Reduction of acrylamide formation in French fries by microwave pre-cooking of potato strips

Abstract

In this study, the effect of microwave pre-cooking of potato strips on the resultant acrylamide levels in French fries was investigated. Control and microwaved (10, 20, and 30 s at 850 W) samples were fried at 150, 170 and 190 °C for predetermined times. Surface and core temperatures of potato strips were acquired during frying, and acrylamide content in the surface and the core regions were determined separately. The results showed that microwave application prior to frying resulted in a marked reduction of acrylamide level in the surface region, whereas a slight increase was noted for the core region. When the potato strips were subjected to frying after a microwave pre-cooking step, acrylamide content in the whole potato strip was reduced by 36%, 41%, and 60% for frying at 150, 170, and 190 °C, respectively, in comparison to the control. Copyright © 2006 Society of Chemical Industry

INTRODUCTION

After the discovery of acrylamide (a possible carcinogen in humans) in certain foods, considerable research has been done to minimize its formation during processing. Acrylamide forms during processes such as frying, baking, and roasting where high-temperature and low-moisture conditions exist. Fried potato products such as potato chips and French fries are among the foods with the highest amounts of acrylamide, probably due to relatively high levels of suspected acrylamide precursors present in potato.1 Interaction between asparagine and reducing sugars, such as glucose and fructose, is widely accepted as the reaction responsible for acrylamide formation.

Numerous studies have been conducted to explore the possibilities of reducing acrylamide levels in French fries. Considering the high level of acrylamide precursors naturally found in potatoes, reducing strategies that involve pretreatment and/or modification of the frying conditions appear to be a logical approach to control acrylamide levels in the final product. While pretreatment steps such as enzyme treatment,2 soaking in water,3 and blanching4 reduce the amount of acrylamide precursors, some other pretreatment steps (e.g., reduction of pH) were intended to make the conditions less favorable for acrylamide formation.5

Effect of frying conditions on the resultant acrylamide levels has been investigated by several researchers,1, 4, 6, 7 and frying time and temperature have been shown to significantly affect acrylamide formation. Haase et al.8 were able to reduce the acrylamide formation in potato chips by half as a result of lowering the oil temperature from 185 to 165 °C. Pedreschi et al.4 also reported 68% and 88% reduction in acrylamide content with a decrease in temperature from 190 °C (3.5 min) to 170 °C (5 min) and to 150 °C (7 min), respectively. Among several factors studied including potato cultivar, soaking in water, and frying oil type, Williams1 identified the time and temperature of frying as the most significant parameters affecting acrylamide formation. Acrylamide levels in overcooked French fries were found to be as high as 10 mg kg⁻¹, also indicating the strong dependence of acrylamide levels on frying time.9 Claeys et al.10 reported that the most straightforward way of reducing acrylamide level is to reduce the time and temperature of frying without compromising the product quality.

Previous research suggests that acrylamide formation can easily be limited by reducing the frying time. In deep fat frying, conduction within the food material is the rate-controlling heat transfer mechanism,11 which implies that frying time can be reduced if the potato strips are cooked before going into the fryer. This would eliminate the need to rely on conduction of heat during frying to render the interior cooked. Then, the frying is performed just to bring about the desirable surface characteristics (crust and color) of the already-cooked strips.

This study aimed to reduce the frying time and hence acrylamide formation by microwave pre-cooking of potato strips prior to frying. Microwave pre-cooking differs from blanching in that the potato strip is rendered cooked volumetrically in a very short time without losing the reducing sugars and asparagine, which are also responsible for the product's desirable surface characteristics. Acrylamide formation in the core and surface regions of potato strips were determined separately. Temperature profiles for the core and surface were obtained during frying to gain insight into the heating behavior of already-cooked and raw potato strips.

MATERIALS AND METHODS

Materials

Sunflower oil (frying medium) and potatoes (Agria) were purchased from a local market. Potato samples were washed and peeled before cutting in 0.85 × 0.85 × 7 cm dimensions by using a French fry cutter.

Pre-treatments

Strips were pre-cooked in a microwave oven for three different time periods (10, 20 and 30 s) before frying. Three potato strips were placed horizontally in the microwave oven (MD 572, Arçelik, Turkey) operating at 850 W. Potato strips without microwave treatment were considered as the control.

Frying conditions

Four strips (pre-cooked in microwave and control) per sampling time were immersed in 5 L of hot oil contained in an electrical fryer (Precisterm, J. P. Selecta, Spain) at each frying temperature (150, 170 and 190 °C). Frying times which provided a thoroughly cooked potato strip at that particular temperature of frying were employed during the experiments. To do this, control and microwaved potato strips were sampled at different time intervals during frying and evaluated sensorily by us. The frying times used for the control and 10, 20, and 30 s microwave-treated samples are given in Table 1 for three different frying temperatures. Experiments were conducted in parallel.

Table 1. Frying times (in minutes) employed during the experiments

Microwave pretreatment time	Frying temperature
	150 °C	170 °C	190 °C
0 s	6	4.5	4
10 s	4	4	3
20 s	3.5	3	2.5
30 s	3	2.5	1.5

Temperature measurement

Temperature at the surface and core of potato strips during frying was measured by inserting two thermocouples (type-T, 36 gauge, Omega Engineering, Stamford, CT, USA) as shown in Fig. 1. The surface thermocouple was placed so that the tip of the thermocouple was flush with the surface. Location of the surface thermocouple was verified after each experiment by visual observation. Time–temperature data were discarded if the position of the thermocouple tip was found to change during the frying process. Temperature profiles obtained from two different surface thermocouples during a frying experiment were found to be similar. Temperature was recorded every second by using a data acquisition system, comprising a digital multimeter and a 20-channel multiplexer (Keithley, Model 2700 DMM and Model 7700, Cleveland, OH, USA).

Figure 1. Schematic representation of experimental set-up used to measure the temperature in potato strips during frying.

Moisture content analysis

Moisture content of samples was determined by drying the samples to constant weight at 105 ± 1 °C.12

Acrylamide analysis

Sample preparation for LC-MS analysis

A surface and a core sample were prepared from each potato strip to determine the acrylamide content (on wet basis) in the surface and the core regions separately. A 2 mm layer from the surface (the surface sample) was removed from each strip using a knife, and the remaining portion of the strip was considered the core sample. Acrylamide analyses were also done in the potato strips after the microwaving step prior to frying to check whether microwave application resulted in any acrylamide formation.

The sample preparation procedure described by Şenyuva and Gökmen13 was used with minor modification. Finely ground sample (∼1 g) was weighed into a 10 mL glass centrifuge tube with a screw cap. The sample was spiked with ¹³C₃-labeled acrylamide to confirm acrylamide. 500 µL Carrez 1 and 500 µL Carrez 2 solution were added and the volume was adjusted to 10 mL with 0.2 mmol L⁻¹ acetic acid. After vortexing for 2 min, the mixture was centrifuged at 10 000 rpm for 10 min at −5 °C to solidify oil. The clear supernatant was quantitatively transferred into a vial, avoiding the top oil layer. It was filtered through a 0.45 µm nylon syringe filter prior to LC-MS analysis.

LC-APCI-MS analysis

LC-APCI-MS analyses were performed using an Agilent 1100 HPLC system (Waldbronn, Germany) consisting of a binary pump, an autosampler and a temperature-controlled column oven, coupled to an Agilent 1100 MS detector equipped with atmospheric pressure chemical ionization (APCI) interface. The analytical separation was performed on an Inertsil ODS-3 column (250 × 4.6 mm, 5 µm) using the isocratic mixture of 0.01 mmol L⁻¹ acetic acid in 0.2% aqueous solution of formic acid at a flow rate of 0.6 mL min⁻¹ at 25 °C. The LC eluent was directed to the MS system after a delay time of 6.5 min using MSD software. Data acquisition was performed in selected ion monitoring (SIM) mode using the following interface parameters: drying gas (N₂, 100 psig) flow of 4 L min⁻¹, nebulizer pressure of 60 psig, drying gas temperatures 325 °C, vaporizer temperature of 425 °C, capillary voltage of 4 kV, corona current of 4 µA, fragmenter voltage of 55 eV. Ions monitored were m/z 72 and 55 for acrylamide and m/z 75 and 58 for ¹³C₃-labeled acrylamide for the quantification of acrylamide in the samples.

RESULTS AND DISCUSSION

At all three frying temperatures, microwave pre-cooking resulted in lower acrylamide content in the whole potato strip in comparison to the control. Acrylamide level of the microwave-treated samples was 36%, 41%, and 60% lower than that of the control for 20, 20 and 30 s microwave times and frying temperatures of 150, 170, and 190 °C, respectively. In addition, acrylamide content of the whole strip appeared to decrease with increasing microwave treatment time (Figs 2, 3, and 4). These results were expected, since the time of frying employed to obtain the same degree of cooking decreased with an increase in pretreatment time. It should be noted that no significant formation of acrylamide (∼50 ng g⁻¹) was detected upon microwave pre-cooking without any frying.

Figure 2. Acrylamide content of the control and microwave pre-cooked samples after frying at 150 °C.

Figure 3. Acrylamide content of the control and microwave pre-cooked samples after frying at 170 °C.

Figure 4. Acrylamide content of the control and microwave pre-cooked samples after frying at 190 °C.

Both the control and microwave pre-cooked samples showed a marked increase in acrylamide content as the frying temperature increased from 150 to 190 °C (Fig. 5). The effect of temperature appeared to be exponential. Reduction of the frying temperature from 190 °C to 170 °C and 150 °C decreased acrylamide formation in the control samples by 73% and 92%, respectively. Similar results were observed in the study of Pedreschi et al.,4 who reported an 88% reduction by lowering the frying temperature from 190 to 150 °C.

Figure 5. Effect of frying temperature on acrylamide formation in the control and microwave pre-cooked whole potato strips.

Surface

Acrylamide formation in the surface region was considerably reduced by microwave pre-cooking of the strips (Figs 2–4). Reduction of acrylamide in the surface of the microwave-pretreated samples may be attributed, along with reduced frying times, to the lower surface temperatures attained during frying of these samples (Figs 6–8). It was observed that the longer the microwave pretreatment time, the lower the surface temperature during frying, and hence less acrylamide formation in this region. This is believed to be brought about by the change in potato structure as a result of pre-cooking. Cooked potato tissue has been reported to be more permeable, presenting less resistance to mass transfer.3 This, in turn, may have resulted in an increase in the rate of diffusion of water from the interior to the surface. As more water is supplied from the interior, more of the energy transferred from the hot oil to the potato strip will be extracted to vaporize that water, and this will prevent the surface temperature from increasing. This finding is in agreement with that of Gökmen et al.,14 who showed that moisture evaporation during frying is an important barrier to internal energy increase. The fact that acrylamide content in the surface region further decreased with increasing microwave treatment time may actually be due to the increased degree of cooking, resulting in a more permeable structure allowing the transport of water at a higher rate.

Figure 6. Surface temperature profiles of potato strips during frying at 150 °C.

Figure 7. Surface temperature profiles of potato strips during frying at 170 °C.

Figure 8. Surface temperature profiles of potato strips during frying at 190 °C.

These findings also suggest that acrylamide reduction obtained by blanching prior to frying should not be attributed only to the removal of acrylamide precursors from the surface, but also to the lower temperatures attained at the surface during frying as a result of cooked potato tissue delivering more moisture from the interior.

Core

Even though the temperature of the core regions of all samples was slightly above 100 °C for the duration of frying experiments, acrylamide formation in the core region upon frying was found to increase slightly with increasing microwave treatment time (Figs 2–4). This finding may be attributed to faster drying of the interior of the microwave-treated samples, resulting in the favorable conditions for Maillard reaction (in terms of moisture content) to develop sooner during the frying process. Microwave pre-heating was reported to potentially lead to increased acrylamide levels in baked cut potato products due to the microwave removing moisture before oven baking.15

The fact that the core temperature remained just above 100 °C throughout all of the experiments indicates that although the water transport to the surface took place at a greater rate in the cooked potato strips, there was still enough water within the core region of these samples which prevented the internal energy increase.

CONCLUSION

The findings of the present study indicate that microwave pre-cooking was effective in reducing acrylamide levels in the surface region of French fries, where most of the acrylamide formation takes place. The reduction was a consequence of the combined effect of reduced frying time and surface temperature. Water transport rate from the interior during frying was shown to play an important role in limiting acrylamide formation in the surface region.

Microwave pre-cooking requires little time and since the reducing sugars and asparagine are retained within the strip, surface characteristics of the final product are not adversely affected. In fact, based on visual observation, microwave pre-cooked samples fried to the same degree of cooking appeared to have more acceptable color, again probably due to the more gentle heat treatment (time-temperature) they experienced during frying. Microwave pretreatment would also fit favorably into the continuous process line for industrial production of French fries.