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

Transcription

1 Journal of the Science of Food and Agriculture J Sci Food Agric 87: (27) Reduction of acrylamide formation in French fries by microwave pre-cooking of potato strips S Belgin Erdoǧdu, 1 Tunç Koray Palazoǧlu, 1 Vural Gökmen, 2 Hamide Z Şenyuva 3 and H İbrahim Ekiz1 1 Department of Food Engineering, University of Mersin, Çiftlikköy, Mersin, Turkey 2 Department of Food Engineering, Hacettepe University, 6532 Beytepe, Ankara, Turkey 3 Ankara Test and Analysis Laboratory, Scientific and Technical Research Council of Turkey, Ankara 633, Turkey 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 (1, 2, and 3 s at 85 W) samples were fried at 15, 17 and 19 C for predetermined times. Surface and temperatures of potato strips were acquired during frying, and acrylamide content in the and the regions were determined separately. The results showed that microwave application prior to frying resulted in a marked reduction of acrylamide level in the region, whereas a slight increase was noted for the 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 6% for frying at 15, 17, and 19 C, respectively, in comparison to the. 26 Society of Chemical Industry Keywords: acrylamide; French fries; frying; microwave pre-cooking 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 acrylamide levels in the final product. While pretreatment steps such as enzyme treatment, 2 soaking in water, 3 and blanching 4 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 19 C (3.5 min) to 17 C(5min)andto 15 C (7 min), respectively. Among several factors studied including potato cultivar, soaking in water, and frying oil type, Williams 1 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 1 mg kg 1, also indicating the strong dependence of acrylamide levels on frying time. 9 Claeys et al. 1 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-ling 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 characteristics (crust and color) of the already-cooked strips. Correspondence to: Tunç KorayPalazoǧlu, Department of Food Engineering, University of Mersin, Çiftlikköy, Mersin, Turkey koray palazoglu@mersin.edu.tr (Received 21 October 25; revised version received 11 January 26; accepted 16 August 26) Published online 31 October 26; DOI: 1.12/jsfa Society of Chemical Industry. J Sci Food Agric /26/$3.

2 SBErdoǧdu et al. This study aimed to reduce the frying time and hence acrylamide formation by microwave precooking of potato strips prior to frying. Microwave precooking 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 characteristics. Acrylamide formation in the and regions of potato strips were determined separately. Temperature profiles for the and 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 cm dimensions by using a French fry cutter. Pre-treatments Strips were pre-cooked in a microwave oven for three different time periods (1, 2 and 3 s) before frying. Three potato strips were placed horizontally in the microwave oven (MD 572, Arçelik, Turkey) operating at 85 W. Potato strips without microwave treatment were considered as the. Frying conditions Four strips (pre-cooked in microwave and ) 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 (15, 17 and 19 C). Frying times which provided a thoroughly cooked potato strip at that particular temperature of frying were employed during the experiments. To do this, and microwaved potato strips were sampled at different time intervals during frying and evaluated sensorily by us. The frying times used for the and 1, 2, and 3 s microwave-treated samples are given in Table 1 for three different frying temperatures. Experiments were conducted in parallel. Temperature measurement Temperature at the and 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 thermocouple was placed so that the tip of Surface thermocouple Core thermocouple Potato strip Fryer Frying time (s) Thermocouples Serial connection Data acquisition unit Figure 1. Schematic representation of experimental set-up used to measure the temperature in potato strips during frying. 134 J Sci Food Agric 87: (27) DOI: 1.12/jsfa

3 Reduction of acrylamide formation in French fries Table 1. Frying times (in minutes) employed during the experiments Frying temperature Microwave pretreatment time 15 C 17 C 19 C s s s s the thermocouple was flush with the. Location of the 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 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 2-channel multiplexer (Keithley, Model 27 DMM and Model 77, Cleveland, OH, USA). Moisture content analysis Moisture content of samples was determined by drying the samples to constant weight at 15 ± 1 C. 12 Acrylamide analysis Sample preparation for LC-MS analysis A and a sample were prepared from each potato strip to determine the acrylamide content (on wet basis) in the and the regions separately. A 2 mm layer from the (the sample) was removed from each strip using a knife, and the remaining portion of the strip was considered the 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 Gökmen 13 was used with minor modification. Finely ground sample ( 1g) was weighed into a 1 ml glass centrifuge tube with a screw cap. The sample was spiked with 13 C 3 -labeled acrylamide to confirm acrylamide. 5 µl Carrez 1 and 5µL Carrez 2 solution were added and the volume was adjusted to 1 ml with.2 mmol L 1 acetic acid. After vortexing for 2 min, the mixture was centrifuged at 1 rpm for 1 min at 5 Cto solidify oil. The clear supernatant was quantitatively transferred into a vial, avoiding the top oil layer. It was filtered through a.45 µm nylon syringe filter prior to LC-MS analysis. LC-APCI-MS analysis LC-APCI-MS analyses were performed using an Agilent 11 HPLC system (Waldbronn, Germany) consisting of a binary pump, an autosampler and a temperature-led column oven, coupled to an Agilent 11 MS detector equipped with atmospheric pressure chemical ionization (APCI) interface. The analytical separation was performed on an Inertsil ODS-3 column (25 4.6mm, 5µm) using the isocratic mixture of.1 mmol L 1 acetic acid in.2% aqueous solution of formic acid at a flow rate of.6ml 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 (N 2, 1 psig) flow of 4 L min 1, nebulizer pressure of 6 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 13 C 3 -labeled acrylamide for the quantification of acrylamide in the samples. RESULTS AND DISCUSSION At all three frying temperatures, microwave precooking resulted in lower acrylamide content in the whole potato strip in comparison to the. Acrylamide level of the microwave-treated samples was 36%, 41%, and 6% lower than that of the for 2, 2 and 3 s microwave times and frying temperatures of 15, 17, and 19 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 ( 5 ng g 1 )was detected upon microwave pre-cooking without any frying. Both the and microwave pre-cooked samples showed a marked increase in acrylamide content as the frying temperature increased from 15 to 19 C (Fig. 5). The effect of temperature appeared to be exponential. Reduction of the frying temperature from 19 C to 17 C and 15 C decreased acrylamide formation in the 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 19 to 15 C Figure 2. Acrylamide content of the and microwave pre-cooked samples after frying at 15 C. J Sci Food Agric 87: (27) 135 DOI: 1.12/jsfa

4 SBErdoǧdu et al Figure 3. Acrylamide content of the and microwave pre-cooked samples after frying at 17 C Figure 4. Acrylamide content of the and microwave pre-cooked samples after frying at 19 C s MW 2 s MW 3 s MW Frying temperature ( C) Figure 5. Effect of frying temperature on acrylamide formation in the and microwave pre-cooked whole potato strips. Surface Acrylamide formation in the region was considerably reduced by microwave pre-cooking of the strips (Figs 2 4). Reduction of acrylamide in the of the microwave-pretreated samples may be attributed, along with reduced frying times, to the lower temperatures attained during frying of these samples (Figs 6 8). It was observed that the longer the microwave pretreatment time, the lower the 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 s MW 2 s MW 3 s MW Figure 6. Surface temperature profiles of potato strips during frying at 15 C s MW 2 s MW 3 s MW Figure 7. Surface temperature profiles of potato strips during frying at 17 C s MW 2 s MW 3 s MW Figure 8. Surface temperature profiles of potato strips during frying at 19 C. resulted in an increase in the rate of diffusion of water from the interior to the. 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 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 region further decreased with increasing microwave treatment time may actually be due to the increased degree of cooking, 136 J Sci Food Agric 87: (27) DOI: 1.12/jsfa

5 Reduction of acrylamide formation in French fries resulting in a more permeable structure allowing the transport of water at a higher rate. 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, but also to the lower temperatures attained at the during frying as a result of cooked potato tissue delivering more moisture from the interior. Core Even though the temperature of the regions of all samples was slightly above 1 C for the duration of frying experiments, acrylamide formation in the 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 temperature remained just above 1 C throughout all of the experiments indicates that although the water transport to the took place at a greater rate in the cooked potato strips, there was still enough water within the 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 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 temperature. Water transport rate from the interior during frying was shown to play an important role in limiting acrylamide formation in the region. Microwave pre-cooking requires little time and since the reducing sugars and asparagine are retained within the strip, 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. REFERENCES 1 Williams JSE, Influence of variety and processing conditions on acrylamide levels in fried potato crisps. Food Chem 9: (25). 2 Zyzak D, Sanders R, Stokanovic M, Tallmadge D, Eberhart B and Ewald D, Acrylamide formation mechanism in heated foods. J Agric Food Chem 51: (23). 3 Grob K, Biedermann M, Biedermann-Brem S, Noti A, Imhof D, Amrein T, et al, French fries with less than 1 mg/kg acrylamide: a collaboration between cooks and analysts. Eur Food Res Technol 217: (23). 4 Pedreschi F, Kaack K and Granby K, Reduction of acrylamide formation in potato strips during frying. Lebensm Wiss Technol 37: (24). 5 Jung MY, Choi DS and Ju JW, A novel technique for limitation of acrylamide formation in fried and baked corn chips and in French fries. Food Chem Toxicol 68: (23). 6 Granda C, Moreira RG and Tichy SE, Reduction of acrylamide formation in potato chips by low-temperature vacuum frying. Food Eng Phys Prop 69: (24). 7 Pedreschi F, Moyano P, Kaack K and Granby K, Color changes and acrylamide formation in fried potato strips. Food Res Int 38:1 9 (25). 8 Haase NU, Matthäus B and Vosmann K, Minimierungsansätze zur Acrylamid-Bildung in pflanzlichen Lebensmittelnaufgezeigt am Beispiel von Kartoffelchips. Dtsch Lebensmitt Rundsch 99:87 9 (23). 9 EU, Opinion of the scientific committee on food on new findings regarding the presence of acrylamide in food, 3 July 22. [Online]. Available: out131 en.pdf. [31 January 25]. 1 Claeys WL, De Vleeschouwer K and Henrickx ME, Quantifying the formation of carcinogens during food processing: acrylamide. Trends in Food Science and Technology 16(5): (25). 11 Hallström B, Skjöldebrand C and Trägårdh C, Heat Transfer and Food Products. Elsevier Applied Science, London (1988). 12 AOAC, Official Methods of Analysis of the Association of Official Analytical Chemists (12th edn). Washington, DC (1975). 13 Şenyuva HZ and Gökmen V, Survey of acrylamide in Turkish foods by an in-house validated LC-MS method. Food Addit Contam 22:24 29 (25). 14 Gökmen V, Palazoǧlu TK and Şenyuva HZ, Relation between the acrylamide formation and time temperature history of and regions of French fries. Journal of Food Engineering 77(4): (26). 15 EU, Information on ways to lower the levels of acrylamide formed in food. Note of the meeting of experts on industrial contaminants in food. Acrylamide Workshop, 2 21 October 23. [Online]. Available: contaminants/acryl guidance.pdf. [31 January 25]. J Sci Food Agric 87: (27) 137 DOI: 1.12/jsfa