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The mechanisms controlling heat and mass transfer on frying of beefburgers. Part 2: The influence of the pan temperature and patty diameter
Journal of Food Engineering 71 (2005) 18–27 www.elsevier.com/locate/jfoodeng
The mechanisms controlling heat and mass transfer on frying of beefburgers. Part 2: The influence of the pan temperature and patty diameter Bea Kova´csne´ Oroszva´ri *, Elena Bayod, Ingegerd Sjo¨holm, Eva Tornberg Department of Food Technology, Engineering and Nutrition, Lund Institute of Technology, P.O. Box 124, S-221 00 Lund, Sweden Received 12 June 2004; accepted 7 October 2004 Available online 22 December 2004
Abstract The effects of varying pan temperature and meat patty diameter on heat (temperature at the centre (5 mm) and 2 mm below the surface) and mass transfer (total, water and fat loss) of beefburgers prepared by double-sided frying were studied. The porosity of the fried beefburgers was determined based on the volume shrinkage and density measurements. The thawing time measured at the centre of the beefburger, total frying time and the final temperature 2 mm below the surface were significantly influenced by pan temperature, whereas the choice of meat raw material was of less importance than the pan temperature for the heat transfer. The characteristics of heat transfer, i.e. thawing and total frying time, were significantly influenced by the original patty diameter, with faster temperature increase in the smaller beefburgers. The most determinant factor for the water flux is the temperature gradient, and for the fat flux, the fat content. The water loss based on the initial water content in the form of drip was about 80% of the water loss even at a pan temperature of 175 C. This means that the pressure-driven water loss is the main mechanism governing the water loss in the frying of beefburgers. The higher the heat penetration by using beefburgers of the smaller diameter and the higher the cooking temperature that induces higher water losses, the faster is the crust formation, which in turn results in less shrinkage and higher porosity of the heat-processed meat. When beefburgers of different diameters were studied the porosity was dependent both on the rate of heat transfer and on the amount of water and fat flux and was higher with increasing water and fat flux. 2004 Elsevier Ltd. All rights reserved. Keywords: Heat and mass transfer; Beefburger; Frying; Contraction; Porosity
1. Introduction During recent decades minced meat products as fast food have become an enormous market. Thus, it is important for the food industry, when the production of such food items increases, to have a good control on production in order to achieve safe and good products. During thermal processing, such as frying, minced meat products loose weight as a result of heat denatur*
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22. E-mail address: [email protected] (B. Kova´csne´ Oroszva´ri). 0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.10.013
ation and contraction of the meat proteins, which, from an industrial point of view, not only means economic losses, but also a lowering of the sensory attributes such as tenderness and juiciness. Therefore, there is a need to understand the mechanisms controlling heat and mass transfer during frying in order to minimise frying losses and frying time, while at the same time ensuring that the temperature at the centre reaches 72 C to guarantee the safety of the product. Cooking loss can be divided into two main components, namely, moisture and fat loss. Moisture can be lost either as vapour through the crust–core interface or as drip, while fat leaves the patty only in the form of drip, since at normal frying temperatures fat does
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27
not vaporise. These phenomena have only partly been investigated in meat products (Dagerskog, 1979a, 1979b; Ngadi, Watts, & Correia, 1997; Sheridan & Shilton, 2002; Shilton, Mallikarjunan, & Sheridan, 2002). The influence of pan temperature on the cooking loss (water and fat loss) of hamburgers fried in a double-sided pan fryer was studied by Dagerskog and Bengtsson (1974). They found that the water loss increased with cooking time and temperature, while the pan temperature (140, 160, 180 and 200 C) had no effect on the fat loss. The influence of the cooking temperature (140 and 180 C) on the development of the centre temperature was also found to be negligible (Dagerskog, 1979a). On the other hand, Pan, Singh, and Rumsey (2000) have found, based on their predictive model, using a sensitivity analysis, that the frying temperature has a major influence on the centre temperature of the patty, where a 10% change in the pan temperature might result in a 20% change in the centre temperature. Although a number of papers have been published on the effect of cooking time and the chemical composition of beefburgers during cooking, no studies have been conducted to investigate the simultaneous effect of different patty diameters, varying raw material and pan temperature, using a factorial experimental design on, heat and mass transfer. In the present study the possibility of discerning between water drip and vapour loss on contact frying by using three different pan temperatures (100, 150 and 175 C) and the possibility to vary water and fat drip loss by using different original patty diameters (3.0, 5.3, 8.1, 10.0 and 12.7 cm) was investigated according to a factorial experimental design. The reason for choosing the low pan temperature of 100 C is to avoid water loss by evaporation and thus to be able to assess mainly water drip loss. The reasoning behind the variation in patty diameter is based on earlier studies of Kova´csne´, Sjo¨holm, and Tornberg (2004), where we showed that patty diameter shrinkage correlated with the fat loss, and on the assumption that the greater the patty diameter the larger the contraction and the higher the fat loss.
2. Materials and methods 2.1. Raw material Three types of beef meat (brisket, rib and shank) were chosen to produce low-, medium- and high-fat content beefburgers (Table 1). All the meat was ground through first an 8-mm and then a 3-mm grinder plate. The lean brisket was divided into two batches, one of which was minced with tallow in order to get the high fat content of fat brisket. After grinding, 1.5% salt was added to the batches and mixed by hand for 5 min to ensure
19
Table 1 Chemical composition of the raw materials Sample
Fat [%]
Std
Water [%]
Std
Fat brisket (FB) Rib (R) Lean brisket (LB) Shank (S)
39.0 16.1 15.3 6.7
1.6 0.5 0.6 0.8
46.4 64.3 64.4 71.1
1.5 1.2 1.3 1.3
good distribution. After the preparation the three different types of minced meat were vacuum packed separately and stored in a freezer until further usage. 2.2. The experimental design The experimental design used in this study is presented in Table 2. Frozen beefburgers (fat brisket, lean brisket and shank) with a diameter of 3 and 10 cm were fried at 100, 150 and 175 C, until a temperature of 72 C was reached at the centre of the patty to determine the influence of the pan temperatures on the heat and mass transfer on frying beefburgers and to distinguish between drip and evaporative water losses (Experiment I). Furthermore, the effect of the original patty diameter (3, 5.3, 8.1, 10 and 12.7 cm) was studied in Experiment II, in which frying was carried out at two pan temperatures (150 and 175 C) using rib meat, with a medium fat content of 16.1%. 2.3. Preparation of the beefburgers Beefburger patties were prepared in the same manner as described in Kova´csne´ et al. (2004). 2.4. Frying set-up The frying set-up that was used for all the experiments in the present investigation was the same as described in Kova´csne´ et al. (2004). 2.5. Data acquisition system The temperature data were collected in the same manner as described in Kova´csne´ et al. (2004). 2.6. Chemical analysis The moisture and the fat content of the beefburgers were determined according to Kova´csne´ et al. (2004). Table 2 Full experimental design with two replicates Experiment
Meat raw material
Pan temperature [C]
Patty diameter [cm]
Ia Ib II
FB, LB, S FB, LB, S R
100, 150, 175 100, 150, 175 150, 175
10 3 3, 5.3, 8.1, 10, 12.7
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2.7. Determination of the density The density of the raw and fried patties was measured using a liquid replacement method described by Pan and Singh (2001). Rapeseed oil with a density of 914.0 kg m3 was used for the density measurement, because its viscosity could slow down oil penetration into the beefburger. The sample was then placed in a glass beaker with a full level of the rapeseed oil. The oil displaced by the sample gave the volume of the beefburger. The density of the sample was calculated as the ratio of its weight and its volume. 2.8. Determination of the water, fat, total cooking loss, diameter shrinkage and porosity The weight of each sample was measured before frying in the frozen state and after frying. The total loss (TL) was calculated as a percentage of the original weight of the beefburger (Eq. (1)). ð1Þ
where TL is the total loss [%], Wb is the weight before [g], and Wa is the weight after frying [g]. Water and fat loss (a and b values, respectively) were calculated as a percentage related to the weight of the raw beefburger (Eqs. (2) and (3)). Wia Wf ¼ 100 100 a b
b ðW b W a Þ aþb AL t72
½g cm2 s1
ð4Þ
½g cm2 s1
ð5Þ
AT ¼ 2r2b p þ 2rb phb
ð6Þ
AL ¼ 2rb phb
ð7Þ
ð8Þ
where S is the patty diameter shrinkage [%], Db is the patty diameter before [mm], and Da is the patty diameter after frying [mm]. Two types of porosity were calculated, one called porosity based on measured shrinkage (es) and the other called porosity based on measured and calculated densities (ed) and they are defined below. The porosity based on measured shrinkage (es) was based on the volume derived from the difference between the calculated meat patty volume related to fat and water loss and the volume calculated from the diameter shrinkage according to Eq. (9). es ¼ 1
V b ððV fb V fa Þ þ ðV wb V wa ÞÞ Va
V b ¼ r2b phb
ð9Þ ð10Þ
V fb ¼
Fi Wb 100 qf
ð11Þ
V fa ¼
F f Wa 100 qf
ð12Þ
V wb ¼
WiWb 100 qw
ð13Þ
V wa ¼
Wf Wa 100 qw
ð14Þ
ð3Þ
where Wf is the water content after frying [%], Wi is the water content of the raw material [%], Ff is the fat content after frying [%]; Fi is the fat content of the raw material [%]. In addition, the flux of water (Wr) and fat (Fr) was calculated, by dividing the water and fat loss by the total (AT) and lateral area (AL) for the water and fat loss, respectively, and by the total frying time (t72) (Eqs. (4) and (5)). rb is the radius of the beefburger before frying [cm] and hb is the height of the beefburger before frying [cm].
Fr ¼
Db Da 100 Db
ð2Þ
Fi b 100 Ff ¼ 100 a b
a ðW b W a Þ aþb AT t72
S¼
WbWa TL ¼ 100 Wb
Wr ¼
The diameter of each sample was measured at intervals of 45 around the circumference in the frozen state before and 5 min after frying. The patty diameter shrinkage (S) was then calculated as a percentage of the original diameter of the patty (Eq. (8)).
V a ¼ r2a pha
ð15Þ
where Vb is the patty volume before frying [cm3], Vfb is the fat volume in the beefburger before frying [cm3], qf is the density of fat [g cm3], Vfa is the fat volume in the beefburger after frying [cm3], Vwb is the water volume in the beefburger before frying [cm3], qw is the density of water [g cm3], Vwa is the water volume in the beefburger after frying [cm3], Va is the patty volume after frying [cm3], ra is the radius of the beefburger after frying [cm] and ha is the height of the beefburger after frying [cm]. The porosity based on measured and calculated densities (ed) was calculated according to the definition described by Miles, Beek, and Veerkamp (1983) and was derived from the density data (Eq. (16)). qmeasured ed ¼ 1 ð16Þ qcalculated
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27 120
ð17Þ
where qmeasured is the measured density of the fried meat patty [kg m3], qcalculated is the density of the fried meat patty calculated from its individual components [kg m3], X wi is the weight fraction of component i and qi is the density of the pure component [kg m3]. The density of the pure components (i) was calculated by using mathematical expressions given by Choi and Okos (1986) (Eq. (17)). The density values of the pure components used in the present study at 22 C were 995.4 kg m3 for water, 916.4 kg m3 for fat and 1318.5 kg m3 for protein.
100 80 o
1 ðX wi =qi Þ
Temperature [ C]
qcalculated ¼ P
21
60 40 20 0 0
40
80
120
160
200
-20 -40
Frying time [s]
2.9. Statistical analysis The statistical analysis was carried out using a statistics program (Minitab, v. 12.2; MINITAB Inc., USA) with the Pearson correlation, linear regression and analysis of variance (ANOVA).
3. Results and discussion 3.1. The influence of the cooking temperature and meat raw material The cooking time to reach 72 C at the centre of the patty by frying at the three processing temperatures (100, 150 and 175 C) varied considerably (Experiment Ia). The longest frying time of 190 s was found for beefburgers with a high fat content fried at the lowest temperature (100 C), and the shortest frying time of 132 s was found in burgers made of the leanest meat fried at 175 C. Typical temperature profiles of a beefburger prepared from the fat brisket (fat content 39%) can be seen in Fig. 1 as a function of the frying time (measured at the centre (5 mm) and 2 mm below the surface) at 100, 150 and 175 C pan temperature. The higher temperature of the plates resulted in shorter thawing and total frying times at the centre of the burgers. The heat transfer measured 2 mm below the surface, when frying was performed at 100 C and no or minimal crust was formed (visual observation), took significantly longer than at the higher temperatures. The present findings are in agreement with the results of Chen, Marks, and Murphy (1999), who also found that the higher heating temperature contributed to faster heat transfer, when the centre temperature of meat patties cooked in an oven at four different air temperatures (135, 163, 191 and 218 C) were compared. In the study of Dagerskog and Bengtsson (1974), on the other hand, the pan temperatures above 140 C had a small influence on the centre temperature. The explanation for the small deviation in the centre temperature profiles was, according to the
100 C_2 mm
150 C_2 mm
175 C_2 mm
100 C_5 mm
150 C_5 mm
175 C_5 mm
Fig. 1. Measured temperature profiles (at the centre (5 mm) and 2 mm below the surface) of a beefburger (D = 10 cm) prepared from fat brisket as a function of the frying time.
authors, the constant driving force since the wet surface temperature cannot exceed 100 C irrespective of the pan temperature applied. In a later study of Dagerskog (1978) the centre temperature was found to increase with increasing pan temperature. A predictive model of the contact cooking process for frying a hamburger patty was developed, and based on the sensitivity analysis (Pan et al., 2000), it was also shown that the higher heating temperature results in a shorter total frying time to reach the same internal temperature. The weight loss during frying varied from 23.4% for beefburgers made of the leanest meat (fat content 6.7%) fried at 100 C to 46.7% for beefburgers prepared from the fatty meat (fat content 39%) fried at 175 C. This is in agreement with the previous study of Kova´csne´ et al. (2004), where the total weight loss variation was between 25% and 45% for the beefburgers fried at 175 C. The influence of the process temperature on the weight loss was shown in the model of Pan et al. (2000). It was found that an 11% change of the heating temperature could cause about an 8% change in the weight loss. However, in our study the cooking temperature had no influence on the total loss. In a later investigation by Pan and Singh (2001) the same positive relationship between the heating temperature and the weight loss was reported, even though the hamburgers (20% fat content) were cooked without a turn-over on a single-sided pan fryer. The authors reported a weight loss of about 45% on a single-sided pan fryer (300 s of frying time) and a weight loss of about of 25% on a double-sided fryer (frying time of 122 s) at the end of cooking, when the centre temperature was about the same (68 C). Comparing the results of Troutt et al. (1992) with the findings of Pan and Singh (2001) the weight loss
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in the study of Troutt et al. (1992) was found to be much less (about 26%), when hamburgers with the same fat content (20%) were fried for about the same time (300 s) at a cooking temperature of 150 C. Beefburgers with a fat content of 39.0% (fat brisket) contributed to higher total and fat loss, as shown earlier (Kova´csne´ et al., 2004). Pan et al. (2000) found a negligible effect of the fat content on the total loss based on the model sensitivity analysis, when the fat content was changed by about 17%. The high dependence of the total and fat loss on the initial fat content of the beefburgers suggests that the fat release is substantially enhanced by the possibility of contact between fat droplets, as suggested by Olsson and Tornberg (1991). The fat loss varied between 1.4% and 26.3%, which is in the same range as found by Sheridan and Shilton (2002), who found fat loss to be between 0% and 20% during infrared cooking of beefburgers. The moisture loss during frying varied from 14.7% for burgers with a high fat content fried at 100 C to 28.1% for meat patties prepared from the leanest meat fried at the highest pan temperature (175 C) used in this investigation. The present results for the water loss are consistent with the results of Kova´csne´ et al. (2004), where the water loss varied between 25% and 30%. The results of the present study are also consistent with the findings of Dagerskog and Bengtsson (1974), who reported that the water loss varied between 27–32%, when the temperature at the centre reached about 72 C. They also found that the water loss tended to be higher at higher cooking temperatures, although the differences were very small. They also reported higher water loss in hamburgers with a higher initial fat content, which is not consistent with our findings. In the fried beefburgers the diameter shrinkage varied from 11% for beefburger made of the leanest meat (shank) fried at the lowest cooking temperature of 100 C to 23.4% for patties with the highest fat content fried at 175 C. Our results in this study are in accordance with the previous research of Kova´csne´ et al. (2004), where the variation in diameter shrinkage was between 11–20.3%, and with the results of Pan and Singh (2001), where the diameter shrinkage varied from about 20.8% for hamburgers with a fat content of 20% fried at 160 C to about 23.5% at 200 C. Water loss related to the initial water content was significantly influenced by pan temperature, but not by the meat raw material. The high cooking temperature contributed to higher water loss related to the initial water content. This suggests that the evaporation of water as induced by the two higher temperatures gives rise to higher water loss, whereas the pressure-driven drip loss of water is independent of the higher degree of shrinkage as observed for the fattiest meat (fat brisket). The patty diameter shrinkage was influenced only by the meat raw material, and a high fat content of the
meat patty contributed to higher diameter shrinkage, as previously shown (Kova´csne´ et al., 2004). These observations tell us that the pressure-induced contraction by the drying of the crust is of less importance than the contraction pressure formed by the denaturation of the connective tissue in the fattier meat. 3.2. The influence of patty diameter, pan temperature and meat raw material (Experiment Ia + Ib) The thawing time measured at the centre of the beefburger varied from 51.3 s for burgers made of the leanest meat with the smallest diameter (D = 3 cm) fried at 175 C, to 98.1 s for meat patties prepared from the fatty meat with the largest diameter (D = 10 cm) fried at 100 C. The total frying time was found to be in the range of 89.7–188.9 s. The shortest cooking time was observed in meat patties made of lean brisket with the smaller diameter and fried at 175 C, while the longest cooking time was in beefburgers prepared from the fatty meat of the larger diameter fried at 100 C. The lowest temperature of 83.7 C 2 mm below the surface was obtained when burgers made of lean brisket with a diameter of 10 cm were fried at 100 C, and the highest temperature of 101.6 C was reached in meat patties prepared from the lean meat with the smaller diameter fried at 175 C. The weight loss on frying varied considerably from 23.4% for the lean burgers with the smaller diameter fried at 100 C to 56.3% for the fatty patties with the smaller diameter at 175 C. The variation in the water loss was between 14.7% in beefburgers made of the fatty meat with the larger diameter fried at 100 C and 36.7% in patties prepared from lean brisket with the smaller diameter fried at 175 C. The fat loss varied considerably from 1.3% in the lean burger to 30.4% in the fatty meat, whereas, the heating temperature and the size of the patty had no significant influence. One of the most important physical changes that occurs in beefburgers during frying is the reduction of its volume due to the protein denaturation squeezing out the fat and the water from the meat patty. In the present investigation the patty diameter shrinkage varied from 9% in lean burgers with the smaller diameter fried at 100 C to 23.4% in fatty patties with the larger diameter fried at the highest cooking temperature (175 C). Very few studies have followed this phenomenon, which is so important for the fat and water transfer in the frying of beefburgers. In Table 3 the results of three-way ANOVA for the effects of pan temperature, patty diameter and meat raw material regarding the characteristics of heat, i.e. thawing time at the centre, total frying time and the final temperature 2 mm below the surface and mass transfer, i.e. total, fat and water loss based on the weight of the raw beefburger, fat and water loss related to the initial
<0.001 18.8 12.5 11.2 0.090 15.1 13.2 0.560 13.9 13.6
15.0
<0.001 <0.001 <0.001 0.012 0.316 <0.001 <0.001 <0.001 48.3 28.3 19.7 72.9 42.9 34.3 6.1 0.299 34.2 6.3 27.9 41.1 43.3 7.1 8.7 0.218 28.3 2.5 28.5 37.4 40.1 2.5 8.3 0.147 <0.001 0.092 <0.001 0.871 <0.001 0.001 0.001 <0.001 32.9 11.4 21.8 49.6 36.4 20.5 6.9 0.145 41.0 13.3 28.9 51.3 47.7 8.8 8.5 0.297 0.001 0.694 0.001 0.464 <0.001 0.280 <0.001 0.021 38.1 12.2 26.7 46.2 44.2 15.2 8.1 0.248 33.6 11.9 22.0 45.6 36.0 11.2 5.2 0.188
39.2 13.0 27.5 59.6 46.1 17.6 9.8 0.228
0.041 0.560 0.179
p-value Fat brisket
75.8 144.3 96.1 77.5 141.6 94.0
Lean brisket Shank
70.6 138.4 94.5 <0.001 <0.001 0.003
p-value 10.0
85.9 164.9 93.3 63.4 118.0 96.4
3.0 p-value
<0.001 <0.001 <0.001 67.2 121.0 98.3
175 150
72.4 135.7 97.8
100
84.3 167.6 88.5
Thawing time, at the centre [s] Total frying time [s] Final temperature 2 mm below the surface (Tf_2 mm) [C] Total loss [%] Fat loss [%] Water loss [%] Fat loss/initial fat content [%] Water loss/initial water content [%] Fat flux [g cm2 s1] · 104 Water flux [g cm2 s1] · 104 Porosity based on measured shrinkage (es) Patty diameter shrinkage [%]
The significance of the effect Meat raw material The significance of the effect Original patty diameter [cm] The significance of the effect Pan temperature [C]
Table 3 The main effects of pan temperature, original patty diameter and meat raw material on the characteristics of heat and mass transfer and patty diameter shrinkage giving least square means and the significance (p-value < 0.05) of the effect using three-way ANOVA
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27
23
fat and water content, respectively, and fat and water flux and patty porosity and diameter shrinkage are summarised. The thawing time was significantly influenced by pan temperature, the original diameter of the patty and meat raw material. The higher pan temperature (175 C), smaller patty diameter (3.0 cm) and beefburger prepared from the meat of shank (fat content 6.7%) contributed to a faster thawing time. However, the original diameter had a greater influence than meat raw material on the heat transfer. Total frying time and the final temperature 2 mm below the surface were significantly influenced by the pan temperature and the original diameter of the patty, pan temperature having the greatest influence. The longer frying time in the case of the larger burgers may be partly caused by the decrease in the pan temperature when the patties were placed on the fryer. To clearly see the contribution of this effect the following experiment was performed. The total cooling surface of the 10 cm patties was simulated using eight D = 3 cm burgers. The centre patty, with the thermocouples placed as described previously, was surrounded by the seven others to achieve a 10 cm diameter circle (thus the significant gap around the centre one allowed heat penetration laterally). The frying time for this configuration was 16% shorter than for the D = 10 cm patties. Thus, the explanation for the faster heat transfer in the smaller meat patty could be that in samples of these dimensions one-dimensional heat transfer has become two-dimensional, because the ratio diameter:thickness is less than 10 (Pan et al., 2000). In this case, heat transfer by radiation and convection at the sides has to be taken into account. Apart from the observations made in the former experiments with regard to mass transfer, it can be noted that the fat loss per se is not influenced by the decrease in patty diameter, whereas this is the case for the water loss. This indicates that the pressure-driven shrinkage generating fat drip loss does not change as markedly with the original patty diameter, as does the water evaporation loss in the smaller patties of 3 cm, due to the two-dimensional heat transfer. As can be seen in Fig. 2, the water loss related to the initial water content increases with increasing frying temperature and with decreasing patty diameter. At 100 C pan temperature the average value for D = 10 cm patties was about 33% and for burgers with a diameter of 3 cm was about 39%. At the end of the frying the temperature 2 mm below the surface (Fig. 3) was about 88 C, well below the boiling point of water, for all diameters. Therefore, it can be assumed that the water losses at the lowest cooking temperature occur mainly in the form of drip. Consequently, as with higher pan temperatures, the meat goes through the same temperature and phase stages, a considerable proportion, about 80%, of the water losses will be water drip. This is a very important observation because it tells us that the
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27
24 60
be noted with regard to the size of the burgers. In the smaller meat patties the temperature was found to be significantly higher (about 100 C) due to the considerable circumferential heat transfer. There was no significant difference in the final temperature 2 mm below the surface between 150 and 175 C frying temperatures or for the types of meat. Water and fat flux, as a function of the different meat raw material and frying temperature for the two different sizes of beefburger are shown in Fig. 4a and b. As can be seen, the flux of water at 100 C has a similar tendency for the two diameters, when water is lost mainly as drip. However, at 150 and 175 C pan temperature
D=3 cm
Water loss/initial water content [%]
D=10 cm 55
50
45
40
35
30 Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
Shank
0.0016
Lean Fat Brisket Brisket
D=3 cm
175 C 175 C 175 C
0.0014
Fig. 2. Water loss related to the initial water content expressed as a function of the frying temperature for fat brisket, lean brisket and shank. Mean values are shown with standard errors.
0.0012
o
o
o
o
150 C 150 C 150 C
o
o
o
105 D=3 cm D=10 cm 100
D=10 cm
0.001 0.0008 0.0006 0.0004 0.0002
95 0
(a)
o
o
100 C 100 C 100 C 0.008 0.007
85
Shank Lean Fat Brisket Brisket o
90
Shank Lean Fat Brisket Brisket o
150 C 150 oC 150 oC
Shank Lean Fat Brisket Brisket
175 oC 175 oC 175 oC
D=3 cm D=10 cm
0.006
80 Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
100oC 100oC 100oC
150oC 150oC 150oC
175oC 175oC 175oC
Fig. 3. Final temperature 2 mm below the surface expressed as a function of the frying temperature for fat brisket, lean brisket and shank. Mean values are shown with standard errors.
Fat flux [gcm-2s -1]
Final temperature 2 mm below the surface [oC]
100 C 100 C 100 C
o
Water flux [gcm-2s -1]
o
0.005 0.004 0.003 0.002 0.001
pressure-driven drip loss of water constitutes the main mechanism for water losses even at the high frying temperature of 175 C. Evidently, any simulation of water losses on frying of hamburger must not neglect to consider this mechanism in the equations. To our knowledge this has not been done so far. With an 88 C (near) surface temperature no significant crust was observed visually on the top and bottom surfaces of the burgers. By increasing the pan temperature significant differences in the final temperature can
0
(b)
Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
100 oC 100 oC 100 oC
150 oC 150 oC 150 oC
175 oC 175 oC 175 oC
Fig. 4. (a) Water flux (g cm2 s1) as a function of different meat raw material and frying temperature for the two different sizes of beefburger. Mean values are shown with standard errors. (b) Fat flux (g cm2 s1) as a function of different meat raw material and frying temperature for the two different sizes of beefburger. Mean values are shown with standard errors.
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27 0.48 D=3 cm Porosity based on measured shrinkage
the smaller diameter beefburgers have a maximal rate of water release for the medium fat burger, when evaporation of water also comes into play. The water flux in fat brisket meat at 100 C is significantly less than in the two least fatty meats and the explanation for this observation could be the availability of water. However, the maximum regarding the water flux for the smaller patties made of lean brisket fried at 150 and 175 C might be explained by the following reasoning. At the higher pan temperature, when crust is formed and at the lower patty diameter of 3 cm and for fattier meat the porosity of the burgers is significantly higher according to Table 3. That means that the water flux due to the increased porosity is higher for burgers at 150 and 175 C pan temperature for the smallest diameter and using fattier meat, as is in fact observed in Fig. 4a. However, for the fattiest meat the water availability is less and therefore the water flux is less for those burgers. In Fig. 4b the fat flux as a function of the different meat raw material and frying temperature for the two different sizes of beefburger is shown. As can be seen, the fat fluxes follow the same trend for all the pan temperatures used in this investigation. As explained previously, the most important determinant for the fat loss is the fat content and not the cooking temperature. Despite the large standard error in the case of the fatty meat, the fat flux is higher in the larger beefburgers. This is the reverse of the case for the water flux as a function of patty diameter. That means that the shrinkage, which is higher in the larger beefburgers, is the most important parameter for the fat release and not the increased heat transfer as in the case of water loss. No significant differences were found regarding shank and lean brisket patties. Shrinkage-based porosity as a function of different meat raw materials and frying temperatures for the two different sizes of beefburger is shown in Fig. 5. Interesting observations can be made from Fig. 5. Firstly, when no marked crust is formed at 100 C pan temperature and when the diameter of the patty is large, the lowest porosity of about 8–10% is obtained. Then the flux of water and fat is relatively in phase (only with 8–10% difference) with the rate of formation of a dried out surface which is not flexible enough to follow the non-filled volumes, created by the fat and water release. It can further be seen in Fig. 5 that the porosity is influenced by the pan temperature, patty diameter and fat content, which in turn reflects the amount of fat loss. It can further be noted that for the fattiest and the smallest beefburger the porosity can reach as much as 40%. Moreover, decreasing the patty diameter gives rise to substantial enhancement of the porosity, and there seems to be a maximum in porosity at a pan temperature of 150 C compared to the other two temperatures.
25
0.42
D=10 cm
0.36 0.3 0.24 0.18 0.12 0.06 0 Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
100 oC 100 oC 100 oC
150 oC 150 oC 150 oC
175 oC 175 oC 175 oC
Fig. 5. Shrinkage-based porosity as a function of different meat raw material and frying temperature for the two different sizes of beefburger. Mean values are shown with standard errors.
3.3. Influence of the diameter and pan temperature (Experiment II) In Table 4 the results of ANOVA for the effects of pan temperature and patty diameter regarding the same parameters as given in the other tables, are summarised. The thawing time at the centre and total frying time was for these two higher temperatures significantly influenced only by the original patty diameter. The thawing time at the centre increased with increasing patty diameter up to the diameter of 10.0 cm and then levelled off. This could be caused by the small ratio of diameter to thickness of a small beefburger patty (diameter/height < 10) making the radial heat transfer significant. Total loss based on the weight of the raw beefburger was significantly influenced by pan temperature and original patty diameter. Fat and water loss based on the weight of the raw beefburger, fat and water loss related to the initial fat and water content, respectively, and the flux of fat was not significantly influenced either by pan temperature or by the original patty diameter. The flux of water was significantly influenced by pan temperature and somewhat less by the original patty diameter. The smaller patty diameter contributed clearly to higher water flux, since the heat transfer through the circumferential area has a major impact in the smaller patties. Beefburgers of three diameters were included in the porosity investigation based on density measurements (8.3, 10 and 12.7 cm). Density-based porosity was significantly influenced only by pan temperature (p = 0.028, Table 4), where the higher cooking temperatures
26
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Table 4 The main effects of pan temperature and original patty diameter on the characteristics of heat and mass transfer, giving least square means and the significance (p-value < 0.05) of the effect using two-way ANOVA Pan temperature [C]
Original patty diameter [cm]
The significance of the effect
150
175
p-value
3.0
5.3
8.1
10.0
12.7
73.1 134.6 93.8 34.1 7.4 25.3 45.8 39.3 12.3 8.9 0.071
67.7 118.9 95.9 38.6 7.8 28.0 48.2 43.6 15.0 11.2 0.147
0.278 0.146 0.463 0.030 0.688 0.056 0.688 0.056 0.249 0.010 0.028
57.5 98.5 99.4 37.6 7.3 28.9 45.0 44.9 7.1 10.8 –
60.1 101.1 89.8 41.0 8.7 26.5 53.8 41.2 14.3 11.7 –
70.6 120.5 93.2 38.7 8.8 27.7 54.5 43.1 17.8 11.2 0.115
82.8 148.3 94.1 33.4 7.5 26.3 46.3 40.9 15.1 8.7 0.143
80.9 165.2 97.9 31.0 5.7 23.9 35.4 37.2 13.8 7.7 0.068
0.011 0.002 0.233 0.021 0.233 0.202 0.233 0.202 0.092 0.019 0.148
0.244
0.289
0.059
0.348
0.300
0.296
0.115
<0.001
9.7
0.185
6.5
11.2
contributed to a larger porosity (Table 4). Wang and Brennan (1995) found also a significant relationship between porosity and drying temperature. The maximal volume of air pore formation during dehydration of potato was larger at higher drying temperatures. During frying of beefburgers, when the temperature at the surface of the patty rises above 102 C (the boiling point of water with solutes), a thin crust starts to form. This crust is characterised by a low thermal conductivity and high porosity (very different from the core region) and acts as a thermally insulating material. At higher frying temperatures the crust formation is faster as a result of higher heat penetration, giving a harder and more porous ÔshellÕ around the internal core region than at lower frying temperatures. It was found that with increasing pan temperature the diameter shrinkage of the patty decreased, which could be due to the resistance of this harder shell to follow the contraction of the core. As can be seen in Fig. 6, the shrinkage-based porosity follows the same trend for the two frying temperatures for both diameters. The highest porosity of about 35% was found for beefburgers with the smallest diameter. For the three medium diameters, however, the size of the beefburger did not influence the porosity. The patty diameter had a significant influence on the porosity, regardless of the pan temperature applied. In the smallest patties the high porosity could be due to the fact that the heating rate was the quickest as was therefore also the formation of crust. This hard crust (shell) developed during the heating process was not able to follow the decline in the volume due to the fat and water migration. For the largest beefburgers, 12.7 cm in diameter, a substantially lower porosity could be due to the long migration distance for the water and fat drip, thereby causing
0.275 10.0
10.3
12.0
13.4
p-value
0.013
0.45 Shrinkage based (filled symbols) and density based (unfilled symbols) porosity at 175 o C and 150 o C
Thawing time, at the centre [s] Total frying time [s] Tf_2 mm [C] Total loss [%] Fat loss [%] Water loss [%] Fat loss/initial fat content [%] Water loss/initial water content [%] Fat flux [g cm2 s1] · 104 Water flux [g cm2 s1] · 104 Porosity based on measured and calculated density (ed) Porosity based on measured shrinkage (es) Patty diameter shrinkage [%]
The significance of the effect
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
2 150 C
4 6 8 10 Patty original diameter [cm] 175 C
150 C
12
14
175 C
Fig. 6. Shrinkage- and density-based porosity as a function of original patty diameter at 150 and 175 C pan temperature. Mean values are shown with standard errors.
less water and fat loss (see Table 4). The density-based porosity was found to be lower than the shrinkagebased porosity. Probably the rapeseed oil could not hinder the oil penetration into the beefburger sufficiently, resulting in the porosity measured in this way being too low. Patty diameter shrinkage was significantly influenced only by the original diameter. The larger diameter of the meat patty evidently contributed to higher shrinkage, as we have observed earlier (Kova´csne´ et al., 2004).
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27
4. Conclusions
References
The higher pan temperature had a significant impact on the water loss but not on the fat loss and patty diameter shrinkage. At 100 C pan frying temperature, when the crust formation is minimal and no significant water evaporation occurs, the water loss occurred mainly in the form of drip. Increasing the pan temperature to 175 C the water flux loss doubled, due to evaporation. But the water loss based on initial water in the form of drip constituted about 80% of the water loss even at 175 C frying pan temperature. This indicates that the pressure-driven drip loss of water is the main mechanism by which water is lost on frying beefburgers. In summary, water drip loss is constant at a given meat composition and evaporation increases with increasing pan temperature. Fat loss increases significantly with fat content and is not influenced to any large extent by the cooking temperature. The temperature increase in the smaller patties was faster than in the larger ones, regardless of the frying temperature, possibly due to the relatively higher circumferential heat transfer. For the same reason water loss was found to be higher in the smaller patties. The fat release, however, was found to be controlled primarily by the patty diameter shrinkage, which is much higher in the larger beefburgers. The higher the heat penetration by using smaller diameters and higher pan temperatures and using fattier meat caused more fat and water losses, which in turn gave rise to higher porosity in the fried patty. Comparing the shrinkage- and density-based porosity, it can be concluded that the density-based porosity was always lower than the shrinkage-based porosity, probably due to oil penetration during the density measurement. However, they both followed the same trend when the porosity of the fried beefburgers of different diameters was studied. This means that at the lowest patty diameter of 3 cm the porosity was the highest, whereas an increase from 4 to 10 cm in patty diameter did not change the porosity until at a diameter of 12.7 cm, it decreased to half its value. This is due to the fact that patty shrinkage is the highest at the largest diameter and at the same time the total losses are the lowest, leaving less space within the beefburger.
Chen, H., Marks, B. P., & Murphy, R. Y. (1999). Modeling coupled heat and mass transfer for convection cooking of chicken patties. Journal of Food Engineering, 42, 139–146. Choi, Y., & Okos, M. R. (1986). Effects of temperature and composition on the thermal properties of foods. In M. Le Maguer & P. Jelen (Eds.), Food engineering and process applications. Transport phenomena (Vol. 1). England: Elsevier Applied Science Publishers Ltd. Dagerskog, M. (1978). Stekning av livsmedel. Studier av det processtekniska underlaget., Ph.D. thesis, Chalmers Tekniska Ho¨gskola, Go¨teborg. Dagerskog, M. (1979a). Pan frying of meat patties, I. A study of heat and mass transfer. Lebensmittel Wissenschaft und Technologie, 12, 217–224. Dagerskog, M. (1979b). Pan frying of meat patties, II. Influence of processing conditions on heat transfer, crust color formation, cooking losses and sensory quality. Lebensmittel Wissenschaft und Technologie, 12, 225–230. Dagerskog, M., & Bengtsson, N. E. (1974). Pan frying of meat patties—relationship among crust formation, yield, composition and processing conditions. Lebensmittel Wissenschaft und Technologie, 7(4), 202–207. Kova´csne´, O. B., Sjo¨holm, I., & Tornberg, E. (2004). The mechanisms controlling heat and mass transfer on frying of beefburgers. I. The influence of the composition and comminution of meat raw material. Journal of Food Engineering, 67(4), 499– 506. Miles, C. A., Beek, G. V., & Veerkamp, C. H. (1983). Calculation of thermophysical properties of foods. In R. Jowitt, F. Escher, B. Hallstro¨m, H. F. T. Meffert, W. E. L. Spiess, & G. Vos (Eds.), Thermophysical properties of foods (pp. 269–312). London: Applied Science Publishers. Ngadi, M. O., Watts, K. C., & Correia, L. R. (1997). Finite element method modelling of moisture transfer in chicken drum during deep-fat frying. Journal of Food Engineering, 32, 11–20. Olsson, A., & Tornberg, E. (1991). Fat-holding in hamburgers as influenced by the different constituents of beef adipose tissue. Food Structure, 10, 333–344. Pan, Z., & Singh, R. P. (2001). Physical and thermal properties of ground beef during cooking. Lebensmittel Wissenschaft und Technologie, 34, 437–444. Pan, Z., Singh, R. P., & Rumsey, T. R. (2000). Predictive modelling of contact-heating process for cooking a hamburger patty. Journal of Food Engineering, 46, 9–19. Sheridan, P. S., & Shilton, N. C. (2002). Analysis of yield while cooking beefburger patties using far infrared radiation. Journal of Food Engineering, 51, 3–11. Shilton, N., Mallikarjunan, P., & Sheridan, P. (2002). Modeling of heat transfer and evaporative mass losses during the cooking of beef patties using far-infrared radiation. Journal of Food Engineering, 55, 217–222. Troutt, E. S., Hunt, M. C., Johnson, D. E., Claus, J. R., Kastner, C. L., Kropf, D. H., & Stroda, S. (1992). Chemical, physical, and sensory characterization of ground beef containing 5–30 percent fat. Journal of Food Science, 57(1), 25–29. Wang, N., & Brennan, J. G. (1995). Changes in structure, density and porosity of potato during dehydration. Journal of Food Engineering, 24, 61–76.
Acknowledgement This work was financially supported by LiFT-Future Technologies for Food Production which is acknowledged with thanks. Thanks are also due to Prof. Petr Dejmek for reading the manuscript and for many good suggestions.
The mechanisms controlling heat and mass transfer on frying of beefburgers. Part 2: The influence of the pan temperature and patty diameter Bea Kova´csne´ Oroszva´ri *, Elena Bayod, Ingegerd Sjo¨holm, Eva Tornberg Department of Food Technology, Engineering and Nutrition, Lund Institute of Technology, P.O. Box 124, S-221 00 Lund, Sweden Received 12 June 2004; accepted 7 October 2004 Available online 22 December 2004
Abstract The effects of varying pan temperature and meat patty diameter on heat (temperature at the centre (5 mm) and 2 mm below the surface) and mass transfer (total, water and fat loss) of beefburgers prepared by double-sided frying were studied. The porosity of the fried beefburgers was determined based on the volume shrinkage and density measurements. The thawing time measured at the centre of the beefburger, total frying time and the final temperature 2 mm below the surface were significantly influenced by pan temperature, whereas the choice of meat raw material was of less importance than the pan temperature for the heat transfer. The characteristics of heat transfer, i.e. thawing and total frying time, were significantly influenced by the original patty diameter, with faster temperature increase in the smaller beefburgers. The most determinant factor for the water flux is the temperature gradient, and for the fat flux, the fat content. The water loss based on the initial water content in the form of drip was about 80% of the water loss even at a pan temperature of 175 C. This means that the pressure-driven water loss is the main mechanism governing the water loss in the frying of beefburgers. The higher the heat penetration by using beefburgers of the smaller diameter and the higher the cooking temperature that induces higher water losses, the faster is the crust formation, which in turn results in less shrinkage and higher porosity of the heat-processed meat. When beefburgers of different diameters were studied the porosity was dependent both on the rate of heat transfer and on the amount of water and fat flux and was higher with increasing water and fat flux. 2004 Elsevier Ltd. All rights reserved. Keywords: Heat and mass transfer; Beefburger; Frying; Contraction; Porosity
1. Introduction During recent decades minced meat products as fast food have become an enormous market. Thus, it is important for the food industry, when the production of such food items increases, to have a good control on production in order to achieve safe and good products. During thermal processing, such as frying, minced meat products loose weight as a result of heat denatur*
Corresponding author. Tel.: +46 46 222 98 15; fax: +46 46 222 46
22. E-mail address: [email protected] (B. Kova´csne´ Oroszva´ri). 0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.10.013
ation and contraction of the meat proteins, which, from an industrial point of view, not only means economic losses, but also a lowering of the sensory attributes such as tenderness and juiciness. Therefore, there is a need to understand the mechanisms controlling heat and mass transfer during frying in order to minimise frying losses and frying time, while at the same time ensuring that the temperature at the centre reaches 72 C to guarantee the safety of the product. Cooking loss can be divided into two main components, namely, moisture and fat loss. Moisture can be lost either as vapour through the crust–core interface or as drip, while fat leaves the patty only in the form of drip, since at normal frying temperatures fat does
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27
not vaporise. These phenomena have only partly been investigated in meat products (Dagerskog, 1979a, 1979b; Ngadi, Watts, & Correia, 1997; Sheridan & Shilton, 2002; Shilton, Mallikarjunan, & Sheridan, 2002). The influence of pan temperature on the cooking loss (water and fat loss) of hamburgers fried in a double-sided pan fryer was studied by Dagerskog and Bengtsson (1974). They found that the water loss increased with cooking time and temperature, while the pan temperature (140, 160, 180 and 200 C) had no effect on the fat loss. The influence of the cooking temperature (140 and 180 C) on the development of the centre temperature was also found to be negligible (Dagerskog, 1979a). On the other hand, Pan, Singh, and Rumsey (2000) have found, based on their predictive model, using a sensitivity analysis, that the frying temperature has a major influence on the centre temperature of the patty, where a 10% change in the pan temperature might result in a 20% change in the centre temperature. Although a number of papers have been published on the effect of cooking time and the chemical composition of beefburgers during cooking, no studies have been conducted to investigate the simultaneous effect of different patty diameters, varying raw material and pan temperature, using a factorial experimental design on, heat and mass transfer. In the present study the possibility of discerning between water drip and vapour loss on contact frying by using three different pan temperatures (100, 150 and 175 C) and the possibility to vary water and fat drip loss by using different original patty diameters (3.0, 5.3, 8.1, 10.0 and 12.7 cm) was investigated according to a factorial experimental design. The reason for choosing the low pan temperature of 100 C is to avoid water loss by evaporation and thus to be able to assess mainly water drip loss. The reasoning behind the variation in patty diameter is based on earlier studies of Kova´csne´, Sjo¨holm, and Tornberg (2004), where we showed that patty diameter shrinkage correlated with the fat loss, and on the assumption that the greater the patty diameter the larger the contraction and the higher the fat loss.
2. Materials and methods 2.1. Raw material Three types of beef meat (brisket, rib and shank) were chosen to produce low-, medium- and high-fat content beefburgers (Table 1). All the meat was ground through first an 8-mm and then a 3-mm grinder plate. The lean brisket was divided into two batches, one of which was minced with tallow in order to get the high fat content of fat brisket. After grinding, 1.5% salt was added to the batches and mixed by hand for 5 min to ensure
19
Table 1 Chemical composition of the raw materials Sample
Fat [%]
Std
Water [%]
Std
Fat brisket (FB) Rib (R) Lean brisket (LB) Shank (S)
39.0 16.1 15.3 6.7
1.6 0.5 0.6 0.8
46.4 64.3 64.4 71.1
1.5 1.2 1.3 1.3
good distribution. After the preparation the three different types of minced meat were vacuum packed separately and stored in a freezer until further usage. 2.2. The experimental design The experimental design used in this study is presented in Table 2. Frozen beefburgers (fat brisket, lean brisket and shank) with a diameter of 3 and 10 cm were fried at 100, 150 and 175 C, until a temperature of 72 C was reached at the centre of the patty to determine the influence of the pan temperatures on the heat and mass transfer on frying beefburgers and to distinguish between drip and evaporative water losses (Experiment I). Furthermore, the effect of the original patty diameter (3, 5.3, 8.1, 10 and 12.7 cm) was studied in Experiment II, in which frying was carried out at two pan temperatures (150 and 175 C) using rib meat, with a medium fat content of 16.1%. 2.3. Preparation of the beefburgers Beefburger patties were prepared in the same manner as described in Kova´csne´ et al. (2004). 2.4. Frying set-up The frying set-up that was used for all the experiments in the present investigation was the same as described in Kova´csne´ et al. (2004). 2.5. Data acquisition system The temperature data were collected in the same manner as described in Kova´csne´ et al. (2004). 2.6. Chemical analysis The moisture and the fat content of the beefburgers were determined according to Kova´csne´ et al. (2004). Table 2 Full experimental design with two replicates Experiment
Meat raw material
Pan temperature [C]
Patty diameter [cm]
Ia Ib II
FB, LB, S FB, LB, S R
100, 150, 175 100, 150, 175 150, 175
10 3 3, 5.3, 8.1, 10, 12.7
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20
2.7. Determination of the density The density of the raw and fried patties was measured using a liquid replacement method described by Pan and Singh (2001). Rapeseed oil with a density of 914.0 kg m3 was used for the density measurement, because its viscosity could slow down oil penetration into the beefburger. The sample was then placed in a glass beaker with a full level of the rapeseed oil. The oil displaced by the sample gave the volume of the beefburger. The density of the sample was calculated as the ratio of its weight and its volume. 2.8. Determination of the water, fat, total cooking loss, diameter shrinkage and porosity The weight of each sample was measured before frying in the frozen state and after frying. The total loss (TL) was calculated as a percentage of the original weight of the beefburger (Eq. (1)). ð1Þ
where TL is the total loss [%], Wb is the weight before [g], and Wa is the weight after frying [g]. Water and fat loss (a and b values, respectively) were calculated as a percentage related to the weight of the raw beefburger (Eqs. (2) and (3)). Wia Wf ¼ 100 100 a b
b ðW b W a Þ aþb AL t72
½g cm2 s1
ð4Þ
½g cm2 s1
ð5Þ
AT ¼ 2r2b p þ 2rb phb
ð6Þ
AL ¼ 2rb phb
ð7Þ
ð8Þ
where S is the patty diameter shrinkage [%], Db is the patty diameter before [mm], and Da is the patty diameter after frying [mm]. Two types of porosity were calculated, one called porosity based on measured shrinkage (es) and the other called porosity based on measured and calculated densities (ed) and they are defined below. The porosity based on measured shrinkage (es) was based on the volume derived from the difference between the calculated meat patty volume related to fat and water loss and the volume calculated from the diameter shrinkage according to Eq. (9). es ¼ 1
V b ððV fb V fa Þ þ ðV wb V wa ÞÞ Va
V b ¼ r2b phb
ð9Þ ð10Þ
V fb ¼
Fi Wb 100 qf
ð11Þ
V fa ¼
F f Wa 100 qf
ð12Þ
V wb ¼
WiWb 100 qw
ð13Þ
V wa ¼
Wf Wa 100 qw
ð14Þ
ð3Þ
where Wf is the water content after frying [%], Wi is the water content of the raw material [%], Ff is the fat content after frying [%]; Fi is the fat content of the raw material [%]. In addition, the flux of water (Wr) and fat (Fr) was calculated, by dividing the water and fat loss by the total (AT) and lateral area (AL) for the water and fat loss, respectively, and by the total frying time (t72) (Eqs. (4) and (5)). rb is the radius of the beefburger before frying [cm] and hb is the height of the beefburger before frying [cm].
Fr ¼
Db Da 100 Db
ð2Þ
Fi b 100 Ff ¼ 100 a b
a ðW b W a Þ aþb AT t72
S¼
WbWa TL ¼ 100 Wb
Wr ¼
The diameter of each sample was measured at intervals of 45 around the circumference in the frozen state before and 5 min after frying. The patty diameter shrinkage (S) was then calculated as a percentage of the original diameter of the patty (Eq. (8)).
V a ¼ r2a pha
ð15Þ
where Vb is the patty volume before frying [cm3], Vfb is the fat volume in the beefburger before frying [cm3], qf is the density of fat [g cm3], Vfa is the fat volume in the beefburger after frying [cm3], Vwb is the water volume in the beefburger before frying [cm3], qw is the density of water [g cm3], Vwa is the water volume in the beefburger after frying [cm3], Va is the patty volume after frying [cm3], ra is the radius of the beefburger after frying [cm] and ha is the height of the beefburger after frying [cm]. The porosity based on measured and calculated densities (ed) was calculated according to the definition described by Miles, Beek, and Veerkamp (1983) and was derived from the density data (Eq. (16)). qmeasured ed ¼ 1 ð16Þ qcalculated
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27 120
ð17Þ
where qmeasured is the measured density of the fried meat patty [kg m3], qcalculated is the density of the fried meat patty calculated from its individual components [kg m3], X wi is the weight fraction of component i and qi is the density of the pure component [kg m3]. The density of the pure components (i) was calculated by using mathematical expressions given by Choi and Okos (1986) (Eq. (17)). The density values of the pure components used in the present study at 22 C were 995.4 kg m3 for water, 916.4 kg m3 for fat and 1318.5 kg m3 for protein.
100 80 o
1 ðX wi =qi Þ
Temperature [ C]
qcalculated ¼ P
21
60 40 20 0 0
40
80
120
160
200
-20 -40
Frying time [s]
2.9. Statistical analysis The statistical analysis was carried out using a statistics program (Minitab, v. 12.2; MINITAB Inc., USA) with the Pearson correlation, linear regression and analysis of variance (ANOVA).
3. Results and discussion 3.1. The influence of the cooking temperature and meat raw material The cooking time to reach 72 C at the centre of the patty by frying at the three processing temperatures (100, 150 and 175 C) varied considerably (Experiment Ia). The longest frying time of 190 s was found for beefburgers with a high fat content fried at the lowest temperature (100 C), and the shortest frying time of 132 s was found in burgers made of the leanest meat fried at 175 C. Typical temperature profiles of a beefburger prepared from the fat brisket (fat content 39%) can be seen in Fig. 1 as a function of the frying time (measured at the centre (5 mm) and 2 mm below the surface) at 100, 150 and 175 C pan temperature. The higher temperature of the plates resulted in shorter thawing and total frying times at the centre of the burgers. The heat transfer measured 2 mm below the surface, when frying was performed at 100 C and no or minimal crust was formed (visual observation), took significantly longer than at the higher temperatures. The present findings are in agreement with the results of Chen, Marks, and Murphy (1999), who also found that the higher heating temperature contributed to faster heat transfer, when the centre temperature of meat patties cooked in an oven at four different air temperatures (135, 163, 191 and 218 C) were compared. In the study of Dagerskog and Bengtsson (1974), on the other hand, the pan temperatures above 140 C had a small influence on the centre temperature. The explanation for the small deviation in the centre temperature profiles was, according to the
100 C_2 mm
150 C_2 mm
175 C_2 mm
100 C_5 mm
150 C_5 mm
175 C_5 mm
Fig. 1. Measured temperature profiles (at the centre (5 mm) and 2 mm below the surface) of a beefburger (D = 10 cm) prepared from fat brisket as a function of the frying time.
authors, the constant driving force since the wet surface temperature cannot exceed 100 C irrespective of the pan temperature applied. In a later study of Dagerskog (1978) the centre temperature was found to increase with increasing pan temperature. A predictive model of the contact cooking process for frying a hamburger patty was developed, and based on the sensitivity analysis (Pan et al., 2000), it was also shown that the higher heating temperature results in a shorter total frying time to reach the same internal temperature. The weight loss during frying varied from 23.4% for beefburgers made of the leanest meat (fat content 6.7%) fried at 100 C to 46.7% for beefburgers prepared from the fatty meat (fat content 39%) fried at 175 C. This is in agreement with the previous study of Kova´csne´ et al. (2004), where the total weight loss variation was between 25% and 45% for the beefburgers fried at 175 C. The influence of the process temperature on the weight loss was shown in the model of Pan et al. (2000). It was found that an 11% change of the heating temperature could cause about an 8% change in the weight loss. However, in our study the cooking temperature had no influence on the total loss. In a later investigation by Pan and Singh (2001) the same positive relationship between the heating temperature and the weight loss was reported, even though the hamburgers (20% fat content) were cooked without a turn-over on a single-sided pan fryer. The authors reported a weight loss of about 45% on a single-sided pan fryer (300 s of frying time) and a weight loss of about of 25% on a double-sided fryer (frying time of 122 s) at the end of cooking, when the centre temperature was about the same (68 C). Comparing the results of Troutt et al. (1992) with the findings of Pan and Singh (2001) the weight loss
22
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27
in the study of Troutt et al. (1992) was found to be much less (about 26%), when hamburgers with the same fat content (20%) were fried for about the same time (300 s) at a cooking temperature of 150 C. Beefburgers with a fat content of 39.0% (fat brisket) contributed to higher total and fat loss, as shown earlier (Kova´csne´ et al., 2004). Pan et al. (2000) found a negligible effect of the fat content on the total loss based on the model sensitivity analysis, when the fat content was changed by about 17%. The high dependence of the total and fat loss on the initial fat content of the beefburgers suggests that the fat release is substantially enhanced by the possibility of contact between fat droplets, as suggested by Olsson and Tornberg (1991). The fat loss varied between 1.4% and 26.3%, which is in the same range as found by Sheridan and Shilton (2002), who found fat loss to be between 0% and 20% during infrared cooking of beefburgers. The moisture loss during frying varied from 14.7% for burgers with a high fat content fried at 100 C to 28.1% for meat patties prepared from the leanest meat fried at the highest pan temperature (175 C) used in this investigation. The present results for the water loss are consistent with the results of Kova´csne´ et al. (2004), where the water loss varied between 25% and 30%. The results of the present study are also consistent with the findings of Dagerskog and Bengtsson (1974), who reported that the water loss varied between 27–32%, when the temperature at the centre reached about 72 C. They also found that the water loss tended to be higher at higher cooking temperatures, although the differences were very small. They also reported higher water loss in hamburgers with a higher initial fat content, which is not consistent with our findings. In the fried beefburgers the diameter shrinkage varied from 11% for beefburger made of the leanest meat (shank) fried at the lowest cooking temperature of 100 C to 23.4% for patties with the highest fat content fried at 175 C. Our results in this study are in accordance with the previous research of Kova´csne´ et al. (2004), where the variation in diameter shrinkage was between 11–20.3%, and with the results of Pan and Singh (2001), where the diameter shrinkage varied from about 20.8% for hamburgers with a fat content of 20% fried at 160 C to about 23.5% at 200 C. Water loss related to the initial water content was significantly influenced by pan temperature, but not by the meat raw material. The high cooking temperature contributed to higher water loss related to the initial water content. This suggests that the evaporation of water as induced by the two higher temperatures gives rise to higher water loss, whereas the pressure-driven drip loss of water is independent of the higher degree of shrinkage as observed for the fattiest meat (fat brisket). The patty diameter shrinkage was influenced only by the meat raw material, and a high fat content of the
meat patty contributed to higher diameter shrinkage, as previously shown (Kova´csne´ et al., 2004). These observations tell us that the pressure-induced contraction by the drying of the crust is of less importance than the contraction pressure formed by the denaturation of the connective tissue in the fattier meat. 3.2. The influence of patty diameter, pan temperature and meat raw material (Experiment Ia + Ib) The thawing time measured at the centre of the beefburger varied from 51.3 s for burgers made of the leanest meat with the smallest diameter (D = 3 cm) fried at 175 C, to 98.1 s for meat patties prepared from the fatty meat with the largest diameter (D = 10 cm) fried at 100 C. The total frying time was found to be in the range of 89.7–188.9 s. The shortest cooking time was observed in meat patties made of lean brisket with the smaller diameter and fried at 175 C, while the longest cooking time was in beefburgers prepared from the fatty meat of the larger diameter fried at 100 C. The lowest temperature of 83.7 C 2 mm below the surface was obtained when burgers made of lean brisket with a diameter of 10 cm were fried at 100 C, and the highest temperature of 101.6 C was reached in meat patties prepared from the lean meat with the smaller diameter fried at 175 C. The weight loss on frying varied considerably from 23.4% for the lean burgers with the smaller diameter fried at 100 C to 56.3% for the fatty patties with the smaller diameter at 175 C. The variation in the water loss was between 14.7% in beefburgers made of the fatty meat with the larger diameter fried at 100 C and 36.7% in patties prepared from lean brisket with the smaller diameter fried at 175 C. The fat loss varied considerably from 1.3% in the lean burger to 30.4% in the fatty meat, whereas, the heating temperature and the size of the patty had no significant influence. One of the most important physical changes that occurs in beefburgers during frying is the reduction of its volume due to the protein denaturation squeezing out the fat and the water from the meat patty. In the present investigation the patty diameter shrinkage varied from 9% in lean burgers with the smaller diameter fried at 100 C to 23.4% in fatty patties with the larger diameter fried at the highest cooking temperature (175 C). Very few studies have followed this phenomenon, which is so important for the fat and water transfer in the frying of beefburgers. In Table 3 the results of three-way ANOVA for the effects of pan temperature, patty diameter and meat raw material regarding the characteristics of heat, i.e. thawing time at the centre, total frying time and the final temperature 2 mm below the surface and mass transfer, i.e. total, fat and water loss based on the weight of the raw beefburger, fat and water loss related to the initial
<0.001 18.8 12.5 11.2 0.090 15.1 13.2 0.560 13.9 13.6
15.0
<0.001 <0.001 <0.001 0.012 0.316 <0.001 <0.001 <0.001 48.3 28.3 19.7 72.9 42.9 34.3 6.1 0.299 34.2 6.3 27.9 41.1 43.3 7.1 8.7 0.218 28.3 2.5 28.5 37.4 40.1 2.5 8.3 0.147 <0.001 0.092 <0.001 0.871 <0.001 0.001 0.001 <0.001 32.9 11.4 21.8 49.6 36.4 20.5 6.9 0.145 41.0 13.3 28.9 51.3 47.7 8.8 8.5 0.297 0.001 0.694 0.001 0.464 <0.001 0.280 <0.001 0.021 38.1 12.2 26.7 46.2 44.2 15.2 8.1 0.248 33.6 11.9 22.0 45.6 36.0 11.2 5.2 0.188
39.2 13.0 27.5 59.6 46.1 17.6 9.8 0.228
0.041 0.560 0.179
p-value Fat brisket
75.8 144.3 96.1 77.5 141.6 94.0
Lean brisket Shank
70.6 138.4 94.5 <0.001 <0.001 0.003
p-value 10.0
85.9 164.9 93.3 63.4 118.0 96.4
3.0 p-value
<0.001 <0.001 <0.001 67.2 121.0 98.3
175 150
72.4 135.7 97.8
100
84.3 167.6 88.5
Thawing time, at the centre [s] Total frying time [s] Final temperature 2 mm below the surface (Tf_2 mm) [C] Total loss [%] Fat loss [%] Water loss [%] Fat loss/initial fat content [%] Water loss/initial water content [%] Fat flux [g cm2 s1] · 104 Water flux [g cm2 s1] · 104 Porosity based on measured shrinkage (es) Patty diameter shrinkage [%]
The significance of the effect Meat raw material The significance of the effect Original patty diameter [cm] The significance of the effect Pan temperature [C]
Table 3 The main effects of pan temperature, original patty diameter and meat raw material on the characteristics of heat and mass transfer and patty diameter shrinkage giving least square means and the significance (p-value < 0.05) of the effect using three-way ANOVA
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27
23
fat and water content, respectively, and fat and water flux and patty porosity and diameter shrinkage are summarised. The thawing time was significantly influenced by pan temperature, the original diameter of the patty and meat raw material. The higher pan temperature (175 C), smaller patty diameter (3.0 cm) and beefburger prepared from the meat of shank (fat content 6.7%) contributed to a faster thawing time. However, the original diameter had a greater influence than meat raw material on the heat transfer. Total frying time and the final temperature 2 mm below the surface were significantly influenced by the pan temperature and the original diameter of the patty, pan temperature having the greatest influence. The longer frying time in the case of the larger burgers may be partly caused by the decrease in the pan temperature when the patties were placed on the fryer. To clearly see the contribution of this effect the following experiment was performed. The total cooling surface of the 10 cm patties was simulated using eight D = 3 cm burgers. The centre patty, with the thermocouples placed as described previously, was surrounded by the seven others to achieve a 10 cm diameter circle (thus the significant gap around the centre one allowed heat penetration laterally). The frying time for this configuration was 16% shorter than for the D = 10 cm patties. Thus, the explanation for the faster heat transfer in the smaller meat patty could be that in samples of these dimensions one-dimensional heat transfer has become two-dimensional, because the ratio diameter:thickness is less than 10 (Pan et al., 2000). In this case, heat transfer by radiation and convection at the sides has to be taken into account. Apart from the observations made in the former experiments with regard to mass transfer, it can be noted that the fat loss per se is not influenced by the decrease in patty diameter, whereas this is the case for the water loss. This indicates that the pressure-driven shrinkage generating fat drip loss does not change as markedly with the original patty diameter, as does the water evaporation loss in the smaller patties of 3 cm, due to the two-dimensional heat transfer. As can be seen in Fig. 2, the water loss related to the initial water content increases with increasing frying temperature and with decreasing patty diameter. At 100 C pan temperature the average value for D = 10 cm patties was about 33% and for burgers with a diameter of 3 cm was about 39%. At the end of the frying the temperature 2 mm below the surface (Fig. 3) was about 88 C, well below the boiling point of water, for all diameters. Therefore, it can be assumed that the water losses at the lowest cooking temperature occur mainly in the form of drip. Consequently, as with higher pan temperatures, the meat goes through the same temperature and phase stages, a considerable proportion, about 80%, of the water losses will be water drip. This is a very important observation because it tells us that the
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27
24 60
be noted with regard to the size of the burgers. In the smaller meat patties the temperature was found to be significantly higher (about 100 C) due to the considerable circumferential heat transfer. There was no significant difference in the final temperature 2 mm below the surface between 150 and 175 C frying temperatures or for the types of meat. Water and fat flux, as a function of the different meat raw material and frying temperature for the two different sizes of beefburger are shown in Fig. 4a and b. As can be seen, the flux of water at 100 C has a similar tendency for the two diameters, when water is lost mainly as drip. However, at 150 and 175 C pan temperature
D=3 cm
Water loss/initial water content [%]
D=10 cm 55
50
45
40
35
30 Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
Shank
0.0016
Lean Fat Brisket Brisket
D=3 cm
175 C 175 C 175 C
0.0014
Fig. 2. Water loss related to the initial water content expressed as a function of the frying temperature for fat brisket, lean brisket and shank. Mean values are shown with standard errors.
0.0012
o
o
o
o
150 C 150 C 150 C
o
o
o
105 D=3 cm D=10 cm 100
D=10 cm
0.001 0.0008 0.0006 0.0004 0.0002
95 0
(a)
o
o
100 C 100 C 100 C 0.008 0.007
85
Shank Lean Fat Brisket Brisket o
90
Shank Lean Fat Brisket Brisket o
150 C 150 oC 150 oC
Shank Lean Fat Brisket Brisket
175 oC 175 oC 175 oC
D=3 cm D=10 cm
0.006
80 Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
100oC 100oC 100oC
150oC 150oC 150oC
175oC 175oC 175oC
Fig. 3. Final temperature 2 mm below the surface expressed as a function of the frying temperature for fat brisket, lean brisket and shank. Mean values are shown with standard errors.
Fat flux [gcm-2s -1]
Final temperature 2 mm below the surface [oC]
100 C 100 C 100 C
o
Water flux [gcm-2s -1]
o
0.005 0.004 0.003 0.002 0.001
pressure-driven drip loss of water constitutes the main mechanism for water losses even at the high frying temperature of 175 C. Evidently, any simulation of water losses on frying of hamburger must not neglect to consider this mechanism in the equations. To our knowledge this has not been done so far. With an 88 C (near) surface temperature no significant crust was observed visually on the top and bottom surfaces of the burgers. By increasing the pan temperature significant differences in the final temperature can
0
(b)
Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
100 oC 100 oC 100 oC
150 oC 150 oC 150 oC
175 oC 175 oC 175 oC
Fig. 4. (a) Water flux (g cm2 s1) as a function of different meat raw material and frying temperature for the two different sizes of beefburger. Mean values are shown with standard errors. (b) Fat flux (g cm2 s1) as a function of different meat raw material and frying temperature for the two different sizes of beefburger. Mean values are shown with standard errors.
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 71 (2005) 18–27 0.48 D=3 cm Porosity based on measured shrinkage
the smaller diameter beefburgers have a maximal rate of water release for the medium fat burger, when evaporation of water also comes into play. The water flux in fat brisket meat at 100 C is significantly less than in the two least fatty meats and the explanation for this observation could be the availability of water. However, the maximum regarding the water flux for the smaller patties made of lean brisket fried at 150 and 175 C might be explained by the following reasoning. At the higher pan temperature, when crust is formed and at the lower patty diameter of 3 cm and for fattier meat the porosity of the burgers is significantly higher according to Table 3. That means that the water flux due to the increased porosity is higher for burgers at 150 and 175 C pan temperature for the smallest diameter and using fattier meat, as is in fact observed in Fig. 4a. However, for the fattiest meat the water availability is less and therefore the water flux is less for those burgers. In Fig. 4b the fat flux as a function of the different meat raw material and frying temperature for the two different sizes of beefburger is shown. As can be seen, the fat fluxes follow the same trend for all the pan temperatures used in this investigation. As explained previously, the most important determinant for the fat loss is the fat content and not the cooking temperature. Despite the large standard error in the case of the fatty meat, the fat flux is higher in the larger beefburgers. This is the reverse of the case for the water flux as a function of patty diameter. That means that the shrinkage, which is higher in the larger beefburgers, is the most important parameter for the fat release and not the increased heat transfer as in the case of water loss. No significant differences were found regarding shank and lean brisket patties. Shrinkage-based porosity as a function of different meat raw materials and frying temperatures for the two different sizes of beefburger is shown in Fig. 5. Interesting observations can be made from Fig. 5. Firstly, when no marked crust is formed at 100 C pan temperature and when the diameter of the patty is large, the lowest porosity of about 8–10% is obtained. Then the flux of water and fat is relatively in phase (only with 8–10% difference) with the rate of formation of a dried out surface which is not flexible enough to follow the non-filled volumes, created by the fat and water release. It can further be seen in Fig. 5 that the porosity is influenced by the pan temperature, patty diameter and fat content, which in turn reflects the amount of fat loss. It can further be noted that for the fattiest and the smallest beefburger the porosity can reach as much as 40%. Moreover, decreasing the patty diameter gives rise to substantial enhancement of the porosity, and there seems to be a maximum in porosity at a pan temperature of 150 C compared to the other two temperatures.
25
0.42
D=10 cm
0.36 0.3 0.24 0.18 0.12 0.06 0 Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
Shank Lean Fat Brisket Brisket
100 oC 100 oC 100 oC
150 oC 150 oC 150 oC
175 oC 175 oC 175 oC
Fig. 5. Shrinkage-based porosity as a function of different meat raw material and frying temperature for the two different sizes of beefburger. Mean values are shown with standard errors.
3.3. Influence of the diameter and pan temperature (Experiment II) In Table 4 the results of ANOVA for the effects of pan temperature and patty diameter regarding the same parameters as given in the other tables, are summarised. The thawing time at the centre and total frying time was for these two higher temperatures significantly influenced only by the original patty diameter. The thawing time at the centre increased with increasing patty diameter up to the diameter of 10.0 cm and then levelled off. This could be caused by the small ratio of diameter to thickness of a small beefburger patty (diameter/height < 10) making the radial heat transfer significant. Total loss based on the weight of the raw beefburger was significantly influenced by pan temperature and original patty diameter. Fat and water loss based on the weight of the raw beefburger, fat and water loss related to the initial fat and water content, respectively, and the flux of fat was not significantly influenced either by pan temperature or by the original patty diameter. The flux of water was significantly influenced by pan temperature and somewhat less by the original patty diameter. The smaller patty diameter contributed clearly to higher water flux, since the heat transfer through the circumferential area has a major impact in the smaller patties. Beefburgers of three diameters were included in the porosity investigation based on density measurements (8.3, 10 and 12.7 cm). Density-based porosity was significantly influenced only by pan temperature (p = 0.028, Table 4), where the higher cooking temperatures
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Table 4 The main effects of pan temperature and original patty diameter on the characteristics of heat and mass transfer, giving least square means and the significance (p-value < 0.05) of the effect using two-way ANOVA Pan temperature [C]
Original patty diameter [cm]
The significance of the effect
150
175
p-value
3.0
5.3
8.1
10.0
12.7
73.1 134.6 93.8 34.1 7.4 25.3 45.8 39.3 12.3 8.9 0.071
67.7 118.9 95.9 38.6 7.8 28.0 48.2 43.6 15.0 11.2 0.147
0.278 0.146 0.463 0.030 0.688 0.056 0.688 0.056 0.249 0.010 0.028
57.5 98.5 99.4 37.6 7.3 28.9 45.0 44.9 7.1 10.8 –
60.1 101.1 89.8 41.0 8.7 26.5 53.8 41.2 14.3 11.7 –
70.6 120.5 93.2 38.7 8.8 27.7 54.5 43.1 17.8 11.2 0.115
82.8 148.3 94.1 33.4 7.5 26.3 46.3 40.9 15.1 8.7 0.143
80.9 165.2 97.9 31.0 5.7 23.9 35.4 37.2 13.8 7.7 0.068
0.011 0.002 0.233 0.021 0.233 0.202 0.233 0.202 0.092 0.019 0.148
0.244
0.289
0.059
0.348
0.300
0.296
0.115
<0.001
9.7
0.185
6.5
11.2
contributed to a larger porosity (Table 4). Wang and Brennan (1995) found also a significant relationship between porosity and drying temperature. The maximal volume of air pore formation during dehydration of potato was larger at higher drying temperatures. During frying of beefburgers, when the temperature at the surface of the patty rises above 102 C (the boiling point of water with solutes), a thin crust starts to form. This crust is characterised by a low thermal conductivity and high porosity (very different from the core region) and acts as a thermally insulating material. At higher frying temperatures the crust formation is faster as a result of higher heat penetration, giving a harder and more porous ÔshellÕ around the internal core region than at lower frying temperatures. It was found that with increasing pan temperature the diameter shrinkage of the patty decreased, which could be due to the resistance of this harder shell to follow the contraction of the core. As can be seen in Fig. 6, the shrinkage-based porosity follows the same trend for the two frying temperatures for both diameters. The highest porosity of about 35% was found for beefburgers with the smallest diameter. For the three medium diameters, however, the size of the beefburger did not influence the porosity. The patty diameter had a significant influence on the porosity, regardless of the pan temperature applied. In the smallest patties the high porosity could be due to the fact that the heating rate was the quickest as was therefore also the formation of crust. This hard crust (shell) developed during the heating process was not able to follow the decline in the volume due to the fat and water migration. For the largest beefburgers, 12.7 cm in diameter, a substantially lower porosity could be due to the long migration distance for the water and fat drip, thereby causing
0.275 10.0
10.3
12.0
13.4
p-value
0.013
0.45 Shrinkage based (filled symbols) and density based (unfilled symbols) porosity at 175 o C and 150 o C
Thawing time, at the centre [s] Total frying time [s] Tf_2 mm [C] Total loss [%] Fat loss [%] Water loss [%] Fat loss/initial fat content [%] Water loss/initial water content [%] Fat flux [g cm2 s1] · 104 Water flux [g cm2 s1] · 104 Porosity based on measured and calculated density (ed) Porosity based on measured shrinkage (es) Patty diameter shrinkage [%]
The significance of the effect
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
2 150 C
4 6 8 10 Patty original diameter [cm] 175 C
150 C
12
14
175 C
Fig. 6. Shrinkage- and density-based porosity as a function of original patty diameter at 150 and 175 C pan temperature. Mean values are shown with standard errors.
less water and fat loss (see Table 4). The density-based porosity was found to be lower than the shrinkagebased porosity. Probably the rapeseed oil could not hinder the oil penetration into the beefburger sufficiently, resulting in the porosity measured in this way being too low. Patty diameter shrinkage was significantly influenced only by the original diameter. The larger diameter of the meat patty evidently contributed to higher shrinkage, as we have observed earlier (Kova´csne´ et al., 2004).
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27
4. Conclusions
References
The higher pan temperature had a significant impact on the water loss but not on the fat loss and patty diameter shrinkage. At 100 C pan frying temperature, when the crust formation is minimal and no significant water evaporation occurs, the water loss occurred mainly in the form of drip. Increasing the pan temperature to 175 C the water flux loss doubled, due to evaporation. But the water loss based on initial water in the form of drip constituted about 80% of the water loss even at 175 C frying pan temperature. This indicates that the pressure-driven drip loss of water is the main mechanism by which water is lost on frying beefburgers. In summary, water drip loss is constant at a given meat composition and evaporation increases with increasing pan temperature. Fat loss increases significantly with fat content and is not influenced to any large extent by the cooking temperature. The temperature increase in the smaller patties was faster than in the larger ones, regardless of the frying temperature, possibly due to the relatively higher circumferential heat transfer. For the same reason water loss was found to be higher in the smaller patties. The fat release, however, was found to be controlled primarily by the patty diameter shrinkage, which is much higher in the larger beefburgers. The higher the heat penetration by using smaller diameters and higher pan temperatures and using fattier meat caused more fat and water losses, which in turn gave rise to higher porosity in the fried patty. Comparing the shrinkage- and density-based porosity, it can be concluded that the density-based porosity was always lower than the shrinkage-based porosity, probably due to oil penetration during the density measurement. However, they both followed the same trend when the porosity of the fried beefburgers of different diameters was studied. This means that at the lowest patty diameter of 3 cm the porosity was the highest, whereas an increase from 4 to 10 cm in patty diameter did not change the porosity until at a diameter of 12.7 cm, it decreased to half its value. This is due to the fact that patty shrinkage is the highest at the largest diameter and at the same time the total losses are the lowest, leaving less space within the beefburger.
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Acknowledgement This work was financially supported by LiFT-Future Technologies for Food Production which is acknowledged with thanks. Thanks are also due to Prof. Petr Dejmek for reading the manuscript and for many good suggestions.