Effect of Microwave Blanching on Acrylamide Content and Quality Attributes of French Fries Sezin Tuta a, Koray Palazoglu a, Vural Gökmen b a Department of Food Engineering, Mersin,Turkiye(sezintuta@gmail.com, koray_palazoglu@yahoo.com) b Department of Food Engineering, Ankara, Turkiye (vgokmen@hacettepe.edu.tr) ABSTRACT Fried potato products are among the foods with the highest levels of acrylamide due to the high level of acrylamide precursors naturally present in potato. Therefore, reducing acrylamide level of fried potato products such as French fries is important. The objective of this study was to employ microwave blanching (instead of conventional blanching) during manufacturing of frozen par-fried potato strips and investigate its effect on acrylamide content and quality attributes (texture, color, oil content) of French fries. Microwave blanching was performed by immersing the potato strips in boiling water and microwaving for 1, 2, 3, and 4 min at 9 W prior to drying, par-frying, cooling and freezing steps. Conventional blanching consisted of two steps: 3 min at 8 C followed by 2 min at 65 C (control sample). Frying times at 18 C were determined by analyzing the samples sensorily for a thoroughly-cooked potato strip with a soft interior and a crispy outer crust. Frying times for the control and 1, 2, 3 and 4 min microwave blanched samples were 5, 4, 3.5, 3 and 2.5 min, respectively. Color measurement was done by digital image analysis, Texture Analyzer was used for texture measurement and oil content was determined using Soxhlet method. An LC- MS method was used for acrylamide analysis. Acrylamide reductions (in comparison to the control) for the samples microwave-blanched for 1, 2, 3, 4 min were 18%, 48%, 71% and 79%, respectively. The samples microwave-blanched for 3 min were the most comparable to the control in terms of quality attributes. The method modification also shortened both blanching and frying times. Therefore, the results of this study may be useful in industrial production of French fries. However, the effect of microwave power should be investigated and optimization of blanching time should be performed for industrial-scale microwave systems. Keywords: French fries; acrylamide; microwave; blanching; quality INTRODUCTION Acrylamide (AA) is a human neurotoxicant and rat tumorigen which is produced in starchy foods when cooked. In April 22, the Swedish National Food Administration highlighted its presence in different foodstuffs [1]. Several researchers have established that the main pathway of formation of acrylamide in foods is linked to the Maillard reaction, and in particular, the amino acid asparagine [2,3]. Fried potatoes are in the food category with probably the highest concentrations of acrylamide recorded so far [4]. Numerous studies were conducted for reducing acrylamide content of fried potato products. Enzyme treatment [5], blanching, soaking in water [6], modifying frying conditions are some of them. Frying time and temperature were reported to be important process parameters affecting acrylamide formation. Researchers reported that lowering frying temperature and time reduced acrylamide level significantly [7,8,9]. Blanching is a unit operation applied prior to freezing, canning, or drying in which fruits or vegetables are heated for the purpose of inactivating enzymes; modifying texture; preserving color, flavor, and nutritional value; and removing trapped air. In the process line for French fry production, the blanching operation is indispensable to the development of on appealing fry of good quality in terms of texture and color. It is through blanching that enzymes are inactivated, sugars leached out, starch gelatinized, and cell separation achieved [1]. Basically the manufacturing process of French fries consists of two blanching steps; short blanching at elevated temperatures (8 1 C) to inactivate enzymes (polyphenol oxidase and peroxidase) followed by a second longer blanching at lower temperatures (65 7 C) to control the reducing sugar content [11]. Hot water and steam are the most commonly used heating media for blanching in industry, but microwave blanching have also been studied [12]. Blanching by electromagnetic energy has been extensively studied. Electromagnetic energy presents advantages over conventional blanching by reduction of process times, energy and water usage and improvement of product quality [13,14,15,16,17]. Therefore, the objective of this study was to investigate the effect of microwave blanching on acrylamide content and quality attributes (texture, color, oil content) of French fries.
MATERIALS & METHODS Raw potatoes were washed, peeled and cut into strips of the desired dimensions (8.5 x 8.5 x 7 mm) by a French fry cutter. Microwave blanching was performed by immersing the potato strips in boiling water and microwaving for 1, 2, 3, and 4 min at 9 W. Blanched potato strips were then dried (5 min at 7 ºC), parfried (1 min at 18 ºC), cooled (2 min at 4 ºC) and frozen (2h at -3 ºC). Control samples were prepared by two step blanching procedure ( 3 min at 8 ºC and 2 min at 65 ºC) and all other steps were the same [11]. Samples were stored at -18 ºC until ready to use. Sensorial analysis (thoroughly cooked potato strip with a soft interior and a crispy outer crust) were done to determine frying times for final frying at 18 C (Table 1). Acrylamide and oil content analysis as well as texture and color measurements were performed on the final product. Color measurement was done by digital image analysis, Texture Analyzer was used for texture measurement and oil content was determined using Soxhlet method [18]. An LC-MS (liquid chromotography-mass spectrometry) method was used for acrylamide analysis. All samples were subjected to moisture content analysis after final frying. Moisture content was determined by drying the samples to constant weight at 15 ± 1 ºC [19]. Temperature data during blanching and frying were collected 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). The differences in color, texture, and oil content of microwave blanched and control samples were statistically evaluated by two-sided t-test (α =.5) using a statistical program (Statistica for Windows, Release 9, 21, Statsoft, Inc., Tulsa, OK). Table 1. Frying time of microwave blanched and control samples at 18 C Blanching time (min) Frying time (min) Control 5 1 4 2 3.5 3 3 4 2.5 RESULTS & DISCUSSION Temperature Profiles of Potatoes during Blanching In order to prevent potato strips from losing heat to the surrounding water, microwave-blanching was performed in hot water (~ 9 ºC). This resulted in a more uniform temperature distribution within the potato strip during microwave blanching in comparison to traditional blanching. Temperature of potato strip during microwave blanching increased faster than conventional method. Conventional and microwave blanching treatments generate different heating profiles (Figure 2). Heat transfer from the hot-water through the strip is the sole mechanism conventional blanching. However, during microwave-blanching in hot water in addition to heat transfer from the water to the strip surface, heat generation within the strip also takes place due to microwave energy. More uniform temperature distribution has been reported as a result of a combined microwave and hot water treatment [4,5].
a Temperature ( C) 1 8 6 4 2 water surface internal 2 4 6 8 1 12 14 16 Time (s) 12 b Temperature ( C) 1 8 6 4 2 water surface internal 5 1 15 2 25 Time (s) Figure 1. Temperature profile of potato strips during blanching a) conventional blanching b) microwave blanching. Acrylamide Content Acrylamide reduction was % 18, % 48, % 71 and % 79 for blanching 1, 2, 3, 4 min with microwave, respectively (Figure 2). No acrylamide was detected in frozen par-fried potato strips. Acrylamide, ng/g 7 6 5 4 3 2 1 Control MW1 MW2 MW3 MW4 Figure 2. Acrylamide content of control and microwave blanched samples.
In our study, the largest reduction in acrylamide formation was observed for the microwave blanching time of 4 minutes. It has been reported that microwave pre-treatment to potato strips reduce the frying time resulting in a decrease in acrylamide level. Acrylamide formation was reported to further decrease with increasing microwave application time [8,9]. 16 14 12 Temperature( ºC) 1 8 6 4 2-2 mw1 mw2 mw3 mw4 control 5 1 15 2 25 3-4 Time (s) Figure 3. Surface temperature profiles of potato strips during final frying. In this research in contrast to conventional method potato strips were heated volumetrically during microwave blanching resulting in a greater degree of cooking. As a result frying time was found to decrease in comparison to control (Table 1). It was observed that the longer the microwave blanching time, the lower the surface temperature during frying (Figure 3), and hence less acrylamide formation. Since potato tissue becomes more permeable upon microwave pre-treatment, less resistance to mass transfer takes place during frying. This results in a moisture transfer from the interior to the surface at a greater rate limiting temperature increase in the surface region. Moisture content of microwave-blanched samples were less than the control and it further decreased with microwave blanching time (Table 2). It was observed that potato strips interior temperature remained slightly above 1 ºC throughout all of the experiments. Table 2. Moisture content of final product Blanching time (min) Moisture Content (%) Control 55.1 ±.44 Mw_1 53.35 ± 5.66 Mw_2 51.12 ± 5.35 Mw_3 47.56 ± 3.99 Mw_4 46.16 ± 5.17
Quality Attributes L*,a*,b* values obtained through digital image analysis are shown in Table 3. L* values for each microwave blanching times, there was no significant difference compared with control (p>.5). But a*, b* values were significantly change at all applied microwave blanching times. Although these values were significantly different in comparison to control, when visually evaluated all samples appeared to be comparable in terms of color. Table 3. L*, a*,b* values of control and microwave blanched samples Blanching time (min) L* a* b* Control 83.68±1.16-17.29±.22 45.65±1.14 Mw_1 83.39±.59-14.3±.69 54.5±3.3 Mw_2 84.27±1.8-19.29±.76 57.11±2.4 Mw_3 84.13±.8-19.64±.87 6.56±2.55 Mw_4 85.83±1.11 19.66±.13 59.19±3.24 Oil contents of control and microwave blanched samples are presented in Table 4. There was significant difference only for 1 minute microwave blanched samples (p<.5). For this sample oil content of microwave-blanched samples were less than that of the control. Table 4. Oil content of control and microwave blanched samples Blanching time (min) Oil Content (%) Control 1.6±.8 Mw_1 7.85±.97 Mw_2 11.63±2.38 Mw_3 13.17±3.16 Mw_4 1.1±.56 Table 5 shows the statistical analysis results for cutting force (F max ) obtained with control and microwaveblanched samples. There was no significant difference only at applied microwave blanching for 3 minutes (p>.5). Table 5. F max values of control and microwave blanched samples Blanching time (min) F max (N) Control 3.52±.22 Mw_1 2.81±.15 Mw_2 2.6±.29 Mw_3 2.73±.5 Mw_4 1.91±.4 CONCLUSION In this study, frying time was reduced by microwave blanching while production of frozen potato strips, and the effect of this application on acrylamide level and quality attributes of French fries was determined. Acrylamide level was significantly reduced for 3 min (% 71) and 4 min (% 79). The method modification also shortened blanching times. Although, the samples microwave blanched for 4 min had the lowest acrylamide level, these samples appeared to be an undesirable structure upon microwave blanching. Hence, the samples that were microwave-blanched for 3 min were the most comparable to the control in terms of quality attributes of the final product. And its structure was better than that of 4 min microwave blanched samples. Therefore, the results of this study may be useful in industrial production of French fries. However, the effect of microwave power should be investigated and optimization of blanching time should be performed for industrial-scale microwave systems. REFERENCES [1] FAO, 27. Proposed draft code of practice for the reduction of acrylamide in food (N6-26). Joint Fao/Who Food Standards Programme Codex Committee OnContaminants in Foods. First Session, Beijing, China, 16 2 April. [2] Mottram, D. S., Wedzicha B. L., Dodson, A. T. 22. Acrylamide is formed in the Maillard Reaction. Nature, 419, 448.
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