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Effect of different home-cooking methods on acrylamide formation in pre-prepared croquettes
Journal of Food Composition and Analysis 56 (2017) 134–139
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Original research article
Effect of different home-cooking methods on acrylamide formation in pre-prepared croquettes _ Joanna Michalak* , Elzbieta Gujska, Marta Czarnowska-Kujawska, Fabian Nowak Department of Food Science, University of Warmia and Mazury in Olsztyn, Heweliusza 6, Olsztyn 10-719, Poland
A R T I C L E I N F O
Article history: Received 14 July 2016 Received in revised form 25 November 2016 Accepted 11 December 2016 Available online 12 December 2016 Keywords: Acrylamide Browning RP-HPLC Ready-to-eat products Pre-prepared croquettes Heating methods Microwave heating Food safety Food composition Food analysis
A B S T R A C T
This study compared the effects of different heating methods such as roasting, pan-frying, deep-frying and microwave treatment on the formation of acrylamide (AA) in ready-to-eat croquettes. The experiment was performed with ten commercially available pre-cooked flour-based croquettes with meat filling for home-cooking reheated according to the information on the labels. The AA content was determined by the reversed phase-high performance liquid chromatography (RP-HPLC) method coupled to a diode array detector (DAD). Browning development and water activity along with free asparagine and sugar content were also monitored. Before preparation, all products showed the lowest (190 mg/kg) acrylamide content. The highest acrylamide content was found when microwave heating was used. The mean AA content in all samples prepared in this way was significantly higher (420 mg/kg) than that of roasting (360 mg/kg), deep-frying (298 mg/kg) or pan-frying (285 mg/kg) (p < 0.05). The manner in which heat is transmitted to a food appears to have a significant impact on the rate of acrylamide formation. Among the domestic methods used, microwave treatment was more favourable for AA formation in products. The use of microwave heating for thermal processing of carbohydrate-rich food should be limited by consumers to prevent excessive acrylamide formation. © 2016 Elsevier Inc. All rights reserved.
1. Introduction Process contaminants are chemical substances which are absent in raw foods or raw materials used to make food products and are only formed when components within the food or raw materials undergo chemical changes during processing. The presence of these contaminants in processed foods cannot be entirely avoided. However, technological processes can be adjusted and optimized in order to reduce the levels of their formation. The impact of chemical contaminants on consumer health and wellbeing is often apparent only after many years of prolonged exposure at low levels (e.g. cancer). An example of such chemical contamination of food is acrylamide (AA) (Anese et al., 2013). It is believed that acrylamide has the potential to increase the risk of cancer. It is formed when foods containing asparagine (a natural amino acid) and sugars (either present naturally or added) are heated at temperatures higher than 120 C (Anese et al., 2013; Geng et al., 2008; Keramat et al., 2011). AA has been found in a wide range of commercially-processed and home-cooked foods, mainly potato- and grain-based products. High levels of this compound
* Corresponding author. E-mail address: [email protected] (J. Michalak). http://dx.doi.org/10.1016/j.jfca.2016.12.006 0889-1575/© 2016 Elsevier Inc. All rights reserved.
have also been found in many ready-to-eat products, which were heat-treated at home. As with commercially processed foods, acrylamide levels in home-prepared foods tend to increase with an increasing cooking time and temperature (Claeys et al., 2005; Dybing et al., 2005; Michalak et al., 2011; Skog et al., 2008; Stadler and Scholz, 2004). Countries and regions differ in food choices and cooking methods. In addition, dietary habits and the time devoted to home cooking are changing due to the increased availability of readymade foods and changes in lifestyles. The number of meals fully prepared at home is decreasing in favour of an increased consumption of pre-prepared and fast foods. A significant source of dietary acrylamide are foods cooked and prepared at home, by catering services or served in restaurants (Anese et al., 2009; Skog et al., 2008). Approximately 50% of acrylamide intake may be derived from such sources, but quantitative data are scarce (Dybing et al., 2005; Stadler and Scholz, 2004). Nevertheless, based on data obtained from different studies, it seems reasonable not to ignore the potential for reducing acrylamide exposure through better home cooking procedures. Not only where to buy the food, but also how to cook it in order to retain as much nutrition as possible should be considered. Therefore, the purpose of the study was to compare the effects
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of different home-cooking methods, such as roasting, pan-frying, deep-frying and microwave treatment on acrylamide formation in some ready-to-eat foods. The study will enable a better understanding of domestic cooking methods causing acrylamide formation in pre-prepared foods.
1.92 0.4
Aspa (mg/100 g)
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0.80 0.1
The experiment was performed with ten commerciallyavailable pre-cooked flour-based croquettes with meat filling for home-cooking (the same kind of products obtained from different producers). The purchased croquettes were similar in size (9 0.5 cm in length, 3 0.5 cm in width and 2 cm in thickness) and weight (120 10 g). The major ingredients of croquettes were wheat flour, water, meat (20–22% pork or beef), rapeseed oil, breadcrumbs (wheat flour, salt, yeast), soy protein, rice flour, salt, onion, egg material, sugar, spices, preservative potassium sorbate, raising agents sodium carbonate, yeast, maltodextrin and flavouring substances. All products were packed in modified atmosphere and were obtained from a local retailer. The samples were heated and analysed immediately after buying. The characteristics of the samples (before final preparation), as reported on labels and own study, are shown in Table 1. 2.2. Sample preparation
Data are expressed as mean values standard deviations (SDs) of ten independent samples analyzed in triplicate (n = 30). Asp: asparagine. a Content based on own studies.
Fructosea (g/100 g)
1.40 0.1 1.80 0.1 15.0 3.0 22.0 5.3 10.0 2.1
Fat (g/100 g) Carbohydrate (g/100 g) Protein (g/100 g) Energy (kJ/100 g)
1090 50 Croquettes before final preparation
Table 1 Croquette characteristics based on labels and own studies.
2.1. Food samples
Glucosea (g/100 g)
Sucrosea (g/100 g)
2. Materials and methods
Directly after purchasing, products were heated with domestic methods according to the information on the labels by pan-frying (5 min at 180 C), deep-frying (5 min at 180 C), roasting (10 min at 200 C) and microwaving (10 min at 200 C). The deep-frying of all croquettes was performed in a Philips HD 6158/55 Deep Fryer (Amsterdam, Netherlands) domestic fryer (200 g in 2 L of hot oil). The oil temperature during frying was monitored by immersing a thermocouple in the fryer. Pan-frying was performed in a saucepan using 100 g portions of products fried in 100 mL of hot oil. The temperature of the heating medium was measured using an automatic temperature control system for domestic appliances. Fresh sunflower oil was used in both experiments. The roasting of croquettes was performed in a domestic oven (Miele H 6560 BP, Gütersloh, Germany) with precise temperature control. Microwaving was performed in a Miele M637-45 ECR STAL microwave (Gütersloh, Germany) with automatic monitoring and control of internal temperature. The microwave heating of croquettes was performed using one of the combination mode programs (Medium high microwave power 77% with low grill power 23%). The temperature, operating power of the microwave and heating time were 200 C, 700 W and 10 min, respectively. After heating, the products were drained with a wire screen and cooled to 20 C at room temperature. All samples were ground and mixed in a blender (8000 FP664 Moulinex, Grupe SEB, Warsaw, Poland) to assure a homogeneous distribution of potential hotspots. 2.3. Reagents and chemicals The acrylamide standard, the standard sugars (fructose, glucose and sucrose) and the nor-leucine were of 99.8%, >99.0% and 99.9% purity, respectively. All chemicals were of HPLC analytical grade and were obtained from Sigma–Aldrich (St. Louis, MO, USA) and from Merck (Darmstadt, Germany). 2.4. Determination of free asparagine The content of free asparagine in the croquettes before final preparation was analysed according to Davies (2002). Asparagine
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was extracted with sulfosalicylic acid. A synthetic amino acid (norleucine) was used as an internal standard. The method involved a post-column derivative of free asparagine with ninhydrin. The coloured amino acid derivative was determined spectrophotometrically at 570 nm. The spectrophotometer used was a Nicolet Evolution 300 (Spectro-Lab, Warsaw, Poland). 2.5. Determination of sugars The sugars content in the croquettes before final preparation was determined. The analysis procedure for fructose, glucose and sucrose was adapted with modifications from Chávez-Servín et al. (2004). Samples (2 g) were dissolved in 20 mL of water and placed in a water bath and stirred at 60 C for 30 min. After cooling at room temperature, 0.5 mL Carrez I and Carrez II and 5 mL of acetonitrile were added. These reagents were used to precipitate the protein and non-sugar fraction. The solution was filled up to 25 mL with water in a volumetric flask and was then left for 1 or 2 h until the complete formation and precipitation of a protein clot. The resulting solution was filtered through filter paper and passed through a Sep-Pak C18 Plus cartridge (Waters Corporation, Milford, MA, USA) previously conditioned with 10 mL of methanol and 10 mL of water. This filtered extract was forced through a 0.45 mm nylon filter (Millipore from Merck) (Darmstadt, Germany) and injected into a high performance liquid chromatography (HPLC) system. A Shimadzu LC-10A series HPLC (Kyoto, Japan) equipped with a differential refractive index detector (RID model RID-20A, Shimadzu) was used. The column was a Luna NH2 5 mm 250 4.6 mm (Phenomenex, Torrance, CA, USA); the mobile phase was an isocratic mixture of acetonitrile–water (75:25, v/v); flow rate 1.0 mL/min; injection volume 50 mL; column temperature 40 C. Peaks were identified by comparing the retention times with those of sugar standards. Calibration curves for each sugar were prepared at seven levels, from 0.100 to 10.0 mg/mL for fructose, glucose and sucrose, all dissolved in water. 2.6. Determination of water activity (aw) The water activity (aw) of the croquettes before and after final preparation was determined, using a resistive electrolytic humidity measuring system (Novasina LabMaster AW, Novatron Scientific Ltd, Horsham, England). 2.7. Determination of acrylamide The acrylamide content in croquettes before and after final preparation was determined with the method developed by Michalak et al. (2013). The sample preparation procedure included the extraction of acrylamide with 80% methanol in water, defatting with hexane, freezing to 18 C for 24 h, centrifugation at 5000 rpm for 10 min (MPW-350R, MPW Med. Instruments, Warsaw, Poland) and clean-up by SPE with Oasis HLB 200 mg cartridges
(Waters, Milford, MA, USA). The determination of AA was performed using the reversed phase-high performance liquid chromatography (RP-HPLC) method. 2.8. Determination of browning The surface colour of the whole croquettes before and after final preparation was measured by a portable spectrophotometer MiniScan EZ (HunterLab, Germany) in units CIE L*a*b*, where L* represents lightness, positive a* = red, negative a* = green, positive b* = yellow and negative b* = blue. The colour was measured at three different positions on the product surface. The colour difference (DE*) was evaluated by comparing the results to the unheated samples. The colour difference (DE*) was calculated by the following formula: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DE ¼ L0 L 2 þ a0 a 2 þ b0 b 2
where L0 , a0 , b0 correspond to the CIE colour parameters of croquettes before final preparation, whereas L*, a*, and b* correspond to the CIE colour parameters of croquettes after final preparation. 2.9. Statistical analysis of data Ten commercial samples of the same kind of croquettes obtained from different producers were analyzed. One commercial sample was represented by two packs of the product from the same production batch. Each sample was analyzed in triplicate. The results were presented as mean values standard deviations (SDs) of ten independent samples, analyzed in triplicate (n = 30). The data were analysed using the Statistica 12.5 software package (StatSoft, Poland). Significant differences were calculated using Duncan’s Multiple range test and were considered statistically significant at the 5% level. Correlations among AA and water activity (aw) level and colour difference (DE*) were determined by Pearson’s correlation analysis at the p < 0.05 confidence level. 3. Results and discussion The composition of the ten products was very similar. No significant differences (p < 0.05) in the contents of sugars, asparagine, fat, protein and energy were found between the croquettes before final preparation (Table 1). Acrylamide was found in all samples. High levels of AA, were already found in prefabricated croquettes before final preparation (Table 2). The relatively high concentrations of acrylamide were due to the fact that these were food products already highly processed and readyto-eat just after heating at home. There is a lack of literature data on the acrylamide content in pre-prepared croquettes. Pedreschi et al. (2008) found the acrylamide content at the level of 370 mg/kg in pre-prepared French fries and 2075 mg/kg after final
Table 2 Acrylamide (AA) content, browning development (DE*) and water activity (aw) level in croquettes. Preparation methods temperature ( C)/time (min)
AA (mg/kg)
aw
Browning development (DE*)
Before final preparation Pan-frying 180/5 Deep-frying 180/5 Roasting 200/10 Microwaving 200/10
190 49d 285 34c 298 31c 360 27b 420 29a
0.979 0.006a 0.964 0.006b 0.963 0.005b 0.943 0.004d 0.952 0.004c
– 30.5 1.8b 31.7 1.1b 46.1 1.3a 16.3 0.8c
Data are expressed as mean values standard deviations (SDs) of ten independent samples analyzed in triplicate (n = 30). AA: acrylamide. aw: water activity. DE*: colour difference. a,b,c,d Mean values in columns with the same letter are not significantly different (p < 0.05).
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preparation. The Food Standards Agency (FSA) has carried out research including tests on pre-cooked, processed and packaged foods, plus chips that were prepared from potatoes. High AA levels were found in home-cooked foods and in processed foods. Acrylamide was found at high levels in “French fries and potato fried, fresh or pre-cooked, sold as ready-to-eat” (average levels of 239–368 mg/kg) and “other potato fried products” (average levels of 606 mg/kg). High AA levels were also found in “composite dishes containing cereals” (average level of 129 mg/kg) (EFSA, 2012; Ûea and Popa, 2015). Traditional Turkish foods also contained Musta high levels of acrylamide (Ölmez et al., 2008). Studies coming from China showed also high values of acrylamide content in ready-toeat Chinese foods (86.3–151 mg/kg) (Wang et al., 2013). In popular prepared Egyptian meals (i.e. fried onion, fried noodles and fried rice) the acrylamide levels reached 309, 311 and 205 mg/kg, respectively (Osman et al., 2015). In the present study, relatively high amounts of acrylamide were found in croquettes treated with pan-frying and deep-frying (285 and 298 mg/kg, respectively). No significant differences (p < 0.05) in AA content were found between the croquettes heated by both frying methods, probably because of using the same conditions (time and temperature) during preparation. However, both mean values of all fried samples were lower than that of roasting (360 mg/kg). The highest acrylamide levels for croquettes (420 mg/kg) were found when these foods were treated with microwave heating (Table 2). The statistical analysis showed significant differences (p < 0.05) in acrylamide content as a function of the heating method (frying, roasting and microwaving). This study showed that the manner in which heat is transmitted to a food appears to have an significant impact on the rate of acrylamide formation. According to the results, the acrylamide content was approx. 21% and 17% higher in the oven-roasted croquettes than in the pan-fried and deep-fried products, respectively. The high levels of acrylamide in oven-method products may be explained by the fact that acrylamide is mainly formed in the outer layer exposed to heat and heat transfer is more efficient during convection heating (oven) than contact heating (pan- and deep-frying). It is believed that the use of ovens with forced air circulation is responsible for faster drying. In consequence, the temperature of the product surface increased, enhancing acrylamide formation, which takes place mainly at the surface and in the near-surface regions. This is because the conditions in those parts of the product become favourable for acrylamide formation as a result of simultaneous drying (Anese et al., 2009; Claus et al., 2008). In addition, higher contents of acrylamide in the oven-roasted croquettes than in the fried were due to higher temperature and heating time. This main effect of temperature and time in heat processing methods on acrylamide formation has been reported by many researchers (Claeys et al., lu, 2008; 2005; Gökmen et al., 2006; Gökmen and Palazog Matthäus et al., 2004; Michalak et al., 2011; Yuan et al., 2007; Zhang et al., 2008). On the other hand, the contents of acrylamide in croquettes treated in a microwave were about 47%, 41% and 17% higher than in those deep-fried, pan-fried and oven-roasted with using the same conditions (time and temperature) of preparation, respectively. The acrylamide content in croquettes after microwave heating was 2–3 times higher than in products before final preparation. Microwave ovens are widely used as tools for rapid food heating in daily life. There are few studies in the literature on the effect of du et al., 2007; microwave heating on acrylamide formation (Erdog Tareke et al., 2002; Ye et al., 2011; Yuan et al., 2007). Some studies have shown that microwave heating might be more favourable to acrylamide formation than conventional heating methods, such as frying and roasting (Michalak et al., 2011; Takatsuki et al., 2004; Tareke et al., 2002; Ye et al., 2011; Yuan et al., 2007; Zhang et al.,
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2008; Zyzak et al., 2003). Microwaves offer a rapid temperature rise in the foods owing to their capacity to generate heat energy inside the food, without requiring any medium as a vehicle for heat transfer. Products with low thermal conductivity may quickly reach high temperatures, which does not occur in conventional heating. Microwave heating provides a favourable medium for the occurrence of acrylamide and probably affects the formation and kinetics of acrylamide distinguishingly due to its extraordinary heating style (Zhang at al., 2008). The authors suggest that AA in food is generated from pyrolytic fragments of asparagine and that this reaction is facilitated by concomitant pyrolysis of Maillardactive dicarbonyl and hydroxycarbonyl precursors (Rydberg et al., 2005). Meanwhile, according to Fernández et al. (2011) microwave heating favours the pyrolysis process. However, other researchers have found no (or not significant) acrylamide formation in foods under microwave heating (Anese et al., 2013; Barutcu et al., 2009; Burch, 2007). Some authors have also reported the possibility of reducing acrylamide in potato fries using microwaves for blanchdu et al., 2007). ing or pre-thawing of frozen potato strips (Erdog Therefore, short exposure to microwaves (blanching and prethawing) can be recommended to reduce the amount of acrylamide formation. In contrast to microwave blanching and pre-thawing, in microwave-reheated food products acrylamide is readily generated at high heating temperature and a higher microwave oven power level. Yuan et al. (2007) demonstrated that microwave power has a great effect on the formation of acrylamide; the greater the power is, the more acrylamide is generated. Thus, in microwave heating studies, contradictory results are found in the literature. Meanwhile, our studies suggest that the specific microwave effect might play a larger role in accelerating acrylamide formation than conventional heating. Therefore, due to high AA formation in foods during microwave heating in domestic ovens microwave roasting may not be recommended in food treatment. Moreover, further research is necessary to investigate the effects of microwave heating on food processes and the quality of different foods. Table 2 shows the water activity level in croquettes before final preparation and after different domestic preparation methods. In our study, all tested croquettes, both pre-prepared and after final preparation, had aw higher than 0.9, suggesting that this parameter was of limited relevance in the context of acrylamide formation. The highest decrease in water activity was observed in croquettes after roasting and microwave heating. However, roasting induced a higher decrease of water activity with (at the same time) a lower acrylamide content than the microwaved samples. There was no statistically significant correlation between acrylamide content and water activity level when all samples were examined together (unheated, conventionally-heated and microwaved samples) (results not shown). A statistical analysis indicated that acrylamide formation and water activity level were negatively correlated in the conventional home-processed (fried and roasted) croquettes (Table 3). The water activity level of the samples was also negatively correlated with the total colour difference (DE*) (Table 3). Table 3 Correlations between AA contents, browning development (DE*) and water activity (aw) in croquettes after different domestic preparation methods excluding the microwave heating.
AA DE*
DE*
aw
0.889*
0.780* 0.698*
AA: acrylamide. aw: water activity. DE*: colour difference. * Correlation coefficients statistically significant at p < 0.05.
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The effect of water activity on acrylamide formation has been widely studied and there are various complexities regarding the effect of water activity. Depending on the water activity range studied, positive or negative correlations between aw and acrylamide content have been observed. Some studies have shown that the faster reduction of the water content in the outer parts of the product, as a result of higher processing temperatures, favours acrylamide formation and results in higher amounts of AA in bread and French fries (Claus et al., 2008; Matthäus et al., 2004). However, Robert et al. (2005) found that acrylamide formation in an equimolar glucose/asparagine model system was not influenced much by water activity. The effect of water activity on acrylamide formation remains unclear, and sometimes contradictory, because the experimental procedures and the physical state of the reactants reported in the literature vary considerably. For example, acrylamide is not formed if water activity is not reduced to a value below 0.8 and maximum acrylamide formation was observed when water activity is about 0.4. Further reduction of water activity tends to decrease the amount of acrylamide (Delatour et al., 2004; Hoenicke and Gatermann, 2005; Mottram et al., 2002; Stadler et al., 2002). The main explanation for an optimum reaction rate at an intermediate aw is that the reactants are diluted at higher water activity, while at lower aw the mobility of reactants is limited, despite their presence in high concentrations (Bassama et al., 2011; Claeys et al., 2005; Lingnert et al., 2002). Clearly, further studies are needed to better understand the role of water in different foods in relation to acrylamide formation. The main pathway for AA formation in foods is the Maillard reaction. The Maillard reaction occurs on the food surface and leads to the production of desirable colour, flavours and aromas (Lingnert et al., 2002; Matthäus and Haase, 2014; Romani et al., 2009). Although surface browning is a typical phenomenon for baked foods, our studies suggest that microwaving might be more favourable to acrylamide formation than conventional heating. Croquette surface browning as a function of acrylamide formation was studied. A colorimeter was used to obtain a more objective assessment of surface colour as a function of heating methods. Table 2 shows AA contents in croquettes after all preparation methods and their total colour difference (DE*). Higher AA content was observed under microwave heating with a lower total colour difference than in all conventional heating methods. Conventional home-processed (fried and roasted) croquettes had varying degrees of browning with corresponding acrylamide concentrations. A statistical analysis indicated that acrylamide concentration showed a significant linear correlation (r = 0.889) with the colour of croquettes represented by the total colour difference (DE*), treated in different conventional heating methods (Table 3). As the samples became darker during frying and roasting, the L* value diminished and a* value increased (results not shown), which resulted in a general increase in the total colour difference (DE*). There was no statistically significant correlation between acrylamide content and total colour difference (DE*) when all samples were examined together (unheated, conventionally-heated and microwaved samples) (results not shown). Colour measurements indicated that the degree of surface browning determination appeared to be a good method for estimating acrylamide formation during conventional heating. For microwave heating, the degree of surface browning should not be used as a method for estimating acrylamide formation. Information on the relationship between browning and acrylamide formation varies widely. Some researchers have reported that high-temperature, long-time treatment of foods is responsible for a great increase in AA levels in foods, without causing significant changes in the colour or texture parameters (Anese et al., 2009). Gökmen and Şenyuva (2006) and Gökmen et al. (2007) found that changes in CIE redness parameter “a*”
during frying were somewhat similar to that of acrylamide concentration in potato chips. However, these two variables could not be linearly correlated with each other. It was also found that acrylamide concentrations were lower in darker potato chips and, since colour continues to develop during the Maillard reaction and acrylamide may start degrading, browning alone should not be used as the sole predictor of acrylamide formation (Taubert et al., 2004). On the other hand, many studies have found a good correlation between the acrylamide content of fried, baking and roasting foods and their colour (Ahrné et al., 2007; Capuano et al., 2009; Mestdagh et al., 2008). Surdyk et al. (2004) reported that more than 99% of acrylamide found in bread after roasting was formed in the crust. A strong correlation between crust colour and acrylamide concentration in crust, when conventional roasting was used, was also reported by Surdyk et al. (2004). In general, it was found that although the colour formation and melanoidin structure are largely unknown, the colour of foods may be used as an indicator of acrylamide formation during food processing. Many researchers have associated the colour with AA formation in fried potatoes (Pedreschi et al., 2006), crisp bread (Mustafa et al., 2005), coffee (Gökmen and Şenyuva, 2006) and wheat flour (Gökmen and Şenyuva, 2006). According to some authors, in contrast to the convection oven, the air in microwaves is not heated, so it cannot brown the surface of most foods. The resulting surface of microwave cooked products remains moist and the development of the expected flavours, colours and AA formation is limited (Anese et al., 2013; Barutcu et al., 2009). This raises questions of the cause of acrylamide formation and whether acrylamide is similarly formed during the microwave heating process as in conventional heating. In our opinion, during cooking in the conventional oven, heat is transferred into the product by its surface. Unlike the air in conventional ovens, the air in microwaves is not heated, so it cannot brown the surface of most foods. In microwave cooking, the conversion of microwave energy into heat is dispersed through the products. Microwave cooking causes less acrylamide formation on the surface region of heated foods where, in conventional heating, most of the acrylamide formation takes place. Most likely, microwave heating can affect acrylamide formation throughout the entire product and thus give a larger total amount of the compound in the final product. Similar du et al. (2007). observations were made by Erdog 4. Conclusions Acrylamide is present in ready–to-eat foods such as croquettes even before the heat treatment process at home. Heating of preprepared food at home (using any method) cannot avoid further AA formation. During heating, when the temperature and duration of cooking are arbitrary, extremely high acrylamide contents in these foods are possible. Therefore, the heat treatment parameters of the products during processing such as frying, roasting or microwaving at home should be controlled and reduced as much as possible. Consumers should follow the rule to cook to a golden colour instead of brown. The food industry is aware of the acrylamide problem and manufacturers are responding accordingly, although home preparation is still an important issue. The manner in which heat is transmitted to a food appears to have an impact on the level of acrylamide formation. In comparison with conventional heating, microwave treatment is more favourable for AA formation in home-cooked products. Microwave heating results in a lighter colour of samples and higher acrylamide formation in the croquettes than conventional heating. The mechanism of these reactions is not clear and further studies are necessary to investigate the formation of acrylamide during microwave heating. However, research suggests that the use of microwave heating for thermal processing of carbohydrate-rich
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food should be limited by consumers in order to prevent excessive acrylamide formation in heat-treated food at home. Acknowledgement This study was financed by the Polish Ministry of Education and Science under research Project No. N N 312 227236. References Ahrné, L., Andersson, C.-G., Floberg, P., Rosén, J., Lingnert, H., 2007. Effect of crust temperature and water content on acrylamide formation during baking of white bread: steam and falling temperature baking. LWT Food Sci. Technol. 40, 1708–1715. Anese, M., Bortolomeazzi, R., Manzocco, L., Manzano, M., Giusto, C., Nicoli, M.C., 2009. Effect of chemical and biological dipping on acrylamide formation and sensory properties in deep-fried potatoes. Food Res. Int. 42, 142–147. Anese, M., Manzocco, L., Calligaris, S., Nicoli, M.C., 2013. Industrially applicable strategies for mitigating acrylamide, furan: and 5-hydroxymethylfurfural in food. J. Agric. Food Chem. 61, 10209–10214. Barutcu, I., Sahin, S., Sumnu, G., 2009. Acrylamide formation in different batter formulations during microwave frying. LWT Food Sci. Technol. 42, 17–22. Bassama, J., Brat, P., Bohuon, P., Hocine, B., Boulanger, R., Günata, Z., 2011. Acrylamide kinetic in plantain during heating process: precursors and effect of water activity. Food Res. Int. 44, 1452–1458. Burch, R., 2007. Examination of the Effect of Domestic Cooking on Acrylamide Levels in FoodFood Standards Agency, London, UK. . Retrieved March 24, 2016 from: http://www.foodbase.org.uk/results.php?f_report_id=46. Capuano, E., Ferrigno, A., Acampa, I., Serpen, A., Açar, Ö.Ç., Gökmen, V., Fogliano, V., 2009. Effect of flour type on Maillard reaction and acrylamide formation during toasting of bread crisp model systems and mitigation strategies. Food Res. Int. 42, 1295–1302. Chávez-Servín, J.L., Castellote, A.I., López-Sabater, M.C., 2004. Analysis of mono- and disaccharides in milk-based formulae by high-performance liquid chromatography with refractive index detection. J. Chromatogr. A 1043, 211–215. Claeys, W.L., De Vleeschouwer, K., Hendrickx, M.E., 2005. Quantifying the formation of carcinogens during food processing: acrylamide. Trends Food Sci. Technol. 16, 181–193. Claus, A., Carle, R., Schieber, A., 2008. Acrylamide in cereal products: a review. J. Cereal Sci. 47, 118–133. Davies, M., 2002. The Biochrom Hand Book of Amino Acid Analysis. Biochrom Ltd, Cambridge, UK. Delatour, T., Périsset, A., Goldmann, T., Riedeker, S., Stadler, R.H., 2004. Improved sample preparation to determine acrylamide in difficult matrixes such as chocolate powder, cocoa, and coffee by liquid chromatography tandem mass spectrometry. J. Agric. Food Chem. 52, 4625–4631. Dybing, E., Farmer, P.B., Andersen, M., Fennell, T.R., Lalljie, S.P.D., Müller, D.J.G., Olin, S., Petersen, B.J., Schlatter, J., Scholz, G., Scimeca, J.A., Slimani, N., Törnqvist, M., Tuijtelaars, S., Verger, P., 2005. Human exposure and internal dose assessments of acrylamide in food. Food Chem. Toxicol. 43, 365–410. EFSA (European Food Safety Authority), 2012. Update on acrylamide levels in food from monitoring years 2007 to 2010. EFSA J. 10 (10), 38. doi:http://dx.doi.org/ 10.2903/j.efsa.2012.2938 2938. Retrieved March 29, 2016 from: www.efsa. europa.eu/efsajournal.. du, S.B., Palazoglu, T.K., Gökmen, V., Şenyuva, H.Z., Ekiz, I., 2007. Reduction of Erdog acrylamide formation in French fries by microwave pre-cooking of potato strips. J. Sci. Food Agric. 87 (1), 133–137. Fernández, Y., Arenillas, A., Menéndez Á, J., 2011. Microwave heating applied to pyrolysis. Advances in Induction and Microwave Heating of Mineral and Organic Material. Chapter in a Book. Collegium, Viena, Austria, pp. 723–752 ISBN 978– 953-307–522-8. Gökmen, V., Şenyuva, H.Z., 2006. Study of colour and acrylamide formation in coffee: wheat flour and potato chips during heating. Food Chem. 99, 238–243. lu, T.K., 2008. Acrylamide formation in foods during thermal Gökmen, V., Palazog processing with a focus on frying. Food Bioprocess Technol. 1, 35–42. lu, T.K., Şenyuva, H.Z., 2006. Relation between the acrylamide Gökmen, V., Palazog formation and time-temperature history of surface and core regions of French fries. J. Food Eng. 77, 972–976. Gökmen, V., Akbudak, B., Serpen, A., Açar, J., Eris, A., 2007. Effects of controlled atmosphere storage and low-dose irradiation on potato tuber components affecting acrylamide and color formations upon frying. Eur. Food Res. Technol. 224, 681–687. Geng, Z., Jiang, R., Chen, M., 2008. Determination of acrylamide in starch-based foods by ion-exclusion liquid chromatography. J. Food Compos. Anal. 21, 178– 182. Hoenicke, K., Gatermann, R., 2005. Studies on the stability of acrylamide in food during storage. J. AOAC Int. 88, 268–273.
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Contents lists available at ScienceDirect
Journal of Food Composition and Analysis journal homepage: www.elsevier.com/locate/jfca
Original research article
Effect of different home-cooking methods on acrylamide formation in pre-prepared croquettes _ Joanna Michalak* , Elzbieta Gujska, Marta Czarnowska-Kujawska, Fabian Nowak Department of Food Science, University of Warmia and Mazury in Olsztyn, Heweliusza 6, Olsztyn 10-719, Poland
A R T I C L E I N F O
Article history: Received 14 July 2016 Received in revised form 25 November 2016 Accepted 11 December 2016 Available online 12 December 2016 Keywords: Acrylamide Browning RP-HPLC Ready-to-eat products Pre-prepared croquettes Heating methods Microwave heating Food safety Food composition Food analysis
A B S T R A C T
This study compared the effects of different heating methods such as roasting, pan-frying, deep-frying and microwave treatment on the formation of acrylamide (AA) in ready-to-eat croquettes. The experiment was performed with ten commercially available pre-cooked flour-based croquettes with meat filling for home-cooking reheated according to the information on the labels. The AA content was determined by the reversed phase-high performance liquid chromatography (RP-HPLC) method coupled to a diode array detector (DAD). Browning development and water activity along with free asparagine and sugar content were also monitored. Before preparation, all products showed the lowest (190 mg/kg) acrylamide content. The highest acrylamide content was found when microwave heating was used. The mean AA content in all samples prepared in this way was significantly higher (420 mg/kg) than that of roasting (360 mg/kg), deep-frying (298 mg/kg) or pan-frying (285 mg/kg) (p < 0.05). The manner in which heat is transmitted to a food appears to have a significant impact on the rate of acrylamide formation. Among the domestic methods used, microwave treatment was more favourable for AA formation in products. The use of microwave heating for thermal processing of carbohydrate-rich food should be limited by consumers to prevent excessive acrylamide formation. © 2016 Elsevier Inc. All rights reserved.
1. Introduction Process contaminants are chemical substances which are absent in raw foods or raw materials used to make food products and are only formed when components within the food or raw materials undergo chemical changes during processing. The presence of these contaminants in processed foods cannot be entirely avoided. However, technological processes can be adjusted and optimized in order to reduce the levels of their formation. The impact of chemical contaminants on consumer health and wellbeing is often apparent only after many years of prolonged exposure at low levels (e.g. cancer). An example of such chemical contamination of food is acrylamide (AA) (Anese et al., 2013). It is believed that acrylamide has the potential to increase the risk of cancer. It is formed when foods containing asparagine (a natural amino acid) and sugars (either present naturally or added) are heated at temperatures higher than 120 C (Anese et al., 2013; Geng et al., 2008; Keramat et al., 2011). AA has been found in a wide range of commercially-processed and home-cooked foods, mainly potato- and grain-based products. High levels of this compound
* Corresponding author. E-mail address: [email protected] (J. Michalak). http://dx.doi.org/10.1016/j.jfca.2016.12.006 0889-1575/© 2016 Elsevier Inc. All rights reserved.
have also been found in many ready-to-eat products, which were heat-treated at home. As with commercially processed foods, acrylamide levels in home-prepared foods tend to increase with an increasing cooking time and temperature (Claeys et al., 2005; Dybing et al., 2005; Michalak et al., 2011; Skog et al., 2008; Stadler and Scholz, 2004). Countries and regions differ in food choices and cooking methods. In addition, dietary habits and the time devoted to home cooking are changing due to the increased availability of readymade foods and changes in lifestyles. The number of meals fully prepared at home is decreasing in favour of an increased consumption of pre-prepared and fast foods. A significant source of dietary acrylamide are foods cooked and prepared at home, by catering services or served in restaurants (Anese et al., 2009; Skog et al., 2008). Approximately 50% of acrylamide intake may be derived from such sources, but quantitative data are scarce (Dybing et al., 2005; Stadler and Scholz, 2004). Nevertheless, based on data obtained from different studies, it seems reasonable not to ignore the potential for reducing acrylamide exposure through better home cooking procedures. Not only where to buy the food, but also how to cook it in order to retain as much nutrition as possible should be considered. Therefore, the purpose of the study was to compare the effects
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of different home-cooking methods, such as roasting, pan-frying, deep-frying and microwave treatment on acrylamide formation in some ready-to-eat foods. The study will enable a better understanding of domestic cooking methods causing acrylamide formation in pre-prepared foods.
1.92 0.4
Aspa (mg/100 g)
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0.80 0.1
The experiment was performed with ten commerciallyavailable pre-cooked flour-based croquettes with meat filling for home-cooking (the same kind of products obtained from different producers). The purchased croquettes were similar in size (9 0.5 cm in length, 3 0.5 cm in width and 2 cm in thickness) and weight (120 10 g). The major ingredients of croquettes were wheat flour, water, meat (20–22% pork or beef), rapeseed oil, breadcrumbs (wheat flour, salt, yeast), soy protein, rice flour, salt, onion, egg material, sugar, spices, preservative potassium sorbate, raising agents sodium carbonate, yeast, maltodextrin and flavouring substances. All products were packed in modified atmosphere and were obtained from a local retailer. The samples were heated and analysed immediately after buying. The characteristics of the samples (before final preparation), as reported on labels and own study, are shown in Table 1. 2.2. Sample preparation
Data are expressed as mean values standard deviations (SDs) of ten independent samples analyzed in triplicate (n = 30). Asp: asparagine. a Content based on own studies.
Fructosea (g/100 g)
1.40 0.1 1.80 0.1 15.0 3.0 22.0 5.3 10.0 2.1
Fat (g/100 g) Carbohydrate (g/100 g) Protein (g/100 g) Energy (kJ/100 g)
1090 50 Croquettes before final preparation
Table 1 Croquette characteristics based on labels and own studies.
2.1. Food samples
Glucosea (g/100 g)
Sucrosea (g/100 g)
2. Materials and methods
Directly after purchasing, products were heated with domestic methods according to the information on the labels by pan-frying (5 min at 180 C), deep-frying (5 min at 180 C), roasting (10 min at 200 C) and microwaving (10 min at 200 C). The deep-frying of all croquettes was performed in a Philips HD 6158/55 Deep Fryer (Amsterdam, Netherlands) domestic fryer (200 g in 2 L of hot oil). The oil temperature during frying was monitored by immersing a thermocouple in the fryer. Pan-frying was performed in a saucepan using 100 g portions of products fried in 100 mL of hot oil. The temperature of the heating medium was measured using an automatic temperature control system for domestic appliances. Fresh sunflower oil was used in both experiments. The roasting of croquettes was performed in a domestic oven (Miele H 6560 BP, Gütersloh, Germany) with precise temperature control. Microwaving was performed in a Miele M637-45 ECR STAL microwave (Gütersloh, Germany) with automatic monitoring and control of internal temperature. The microwave heating of croquettes was performed using one of the combination mode programs (Medium high microwave power 77% with low grill power 23%). The temperature, operating power of the microwave and heating time were 200 C, 700 W and 10 min, respectively. After heating, the products were drained with a wire screen and cooled to 20 C at room temperature. All samples were ground and mixed in a blender (8000 FP664 Moulinex, Grupe SEB, Warsaw, Poland) to assure a homogeneous distribution of potential hotspots. 2.3. Reagents and chemicals The acrylamide standard, the standard sugars (fructose, glucose and sucrose) and the nor-leucine were of 99.8%, >99.0% and 99.9% purity, respectively. All chemicals were of HPLC analytical grade and were obtained from Sigma–Aldrich (St. Louis, MO, USA) and from Merck (Darmstadt, Germany). 2.4. Determination of free asparagine The content of free asparagine in the croquettes before final preparation was analysed according to Davies (2002). Asparagine
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was extracted with sulfosalicylic acid. A synthetic amino acid (norleucine) was used as an internal standard. The method involved a post-column derivative of free asparagine with ninhydrin. The coloured amino acid derivative was determined spectrophotometrically at 570 nm. The spectrophotometer used was a Nicolet Evolution 300 (Spectro-Lab, Warsaw, Poland). 2.5. Determination of sugars The sugars content in the croquettes before final preparation was determined. The analysis procedure for fructose, glucose and sucrose was adapted with modifications from Chávez-Servín et al. (2004). Samples (2 g) were dissolved in 20 mL of water and placed in a water bath and stirred at 60 C for 30 min. After cooling at room temperature, 0.5 mL Carrez I and Carrez II and 5 mL of acetonitrile were added. These reagents were used to precipitate the protein and non-sugar fraction. The solution was filled up to 25 mL with water in a volumetric flask and was then left for 1 or 2 h until the complete formation and precipitation of a protein clot. The resulting solution was filtered through filter paper and passed through a Sep-Pak C18 Plus cartridge (Waters Corporation, Milford, MA, USA) previously conditioned with 10 mL of methanol and 10 mL of water. This filtered extract was forced through a 0.45 mm nylon filter (Millipore from Merck) (Darmstadt, Germany) and injected into a high performance liquid chromatography (HPLC) system. A Shimadzu LC-10A series HPLC (Kyoto, Japan) equipped with a differential refractive index detector (RID model RID-20A, Shimadzu) was used. The column was a Luna NH2 5 mm 250 4.6 mm (Phenomenex, Torrance, CA, USA); the mobile phase was an isocratic mixture of acetonitrile–water (75:25, v/v); flow rate 1.0 mL/min; injection volume 50 mL; column temperature 40 C. Peaks were identified by comparing the retention times with those of sugar standards. Calibration curves for each sugar were prepared at seven levels, from 0.100 to 10.0 mg/mL for fructose, glucose and sucrose, all dissolved in water. 2.6. Determination of water activity (aw) The water activity (aw) of the croquettes before and after final preparation was determined, using a resistive electrolytic humidity measuring system (Novasina LabMaster AW, Novatron Scientific Ltd, Horsham, England). 2.7. Determination of acrylamide The acrylamide content in croquettes before and after final preparation was determined with the method developed by Michalak et al. (2013). The sample preparation procedure included the extraction of acrylamide with 80% methanol in water, defatting with hexane, freezing to 18 C for 24 h, centrifugation at 5000 rpm for 10 min (MPW-350R, MPW Med. Instruments, Warsaw, Poland) and clean-up by SPE with Oasis HLB 200 mg cartridges
(Waters, Milford, MA, USA). The determination of AA was performed using the reversed phase-high performance liquid chromatography (RP-HPLC) method. 2.8. Determination of browning The surface colour of the whole croquettes before and after final preparation was measured by a portable spectrophotometer MiniScan EZ (HunterLab, Germany) in units CIE L*a*b*, where L* represents lightness, positive a* = red, negative a* = green, positive b* = yellow and negative b* = blue. The colour was measured at three different positions on the product surface. The colour difference (DE*) was evaluated by comparing the results to the unheated samples. The colour difference (DE*) was calculated by the following formula: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DE ¼ L0 L 2 þ a0 a 2 þ b0 b 2
where L0 , a0 , b0 correspond to the CIE colour parameters of croquettes before final preparation, whereas L*, a*, and b* correspond to the CIE colour parameters of croquettes after final preparation. 2.9. Statistical analysis of data Ten commercial samples of the same kind of croquettes obtained from different producers were analyzed. One commercial sample was represented by two packs of the product from the same production batch. Each sample was analyzed in triplicate. The results were presented as mean values standard deviations (SDs) of ten independent samples, analyzed in triplicate (n = 30). The data were analysed using the Statistica 12.5 software package (StatSoft, Poland). Significant differences were calculated using Duncan’s Multiple range test and were considered statistically significant at the 5% level. Correlations among AA and water activity (aw) level and colour difference (DE*) were determined by Pearson’s correlation analysis at the p < 0.05 confidence level. 3. Results and discussion The composition of the ten products was very similar. No significant differences (p < 0.05) in the contents of sugars, asparagine, fat, protein and energy were found between the croquettes before final preparation (Table 1). Acrylamide was found in all samples. High levels of AA, were already found in prefabricated croquettes before final preparation (Table 2). The relatively high concentrations of acrylamide were due to the fact that these were food products already highly processed and readyto-eat just after heating at home. There is a lack of literature data on the acrylamide content in pre-prepared croquettes. Pedreschi et al. (2008) found the acrylamide content at the level of 370 mg/kg in pre-prepared French fries and 2075 mg/kg after final
Table 2 Acrylamide (AA) content, browning development (DE*) and water activity (aw) level in croquettes. Preparation methods temperature ( C)/time (min)
AA (mg/kg)
aw
Browning development (DE*)
Before final preparation Pan-frying 180/5 Deep-frying 180/5 Roasting 200/10 Microwaving 200/10
190 49d 285 34c 298 31c 360 27b 420 29a
0.979 0.006a 0.964 0.006b 0.963 0.005b 0.943 0.004d 0.952 0.004c
– 30.5 1.8b 31.7 1.1b 46.1 1.3a 16.3 0.8c
Data are expressed as mean values standard deviations (SDs) of ten independent samples analyzed in triplicate (n = 30). AA: acrylamide. aw: water activity. DE*: colour difference. a,b,c,d Mean values in columns with the same letter are not significantly different (p < 0.05).
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preparation. The Food Standards Agency (FSA) has carried out research including tests on pre-cooked, processed and packaged foods, plus chips that were prepared from potatoes. High AA levels were found in home-cooked foods and in processed foods. Acrylamide was found at high levels in “French fries and potato fried, fresh or pre-cooked, sold as ready-to-eat” (average levels of 239–368 mg/kg) and “other potato fried products” (average levels of 606 mg/kg). High AA levels were also found in “composite dishes containing cereals” (average level of 129 mg/kg) (EFSA, 2012; Ûea and Popa, 2015). Traditional Turkish foods also contained Musta high levels of acrylamide (Ölmez et al., 2008). Studies coming from China showed also high values of acrylamide content in ready-toeat Chinese foods (86.3–151 mg/kg) (Wang et al., 2013). In popular prepared Egyptian meals (i.e. fried onion, fried noodles and fried rice) the acrylamide levels reached 309, 311 and 205 mg/kg, respectively (Osman et al., 2015). In the present study, relatively high amounts of acrylamide were found in croquettes treated with pan-frying and deep-frying (285 and 298 mg/kg, respectively). No significant differences (p < 0.05) in AA content were found between the croquettes heated by both frying methods, probably because of using the same conditions (time and temperature) during preparation. However, both mean values of all fried samples were lower than that of roasting (360 mg/kg). The highest acrylamide levels for croquettes (420 mg/kg) were found when these foods were treated with microwave heating (Table 2). The statistical analysis showed significant differences (p < 0.05) in acrylamide content as a function of the heating method (frying, roasting and microwaving). This study showed that the manner in which heat is transmitted to a food appears to have an significant impact on the rate of acrylamide formation. According to the results, the acrylamide content was approx. 21% and 17% higher in the oven-roasted croquettes than in the pan-fried and deep-fried products, respectively. The high levels of acrylamide in oven-method products may be explained by the fact that acrylamide is mainly formed in the outer layer exposed to heat and heat transfer is more efficient during convection heating (oven) than contact heating (pan- and deep-frying). It is believed that the use of ovens with forced air circulation is responsible for faster drying. In consequence, the temperature of the product surface increased, enhancing acrylamide formation, which takes place mainly at the surface and in the near-surface regions. This is because the conditions in those parts of the product become favourable for acrylamide formation as a result of simultaneous drying (Anese et al., 2009; Claus et al., 2008). In addition, higher contents of acrylamide in the oven-roasted croquettes than in the fried were due to higher temperature and heating time. This main effect of temperature and time in heat processing methods on acrylamide formation has been reported by many researchers (Claeys et al., lu, 2008; 2005; Gökmen et al., 2006; Gökmen and Palazog Matthäus et al., 2004; Michalak et al., 2011; Yuan et al., 2007; Zhang et al., 2008). On the other hand, the contents of acrylamide in croquettes treated in a microwave were about 47%, 41% and 17% higher than in those deep-fried, pan-fried and oven-roasted with using the same conditions (time and temperature) of preparation, respectively. The acrylamide content in croquettes after microwave heating was 2–3 times higher than in products before final preparation. Microwave ovens are widely used as tools for rapid food heating in daily life. There are few studies in the literature on the effect of du et al., 2007; microwave heating on acrylamide formation (Erdog Tareke et al., 2002; Ye et al., 2011; Yuan et al., 2007). Some studies have shown that microwave heating might be more favourable to acrylamide formation than conventional heating methods, such as frying and roasting (Michalak et al., 2011; Takatsuki et al., 2004; Tareke et al., 2002; Ye et al., 2011; Yuan et al., 2007; Zhang et al.,
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2008; Zyzak et al., 2003). Microwaves offer a rapid temperature rise in the foods owing to their capacity to generate heat energy inside the food, without requiring any medium as a vehicle for heat transfer. Products with low thermal conductivity may quickly reach high temperatures, which does not occur in conventional heating. Microwave heating provides a favourable medium for the occurrence of acrylamide and probably affects the formation and kinetics of acrylamide distinguishingly due to its extraordinary heating style (Zhang at al., 2008). The authors suggest that AA in food is generated from pyrolytic fragments of asparagine and that this reaction is facilitated by concomitant pyrolysis of Maillardactive dicarbonyl and hydroxycarbonyl precursors (Rydberg et al., 2005). Meanwhile, according to Fernández et al. (2011) microwave heating favours the pyrolysis process. However, other researchers have found no (or not significant) acrylamide formation in foods under microwave heating (Anese et al., 2013; Barutcu et al., 2009; Burch, 2007). Some authors have also reported the possibility of reducing acrylamide in potato fries using microwaves for blanchdu et al., 2007). ing or pre-thawing of frozen potato strips (Erdog Therefore, short exposure to microwaves (blanching and prethawing) can be recommended to reduce the amount of acrylamide formation. In contrast to microwave blanching and pre-thawing, in microwave-reheated food products acrylamide is readily generated at high heating temperature and a higher microwave oven power level. Yuan et al. (2007) demonstrated that microwave power has a great effect on the formation of acrylamide; the greater the power is, the more acrylamide is generated. Thus, in microwave heating studies, contradictory results are found in the literature. Meanwhile, our studies suggest that the specific microwave effect might play a larger role in accelerating acrylamide formation than conventional heating. Therefore, due to high AA formation in foods during microwave heating in domestic ovens microwave roasting may not be recommended in food treatment. Moreover, further research is necessary to investigate the effects of microwave heating on food processes and the quality of different foods. Table 2 shows the water activity level in croquettes before final preparation and after different domestic preparation methods. In our study, all tested croquettes, both pre-prepared and after final preparation, had aw higher than 0.9, suggesting that this parameter was of limited relevance in the context of acrylamide formation. The highest decrease in water activity was observed in croquettes after roasting and microwave heating. However, roasting induced a higher decrease of water activity with (at the same time) a lower acrylamide content than the microwaved samples. There was no statistically significant correlation between acrylamide content and water activity level when all samples were examined together (unheated, conventionally-heated and microwaved samples) (results not shown). A statistical analysis indicated that acrylamide formation and water activity level were negatively correlated in the conventional home-processed (fried and roasted) croquettes (Table 3). The water activity level of the samples was also negatively correlated with the total colour difference (DE*) (Table 3). Table 3 Correlations between AA contents, browning development (DE*) and water activity (aw) in croquettes after different domestic preparation methods excluding the microwave heating.
AA DE*
DE*
aw
0.889*
0.780* 0.698*
AA: acrylamide. aw: water activity. DE*: colour difference. * Correlation coefficients statistically significant at p < 0.05.
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The effect of water activity on acrylamide formation has been widely studied and there are various complexities regarding the effect of water activity. Depending on the water activity range studied, positive or negative correlations between aw and acrylamide content have been observed. Some studies have shown that the faster reduction of the water content in the outer parts of the product, as a result of higher processing temperatures, favours acrylamide formation and results in higher amounts of AA in bread and French fries (Claus et al., 2008; Matthäus et al., 2004). However, Robert et al. (2005) found that acrylamide formation in an equimolar glucose/asparagine model system was not influenced much by water activity. The effect of water activity on acrylamide formation remains unclear, and sometimes contradictory, because the experimental procedures and the physical state of the reactants reported in the literature vary considerably. For example, acrylamide is not formed if water activity is not reduced to a value below 0.8 and maximum acrylamide formation was observed when water activity is about 0.4. Further reduction of water activity tends to decrease the amount of acrylamide (Delatour et al., 2004; Hoenicke and Gatermann, 2005; Mottram et al., 2002; Stadler et al., 2002). The main explanation for an optimum reaction rate at an intermediate aw is that the reactants are diluted at higher water activity, while at lower aw the mobility of reactants is limited, despite their presence in high concentrations (Bassama et al., 2011; Claeys et al., 2005; Lingnert et al., 2002). Clearly, further studies are needed to better understand the role of water in different foods in relation to acrylamide formation. The main pathway for AA formation in foods is the Maillard reaction. The Maillard reaction occurs on the food surface and leads to the production of desirable colour, flavours and aromas (Lingnert et al., 2002; Matthäus and Haase, 2014; Romani et al., 2009). Although surface browning is a typical phenomenon for baked foods, our studies suggest that microwaving might be more favourable to acrylamide formation than conventional heating. Croquette surface browning as a function of acrylamide formation was studied. A colorimeter was used to obtain a more objective assessment of surface colour as a function of heating methods. Table 2 shows AA contents in croquettes after all preparation methods and their total colour difference (DE*). Higher AA content was observed under microwave heating with a lower total colour difference than in all conventional heating methods. Conventional home-processed (fried and roasted) croquettes had varying degrees of browning with corresponding acrylamide concentrations. A statistical analysis indicated that acrylamide concentration showed a significant linear correlation (r = 0.889) with the colour of croquettes represented by the total colour difference (DE*), treated in different conventional heating methods (Table 3). As the samples became darker during frying and roasting, the L* value diminished and a* value increased (results not shown), which resulted in a general increase in the total colour difference (DE*). There was no statistically significant correlation between acrylamide content and total colour difference (DE*) when all samples were examined together (unheated, conventionally-heated and microwaved samples) (results not shown). Colour measurements indicated that the degree of surface browning determination appeared to be a good method for estimating acrylamide formation during conventional heating. For microwave heating, the degree of surface browning should not be used as a method for estimating acrylamide formation. Information on the relationship between browning and acrylamide formation varies widely. Some researchers have reported that high-temperature, long-time treatment of foods is responsible for a great increase in AA levels in foods, without causing significant changes in the colour or texture parameters (Anese et al., 2009). Gökmen and Şenyuva (2006) and Gökmen et al. (2007) found that changes in CIE redness parameter “a*”
during frying were somewhat similar to that of acrylamide concentration in potato chips. However, these two variables could not be linearly correlated with each other. It was also found that acrylamide concentrations were lower in darker potato chips and, since colour continues to develop during the Maillard reaction and acrylamide may start degrading, browning alone should not be used as the sole predictor of acrylamide formation (Taubert et al., 2004). On the other hand, many studies have found a good correlation between the acrylamide content of fried, baking and roasting foods and their colour (Ahrné et al., 2007; Capuano et al., 2009; Mestdagh et al., 2008). Surdyk et al. (2004) reported that more than 99% of acrylamide found in bread after roasting was formed in the crust. A strong correlation between crust colour and acrylamide concentration in crust, when conventional roasting was used, was also reported by Surdyk et al. (2004). In general, it was found that although the colour formation and melanoidin structure are largely unknown, the colour of foods may be used as an indicator of acrylamide formation during food processing. Many researchers have associated the colour with AA formation in fried potatoes (Pedreschi et al., 2006), crisp bread (Mustafa et al., 2005), coffee (Gökmen and Şenyuva, 2006) and wheat flour (Gökmen and Şenyuva, 2006). According to some authors, in contrast to the convection oven, the air in microwaves is not heated, so it cannot brown the surface of most foods. The resulting surface of microwave cooked products remains moist and the development of the expected flavours, colours and AA formation is limited (Anese et al., 2013; Barutcu et al., 2009). This raises questions of the cause of acrylamide formation and whether acrylamide is similarly formed during the microwave heating process as in conventional heating. In our opinion, during cooking in the conventional oven, heat is transferred into the product by its surface. Unlike the air in conventional ovens, the air in microwaves is not heated, so it cannot brown the surface of most foods. In microwave cooking, the conversion of microwave energy into heat is dispersed through the products. Microwave cooking causes less acrylamide formation on the surface region of heated foods where, in conventional heating, most of the acrylamide formation takes place. Most likely, microwave heating can affect acrylamide formation throughout the entire product and thus give a larger total amount of the compound in the final product. Similar du et al. (2007). observations were made by Erdog 4. Conclusions Acrylamide is present in ready–to-eat foods such as croquettes even before the heat treatment process at home. Heating of preprepared food at home (using any method) cannot avoid further AA formation. During heating, when the temperature and duration of cooking are arbitrary, extremely high acrylamide contents in these foods are possible. Therefore, the heat treatment parameters of the products during processing such as frying, roasting or microwaving at home should be controlled and reduced as much as possible. Consumers should follow the rule to cook to a golden colour instead of brown. The food industry is aware of the acrylamide problem and manufacturers are responding accordingly, although home preparation is still an important issue. The manner in which heat is transmitted to a food appears to have an impact on the level of acrylamide formation. In comparison with conventional heating, microwave treatment is more favourable for AA formation in home-cooked products. Microwave heating results in a lighter colour of samples and higher acrylamide formation in the croquettes than conventional heating. The mechanism of these reactions is not clear and further studies are necessary to investigate the formation of acrylamide during microwave heating. However, research suggests that the use of microwave heating for thermal processing of carbohydrate-rich
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