This study investigates the effects of repeated microwave frying at different power levels (360W, 600W, 900W) and deep-fat frying on sunflower oil and French fries. The intermittent batch frying cycle was repeated 15 times a day during five consecutive days. The fatty acid profile and physicochemical properties including free fatty acid (FFA), extinction coefficient (K270), total polar compound (TPC), color, viscosity, refractive index of the sunflower oil were determined each day. At the end of the frying period, the highest values of viscosity (76.29cp) and refractive index (1.4738) were detected in microwave frying at 900W power level. TPC level exceeded 25% after the third day of microwave frying at all power levels. The FFA values during microwave frying increased progressively from 0.157% to 0.320- 0.379% on the fifth day. The loss of polyunsaturated fatty acids was 37-53% more in the case of microwave frying. The oil quality during microwave frying did not have a significant impact on the oil absorption and total color change of the French fries. Microwave frying, even at higher levels, provided lower oil (8.60-12.32%wb) and moisture contents (35.47-41.24%) compared to deep frying. Microwave frying caused longer processing time and significantly higher levels of degradation of the sunflower oil at all power levels compared to deep frying. However, microwave frying has the advantage of reducing oil absorption. The oil content of French fries was lowered by 20-33% (wb) at the highest power level.
The popularity of processed potato products, particularly frozen French fries is increasing worldwide. Consumer demand for healthier products has caused a significant increase in research in the area of fried foods. Many of these studies have focused on the reduction of oil uptake (Ziaiifar
Microwave frying is proposed as a new technology for improving fried food quality which may be an alternative method to traditional frying (Sahin
Sunflower oil, which contains a high (around 71%) polyunsaturated fatty acid (PUFA) content (Zambiazi
Frozen, par-fried French fries were purchased from local supermarket. The sunflower oil used in finish frying was obtained from a local oil company (Çotanak, Ordu, Turkey). A fatty acid methyl ester (FAME) standard mixture (Restek Food Industry FAME Mix, cat# 35077) was purchased from Superchrom (Milan, Italy). All other chemicals were obtained from Merck (Darmstadt, Germany) and Sigma Chemicals Co. (St. Louis, United States) and were of analytical grade.
Finish frying was carried out in a domestic microwave oven (Bosch HMT84G421) and in a commercial bench-top deep-fat fryer (Tefal, France) with 2.1lt oil capacity. For microwave frying, 2L of sunflower oil was placed in a glass container with a total capacity of 4lt and heated from room temperature to a frying temperature of 180±1 ºC by using 360W, 600W or 900W power levels. A fiber optic temperature probe (FISO Technologies, Inc, Quebec, Canada) was used to follow the oil temperature. The initial heating time of the oil from room temperature to 180±1 ºC lasted 65, 33 and 25 minutes at the power levels of 360W, 600W or 900W, respectively. At each power level, French fries (50 g) were fried for 3 min. The power level was maintained during this time. After each batch, the oil was allowed to cool for 15 min and then reheated to 180±1 ºC. This intermittent batch frying cycle was repeated 15 times a day for five consecutive days. At the end of each day, the oil was covered and left at room temperature overnight. Deep-fat frying was performed using the same frying cycle as the microwave frying. It took 9 min to heat the oil in the fryer from room temperature to 180±1 ºC. French fries were fried at 180±1 ºC for 3 min in 2L of sunflower oil. The average treatment time that included initial heating, frying, cooling and reheating of oil, took 5.6 hours on the first day of deep-fat frying while the same frying cycle lasted 7.4, 7.5 and 11.6 hours on the first day of microwave frying at power levels of 900W, 600W and 360W, respectively. The potato samples fried at the end of the first, third and fifth days were used for further analysis. 100ml of oil sample from microwave frying at power levels of 360W (MF360), 600W (MF600), 900W (MF900) and from deep-fat frying (DF) at 180±1 ºC were collected, put in sealed glass and kept at -18 °C for further analysis at the end of each day. Since there was no replenishment of oil, the amount of oil remaining reduced to 1.5lt at the end of the frying period. Initially, the ratio between the surface of the glass container and the oil volume was 0.24cm-1 and changed to 0.33cm-1 after the fifth frying operation. The surface to oil ratio in the fryer changed from the initial value of 0.21cm-1 to 0.27cm-1 after the fifth frying operation.
The free fatty acid (FFA) content and the extinction coefficient (K270) were determined according to AOCS official methods of Ca 5a-40 and Ch 5-91, respectively (AOCS,
where:
Kλ = specific extinction at wavelength λ;
Eλ = extinction measured at wavelength λ;
c = concentration of the solution in g/100 ml;
s = path length of the quartz cell in cm;
Fatty acids were evaluated as their methyl esters according to the AOAC method 996.06 (AOAC,
A Testo 270 instrument (Lenzkirch, Germany) was used to obtain the TPC values of sunflower oil. The sensor was calibrated using the reference oil having a TPC value of 3.5±0.5% according to the manufacturer’s instructions. Before TPC measurement, the sensor was immersed in the reference oil heated to approximately 50 ºC for reference value adjustment. Refractive indexes of the oil samples were determined by using a hand-held refractometer (Krüss, DR201-95), calibrated against pure water at 25 °C. Oil color parameters of redness (R) and yellowness (Y) were measured by Lovibond Tintometer (PFX 880). A Vibro Viscometer (SV-10 Series, A&D Company, Limited, Japan) was used to obtain the viscosity (cp) of the oil samples at 25 °C.
The moisture content of fried French fries was determined in a forced convection oven (NST-120, Ankara, Turkey) at 105 °C up to constant weight (AOAC,
where, L0, a0 and b0 are reference values belonging to the color of frozen par-fried French fries (L0 = 77.8, a0 =1.3, b0 =12.9).
The frying experiments were carried out in triplicate under the same experimental conditions and the results were expressed as mean ± standard deviation (SD). The results were analyzed by ANOVA (Analysis of Variance) followed by Tukey’s multiple comparison test (p ˂0.05) (Minitab, version 17).
Changes in free fatty acid (FFA) values in sunflower oil during microwave frying at 360W (MF360). 600W (MF600). 900W (MF900) and deep-fat frying (DF) of frozen French fries at 180 °C.
Error bars indicate the SD of the mean of three replicates (n=3). Means containing different letters in the same day are significantly different (p < 0.05).
The evolution of K270 values during frying are displayed in
Changes in K270 values in sunflower oil during microwave frying at 360W (MF360). 600W (MF600). 900W (MF900) and deep-fat frying (DF) of frozen French fries at 180 °C.
Error bars indicate the SD of the mean of three replicates (n=3). Means containing different letters in the same day are significantly different (p < 0.05).
The effects of different treatments on the refractive index and viscosity of oil samples are given in
Refractive index (25 °C) and viscosity (cp) (25 °C) of sunflower oil during microwave frying at 360W (MF360), 600W (MF600), 900W (MF900) and deep-fat frying (DF) of frozen French fries at 180 °C
Days of frying | Refractive Index (25°C) | Viscosity (cp) (25°C) | ||||||
---|---|---|---|---|---|---|---|---|
DF | MF360 | MF600 | MF900 | DF | MF360 | MF600 | MF900 | |
0 | 1.4710±0.00A |
1.4710±0.00Aa | 1.4710±0.00Aa | 1.4710±0.00Aa | 37.67±0.03Aa | 37.36±0.03Aa | 37.67±0.03Aa | 37.67±0.03Aa |
1 | 1.4732±0.00Cd | 1.4720±0.00Ba | 1.4723±0.00Bb | 1.4731±0.00Bc | 39.74±0.02Bb | 45.20±0.06Bd | 38.80±0.05Ba | 43.61±0.02Bc |
2 | 1.4732±0.00Cc | 1.4720±0.00Ba | 1.4725±0.00Bb | 1.4736±0.00Cd | 41.45±0.04Ca | 53.76±0.13Cd | 47.41±0.09Cb | 50.84±0.03Cc |
3 | 1.4707±0.00Aa | 1.4721±0.00Bb | 1.4729±0.00Cc | 1.4739±0.00Cd | 44.53±0.03Da | 60.29±0.07Dc | 55.57±0.12Db | 61.42±0.01Dd |
4 | 1.4708±0.00Aa | 1.4722±0.00Bb | 1.4731±0.00Cc | 1.4736±0.00Cd | 45.42±0.02Ea | 65.23±0.11Ec | 61.19±0.09Eb | 67.44±0.08Ed |
5 | 1.4725±0.00Bb | 1.4722±0.00Ba | 1.4731±0.00Cc | 1.4738±0.00Cd | 48.51±0.03Fa | 69.56±0.11Fc | 67.76±0.11Fb | 76.29±0.06Fd |
Capital letters shown in the same column compare the frying days for each of the frying conditions.
Small letters shown in the same row compare the frying conditions per frying day (p < 0.05).
A different variety of polar compounds, oxidized and dimerized triglycerides, FFAs, monoglycerides, diglycerides, dimers, trimers tetramers etc. are formed during frying (Velasco
Changes in total polar compound (TPC) (%) in sunflower oil during different frying treatments (▲) DF y=0.15x+10.6 R2=0.988. (■) MF360 y=0.41x+14.75 R2=0.956. (Δ) MF600 y=0.41x+10.67 R2=0.989. (○) MF900 y=0.43x+11.30 R2=0.994.
Color is one of the extensively used physical parameters to evaluate frying oil quality. There was a significant rise in redness (R) and yellowness (Y) values, especially after the initial day of both frying methods (
Color parameters (R, Y) of sunflower oil during microwave frying at 360W (MF360), 600W (MF600), 900W (MF900) and deep-fat frying (DF) of frozen French fries at 180 °C
Days of frying | R | Y | ||||||
---|---|---|---|---|---|---|---|---|
DF | MF360 | MF600 | MF900 | DF | MF360 | MF600 | MF900 | |
0 | 0.97±0.06A |
0.97±0.06Aa | 0.97±0.06Aa | 0.97±0.06Aa | 15.00±0.00Ba | 15.00±0.00Aa | 15.00±0.00Aa | 15.00±0.00Aa |
1 | 1.10±0.00Aa | 1.40±0.00Bb | 1.10±0.00Aa | 1.60±0.14Bb | 14.00±0.00Ab | 16.00±0.00Bc | 13.00±0.00Aa | 16.00±0.00Ac |
2 | 1.63±0.06Ba | 2.45±0.07Cc | 2.00±0.00Bb | 2.30±0.00Cc | 18.27±0.46Ca | 39.00±0.00Cd | 27.00±0.00Bb | 35.00±0.00Bc |
3 | 2.30±0.00Ca | 3.25±0.07Dc | 2.55±0.07Cab | 2.80±0.14Db | 24.30±0.00Da | 57.00±0.00Dd | 39.00±0.00Cb | 51.00±0.00Cc |
4 | 3.10±0.00Da | 4.45±0.07Eb | 2.93±0.26Ca | 3.00±0.14Da | 39.00±0.00Ea | 70.00±0.00Ed | 49.90±2.20Db | 57.00±0.00Cc |
5 | 5.40±0.00Ed | 4.65±0.07Ec | 3.77±0.06Da | 4.40±0.00Eb | 71.35±0.35Fa | 70.00±0.00Ea | 69.50±0.00Ea | 63.50±0.46Ca |
Capital letters shown in the same column compare the frying days for each of the frying conditions.
Small letters shown in the same row compare the frying conditions per frying day (p < 0.05).
The fatty acid composition of oil is essential from a nutritional point of view. A high level of polyunsaturated fatty acids (PUFAs) is desirable for human health; on the other hand, PUFAs are known to be unstable under high temperature conditions. The fatty acid compositions of the oil samples are given in
Quantitation of fatty acids (wt% in oil) of untreated, DF, MF360, MF600 and MF900 sunflower oil samples
Sunflower oil | Days of frying | Total content of FAMEs | Percentage of main FAMEs | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Saturated | Unsaturated | Palmitic | Stearic | Oleic | Linoleic | Others | |||||
Mono | Poly | C16:0 | C18:0 | C18:1 | C18:2 | Saturated | Unsaturated | ||||
Untreated | 0.0 | 11.7 | 28.4 | 59.9 | 6.1 | 4.1 | 28.1 | 59.8 | 1.5 | 0.4 | |
Deep-fat Frying | 1.0 | 12.5 | 29.0 | 58.5 | 6.7 | 4.2 | 28.7 | 58.4 | 1.6 | 0.5 | |
2.0 | 13.1 | 29.5 | 57.4 | 7.3 | 4.2 | 29.1 | 57.2 | 1.6 | 0.5 | ||
3.0 | 13.9 | 29.6 | 56.6 | 8.1 | 4.1 | 29.2 | 56.4 | 1.6 | 0.5 | ||
4.0 | 14.8 | 30.2 | 55.1 | 9.0 | 4.2 | 29.8 | 54.8 | 1.6 | 0.6 | ||
5.0 | 16.2 | 31.2 | 52.7 | 10.1 | 4.3 | 30.8 | 52.4 | 1.7 | 0.6 | ||
Microwave Frying | 360W | 1.0 | 13.2 | 30.0 | 56.8 | 7.0 | 4.4 | 29.6 | 56.6 | 1.8 | 0.6 |
2.0 | 14.4 | 30.9 | 54.7 | 8.0 | 4.5 | 30.5 | 54.5 | 1.9 | 0.7 | ||
3.0 | 15.2 | 31.2 | 53.6 | 8.8 | 4.5 | 30.8 | 53.3 | 1.8 | 0.7 | ||
4.0 | 16.2 | 32.3 | 51.5 | 9.6 | 4.7 | 31.8 | 51.2 | 2.0 | 0.8 | ||
5.0 | 17.1 | 33.0 | 49.9 | 10.3 | 4.7 | 32.6 | 49.6 | 2.0 | 0.7 | ||
600W | 1.0 | 12.6 | 29.1 | 58.2 | 6.9 | 4.1 | 28.7 | 58.0 | 1.6 | 0.6 | |
2.0 | 13.6 | 30.1 | 56.3 | 7.7 | 4.2 | 29.7 | 56.1 | 1.6 | 0.6 | ||
3.0 | 15.2 | 31.5 | 53.3 | 8.7 | 4.6 | 31.1 | 53.1 | 1.9 | 0.7 | ||
4.0 | 16.1 | 32.2 | 51.7 | 9.6 | 4.6 | 31.8 | 51.4 | 1.9 | 0.7 | ||
5.0 | 17.1 | 32.9 | 50.0 | 10.5 | 4.7 | 32.5 | 49.7 | 2.0 | 0.7 | ||
900W | 1.0 | 13.1 | 29.8 | 57.1 | 7.0 | 4.4 | 29.4 | 56.9 | 1.8 | 0.6 | |
2.0 | 13.9 | 30.4 | 55.7 | 7.8 | 4.4 | 30.0 | 55.5 | 1.8 | 0.7 | ||
3.0 | 15.2 | 31.5 | 53.3 | 8.8 | 4.6 | 31.1 | 53.0 | 1.9 | 0.7 | ||
4.0 | 16.4 | 32.4 | 51.2 | 9.8 | 4.7 | 31.9 | 50.9 | 2.0 | 0.7 | ||
5.0 | 17.8 | 33.4 | 48.8 | 10.9 | 4.8 | 32.9 | 48.5 | 2.1 | 0.8 |
Estimated quantitation of FAME (wt% in oil) of untreated, DF, MF360, MF600 and MF900 sunflower oil samples
Sunflower oil | Days of frying | Total content of FAMEs | Percentage of main FAMEs | Estimated nonpolar FAME | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Saturated | Unsaturated | Palmitic | Stearic | Oleic | Linoleic | Others | ||||||
Mono | Poly | C16:0 | C18:0 | C18:1 | C18:2 | Saturated | Unsaturated | |||||
Untreated | 0.0 | 11.7 | 28.4 | 59.9 | 6.1 | 4.1 | 28.1 | 59.8 | 1.5 | 0.4 | 100.0 | |
Deep-fat Frying | 1.0 | 11.7 | 27.2 | 54.9 | 6.1 | 4.1 | 26.9 | 54.7 | 1.5 | 0.5 | 93.8 | |
2.0 | 11.7 | 26.3 | 51.2 | 6.1 | 4.1 | 26.0 | 51.0 | 1.5 | 0.5 | 89.2 | ||
3.0 | 11.7 | 24.9 | 47.7 | 6.1 | 4.1 | 24.6 | 47.5 | 1.5 | 0.5 | 84.3 | ||
4.0 | 11.7 | 23.9 | 43.6 | 6.1 | 4.1 | 23.6 | 43.4 | 1.5 | 0.5 | 79.2 | ||
5.0 | 11.7 | 22.6 | 38.1 | 6.1 | 4.1 | 22.3 | 38.1 | 1.5 | 0.5 | 72.6 | ||
Microwave Frying | 360W | 1.0 | 11.7 | 26.6 | 50.4 | 6.1 | 4.1 | 26.3 | 50.2 | 1.5 | 0.5 | 88.7 |
2.0 | 11.7 | 25.0 | 44.3 | 6.1 | 4.1 | 24.7 | 44.1 | 1.5 | 0.5 | 81.1 | ||
3.0 | 11.7 | 24.0 | 41.2 | 6.1 | 4.1 | 23.6 | 40.9 | 1.5 | 0.6 | 76.9 | ||
4.0 | 11.7 | 23.3 | 37.2 | 6.1 | 4.1 | 23.0 | 37.0 | 1.5 | 0.6 | 72.2 | ||
5.0 | 11.7 | 22.6 | 34.1 | 6.1 | 4.1 | 22.3 | 34.0 | 1.5 | 0.5 | 68.5 | ||
600W | 1.0 | 11.7 | 27.0 | 54.0 | 6.1 | 4.1 | 26.6 | 53.8 | 1.5 | 0.6 | 92.6 | |
2.0 | 11.7 | 25.9 | 48.4 | 6.1 | 4.1 | 25.5 | 48.2 | 1.5 | 0.5 | 86.0 | ||
3.0 | 11.7 | 24.3 | 41.1 | 6.1 | 4.1 | 23.9 | 40.9 | 1.5 | 0.5 | 77.1 | ||
4.0 | 11.7 | 23.3 | 37.5 | 6.1 | 4.1 | 23.0 | 37.3 | 1.5 | 0.5 | 72.5 | ||
5.0 | 11.7 | 22.5 | 34.1 | 6.1 | 4.1 | 22.2 | 33.9 | 1.5 | 0.5 | 68.3 | ||
900W | 1.0 | 11.7 | 26.6 | 51.0 | 6.1 | 4.1 | 26.3 | 50.8 | 1.5 | 0.5 | 89.3 | |
2.0 | 11.7 | 25.6 | 46.9 | 6.1 | 4.1 | 25.2 | 46.7 | 1.5 | 0.6 | 84.1 | ||
3.0 | 11.7 | 24.2 | 40.9 | 6.1 | 4.1 | 23.9 | 40.7 | 1.5 | 0.5 | 76.9 | ||
4.0 | 11.7 | 23.0 | 36.4 | 6.1 | 4.1 | 22.7 | 36.2 | 1.5 | 0.5 | 71.2 | ||
5.0 | 11.7 | 21.9 | 32.0 | 6.1 | 4.1 | 21.6 | 31.8 | 1.5 | 0.5 | 65.6 |
During microwave frying, the sunflower oil sample had higher values of FFA, K270, TPC, and viscosity and lower content of PUFA compared to the conventional frying method. The oil samples were exposed to higher temperatures for longer periods and also to microwave energy during microwave frying. It is recorded in the literature that when the temperature was kept below 40 °C, microwave energy alone had no effect on oil degradation (Albi
Frozen par-fried French fries have initial moisture and oil contents of 71.7% (wb) and 4.1% (wb), respectively. The oil content, moisture content and ΔE values of fried French fries are represented in
Moisture content. oil content and ΔE values of French fries during microwave frying at 360W (MF360). 600W (MF600). 900W (MF900) and deep-fat frying (DF)
Days of frying | DF | MF360 | MF600 | MF900 | |
---|---|---|---|---|---|
1 | 52.95±0.98B |
41.41±0.85Ab | 37.20±0.51Aa | 35.47±0.64Aa | |
3 | 51.22±0.65ABd | 47.68±0.45Bc | 41.24±0.43Bb | 37.73±0.30Aa | |
5 | 49.15±1.02Ab | 48.70±1.22Bb | 40.61±0.02Ba | 39.58±1.07Aa | |
1 | 13.04±1.40Ac | 12.32±0.59Ab | 9.39±0.29Aa | 10.50±0.53Aa | |
3 | 11.81±0.23Ac | 10.25±0.09Ab | 8.60±0.74Aa | 8.60±1.02Aa | |
5 | 12.79±0.78Ac | 9.76±1.03Ab | 8.78±1.17Aab | 8.61±0.91Aa | |
1 | 25.96±1.44Aa | 29.25±1.90Aab | 30.28±1.27Aab | 31.76±1.65Ab | |
3 | 30.55±1.77Ba | 30.68±1.98Aa | 29.78±1.74Aa | 31.09±1.32Aa | |
5 | 31.69±1.15Ba | 28.29±1.64Aa | 30.75±1.82Aa | 32.65±1.65Aa |
Capital letters shown in the same column compare the frying days for each of the frying conditions.
Small letters shown in the same row compare the frying conditions per frying day (p < 0.05).
Color is another quality criterion of deep fat fried products. The total color change (ΔE) of microwave fried samples increased with an increasing microwave power level on the initial and final days of frying, although this difference was not found to be statistically significant. The ΔE value of deep-fat fried samples increased gradually with increasing frying days while that of the microwave fried ones stayed nearly constant. The color changes of foods during frying are reported to be influenced by some factors like the chemical browning reactions in food, oil absorbed by food, time and temperature of frying process, etc. (Loewe,
Microwave frying increased the rate of sunflower oil degradation compared to deep-fat frying. The TPC level reached 39-42% at the end of microwave frying trials while it was still under 25% at the end of the deep-fat frying period. Likewise, the loss in the amount of polyunsaturated fatty acids was higher in the case of microwave frying. However, microwave frying provided an advantage in French fry quality by lowering the oil content. Oil degradation did not have a significant effect on the oil uptake during both frying methods. Microwaves are reported to offer many advantages in certain food processing operations with regards to time, space and energy savings, as it ensures the preservation of nutritional value, process control, and selective heating. However, the longer period of time required to heat the oil to frying temperature was one of the major disadvantages of using microwave energy during a repetitive frying process. Cleaning the oil that splashes out into the microwave also requires extra time after each frying process. More importantly, microwave frying resulted in significantly higher levels of degradation of sunflower oil at all power levels. The differences in the heating mechanisms together with the duration of the process are thought to be important factors in this respect. Especially during the initial days of frying, these disadvantages may be minimized to some extent by using higher power levels. In addition, the oil absorption of French fries decreased by increasing the microwave power level. However, on the last day of treatment, the higher power level caused higher values of TPC, viscosity and a higher reduction in PUFA contents in the oil. This study showed that both the degradation rate of sunflower oil and the quality of French fries were influenced by the level of power during microwave frying and the highest power level provided better results during the initial days of a repetitive frying process.
The authors wish to thank Altaş Yağ Sanayi (Çotanak) for their help in performing the color experiments. This work is funded by Ordu University (TF 1460).