and 2.99 for low-, medium- and high-power settings, respectively.
161
C18:2rC16:0 ratios in microwave heated oils are shown in Table 2. The ratio was expected to decrease due to a decline in linoleic acid by oxidation. It was considered to be a reliable indicator of lipid oxidation during the heating process. A decrease in the relative percentage of unsaturated fatty acids and an increase in the relative percentage of saturated fatty acids occurs when the oils are heated ŽTan & Che Man, 1999a.. This pattern was observed in RBDPOO after heating at different microwave power settings. In the present study, the C18:2rC16:0 for microwave-heated oil decreased with increasing heating time. Microwave-heated RBDPOO exhibited a dominant exotherm Žat approx. y3 to y5 ⬚C. after cooling in the DSC, as shown in Fig. 2 for medium-power setting. From this figure, it is apparent that the DSC traces are affected in a systematic way by the increase in heating time. As the time of heating increased, the temperature of the peak Žshoulder peak with lower-temperature., Tc , shifted to lower temperatures ŽTable 3.. The same phenomena were observed in oil samples heated at low- and high-power settings. We also observed the same phenomenon in our previous paper ŽTan & Che Man, 1999b.. Fig. 3 showed the melting curves of
Fig. 2. DSC cooling curves of refined-bleached-deodorised palm olein samples heated at medium-power setting with different heating times Ž4, 8, 12, 16 and 20 min.. Abbre¨ iation: Tc , temperature of the lower-temperature shoulder peak.
162
C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163 Table 4 Pearson correlation coefficient Ž n s 5. between DSC lower-temperature shoulder peak temperature ŽTc ., heating time and wet chemical methodsa Cooling curve, Tc Ž⬚C. Low Heating time PV AnV FFA IV C18:2rC16:0
Medium UU
y0.9821 UU y0.9923 UU y0.9917 U y0.9178 U 0.8967 U 0.9561
High UU
y0.9727 0.4666 U y0.9358 U y0.9569 U 0.9366 UU 0.9827
UU
y0.9887 0.3576 U y0.9176 UU y0.9841 U 0.9536 UU 0.9803
U
Significance at the 0.05 level Ž P- 0.05.. Significance at the 0.01 level Ž P- 0.01.. a Abbre¨ iations: see Table 1.
UU
Fig. 3. DSC melting curves of refined-bleached-deodorised palm olein samples heated at medium-power setting with different heating times Ž4, 8, 12, 16 and 20 min.. Abbre¨ iation: Tm , temperature of trough Žexothermic peak..
microwave heated RBDPOO at various heating periods for medium-power settings. These melting curves revealed a major endotherm Žat ; 3᎐5 ⬚C., mainly due to the melting of the olein fraction. In general, as the time of heating increased, the temperature of the trough Žexothermic peak., Tm , moved to lower temperatures. The same observations were perceived in oil samples heated at low- and high-power settings. In microwave heated oils, the oxidation products, such as hydroperoxides, aldehydes, polar compounds, dimers and polymers, increased during the heating operation. The presence of these compounds may also disturb the rearrangement of different polymorphic forms in oil. As their levels increase, such compounds would contribute to the changes in DSC cooling and melting curves. It is well known that the presence of free fatty acids, partial glycerides and oxidation products in oil tend to shift the melting range to a lower temperature ŽChe Man & Swe, 1995.. The presence of oxidation products in oil also tends to shift the crystallisation range to a lower temperature ŽTan & Che Man, 1999a,b.. Therefore, the same phenomenon is expected for changes in DSC cooling and melting peak parameters for microwave-heated oil. In general, the effect of microwave heating on the peak temperature for RBDPOO was entirely dependent upon the heating
power setting; the high-power setting produced the biggest shift in peak temperature, followed by mediumand low-power settings. The coefficients of correlation matrix between each of the DSC curve parameter ŽTc and Tm . and standard chemical methods for RBDPOO are shown in Tables 4 and 5. Generally, the DSC peak temperatures showed excellent correlations with the heating periods and other standard chemical methods at all heating power settings. However, there were poor correlations in between each of the DSC curve parameters and PV at medium- and high-power settings. This may due to the rapid decomposition of hydroperoxides to secondary oxidation products at medium- and high-power settings. In particular, the high correlation Ž P- 0.01. found between each of the DSC curve parameters and heating periods, suggests that the DSC can be recommended as an appropriate objective method for evaluating the extent of oil deterioration during microwave heating. The DSC method can, therefore, be employed as a time᎐microwave power indicator during microwave heating. Although there is a high correlation between each DSC curve parameter and standard chemical parameters, it would be more meaningful to provide some information on the relationship between the oil temTable 5 Pearson correlation coefficient Ž n s 5. between DSC exothermic peak temperature ŽTm ., heating time and wet chemical methodsa Melting curve, Tm Ž⬚C. Low Heating time PV AnV FFA IV C18:2rC16:0 U
Medium UU
y0.9988 UU y0.9973 UU y0.9935 U y0.9655 U 0.9511 U 0.9416
y0.9729 0.4634 U y0.9731 UU y0.9598 U 0.9372 UU 0.9833
Significance at the 0.05 level Ž P- 0.05.. Significance at the 0.01 level Ž P- 0.01.. a Abbre¨ iations: see Table 1.
UU
High UU
UU
y0.9884 0.3570 U y0.8918 UU y0.9686 UU 0.9886 UU 0.9900
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Table 6 Summary of regression analysis Ž n s 15. on the relationship between temperature᎐time᎐DSC parameter and wet chemical methodsa Regression analysis Ž R2 , coefficient of determination.
PV AnV FFA IV C18:2rC16:0
Temperature᎐time᎐DSC lowertemperature shoulder peak temperature
Temperature᎐time᎐DSC exothermic peak temperature
0.4501 UU 0.8735 UU 0.9569 UU 0.9339 UU 0.9483
0.6873 U 0.5676 UU 0.8474 UU 0.8494 UU 0.8612
UU
U
Significance at the 0.05 level Ž P- 0.05.. Significance at the 0.01 level Ž P- 0.01.. a Abbre¨ iations: see Table 1.
UU
perature᎐time᎐DSC curve parameter combination and wet chemical methods. The results of the regression analysis of this relationship are given in Table 6. The end-temperatures recorded in the microwave-heated oils were used in the regression analysis. The regression analysis showed that the oil temperature᎐ time᎐DSC curve parameter combination was well correlated to standard chemical methods Žexcept for PV., as judged by the coefficient of determination Ž R 2 . values. This observation revealed that, with a known oil temperature, heating time and DSC curve parameter, we could predict the AnV, FFA, IV or C18:2rC16:0 in microwave-heated RBDPOO . In summary, exposing the RBDPOO to various microwave power settings and heating periods caused the formation of hydroperoxides and secondary oxidation products, hydrolysis of FFA, decreased the level of unsaturated fatty acids and thus IV, and changes in DSC cooling and melting parameters. From the results of this study, we also concluded that DSC can be utilised as an efficient and convenient method for monitoring the extent of oil deterioration in RBDPOO , with no need for chemical and dedicated skills.
Acknowledgements This research work was supported by Universiti Putra Malaysia ŽIRPA Project No. 03-02-04-003.. References Albi, T., Lanzon, M. C., & Leon, ´ A., Guinda, A., Perez-Camino, ´ ´ M. Ž1997a.. Microwave and conventional heating effects on some physical and chemical parameters of edible fats. Journal of Agricultural and Food Chemistry, 45, 3000᎐3003. Albi, T., Lanzon, M. C. ´ A., Guinda, A., Leon, ´ M., & Perez-Camino, ´ Ž1997b.. Microwave and conventional heating effects on thermoxidative degradation of edible fats. Journal of Agricultural and Food Chemistry, 45, 3795᎐3798. Anon Ž1995.. 7 Seriesr UNIX DSC7 Users Manual. Software Version 4.0, Perkin-Elmer Corporation, Norwalk, CT, USA. AOCS Ž1993.. Official methods and recommended practices of the
American Oil Chemists’ Society Ž4th ed... Champaign, Illinois: American Oil Chemists’ Society, ed. D. Firestone. Cebula, D. J., & Smith, K. W. Ž1992.. Differential scanning calorimetry of confectionery fats. Part IIᎏeffects of blends and minor components. Journal of the American Oil Chemists’ Society, 69, 992᎐998. Che Man, Y. B., & Swe, P. Z. Ž1995.. Thermal analysis of failed-batch palm oil by differential scanning calorimetry. Journal of the American Oil Chemists’ Society, 72, 1529᎐1532. Duncan, D. B. Ž1955.. Multiple range and multiple F-test. Biometrics, 11, 1᎐42. Fristch, C. W. Ž1981.. Measurements of frying fat deterioration: a brief review. Journal of the American Oil Chemists’ Society, 58, 272᎐274. Mudgett, R. E. Ž1989.. Microwave food processing. Food Technology, 43, 117᎐126. Paul, S., & Mittal, G. S. Ž1997.. Regulating the use of degraded oilrfat in deep-fatroil food frying. Critical Re¨ iews in Food Science and Nutrition, 37, 635᎐662. Robards, K., Kerr, A. F., & Patsalides, E. Ž1988.. Rancidity and its measurement in edible oils and snack foods. Analyst, 113, 213᎐222. Rosenberg, U., & Bogl, ¨ W. Ž1987a.. Microwave thawing, drying, and baking in the food industry. Food Technology, 41Ž6., 85᎐91. Rosenberg, U., & Bogl, ¨ W. Ž1987b.. Microwave pasteurization, sterilization, blanching, and pest control in the food industry. Food Technology, 41Ž6., 92᎐99. SAS Ž1989.. Statistical analysis system user’s guide: basic statistics. SAS Institute, Cary, USA. Tan, C. P., & Che Man, Y. B. Ž1999a.. Differential scanning calorimetric analysis for monitoring the oxidation of heated oils. Food Chemistry, 67, 177᎐184. Tan, C. P., & Che Man, Y. B. Ž1999b.. Quantitative differential scanning calorimetric analysis for determining total polar compounds in heated oils. Journal of the American Oil Chemists’ Society, 76, 1047᎐1057. Tan, C. P., & Che Man, Y. B. Ž2000.. Differential scanning calorimetric analysis of edible oils: comparison of thermal properties and chemical composition. Journal of the American Oil Chemists’ Society, 77, 143᎐155. Yoshida, H., & Kajimoto, G. Ž1994.. Microwave heating affects composition and oxidative stability of sesame Ž Sesamum indicum. oil. Journal of Food Science, 59, 613᎐616, 625. Yoshida, H., & Takagi, S. Ž1997.. Microwave roasting and positional distribution of fatty acids of phospholipids in soybeans ŽGlycine max L... Journal of the American Oil Chemists’ Society, 74, 915᎐921. Yoshida, H., Hirooka, N., & Kajimoto, G. Ž1990.. Microwave energy effects on quality of some seed oils. Journal of Food Science, 55, 1412᎐1416.
Effects of microwave heating on the quality characteristics and thermal properties of RBD palm olein C.P. Tana , Y.B. Che Mana,U , S. Jinap b, M.S.A. Yusoff c a
Department of Food Technology, Faculty of Food Science and Biotechnology, Uni¨ ersiti Putra Malaysia, 43400 UPM Serdang, Selangor D.E., Malaysia b Department of Food Science, Faculty of Food Science and Biotechnology, Uni¨ ersiti Putra Malaysia, 43400 UPM Serdang, Selangor D.E., Malaysia c Malaysian Palm Oil Board, No. 6 Persiaran Institusi, Bandar Baru Bangi, 43000, Kajang, Selangor D.E., Malaysia
Abstract In the present work, the influence of microwave power Žlow-, medium- and high-power settings. and heating time on lipid deterioration produced during the microwave heating of RBD palm olein ŽRBDPOO . was evaluated. The changes in thermal profiles by differential scanning calorimetry ŽDSC. were studied in comparison to the changes in chemical parameters. The DSC method was based on the cooling and melting curves of oils at a scanning rate of 5 ⬚Crmin. The chemical evaluation of the oils was based on free fatty acid content, C18:2rC16:0 ratio, peroxide, iodine, and anisidine values. The DSC results were explained on the basis of the endo- or exothermic peak temperatures. A statistical comparative study was carried out on the DSC and chemical parameters. The results indicate that there is a good correlation between the DSC and chemical methods. Based on the results obtained, the DSC appears to be a useful instrumental method in monitoring the oxidation of microwave heated oils, and it may have the potential to replace the time- and chemical-consuming standard chemical methods. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Microwave heating; Differential scanning calorimetry ŽDSC.; Refined-bleached-deodorised palm olein; Oxidation; Chemical analyses; Quality characteristics Industrial rele¨ ance: Microwave heating has become an essential tool in home meal preparation and in the institutional food service sector. Few studies exist on the influence of microwave heating on the composition and quality of edible oils especially refined-bleached-deodorized palm olein ŽRBDPOo .. Consequently the objective of this work was to investigate the effect of microwave heating Žin combination with microwave power and heating time. on RBDPOo . Exposing RBDPOo to various microwave power settings and heating periods caused the formation of hydroperoxides and secondary oxidation products, hydrolysis of free fatty acids, decreased level of unsaturated fatty acids and iodine as well as changes in the differential scanning calorimeter cooling and melting parameters.
1. Introduction The application of microwave heating for both home meal preparation and institutional food service trade has increased because of its speed and convenience ŽMudgett, 1989; Yoshida & Takagi, 1997.. Today, microwave heating offers the food industry the opportunity to develop new food products with high nutritional U
Corresponding author. Tel.: q603-89486101, ext. 3468; fax: q603-89423552. E-mail address: [email protected] ŽY.B. Che Man..
and sensory quality, novel texture, food safety, and an extended shelf-life. In the food industry, microwave heating operations have been increasingly successful in baking, blanching, cooking, drying, pasteurisation, sterilisation and thawing of various food products ŽRosenberg & Bogl, ¨ 1987a,b.. It is important to recognise that the microwave heating characteristics of food products may vary considerably with the processing frequency and with the temperature, chemical composition, and physical state of the product ŽMudgett, 1989.. The chemical constituents of oils that degrade during microwave heating do so at rates that vary with heating
1466-8564r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 6 - 8 5 6 4 Ž 0 2 . 0 0 0 2 6 - 7
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C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163
temperature and time in a similar manner to many other type of different domestic processing methods Že.g. frying, steaming and roasting.. Suitable quality parameters can, therefore, be used as time᎐temperature integrators of quality deterioration of oils during microwave heating. Such quality parameters have been reviewed by many researchers ŽFristch, 1981; Robards, Kerr & Patsalides, 1988; Paul & Mittal, 1997.. However, many of these are chemical intensive methods. Also, the methods for measuring such components can be relatively complex and time-consuming, which could be a major drawback in industry applications. Instrumental methods involving a simpler technique, with a fast method for determining the change in oil quality is, therefore, required. Differential scanning calorimetry ŽDSC. has long been available as an instrumental method to the oils and fats researcher ŽCebula & Smith, 1992.. This technique is used for studying various heat-related phenomena in materials by monitoring associated changes in enthalpy. Recently, we used this technique to monitor oxidation process and determine total polar compounds in heated oils ŽTan & Che Man, 1999a,b.. Few studies concerning the influence of microwave on composition and quality of edible oils have been reported. Recently, the results of the effects of microwave treatments on the thermoxidative degradation and some physical and chemical parameters of edible oils were published in two separated papers ŽAlbi, Lanzon, & Leon, ´ Guinda, Perez-Camino ´ ´ 1997a; Albi, Lanzon, 1997b.. The ´ Guinda, Leon ´ & Perez-Camino, ´ effects of microwave and conventional heating on physical and chemical parameters of five edible oils and fats Žvirgin olive oil, olive oil, sunflower oil, high oleic sunflower oil and lard. were investigated by Albi and co-workers Ž1997a.. Meanwhile, the effects of microwave treatments on the thermoxidative degradation of five edible oils and fats Žsunflower oil, high oleic sunflower oil, virgin olive oil, olive oil, and lard. were determined by Albi et al. Ž1997b.. However, little has been reported about how microwave heating affects the quality of the refined-bleached-deodorised palm olein ŽRBDPOO .. Consequently, the primary objective of this research was to investigate the effect of microwave heating in combination with microwave power and heating time in terms of RBDPOO quality. A secondary objective was to investigate the correlation between DSC curve parameters and other standard chemical methods in microwave heated RBDPOO .
and was used in all experiments. All chemicals and solvents used were of analytical grade unless otherwise specified. 2.2. Microwa¨ e treatments A domestic microwave oven ŽNational, Model NN5656F, Matsushita Electric Industrial Co., Ltd., Osaka, Japan. was used in this study. In order to standardise the operation of the microwave oven for each heating test, some additional steps were done between tests. Between tests, the oven door was opened and an electrical fan was used to blow air into the oven cavity Žmax. 23 l. for 5 min to facilitate the cooling process. Thus, the temperature of the oven was reduced to approximately 30 " 2 ⬚C between tests. All oil samples were simultaneously exposed at a frequency of 2450 MHz. There were three settings of heating corresponding to low-, medium- and high-power, and when operated at high power, it provided approximately 900 W. Eighty grams of oil were divided into two 40-ml amber bottles Ž40 g each; 4 cm internal diameter., which were placed at equal distances on the turntable rotary plate of the microwave oven. The oil samples were heated for various periods Ž4, 8, 12, 16 and 20 min. at each power setting. The two samples were combined after microwave treatment and before analysis. The final oil temperatures at various heating times and power settings were measured by inserting a calibrated chromel᎐alumel thermocouple into the oil samples immediately after removal from the microwave oven. These temperature data are presented in Fig. 1. All oil samples were stored under nitrogen at y18 ⬚C for further analyses. Analysis of oil was carried out immediately after the heating experiments and completed within 2 weeks. 2.3. Standard chemical analyses The AOCS Official Methods were employed for the determination of free fatty acid ŽFFA. content, iodine ŽIV., anisidine ŽAnV. and peroxide ŽPV. values in the oil samples ŽAOCS, 1993.. The fatty acid compositions of the microwave-heated oil were analysed by gas chromatography ŽGC. after transesterification. GC analysis was performed as described previously for edible oils ŽTan & Che Man, 2000.. The ratios of C18:2rC16:0 were then calculated. 2.4. DSC analysis
2. Materials and methods 2.1. Materials RBDPOO was purchased from a local grocery store
A Perkin-Elmer differential scanning calorimeter, DSC-7 ŽPerkin-Elmer Corp., Norwalk, CT, USA. was used for the thermal analysis of oil samples. Purified nitrogen Ž99.999% purity. was the purge gas for the dry box and flowed at approximately 20 mlrmin. The DSC
C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163
159
Fig. 1. Comparison of oil temperatures heated in microwave oven at three different power settings with different heating times.
instrument was calibrated with indium and n-dodecane. Samples of approximately 6᎐12 mg were weighed into aluminium pans to the nearest 0.1 mg, and the covers were hermetically sealed into place. An empty, hermetically sealed aluminium pan was used as reference. Prior to analysis of samples, the baseline was obtained with an empty, hermetically sealed aluminium pan. RBDPOO samples were subjected to the following temperature program: 80 ⬚C isotherm for 5 min, cooled at 5 ⬚Crmin to y80 ⬚C and held for 5 min. The same sample was then heated from y80 to 80 ⬚C at the same rate. The cooling curves of the RBDPOO samples revealed one major crystallisation peak with two lowtemperature shoulder peaks. The temperature of the peak Žshoulder peak with lower-temperature., Tc , was obtained by analysing the DSC cooling curve with the 7 SeriesrUNIX DSC software library ŽAnon, 1995.. While the melting curves showed one major endothermic peak, which corresponded in temperature to the melting of olein fraction ŽTan and Che Man, 2000. and a small endotherm region at lower temperature consisted of a plateau with a pair of shoulder peaks. We speculated that there was a chain rearrangement in the crystallites and thus a smaller fraction of RBDPOO melted at a low temperature. A trough Žexotherm. was observed in between the two shoulder peaks. The temperature of the trough Žexothermic peak., Tm , was obtained by analysing the DSC melting curve with the same software library ŽAnon, 1995.. 2.5. Statistical analysis All experiments andror measurements were duplicated. The data were analysed using the SASrSTAT release 6.08 program ŽSAS Institute, Cary, NC, USA.. To examine the influence of microwave heating, data
from the oils were analysed as a 3 = 5 factorial arrangement of treatments with microwave power Žlow, medium and high. and heating time Ž4, 8, 12, 16 and 20 min. as main effects using the ANOVA procedure ŽSAS, 1989.. Differences between microwave powers and between times of heating in oil quality variables were tested for significance using a one-way analysis of variance, whereas for interaction between the two main effects Žmicrowave power= heating time. a two-way analysis of variance was used. When variance analysis revealed a significant effect, differences between heating time were tested using Duncan’s multiple range test ŽDuncan, 1955.. The relationships between each of the DSC curve parameters and standard chemical methods were determined by Pearson’s correlation analysis with the SAS program. The coefficient of determination Ž R 2 . was calculated for the relationship between each of the oil temperature᎐time᎐DSC curve parameter combinations and standard chemical methods by using the SAS REG procedure ŽSAS, 1989..
3. Results and discussion The initial characteristics of unheated RBDPOO used in this study are shown in Table 1. In this study, PV, AnV, FFA, IV and C18:2rC16:0 ratios were employed to measure heated oil deterioration. The major chemical and DSC parameter changes affected by microwave power setting and heating time are given in Tables 2 and 3. Data regarding the chemical changes of the oils are in general agreement with those published in the literature ŽYoshida, Hirooka & Kajimoto, 1990; Yoshida & Kajimoto 1994.. Microwave power from low to high has been shown to greatly influence the quality of oils. In general, microwave heating had significant
C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163
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Table 1 Characteristics of unheated refined-bleached-deodorised palm olein used in experiments a Characteristics of the oil
Value
Peroxide value Žmeqrkg oil. Anisidine value Free fatty acid Ž%. Iodine value Žg of I2r100 g oil. C18:2rC16:0 ratio DSC lower-temperature shoulder peak, Tc Ž⬚C. DSC exothermic peak, Tm Ž⬚C.
2.12" 0.30 1.37" 0.04 0.09" 0.00 59.92" 0.02 0.28" 0.00 y54.49" 0.10 y34.58" 0.18
a
Each value in the table represents the mean " S.D. of four measurements from two replicates. Abbre¨ iations: C18:2rC16:0, ratio of linoleic acidrpalmitic acid.
Ž P- 0.05. effect on these quality parameters. As expected, oil samples heated at high-power setting showed significantly Ž P- 0.05. more heat abuse than oil samples heated at medium- and low-power settings. Highly significant Ž P- 0.05. increases or decreases for PV, AnV, FFA, IV, C18:2rC16:0, DSC peak temperatures Žboth Tc and Tm . were noticed for oil samples heated at high-power settings. Generally, the experimental results of the AnV and FFA showed an increase with heating time. In contrast, the IV, C18:2rC16:0, DSC peak temperatures Žboth Tc and Tm . decreased with the increasing time of heating. The variance analysis indicated that oil quality was significantly Ž P- 0.05. affected by both microwave power and heating time. In this study, the interaction between microwave power= heating time was significant Ž P- 0.05.. The changes in PV are given in Table 2. Edible oils with a PV of 7.5 meqrkg have been deemed as unacceptable from a sensory viewpoint by some investigators ŽRobards et al., 1988.. The PV values at each power setting were all below this critical value. At
low-power setting, the PV increased gradually. PV for the oil samples heated at medium- and high-power settings exhibited two Žboth at 4 and 20 min of heating. maxima. Another distinct observation clearly demonstrated that the formation of hydroperoxides was more pronounced at a low-power setting than at mediumand high-power settings. This resulted from rapid decomposition of hydroperoxides to secondary oxidation products at elevated temperatures. They are extremely unstable and decompose via fission, dehydration, and the formation of free radicals to form a variety of chemical products, such as alcohols, aldehydes, ketones, acids, dimers, trimers, polymers, and cyclic compounds. Because hydroperoxides are unstable and do not accumulate in heated oil, PV is not a good indicator of heated oil deterioration ŽFristch, 1981.. The AnV values of microwave-heated oils are shown in Table 2. RBDPOO was less susceptible to oxidation because of lower levels of highly unsaturated fatty acids Že.g. linoleic and linolenic acids.. Generally, the oil AnV increased slowly at the low-power setting and
Table 2 Changes in chemical characteristics of refined-bleached-deodorised palm olein during microwave heating a Power
Heating time Žmin.
Peroxide value Žmeqrkg oil.
Low
4 8 12 16 20
2.32" 0.06e 2.61" 0.07d 2.75 " 0.02c 3.05" 0.14b 3.20" 0.06a
Medium
4 8 12 16 20
High
4 8 12 16 20
Anisidine value
Free fatty acid Ž%.
Iodine value Žg of I2 r100 g oil.
C18:2r16:0
1.72" 0.03e 1.92" 0.02d 2.16" 0.03c 2.34" 0.09b 2.46" 0.03a
0.09" 0.00c 0.09" 0.00c 0.10" 0.01bc 0.11" 0.01ab 0.12" 0.01a
59.46" 0.10a 59.37" 0.06a 59.19" 0.30a 58.79" 0.17b 58.28" 0.09c
0.28" 0.00a 0.27" 0.00b 0.27" 0.00bc 0.26" 0.00cd 0.26" 0.00d
4.18" 0.06c 5.43" 0.16a 5.09" 0.05b 3.29" 0.11d 3.42" 0.13d
1.72" 0.05e 5.39" 0.10d 9.04" 0.06c 14.91" 0.02b 15.37" 0.06a
0.10" 0.01c 0.11" 0.01bc 0.11" 0.00b 0.12" 0.01a 0.13" 0.01a
59.60" 0.22a 59.39" 0.18ab 58.62" 0.12bc 58.41" 1.24cd 57.63" 0.08d
0.27" 0.00a 0.27" 0.00b 0.26" 0.00b 0.26" 0.00c 0.26" 0.00c
4.85" 0.31b 5.73" 0.04a 3.63" 0.04c 3.05" 0.02d 4.72" 0.14b
3.42" 0.06e 8.77" 0.02d 14.31" 0.11c 15.12" 0.15b 16.38" 0.21a
0.09" 0.00d 0.11" 0.00c 0.12" 0.01b 0.13" 0.00ab 0.14" 0.01a
59.36" 0.00a 59.14" 0.64a 57.97 " 0.71b 57.31" 0.65b 56.37" 0.59c
0.27" 0.00a 0.27" 0.01ab 0.26" 0.01bc 0.25" 0.01cd 0.24" 0.00d
a Each value in the table represents the mean " S.D. of four measurements from two replicates. For each heating power, means within each column with different superscripts are significantly Ž P- 0.05. different.
C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163 Table 3 Changes in characteristics of refined-bleached-deodorised palm olein during microwave heating a Power
Heating
DSC peak temperature Ž⬚C. Cooling curve, Tc
Melting curve, Tm
Low
4 8 12 16 20
y56.60" 0.32a y58.05" 0.13b y59.04" 0.17b y60.52" 0.18c y60.76" 0.05d
y35.20" 0.02a y35.58" 0.03b y35.89" 0.04c y36.31" 0.03d y36.60" 0.06e
Medium
4 8 12 16 20
y59.52 " 0.12a y60.32" 0.05b y60.41" 0.06c y60.80" 0.09d y61.38" 0.08d
y35.87" 0.06a y36.70" 0.06 y37.42" 0.04c y37.80" 0.05d y38.04" 0.10e
High
4 8 12 16 20
y59.94" 0.06a y60.76" 0.05b y60.96" 0.28b y61.74" 0.10c y62.26" 0.08d
y36.38" 0.08a y36.89" 0.04b y37.57" 0.14c y38.18" 0.13d y39.31" 0.12e
Abbre¨ iation: Tc , temperature of the lower-temperature shoulder peak; Tm , temperature of trough Žexothermic peak.. a Each value in the table represents the mean " S.D. of four measurements from two replicates. For each heating power, means within each column with different superscripts are significantly Ž P0.05. different.
more rapidly at the medium- and high-power settings. As compared to PV, the AnV is a more reliable and meaningful test, because it measures the accumulation of secondary oxidation products Žaldehyde compounds.. Changes in the FFA of oil samples during microwave heating are shown in Table 2. Although the FFA content is an index of hydrolytic rancidity, it was nevertheless measured, as free acids contribute to the development of off-flavours and off-odours in the oil. There were no significant differences Ž P) 0.05. in the FFA contents until 8 min of heating at a low-power setting. The FFA of the oil samples increased slightly with longer heating time, particularly those heated at a high-power setting. These unexpected results showed that the FFA contents increased significantly Ž P- 0.05. due to production of fatty acids by microwave energy. The IV for microwave heated oils gradually decreased with increasing heating power settings and times ŽTable 2.. The reductions in IV were highest for oil samples heated at the high-power setting. A decrease in IV is consistent with the decreasing number of double bonds in the oil as it becomes oxidised. Thus, the reduction in IV during heating is often taken as a measure of oil deterioration ŽFristch, 1981.. The analysis is analogous to that for the reduction of unsaturated fatty acids, but gives more weight to the higher unsaturated oil samples. The changes in IV over 20 min of microwave heating were 1.18, 1.97 and 2.99 for low-, medium- and high-power settings, respectively.
161
C18:2rC16:0 ratios in microwave heated oils are shown in Table 2. The ratio was expected to decrease due to a decline in linoleic acid by oxidation. It was considered to be a reliable indicator of lipid oxidation during the heating process. A decrease in the relative percentage of unsaturated fatty acids and an increase in the relative percentage of saturated fatty acids occurs when the oils are heated ŽTan & Che Man, 1999a.. This pattern was observed in RBDPOO after heating at different microwave power settings. In the present study, the C18:2rC16:0 for microwave-heated oil decreased with increasing heating time. Microwave-heated RBDPOO exhibited a dominant exotherm Žat approx. y3 to y5 ⬚C. after cooling in the DSC, as shown in Fig. 2 for medium-power setting. From this figure, it is apparent that the DSC traces are affected in a systematic way by the increase in heating time. As the time of heating increased, the temperature of the peak Žshoulder peak with lower-temperature., Tc , shifted to lower temperatures ŽTable 3.. The same phenomena were observed in oil samples heated at low- and high-power settings. We also observed the same phenomenon in our previous paper ŽTan & Che Man, 1999b.. Fig. 3 showed the melting curves of
Fig. 2. DSC cooling curves of refined-bleached-deodorised palm olein samples heated at medium-power setting with different heating times Ž4, 8, 12, 16 and 20 min.. Abbre¨ iation: Tc , temperature of the lower-temperature shoulder peak.
162
C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163 Table 4 Pearson correlation coefficient Ž n s 5. between DSC lower-temperature shoulder peak temperature ŽTc ., heating time and wet chemical methodsa Cooling curve, Tc Ž⬚C. Low Heating time PV AnV FFA IV C18:2rC16:0
Medium UU
y0.9821 UU y0.9923 UU y0.9917 U y0.9178 U 0.8967 U 0.9561
High UU
y0.9727 0.4666 U y0.9358 U y0.9569 U 0.9366 UU 0.9827
UU
y0.9887 0.3576 U y0.9176 UU y0.9841 U 0.9536 UU 0.9803
U
Significance at the 0.05 level Ž P- 0.05.. Significance at the 0.01 level Ž P- 0.01.. a Abbre¨ iations: see Table 1.
UU
Fig. 3. DSC melting curves of refined-bleached-deodorised palm olein samples heated at medium-power setting with different heating times Ž4, 8, 12, 16 and 20 min.. Abbre¨ iation: Tm , temperature of trough Žexothermic peak..
microwave heated RBDPOO at various heating periods for medium-power settings. These melting curves revealed a major endotherm Žat ; 3᎐5 ⬚C., mainly due to the melting of the olein fraction. In general, as the time of heating increased, the temperature of the trough Žexothermic peak., Tm , moved to lower temperatures. The same observations were perceived in oil samples heated at low- and high-power settings. In microwave heated oils, the oxidation products, such as hydroperoxides, aldehydes, polar compounds, dimers and polymers, increased during the heating operation. The presence of these compounds may also disturb the rearrangement of different polymorphic forms in oil. As their levels increase, such compounds would contribute to the changes in DSC cooling and melting curves. It is well known that the presence of free fatty acids, partial glycerides and oxidation products in oil tend to shift the melting range to a lower temperature ŽChe Man & Swe, 1995.. The presence of oxidation products in oil also tends to shift the crystallisation range to a lower temperature ŽTan & Che Man, 1999a,b.. Therefore, the same phenomenon is expected for changes in DSC cooling and melting peak parameters for microwave-heated oil. In general, the effect of microwave heating on the peak temperature for RBDPOO was entirely dependent upon the heating
power setting; the high-power setting produced the biggest shift in peak temperature, followed by mediumand low-power settings. The coefficients of correlation matrix between each of the DSC curve parameter ŽTc and Tm . and standard chemical methods for RBDPOO are shown in Tables 4 and 5. Generally, the DSC peak temperatures showed excellent correlations with the heating periods and other standard chemical methods at all heating power settings. However, there were poor correlations in between each of the DSC curve parameters and PV at medium- and high-power settings. This may due to the rapid decomposition of hydroperoxides to secondary oxidation products at medium- and high-power settings. In particular, the high correlation Ž P- 0.01. found between each of the DSC curve parameters and heating periods, suggests that the DSC can be recommended as an appropriate objective method for evaluating the extent of oil deterioration during microwave heating. The DSC method can, therefore, be employed as a time᎐microwave power indicator during microwave heating. Although there is a high correlation between each DSC curve parameter and standard chemical parameters, it would be more meaningful to provide some information on the relationship between the oil temTable 5 Pearson correlation coefficient Ž n s 5. between DSC exothermic peak temperature ŽTm ., heating time and wet chemical methodsa Melting curve, Tm Ž⬚C. Low Heating time PV AnV FFA IV C18:2rC16:0 U
Medium UU
y0.9988 UU y0.9973 UU y0.9935 U y0.9655 U 0.9511 U 0.9416
y0.9729 0.4634 U y0.9731 UU y0.9598 U 0.9372 UU 0.9833
Significance at the 0.05 level Ž P- 0.05.. Significance at the 0.01 level Ž P- 0.01.. a Abbre¨ iations: see Table 1.
UU
High UU
UU
y0.9884 0.3570 U y0.8918 UU y0.9686 UU 0.9886 UU 0.9900
C.P. Tan et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 3 (2002) 157᎐163
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Table 6 Summary of regression analysis Ž n s 15. on the relationship between temperature᎐time᎐DSC parameter and wet chemical methodsa Regression analysis Ž R2 , coefficient of determination.
PV AnV FFA IV C18:2rC16:0
Temperature᎐time᎐DSC lowertemperature shoulder peak temperature
Temperature᎐time᎐DSC exothermic peak temperature
0.4501 UU 0.8735 UU 0.9569 UU 0.9339 UU 0.9483
0.6873 U 0.5676 UU 0.8474 UU 0.8494 UU 0.8612
UU
U
Significance at the 0.05 level Ž P- 0.05.. Significance at the 0.01 level Ž P- 0.01.. a Abbre¨ iations: see Table 1.
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perature᎐time᎐DSC curve parameter combination and wet chemical methods. The results of the regression analysis of this relationship are given in Table 6. The end-temperatures recorded in the microwave-heated oils were used in the regression analysis. The regression analysis showed that the oil temperature᎐ time᎐DSC curve parameter combination was well correlated to standard chemical methods Žexcept for PV., as judged by the coefficient of determination Ž R 2 . values. This observation revealed that, with a known oil temperature, heating time and DSC curve parameter, we could predict the AnV, FFA, IV or C18:2rC16:0 in microwave-heated RBDPOO . In summary, exposing the RBDPOO to various microwave power settings and heating periods caused the formation of hydroperoxides and secondary oxidation products, hydrolysis of FFA, decreased the level of unsaturated fatty acids and thus IV, and changes in DSC cooling and melting parameters. From the results of this study, we also concluded that DSC can be utilised as an efficient and convenient method for monitoring the extent of oil deterioration in RBDPOO , with no need for chemical and dedicated skills.
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