- Email: [email protected]
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 (2005) 499–506 www.elsevier.com/locate/jfoodeng
The mechanisms controlling heat and mass transfer on frying of beefburgers. I. The influence of the composition and comminution of meat raw material Bea Kova´csne´ Oroszva´ri *, 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 15 September 2003; revised 15 March 2004; accepted 17 May 2004
Abstract Heat and mass transfer in minced meat patties (D = 100 mm, H = 10 mm) were studied during frying from the frozen state (20 C) to a centre temperature of 72 C in a double-sided pan fryer. The chemical composition of the meat raw material was varied to study the effect of the water, fat and connective tissue content and the water–protein ratio on the mass transfer (total loss, fat and water losses, shrinkage of the fried patties) and the heat transfer, by recording the time–temperature course at the centre and 2 mm below the surface. The higher initial water content in the meat patties contributed to a faster thawing time in both the core and 2 mm below the surface of the hamburger. The results showed that the higher the shrinkage in diameter of the meat patties, the more fat and total losses, which in turn was related to the amount of connective tissue. This investigation suggests that water and fat losses influence heat transfer and they should be separated. The latter influences mainly the heat transfer in the central core of the thawed beefburgers, whereas in the neighbourhood of crust formation at the surface of the burger, water evaporation losses mainly prolong the frying time. The degree of comminution had no significant effect on the heat and mass transfer. 2004 Elsevier Ltd. All rights reserved. Keywords: Beefburger; Connective tissue content; Frying time; Fat and water loss; Heat and mass transfer
1. Introduction Thermal processing must be adequate to destroy both spoilage and toxigenic microorganisms, but overprocessing should be avoided because of its undesirable effects on the flavour, texture, and nutritive value of meat foods. Therefore, numerous numerical models of beef patties have been developed over recent years in an attempt to predict temperature histories (Dagerskog, 1979; Ikediala, Correia, Fenton, & Ben-Abdallah, 1996; Pan, 1998; Zorrilla & Singh, 2000). To understand the *
Corresponding author. Tel.: +46 46 222 98 15; fax: +46 46 222 46
22. E-mail address: Kova´csne´ Oroszva´ri).
[email protected]
(B.
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.05.017
process, it is often more useful to start by developing one-dimensional models, following then up with multidimensional models that are closer to the real situation (Zorrilla & Singh, 2003). To avoid the computational complexities of a coupled heat and mass transfer model, shrinkage is often neglected (Chen, Marks, & Murphy, 1999; Huang & Mittal, 1995; Ikediala et al., 1996; Sterner, 2003; Teixeira & Tobinaga, 1998). Severe shrinkage of hamburger patties during heat treatment has been reported, however (Berry, 1992; Troutt et al., 1992). Relatively few models of coupled heat and mass transfer during thermal processing of beefburgers have been proposed to date (Pan, Singh, & Rumsey, 2000; Sterner, 2003). A great deal of work has been done on investigating how the chemical and physical properties of beefburgers
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
500
influence heat and mass transfer during heat treatments (e.g. Andersson, Andersson, & Tornberg, 2000; Dagerskog, 1978; Housova´ & Topinka, 1985; Ikediala et al., 1996; Pan & Singh, 2001; Troutt et al., 1992). However, there is a lack in the literature of how the heterogeneous structural properties of beefburgers contribute to the mechanisms of heat and mass transfer during frying. By means of these mechanisms perhaps an improved computer modelling of heat and mass transfer on frying of beefburgers could be developed. In order to successfully predict the various changes (structural-, thermal-, compositional-) in beefburgers during frying we will in this investigation try to identify some of the relevant factors and quantify their effect. These factors may be physical (e.g. comminution) or chemical (e.g. water–protein ratio) in nature. In addition to the commonly used factors, phenomena such as shrinkage, occurring during the heat treatment, might play an important role in the thermal and mass transfer processes.
2. Materials and methods 2.1. Raw material Four types of meat raw material were chosen to give different fat, water–protein ratios and connective tissue content in the beefburgers (Table 1). The contents of water (NMKL, 1991), fat (Croon, Fuchs, & Torelm, 1985), protein (AOAC, 1993) and hydroxyproline (NMKL, 1988), (which gives the connective tissue content by multiplying the hydroxyproline content by a factor of 8) were determined for all the raw materials. Each type of meat was first ground through an 8-mm grinder plate. The meat batter was then split into two halves, of which one was ground once more through a 3-mm grinder plate. In some samples 10% extra water was added (90% meat and 10% water). All the batters were mixed with 1.5% NaCl for 2 min at low speed in a Hobart mixer. After grinding the pH was measured for all the meat types, using a pH meter (PHM 62 Standard pH meter). The ground meat was then vacuum packed separately and stored at 45 C until use.
2.2. The experimental design A full factorial experimental design was constructed with three factors (raw material, degree of comminution and added water) and the corresponding levels (4, 2 and 2, respectively), which are presented in Table 2. Frying experiments were thus conducted on 16 different types of beefburgers. 2.3. Preparation of beefburgers Beefburgers, each 92 g in weight, 10 mm thick and 100 mm in diameter, were made. Three plastic cylinders, 100 mm in diameter, of three different heights (2, 3 and 5 mm) were used to form beefburgers 10 mm thick. Meat patties had three layers each, to ensure the position of the thermocouples exactly at the centre and 2 mm below the surface. When the layers were made, they were placed in a freezer (45 C) for about 30 min, so it was easy to separate the meat layers from the rings. After weighing of the three layers, the thermocouples were placed between the semi-thawed meat layers to provide a better union. After the placement of the thermocouples, beefburgers were frozen another time for, at least, 2 h until they reached 20 C at the centre. Preparing the beefburgers in that way the meat layers could freeze together with the thermocouples, they were not separated (visual observation). For the temperature measurements two K-type thermocouples (T/T-40-K, Pentronic AB, Sweden) were used, one at the centre and one 2 mm below the surface. The diameter of each thermo wire was 40 gauges. 2.4. Frying procedure A rectangular shaped, double-sided pan fryer made of Teflon-coated aluminium, with heating surfaces of 220 · 220 mm was used. They were heated by resistance wire in close contact with their back side, giving negligible temperature gradients over the central part of the surface. Aluminium was preferred over iron because of its higher thermal conductivity. Thin copper constantan thermocouple spears were bored into the aluminium pans, 1 mm under the heating surface, for temperature
Table 1 Chemical composition and pH of the raw materials Sample
Raw material
Water [%]
Fat [%]
Protein [%]
Connective tissue [%]
Water protein ratio
pH
Brisket fat Brisket fat + 10% water Brisket lean Brisket lean + 10% water Rib Rib + 10% water Shank Shank + 10% water
1 1 2 2 3 3 4 4
57.9 62.1 66.3 69.7 70.3 73.3 72.7 75.4
25.9 23.3 14.5 13.0 9.1 8.2 6.1 5.5
15.6 14.0 19.5 17.6 19.3 17.4 20.2 18.2
2.2 1.9 2.3 2.1 1.1 1.0 2.2 2.0
3.7 4.4 3.4 4.0 3.6 4.2 3.6 4.1
5.71 5.71 5.77 5.77 5.62 5.62 5.82 5.82
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506 Table 2 Full factorial design Factor
Level
Raw material Degree of comminution Added water
Brisket fat, brisket lean, rib, shank 3 mm, 8 mm 0%, 10%
control and measurement immediately under the centre of the meat patty being heated. Using a PID-regulator with a tyristor controlled output; the effective pan temperature could thus be controlled within ±2 C. The frozen beefburgers were placed in the middle of the heated surfaces and fried one by one. The heating temperatures of the top and bottom plate could be set separately. A distance of 10 mm between the two heating plates was maintained during frying by means of a Teflon frame with outer dimensions of 200 · 200 mm and a height of 10 mm. In this way, the weight of the top heating plate did not give rise to an extra pressure exerted on the beefburger during frying. Maintaining this constant distance between the frying plates ensured that the beefburgers contracted mainly in two dimensions thereby minimising the chance of changing the position of the thermocouple during shrinkage of the beefburger. The frozen beefburgers (20 ± 1 C initial temperature) were fried at the original frying pan temperature of 175 C until a temperature of 72 C at the centre was reached. Immediately after the frozen patty was placed on the heating surface the temperature of the fryer dropped by 40 C. The actual, final pan temperature was 20 C lower than 175 C and due to the short cooking time, the initial temperature of 175 C was not reached again. When the frying was terminated, the beefburgers were placed on a piece of absorbent paper to ensure that the fat and water at the surfaces were included in the losses. 2.5. Data acquisition system A personal computer, an AAC-2 thermo logger (INTAB Interface-Teknik AB, Sweden), and software (EasyView32 v.4, INTAB Interface-Teknik AB, Sweden) was used to collect the temperature data at a frequency of 1 Hz. 2.6. Water and fat analysis of the fried beefburgers The contents of water and fat were measured in the fried beefburger patties in order to be able to calculate the water and fat losses, respectively. For the determination of the moisture content of the fried beefburger 5 g of sample was dried at 105 C for 16 h or to constant weight. The fat content of the fried beefburger was determined according to Motarjemi (1988) as follows: the fried beefburger patty was cut into four pieces and four representative samples for the water and the fat determi-
501
nation were taken from the two crossing diagonal lines of the beefburger (5 g). After the determination of the water content of the fried meat patty, the dried material was crushed in a mortar with chloroform for 5 min. The mixture was then filtered through a glass filter and the fat was eluted with fresh chloroform by suction from a water pump for 10 min. The glass filter containing the de-fatted dried meat was dried in an oven at 105 C for one hour. The fat content of the fried beefburger was calculated by subtracting the weight of the fat-free substance from the weight of the dried meat. 2.7. Determination of the water, fat, total cooking loss and diameter shrinkage The weight of each sample was measured in the frozen state before and after the end of frying. The total loss was calculated as a percentage of the original weight of the beefburger (Eq. (1)). TL ¼
WbWa 100 Wb
ð1Þ
where TL is the total loss [%], Wb is the weight before [g], Wa is the weight after frying [g]. Water and fat loss (a and b values) was calculated as a percentage related to the weight of the raw beefburger (Eqs. (2) and (3)). Wf ¼
Wia 100 100 a b
ð2Þ
Ff ¼
Fi b 100 100 a b
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 [%], a is the water loss [%], b is the fat loss [%]. The diameter of each sample was measured at eight different points in the frozen state before, and 5 min after the end of frying, and averaged. The diameter shrinkage was then calculated as a percentage of the original diameter of the patty (Eq. (4)). S¼
Db Da 100 Db
ð4Þ
where S is the diameter shrinkage [%], Db is the diameter before [mm], Da is the diameter after flying [mm]. 2.8. 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, One Way Analysis of Variance (ANOVA) and Principle Component Analysis (PCA).
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
502
3. Results and discussion Typical temperature curves obtained for the centre and 2 mm below the surface during frying of beefburgers are given in Fig. 1a and b. These curves were divided into different, relevant time intervals, which were used as the characteristics of the heat transfer (Table 3). Hamm and Deatherage (1960) found that the denaturation of the myofibrillar proteins begins between 30 and 40 C, therefore 36 C was chosen for the determination of the intervals t0–t36 and t36–t72, as there was usually a little change in the slope at 35–36 C. According to Bendall and Restall (1983) the collagen fibrils of the connective tissue start to shrink between 58 and 64 C and continue to do so at higher temperatures. Therefore the temperature of 60 C was chosen to divide the 2 mm curve into two intervals, as the slope had a tendency to change at this temperature. The lateral structural changes in the form of diameter shrinkage of the beefburger are presented in Table 3. No changes in the thickness of the hamburgers were observed as the meat patty was restricted by the way the
80 70
Temperature [°C]
60 50
t0
40
t 0-t 36
t36-t 72
30 20 10 0 -10 -20 -30 0
(a)
20
40
60
80
100
120
140
160
Time [s] 120
Temperature [°C]
100 80
t0_2mm t 0-t 60
60
t 60-t T
f
40 20 0 -20
(b)
0
20
40
60
80
100
120
140
160
Time [s]
Fig. 1. Typical measured temperatures as a function of time, at the centre of the beefburger (a) and at 2 mm below the surface (b).
frying was performed (see methods). Tf represents the temperature (C) 2 mm below the surface of the hamburger at the end of the frying, when the centre temperature was 72 C. Total frying time (s) is the time in seconds until the temperature of 72 C is reached at the centre. According to Fig. 1 and Table 3 the thawing time constitutes the longest part of the frying time for the centre of the beefburger, whereas for 2 mm below the surface this is the case for the final temperature interval, i.e. t60–tTf. This shows that for this type of frying of frozen beefburgers, the time for thawing dominates at the centre, while crust formation and water evaporation takes most time near the surface of the burger. For the temperature profile at 2 mm below the surface there is a large variation of times and temperatures (Table 3), as evidenced in the standard deviations, which are much larger for the 2 mms registrations than for those measured at the centre of the beefburger. One reason could be the difficulty to keep the thermocouple in place at exactly 2 mm below the surface, due to less material to support it, but it also suggests that the crust formation can proceed in an uneven way. This might also be the reason for the fact that no significant influence of the raw material on the heat transfer 2 mm below the surface of the beefburgers can be observed, as shown by p-values above 0.05 for t0–2 mm, t0–t60, t60–tTf and Tf. Surprisingly, the thawing time, t0, for the centre of the beefburger was not influenced by the raw material, which is mainly a variation in fat content. By contrast, the raw material played an important role in the total frying time in such a way that the higher the fat content of the meat patties the more the heat transfer was retarded. However, the heat transfer above 0 C and the diameter shrinkage are significantly influenced by the composition of the raw material. For the mass transfer, i.e. fat, water and total losses, the influence of the raw material can be seen in Fig. 2. A higher initial fat content results in a higher fat and total loss, whereas the water losses are substantially less influenced by the different raw materials. This is in agreement with Dagerskog and Bengtsson (1974) and Olsson and Tornberg (1991), among others. There was a positive, quadratic relationship between fat loss (expressed as a percentage based on the weight of the beefburgers) and initial fat content (r2 = 0.97) and this is in agreement with earlier studies (Andersson et al., 2000). As suggested by these authors, beefburgers with higher initial fat content can create larger fat pools, which help the fat to migrate out of the inner to the outer part of the beefburger. There was, however, no significant influence of the initial amount of water on water losses during the heat treatment, the water losses being almost the same, an average of 30% (expressed as a percentage based on the weight of the beefburgers), for all kinds of meat (Fig. 2).
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
503
Table 3 The effect of the raw material on the characteristics of the heat transfer and diameter shrinkage, using One-way Analysis of Variance giving least square means, standard deviations and the significance (p-value < 0.05) of the effect Time [s] to reach certain temperature [ti], final temperature [C] and shrinkage [%]
Thawing time, t0 [s] Time between 0 and 36 C, t0–t36 [s] Time between 36 and 72 C, t36–t72 [s] Thawing time 2 mm below the surface, t0–2 mm [s] Time between 0 and 60 C 2 mm below the surface, t0–t60 [s] Time between 60 and end temperature 2 mm below the surface, t60–tTf [s] End temperature 2 mm below surface Tf [C] Total frying time [s] Diameter shrinkage [%]
Brisket fat (n = 5)
Brisket lean (n = 8)
Rib (n = 5)
Mean
Std
Shank (n = 5)
The significance of the effect p-value
Mean
Std
Mean
Std
Mean
Std
83.8 32.2 35.4 21.5 44.3
1.9 1.5 4.4 6.8 9.8
78.8 35.6 37.5 28.3 58.4
5.5 1.8 4.5 5.4 12.7
85.5 19.1 24.3 22.4 48.2
6.1 6.5 6.2 13.1 27.5
77.1 16.3 24.0 18.5 30.8
12.0 9.1 12.5 7.3 32.0
0.205 <0.001 0.006 0.223 0.215
87.6
15.6
65.9
11.2
67.9
40.6
75.1
24.4
0.434
96.7 151.4 19.6
5.1 3.6 0.5
94.8 151.9 20.3
4.7 9.4 3.0
89.9 128.9 12.8
9.0 6.4 2.6
96.4 117.4 11.0
14.0 9.8 2.9
0.635 <0.001 <0.001
45
Total loss [%]
40 35 30 no added water
25
added water 20 0
Fig. 2. Fat, water, and total losses as percentages with standard errors for each type of beefburger (shank, rib, brisket lean and brisket fat).
Meat patties containing 10% extra water had significantly greater losses regardless the type of meat used (Fig. 3). It seems that added water is more loosely bound in the protein matrix than the water embedded in the myofibrillar system of the meat. By contrast, fat losses (based on the initial amount of fat) of the beefburgers with 10% added water were significantly less. The explanation could be in accordance with the reasoning of Andersson et al. (2000) that with extra water there is proportionally less fat in the meat patty at the beginning of the frying process, thereby creating longer distances between fat cells. This in turn lowers the probability of fat cells to meet each other and form channels for further migration of the fat out of the inner part of the beefburger to the surface. In order to see how the characteristics of heat and mass transfer depend on each other Principal Component Analysis (PCA) was performed. The loading plot of the two first components (the degree of variance explained was 68.7%) is shown in Fig. 4 for the centre temperature, where the different frying times and losses can be compared. Three groups, which are interrelated, can be identified according to the loading plot in Fig. 4. Group 1 consists of thawing time t0, water–protein ratio (W/P),
5
10
15
20
25
30
Initial fat content [%]
Fig. 3. Total losses as a function of the initial fat content in beefburgers with and without added water. The error bars represent the standard error.
added water and water losses. Group 2 represents the total loss, connective tissue content, diameter shrinkage, and the two temperature–time intervals of t0–t36 and t36–t72. Fat content, fat losses and raw material represents group 3. Thawing time t0 is inversely correlated with the W/P ratio (r = 0.573, p < 0.05), added water (r = 0.560, p < 0.05) and water losses (r = 0.348, p > 0.05) and this is confirmed for the W/P ratio both for the centre temperature and for 2 mm below the surface by linear regression (Fig. 5). The higher the initial water content of the burgers the less time is needed to thaw the meat patties. This seems to be contradictory because the higher the water contents the more latent heat is necessary to thaw the ice. However, comparing the thermal conductivity of water, ice, fat, and protein in the interval of 25 to 0 C, ice has approximately 5 times better thermal conductivity than water, fat or protein (Heldman, 1992, Chapter 6). Beefburger is a mixture of minced meat, fat, salt and other ingredients. It consists of a coarse and heterogeneous structure, for example, almost intact meat fibres
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506 1.0
Added water W/P
Second Factor
Water loss 0.5
Raw material Total loss
Grinding rate
Shrinkage t0-t36 0.0
t36-t72
Connective tissue content
Fat loss
-0.5
Fat content -1.0
t0 -0.5
0.0
0.5
First Factor
Thawing times [s]
Fig. 4. Loading plot of the two first factors in PCA analysis for frying times at the centre and the different losses (68.7% of the data variance was explained by the first and the second factor).
100 90 80 70 60
R2 = 0.5966
50 40 30 20 10 0 3.35
R2 = 0.6053 3.55
3.75
3.95
4.15
4.35
Water-protein ratio (W/P)
Fig. 5. Thawing times 2 mm below the surface (j, h) and at the centre (d, ) as a function of the water–protein ratio (W/P), with 10% extra added water (unfilled symbols).
(up to 70%), are found to compose its structure, when meat is comminuted with a 3-mm grinder plate (Andersson et al., 2000). The fat in the burger is held mainly in aggregates of fat cells and separate fat cells, which are situated in the fatty tissue surrounded by a connective tissue shell. During the heating first the fat melts, being totally melted around 40 C (Deman, Deman, & Blackman, 1983). Then the collagen, which is the major part of the connective tissue, starts to contract at about 60 C, pressing the fat out of the cells (Andersson et al., 2000). In the present study a positive relationship between the shrinkage and the connective tissue content could also be found, as shown in Fig. 4 (r = 0.505, p < 0.05). Positive correlations were also found between the connective tissue content, t0–t36 (r = 0.538, p < 0.05), t36–t72 (r = 0.614, p < 0.05), shrinkage (r = 0.505, p < 0.05) and total loss (r = 0.300, p > 0.05) and fat loss (r = 0.627, p < 0.05) (Fig. 4). So the fattier the meat the longer the frying times needed to reach the end temperatures of 36 and 72 C. This is probably due to the fact that the thermal conductivity of fat is lower than that of
the other components. These frying times are also related to the shrinkage per se, which implies that the active contraction of the system hinders the heat transfer (r = 0.639, p < 0.05), especially when the fat is released. The dominating effect of water compared to fat on thermal conductivity has also been documented by Muzilla, Unklesbay, Unklesbay, and Helsel (1990), Ziegler, Rizvi, and Acton (1987) and Cuevas and Cheryan (1978). The present finding that the frying time is longer for the fattier meat opposes the results of Troutt et al. (1992). Another PCA analysis was performed for different frying times and losses at 2 mm below the frying surface. Similar negative correlations were found between the thawing time t0–2 mm and W/P and added water (Fig. 6), as were found for the centre temperature in Fig. 4. However, the heat transfer, as characterised by t60–tTf and Tf, was observed to be more positively correlated with added water and W/P, than it was for the centre temperature. Those burgers containing extra water initially needed more time to reach the final temperature from 60 C and lost more water during frying. Evidently, water content and losses come more into play for the heat transfer nearer the surface than at the centre, where fat content and losses were more important. Water has a better ability to conduct heat compared to water vapour and the other components. Since there is less water and more vapour in the patty conducting heat in the zone near the crust and as the crust zone moves, then the thermal conductivity is reduced. Therefore it takes longer to reach the final temperature 2 mm below the surface with added water. On the other hand, the heat transfer characterised by t0–t60 was observed to be negatively correlated with the amount of water added. This is not surprising, since in that temperature–time interval the water is present in liquid form, which has a good thermal conductivity. Therefore it takes less time to reach 60 C from 0 C 2 mm below the surface with 10% extra water. W/P Added water t60-tTf Water loss
0.5
Second Factor
504
Tf
Total loss
0.0
Shrinkage Grinding rate Connective tissue content Fat loss
Raw material
Fat content
-0.5
t0-t60t0-2mm -0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
First Factor
Fig. 6. Loading plot of the two first factors in PCA analysis for frying times 2 mm below the surface and the different losses (55.4% of the data variance was explained by the first and the second factor).
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
Based on the results of the experiments, the degree of comminution showed no significant effect on the heat and mass transfer. This implies that the coarseness of the product does not influence the heat and mass transfer as much as anticipated in comparison with the fat content and fat losses.
4. Conclusions Temperature profiles at the centre and 2 mm below the surface were registered during frying of initially frozen beefburgers made of different meat raw material. For both the core and 2 mm below the surface of the hamburger the thawing time is inversely related to the water content and water losses. This means that the longer the thawing times the less are the water losses. Core: The thawing time (about 76 s) is the longest part of the frying time of 133 s for the centre of the beefburger. For the heating of the core of a hamburger from 0 to 72 C lower losses are to be favoured with regard to quick heat transfer, especially fat losses, which are governed mainly by the initial fat content and the degree of shrinkage. The inter-connections of all these parameters show the interaction between heat and mass transfer. The degree of comminution in this study had no significant effect on the heat and mass transfer. Crust: For the heating of the crust, and in its neighbourhood (2 mm below the surface) fat losses and shrinkage have no influence at all, whereas the time to reach the final temperature (t60–tTf), being the largest part of the frying time in this region of the hamburger (about 77 s), was controlled mainly by the water content.
Acknowledgment This work was financially supported by LiFT-Future Technologies for Food Production.
References Andersson, A., Andersson, K., & Tornberg, E. (2000). A comparison of fat-holding between beefburgers and emulsion sausages. Journal of the Science of Food and Agriculture, 80, 555–560. AOAC Official Method 992.15 (first action 1992) (1993). Crude protein in meat and meat products including pet foods. Combustion method. Journal of AOAC International, 76, 787. Bendall, J. R., & Restall, D. J. (1983). The cooking of single myofibres, small myofibre bundles and muscle strips from beef M. psoas and M. sternomandibularis muscles at varying heating rates and temperatures. Meat Science, 8, 93–117. Berry, B. W. (1992). Low fat level effects on sensory, shear, cooking, and chemical properties of ground beef patties. Journal of Food Science, 57, 537–540.
505
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. Croon, L. B., Fuchs, G., & Torelm, I. (1985). Besta¨mning av fett i ko¨ttvaror med NMR. SLV rapport 1985:9. National Food Administration, Uppsala, Sweden. Cuevas, R., & Cheryan, M. (1978). Thermal conductivity of liquid foods––A review. Journal of Food Process Engineering, 2, 283– 306. Dagerskog, M. (1978). Stekning av livsmedel. Studier av det processtekniska underlaget. Ph.D. thesis, Chalmers Tekniska Ho¨gskola, Go¨teborg. Dagerskog, M. (1979). Pan frying of meat patties. I. A study of heat and mass transfer. Lebensmittel Wissenschaft und Technologie, 12, 217–224. Dagerskog, M., & Bengtsson, N. E. (1974). Pan frying of meat pattiesrelationship among crust formation, yield, composition and processing conditions. Lebensmittel Wissenschaft und Technologie, 7(4), 202–207. Deman, J. M., Deman, L., & Blackman, B. (1983). Melting-point determination of fat products. Journal of the American-Oil ChemistsÕ Society, 60(1), 15–18. Hamm, R., & Deatherage, F. E. (1960). Changes in hydration, solubility and charges of muscle proteins during heating of meat. Food Research, 25, 587. Heldman, D. R. (1992). Food freezing. In D. R. Heldman & D. B. Lund (Eds.), Handbook of Food Engineering (pp. 277–315). New York: Marcel Dekker Inc. Housova´, J., & Topinka, P. (1985). Heat transfer during contact cooking of minced meat patties. Journal of Food Engineering, 4, 169–188. Huang, E., & Mittal, G. S. (1995). Meatball cooking-modeling and simulation. Journal of Food Engineering, 24, 87–100. Ikediala, J. N., Correia, L. R., Fenton, G. A., & Ben-Abdallah, N. (1996). Finite element modeling of heat transfer in meat patties during single-sided pan-frying. Journal of Food Science, 61(4), 796–802. Motarjemi, Y. (1988). A study of some physical properties of water in foodstuffs. Water activity, water binding and water diffusivity in minced meat products. Ph.D. thesis, Lund University, Lund. Muzilla, M., Unklesbay, N., Unklesbay, K., & Helsel, Z. (1990). Effect of moisture content on density, heat capacity and conductivity of restructured pork/soy hull mixtures. Journal of Food Science, 55(6), 1491–1493. NMKL (1988). Hydroxyproline. Colorimetric determination as a measure of collagen in meat and meat products. Method No. 127. Nordic Committee on Food Analysis. NMKL (1991). Moisture and ash. Gravimetric determination in meat and meat products. Method No. 23, 3rd ed. Nordic Committee on Food Analysis. 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. (1998). Predictive modeling and optimization of hamburger patty contact-cooking process. Ph.D. thesis, University of California, Davis. 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. Pan, Z., & Singh, R. P. (2001). Physical and thermal properties of ground beef during cooking. Lebensmittel Wissenschaft und Technologie, 34, 437–444. Sterner, D. (2003). Heat and mass transfer investigations of relevance for food engineering applications. Ph.D. thesis, Lund University, Lund. Teixeira, M. B. F., & Tobinaga, S. (1998). A diffusion model for describing water transport in round squid mantle during drying
506
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
with a moisture-dependent effective diffusivity. Journal of Food Engineering, 36, 169–181. 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 to 30 percent fat. Journal of Food Science, 57(1), 25–29. Ziegler, G. R., Rizvi, S. S. H., & Acton, J. C. (1987). Relationship of water content to textural characteristics, water activity, and
thermal conductivity of some commercial sausages. Journal of Food Science, 52(4), 901–905. Zorrilla, S. E., & Singh, R. P. (2000). Heat transfer in meat patties during double-sided cooking. Food Science and Technology Research, 6, 130–135. Zorrilla, S. E., & Singh, R. P. (2003). Heat transfer in double-sided cooking of meat patties considering two-dimensional geometry and radial shrinkage. Journal of Food Engineering, 57, 57–65.
The mechanisms controlling heat and mass transfer on frying of beefburgers. I. The influence of the composition and comminution of meat raw material Bea Kova´csne´ Oroszva´ri *, 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 15 September 2003; revised 15 March 2004; accepted 17 May 2004
Abstract Heat and mass transfer in minced meat patties (D = 100 mm, H = 10 mm) were studied during frying from the frozen state (20 C) to a centre temperature of 72 C in a double-sided pan fryer. The chemical composition of the meat raw material was varied to study the effect of the water, fat and connective tissue content and the water–protein ratio on the mass transfer (total loss, fat and water losses, shrinkage of the fried patties) and the heat transfer, by recording the time–temperature course at the centre and 2 mm below the surface. The higher initial water content in the meat patties contributed to a faster thawing time in both the core and 2 mm below the surface of the hamburger. The results showed that the higher the shrinkage in diameter of the meat patties, the more fat and total losses, which in turn was related to the amount of connective tissue. This investigation suggests that water and fat losses influence heat transfer and they should be separated. The latter influences mainly the heat transfer in the central core of the thawed beefburgers, whereas in the neighbourhood of crust formation at the surface of the burger, water evaporation losses mainly prolong the frying time. The degree of comminution had no significant effect on the heat and mass transfer. 2004 Elsevier Ltd. All rights reserved. Keywords: Beefburger; Connective tissue content; Frying time; Fat and water loss; Heat and mass transfer
1. Introduction Thermal processing must be adequate to destroy both spoilage and toxigenic microorganisms, but overprocessing should be avoided because of its undesirable effects on the flavour, texture, and nutritive value of meat foods. Therefore, numerous numerical models of beef patties have been developed over recent years in an attempt to predict temperature histories (Dagerskog, 1979; Ikediala, Correia, Fenton, & Ben-Abdallah, 1996; Pan, 1998; Zorrilla & Singh, 2000). To understand the *
Corresponding author. Tel.: +46 46 222 98 15; fax: +46 46 222 46
22. E-mail address: Kova´csne´ Oroszva´ri).
[email protected]
(B.
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.05.017
process, it is often more useful to start by developing one-dimensional models, following then up with multidimensional models that are closer to the real situation (Zorrilla & Singh, 2003). To avoid the computational complexities of a coupled heat and mass transfer model, shrinkage is often neglected (Chen, Marks, & Murphy, 1999; Huang & Mittal, 1995; Ikediala et al., 1996; Sterner, 2003; Teixeira & Tobinaga, 1998). Severe shrinkage of hamburger patties during heat treatment has been reported, however (Berry, 1992; Troutt et al., 1992). Relatively few models of coupled heat and mass transfer during thermal processing of beefburgers have been proposed to date (Pan, Singh, & Rumsey, 2000; Sterner, 2003). A great deal of work has been done on investigating how the chemical and physical properties of beefburgers
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
500
influence heat and mass transfer during heat treatments (e.g. Andersson, Andersson, & Tornberg, 2000; Dagerskog, 1978; Housova´ & Topinka, 1985; Ikediala et al., 1996; Pan & Singh, 2001; Troutt et al., 1992). However, there is a lack in the literature of how the heterogeneous structural properties of beefburgers contribute to the mechanisms of heat and mass transfer during frying. By means of these mechanisms perhaps an improved computer modelling of heat and mass transfer on frying of beefburgers could be developed. In order to successfully predict the various changes (structural-, thermal-, compositional-) in beefburgers during frying we will in this investigation try to identify some of the relevant factors and quantify their effect. These factors may be physical (e.g. comminution) or chemical (e.g. water–protein ratio) in nature. In addition to the commonly used factors, phenomena such as shrinkage, occurring during the heat treatment, might play an important role in the thermal and mass transfer processes.
2. Materials and methods 2.1. Raw material Four types of meat raw material were chosen to give different fat, water–protein ratios and connective tissue content in the beefburgers (Table 1). The contents of water (NMKL, 1991), fat (Croon, Fuchs, & Torelm, 1985), protein (AOAC, 1993) and hydroxyproline (NMKL, 1988), (which gives the connective tissue content by multiplying the hydroxyproline content by a factor of 8) were determined for all the raw materials. Each type of meat was first ground through an 8-mm grinder plate. The meat batter was then split into two halves, of which one was ground once more through a 3-mm grinder plate. In some samples 10% extra water was added (90% meat and 10% water). All the batters were mixed with 1.5% NaCl for 2 min at low speed in a Hobart mixer. After grinding the pH was measured for all the meat types, using a pH meter (PHM 62 Standard pH meter). The ground meat was then vacuum packed separately and stored at 45 C until use.
2.2. The experimental design A full factorial experimental design was constructed with three factors (raw material, degree of comminution and added water) and the corresponding levels (4, 2 and 2, respectively), which are presented in Table 2. Frying experiments were thus conducted on 16 different types of beefburgers. 2.3. Preparation of beefburgers Beefburgers, each 92 g in weight, 10 mm thick and 100 mm in diameter, were made. Three plastic cylinders, 100 mm in diameter, of three different heights (2, 3 and 5 mm) were used to form beefburgers 10 mm thick. Meat patties had three layers each, to ensure the position of the thermocouples exactly at the centre and 2 mm below the surface. When the layers were made, they were placed in a freezer (45 C) for about 30 min, so it was easy to separate the meat layers from the rings. After weighing of the three layers, the thermocouples were placed between the semi-thawed meat layers to provide a better union. After the placement of the thermocouples, beefburgers were frozen another time for, at least, 2 h until they reached 20 C at the centre. Preparing the beefburgers in that way the meat layers could freeze together with the thermocouples, they were not separated (visual observation). For the temperature measurements two K-type thermocouples (T/T-40-K, Pentronic AB, Sweden) were used, one at the centre and one 2 mm below the surface. The diameter of each thermo wire was 40 gauges. 2.4. Frying procedure A rectangular shaped, double-sided pan fryer made of Teflon-coated aluminium, with heating surfaces of 220 · 220 mm was used. They were heated by resistance wire in close contact with their back side, giving negligible temperature gradients over the central part of the surface. Aluminium was preferred over iron because of its higher thermal conductivity. Thin copper constantan thermocouple spears were bored into the aluminium pans, 1 mm under the heating surface, for temperature
Table 1 Chemical composition and pH of the raw materials Sample
Raw material
Water [%]
Fat [%]
Protein [%]
Connective tissue [%]
Water protein ratio
pH
Brisket fat Brisket fat + 10% water Brisket lean Brisket lean + 10% water Rib Rib + 10% water Shank Shank + 10% water
1 1 2 2 3 3 4 4
57.9 62.1 66.3 69.7 70.3 73.3 72.7 75.4
25.9 23.3 14.5 13.0 9.1 8.2 6.1 5.5
15.6 14.0 19.5 17.6 19.3 17.4 20.2 18.2
2.2 1.9 2.3 2.1 1.1 1.0 2.2 2.0
3.7 4.4 3.4 4.0 3.6 4.2 3.6 4.1
5.71 5.71 5.77 5.77 5.62 5.62 5.82 5.82
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506 Table 2 Full factorial design Factor
Level
Raw material Degree of comminution Added water
Brisket fat, brisket lean, rib, shank 3 mm, 8 mm 0%, 10%
control and measurement immediately under the centre of the meat patty being heated. Using a PID-regulator with a tyristor controlled output; the effective pan temperature could thus be controlled within ±2 C. The frozen beefburgers were placed in the middle of the heated surfaces and fried one by one. The heating temperatures of the top and bottom plate could be set separately. A distance of 10 mm between the two heating plates was maintained during frying by means of a Teflon frame with outer dimensions of 200 · 200 mm and a height of 10 mm. In this way, the weight of the top heating plate did not give rise to an extra pressure exerted on the beefburger during frying. Maintaining this constant distance between the frying plates ensured that the beefburgers contracted mainly in two dimensions thereby minimising the chance of changing the position of the thermocouple during shrinkage of the beefburger. The frozen beefburgers (20 ± 1 C initial temperature) were fried at the original frying pan temperature of 175 C until a temperature of 72 C at the centre was reached. Immediately after the frozen patty was placed on the heating surface the temperature of the fryer dropped by 40 C. The actual, final pan temperature was 20 C lower than 175 C and due to the short cooking time, the initial temperature of 175 C was not reached again. When the frying was terminated, the beefburgers were placed on a piece of absorbent paper to ensure that the fat and water at the surfaces were included in the losses. 2.5. Data acquisition system A personal computer, an AAC-2 thermo logger (INTAB Interface-Teknik AB, Sweden), and software (EasyView32 v.4, INTAB Interface-Teknik AB, Sweden) was used to collect the temperature data at a frequency of 1 Hz. 2.6. Water and fat analysis of the fried beefburgers The contents of water and fat were measured in the fried beefburger patties in order to be able to calculate the water and fat losses, respectively. For the determination of the moisture content of the fried beefburger 5 g of sample was dried at 105 C for 16 h or to constant weight. The fat content of the fried beefburger was determined according to Motarjemi (1988) as follows: the fried beefburger patty was cut into four pieces and four representative samples for the water and the fat determi-
501
nation were taken from the two crossing diagonal lines of the beefburger (5 g). After the determination of the water content of the fried meat patty, the dried material was crushed in a mortar with chloroform for 5 min. The mixture was then filtered through a glass filter and the fat was eluted with fresh chloroform by suction from a water pump for 10 min. The glass filter containing the de-fatted dried meat was dried in an oven at 105 C for one hour. The fat content of the fried beefburger was calculated by subtracting the weight of the fat-free substance from the weight of the dried meat. 2.7. Determination of the water, fat, total cooking loss and diameter shrinkage The weight of each sample was measured in the frozen state before and after the end of frying. The total loss was calculated as a percentage of the original weight of the beefburger (Eq. (1)). TL ¼
WbWa 100 Wb
ð1Þ
where TL is the total loss [%], Wb is the weight before [g], Wa is the weight after frying [g]. Water and fat loss (a and b values) was calculated as a percentage related to the weight of the raw beefburger (Eqs. (2) and (3)). Wf ¼
Wia 100 100 a b
ð2Þ
Ff ¼
Fi b 100 100 a b
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 [%], a is the water loss [%], b is the fat loss [%]. The diameter of each sample was measured at eight different points in the frozen state before, and 5 min after the end of frying, and averaged. The diameter shrinkage was then calculated as a percentage of the original diameter of the patty (Eq. (4)). S¼
Db Da 100 Db
ð4Þ
where S is the diameter shrinkage [%], Db is the diameter before [mm], Da is the diameter after flying [mm]. 2.8. 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, One Way Analysis of Variance (ANOVA) and Principle Component Analysis (PCA).
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
502
3. Results and discussion Typical temperature curves obtained for the centre and 2 mm below the surface during frying of beefburgers are given in Fig. 1a and b. These curves were divided into different, relevant time intervals, which were used as the characteristics of the heat transfer (Table 3). Hamm and Deatherage (1960) found that the denaturation of the myofibrillar proteins begins between 30 and 40 C, therefore 36 C was chosen for the determination of the intervals t0–t36 and t36–t72, as there was usually a little change in the slope at 35–36 C. According to Bendall and Restall (1983) the collagen fibrils of the connective tissue start to shrink between 58 and 64 C and continue to do so at higher temperatures. Therefore the temperature of 60 C was chosen to divide the 2 mm curve into two intervals, as the slope had a tendency to change at this temperature. The lateral structural changes in the form of diameter shrinkage of the beefburger are presented in Table 3. No changes in the thickness of the hamburgers were observed as the meat patty was restricted by the way the
80 70
Temperature [°C]
60 50
t0
40
t 0-t 36
t36-t 72
30 20 10 0 -10 -20 -30 0
(a)
20
40
60
80
100
120
140
160
Time [s] 120
Temperature [°C]
100 80
t0_2mm t 0-t 60
60
t 60-t T
f
40 20 0 -20
(b)
0
20
40
60
80
100
120
140
160
Time [s]
Fig. 1. Typical measured temperatures as a function of time, at the centre of the beefburger (a) and at 2 mm below the surface (b).
frying was performed (see methods). Tf represents the temperature (C) 2 mm below the surface of the hamburger at the end of the frying, when the centre temperature was 72 C. Total frying time (s) is the time in seconds until the temperature of 72 C is reached at the centre. According to Fig. 1 and Table 3 the thawing time constitutes the longest part of the frying time for the centre of the beefburger, whereas for 2 mm below the surface this is the case for the final temperature interval, i.e. t60–tTf. This shows that for this type of frying of frozen beefburgers, the time for thawing dominates at the centre, while crust formation and water evaporation takes most time near the surface of the burger. For the temperature profile at 2 mm below the surface there is a large variation of times and temperatures (Table 3), as evidenced in the standard deviations, which are much larger for the 2 mms registrations than for those measured at the centre of the beefburger. One reason could be the difficulty to keep the thermocouple in place at exactly 2 mm below the surface, due to less material to support it, but it also suggests that the crust formation can proceed in an uneven way. This might also be the reason for the fact that no significant influence of the raw material on the heat transfer 2 mm below the surface of the beefburgers can be observed, as shown by p-values above 0.05 for t0–2 mm, t0–t60, t60–tTf and Tf. Surprisingly, the thawing time, t0, for the centre of the beefburger was not influenced by the raw material, which is mainly a variation in fat content. By contrast, the raw material played an important role in the total frying time in such a way that the higher the fat content of the meat patties the more the heat transfer was retarded. However, the heat transfer above 0 C and the diameter shrinkage are significantly influenced by the composition of the raw material. For the mass transfer, i.e. fat, water and total losses, the influence of the raw material can be seen in Fig. 2. A higher initial fat content results in a higher fat and total loss, whereas the water losses are substantially less influenced by the different raw materials. This is in agreement with Dagerskog and Bengtsson (1974) and Olsson and Tornberg (1991), among others. There was a positive, quadratic relationship between fat loss (expressed as a percentage based on the weight of the beefburgers) and initial fat content (r2 = 0.97) and this is in agreement with earlier studies (Andersson et al., 2000). As suggested by these authors, beefburgers with higher initial fat content can create larger fat pools, which help the fat to migrate out of the inner to the outer part of the beefburger. There was, however, no significant influence of the initial amount of water on water losses during the heat treatment, the water losses being almost the same, an average of 30% (expressed as a percentage based on the weight of the beefburgers), for all kinds of meat (Fig. 2).
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
503
Table 3 The effect of the raw material on the characteristics of the heat transfer and diameter shrinkage, using One-way Analysis of Variance giving least square means, standard deviations and the significance (p-value < 0.05) of the effect Time [s] to reach certain temperature [ti], final temperature [C] and shrinkage [%]
Thawing time, t0 [s] Time between 0 and 36 C, t0–t36 [s] Time between 36 and 72 C, t36–t72 [s] Thawing time 2 mm below the surface, t0–2 mm [s] Time between 0 and 60 C 2 mm below the surface, t0–t60 [s] Time between 60 and end temperature 2 mm below the surface, t60–tTf [s] End temperature 2 mm below surface Tf [C] Total frying time [s] Diameter shrinkage [%]
Brisket fat (n = 5)
Brisket lean (n = 8)
Rib (n = 5)
Mean
Std
Shank (n = 5)
The significance of the effect p-value
Mean
Std
Mean
Std
Mean
Std
83.8 32.2 35.4 21.5 44.3
1.9 1.5 4.4 6.8 9.8
78.8 35.6 37.5 28.3 58.4
5.5 1.8 4.5 5.4 12.7
85.5 19.1 24.3 22.4 48.2
6.1 6.5 6.2 13.1 27.5
77.1 16.3 24.0 18.5 30.8
12.0 9.1 12.5 7.3 32.0
0.205 <0.001 0.006 0.223 0.215
87.6
15.6
65.9
11.2
67.9
40.6
75.1
24.4
0.434
96.7 151.4 19.6
5.1 3.6 0.5
94.8 151.9 20.3
4.7 9.4 3.0
89.9 128.9 12.8
9.0 6.4 2.6
96.4 117.4 11.0
14.0 9.8 2.9
0.635 <0.001 <0.001
45
Total loss [%]
40 35 30 no added water
25
added water 20 0
Fig. 2. Fat, water, and total losses as percentages with standard errors for each type of beefburger (shank, rib, brisket lean and brisket fat).
Meat patties containing 10% extra water had significantly greater losses regardless the type of meat used (Fig. 3). It seems that added water is more loosely bound in the protein matrix than the water embedded in the myofibrillar system of the meat. By contrast, fat losses (based on the initial amount of fat) of the beefburgers with 10% added water were significantly less. The explanation could be in accordance with the reasoning of Andersson et al. (2000) that with extra water there is proportionally less fat in the meat patty at the beginning of the frying process, thereby creating longer distances between fat cells. This in turn lowers the probability of fat cells to meet each other and form channels for further migration of the fat out of the inner part of the beefburger to the surface. In order to see how the characteristics of heat and mass transfer depend on each other Principal Component Analysis (PCA) was performed. The loading plot of the two first components (the degree of variance explained was 68.7%) is shown in Fig. 4 for the centre temperature, where the different frying times and losses can be compared. Three groups, which are interrelated, can be identified according to the loading plot in Fig. 4. Group 1 consists of thawing time t0, water–protein ratio (W/P),
5
10
15
20
25
30
Initial fat content [%]
Fig. 3. Total losses as a function of the initial fat content in beefburgers with and without added water. The error bars represent the standard error.
added water and water losses. Group 2 represents the total loss, connective tissue content, diameter shrinkage, and the two temperature–time intervals of t0–t36 and t36–t72. Fat content, fat losses and raw material represents group 3. Thawing time t0 is inversely correlated with the W/P ratio (r = 0.573, p < 0.05), added water (r = 0.560, p < 0.05) and water losses (r = 0.348, p > 0.05) and this is confirmed for the W/P ratio both for the centre temperature and for 2 mm below the surface by linear regression (Fig. 5). The higher the initial water content of the burgers the less time is needed to thaw the meat patties. This seems to be contradictory because the higher the water contents the more latent heat is necessary to thaw the ice. However, comparing the thermal conductivity of water, ice, fat, and protein in the interval of 25 to 0 C, ice has approximately 5 times better thermal conductivity than water, fat or protein (Heldman, 1992, Chapter 6). Beefburger is a mixture of minced meat, fat, salt and other ingredients. It consists of a coarse and heterogeneous structure, for example, almost intact meat fibres
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506 1.0
Added water W/P
Second Factor
Water loss 0.5
Raw material Total loss
Grinding rate
Shrinkage t0-t36 0.0
t36-t72
Connective tissue content
Fat loss
-0.5
Fat content -1.0
t0 -0.5
0.0
0.5
First Factor
Thawing times [s]
Fig. 4. Loading plot of the two first factors in PCA analysis for frying times at the centre and the different losses (68.7% of the data variance was explained by the first and the second factor).
100 90 80 70 60
R2 = 0.5966
50 40 30 20 10 0 3.35
R2 = 0.6053 3.55
3.75
3.95
4.15
4.35
Water-protein ratio (W/P)
Fig. 5. Thawing times 2 mm below the surface (j, h) and at the centre (d, ) as a function of the water–protein ratio (W/P), with 10% extra added water (unfilled symbols).
(up to 70%), are found to compose its structure, when meat is comminuted with a 3-mm grinder plate (Andersson et al., 2000). The fat in the burger is held mainly in aggregates of fat cells and separate fat cells, which are situated in the fatty tissue surrounded by a connective tissue shell. During the heating first the fat melts, being totally melted around 40 C (Deman, Deman, & Blackman, 1983). Then the collagen, which is the major part of the connective tissue, starts to contract at about 60 C, pressing the fat out of the cells (Andersson et al., 2000). In the present study a positive relationship between the shrinkage and the connective tissue content could also be found, as shown in Fig. 4 (r = 0.505, p < 0.05). Positive correlations were also found between the connective tissue content, t0–t36 (r = 0.538, p < 0.05), t36–t72 (r = 0.614, p < 0.05), shrinkage (r = 0.505, p < 0.05) and total loss (r = 0.300, p > 0.05) and fat loss (r = 0.627, p < 0.05) (Fig. 4). So the fattier the meat the longer the frying times needed to reach the end temperatures of 36 and 72 C. This is probably due to the fact that the thermal conductivity of fat is lower than that of
the other components. These frying times are also related to the shrinkage per se, which implies that the active contraction of the system hinders the heat transfer (r = 0.639, p < 0.05), especially when the fat is released. The dominating effect of water compared to fat on thermal conductivity has also been documented by Muzilla, Unklesbay, Unklesbay, and Helsel (1990), Ziegler, Rizvi, and Acton (1987) and Cuevas and Cheryan (1978). The present finding that the frying time is longer for the fattier meat opposes the results of Troutt et al. (1992). Another PCA analysis was performed for different frying times and losses at 2 mm below the frying surface. Similar negative correlations were found between the thawing time t0–2 mm and W/P and added water (Fig. 6), as were found for the centre temperature in Fig. 4. However, the heat transfer, as characterised by t60–tTf and Tf, was observed to be more positively correlated with added water and W/P, than it was for the centre temperature. Those burgers containing extra water initially needed more time to reach the final temperature from 60 C and lost more water during frying. Evidently, water content and losses come more into play for the heat transfer nearer the surface than at the centre, where fat content and losses were more important. Water has a better ability to conduct heat compared to water vapour and the other components. Since there is less water and more vapour in the patty conducting heat in the zone near the crust and as the crust zone moves, then the thermal conductivity is reduced. Therefore it takes longer to reach the final temperature 2 mm below the surface with added water. On the other hand, the heat transfer characterised by t0–t60 was observed to be negatively correlated with the amount of water added. This is not surprising, since in that temperature–time interval the water is present in liquid form, which has a good thermal conductivity. Therefore it takes less time to reach 60 C from 0 C 2 mm below the surface with 10% extra water. W/P Added water t60-tTf Water loss
0.5
Second Factor
504
Tf
Total loss
0.0
Shrinkage Grinding rate Connective tissue content Fat loss
Raw material
Fat content
-0.5
t0-t60t0-2mm -0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
First Factor
Fig. 6. Loading plot of the two first factors in PCA analysis for frying times 2 mm below the surface and the different losses (55.4% of the data variance was explained by the first and the second factor).
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
Based on the results of the experiments, the degree of comminution showed no significant effect on the heat and mass transfer. This implies that the coarseness of the product does not influence the heat and mass transfer as much as anticipated in comparison with the fat content and fat losses.
4. Conclusions Temperature profiles at the centre and 2 mm below the surface were registered during frying of initially frozen beefburgers made of different meat raw material. For both the core and 2 mm below the surface of the hamburger the thawing time is inversely related to the water content and water losses. This means that the longer the thawing times the less are the water losses. Core: The thawing time (about 76 s) is the longest part of the frying time of 133 s for the centre of the beefburger. For the heating of the core of a hamburger from 0 to 72 C lower losses are to be favoured with regard to quick heat transfer, especially fat losses, which are governed mainly by the initial fat content and the degree of shrinkage. The inter-connections of all these parameters show the interaction between heat and mass transfer. The degree of comminution in this study had no significant effect on the heat and mass transfer. Crust: For the heating of the crust, and in its neighbourhood (2 mm below the surface) fat losses and shrinkage have no influence at all, whereas the time to reach the final temperature (t60–tTf), being the largest part of the frying time in this region of the hamburger (about 77 s), was controlled mainly by the water content.
Acknowledgment This work was financially supported by LiFT-Future Technologies for Food Production.
References Andersson, A., Andersson, K., & Tornberg, E. (2000). A comparison of fat-holding between beefburgers and emulsion sausages. Journal of the Science of Food and Agriculture, 80, 555–560. AOAC Official Method 992.15 (first action 1992) (1993). Crude protein in meat and meat products including pet foods. Combustion method. Journal of AOAC International, 76, 787. Bendall, J. R., & Restall, D. J. (1983). The cooking of single myofibres, small myofibre bundles and muscle strips from beef M. psoas and M. sternomandibularis muscles at varying heating rates and temperatures. Meat Science, 8, 93–117. Berry, B. W. (1992). Low fat level effects on sensory, shear, cooking, and chemical properties of ground beef patties. Journal of Food Science, 57, 537–540.
505
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. Croon, L. B., Fuchs, G., & Torelm, I. (1985). Besta¨mning av fett i ko¨ttvaror med NMR. SLV rapport 1985:9. National Food Administration, Uppsala, Sweden. Cuevas, R., & Cheryan, M. (1978). Thermal conductivity of liquid foods––A review. Journal of Food Process Engineering, 2, 283– 306. Dagerskog, M. (1978). Stekning av livsmedel. Studier av det processtekniska underlaget. Ph.D. thesis, Chalmers Tekniska Ho¨gskola, Go¨teborg. Dagerskog, M. (1979). Pan frying of meat patties. I. A study of heat and mass transfer. Lebensmittel Wissenschaft und Technologie, 12, 217–224. Dagerskog, M., & Bengtsson, N. E. (1974). Pan frying of meat pattiesrelationship among crust formation, yield, composition and processing conditions. Lebensmittel Wissenschaft und Technologie, 7(4), 202–207. Deman, J. M., Deman, L., & Blackman, B. (1983). Melting-point determination of fat products. Journal of the American-Oil ChemistsÕ Society, 60(1), 15–18. Hamm, R., & Deatherage, F. E. (1960). Changes in hydration, solubility and charges of muscle proteins during heating of meat. Food Research, 25, 587. Heldman, D. R. (1992). Food freezing. In D. R. Heldman & D. B. Lund (Eds.), Handbook of Food Engineering (pp. 277–315). New York: Marcel Dekker Inc. Housova´, J., & Topinka, P. (1985). Heat transfer during contact cooking of minced meat patties. Journal of Food Engineering, 4, 169–188. Huang, E., & Mittal, G. S. (1995). Meatball cooking-modeling and simulation. Journal of Food Engineering, 24, 87–100. Ikediala, J. N., Correia, L. R., Fenton, G. A., & Ben-Abdallah, N. (1996). Finite element modeling of heat transfer in meat patties during single-sided pan-frying. Journal of Food Science, 61(4), 796–802. Motarjemi, Y. (1988). A study of some physical properties of water in foodstuffs. Water activity, water binding and water diffusivity in minced meat products. Ph.D. thesis, Lund University, Lund. Muzilla, M., Unklesbay, N., Unklesbay, K., & Helsel, Z. (1990). Effect of moisture content on density, heat capacity and conductivity of restructured pork/soy hull mixtures. Journal of Food Science, 55(6), 1491–1493. NMKL (1988). Hydroxyproline. Colorimetric determination as a measure of collagen in meat and meat products. Method No. 127. Nordic Committee on Food Analysis. NMKL (1991). Moisture and ash. Gravimetric determination in meat and meat products. Method No. 23, 3rd ed. Nordic Committee on Food Analysis. 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. (1998). Predictive modeling and optimization of hamburger patty contact-cooking process. Ph.D. thesis, University of California, Davis. 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. Pan, Z., & Singh, R. P. (2001). Physical and thermal properties of ground beef during cooking. Lebensmittel Wissenschaft und Technologie, 34, 437–444. Sterner, D. (2003). Heat and mass transfer investigations of relevance for food engineering applications. Ph.D. thesis, Lund University, Lund. Teixeira, M. B. F., & Tobinaga, S. (1998). A diffusion model for describing water transport in round squid mantle during drying
506
B. Kova´csne´ Oroszva´ri et al. / Journal of Food Engineering 67 (2005) 499–506
with a moisture-dependent effective diffusivity. Journal of Food Engineering, 36, 169–181. 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 to 30 percent fat. Journal of Food Science, 57(1), 25–29. Ziegler, G. R., Rizvi, S. S. H., & Acton, J. C. (1987). Relationship of water content to textural characteristics, water activity, and
thermal conductivity of some commercial sausages. Journal of Food Science, 52(4), 901–905. Zorrilla, S. E., & Singh, R. P. (2000). Heat transfer in meat patties during double-sided cooking. Food Science and Technology Research, 6, 130–135. Zorrilla, S. E., & Singh, R. P. (2003). Heat transfer in double-sided cooking of meat patties considering two-dimensional geometry and radial shrinkage. Journal of Food Engineering, 57, 57–65.