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Effect of crust temperature and water content on acrylamide formation during baking of white bread: Steam and falling temperature baking
ARTICLE IN PRESS
LWT 40 (2007) 1708–1715 www.elsevier.com/locate/lwt
Effect of crust temperature and water content on acrylamide formation during baking of white bread: Steam and falling temperature baking Lilia Ahrne´a,, Claes-Go¨ran Anderssona, Per Floberga, Johan Rose´nb, Hans Lingnerta a
SIK—The Swedish Institute for Food and Biotechnology, P.O. Box 5401, SE-402 29 Go¨teborg, Sweden b National Food Administration, Box 622, SE-751 26 Uppsala, Sweden Received 11 August 2006; received in revised form 22 December 2006; accepted 4 January 2007
Abstract The effect of crust temperature and water content on acrylamide formation was studied during the baking of white bread. To assess the effect of over-baking, we used a full factorial experimental design in which the baking time was increased by 5 and 10 min at each baking temperature. Additional experiments were performed with steam baking and falling temperature baking. Immediately after baking, the crust was divided into the outer and inner crust fractions, and the water content and acrylamide concentration of each fraction was measured. The outer crust had a significantly lower water content and higher acrylamide concentration than the inner crust did. Crust temperature in combination with water content had a significant effect on acrylamide formation, higher temperatures resulting in higher acrylamide concentrations. However, at very high temperatures and lower water contents, acrylamide concentration was observed to decrease, though the bread colour was then unacceptable for consumption. Steam and falling temperature baking, on the other hand, decreased the acrylamide content while producing bread crust with an acceptable colour. The lowest acrylamide values and an acceptable crust colour were produced by steam baking. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Baking; Acrylamide; Steam baking; Crust; Temperature; Water content
1. Introduction Bread baking is a complex process involving many physical and chemical changes. The optimum baking process depends on the type of bread to be baked and the desired bread characteristics. The surface colour of bread is an important quality associated with aroma, texture, and appearance characteristics important to consumers. Surface colour is often used as indicator of baking completion. Bread colour develops late in baking, simultaneously with crust formation, and arises from chemical reactions such as the Maillard reaction and sugar caramelization. The extent of these chemical reactions is largely influenced by the physical mechanisms of heat and water transport during baking. Thus, bread crust colour is influenced by the dough recipe and by the processing conditions during baking, i.e., time, temperaCorresponding author. Tel.: +46 31 33 55 600; fax: +46 31 833 782.
E-mail address: [email protected] (L. Ahrne´).
ture, air velocity, relative humidity, and rate of heat transfer. The Maillard reaction is important for the formation of colour and aroma in the bread crust, but may also be associated with the formation of toxic compounds, such as acrylamide (Mottram, Wedzicha, & Dodson, 2002; Stadler, Blank, Varga, Robert, & Riediker, 2002; Zyzak et al., 2003). Since the Swedish National Food Administration announced in April 2002 that acrylamide had been found in food products, research has been conducted worldwide to attain a better understanding of acrylamide formation mechanisms and to find ways to reduce such formation. Acrylamide has been found in substantial amounts in many different food products, mainly of plant origin, processed at temperatures above 100–120 1C. The highest amounts of acrylamide have been found in French Fries, potato crisps, and crisp bread (Tareke, Rydberg, Karlsson, Eriksson, & Tornqvist, 2002). Later studies, taking intake data into account, have shown that the main food categories responsible for acrylamide intake are potato products,
0023-6438/$30.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.01.010
ARTICLE IN PRESS L. Ahrne´ et al. / LWT 40 (2007) 1708–1715
cereal products (including bread), and coffee (Svensson et al., 2003; Matthys et al., 2005). CIAA, the Confederation of the Food and Drink Industries in the Europe, published in 2005 and updated in October 2006, the ‘‘Acrylamide Toolbox’’ containing relevant information about acrylamide in foods and ways to reduce its formation (www. ciaa.be). Several approaches to reducing the acrylamide content of bread have been reported in the literature in recent years. Haase, Matthaeus, and Vosmann (2003) discussed the formation of acrylamide in baked products, and identified flour milling intensity and baking temperature as important factors affecting acrylamide concentration in bread. Springer, Fischer, Lehrack, and Freund (2003) reported a 50% reduction in the acrylamide content by changing the temperature/moisture profile during the baking process. Surdyk, Rosen, Andersson, and Aman (2004) examined the effect of aspargine and fructose on acrylamide formation in white leavened bread at baking temperatures over 200 1C, finding a strong correlation between crust colour and acrylamide formation when the recipe remained constant. However, when a flour with a lower ash content was used, a lower acrylamide content and a similar crust colour were obtained. Fredriksson, Tallving, Rosen, and Aman (2004) suggested that extensive fermentation with yeast may be one strategy for reducing the acrylamide content of bread. Brathen and Knutsen (2005) examined the effect of time and temperature on the formation of acrylamide in bread, flat bread, dry starch systems, and dried rye-based flat bread. Mustafa, Andersson, Rosen, Kamal-Eldin, and Aman (2005) studied the effects of baking time and temperature and of changing the recipe for yeast-leavened whole-grain rye crisp bread (by adding fructose, asparagine, and oat bran concentrate) on its acrylamide content and colour. Brathen, Kita, Knutsen, and Wicklund (2005) and Fink, Andersson, Rosen, and Aman (2006) observed that the addition of glycine to dough significantly reduced the acrylamide content of both flat bread and bread crust. Baking temperature is an important parameter influencing acrylamide formation. Although the temperature of importance for the formation of acrylamide is the exact temperature in the bread surface, most studies only report the oven temperature. The heat is transported from the oven air to the bread, and since bread is a poor heat conductor, a temperature and water profile arises in the bread. This temperature profile and its development over time during baking are strongly influenced by the rate of heat transfer from the oven to the bread surface, and by the thermal and structural properties of the dough/bread that determine heat and water transport inside the bread. Bread crust is formed at the end of the baking process, when the bread surface temperature is over 100 1C and water loss in the bread surface is considerable. The crust fraction in contact with the bread crumb may become dried, as is typical of a crust, but may have poor colour development compared with the outer surface. The extension of
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chemical reactions, including the formation of acrylamide or colour, depend on the temperature and water distribution in the crust during baking. Very little research has examined the effect of the in situ water content and temperature of the bread crust during baking and their effect on acrylamide formation. This present research aimed to determine the effect of crust temperature and water content on acrylamide formation in bread crust, and to suggest ways to reduce it. 2. Material and methods 2.1. Recipe Wheat flour (Bagarns Ba¨sta, 18% protein, 0.45% ash, 0.017% aspargine, 0.013% glucose, and 0.005% fructose) obtained from Nordmills (Uppsala, Sweden), was used in this study. Bread dough was prepared as described in Table 1. The yeast was dissolved in 20 1C water and then added to the dry ingredients. All the ingredients were mixed for 2 min at slow speed and then 5 min at high speed in a spiral kneader (CDE Freviglio, Italy). Portions of dough, each weighing 200 g, were placed in rectangular baking tins and allowed to rest for 10 min at ambient temperature before proofing for 45 min at 35 1C and 80% RH. 2.2. Baking process The fermented dough was baked in the baking tins in a deck oven without air circulation (Dahlen-Nova, Sveba Dahlen AB, Fristad, Sweden) following the 23 factorial design with three central points as described in Table 2. During baking, the temperature was measured in the bread crust at depths of 1 and 2 mm from the surface and in the centre of the bread. Very thin copperconstantan thermocouples, 0.07 mm in diameter (type T; Pentronic AB, Sweden), connected to a logger and a computer were used to record the temperature. Additional experiments were done in a convection oven (Dahlen S400; Sveba Dahlen AB, Fristad, Sweden) to assess the effect of steam on the water content of the crust and on acrylamide formation. Steam was injected into the oven after 5, 10, and 15 min and retained in it until the end Table 1 Recipe for the bread dough Recipe
Weight (g)
Wheat flour Water Yeast Salt Improver (Lecimax 2000, Nordbakels, Sweden)
1850 1000 90 18.5 18.5
Total
2977
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1710 Table 2 Experimental design Exp no.
T oven (1C)
Baking timea (min)
1 2 3 4 5 6 7 8 9 10 11 12
200 230 260 200 230 260 200 230 260 230 230 230
tb (15) tb (12) tb (10) tb+5 (20) tb+5 (17) tb+5 (15) tb+10 (25) tb+10 (22) tb+10 (20) tb+5 (17) tb+5 (17) tb+5 (17)
a tb is the baking time necessary to reach a fully developed crumb, i.e. for the centre temperature of the bread to reach 98 1C.
of baking (20 min). Since steam reduced the crust temperature, other experiments were performed without steam but in which the oven temperature was adjusted to produce a temperature history curve in the bread similar to that occurring during steam baking. Table 3 shows a summary of the experiments performed.
2.3. Analyses of the bread Seven loaves of bread made from 200-g dough portions were baked in each experiment. Immediately after baking, the crust was separated from the crumb in six loaves. Two fractions of crust were obtained using a razor blade, crust I consisting of the outer surface crust, and crust II consisting of the inner crust in contact with the crumb. The thickness, water content, and water activity were measured in each crust fraction immediately after separation. Samples of each crust fraction were also collected for acrylamide analysis. The colour was measured at three different positions on the bread surface using a Minolta CR-10 camera. The L*a*b* colour space analysis method was used, where L* represents lightness and a* and b* the chromaticity co-ordinates. The result was reported in the form of a colour difference, dE ab , as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dE ab ¼ ðdL Þ2 þ ðda Þ2 þ ðdb Þ2 , where dL*, da*, and db* are the differences between the L*, a*, and b* values of the sample and the reference (a white ceramic plate for which L ¼ 93.4, a ¼ –1.8, and b ¼ 4.4). The thickness of the crust was measured using a digital calliper in three different places. Total water loss during baking, i.e., g water lost/g initial weight, was determined by measuring the weight of the whole loaf before and after baking. Water content was measured using the AOAC method (drying at 70 1C for 16 h) and water activity in an AquaLab Series 3 water activity metre (Decagon Devices, Pullman, WA, USA).
Table 3 Experiments performed to assess the effect of steam. Baking was performed at 200 1C, and the total baking time was 20 min Experiment
Steam (t1t2)
A B C D E F G
No steam 5–20 10–20 15–20 No steam; oven temperature decreased after 5 min baking No steam; oven temperature decreased after 10 min baking No steam; oven temperature decreased after 15 min baking
t1, time (min) when steam was turned on; t2, time (min) when steam was turned off; T(t), temperature time curve.
The acrylamide content was analysed with liquid chromatography tandem mass spectrometry (LCMS MS) using electrospray ionization. The dried and milled samples were extracted with water at room temperature, and deuterium-labelled acrylamide (Polymer Source Inc., Dorval, QC, Canada) was added as an internal standard. Two types of solid-phase extraction columns, Isolute Multimode (1 g) and ENV+ (1 g) (International Sorbent Technology, Hengoed, Mid Glamorgan, UK), were used to make an extract pure and concentrated enough to enable the analysis of solid matrices down to 2 mg kg–1. The extract was then injected three times into the LCMSMS system. Validation data in the 5–1000 mg kg–1 interval, obtained using spiked samples of mashed raw potatoes, were excellent with relative standard deviations of 29% and bias of 72%, thus, the limit of quantification (LOQ) was 5 mg kg1. Limit of detection (LOD) was 2 mg kg1. The laboratory has also participated in several proficiency tests for acrylamide in food. Most of the samples (n ¼ 9) represented cereals or bread in the 4.9711 mg kg–1 range. The obtained z-scores ranged from 0.55 to 0.60 (the limits for acceptable z-scores are 72.0.), indicating a relevant working range and the applicability of the method in the present work. A detailed description of the work-up procedure, validation results, and results of the proficiency tests has been published earlier (Fohgelberg, Rose´n, Hellena¨s, & Abramsson-Zetterberg, 2005), as has a description of the LCMSMS procedure (Rose´n & Hellena¨s, 2002). 3. Results and discussion 3.1. Traditional baking The effect of crust temperature and water content on acrylamide formation was studied during the baking of white bread in a deck oven without air circulation at 200, 230, and 260 1C. At each temperature, the shortest baking time was determined as that necessary to obtain a fully developed crumb, i.e., the time needed for the centre of the bread to reach a temperature of 98 1C. To assess the effect of over-baking, the shortest baking time was increased by 5
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Baking time/ baking temperature
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200°C
230°C
260°C
15 min
12 min
10 min
25 min
22 min
20 min
tb
tb+10
Fig. 1. Pictures of the bread baked at 200, 230, and 260 1C at the longest and shortest baking time chosen. tb is the baking time necessary to reach a fully developed crumb, i.e. for the centre temperature of the bread to reach 98 1C; tb+10 is the tb+10 min.
and 10 min. Fig. 1 gives an idea of the colour of bread baked at 200, 230, and 260 1C for the longest and shortest baking times chosen. The higher the baking temperature, the shorter the baking time needed to bake the crumb fully. The low thermal conductivity of the bread causes a low rate of conductive heat transfer to the centre of the bread, while the temperature at the surface, particularly influenced by radiative heat transfer from the oven walls to the bread surface, rises very rapidly. At 260 1C, baking for the time needed to bake the crumb fully creates an unacceptable crust colour. Fig. 2 shows in greater detail the temperature profile in the crumb and crust (1 and 2 mm) when baking bread at 200 and 260 1C. As the moisture at the surface evaporates and the surface dries out, the surface temperature rises towards that of the oven. Decreasing the oven temperature by 60 1C, from 260 to 200 1C, increased the time needed for the centre bread crumb to reach 98 1C from 10 to 15 min, at which time the crust temperature reached 200 and 150 1C, respectively. As baking takes place at atmospheric pressure, the centre temperature of the bread does not exceed 100 1C. Although the crust is thin, a difference of approximately 10 1C was observed between the outer (1 mm from the surface) and inner (2 mm from the surface) crust fractions at all baking temperatures tested. This temperature difference, together with restricted water evaporation from the inner crust, results in a difference between the water contents of the two crust fractions. Fig. 3 shows a significant difference of approximately 2% (g water/100 g crust) between the water contents of the two crust fractions. As expected, a lower water content was observed in the case of higher crust temperatures and longer baking times. Fig. 4 shows the relationship between the maximum temperature in the two crust fractions and their final water
content: higher water content was observed at lower crust temperatures, while the water content in the crust tends to level out towards 2–3% (g water/100 g crust) at higher crust temperatures. In accordance with the results reported by Surdyk et al.(2004) and Brathen and Knutsen (2005), we also observed a strong correlation between acrylamide formation and the baking temperature and time. Fig. 5 shows an increase in the amount of acrylamide formed in the crust with increased baking temperature; no acrylamide was observed in the bread crumb (i.e. o2 mg kg1). The highest concentration of acrylamide was observed in the case of baking at 260 1C for 15 min. However, in bread baked at 260 1C for 20 min, acrylamide concentration was lower, being lower in the outer than in the inner crust. Bagdonaite and Murkovic (2004) have reported similar results for roasting coffee, where beans roasted at 260 1C for 5 min contained the most acrylamide, and increased roasting time reduced the acrylamide concentration. Excluding the results obtained at 260 1C, the acrylamide concentration in the inner crust fraction (crust II) was 25–75% of that in the outer fraction (crust I) for baking at 200 and 230 1C. In Fig. 6 the concentration of acrylamide in crusts I and II is plotted against the respective maximum temperatures and water contents. Fig. 6a shows that the lowest crust temperature at which acrylamide was measured in these experiments was approximately 150 1C. A trend line drawn in Fig. 6 indicates that the formation of acrylamide apparently starts at approximately 120–130 1C. The highest acrylamide concentration (230718 mg kg1 crust) was observed in crust II at 221 1C (Fig. 6a), while crust I at a concurrent temperature of 231 1C had an acrylamide concentration of only 135717 mg kg1 crust. Degradation of acrylamide may have occurred in this temperature
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Oven 200°C
200 150 100 1 mm crust 2 mm crust center
50 0
0
5
10 15 20 25 Baking time (min)
Oven 260°C
250 Temperature (°C)
Temperature (°C)
250
200 150 100 2 mm crust 1 mm crust center
50 0
30
0
5
10 15 20 Baking time (min)
25
Fig. 2. Temperature profile in the crumb and crust (1 and 2 mm) during the baking of white bread at 200 and 260 1C.
water content (g water/100 g crust)
14
14 200°C
12
230°C
12
14
10
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0 0
5
10
15
20
25
0 0
30
5
10
15
20
Baking time (min)
Baking time (min)
25
30
260°C
12
0
5
10 15 20 Baking time (min)
25
30
water content (g water/100 g crust)
Fig. 3. Water content in the crust I (&; 1 mm from the surface) and crust II (’; 2 mm from the surface) during the baking of bread at 200, 230 and 260 1C.
12 10 8 6 4 2 0 120
140
160
180
200
220
240
Maximum temperature in the crust (°C) Fig. 4. Relationship between the maximum temperature in the two crust fractions and their final water content (& crust I and ’ crust II).
range; however, the water content seems to play an important role in any such degradation process. Fig. 6b shows increased acrylamide concentrations with decreased water content down to 4% (g water/100 g crust), which seems largely correlated to the increased temperature (Fig. 6a). However, thereafter, as the crust temperature continues to increase and the water content to decrease, the acrylamide concentration tends to decrease. Fig. 6b indicates great variation and a trend towards decreased
acrylamide concentration with decreased crust water content in the 4–2% (g water/100 g crust) range. However, it is difficult to separate the effect of temperature from that of water content. The lower acrylamide concentration produced at a higher temperature was observed in a crust having a 2.7% (g water/100 g crust) water content. A strong correlation between colour and acrylamide concentration, when varying baking time and temperature but using a constant recipe, has been reported by Surdyk et al.(2004). This is supported by our results as long as the colour is within the acceptable limits. Our results indicated a strong correlation between crust temperature and colour (Fig. 7a); meaning that acrylamide concentration and colour are well correlated only up to a colour value of DE ¼ 65, after which acrylamide content decreases simultaneously with water content (Fig. 7b). A crust colour value above DE ¼ 60 is, however, considered unacceptable for consumption. 3.2. Steam and falling temperature baking Steam baking is commonly used during the initial stages of baking to improve crust texture and colour. Previous studies showed that baking cakes in highly humid air lightened the cake crust colour and raised the final water content of the cake (Xue & Walker, 2003). Since steam can influence both the water content and crust colour, in the present study, steam was supplied to the oven at different
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Acrylamide (µg/kg)
250
200°C
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230°C
200
260°C
150 100 50 0 10
15
20
25
30 10
15
Baking time (min)
25 10
20
15
20
25
Baking time (min)
Baking time (min)
Fig. 5. Amount of acrylamide formed in the crust I (&) and crust II (’) during baking at 200, 230 and 260 1C.
Acrylamide (µg/kg)
300 250
a
b
200 150 100 50 0 120
140
160
180
200
220
240 0
2
4
6
8
10
12
water content in the crust (g water/100 g crust)
Maximum temperature in the crust (ºC)
b 240
200
220 200
150
180
100
160
50
140
0
120 40
50
60
70
80
Crust colour (dE)
9 8 7 6 5 4 3 2 1 0
250 Acrylamide (µg/kg)
250
Crust temperature (°C)
Acrylamide (µg/kg)
a
200 150 100 50 0
45
50 55 60 65 70 Crust colour (dE)
75
Water content (g/100 g crust)
Fig. 6. Amount of acrylamide formed in the crust I (&) and crust II (’) against the respective (a) maximum temperatures and (b) water contents.
Fig. 7. Relationship between crust colour, acrylamide formation, crust temperature and water content. (a) ’ Acrylamide content and & maximum temperature in the crust; (b) ’ acrylamide content and n water content in the crust.
stages of baking, and its effect on acrylamide formation evaluated. Fig. 8 presents an example of the effect of steam on the temperature of the bread crust (at depths of 1 and 2 mm) and crumb during baking. In this case, steam was injected into the oven after 10 min of baking (experiment C) and retained until the end of baking (20 min). Steam baking decreased both the crust temperature and the temperature difference between the outer and inner crust, resulting in a lower acrylamide concentration (Fig. 9) than in baking without steam. Baking with steam for a longer time (the last 15 min of baking; experiment B) produced a lower acrylamide concentration. The use of steam for the last 10 min of
baking (experiment C) reduced the concentration of acrylamide by almost 50% compared with the level when baking without steam (experiment A). Although no local water measurements were possible during steam baking, we expected that steam would increase the crust water content, resulting in better heat transfer than in the dry crust. To imitate the decreased temperature caused by steam, and to further elucidate the effect of the water content of the crust, additional experiments were performed that involved decreasing the oven temperature (experiments E, F, and G). Fig. 10 shows an example of the crust temperature in experiment C (with steam) and in experiments E and F (temperature lowered after 5 and 10 min of baking,
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respectively). Lowering the oven temperature after 5, 10 and 15 min of baking reduced the acrylamide concentration by 67%, 36%, 35%, respectively (Fig. 9), compared with that produced by constant temperature baking. Fig. 11 presents the colour differences between breads produced under different baking conditions. Using steam, it was possible to bake bread to approximately the same colour level as that produced by traditional baking, but with considerably lower acrylamide levels (compare A with C and D). With falling temperature baking, however, though acrylamide levels are lowered, the bread colour is lighter (compare F and G with A).
pressure and the crumb temperature did not exceed 100 1C. A significant difference in temperature, water content, and acrylamide concentration was observed 200 Crust temperature (°C)
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Falling - exp F
160 Falling - exp E
Steam - exp C
120 80 40
4. Conclusions This study has shown that acrylamide formation during baking in yeast-leavened white bread occurs in the bread crust. No acrylamide was measured in the crumb (i.e. o2 mg kg1), since baking was done at atmospheric
0 0.0
5.0
10.0
15.0
20.0
Baking time (min) Fig. 10. Temperature in crust I during baking at steam (experiment C) and falling temperature baking (experiment E and F).
250 60 2 mm
crurst colour (dE)
Temperature (°C)
200 150 1 mm 100
center
Steam baking C
50 B
Traditional baking
D
A G
E
F
40
Falling temperature baking
50 30 0
0 0
5
10 15 Baking time (min)
20
20
40
25
Fig. 8. Temperature profile in the bread crust (1 and 2 mm) and crumb during the steam baking (experiment C; Table 3). Oven temperature was 200 1C and the total baking time 20 min.
60 80 100 Acrylamide (µg/kg)
AA crust I
Acrylamide (µg/kg)
120
AA crust II
100 80 60 40 20 0
No steam
B
C Steam
140
Fig. 11. Relationship between colour and acrylamide formation during steam and falling temperature baking. A: Traditional baking; B, C and D: (closed symbols) experiments with steam and E, F and G: falling temperature baking (open symbols).
140
A
120
D
E
F
G
Falling temperature
Fig. 9. Amount of acrylamide formed in crusts I and II during steam and falling temperature baking (see Table 3).
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between the outer and inner crust fractions of the bread. For baking at 200 and 230 1C, the acrylamide concentration in the outer crust was 2–3 times higher than in the inner fraction. Baking at 260 1C decreased the acrylamide concentration with longer baking times but produced an unacceptably dark colour; this finding strongly agrees with those of previous studies of coffee roasting. Examining the correlations between acrylamide concentration, maximum crust temperature, crust colour, and crust water content revealed that both crust temperature and crust water content significantly affect acrylamide formation. However, the relatively lower values of acrylamide observed in the case of high-temperature baking and low water contents are unacceptable from a consumer standpoint, as the bread colour is far too dark and other sensory attributes of the bread are unacceptable. Steam and falling temperature baking are simple modifications of traditional baking that can produce bread with an acceptable crust colour, and significantly reduce its acrylamide content. The lowest acrylamide values and an acceptable crust colour were produced by steam baking. References Bagdonaite, K., & Murkovic, M. (2004). Factors affecting the formation of acrylamide in coffee. Czech Journal of Food Sciences, 22, 22–24. Brathen, E., Kita, A., Knutsen, S. H., & Wicklund, T. (2005). Addition of glycine reduces the content of acrylamide in cereal and potato products. Journal of Agricultural and Food Chemistry, 53(8), 3259–3264. Brathen, E., & Knutsen, S. H. (2005). Effect of temperature and time on the formation of acrylamide in starch-based and cereal model systems, flat breads and bread. Food Chemistry, 92(4), 693–700. Fink, M., Andersson, R., Rosen, J., & Aman, P. (2006). Effect of added asparagine and glycine on acrylamide content in yeast-leavened bread. Cereal Chemistry, 83(2), 218–222. Fohgelberg, P., Rose´n, J., Hellena¨s, K. E., & Abramsson-Zetterberg, L. (2005). The acrylamide intake via some common baby food for children in Sweden during their first year of life—an improved method
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for analysis of acrylamide. Food and Chemical Toxicology, 43(6), 951–959. Fredriksson, H., Tallving, J., Rosen, J., & Aman, P. (2004). Fermentation reduces free asparagine in dough and acrylamide content in bread. Cereal Chemistry, 81(5), 650–653. Haase, N., Matthaeus, B., & Vosmann, K. (2003). Acrylamide in baked products—state of the art. Getreide, Mehl und Brot, 57(3), 180–184. Matthys, C., Bilau, M., Govaert, Y., Moons, E., De Henauw, S., & Willems, J. L. (2005). Risk assessment of dietary acrylamide intake in Flemish adolescents. Food and Chemical Toxicology, 43(2), 271–278. Mottram, D. S., Wedzicha, B. L., & Dodson, A. T. (2002). Acrylamide is formed in the Mailard reaction. Nature, 419, 448–449. Mustafa, A., Andersson, R., Rosen, J., Kamal-Eldin, A., & Aman, P. (2005). Factors influencing acrylamide content and color in rye crisp bread. Journal of Agricultural and Food Chemistry, 53(15), 5985–5989. Rose´n, J., & Hellena¨s, K.-E. (2002). Analysis of acrylamide in cooked foods by liquid chromatography tandem mass spectrometry. Analyst, 127, 880–882. Springer, M., Fischer, T., Lehrack, A., & Freund, W. (2003). Development of acrylamide in baked products. Getreide Mehl und Brot, 57(5), 274–278. Stadler, R. H., Blank, N., Varga, F., Robert, F., & Riediker, S. (2002). Acrylamide from Maillard reaction products. Nature, 419(6906), 449–450. Surdyk, N., Rosen, J., Andersson, R., & Aman, P. (2004). Effects of asparagine, fructose, and baking conditions on acrylamide content in yeast-leavened wheat bread. Journal of Agricultural and Food Chemistry, 52(7), 2047–2051. Svensson, K., Abramsson, L., Becker, W., Glynn, A., Hellenas, K.-E., Lind, Y., et al. (2003). Dietary intake of acrylamide in Sweden. Food and Chemical Toxicology, 41(11), 1581–1586. 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(17), 4998–5006. Xue, J., & Walker, C. E. (2003). Humidity change and its effects on baking in an electrically heated air jet impingement oven. Food Research International, 36(6), 561–569. Zyzak, D. V., Sanders, R. A., Stojanovic, M., Tallmadge, D. H., Eberhart, B. L., Ewald, D. K., et al. (2003). Acrylamide formation mechanism in heated foods. Journal of Agricultural and Food Chemistry, 51(16), 4782–4787.
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Effect of crust temperature and water content on acrylamide formation during baking of white bread: Steam and falling temperature baking Lilia Ahrne´a,, Claes-Go¨ran Anderssona, Per Floberga, Johan Rose´nb, Hans Lingnerta a
SIK—The Swedish Institute for Food and Biotechnology, P.O. Box 5401, SE-402 29 Go¨teborg, Sweden b National Food Administration, Box 622, SE-751 26 Uppsala, Sweden Received 11 August 2006; received in revised form 22 December 2006; accepted 4 January 2007
Abstract The effect of crust temperature and water content on acrylamide formation was studied during the baking of white bread. To assess the effect of over-baking, we used a full factorial experimental design in which the baking time was increased by 5 and 10 min at each baking temperature. Additional experiments were performed with steam baking and falling temperature baking. Immediately after baking, the crust was divided into the outer and inner crust fractions, and the water content and acrylamide concentration of each fraction was measured. The outer crust had a significantly lower water content and higher acrylamide concentration than the inner crust did. Crust temperature in combination with water content had a significant effect on acrylamide formation, higher temperatures resulting in higher acrylamide concentrations. However, at very high temperatures and lower water contents, acrylamide concentration was observed to decrease, though the bread colour was then unacceptable for consumption. Steam and falling temperature baking, on the other hand, decreased the acrylamide content while producing bread crust with an acceptable colour. The lowest acrylamide values and an acceptable crust colour were produced by steam baking. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Baking; Acrylamide; Steam baking; Crust; Temperature; Water content
1. Introduction Bread baking is a complex process involving many physical and chemical changes. The optimum baking process depends on the type of bread to be baked and the desired bread characteristics. The surface colour of bread is an important quality associated with aroma, texture, and appearance characteristics important to consumers. Surface colour is often used as indicator of baking completion. Bread colour develops late in baking, simultaneously with crust formation, and arises from chemical reactions such as the Maillard reaction and sugar caramelization. The extent of these chemical reactions is largely influenced by the physical mechanisms of heat and water transport during baking. Thus, bread crust colour is influenced by the dough recipe and by the processing conditions during baking, i.e., time, temperaCorresponding author. Tel.: +46 31 33 55 600; fax: +46 31 833 782.
E-mail address: [email protected] (L. Ahrne´).
ture, air velocity, relative humidity, and rate of heat transfer. The Maillard reaction is important for the formation of colour and aroma in the bread crust, but may also be associated with the formation of toxic compounds, such as acrylamide (Mottram, Wedzicha, & Dodson, 2002; Stadler, Blank, Varga, Robert, & Riediker, 2002; Zyzak et al., 2003). Since the Swedish National Food Administration announced in April 2002 that acrylamide had been found in food products, research has been conducted worldwide to attain a better understanding of acrylamide formation mechanisms and to find ways to reduce such formation. Acrylamide has been found in substantial amounts in many different food products, mainly of plant origin, processed at temperatures above 100–120 1C. The highest amounts of acrylamide have been found in French Fries, potato crisps, and crisp bread (Tareke, Rydberg, Karlsson, Eriksson, & Tornqvist, 2002). Later studies, taking intake data into account, have shown that the main food categories responsible for acrylamide intake are potato products,
0023-6438/$30.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.01.010
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cereal products (including bread), and coffee (Svensson et al., 2003; Matthys et al., 2005). CIAA, the Confederation of the Food and Drink Industries in the Europe, published in 2005 and updated in October 2006, the ‘‘Acrylamide Toolbox’’ containing relevant information about acrylamide in foods and ways to reduce its formation (www. ciaa.be). Several approaches to reducing the acrylamide content of bread have been reported in the literature in recent years. Haase, Matthaeus, and Vosmann (2003) discussed the formation of acrylamide in baked products, and identified flour milling intensity and baking temperature as important factors affecting acrylamide concentration in bread. Springer, Fischer, Lehrack, and Freund (2003) reported a 50% reduction in the acrylamide content by changing the temperature/moisture profile during the baking process. Surdyk, Rosen, Andersson, and Aman (2004) examined the effect of aspargine and fructose on acrylamide formation in white leavened bread at baking temperatures over 200 1C, finding a strong correlation between crust colour and acrylamide formation when the recipe remained constant. However, when a flour with a lower ash content was used, a lower acrylamide content and a similar crust colour were obtained. Fredriksson, Tallving, Rosen, and Aman (2004) suggested that extensive fermentation with yeast may be one strategy for reducing the acrylamide content of bread. Brathen and Knutsen (2005) examined the effect of time and temperature on the formation of acrylamide in bread, flat bread, dry starch systems, and dried rye-based flat bread. Mustafa, Andersson, Rosen, Kamal-Eldin, and Aman (2005) studied the effects of baking time and temperature and of changing the recipe for yeast-leavened whole-grain rye crisp bread (by adding fructose, asparagine, and oat bran concentrate) on its acrylamide content and colour. Brathen, Kita, Knutsen, and Wicklund (2005) and Fink, Andersson, Rosen, and Aman (2006) observed that the addition of glycine to dough significantly reduced the acrylamide content of both flat bread and bread crust. Baking temperature is an important parameter influencing acrylamide formation. Although the temperature of importance for the formation of acrylamide is the exact temperature in the bread surface, most studies only report the oven temperature. The heat is transported from the oven air to the bread, and since bread is a poor heat conductor, a temperature and water profile arises in the bread. This temperature profile and its development over time during baking are strongly influenced by the rate of heat transfer from the oven to the bread surface, and by the thermal and structural properties of the dough/bread that determine heat and water transport inside the bread. Bread crust is formed at the end of the baking process, when the bread surface temperature is over 100 1C and water loss in the bread surface is considerable. The crust fraction in contact with the bread crumb may become dried, as is typical of a crust, but may have poor colour development compared with the outer surface. The extension of
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chemical reactions, including the formation of acrylamide or colour, depend on the temperature and water distribution in the crust during baking. Very little research has examined the effect of the in situ water content and temperature of the bread crust during baking and their effect on acrylamide formation. This present research aimed to determine the effect of crust temperature and water content on acrylamide formation in bread crust, and to suggest ways to reduce it. 2. Material and methods 2.1. Recipe Wheat flour (Bagarns Ba¨sta, 18% protein, 0.45% ash, 0.017% aspargine, 0.013% glucose, and 0.005% fructose) obtained from Nordmills (Uppsala, Sweden), was used in this study. Bread dough was prepared as described in Table 1. The yeast was dissolved in 20 1C water and then added to the dry ingredients. All the ingredients were mixed for 2 min at slow speed and then 5 min at high speed in a spiral kneader (CDE Freviglio, Italy). Portions of dough, each weighing 200 g, were placed in rectangular baking tins and allowed to rest for 10 min at ambient temperature before proofing for 45 min at 35 1C and 80% RH. 2.2. Baking process The fermented dough was baked in the baking tins in a deck oven without air circulation (Dahlen-Nova, Sveba Dahlen AB, Fristad, Sweden) following the 23 factorial design with three central points as described in Table 2. During baking, the temperature was measured in the bread crust at depths of 1 and 2 mm from the surface and in the centre of the bread. Very thin copperconstantan thermocouples, 0.07 mm in diameter (type T; Pentronic AB, Sweden), connected to a logger and a computer were used to record the temperature. Additional experiments were done in a convection oven (Dahlen S400; Sveba Dahlen AB, Fristad, Sweden) to assess the effect of steam on the water content of the crust and on acrylamide formation. Steam was injected into the oven after 5, 10, and 15 min and retained in it until the end Table 1 Recipe for the bread dough Recipe
Weight (g)
Wheat flour Water Yeast Salt Improver (Lecimax 2000, Nordbakels, Sweden)
1850 1000 90 18.5 18.5
Total
2977
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1710 Table 2 Experimental design Exp no.
T oven (1C)
Baking timea (min)
1 2 3 4 5 6 7 8 9 10 11 12
200 230 260 200 230 260 200 230 260 230 230 230
tb (15) tb (12) tb (10) tb+5 (20) tb+5 (17) tb+5 (15) tb+10 (25) tb+10 (22) tb+10 (20) tb+5 (17) tb+5 (17) tb+5 (17)
a tb is the baking time necessary to reach a fully developed crumb, i.e. for the centre temperature of the bread to reach 98 1C.
of baking (20 min). Since steam reduced the crust temperature, other experiments were performed without steam but in which the oven temperature was adjusted to produce a temperature history curve in the bread similar to that occurring during steam baking. Table 3 shows a summary of the experiments performed.
2.3. Analyses of the bread Seven loaves of bread made from 200-g dough portions were baked in each experiment. Immediately after baking, the crust was separated from the crumb in six loaves. Two fractions of crust were obtained using a razor blade, crust I consisting of the outer surface crust, and crust II consisting of the inner crust in contact with the crumb. The thickness, water content, and water activity were measured in each crust fraction immediately after separation. Samples of each crust fraction were also collected for acrylamide analysis. The colour was measured at three different positions on the bread surface using a Minolta CR-10 camera. The L*a*b* colour space analysis method was used, where L* represents lightness and a* and b* the chromaticity co-ordinates. The result was reported in the form of a colour difference, dE ab , as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dE ab ¼ ðdL Þ2 þ ðda Þ2 þ ðdb Þ2 , where dL*, da*, and db* are the differences between the L*, a*, and b* values of the sample and the reference (a white ceramic plate for which L ¼ 93.4, a ¼ –1.8, and b ¼ 4.4). The thickness of the crust was measured using a digital calliper in three different places. Total water loss during baking, i.e., g water lost/g initial weight, was determined by measuring the weight of the whole loaf before and after baking. Water content was measured using the AOAC method (drying at 70 1C for 16 h) and water activity in an AquaLab Series 3 water activity metre (Decagon Devices, Pullman, WA, USA).
Table 3 Experiments performed to assess the effect of steam. Baking was performed at 200 1C, and the total baking time was 20 min Experiment
Steam (t1t2)
A B C D E F G
No steam 5–20 10–20 15–20 No steam; oven temperature decreased after 5 min baking No steam; oven temperature decreased after 10 min baking No steam; oven temperature decreased after 15 min baking
t1, time (min) when steam was turned on; t2, time (min) when steam was turned off; T(t), temperature time curve.
The acrylamide content was analysed with liquid chromatography tandem mass spectrometry (LCMS MS) using electrospray ionization. The dried and milled samples were extracted with water at room temperature, and deuterium-labelled acrylamide (Polymer Source Inc., Dorval, QC, Canada) was added as an internal standard. Two types of solid-phase extraction columns, Isolute Multimode (1 g) and ENV+ (1 g) (International Sorbent Technology, Hengoed, Mid Glamorgan, UK), were used to make an extract pure and concentrated enough to enable the analysis of solid matrices down to 2 mg kg–1. The extract was then injected three times into the LCMSMS system. Validation data in the 5–1000 mg kg–1 interval, obtained using spiked samples of mashed raw potatoes, were excellent with relative standard deviations of 29% and bias of 72%, thus, the limit of quantification (LOQ) was 5 mg kg1. Limit of detection (LOD) was 2 mg kg1. The laboratory has also participated in several proficiency tests for acrylamide in food. Most of the samples (n ¼ 9) represented cereals or bread in the 4.9711 mg kg–1 range. The obtained z-scores ranged from 0.55 to 0.60 (the limits for acceptable z-scores are 72.0.), indicating a relevant working range and the applicability of the method in the present work. A detailed description of the work-up procedure, validation results, and results of the proficiency tests has been published earlier (Fohgelberg, Rose´n, Hellena¨s, & Abramsson-Zetterberg, 2005), as has a description of the LCMSMS procedure (Rose´n & Hellena¨s, 2002). 3. Results and discussion 3.1. Traditional baking The effect of crust temperature and water content on acrylamide formation was studied during the baking of white bread in a deck oven without air circulation at 200, 230, and 260 1C. At each temperature, the shortest baking time was determined as that necessary to obtain a fully developed crumb, i.e., the time needed for the centre of the bread to reach a temperature of 98 1C. To assess the effect of over-baking, the shortest baking time was increased by 5
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Baking time/ baking temperature
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200°C
230°C
260°C
15 min
12 min
10 min
25 min
22 min
20 min
tb
tb+10
Fig. 1. Pictures of the bread baked at 200, 230, and 260 1C at the longest and shortest baking time chosen. tb is the baking time necessary to reach a fully developed crumb, i.e. for the centre temperature of the bread to reach 98 1C; tb+10 is the tb+10 min.
and 10 min. Fig. 1 gives an idea of the colour of bread baked at 200, 230, and 260 1C for the longest and shortest baking times chosen. The higher the baking temperature, the shorter the baking time needed to bake the crumb fully. The low thermal conductivity of the bread causes a low rate of conductive heat transfer to the centre of the bread, while the temperature at the surface, particularly influenced by radiative heat transfer from the oven walls to the bread surface, rises very rapidly. At 260 1C, baking for the time needed to bake the crumb fully creates an unacceptable crust colour. Fig. 2 shows in greater detail the temperature profile in the crumb and crust (1 and 2 mm) when baking bread at 200 and 260 1C. As the moisture at the surface evaporates and the surface dries out, the surface temperature rises towards that of the oven. Decreasing the oven temperature by 60 1C, from 260 to 200 1C, increased the time needed for the centre bread crumb to reach 98 1C from 10 to 15 min, at which time the crust temperature reached 200 and 150 1C, respectively. As baking takes place at atmospheric pressure, the centre temperature of the bread does not exceed 100 1C. Although the crust is thin, a difference of approximately 10 1C was observed between the outer (1 mm from the surface) and inner (2 mm from the surface) crust fractions at all baking temperatures tested. This temperature difference, together with restricted water evaporation from the inner crust, results in a difference between the water contents of the two crust fractions. Fig. 3 shows a significant difference of approximately 2% (g water/100 g crust) between the water contents of the two crust fractions. As expected, a lower water content was observed in the case of higher crust temperatures and longer baking times. Fig. 4 shows the relationship between the maximum temperature in the two crust fractions and their final water
content: higher water content was observed at lower crust temperatures, while the water content in the crust tends to level out towards 2–3% (g water/100 g crust) at higher crust temperatures. In accordance with the results reported by Surdyk et al.(2004) and Brathen and Knutsen (2005), we also observed a strong correlation between acrylamide formation and the baking temperature and time. Fig. 5 shows an increase in the amount of acrylamide formed in the crust with increased baking temperature; no acrylamide was observed in the bread crumb (i.e. o2 mg kg1). The highest concentration of acrylamide was observed in the case of baking at 260 1C for 15 min. However, in bread baked at 260 1C for 20 min, acrylamide concentration was lower, being lower in the outer than in the inner crust. Bagdonaite and Murkovic (2004) have reported similar results for roasting coffee, where beans roasted at 260 1C for 5 min contained the most acrylamide, and increased roasting time reduced the acrylamide concentration. Excluding the results obtained at 260 1C, the acrylamide concentration in the inner crust fraction (crust II) was 25–75% of that in the outer fraction (crust I) for baking at 200 and 230 1C. In Fig. 6 the concentration of acrylamide in crusts I and II is plotted against the respective maximum temperatures and water contents. Fig. 6a shows that the lowest crust temperature at which acrylamide was measured in these experiments was approximately 150 1C. A trend line drawn in Fig. 6 indicates that the formation of acrylamide apparently starts at approximately 120–130 1C. The highest acrylamide concentration (230718 mg kg1 crust) was observed in crust II at 221 1C (Fig. 6a), while crust I at a concurrent temperature of 231 1C had an acrylamide concentration of only 135717 mg kg1 crust. Degradation of acrylamide may have occurred in this temperature
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Oven 200°C
200 150 100 1 mm crust 2 mm crust center
50 0
0
5
10 15 20 25 Baking time (min)
Oven 260°C
250 Temperature (°C)
Temperature (°C)
250
200 150 100 2 mm crust 1 mm crust center
50 0
30
0
5
10 15 20 Baking time (min)
25
Fig. 2. Temperature profile in the crumb and crust (1 and 2 mm) during the baking of white bread at 200 and 260 1C.
water content (g water/100 g crust)
14
14 200°C
12
230°C
12
14
10
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0 0
5
10
15
20
25
0 0
30
5
10
15
20
Baking time (min)
Baking time (min)
25
30
260°C
12
0
5
10 15 20 Baking time (min)
25
30
water content (g water/100 g crust)
Fig. 3. Water content in the crust I (&; 1 mm from the surface) and crust II (’; 2 mm from the surface) during the baking of bread at 200, 230 and 260 1C.
12 10 8 6 4 2 0 120
140
160
180
200
220
240
Maximum temperature in the crust (°C) Fig. 4. Relationship between the maximum temperature in the two crust fractions and their final water content (& crust I and ’ crust II).
range; however, the water content seems to play an important role in any such degradation process. Fig. 6b shows increased acrylamide concentrations with decreased water content down to 4% (g water/100 g crust), which seems largely correlated to the increased temperature (Fig. 6a). However, thereafter, as the crust temperature continues to increase and the water content to decrease, the acrylamide concentration tends to decrease. Fig. 6b indicates great variation and a trend towards decreased
acrylamide concentration with decreased crust water content in the 4–2% (g water/100 g crust) range. However, it is difficult to separate the effect of temperature from that of water content. The lower acrylamide concentration produced at a higher temperature was observed in a crust having a 2.7% (g water/100 g crust) water content. A strong correlation between colour and acrylamide concentration, when varying baking time and temperature but using a constant recipe, has been reported by Surdyk et al.(2004). This is supported by our results as long as the colour is within the acceptable limits. Our results indicated a strong correlation between crust temperature and colour (Fig. 7a); meaning that acrylamide concentration and colour are well correlated only up to a colour value of DE ¼ 65, after which acrylamide content decreases simultaneously with water content (Fig. 7b). A crust colour value above DE ¼ 60 is, however, considered unacceptable for consumption. 3.2. Steam and falling temperature baking Steam baking is commonly used during the initial stages of baking to improve crust texture and colour. Previous studies showed that baking cakes in highly humid air lightened the cake crust colour and raised the final water content of the cake (Xue & Walker, 2003). Since steam can influence both the water content and crust colour, in the present study, steam was supplied to the oven at different
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Acrylamide (µg/kg)
250
200°C
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230°C
200
260°C
150 100 50 0 10
15
20
25
30 10
15
Baking time (min)
25 10
20
15
20
25
Baking time (min)
Baking time (min)
Fig. 5. Amount of acrylamide formed in the crust I (&) and crust II (’) during baking at 200, 230 and 260 1C.
Acrylamide (µg/kg)
300 250
a
b
200 150 100 50 0 120
140
160
180
200
220
240 0
2
4
6
8
10
12
water content in the crust (g water/100 g crust)
Maximum temperature in the crust (ºC)
b 240
200
220 200
150
180
100
160
50
140
0
120 40
50
60
70
80
Crust colour (dE)
9 8 7 6 5 4 3 2 1 0
250 Acrylamide (µg/kg)
250
Crust temperature (°C)
Acrylamide (µg/kg)
a
200 150 100 50 0
45
50 55 60 65 70 Crust colour (dE)
75
Water content (g/100 g crust)
Fig. 6. Amount of acrylamide formed in the crust I (&) and crust II (’) against the respective (a) maximum temperatures and (b) water contents.
Fig. 7. Relationship between crust colour, acrylamide formation, crust temperature and water content. (a) ’ Acrylamide content and & maximum temperature in the crust; (b) ’ acrylamide content and n water content in the crust.
stages of baking, and its effect on acrylamide formation evaluated. Fig. 8 presents an example of the effect of steam on the temperature of the bread crust (at depths of 1 and 2 mm) and crumb during baking. In this case, steam was injected into the oven after 10 min of baking (experiment C) and retained until the end of baking (20 min). Steam baking decreased both the crust temperature and the temperature difference between the outer and inner crust, resulting in a lower acrylamide concentration (Fig. 9) than in baking without steam. Baking with steam for a longer time (the last 15 min of baking; experiment B) produced a lower acrylamide concentration. The use of steam for the last 10 min of
baking (experiment C) reduced the concentration of acrylamide by almost 50% compared with the level when baking without steam (experiment A). Although no local water measurements were possible during steam baking, we expected that steam would increase the crust water content, resulting in better heat transfer than in the dry crust. To imitate the decreased temperature caused by steam, and to further elucidate the effect of the water content of the crust, additional experiments were performed that involved decreasing the oven temperature (experiments E, F, and G). Fig. 10 shows an example of the crust temperature in experiment C (with steam) and in experiments E and F (temperature lowered after 5 and 10 min of baking,
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respectively). Lowering the oven temperature after 5, 10 and 15 min of baking reduced the acrylamide concentration by 67%, 36%, 35%, respectively (Fig. 9), compared with that produced by constant temperature baking. Fig. 11 presents the colour differences between breads produced under different baking conditions. Using steam, it was possible to bake bread to approximately the same colour level as that produced by traditional baking, but with considerably lower acrylamide levels (compare A with C and D). With falling temperature baking, however, though acrylamide levels are lowered, the bread colour is lighter (compare F and G with A).
pressure and the crumb temperature did not exceed 100 1C. A significant difference in temperature, water content, and acrylamide concentration was observed 200 Crust temperature (°C)
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Falling - exp F
160 Falling - exp E
Steam - exp C
120 80 40
4. Conclusions This study has shown that acrylamide formation during baking in yeast-leavened white bread occurs in the bread crust. No acrylamide was measured in the crumb (i.e. o2 mg kg1), since baking was done at atmospheric
0 0.0
5.0
10.0
15.0
20.0
Baking time (min) Fig. 10. Temperature in crust I during baking at steam (experiment C) and falling temperature baking (experiment E and F).
250 60 2 mm
crurst colour (dE)
Temperature (°C)
200 150 1 mm 100
center
Steam baking C
50 B
Traditional baking
D
A G
E
F
40
Falling temperature baking
50 30 0
0 0
5
10 15 Baking time (min)
20
20
40
25
Fig. 8. Temperature profile in the bread crust (1 and 2 mm) and crumb during the steam baking (experiment C; Table 3). Oven temperature was 200 1C and the total baking time 20 min.
60 80 100 Acrylamide (µg/kg)
AA crust I
Acrylamide (µg/kg)
120
AA crust II
100 80 60 40 20 0
No steam
B
C Steam
140
Fig. 11. Relationship between colour and acrylamide formation during steam and falling temperature baking. A: Traditional baking; B, C and D: (closed symbols) experiments with steam and E, F and G: falling temperature baking (open symbols).
140
A
120
D
E
F
G
Falling temperature
Fig. 9. Amount of acrylamide formed in crusts I and II during steam and falling temperature baking (see Table 3).
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between the outer and inner crust fractions of the bread. For baking at 200 and 230 1C, the acrylamide concentration in the outer crust was 2–3 times higher than in the inner fraction. Baking at 260 1C decreased the acrylamide concentration with longer baking times but produced an unacceptably dark colour; this finding strongly agrees with those of previous studies of coffee roasting. Examining the correlations between acrylamide concentration, maximum crust temperature, crust colour, and crust water content revealed that both crust temperature and crust water content significantly affect acrylamide formation. However, the relatively lower values of acrylamide observed in the case of high-temperature baking and low water contents are unacceptable from a consumer standpoint, as the bread colour is far too dark and other sensory attributes of the bread are unacceptable. Steam and falling temperature baking are simple modifications of traditional baking that can produce bread with an acceptable crust colour, and significantly reduce its acrylamide content. The lowest acrylamide values and an acceptable crust colour were produced by steam baking. References Bagdonaite, K., & Murkovic, M. (2004). Factors affecting the formation of acrylamide in coffee. Czech Journal of Food Sciences, 22, 22–24. Brathen, E., Kita, A., Knutsen, S. H., & Wicklund, T. (2005). Addition of glycine reduces the content of acrylamide in cereal and potato products. Journal of Agricultural and Food Chemistry, 53(8), 3259–3264. Brathen, E., & Knutsen, S. H. (2005). Effect of temperature and time on the formation of acrylamide in starch-based and cereal model systems, flat breads and bread. Food Chemistry, 92(4), 693–700. Fink, M., Andersson, R., Rosen, J., & Aman, P. (2006). Effect of added asparagine and glycine on acrylamide content in yeast-leavened bread. Cereal Chemistry, 83(2), 218–222. Fohgelberg, P., Rose´n, J., Hellena¨s, K. E., & Abramsson-Zetterberg, L. (2005). The acrylamide intake via some common baby food for children in Sweden during their first year of life—an improved method
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for analysis of acrylamide. Food and Chemical Toxicology, 43(6), 951–959. Fredriksson, H., Tallving, J., Rosen, J., & Aman, P. (2004). Fermentation reduces free asparagine in dough and acrylamide content in bread. Cereal Chemistry, 81(5), 650–653. Haase, N., Matthaeus, B., & Vosmann, K. (2003). Acrylamide in baked products—state of the art. Getreide, Mehl und Brot, 57(3), 180–184. Matthys, C., Bilau, M., Govaert, Y., Moons, E., De Henauw, S., & Willems, J. L. (2005). Risk assessment of dietary acrylamide intake in Flemish adolescents. Food and Chemical Toxicology, 43(2), 271–278. Mottram, D. S., Wedzicha, B. L., & Dodson, A. T. (2002). Acrylamide is formed in the Mailard reaction. Nature, 419, 448–449. Mustafa, A., Andersson, R., Rosen, J., Kamal-Eldin, A., & Aman, P. (2005). Factors influencing acrylamide content and color in rye crisp bread. Journal of Agricultural and Food Chemistry, 53(15), 5985–5989. Rose´n, J., & Hellena¨s, K.-E. (2002). Analysis of acrylamide in cooked foods by liquid chromatography tandem mass spectrometry. Analyst, 127, 880–882. Springer, M., Fischer, T., Lehrack, A., & Freund, W. (2003). Development of acrylamide in baked products. Getreide Mehl und Brot, 57(5), 274–278. Stadler, R. H., Blank, N., Varga, F., Robert, F., & Riediker, S. (2002). Acrylamide from Maillard reaction products. Nature, 419(6906), 449–450. Surdyk, N., Rosen, J., Andersson, R., & Aman, P. (2004). Effects of asparagine, fructose, and baking conditions on acrylamide content in yeast-leavened wheat bread. Journal of Agricultural and Food Chemistry, 52(7), 2047–2051. Svensson, K., Abramsson, L., Becker, W., Glynn, A., Hellenas, K.-E., Lind, Y., et al. (2003). Dietary intake of acrylamide in Sweden. Food and Chemical Toxicology, 41(11), 1581–1586. 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(17), 4998–5006. Xue, J., & Walker, C. E. (2003). Humidity change and its effects on baking in an electrically heated air jet impingement oven. Food Research International, 36(6), 561–569. Zyzak, D. V., Sanders, R. A., Stojanovic, M., Tallmadge, D. H., Eberhart, B. L., Ewald, D. K., et al. (2003). Acrylamide formation mechanism in heated foods. Journal of Agricultural and Food Chemistry, 51(16), 4782–4787.