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Acrylamide reduction under different pre-treatments in French fries
Journal of Food Engineering 79 (2007) 1287–1294 www.elsevier.com/locate/jfoodeng
Acrylamide reduction under different pre-treatments in French fries Franco Pedreschi
a,*
, Karl Kaack b, Kit Granby c, Elizabeth Troncoso
d
a
c
Departmento de Ciencia y Tecnologia de los Alimentos, Facultad Tecnologica, Universidad de Santiago de Chile (USACH), Av. Ecuador 3769, Santiago, Chile b Department of Horticulture, Danish Institute of Agricultural Sciences, Kirstinebjergvej 10, 5792 Aarslev, Denmark Department of Food Chemistry, Danish Institute for Food and Veterinary Research, Moerkhoej Bygade 19, 2860 Soeborg, Denmark d Departamento de Ingenieria Quimica, Universidad de Santiago de Chile (USACH), P.O. Box 10233, Santiago, Chile Received 7 November 2005; accepted 11 April 2006 Available online 3 May 2006
Abstract Acrylamide formation in French fries was investigated in relation under different processing conditions and the content of glucose and asparagine of the strips before frying. Potato strips (0.8 · 0.8 · 5 cm) of Bintje variety were fried at 150, 170 and 190 C until reaching moisture contents of 40 g water/100 g (total basis). Prior to frying, potato strips were treated in one of the following ways: (i) immersed in distilled water for 0 min (control), 60 min and 120 min; (ii) immersed in a citric acid solution of 10 g/L for an hour; (iii) immersed in a sodium pyrophosphate solution of 10 g/L for an hour; (iii) blanched in hot water at six different time-temperature combinations (50 C for 40 and 80 min; 70 C for 10 and 45 min; 90 C for 3 and 10 min). Acrylamide content was determined in French fries while the glucose and asparagine content in the potato strips before frying. Immersed strips in water for 120 min showed a reduction of acrylamide formation of 33%, 21% and 27% at 150, 170 and 190 C, respectively, when they were compared against the control. Potato strips blanched at 50 C for 80 min had the lowest acrylamide content when compared against strips blanched at different conditions and fried at the same temperature (135, 327 and 564 lm acrylamide/kg for 150, 170 and 190 C, respectively). Potato strip immersion in citric acid solution of 10 g/L reduced much more the acrylamide formation after frying than the strip immersion in sodium pyrophosphate solution of 10 g/L (53% vs. 17%, respectively—average values for the three temperatures tested). Acrylamide formation decreased dramatically as the frying temperature decreased from 190 to 150 C for all the pre-treatments tested. Color represented by the total color difference showed high correlation (r2 of 0.854) with the acrylamide content of French fries. 2006 Elsevier Ltd. All rights reserved. Keywords: Potato strips; Frying; Acrylamide; Color; French fries; Glucose; Asparagine
1. Introduction Potato (Solanun tuberosum L.) is one of the words’s major staple food crops. In 2003, 310 · 1012 ton potatoes were produced (FAO, 2005). Potatoes are grown in approximately 80% of all countries and worldwide production stands in excess of 300 millions tons/year. US produce over 17.4 billion pounds of frozen and French fried potato products per year (National Potato Council, 1988). Large
*
Corresponding author. Tel.: +56 9359 1679; fax: +56 2682 3536. E-mail address: [email protected] (F. Pedreschi).
0260-8774/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.04.014
variation in suitability of potato (S. tuberosum) for processing of crisp and French fries have special quality demands compared to ware potatoes. Deep fat frying is extensively used in food processing both industrially and at home, and fried potato products are one of its largest applications. Frying of potato strips is based on heat transfer from the hot oil, which results in water removal and oil uptake by the piece (Aguilera & Gloria-Hernadez, 2000). Since French fries contain almost 15% fat, pressure to reduce the lipid content of diets has prompted many studies on 10 mechanisms of fat absorption during frying. The desirable characteristics of most fried foods are derived from the formation of a composite
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structure: a crispy outer, porous, and oily outer layer of crust, and a moist cooked interior or core, whose microstructures have been formed during the process (Rosen & Hellena¨s, 2002). It has been confirmed that a wide range of cooked food—prepared industrially, in catering or at home—contain acrylamide levels (lg/kg). This includes staple foods like bread, fried potatoes and coffee as wee as specially products like potato chips, biscuits, French fries, bread, and a range of other heat-processed products. Acrylamide has been classified as probably carcinogenic in humans (Rosen & Hellena¨s, 2002; Tareke, Rydberg, Karlsson, Eriksson, & Tornqvist, 2002). In April 2002, Swedish researchers shocked the food safety world when they presented preliminary findings of acrylamide in some fried and baked foods, most notably potato chips and French fries, at levels of 30–2300 lm/kg. As acrylamide has not been detected in unheated or boiled foods, it was considered to be formed during heating at high temperatures. They attributed this fact to the higher temperatures reached in Maillard non-enzymatic browning reactions required for desirable color, flavor and aroma production (Coughlin, 2003). The data published so far indicate that a temperature >100 C is required for acrylamide formation (Becalski, Lau, Lewis, & Seaman, 2003). Acrylamide is formed by heating above 120 C certain starch-based foods, such as potato chips, French fries, bread and processed cereals (Tareke et al., 2002). Recently an analytical method for analyzing acrylamide in coffee was validated (Granby & Fagt, 2004). Reducing acrylamide in food industry wide can only help the public perception about safety, which has suffered in recent years. Studies indicate that acrylamide causes cancer in rats. Among several deep-fat frying technologies, vacuum frying has a significant strategic importance for future fried manufacturing and in reducing acrylamide formation (Garayo & Moreira, 2002; Granda, Moreira, & Tichy, 2004). Some authors diminished acrylamide formation in fried snacks products by adding amino acids such as lysine, glycine and cysteine (Tae Kim, Hwang, & Joo Lee, 2005). On the other hand, lowering the pH with citric acid before frying was an efficient way to considerably diminish acrylamide formation in French fries (Jung, Choi, & Ju, 2003). Some authors reported that by lowering frying temperature at atmospheric pressure of potato chips from 185 to 165 C, it was possible to reduce the acrylamide formation to a half (Haase, Mattha¨us, & Vosmann, 2003; Pedreschi, Kaack, & Granby, 2004; Pedreschi, Kaack, & Granby, 2006). These results suggest that there may be ways to reduce or prevent acrylamide formation by changing production and preparation methods. It has been stated that acrylamide is generated during a side reaction of the Maillard reaction. Crucial participants in this reaction are an amino acid (asparagine) and reducing sugars (fructose and glucose) (Mottram & Wedzicha, 2002). Asaparagine provides the backbone of the acrylamide molecule, while reducing sugars are essential co-
reactants in the formation of the N-glycoside intermediates, which lead to the formation of acrylamide. Fried products, especially French fries and crisps, belong to the food category with probably the highest concentration of acrylamide recorded so far. The reason for this strong susceptibility to acrylamide formation is the abundance of free asparagine present in potato (Zyzak et al., 2003). The acrylamide formation only takes place at temperature above 100 C (Mathau¨s, Haase, & Vosmann, 2004), which makes the frying process and ideal condition. On the other hand, is essential for its contribution to the color and flavor of fried potatoes. Obviously, acrylamide formation will largely be influenced by the potato composition, particularly with regard to its sugar and amino acid content (Fishelier, Hartmann, Fiscalini, & Grob, 2005). Both, potato variety and field site had a noticeable influence upon formation. Acrylamide formation in fried potatoes is related to raw material (potato variety and field site) and the production process (Haase et al., 2003). Acrylamide appears to form as a result of a reaction between specific amino acids, including asparagine, and sugars found in foods reaching high temperatures during cooking processes. The process is known as the Maillard reaction and occurs at temperatures above 100 C. Variation in suitability of potato tubers for processing is not influenced only by cultivar and storage conditions, but also by differences in normal cultural practice and growing conditions. A low amount of reducing sugars in the tuber is necessary to prevent the non-enzymatic Maillard reaction between sugars and free amino acids during frying (Daheny, 1986). The Maillard reaction is responsible for the development of undesirable dark colored compounds melanoidins with bitter taste. Recently, it was discovered that the potential carcinogenic compound acrylamide also is formed in potatoes at high temperatures (Tareke et al., 2002) and that the precursors for melanoidins and acrylamide might be the same (Mottram & Wedzicha, 2002; Stadler et al., 2002). Sugars accumulate in potato tubers, when there is an imbalance between starch degradation, starch synthesis, and respiration of carbohydrates. Storage temperature and physiological age of the tubers are the most important factors that affect this process of sweetening. Potatoes aimed for processing are stored at relatively high temperature (e.g. 8 C). In practice a limit of 1.5–2.0 mg/g of fresh weight of reducing sugars in potato tubers is used as an indicator for suitability for processing (Burton, 1969). Beside diversity in storability between cultivars, large variation is often found between different potato lots/fields of the same cultivar within and between years (Olsson, Svensson, & Roslund, 2004), and hence they should be managed accordingly. Normally, the size of the total free amino acid pool in potato tubers exceeds that of the total reducing sugar pool, and it would be expected that only the latter metabolites limit the degree of color production during storage correlated with darker fry color with an upturn in reducing sugar content (Brierley, Bonner, & Cobb, 1996).
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Besides, fried potato color is the result of Maillard reaction as well, that depends on the superficial reducing sugar content, and the temperature and frying period (Ma´rquez & An˜o´n, 1986). Color of fried potatoes has been measured usually in units L*a*b* using either a colorimeter or specific data acquisition and image processing systems. L*a*b* is an international standard for color measurements, adopted by the Commission Internationale d’Eclairage (CIE) in 1976. L* is the luminance or lightness component, which ranges from 0 to 100, and parameters a* (from green to red) and b* (from blue to yellow) are the two chromatic components, which range from 120 to 120 (Hunt, 1991). In the L*a*b* space, the color perception is uniform which means that the Euclidean distance between two colors corresponds approximately to the color difference perceived by the human eye (Weißhaar & Gutsche, 2002). Recently, research has focused on possible mechanisms of acrylamide formation in foods (Yaylayan, Wnorowski, & Perez Locas, 2003). Some international research groups have separately confirmed a major Maillard reaction pathway for acrylamide formation (Mottram & Wedzicha, 2002; Stadler et al., 2002; Weißhaar & Gutsche, 2002). Significant amounts of acrylamide are formed by the hightemperature reaction of glucose and the common amino acid asparagine (Coughlin, 2003). Since potato products are specially high in asparagine, it is now thought that this Maillard reaction is most likely responsible for the majority of the acrylamide found in potato chips and French fries. Another mechanism of acrylamide formation has been recently proposed (Yaylayan et al., 2003). Asparagine needs carbohydrates to generate acrylamide. Potential of acrylamide formation is strongly related to the sugar content such as glucose and fructose (Biedermann, Biedermann-Brem, Noti, & Grob, 2002; Pedreschi et al., 2006). For instance, some authors reported that the reduction of the sugar content by blanching or soaking could decrease acrylamide concentration by about 60% in potato chips (Haase et al., 2003; Pedreschi et al., 2004). Potato variety, field site and processing conditions (pre-treatments, temperatures and times) had a noticeable influence upon acrylamide formation. Foods rich in both asparagine and glucose are largely derived from plant sources such as potatoes. Asparagine is the free amino acid present in the highest amount in potatoes (93.9 mg/100 g) (Martin & Ames, 2001). Asparagine content in potatoes depends on factors like variety, location, fertilization, storage and processing (Davies, 1977; Hippe, 1988). Maillard reaction today is seen as the major mechanism formation of acrylamide in foods; the reaction of reducing sugars with free asparagine. The cooking process per se— baking, frying, microwaving—as well as the temperature itself seems to be of limited influence. It is the thermal input that is crucial: i.e. the temperature and heating time to which the product is subjected. The objective of this work was to study the effect of different pre-treatments over the acrylamide content of potato strips after frying and the glucose and aparagine content before frying.
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2. Materials and methods 2.1. Materials Potatoes (variety Bintje, 78 g/100 g of dry solids, higher diameter P12 cm) and vegetable oil (Fritao, Denmark) were the raw materials. Potatoes stored at 8 C and 95% of relative humidity were washed and peeled in an industrial peeler IMC (model M591E4, England). Strips of cross sections of 0.8 · 0.8 cm2 were cut from the pith of the parenchymatous region of potato tubers. A ruler and a knife were used to provide strips with a length of 5 cm. 2.2. Pre-treatments Strips were rinsed immediately after cutting for 1 min in distilled water to eliminate some starch material adhering to the surface prior to frying. Then, 30 potato strips were immersed in 1 L of distilled water for the following times: 60 min and 120 min before frying. Rinsed strips in water without the water immersing treatment were considered as the control. Additionally, 30 raw potato strips were immersed for 60 min either in 1 L of 10 g/L citric acid (J.T. Baker, Deventer, Holland) solution or in 1 L of 10 g/L sodium tetrapyrophosphate (Sigma-Aldrich, St. Louis, MO, US) solution. All experiments were run in triplicate. Finally, Blanching was accomplished by immersing 30 raw potato strips in 10 L of distilled water (ratio of potato to water (g/g) of 0.012). The following temperature–time blanching treatments were applied over the potato strips: (i) 50 C for 40 min, (ii) 50 C for 80 min, (iii) 70 C for 10 min, (iv) 70 C for 45 min, (v) 90 C for 3 min, (vi) 90 C for 10 min. 2.3. Frying conditions Thirty strips of each pre-treatment were fried in an industrial fryer containing 100 L of oil at the following temperature–time conditions: (i) 150 C for 11 min, (ii) 170 C for 8.5 min, (iii) 190 C for 6.5 min. These conditions allowed the fried strips to reach final moisture contents of 40 g water/100 g (wet basis). Frying temperature was maintained constant since the potato mass to oil mass ratio (g/g) was kept very low (0.001333). 2.4. Analysis For sugar analysis, pre-treated samples (soaked in water, immersed in acid or blanched) of 40 g of randomly selected potato slices were frozen in a freezer at 24 C and then freeze dried at 22 C in a Martin Christ freeze drier (c 1–20, Osterode, Germany). The dried material was treated in a micro hammer mill (Culatti, Bie and Berntsen, Rødovre, Denmark) equipped with a 1 mm sieve. Sugars were extracted from 100 mg freeze dried material by adding 50 mL ultra pure boiling water generated by
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an Elgastat Maxima Analytica water purification system (Elga Ltd., England). The extracts were kept at room temperature for 3 h and then filtered through a 0.45 lm AcetatePlus Cameo filter and diluted with water. Separation, identification and quantification were carried out as described by (Kaack, Christensen, Hansen, & Grevsen, 2004) using analytical high performance anion exchange chromatography (HPAEC) according to the method described by (Campbell et al., 1997) with a few modifications. Amino acids were extracted with an acetate buffer at pH 7.0 in water, derivatization of the amino acid hydrolysate with 6-aminoquinoline-hemi-succinylcaramin and quantification using reverse phase HPLC and gradient elution according to (Cohen & Michaud, 1993). Measurement of sugars and amino acids were carried out in two replicates. For acrylamide analysis, acrylamide (2-propene amide) [CAS No. 79-06-1] (>99.5%) was obtained from SigmaAldrich (St. Louis, MO, US). Labelled d3-acrylamide (>98%) was from Polymer Source Inc. (Dorval, Quebec Canada). The SPE columns were Isolute Multimode 300 mg from International Sorbent Technology (Hengoed, Mid Glamorgan, UK). Mini uniprep Teflon filter vials 500 lL, filter pore size 0.45 lm, Whatman Int. Ltd (Kent, UK). The water used was MilliQ water (Millipore Corp., Bedford, MA, USA). The acetonitril was of HPLC grade from Rathburn Chemicals (Walkerburn, Scotland). Formic acid for the eluent (0.1% in water) was from Merck (Darmstadt, Germany). All stock solutions of acrylamide and d3acrylamide (1000 and 10 lg mL1) as well as calibration standards (2–30 ng L1) were prepared in water and kept at 18 C until use. Four grams of homogenised potato were extracted with 40.0 mL MilliQ water by an Ultra-turrax mixer (Janke & Kunkel, Staufen, Germany) (after addition of 200 lL d3-acrylamide 10 lg/mL as internal standard). Each analytical batch included 1–2 spiked samples for recovery measurements. The samples were centrifuged for 10 min at 3500 rpm (Hereaus Sepatech Megafuge 3.0R) (Osterode, Germany). The clean up was made on 300 mg Isolute Multimode SPE columns (IST), using an ASPEC TM XLi automatic SPE clean up system (Gilson Inc., Middleton, WI, US). The SPE columns were conditioned with acetonitrile (1 mL) and water (2 * 2 mL). The first 500 lL was discharged and the following 400 lL of sample was collected in Mini uniprep Teflon filter HPLC vials. A HP1100 HPLC system (Agilent Technologies, Palo Alto, CA, USA) was used for acrylamide separation on a Hypercarb column, 5 lm, 50 mm * 2.1 mm (ThermoHypersil, Cheshire, UK, www.thermohypersil.co.uk) after a guard column (Phenomenex SecurityGuardTM, C18 ODS, 4 mm · 2.0 mm, Cheshire, UK). Ten microliters was injected and eluted with 0.1% formic acid in water at a flow of 250 lL min1. The MS/MS detection was performed on a Quattro Ultima triple quadrupole instrument with masslynx software (Micromass Ltd., Manchester, UK). The electrospray was operated in the positive ion mode, and the capillary was set to 3.0 kV, the cone voltage
was 31 V, and the collision energy 10 eV. The source temperature was set at 120 C and the desolvation temperature at 400 C. Nitrogen was used as nebulizer gas (flow 500 L h1) and desolvation gas (flow 150 L h1), and argon was used as collision gas at a pressure of 2.3e3 mbar. The multiple reaction monitoring (MRM) mode of the degradation patterns m/z 72 ! 55 (acrylamide) and m/z 75 ! 58 (d3-acrylamide) were used for quantification. Acrylamide analyses were done in a laboratory accredited for acrylamide analysis in foods by The Danish Accreditation Body. Fried potato strip color was measured using a Minolta Chromo Meter CR 200b attached to a data-processor DP-100 using the CIE Lab L*, a* and b* color scale. Triplicate readings were carried out at 20 C on each three equidistant locations of each strip (in the four bigger sides) and the mean value was recorded. Total color change was cal2 2 2 1=2 culated by: DE ¼ ððL0 L Þ þ ða0 a Þ þ ðb0 b Þ Þ . L*, a* and b* represent the parameters lightness, redness and yellowness of the CIE Lab color scale measured in potato strips fried at determined times. The same values with the subscript zero represent the parameters corresponding to potato strips without frying. 2.5. Statistical analysis Analysis of variance was carried out using Statgraphic Statistical Package (Statistical Graphics Corporation, Version 4, Rockville, USA) including multiple range tests (P > 0.05) for separation of least square means. 3. Results and discussion 3.1. Immersion in water Neither the glucose (Fig. 1A) nor the asparagine content (Fig. 1B) of the immersed potato strips diminished significantly (P > 0.5) after being immersed up to 120 min in distilled water. However, both, the fried control and water immersed samples, showed a significant decrease (P > 0.05) in acrylamide formation as the frying temperature decreased from 190 to 150 C and as the immersion time in distilled water increased (Fig. 2). The reduction of the frying temperature from 190 C to 170 C and to 150 C, decreased acrylamide formation with 21% and 66%, respectively (average values for control and water soaked samples). These results are in agreement with those found by other authors (Haase et al., 2003; Pedreschi et al., 2004). Average acrylamide contents for control and immersed strips were 1416, 3259 and 4134 lg/kg after frying at 150, 170 and 190 C, respectively. For the three temperatures tested, acrylamide formation was higher in the control than in immersed samples. However, Fig. 1A and B showed that not only glucose content but also asparagine which are acrylamide precursors did not diminish significantly (P < 0.05) even during the more drastic water immersing treatment (120 min). Immersed samples in dis-
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Glucose content (%dry basis)
A 1.5
1.0
0.5
0.0 Control (0 min)
60 min
120 min
Asparagine content (g/kg dry basis)
Soaking time in distilled water (min)
18
B 15 12 9 6 3 0 Control (0 min)
60 min
120 min
Soaking time in distilled water (min)
Fig. 1. (A) Glucose content of potato strips soaked 0 min (control), 60 min and 120 min in distilled water before frying. (B) Asparagine content of potato strips soaked 0 min (control), 60 min and 120 min in distilled water before frying.
Control Soaking time: 60 min Soaking time: 120 min 4000
The effect of immersing potato strips in citric acid and sodium pyrophosphate solutions of 10 g/L over acrylamide formation after frying was also studied. There were no significant differences (P > 0.05) in the glucose (Fig. 3A) and asparagine (Fig. 3B) content of the control sample and those immersed in citric acid and sodium pyrophosphate solutions. Acrylamide formation increased drastically with increasing frying temperature not only for the potato strips immersed in citric acid solution but also for those immersed in sodium pyrophosphate solution (Fig. 4). Immersion in citric acid solution was a more effective pre-treatment in reducing acrylamide formation in French fries than immersion in sodium pyrophosphate solution. Strip immersion in sodium pyrophosphate solution of 10 g/L did not reduce significantly (P > 0.05) acrylamide formation with respect to the control, for the frying temperatures of 150, 170 and 190 C, respectively. The final acrylamide contents of the French fries previously immersed in this solution were 1180, 3427 and 4028 lg/kg for 150, 170 and 190 C, respectively (Fig. 4). On the other hand, strip immersion in citric acid solution of 10 g/L reduced significantly (P > 0.05) acrylamide formation in 86%, 47% and 28% with respect to the control,
2.0
A 1.5
1.0
0.5
0.0 2000
Control (distilled Citric Acid (10 g/L) TetraPyrophosphate water) Na (10 g/L) Soaking treatment for 60 min
0
150
170 Frying temperature (°C)
190
Fig. 2. Acrylamide content of potato strips soaked 0 min (control), 60 min and 120 min in distilled water after being fried at 150, 170 and 190 C.
tilled water for 60 min showed a reduction in acrylamide formation of 16%, 15% and 9% at 150, 170 and 190 C, respectively, when they were compared against the control. Besides, immersed samples for 120 min showed a significant reduction (P > 0.05) in acrylamide formation of 32%, 21% and 27% at 150, 170 and 190 C, respectively, when they were compared against the control. These results are coincident with those published by other authors who reported that dipping potato strips in distilled water for 1 h induced almost 25% reduction of acrylamide formation in French fries after frying at 190 C (Jung et al., 2003).
Asparagine content (g/kg dry basis)
Acrylamide content (μg/kg)
6000
3.2. Immersion in citric acid and sodium pyrophosphate solutions
Glucose content (% dry basis)
2.0
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18
B 15 12 9 6 3 0 Control (distilled water)
Citric Acid (10 g/L)
TetraPyrophosphate Na (10 g/L)
Soaking treatment for 60 min
Fig. 3. (A) Glucose content of control and potato strips dipped in sodium pyrophosphate and citric acid solutions of 10 g/L for 60 min. (B) Asparagine content of control and potato strips dipped in sodium pyrophosphate and citric acid solutions of 10 g/L for 60 min. Control corresponds to potato strips dipped in distilled water for 60 min.
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F. Pedreschi et al. / Journal of Food Engineering 79 (2007) 1287–1294 2.0 Control
Glucose content (%dry basis)
Acrylamide content (μg/kg)
6000 TetraPyrophosphate Na (10 g/L) Citric Acid (10 g/L)
4000
2000
0 150
170 Frying temperature (°C)
A 1.5
1.0
0.5
0.0
190
Control
(50, 40)
(50,80)
(70,10)
(70,45)
(90,3)
(90,10)
Blanching treatment 18
B 15 12 9 6 3 0
Control
(50, 40)
(50, 80)
(70, 10)
(70, 45)
(90, 3)
(90,10)
Blanching treatment
Fig. 5. (A) Glucose content of potato strips blanched in hot water at different temperature–time combinations before frying. (B) Asparagine content of potato strips blanched in hot water at different temperature– time combinations before frying. First numbers inside parenthesis indicate the blanching temperature (C); second numbers indicate the blanching time (min). Control corresponds to unblanched potato strips. 6000
Acrylamide content (μg/kg)
for the frying temperatures of 150, 170 and 190 C, respectively. Other authors reported that dipping potato strips in a 10 g/L citric acid solution induced 73.1% reduced acrylamide formation in potato strips fried at 190 C (Jung et al., 2003). However in our frying experiments the percentage of acrylamide reduction (from 86% to 28%) by the immersion in the citric acid solution previous to frying diminished as the oil temperature increased. In our experiments this pre-treatment reduced acrylamide formation in potato strips fried at 190 C in only 28%. This result is in agreement to that found by Jung et al. (2003) for potato chips previously immersed in a citric acid solution of 10 g/L. In this case acrylamide formation was reduced significantly (P > 0.05) in potato slices fried at 150 C (70% with respect to the control). However, when the pre-treated potato slices were fried at 170 and 190 C, there were no detected significant differences between the acrylamide content of these samples and that of the control. This reduction of acrylamide formation in French fries by dipping the potato strips in citric acid solutions was attributed to both pH lowering and leaching out of free asparagine and the reducing sugars from the surface layer of potato cuts to the solutions (Jung et al., 2003). These authors also explain the mechanism by which lowering the pH of the potatoes reduces acrylamide formation after frying. Since in this case there is no almost removal of acrylamide precursors during soaking in citric acid solution (glucose and asparagine), other mechanism different than Maillard reaction probably operates to reduce acrylamide formation after frying.
Asparagine content (g/kg dry basis)
Fig. 4. Acrylamide content of control and potato strips dipped in sodium pyrophosphate and citric acid solutions of 10 g/L for 60 min after being fried at 150, 170 and 190 C. Control corresponds to potato strips dipped in distilled water for 60 min.
Control (50,40) (50,80)
4000
(70,10) (70,45) (90,3) (90,10)
2000
0 150
170 Frying temperature (°C)
190
Fig. 6. Acrylamide content of potato strips blanched at different temperature–time combinations after being fried at 150, 170 and 190 C. First numbers inside parenthesis indicate the blanching temperature (C); second numbers indicate the blanching time (min). Control corresponds to unblanched potato strips.
3.3. Blanching Not only glucose but also asparagine content (Fig. 5A and B) decreased drastically as the temperature and time of blanching increased leading to French fries with less acrylamide content after frying. Acrylamide formation increased significantly (P > 0.05) in blanched samples when the frying temperature was increased (Fig. 6). For instance, acrylamide contents were 287, 1338 and 2128 lg/kg after
frying at 150, 170 and 190 C, respectively, in the case of potato slices blanched at 70 C for 10 min. Long time blanching treatments such as that of 50 C for 80 min and 70 C for 45 min resulted in the lowest levels of acrylamide formation (342 and 538 lg/kg as average values for the three frying temperatures tested). These two blanching treatments, after frying at 190 C, lead to the lowest acrylamide contents (564 and 883 lg/kg, respectively). Blanching
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in general, removes much more glucose and asparagine from the potato slices than the water immersing treatment consequently leading, to significant lower acrylamide formation in the resultant fried potatoes (Pedreschi et al., 2004). A reduction of the sugar content by blanching could reduce the acrylamide concentration by about 60% according to the raw material (potato variety and field site) and the production process variables (e.g. blanching conditions and frying temperatures) (Haase et al., 2003). Fried potato color is the result of Maillard, non-enzymatic browning reactions that depends on the superficial reducing sugar content, and the temperature and frying period (Ma´rquez & An˜o´n, 1986). Acrylamide concentration showed a good linear correlation (r2 = 0.854) with the color of the fried potato strips represented by the total color difference DE pre-treated in different ways (final moisture content 40 g/100 g total basis). As the samples get darker during frying, the L* value diminished and a* value increased (results not shown) which results in a general increase of the total color difference DE. As the frying temperature increase from 150 to 190 C, the resultant French fries get more red and darker as a result of nonenzymatic browning reactions that are highly dependant on oil temperature. Blanching reduces the DE value of potato chips due to the leaching out of reducing sugars previous to frying inhibiting in this way non-enzymatic browning reactions and leading to lighter and less red French fries. These results agree wit those found by other authors for potato chips and French fries. In many cases there is a good correlation between the acrylamide content of the fried potatoes and their color since acrylamide formation seems to be high associated to Maillard reaction. However, there are some cases as that in which potato pieces were previously soaked with citric acid in which acrylamide formation diminished after frying even though acrylamide precursors such as glucose and asparagine remained the same as in untreated potato and no changes in color are observed with respect to control samples (not soaked in citric acid). In this case, the mechanism of acrylamide formations seems not to be related to the Maillard reaction. Fig. 7 showed that DE presented good correlation with the acrylamide content of the fried potato strips previously
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treated in the different ways mentioned in this research (r2 = 0.85). As the frying temperature increase from 150 to 190 C, the resultant chips get more red and darker as a result of non-enzymatic browning reactions that are highly dependant on oil temperature and frying time. Blanching reduces the DE values of French fries probably due to the leaching out of reducing sugars previous to frying inhibiting in this way non-enzymatic browning reactions and leading to lighter and less red French fries (Pedreschi et al., 2006; Pedreschi, Moyano, Kaack, & Granby, 2005). Lightness of French fries decreased as the acrylamide formation increased since the pieces get darker as a result of Maillar reactions. Besides, the redness chromatic component of French fries increased with the acrylamide formation since the pieces 10 to get more red as the Maillard non-enzymatic reaction develops. 4. Conclusions Immersion of potato strips in distilled water decreased the acrylamide formation after frying without reducing significantly their glucose and asparagine content. The previously reported citric acid immersion effect in acrylamide reduction in French fries was confirmed in this research. On the other hand, there was not detected any effect of sodium tetrapyrophosphate in reducing acrylamide formation after frying. For all the pre-treatments tested, acrylamide formation diminished significantly (P < 0.05) as the frying temperature decreased from 190 to 150 C. Blanching lead to a significant (P < 0.05) reduction of acrylamide formation in potato strips after frying at the three frying temperatures tested. The longer the immersion time the lower acrylamide formation after frying, and the lower glucose and asparagine content in potato strips before frying. There was detected a linear correlation between parameter total color difference DE and acrylamide formation in French fries suggesting a relationship between the acrylamide formation and the degree of non-enzymatic browning developed during frying according to the pre-treatments employed. Acknowledgements
Acrylamide content (μg/kg)
5000
Authors acknowledge financial support from FONDECYT Project No. 1030411 and the Danish Ministry for Food, Agriculture and Fisheries (Project: Reduction of the Formation and Occurrence of Acrylamide in Food). Collaboration of Lone Borum is highly appreciated.
R2 = 0.854
4000
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10
15
20
25
30
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(ΔE)
Fig. 7. Acrylamide content vs. color difference parameter DE for potato strips fried at 150, 170 and 190 C for all the pre-treatments tested.
Aguilera, J. M., & Gloria-Hernadez, H. (2000). Oil absorption during frying of frozen parfried potatoes. Journal of Food Science, 65, 446–1479. Becalski, A., Lau, B. P., Lewis, D., & Seaman, S. W. (2003). Acrylamide in foods: occurrence, sources, and modeling. Journal of Agricultural and Food Chemistry, 51, 802–808.
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Biedermann, M., Biedermann-Brem, S., Noti, A., & Grob, K. (2002). Methods for determining the potential of acrylamide formation and its elimination in raw materials for food preparation, such as potatoes. Mitteilungen aus Lebensmitteluntersuchung und Hygiene, 93, 653–667. Brierley, E. R., Bonner, P. L. R., & Cobb, A. H. (1996). Factors influencing the free amino acid content of potato (Solanum tuberosum L.) tubers during prolonged storage. Journal of Science Food and Agriculture, 70, 5515–5525. Burton, W. G. (1969). The sugar balance in some bristish potato varieties during storage. II. The effects of tuber age, previous storage temperature, and intermittent refrigeration upon low-temperature sweetening. European Potato Journal, 12, 81–95. Campbell, J. M., Bauer, L. L., Fahey, C. C., Hogarth, A. J., Wolf, B. W., & Hunter, D. E. (1997). Selected fructooligosaccharide (1-kestose, nystose, and 1F-beta-fructofuranosylnystose) composition of foods and feeds. Journal of Agricultural and Food Chemistry, 45, 3076–3082. Cohen, S. A., & Michaud, D. P. (1993). Synthesis of a fluorescent derivating reagent, 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate, and its application for the analysis of hydrolysate amino acids via high performance liquid chromatography. Analytical Biochemistry, 211, 279–287. Coughlin, J. R. (2003). Acrylamide: What we have learned so far. Food Technology, 57(2), 100. Daheny, S. (1986). Maillard reactions: nonenzymatic browning in food systems with special reference to develop of flavor. Advance Food Research, 30, 77–138. Davies, A. M. C. (1977). The free amino acids of tuber varieties grown in England and Ireland. Potato Research, 20, 9–21. Fishelier, K., Hartmann, A., Fiscalini, A., & Grob, K. (2005). European Food Research of Technology, 221, 376–381. Food and agriculture organization of the United Nations. FAO statistical databases. http://faostat.fao.org (accessed July 205). Garayo & Moreira (2002). Vacuum frying of potato chips. Journal of Food Engineering, 55, 181–191. Granby, K., & Fagt, S. (2004). Analysis of acrylamide in coffee and dietary exposure to acrylamide from coffee. Analytica Chemica Acta, 52, 177–182. Granda, C., Moreira, R. G., & Tichy, S. E. (2004). Reduction of acrylamide formation in potato chips by low-temperature vacuum frying. Journal of Food Science, 69, 405–441. Haase, N. U., Mattha¨us, B., & Vosmann, K. (2003). Minimierungsansa¨tze zur Acrylamid-Bildung in pflanzlichen Lebensmitteln-aufgezeigt am Beispiel von Kartoffelchips. Deutsche Lebensmittel-Rundschau, 99, 87–90. Hippe, J. (1988). HPLC-analysis of the concentrations of free asparagine and glutamine in potato tubers grown in varying amounts of nitrogen. Potato Research, 31, 535–540. Hunt, R. G. W. (1991). Measuring color (2nd ed.). Chichester: Ellis Horwood Ltd., p. 313. Jung, M. Y., Choi, D. S., & Ju, J. W. (2003). A novel technique for limitation of acrylamide formation in fried and baked corn chips and in French fries. Journal of Food Science, 68, 1287–1290.
Kaack, K., Christensen, L. P. C., Hansen, S. L., & Grevsen, K. (2004). Non-structural carbohydrates in processed soft fried onion (Allium cepa L.). European Food Research and Technology, 218(4), 372–379. Ma´rquez, G., & An˜o´n, M. C. (1986). Influence of reducing sugars and amino acids in the color development of fried potatoes. Journal of Food Science, 51, 157–160. Martin, F. L., & Ames, J. M. (2001). Formation of Strecker aldehydes and pyrazines in a fried potato model system. Journal of Agricultural and Food Chemistry, 49, 3885–3892. Mathau¨s, B., Haase, N. U., & Vosmann, K. (2004). Factors affecting the concentration of acrylamide during deep-fat frying of potatoes. European Journal of Lipid Science Technology, 106, 793–801. Mottram, D. S., & Wedzicha, B. L. (2002). Acrylamide is formed in the Maillard reaction. Nature, 419, 448–449. National Potato Council (1988). National Potato Council 1988 Potato Statistical Yearbook. National Potato Council, Englewood, CO. Olsson, K., Svensson, R., & Roslund, C. A. (2004). Tuber components affecting acrylamide formation and color in fried potato: Variation variety per year, storage temperature and storage time. Journal of Food Science and Agriculture, 84, 447–458. Pedreschi, F., Kaack, K., & Granby, K. (2004). Reduction of acrylamide formation in fried potato slices. Lebensmittel-Wissenschaft und-Technologie, 37, 679–685. Pedreschi, F., Kaack, K., & Granby, K. (2006). Acrylamide content and color development in fried potato strips. Food Research International, 39, 40–46. Pedreschi, F., Moyano, P., Kaack, K., & Granby, K. (2005). Color changes and aceylamide formation in fried potato slices. Food Research International, 38, 1–9. Rosen, J., & Hellena¨s, K. E. (2002). Analysis of acrylamide in cooked foods by liquid chromatography tandem mass spectrometry. Analyst, 127, 880–882. Stadler, R. H., Blank, I., Varga, N., Robert, F., Hau, J., Guy, A., et al. (2002). Acrylamide from Maillard reaction products. Nature, 419, 449–450. Tae Kim, C., Hwang, E., & Joo Lee, H. (2005). Reducing acrylamide in fried snack products by adding amino acids. Journal of Food Science, 70, 354–358. Tareke, E., Rydberg, P., Karlsson, P., Eriksson, S., & Tornqvist, M. (2002). Analysis of acrylamide, a carcinogen formed in heated foodstuffs. Journal of Agricultural and Food Chemistry, 50, 4998–5006. Weißhaar, R., & Gutsche, B. (2002). Formation of acrylamide in heated potato products—model experiments pointing to asparagine as precursor. Deutsche Lebensmittel-Rundschau, 98, 397–400. Yaylayan, V. A., Wnorowski, A., & Perez Locas, C. (2003). Why asparagine needs carbohydrates to generate acrylamide. Journal of Agricultural and Food Chemistry, 51, 1753–1757. Zyzak, D., Sanders, R. A., Stojanovic, M., Tallmadge, D. H., Ebehart, L., Ewald, D. K., et al. (2003). Acrylamide. Acrylamide formation mechanism in heated foods. Journal of Agricultural and Food Chemistry, 51, 4782–4787.
Acrylamide reduction under different pre-treatments in French fries Franco Pedreschi
a,*
, Karl Kaack b, Kit Granby c, Elizabeth Troncoso
d
a
c
Departmento de Ciencia y Tecnologia de los Alimentos, Facultad Tecnologica, Universidad de Santiago de Chile (USACH), Av. Ecuador 3769, Santiago, Chile b Department of Horticulture, Danish Institute of Agricultural Sciences, Kirstinebjergvej 10, 5792 Aarslev, Denmark Department of Food Chemistry, Danish Institute for Food and Veterinary Research, Moerkhoej Bygade 19, 2860 Soeborg, Denmark d Departamento de Ingenieria Quimica, Universidad de Santiago de Chile (USACH), P.O. Box 10233, Santiago, Chile Received 7 November 2005; accepted 11 April 2006 Available online 3 May 2006
Abstract Acrylamide formation in French fries was investigated in relation under different processing conditions and the content of glucose and asparagine of the strips before frying. Potato strips (0.8 · 0.8 · 5 cm) of Bintje variety were fried at 150, 170 and 190 C until reaching moisture contents of 40 g water/100 g (total basis). Prior to frying, potato strips were treated in one of the following ways: (i) immersed in distilled water for 0 min (control), 60 min and 120 min; (ii) immersed in a citric acid solution of 10 g/L for an hour; (iii) immersed in a sodium pyrophosphate solution of 10 g/L for an hour; (iii) blanched in hot water at six different time-temperature combinations (50 C for 40 and 80 min; 70 C for 10 and 45 min; 90 C for 3 and 10 min). Acrylamide content was determined in French fries while the glucose and asparagine content in the potato strips before frying. Immersed strips in water for 120 min showed a reduction of acrylamide formation of 33%, 21% and 27% at 150, 170 and 190 C, respectively, when they were compared against the control. Potato strips blanched at 50 C for 80 min had the lowest acrylamide content when compared against strips blanched at different conditions and fried at the same temperature (135, 327 and 564 lm acrylamide/kg for 150, 170 and 190 C, respectively). Potato strip immersion in citric acid solution of 10 g/L reduced much more the acrylamide formation after frying than the strip immersion in sodium pyrophosphate solution of 10 g/L (53% vs. 17%, respectively—average values for the three temperatures tested). Acrylamide formation decreased dramatically as the frying temperature decreased from 190 to 150 C for all the pre-treatments tested. Color represented by the total color difference showed high correlation (r2 of 0.854) with the acrylamide content of French fries. 2006 Elsevier Ltd. All rights reserved. Keywords: Potato strips; Frying; Acrylamide; Color; French fries; Glucose; Asparagine
1. Introduction Potato (Solanun tuberosum L.) is one of the words’s major staple food crops. In 2003, 310 · 1012 ton potatoes were produced (FAO, 2005). Potatoes are grown in approximately 80% of all countries and worldwide production stands in excess of 300 millions tons/year. US produce over 17.4 billion pounds of frozen and French fried potato products per year (National Potato Council, 1988). Large
*
Corresponding author. Tel.: +56 9359 1679; fax: +56 2682 3536. E-mail address: [email protected] (F. Pedreschi).
0260-8774/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.04.014
variation in suitability of potato (S. tuberosum) for processing of crisp and French fries have special quality demands compared to ware potatoes. Deep fat frying is extensively used in food processing both industrially and at home, and fried potato products are one of its largest applications. Frying of potato strips is based on heat transfer from the hot oil, which results in water removal and oil uptake by the piece (Aguilera & Gloria-Hernadez, 2000). Since French fries contain almost 15% fat, pressure to reduce the lipid content of diets has prompted many studies on 10 mechanisms of fat absorption during frying. The desirable characteristics of most fried foods are derived from the formation of a composite
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structure: a crispy outer, porous, and oily outer layer of crust, and a moist cooked interior or core, whose microstructures have been formed during the process (Rosen & Hellena¨s, 2002). It has been confirmed that a wide range of cooked food—prepared industrially, in catering or at home—contain acrylamide levels (lg/kg). This includes staple foods like bread, fried potatoes and coffee as wee as specially products like potato chips, biscuits, French fries, bread, and a range of other heat-processed products. Acrylamide has been classified as probably carcinogenic in humans (Rosen & Hellena¨s, 2002; Tareke, Rydberg, Karlsson, Eriksson, & Tornqvist, 2002). In April 2002, Swedish researchers shocked the food safety world when they presented preliminary findings of acrylamide in some fried and baked foods, most notably potato chips and French fries, at levels of 30–2300 lm/kg. As acrylamide has not been detected in unheated or boiled foods, it was considered to be formed during heating at high temperatures. They attributed this fact to the higher temperatures reached in Maillard non-enzymatic browning reactions required for desirable color, flavor and aroma production (Coughlin, 2003). The data published so far indicate that a temperature >100 C is required for acrylamide formation (Becalski, Lau, Lewis, & Seaman, 2003). Acrylamide is formed by heating above 120 C certain starch-based foods, such as potato chips, French fries, bread and processed cereals (Tareke et al., 2002). Recently an analytical method for analyzing acrylamide in coffee was validated (Granby & Fagt, 2004). Reducing acrylamide in food industry wide can only help the public perception about safety, which has suffered in recent years. Studies indicate that acrylamide causes cancer in rats. Among several deep-fat frying technologies, vacuum frying has a significant strategic importance for future fried manufacturing and in reducing acrylamide formation (Garayo & Moreira, 2002; Granda, Moreira, & Tichy, 2004). Some authors diminished acrylamide formation in fried snacks products by adding amino acids such as lysine, glycine and cysteine (Tae Kim, Hwang, & Joo Lee, 2005). On the other hand, lowering the pH with citric acid before frying was an efficient way to considerably diminish acrylamide formation in French fries (Jung, Choi, & Ju, 2003). Some authors reported that by lowering frying temperature at atmospheric pressure of potato chips from 185 to 165 C, it was possible to reduce the acrylamide formation to a half (Haase, Mattha¨us, & Vosmann, 2003; Pedreschi, Kaack, & Granby, 2004; Pedreschi, Kaack, & Granby, 2006). These results suggest that there may be ways to reduce or prevent acrylamide formation by changing production and preparation methods. It has been stated that acrylamide is generated during a side reaction of the Maillard reaction. Crucial participants in this reaction are an amino acid (asparagine) and reducing sugars (fructose and glucose) (Mottram & Wedzicha, 2002). Asaparagine provides the backbone of the acrylamide molecule, while reducing sugars are essential co-
reactants in the formation of the N-glycoside intermediates, which lead to the formation of acrylamide. Fried products, especially French fries and crisps, belong to the food category with probably the highest concentration of acrylamide recorded so far. The reason for this strong susceptibility to acrylamide formation is the abundance of free asparagine present in potato (Zyzak et al., 2003). The acrylamide formation only takes place at temperature above 100 C (Mathau¨s, Haase, & Vosmann, 2004), which makes the frying process and ideal condition. On the other hand, is essential for its contribution to the color and flavor of fried potatoes. Obviously, acrylamide formation will largely be influenced by the potato composition, particularly with regard to its sugar and amino acid content (Fishelier, Hartmann, Fiscalini, & Grob, 2005). Both, potato variety and field site had a noticeable influence upon formation. Acrylamide formation in fried potatoes is related to raw material (potato variety and field site) and the production process (Haase et al., 2003). Acrylamide appears to form as a result of a reaction between specific amino acids, including asparagine, and sugars found in foods reaching high temperatures during cooking processes. The process is known as the Maillard reaction and occurs at temperatures above 100 C. Variation in suitability of potato tubers for processing is not influenced only by cultivar and storage conditions, but also by differences in normal cultural practice and growing conditions. A low amount of reducing sugars in the tuber is necessary to prevent the non-enzymatic Maillard reaction between sugars and free amino acids during frying (Daheny, 1986). The Maillard reaction is responsible for the development of undesirable dark colored compounds melanoidins with bitter taste. Recently, it was discovered that the potential carcinogenic compound acrylamide also is formed in potatoes at high temperatures (Tareke et al., 2002) and that the precursors for melanoidins and acrylamide might be the same (Mottram & Wedzicha, 2002; Stadler et al., 2002). Sugars accumulate in potato tubers, when there is an imbalance between starch degradation, starch synthesis, and respiration of carbohydrates. Storage temperature and physiological age of the tubers are the most important factors that affect this process of sweetening. Potatoes aimed for processing are stored at relatively high temperature (e.g. 8 C). In practice a limit of 1.5–2.0 mg/g of fresh weight of reducing sugars in potato tubers is used as an indicator for suitability for processing (Burton, 1969). Beside diversity in storability between cultivars, large variation is often found between different potato lots/fields of the same cultivar within and between years (Olsson, Svensson, & Roslund, 2004), and hence they should be managed accordingly. Normally, the size of the total free amino acid pool in potato tubers exceeds that of the total reducing sugar pool, and it would be expected that only the latter metabolites limit the degree of color production during storage correlated with darker fry color with an upturn in reducing sugar content (Brierley, Bonner, & Cobb, 1996).
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Besides, fried potato color is the result of Maillard reaction as well, that depends on the superficial reducing sugar content, and the temperature and frying period (Ma´rquez & An˜o´n, 1986). Color of fried potatoes has been measured usually in units L*a*b* using either a colorimeter or specific data acquisition and image processing systems. L*a*b* is an international standard for color measurements, adopted by the Commission Internationale d’Eclairage (CIE) in 1976. L* is the luminance or lightness component, which ranges from 0 to 100, and parameters a* (from green to red) and b* (from blue to yellow) are the two chromatic components, which range from 120 to 120 (Hunt, 1991). In the L*a*b* space, the color perception is uniform which means that the Euclidean distance between two colors corresponds approximately to the color difference perceived by the human eye (Weißhaar & Gutsche, 2002). Recently, research has focused on possible mechanisms of acrylamide formation in foods (Yaylayan, Wnorowski, & Perez Locas, 2003). Some international research groups have separately confirmed a major Maillard reaction pathway for acrylamide formation (Mottram & Wedzicha, 2002; Stadler et al., 2002; Weißhaar & Gutsche, 2002). Significant amounts of acrylamide are formed by the hightemperature reaction of glucose and the common amino acid asparagine (Coughlin, 2003). Since potato products are specially high in asparagine, it is now thought that this Maillard reaction is most likely responsible for the majority of the acrylamide found in potato chips and French fries. Another mechanism of acrylamide formation has been recently proposed (Yaylayan et al., 2003). Asparagine needs carbohydrates to generate acrylamide. Potential of acrylamide formation is strongly related to the sugar content such as glucose and fructose (Biedermann, Biedermann-Brem, Noti, & Grob, 2002; Pedreschi et al., 2006). For instance, some authors reported that the reduction of the sugar content by blanching or soaking could decrease acrylamide concentration by about 60% in potato chips (Haase et al., 2003; Pedreschi et al., 2004). Potato variety, field site and processing conditions (pre-treatments, temperatures and times) had a noticeable influence upon acrylamide formation. Foods rich in both asparagine and glucose are largely derived from plant sources such as potatoes. Asparagine is the free amino acid present in the highest amount in potatoes (93.9 mg/100 g) (Martin & Ames, 2001). Asparagine content in potatoes depends on factors like variety, location, fertilization, storage and processing (Davies, 1977; Hippe, 1988). Maillard reaction today is seen as the major mechanism formation of acrylamide in foods; the reaction of reducing sugars with free asparagine. The cooking process per se— baking, frying, microwaving—as well as the temperature itself seems to be of limited influence. It is the thermal input that is crucial: i.e. the temperature and heating time to which the product is subjected. The objective of this work was to study the effect of different pre-treatments over the acrylamide content of potato strips after frying and the glucose and aparagine content before frying.
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2. Materials and methods 2.1. Materials Potatoes (variety Bintje, 78 g/100 g of dry solids, higher diameter P12 cm) and vegetable oil (Fritao, Denmark) were the raw materials. Potatoes stored at 8 C and 95% of relative humidity were washed and peeled in an industrial peeler IMC (model M591E4, England). Strips of cross sections of 0.8 · 0.8 cm2 were cut from the pith of the parenchymatous region of potato tubers. A ruler and a knife were used to provide strips with a length of 5 cm. 2.2. Pre-treatments Strips were rinsed immediately after cutting for 1 min in distilled water to eliminate some starch material adhering to the surface prior to frying. Then, 30 potato strips were immersed in 1 L of distilled water for the following times: 60 min and 120 min before frying. Rinsed strips in water without the water immersing treatment were considered as the control. Additionally, 30 raw potato strips were immersed for 60 min either in 1 L of 10 g/L citric acid (J.T. Baker, Deventer, Holland) solution or in 1 L of 10 g/L sodium tetrapyrophosphate (Sigma-Aldrich, St. Louis, MO, US) solution. All experiments were run in triplicate. Finally, Blanching was accomplished by immersing 30 raw potato strips in 10 L of distilled water (ratio of potato to water (g/g) of 0.012). The following temperature–time blanching treatments were applied over the potato strips: (i) 50 C for 40 min, (ii) 50 C for 80 min, (iii) 70 C for 10 min, (iv) 70 C for 45 min, (v) 90 C for 3 min, (vi) 90 C for 10 min. 2.3. Frying conditions Thirty strips of each pre-treatment were fried in an industrial fryer containing 100 L of oil at the following temperature–time conditions: (i) 150 C for 11 min, (ii) 170 C for 8.5 min, (iii) 190 C for 6.5 min. These conditions allowed the fried strips to reach final moisture contents of 40 g water/100 g (wet basis). Frying temperature was maintained constant since the potato mass to oil mass ratio (g/g) was kept very low (0.001333). 2.4. Analysis For sugar analysis, pre-treated samples (soaked in water, immersed in acid or blanched) of 40 g of randomly selected potato slices were frozen in a freezer at 24 C and then freeze dried at 22 C in a Martin Christ freeze drier (c 1–20, Osterode, Germany). The dried material was treated in a micro hammer mill (Culatti, Bie and Berntsen, Rødovre, Denmark) equipped with a 1 mm sieve. Sugars were extracted from 100 mg freeze dried material by adding 50 mL ultra pure boiling water generated by
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an Elgastat Maxima Analytica water purification system (Elga Ltd., England). The extracts were kept at room temperature for 3 h and then filtered through a 0.45 lm AcetatePlus Cameo filter and diluted with water. Separation, identification and quantification were carried out as described by (Kaack, Christensen, Hansen, & Grevsen, 2004) using analytical high performance anion exchange chromatography (HPAEC) according to the method described by (Campbell et al., 1997) with a few modifications. Amino acids were extracted with an acetate buffer at pH 7.0 in water, derivatization of the amino acid hydrolysate with 6-aminoquinoline-hemi-succinylcaramin and quantification using reverse phase HPLC and gradient elution according to (Cohen & Michaud, 1993). Measurement of sugars and amino acids were carried out in two replicates. For acrylamide analysis, acrylamide (2-propene amide) [CAS No. 79-06-1] (>99.5%) was obtained from SigmaAldrich (St. Louis, MO, US). Labelled d3-acrylamide (>98%) was from Polymer Source Inc. (Dorval, Quebec Canada). The SPE columns were Isolute Multimode 300 mg from International Sorbent Technology (Hengoed, Mid Glamorgan, UK). Mini uniprep Teflon filter vials 500 lL, filter pore size 0.45 lm, Whatman Int. Ltd (Kent, UK). The water used was MilliQ water (Millipore Corp., Bedford, MA, USA). The acetonitril was of HPLC grade from Rathburn Chemicals (Walkerburn, Scotland). Formic acid for the eluent (0.1% in water) was from Merck (Darmstadt, Germany). All stock solutions of acrylamide and d3acrylamide (1000 and 10 lg mL1) as well as calibration standards (2–30 ng L1) were prepared in water and kept at 18 C until use. Four grams of homogenised potato were extracted with 40.0 mL MilliQ water by an Ultra-turrax mixer (Janke & Kunkel, Staufen, Germany) (after addition of 200 lL d3-acrylamide 10 lg/mL as internal standard). Each analytical batch included 1–2 spiked samples for recovery measurements. The samples were centrifuged for 10 min at 3500 rpm (Hereaus Sepatech Megafuge 3.0R) (Osterode, Germany). The clean up was made on 300 mg Isolute Multimode SPE columns (IST), using an ASPEC TM XLi automatic SPE clean up system (Gilson Inc., Middleton, WI, US). The SPE columns were conditioned with acetonitrile (1 mL) and water (2 * 2 mL). The first 500 lL was discharged and the following 400 lL of sample was collected in Mini uniprep Teflon filter HPLC vials. A HP1100 HPLC system (Agilent Technologies, Palo Alto, CA, USA) was used for acrylamide separation on a Hypercarb column, 5 lm, 50 mm * 2.1 mm (ThermoHypersil, Cheshire, UK, www.thermohypersil.co.uk) after a guard column (Phenomenex SecurityGuardTM, C18 ODS, 4 mm · 2.0 mm, Cheshire, UK). Ten microliters was injected and eluted with 0.1% formic acid in water at a flow of 250 lL min1. The MS/MS detection was performed on a Quattro Ultima triple quadrupole instrument with masslynx software (Micromass Ltd., Manchester, UK). The electrospray was operated in the positive ion mode, and the capillary was set to 3.0 kV, the cone voltage
was 31 V, and the collision energy 10 eV. The source temperature was set at 120 C and the desolvation temperature at 400 C. Nitrogen was used as nebulizer gas (flow 500 L h1) and desolvation gas (flow 150 L h1), and argon was used as collision gas at a pressure of 2.3e3 mbar. The multiple reaction monitoring (MRM) mode of the degradation patterns m/z 72 ! 55 (acrylamide) and m/z 75 ! 58 (d3-acrylamide) were used for quantification. Acrylamide analyses were done in a laboratory accredited for acrylamide analysis in foods by The Danish Accreditation Body. Fried potato strip color was measured using a Minolta Chromo Meter CR 200b attached to a data-processor DP-100 using the CIE Lab L*, a* and b* color scale. Triplicate readings were carried out at 20 C on each three equidistant locations of each strip (in the four bigger sides) and the mean value was recorded. Total color change was cal2 2 2 1=2 culated by: DE ¼ ððL0 L Þ þ ða0 a Þ þ ðb0 b Þ Þ . L*, a* and b* represent the parameters lightness, redness and yellowness of the CIE Lab color scale measured in potato strips fried at determined times. The same values with the subscript zero represent the parameters corresponding to potato strips without frying. 2.5. Statistical analysis Analysis of variance was carried out using Statgraphic Statistical Package (Statistical Graphics Corporation, Version 4, Rockville, USA) including multiple range tests (P > 0.05) for separation of least square means. 3. Results and discussion 3.1. Immersion in water Neither the glucose (Fig. 1A) nor the asparagine content (Fig. 1B) of the immersed potato strips diminished significantly (P > 0.5) after being immersed up to 120 min in distilled water. However, both, the fried control and water immersed samples, showed a significant decrease (P > 0.05) in acrylamide formation as the frying temperature decreased from 190 to 150 C and as the immersion time in distilled water increased (Fig. 2). The reduction of the frying temperature from 190 C to 170 C and to 150 C, decreased acrylamide formation with 21% and 66%, respectively (average values for control and water soaked samples). These results are in agreement with those found by other authors (Haase et al., 2003; Pedreschi et al., 2004). Average acrylamide contents for control and immersed strips were 1416, 3259 and 4134 lg/kg after frying at 150, 170 and 190 C, respectively. For the three temperatures tested, acrylamide formation was higher in the control than in immersed samples. However, Fig. 1A and B showed that not only glucose content but also asparagine which are acrylamide precursors did not diminish significantly (P < 0.05) even during the more drastic water immersing treatment (120 min). Immersed samples in dis-
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Glucose content (%dry basis)
A 1.5
1.0
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0.0 Control (0 min)
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120 min
Asparagine content (g/kg dry basis)
Soaking time in distilled water (min)
18
B 15 12 9 6 3 0 Control (0 min)
60 min
120 min
Soaking time in distilled water (min)
Fig. 1. (A) Glucose content of potato strips soaked 0 min (control), 60 min and 120 min in distilled water before frying. (B) Asparagine content of potato strips soaked 0 min (control), 60 min and 120 min in distilled water before frying.
Control Soaking time: 60 min Soaking time: 120 min 4000
The effect of immersing potato strips in citric acid and sodium pyrophosphate solutions of 10 g/L over acrylamide formation after frying was also studied. There were no significant differences (P > 0.05) in the glucose (Fig. 3A) and asparagine (Fig. 3B) content of the control sample and those immersed in citric acid and sodium pyrophosphate solutions. Acrylamide formation increased drastically with increasing frying temperature not only for the potato strips immersed in citric acid solution but also for those immersed in sodium pyrophosphate solution (Fig. 4). Immersion in citric acid solution was a more effective pre-treatment in reducing acrylamide formation in French fries than immersion in sodium pyrophosphate solution. Strip immersion in sodium pyrophosphate solution of 10 g/L did not reduce significantly (P > 0.05) acrylamide formation with respect to the control, for the frying temperatures of 150, 170 and 190 C, respectively. The final acrylamide contents of the French fries previously immersed in this solution were 1180, 3427 and 4028 lg/kg for 150, 170 and 190 C, respectively (Fig. 4). On the other hand, strip immersion in citric acid solution of 10 g/L reduced significantly (P > 0.05) acrylamide formation in 86%, 47% and 28% with respect to the control,
2.0
A 1.5
1.0
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0.0 2000
Control (distilled Citric Acid (10 g/L) TetraPyrophosphate water) Na (10 g/L) Soaking treatment for 60 min
0
150
170 Frying temperature (°C)
190
Fig. 2. Acrylamide content of potato strips soaked 0 min (control), 60 min and 120 min in distilled water after being fried at 150, 170 and 190 C.
tilled water for 60 min showed a reduction in acrylamide formation of 16%, 15% and 9% at 150, 170 and 190 C, respectively, when they were compared against the control. Besides, immersed samples for 120 min showed a significant reduction (P > 0.05) in acrylamide formation of 32%, 21% and 27% at 150, 170 and 190 C, respectively, when they were compared against the control. These results are coincident with those published by other authors who reported that dipping potato strips in distilled water for 1 h induced almost 25% reduction of acrylamide formation in French fries after frying at 190 C (Jung et al., 2003).
Asparagine content (g/kg dry basis)
Acrylamide content (μg/kg)
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3.2. Immersion in citric acid and sodium pyrophosphate solutions
Glucose content (% dry basis)
2.0
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18
B 15 12 9 6 3 0 Control (distilled water)
Citric Acid (10 g/L)
TetraPyrophosphate Na (10 g/L)
Soaking treatment for 60 min
Fig. 3. (A) Glucose content of control and potato strips dipped in sodium pyrophosphate and citric acid solutions of 10 g/L for 60 min. (B) Asparagine content of control and potato strips dipped in sodium pyrophosphate and citric acid solutions of 10 g/L for 60 min. Control corresponds to potato strips dipped in distilled water for 60 min.
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Glucose content (%dry basis)
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6000 TetraPyrophosphate Na (10 g/L) Citric Acid (10 g/L)
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0 150
170 Frying temperature (°C)
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0.5
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190
Control
(50, 40)
(50,80)
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Control
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(50, 80)
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(70, 45)
(90, 3)
(90,10)
Blanching treatment
Fig. 5. (A) Glucose content of potato strips blanched in hot water at different temperature–time combinations before frying. (B) Asparagine content of potato strips blanched in hot water at different temperature– time combinations before frying. First numbers inside parenthesis indicate the blanching temperature (C); second numbers indicate the blanching time (min). Control corresponds to unblanched potato strips. 6000
Acrylamide content (μg/kg)
for the frying temperatures of 150, 170 and 190 C, respectively. Other authors reported that dipping potato strips in a 10 g/L citric acid solution induced 73.1% reduced acrylamide formation in potato strips fried at 190 C (Jung et al., 2003). However in our frying experiments the percentage of acrylamide reduction (from 86% to 28%) by the immersion in the citric acid solution previous to frying diminished as the oil temperature increased. In our experiments this pre-treatment reduced acrylamide formation in potato strips fried at 190 C in only 28%. This result is in agreement to that found by Jung et al. (2003) for potato chips previously immersed in a citric acid solution of 10 g/L. In this case acrylamide formation was reduced significantly (P > 0.05) in potato slices fried at 150 C (70% with respect to the control). However, when the pre-treated potato slices were fried at 170 and 190 C, there were no detected significant differences between the acrylamide content of these samples and that of the control. This reduction of acrylamide formation in French fries by dipping the potato strips in citric acid solutions was attributed to both pH lowering and leaching out of free asparagine and the reducing sugars from the surface layer of potato cuts to the solutions (Jung et al., 2003). These authors also explain the mechanism by which lowering the pH of the potatoes reduces acrylamide formation after frying. Since in this case there is no almost removal of acrylamide precursors during soaking in citric acid solution (glucose and asparagine), other mechanism different than Maillard reaction probably operates to reduce acrylamide formation after frying.
Asparagine content (g/kg dry basis)
Fig. 4. Acrylamide content of control and potato strips dipped in sodium pyrophosphate and citric acid solutions of 10 g/L for 60 min after being fried at 150, 170 and 190 C. Control corresponds to potato strips dipped in distilled water for 60 min.
Control (50,40) (50,80)
4000
(70,10) (70,45) (90,3) (90,10)
2000
0 150
170 Frying temperature (°C)
190
Fig. 6. Acrylamide content of potato strips blanched at different temperature–time combinations after being fried at 150, 170 and 190 C. First numbers inside parenthesis indicate the blanching temperature (C); second numbers indicate the blanching time (min). Control corresponds to unblanched potato strips.
3.3. Blanching Not only glucose but also asparagine content (Fig. 5A and B) decreased drastically as the temperature and time of blanching increased leading to French fries with less acrylamide content after frying. Acrylamide formation increased significantly (P > 0.05) in blanched samples when the frying temperature was increased (Fig. 6). For instance, acrylamide contents were 287, 1338 and 2128 lg/kg after
frying at 150, 170 and 190 C, respectively, in the case of potato slices blanched at 70 C for 10 min. Long time blanching treatments such as that of 50 C for 80 min and 70 C for 45 min resulted in the lowest levels of acrylamide formation (342 and 538 lg/kg as average values for the three frying temperatures tested). These two blanching treatments, after frying at 190 C, lead to the lowest acrylamide contents (564 and 883 lg/kg, respectively). Blanching
F. Pedreschi et al. / Journal of Food Engineering 79 (2007) 1287–1294
in general, removes much more glucose and asparagine from the potato slices than the water immersing treatment consequently leading, to significant lower acrylamide formation in the resultant fried potatoes (Pedreschi et al., 2004). A reduction of the sugar content by blanching could reduce the acrylamide concentration by about 60% according to the raw material (potato variety and field site) and the production process variables (e.g. blanching conditions and frying temperatures) (Haase et al., 2003). Fried potato color is the result of Maillard, non-enzymatic browning reactions that depends on the superficial reducing sugar content, and the temperature and frying period (Ma´rquez & An˜o´n, 1986). Acrylamide concentration showed a good linear correlation (r2 = 0.854) with the color of the fried potato strips represented by the total color difference DE pre-treated in different ways (final moisture content 40 g/100 g total basis). As the samples get darker during frying, the L* value diminished and a* value increased (results not shown) which results in a general increase of the total color difference DE. As the frying temperature increase from 150 to 190 C, the resultant French fries get more red and darker as a result of nonenzymatic browning reactions that are highly dependant on oil temperature. Blanching reduces the DE value of potato chips due to the leaching out of reducing sugars previous to frying inhibiting in this way non-enzymatic browning reactions and leading to lighter and less red French fries. These results agree wit those found by other authors for potato chips and French fries. In many cases there is a good correlation between the acrylamide content of the fried potatoes and their color since acrylamide formation seems to be high associated to Maillard reaction. However, there are some cases as that in which potato pieces were previously soaked with citric acid in which acrylamide formation diminished after frying even though acrylamide precursors such as glucose and asparagine remained the same as in untreated potato and no changes in color are observed with respect to control samples (not soaked in citric acid). In this case, the mechanism of acrylamide formations seems not to be related to the Maillard reaction. Fig. 7 showed that DE presented good correlation with the acrylamide content of the fried potato strips previously
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treated in the different ways mentioned in this research (r2 = 0.85). As the frying temperature increase from 150 to 190 C, the resultant chips get more red and darker as a result of non-enzymatic browning reactions that are highly dependant on oil temperature and frying time. Blanching reduces the DE values of French fries probably due to the leaching out of reducing sugars previous to frying inhibiting in this way non-enzymatic browning reactions and leading to lighter and less red French fries (Pedreschi et al., 2006; Pedreschi, Moyano, Kaack, & Granby, 2005). Lightness of French fries decreased as the acrylamide formation increased since the pieces get darker as a result of Maillar reactions. Besides, the redness chromatic component of French fries increased with the acrylamide formation since the pieces 10 to get more red as the Maillard non-enzymatic reaction develops. 4. Conclusions Immersion of potato strips in distilled water decreased the acrylamide formation after frying without reducing significantly their glucose and asparagine content. The previously reported citric acid immersion effect in acrylamide reduction in French fries was confirmed in this research. On the other hand, there was not detected any effect of sodium tetrapyrophosphate in reducing acrylamide formation after frying. For all the pre-treatments tested, acrylamide formation diminished significantly (P < 0.05) as the frying temperature decreased from 190 to 150 C. Blanching lead to a significant (P < 0.05) reduction of acrylamide formation in potato strips after frying at the three frying temperatures tested. The longer the immersion time the lower acrylamide formation after frying, and the lower glucose and asparagine content in potato strips before frying. There was detected a linear correlation between parameter total color difference DE and acrylamide formation in French fries suggesting a relationship between the acrylamide formation and the degree of non-enzymatic browning developed during frying according to the pre-treatments employed. Acknowledgements
Acrylamide content (μg/kg)
5000
Authors acknowledge financial support from FONDECYT Project No. 1030411 and the Danish Ministry for Food, Agriculture and Fisheries (Project: Reduction of the Formation and Occurrence of Acrylamide in Food). Collaboration of Lone Borum is highly appreciated.
R2 = 0.854
4000
3000
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References 1000
0 0
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10
15
20
25
30
35
(ΔE)
Fig. 7. Acrylamide content vs. color difference parameter DE for potato strips fried at 150, 170 and 190 C for all the pre-treatments tested.
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