Food Research International 37 (2004) 489–495 www.elsevier.com/locate/foodres
Bread baking in halogen lamp–microwave combination oven Semin Ozge Keskin, Gulum Sumnu *, Serpil Sahin Middle East Technical University, Food Engineering Department, Ankara 06531, Turkey Received 5 August 2003; accepted 10 October 2003
Abstract The main objective of the study was to compare the effects of halogen lamp–microwave combination baking on quality of breads with other baking methods (conventional, microwave and halogen lamp baking). It was also aimed to improve the quality of microwave baked breads by using combination oven. Weight loss, specific volume, firmness and color of the breads were measured as quality parameters. Halogen lamp–microwave combination oven reduced the conventional baking time of breads by about 75%. Microwave heating was found to be the dominant mechanism in halogen lamp–microwave combination baking in terms of affecting weight loss and texture development. Increasing in halogen lamp power reduced specific volume and increased weight loss, firmness and DE values of breads in combination baking. Breads baked in halogen lamp–microwave combination oven had specific volume and color values comparable with the conventionally baked breads but their weight loss and firmness values were still higher. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Baking; Bread; Halogen lamp; Microwaves; Near-infrared
1. Introduction Halogen lamp–microwave combination heating is a new technology that combines the time saving advantage of microwave heating with the browning and crisping advantages of halogen lamp heating. There is no study on halogen lamp–microwave combination heating in literature, but some other combination heating methods were studied to be an alternative to conventional heating, such as infrared and hot air assisted microwave heating (Datta & Ni, 2002), microwave–hot air combination heating (Lu, Tang, & Liang, 1998), and microwave–impingement combination heating (Smith, 1986; Walker & Li, 1993). There is still a lack of understanding on the fundamentals of heat and moisture transport in foods when adding infrared or hot air to microwaves. Microwave heating modifies the transport processes due to internal pressures developed from evaporation. It was shown that the pressure driven moisture migration during microwave and spouted bed combined drying of diced apples resulted in a high *
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[email protected] (G. Sumnu). 0963-9969/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2003.10.001
drying rate (Feng, Tang, Cavalieri, & Plumb, 2001). Such pressure-driven flow depends on the structure and physical properties of the food material. When infrared is added to microwave heating, the already complex transport processes are modified significantly. It is expected that the power level and penetration of infrared energy would be significant parameters in such a process, but the effects of these parameters have not been identified in quantitative engineering terms (Datta & Ni, 2002). Conventional formulations of bread develop unacceptable textures when baked in microwave oven (Ovadia & Walker, 1996). The exterior parts of the bread are tough while inner parts are firm (Shukla, 1993). The reasons for firm texture in microwave-baked breads were high moisture loss, interactions of microwave with gluten and high amylose leaching during baking (Higo & Noguchi, 1987; Shukla, 1993). The biggest difference between convection ovens and microwave ovens is the inability of the microwave ovens to induce browning. Maillard browning is responsible for the production of many flavored and colored compounds but the cool ambient temperature inside a microwave oven causes surface cooling of microwave-baked products and low surface temperature prevents Maillard browning
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reactions to occur (Decareau, 1992). Halogen lamp heating provides near-infrared radiation and its region in the electromagnetic spectrum is near the visible light with high frequency and low penetration depth. Therefore, the radiation is focused at the surface which can cause the surface temperature of breads to reach the required values for browning. Halogen lamp baking combined with microwave baking may be a solution to reduce the problems in microwave baked products. Comparison of the effects of baking methods on quality parameters of breads will provide insights to understand the heating mechanism of different ovens. The main objective of this study was to assess the quality of breads baked in halogen–microwave combination oven. In addition, the effect of halogen–microwave combination baking on quality of breads were compared with other baking methods (conventional, microwave, halogen lamp baking). Moreover, it was aimed to explore the possibility of using this new technology for improving the quality of microwave baked products.
2. Materials and methods 2.1. Preparation of dough Bread flour contains 32% wet gluten, 13.1% moisture and 0.55% ash. The composition of the prepared dough was on flour weight basis; 100% flour, 8% sugar, 6% milk powder, 2% salt, 3% yeast, 8% margarine, 55% water. Dough was prepared by using straight dough method. First of all, the dry ingredients were mixed. Yeast was dissolved in water at 30 °C. Margarine was melted and added to the dry ingredients in liquid phase together with dissolved yeast. All the ingredients were mixed by a mixer (Kitchen Aid, 5K45SS, USA) for 3 min. After complete mixing of the dough, it was placed into the incubator (N€ uve EN 400, Turkey) at 30 °C and 85% RH for fermentation. The total duration of the fermentation was 105 min. After the first 70 min, the dough was taken out of the incubator, punched and placed into the incubator again. A second punch took place in another 35 min. The dough was divided into 50 g pieces after fermentation. Each piece was shaped and placed into the incubator for the last time for 20 min under the same incubation conditions. 2.2. Conventional baking Conventional baking was performed in a commercial electrical oven (Arcßelik ARMF 4 Plus, Turkey). The prepared dough samples were baked at 175, 200 and 225 °C for 12, 13 and 14 min. The oven was preheated to the set temperatures before placing the dough samples into it. Four breads were baked at a time.
2.3. Microwave baking The halogen lamp–microwave combination oven (Advantium ovenTM , General Electric Company, Louisville, KY, USA) was used by only operating the microwave power. The power of microwave oven has been determined as 706 W by using IMPI 2-liter test (Buffler, 1993). The independent variables were microwave power and baking time. Dough samples were baked at 50% power for 0.5, 0.75, 1.0, 1.5 and 2.0 min; and at 100% power for 0.5, 0.75 and 1.0 min. Only one bread (50 g) was baked at a time. 2.4. Halogen lamp–microwave combination baking Halogen lamp–microwave combination oven (Advantium ovenTM , General Electric Company, Louisville, KY, USA) combines microwave and halogen lamp heating in the oven. There was a rotary table in the oven to improve heating uniformity of samples. Halogen lamps at the top and bottom were operated at the same power during halogen lamp baking. Halogen lamp at the top was located 15 cm above the bread surface while the halogen lamp at the bottom was just under the rotary table. The prepared dough samples were baked at 60% halogen power–50% microwave power for 1.5, 2.0, 2.5 and 3.0 min; at 70% halogen power–30% microwave power for 2.5, 3.0, 3.5 and 4.0 min; at 60% halogen power–30% microwave power for 3.0, 3.5, 4.0 and 4.5 min; at 50% halogen power–30% microwave power and at 40% halogen power–30% microwave power for 3.5, 4.0, 4.5 and 5.0 min. Only one bread (50 g) was baked at a time. 2.5. Halogen lamp baking Halogen lamp–microwave combination oven (Advantium ovenTM , General Electric Company, Louisville, KY, USA) was used by only operating the halogen lamps. Dough samples were baked at 50% power for 10, 11, 12, and 13 min, at 60% power for 7, 8, 9, and 10 min and at 70% power for 6, 7, 8 and 9 min. Only one bread was baked at a time. 2.6. Bread analysis Weight loss measurements were done just after baking and calculated by subtracting the weight of bread after baking from weight of dough and dividing this term by the weight of dough and given in percentage. Bread specific volume was determined by the rape seed displacement method (AACC, 1988). Firmness of breads was measured using a universal testing machine (Lloyd Instruments LR 30K, UK). Breads were compressed for 25% at a speed of 55 mm/min.
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At least three replications were used for each experimental condition.
20
Weight loss (%)
Bread samples were prepared according to the method of AACC (AACC, 1988). Crust color of the bread samples was measured using a Minolta color reader (CR-10, Japan) using the Hunter L , a , and b color scale. Triplicate readings were carried out at room temperature from different positions of bread crust, and mean value was recorded. Total color change (DE) was calculated from the following equation in which dough was used as the reference point, whose L , a , b value was denoted by L0 , a0 and b0 h i1=2 DE ¼ ðL L0 Þ2 þ ða a0 Þ2 þ ðb b0 Þ2 :
491
15
10
5
0 0.5
1
1.5
2
Baking time (min) Fig. 2. Weight losses of breads during microwave baking at different powers. (r) 50%; (j) 100%.
3. Results and discussion 35 30
Weight loss (%)
Weight loss of breads baked in all types of ovens increased linearly with baking time (Figs. 1–3). It was previously shown by other researchers that breads (Sahin, Sumnu, & Zincirkiran, 2002) and cakes (Sumnu, Ndife, & Bayindirli, 1999) lost weight linearly during baking in conventional and microwave oven. The increase in temperature did not significantly increase the weight loss of breads (Fig. 1). However, the increase in halogen power increased the weight loss which is an index of moisture loss. This was due to the subjection of samples to more radiation in the presence of halogen lamp powers. Similarly, breads baked in microwave oven at 100% power had higher rate of weight loss as compared to 50% power because as power was increased more microwaves were coupled to the bread samples during the same baking time resulting in more heating (Fig. 2). The higher the microwave and halogen lamp power, the higher the weight loss was observed in
25 20 15 10 5 0 1.5
2
2.5
3
3.5
4
4.5
5
Baking time (min) Fig. 3. Weight losses of breads during halogen lamp–microwave combination baking at different halogen lamp (H) and microwave (MW) powers. (r) H, 60% and MW, 50%; (j) H, 70% and MW, 30%; (N) H, 60% and MW, 30%; (d) H, 50% and MW, 30%; (*) H, 40% and MW, 30%.
10
Weight loss (%)
9 8 7 6 5 4 3 2 6
8
10
12
14
Baking time (min) Fig. 1. Weight losses of breads during conventional oven and halogen lamp baking at different temperatures and oven powers, respectively. (N) 175 °C; (*) 200 °C; (d) 225 °C; (r) 50%; (j) 60%; (M) 70%.
halogen lamp–microwave combination oven (Fig. 3). As can be seen in Fig. 3 weight loss was the highest for breads baked with 50% microwave and 60% halogen lamp power. This showed that in halogen lamp–microwave combination baking, the microwave power was more effective on weight loss than halogen lamp power. Specific volume of conventionally baked breads showed a maximum during baking (Fig. 4). This trend was also observed by other researchers (He & Hoseney, 1992). Modification of the starch–gluten matrix is required for optimum dough development and gas retention. Starch–gluten matrix requires sufficient crumb temperature and baking time for the occurrence of complex reactions (Pomeranz & Shellenberger, 1971). The trend of volume expansion peak during microwave
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1.7
Specific volume (ml/g)
Specific volume (ml/g)
1.66 1.6 1.5 1.4 1.3
1.64 1.62 1.60 1.58 1.56 1.54 1.52
1.2
1.50 1.48
1.1 6
8
10
12
1.5
14
2
2.5
3
3.5
4
4.5
5
Baking time (min)
Baking time (min) Fig. 4. Changes of specific volume of breads during conventional oven and halogen lamp baking at different temperatures and oven powers, respectively. (N) 175 °C; (*) 200 °C; (d) 225 °C; (r) 50%; (j) 60%; (M) 70%.
Fig. 6. Changes of specific volume of breads during halogen lamp– microwave combination baking at different halogen lamp (H) and microwave (MW) powers. (r) H, 60% and MW, 50%; (j) H, 70% and MW, 30%; (N) H, 60% and MW, 30%; (d) H, 50% and MW, 30%; (*) H, 40% and MW, 30%.
baking was similar to the one obtained during conventional baking (Fig. 5). This volume peak was not be seen in halogen lamp oven because of the different heating mechanism of halogen lamp oven. Specific volume of breads baked in halogen lamp oven increased with baking time (Fig. 4). Since halogen lamp heating provides near-infrared radiation which means low penetration depth, radiation was accumulated at the surface. This might cause sudden formation of a thick crust which might reduce the transfer of heat to the inner parts and formation of starch–gluten matrix with high strength might be retarded. Specific volume of breads baked in halogen lamp–microwave combination oven decreased as baking time increased (Fig. 6). As halogen power increased volume of the breads decreased. Since halogen lamp heating provided focusing of radiation at the surface, the thicker crust formed immediately at the surface of the samples, retarded the expansion of the crumb.
Crumb firmness of breads decreased during halogen lamp baking (Fig. 7). Firmness of breads baked in conventional and halogen lamp oven was found to be negatively correlated with its specific volume (r ¼ 0:94 and r ¼ 0:90, respectively). However the specific volume of breads was not found to be correlated with firmness during microwave and halogen lamp–microwave combination baking. Fig. 8 shows the variation of firmness of breads baked in microwave oven. Firmness of breads increased sharply during baking at 100% power which can be related to high rate of moisture loss. The increase in halogen power did not significantly affect the firmness but the increase in microwave power from 30% to 50% increased the crumb firmness extremely showing that microwave heating was more dominant in affecting firmness in halogen lamp–micro-
1.4 1.3
2.2
Firmness (N)
Specific volume (ml/g)
1.2
2.0 1.8 1.6
1.1 1.0 0.9
1.4 0.8
1.2 0.7
1.0 0.6 6
0.8 0.5
1
1.5
2
8
10
12
14
Baking time (min)
Baking time (min) Fig. 5. Changes of specific volume of breads during microwave baking at different powers. (r) 50%; (j)100%.
Fig. 7. Changes of firmness of breads during conventional and halogen lamp baking at different temperatures and oven powers, respectively. (N) 175 °C; (*) 200 °C; (d) 225 °C; (r) 50%; (j) 60%; (M) 70%.
20
60
15
55
10
50
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∆E
Firmness (N)
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45
5
40
0 0.5
1
1.5
2
Baking time (min)
35 6
8
10
Fig. 8. Changes of firmness of breads during microwave baking at different powers. (r) 50%; (j) 100%.
Fig. 10. Changes of color (DE value) of breads during conventional and halogen lamp baking at different temperatures and oven powers, respectively. (N) 175 °C; (*) 200 °C; (d) 225 °C; (r) 50%; (j) 60%; (M) 70%.
50 40 30 20 10 0 1.5
2
2.5
3
3.5
4
4.5
5
-10
Baking time (min) Fig. 11. Changes of color (DE value) of breads during halogen lamp– microwave combination baking at different at different halogen lamp (H) and microwave (MW) powers. (r) H, 60% and MW, 50%; (j) H, 70% and MW, 30%; (N) H, 60% and MW, 30%; (d) H, 50% and MW, 30%; (*) H, 40% and MW, 30%.
50 45 40
Firmness (N)
14
Baking time (min)
∆E
wave combination oven (Fig. 9). The sudden increase in the firmness of breads at the final stages of baking can be explained by the extreme drying of breads. DE value of the breads baked in conventional and halogen lamp ovens increased linearly with baking time (Fig. 10). Higher baking temperatures or halogen power provided the achievement of browning at the surface of breads in a shorter time. As expected, there was no significant difference between DE values of breads baked in microwave oven with different baking times and oven powers which ranged between 2 and 5. All microwave treated samples had similar colors with the dough, therefore their DE values were very close to zero. DE value of breads baked in halogen lamp–microwave combination oven increased with baking time and halogen lamp oven power (Fig. 11). The increase in halogen power might increase surface temperatures of breads, which might affect the crust color formation. Halogen
12
35 30 25 20 15 10 5 0 1.5
2
2.5
3
3.5
4
4.5
Baking time (min) Fig. 9. Changes of firmness of breads during halogen lamp–microwave combination baking at different halogen lamp (H) and microwave (MW) powers. (r) H, 60% and MW, 50%; (j) H, 70% and MW, 30%; (N) H, 60% and MW, 30%; (d) H, 50% and MW, 30%; (*) H, 40% and MW, 30%.
lamp heating is known to provide low penetration depth and concentrate radiation at the surface, so the surface temperature can reach the required values for browning. Therefore, with halogen lamp–microwave combination heating, similar DE values with the conventionally baked breads could be achieved. In order to compare the differences in quality of breads baked in different ovens the optimum baking conditions giving the least firm, the most voluminous breads with acceptable crust color were determined for different ovens. The selected conditions were 13 min of baking at 200 °C for conventional oven, 0.75 min at 100% power for microwave oven and 10 min at 60% halogen power for halogen lamp ovens. In halogen
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lamp–microwave combination baking breads baked at 70% halogen lamp power and 30% microwave power for 3 min had DE value and specific volume similar to conventionally baked breads but the firmness of breads were still higher as compared to conventionally baked ones. When the baking time was extended darker colors could be observed but specific volume decreased and firmness increased in an unacceptably great extend. Table 1 shows the effects of different baking methods on quality parameters of breads. When the weight loss of breads baked in different ovens were compared microwave and halogen lamp–microwave combination baking resulted in greater weight loss of breads during processing as compared to conventional baking (Table 1). Microwave heating provided high moisture loss because of high internal pressure and concentration gradients which increased the flow of liquid through the food to the boundary. Moreover, the high power of the halogen lamp selected for halogen lamp–microwave combination oven provided high moisture loss. Breads baked in microwave oven had the highest specific volume as compared to other methods (Table 1). This is due to the fact that significant internal pressure created might result in a puffing effect and a high volume. It was shown that the internal pressure build-up in microwave and spouted bed combined drying resulted in a dominant pressure-driven flow in diced apples (Feng et al., 2001). Similarly, the internal vapor pressure produced by microwave heating caused expansion and puffing of carrots during microwave drying (Lin, Durance, & Scaman, 1998). Breads baked in halogen– microwave combination oven had specific volume similar to the conventionally baked ones ranging between that of microwave and halogen lamp treated samples. Both microwave and halogen lamp–microwave combination baking caused the breads to have the firmer texture (Table 1). Since microwave heating was found to be the dominant mechanism in combination heating in terms of texture development, it was not surprising to obtain the similar firmness values of breads baked in halogen lamp–microwave combination oven to that breads baked in microwave oven. Microwave baked breads had the lowest DE values which corresponded to a similar color value with the dough. This can be explained by the short baking times and low temperatures common to microwave proTable 1 Effects of different baking methods on quality parameters of breads Baking methods
Conventional Microwave Halogen lamp Halogen lamp–micro wave combination
Quality parameters Weight loss (%)
Specific volume (ml/g)
Firmness (N)
DE
4.06 10.80 8.20 17.86
1.60 2.04 1.44 2.57
0.67 2.88 0.80 3.05
47.7 3.0 55.6 25.6
cessing which did not promote browning reactions. On the contrary, high halogen powers for halogen lamp– microwave combination oven provided the achievement of desired color at the surface and similar DE values with the conventionally baked breads could be achieved (Table 1). 4. Conclusions As baking time and oven power increased weight loss of breads increased for all oven types. Microwave baked breads had higher specific volume as compared to others. The increase in halogen lamp power decreased specific volume of breads but increased weight loss, firmness and DE values of breads in halogen lamp– microwave combination baking. Microwave heating was found to be the dominant mechanism in halogen lamp– microwave combination baking in terms of affecting weight loss and texture development. By the usage of halogen lamp–microwave combination baking, the time saving advantage of microwave baking was combined with the crust browning advantage of halogen lamp heating and baking time was significantly reduced. Halogen lamp–microwave combination oven can be recommended to solve especially the crust color problem of microwave baked breads. It is not advisable to bake breads by using only halogen lamp mode of the oven because of the formation of the very thick bread crust. Halogen lamp–microwave combination baking provided specific volume and crust color similar to the conventionally baked products but the weight loss and firmness of breads were still higher as compared to conventionally baked ones. Further research is required to reduce the firmness of breads baked in this oven.
Acknowledgements General Electric Company is greatly acknowledged for donation of the halogen lamp–microwave combination oven (AdvantiumTM oven). This research was supported by Middle East Technical University (BAP2003-07-02-00-69).
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