Heat Transfer Properties, Moisture Loss, Product Yield, and Soluble Proteins
in Chicken Breast Patties During Air Convection Cooking
R. Y. Murphy,*,1 E. R. Johnson,* L. K. Duncan,† E. C. Clausen,‡ M. D. Davis,* and J. A. March§
*Department of Biological and Agricultural Engineering; †Department of Mathematical Sciences, ‡Department of Chemical
Engineering; and §Center of Excellence in Poultry Science, University of Arkansas, Fayetteville, Arkansas 72701
obtained for different cooking conditions. Air humidity
in the oven affected the heat transfer coefficient. The moisture loss in the cooked products increased with increasing
the final product temperature and the oven air temperature. The soluble proteins in the cooked patties decreased
with increasing the final product temperature. Increasing
humidity increased heat transfer coefficient and therefore
reduced cooking time. Reducing oven temperature, reducing internal temperature, and increasing air humidity
increased the product yield. Soluble proteins might be
used as an indicator for the degree of cooking. The results
from this study are important for evaluating commercial
thermal processes and improving product yields.
ABSTRACT Chicken breast patties were processed in
an air convection oven at air temperatures of 149 to 218
C, air velocities of 7.1 to 12.7 m3/min, and air relative
humidities of 40 to 95%. The air humidity was controlled
via introducing steam into the oven. The patties were
processed to a final center temperature of 50 to 80 C. Heat
flux, heat transfer coefficient, moisture loss in the cooked
chicken patties, the product yield, and the changes of
soluble proteins in the product were evaluated for the
cooking system. During cooking, heat flux varied with
the processing time. Heat flux increased with increasing
air humidity. The effective heat transfer coefficient was
(Key words: thermophysical property, protein, thermal processing, yield, poultry)
2001 Poultry Science 80:508–514
Air convection ovens are currently operated with a
specified oven temperature, air humidity, air circulation
rate, and conveyor speed. As the chicken products travel
down the line, they are cooked until a specified core
(center or internal) temperature is reached or exceeded
as determined by sampling the product as it exits the
oven. The minimum core temperature is set so as to
assure adequate thermal destruction of food-borne
pathogens because overcooking increases energy costs
and compromises product quality and yield.
The information relating to heat flux (heat transfer
rate per unit area) and heat transfer coefficients during
air convection cooking is lacking in the literature. Air
convection cooking is a process that involves simultaneous heat and mass transfer. Heat is transferred from the
hot air to the product, while moisture is absorbed (high
humidity cooking) in the product or evaporated (low
humidity cooking) from the product. This simultaneous
heat and mass transfer process makes theoretical evaluations of the thermal treatment more complicated.
Dagerskog (1979a,b), Holtz and Skjöldebrand (1986),
Housova and Topinka (1985), and Ikediala et al. (1996)
published studies on heat and mass transfer modeling
for ground beef patties in convection oven cooking and
pan-frying process. However, there is limited application of models in commercial cooking processes because
most published studies on thermal process modeling
were established by assuming constant material proper-
INTRODUCTION
Air convection cooking is widely used in the commercial processing of poultry products. In such ovens, the
oven temperature is much higher than the designated
final product temperature for pathogen destruction.
Cooking time needed for a product to achieve the required internal temperature is not only dependent on
the oven operating conditions but also on the properties
of the product.
According to the USDA-FSIS (1973), an internal temperature of 71.1 C is required for poultry to be labeled
fully cooked. The current rule by the USDA-FSIS (1999)
allows the use of plant-specific processing procedures.
Although longer cooking times ensure that required internal temperatures are met, longer cooking times also
increase processing costs and decrease product yield.
Therefore, the development of an optimal heat treatment
process hinges on the understanding of the heat and
mass transfer processes present inside and outside the
product in addition to the understanding of the pathogen lethality in the food system under consideration.
Received for publication April 10, 2000.
Accepted for publication November 20, 2000.
1
To whom correspondence should be addressed: rymurph@comp.
uark.edu.
508
THERMAL PROCESSING OF CHICKEN PATTY IN A PILOT OVEN
ties. Dagerskog (1979a) reported relationships for density, thermal conductivity, and heat capacity as functions
of meat patty temperature, moisture and fat contents
during pan-fry cooking of hamburgers. The thermal and
physical properties of a product vary with temperature
and moisture content, which adds to the complexity of
the process (Murphy et al., 1998; Murphy and Marks,
2000).
In air convection cooking, convective heat transfer occurs between a solid food and the surrounding air. The
heat transfer coefficient varies during the process. Steam
introduced during cooking complicates the determination of the heat transfer coefficient and results in an
increased heat transfer rate.
Heat-induced changes in protein solubility also relate
to changes in the water-holding capacity of meat (Bouton
and Harris, 1972). Due to protein changes with heating,
water content within the meat myofibrils in the narrow
channels between the filaments changes as meat shrinks
within the tissue matrix (Bertola et al., 1994), resulting
in moisture loss with heating. Meat can shrink in two
dimensions (length and width), while expanding in the
third dimension (Offer et al., 1984; Rowe, 1974). The
extent of meat shrinkage and expansion varies with different muscles and the meat temperature (Winegarden
et al., 1952). The change in moisture content contributes
to the loss of product yield. The change in moisture
content also contributes to changes in sarcomere length
and juiciness with temperature (Laakkonen et al., 1970;
Bouton et al., 1975).
The objective of this study was to evaluate the effect
of oven operating conditions on the heat transport properties, product moisture, product yield, and soluble proteins during air convection cooking in a pilot-scale operation.
MATERIALS AND METHODS
Ground Chicken Breast Meat
Chicken breast meat was obtained from a local processor. A proximate analysis based on AOAC (1990) methods was conducted. The total water content was approximately 71% (g/g, wet basis), as found using an oven
drying method at 110 C for 24 h. The total protein content
was approximately 96% (g/g, dry basis), found using
the Kjeldahl2 method. The total lipids content was approximately 0.2% (g/g, dry basis), as found by the Soxhlet3 method. The total ash content was approximately
2.1% (g/g, dry basis), as found when using a gravimetric
method and heating the sample at 550 C in a muffle
furnace for 24 h.
2
Labconco Corporation, Kansas City, MO 64132.
Soxtec System HT, 1043 Extraction Unit, Fisher Scientific, Itasca,
IL 60143.
4
Lab model 102, Stein, Inc., Sandusky, OH 44870.
5
RDF Cooperation, Hudson, NH 03051.
3
509
Thermal Processing
Ground chicken breast meat was formed into patties
(127 mm diameter × 12.7 mm thickness) with a plastic
former and then stored in a refrigerator at 4 C for approximately 4 h. The refrigerated patties were cooked at various combinations of oven air temperature (149 to 218
C), air velocity (7.1 to 12.7 m3/min), and air humidity
(40 to 95%). After thermal treatment, the patties were
immediately transported to the outside of the oven,
placed in a plastic bag, and immersed in an ice-water
cooling bath.
An air convection oven4 was used in this study to cook
the chicken patties. Steam was supplied to the oven to
regulate the oven humidity. The dry bulb and wet bulb
temperatures were used to monitor the air humidity in
the oven and control the steam flow. A fan pulled air
and steam from the oven and circulated it past an electric
heater that regulated the oven temperature. Heated air
and steam returned through 612 holes that were evenly
distributed above and below the belt on five distribution
boxes. Condensed steam and drip were drained from
the bottom of the oven. The total conveyer length of the
oven was 6 m, and the cooking chamber was 1.5 m wide
and 1.5 m long. Because the pilot-scale oven length was
shorter than in commercial oven systems, the conveyor
moved forward and backward in the oven during thermal processing so that the patties were uniformly heated,
as opposed to moving the conveyor at much slower
speeds than used commercially.
Heat Flux and Heat Transfer Coefficient
During thermal processing, the transient heat flux during thermal processing was measured using a micro-foil
heat flux sensor5 based on the temperature difference
between the product surface and the cooking air. The
apparent heat transfer coefficient during cooking was
calculated using a graphical method of unsteady-state
conduction in a flat plate (Geankoplis, 1993).
Moisture Loss and Soluble Proteins
Patties were individually processed at an oven air
temperature of 163 C (325 F), 177 C (350 F), 190 C (375
F), 204 C (400 F), or 218 C (425 F) to a final center temperature of 50.0, 55.0, 60.0, 65.0, 70.0 75.0, or 80.0 (± 0.5) C.
Type K thermocouples were placed at the patty center
(parallel to the patty diameter), on the top and bottom
surface, and in the air to monitor the temperature via a
data acquisition system.
Cooked and then cooled patties were ground by a food
processor and used for analyzing the moisture losses and
soluble proteins. The moisture loss was calculated from
the moisture content changes of the chicken patties prior
to and after cooking using the oven drying method at
110 C for 3.5 h. The total soluble proteins in chicken
patties were determined by the Bradford method (Murphy and Marks, 1999).
510
MURPHY ET AL.
FIGURE 1. Thermal profile of a patty cooked in a pilot-scale air
convection oven at an oven air temperature of 117 C and an air velocity
of 12.7 m3/min.
Product Yield
The patties (127 mm diameter × 12.7 mm thickness)
were processed at an oven air temperature of 149 C, an
air velocity of 7.1, 9.9, or 12.7 m3/min, and an air relative
humidity of 50 or 95%. The processed patties were placed
in individual plastic bags and cooled in an ice-water
bath. After the patties were cooled, the outsides of the
bagged patties were dried and weighed to determine
product mass. The product yield was calculated as the
percentage change in mass prior to and after cooking.
Statistics
Three replicated experiments were conducted. Statistical analysis was performed with SAS6 software (release
8.1 copyright 1999 to 2000). For moisture losses and total
soluble proteins, standard least squares regressions were
used to analyze the effect of oven air temperature and
product temperature on moisture losses and total soluble proteins in the patties. For product yield, means
were evaluated using the ANOVA and the student ttest. Second order polynomial regressions (Y = a Tp2 +
b Tp + c) were used to relate the product yield to product
internal temperature. The Y represented the product
yield in percentage of mass (g/100 g, wet basis), Tp was
the patty center temperature in degrees C, and a, b, and
c were the constants of the regression.
environment surrounding the patty. The conveyor belt
carried the patty into the cooking chamber where the
patty was cooked to a designated final center temperature.
During cooking, the surface and center temperatures
of the product increased with time. As the cooking progressed, the surface temperature of the patty increased
faster at the beginning and then gradually slowed. The
center temperature gradually increased with time.
Throughout the cooking process, the surface temperature of the patty was always higher than the center temperature. Therefore a substantial temperature gradient
occurred within the patty cooked in the air convection
oven. This temperature gradient varied with cooking
conditions such as air temperature, air humidity, and
air velocity.
A greater temperature difference was observed between the patty surface and the center while cooking at
lower air relative humidity. For the product to reach the
same final center temperature, a longer cooking time
was thus required when cooking at low humidity than
at high humidity.
For the product to reach the same final center temperature, increasing oven air temperature reduced cooking
time. When a patty was cooked at a higher oven air
temperature, a greater temperature gradient was also
observed between the patty surface and the patty center.
Therefore, a greater average temperature was achieved
in the patties cooked at a high oven temperature than
at low oven temperature.
Heat Flux and Heat Transfer Coefficient
Figure 2 shows the heat flux for the patty processed
at an oven air temperature of 177 C, an air velocity of
9.9 m3/min, and air relative humidities of 40, 80, and
95%. During thermal processing, the heat flux varied
with cooking time. The maximum heat flux was 5,000
Watt/m2, 10,000 Watt/m2, and 20,000 Watt/m2, respectively, for the patties processed at air humidities of 40,
80, and 95%. During cooking, humidity in the oven air
RESULTS AND DISCUSSION
Thermal Profiles
Figure 1 illustrates the thermal profile while the patty
was cooked in the pilot-scale air convection oven. The
air temperature in the Figure 1 is the temperature recorded by the thermocouple probes placed in the air
6
SAS Institute威 Inc., Cary, NC 27513.
FIGURE 2. Changes in heat flux for the patty cooked at an air
temperature of 177 C and an air velocity of 9.9 m3/min.
511
THERMAL PROCESSING OF CHICKEN PATTY IN A PILOT OVEN
FIGURE 3. Apparent heat transfer coefficient (h) for patties cooked
at an air temperature of 177 C.
substantially affected the heat transfer rate. The heat
flux increased with increasing relative humidity of the
cooking air. At a relative humidity of 40%, the heat
flux was small and fairly constant; however, at relative
humidities of 80 to 95%, the heat flux was three to six
times higher than at relative humidities of 40%. This
result occurred because introducing steam increased the
heat transfer rate.
Thermal properties of food materials are necessary for
engineering analysis and design of food processing. The
heat transfer coefficient is essential to estimate the course
of the thermal processing of food materials. Accurate
estimation of heat transfer coefficients in thermal processing depends on oven operating conditions (air velocity, temperature, and humidity) and thermal properties
(thermal conductivity, heat capacity, and thermal diffusivity) of the product. Thermal conductivity and heat
capacity of the chicken breast meat have been reported
in previous studies (Murphy et al., 1998; Murphy and
Marks, 1999).
Based on unsteady state heat transfer through a flat
plate (Geankoplis, 1993), the effective heat transfer coefficient was calculated for the patties cooked at an oven
air temperature of 177 C. At an air velocity of 12.7 m3/
min, the heat transfer coefficient increased from 25 to
250 Watt/(m2 K) with increasing air humidity from 40
to 95% (Figure 3). By increasing the air humidity from
40 to 80%, the heat transfer coefficient tripled (Figure
3). Further increasing the air humidity from 40 to 90%,
the heat transfer coefficient increased nearly 10-fold.
Therefore, steam introduction increased the heat transfer
FIGURE 4. Standard least squares model for the moisture loss in the
chicken patties cooked at air temperatures of 163 to 218 C and an air
velocity of 9.9 m3/min, without steam present. Moisture loss (%) =
35.5499 − 0.0713 (oven air temperature) − 0.7420 (patty temperature) +
0.0085 (patty temperature)2; R2 = 0.6790.
coefficient and reduced the cooking time. This result is
consistent with that of the heat flux. Figure 3 also shows
a slight increase in the heat transfer coefficient when
increasing the air velocity from 7.1 to 12.7 m3/min. This
result agreed with most of literature models in which
air velocity positively affected heat transfer coefficients
(Geankoplis, 1993).
Moisture Content
The effect of oven air temperature on moisture loss in
the chicken patties was evaluated at an air velocity of
9.9 m3/min, without steam present, at five different oven
air temperatures from 163 to 218 C. Standard least
squares regression was used to relate the moisture losses
to the oven air and patty temperatures. The parameter
estimates are listed in the Table 1. Moisture losses in
cooked chicken patties were linearly related to the oven
air temperature and quadratically related to the patty
temperature. For the same final product temperature,
increasing the oven air temperature significantly reduced the moisture loss in the cooked chicken patties.
This reduction was due to a shorter cooking time
achieved at a higher oven air temperature. Increasing
product temperatures significantly increased the moisture losses. At the same oven air temperature, the moisture loss in the cooked chicken patties increased three to
five times with increasing the final product temperature
from 50 to 80 C (Figure 4). At an oven air temperature of
163 C, the moisture loss increased from 5 to 25% with
increasing the product temperature from 50 to 80 C.
TABLE 1. Parameter estimates for the standard least squares regression model for moisture losses1
Parameter
Parameter estimates
Standard error
t-value
df
P-value
Intercept
Ta
Tp
35.5499
−0.0713
−0.7420
13.2609
0.0139
0.4065
2.68
−5.11
−1.83
1
1
1
0.0086
<0.0001
0.0709
0.0085
0.0031
2.72
1
0.0076
Tp2
Ta = oven air temperature; Tp = patty center temperature. Replicates = 3. R = 0.6790.
1
2
512
MURPHY ET AL.
At an oven air temperature of 218 C, the moisture loss
increased from 5 to 15% with increasing the product
temperature from 50 to 80 C.
Moisture loss of a product depended on the mass
transfer process during thermal treatment. Mass transfer
due to heat treatments is quite common in food processing. Porosity of a product affected mass transfer during thermal processing (Moreau and Rosenberg, 1999).
Increased porosity increases moisture loss. Murphy and
Marks (1999) reported the changes in porosity of the
chicken breast patties during heat treatment. For the
heat-treated chicken breast patties, higher porosity was
obtained at a higher product temperature (Murphy and
Marks, 1999a).
Product Yield
The studies were also conducted to evaluate the effect
of air velocity and humidity on the product yield during
thermal processing. Among the air velocities of 7.1, 9.9,
and 12.7 m3/min, no significant differences (by ANOVA
at P = 0.05) were obtained on product yield for chicken
patties cooked at an air temperature of 149 C and a
relative humidity of 50 to 95% (data not shown). In order
to determine the effect of air velocity on product yield,
further study is needed to include a broader range (<
7.1 or > 9.9 m3/min) of air velocities.
Significant differences (P < 0.05) were obtained on
product yield between the patties cooked at a humidity
of 50 and 95%. During thermal processing, increasing
air humidity increased the product yield (Figure 5). The
effect of air humidity on the product yield of chicken
patties was related to the product internal temperatures.
At an internal temperature of 55 C, increasing the relative humidity from 50 to 95% increased the product yield
less than 5%. However, at an internal temperature of
80 C, increasing the relative humidity from 50 to 95%
increased the product yield more than 10%.
The product yield is related to the moisture loss during
cooking. Figure 5 also shows that the product yield is a
function of product internal temperature. Increasing the
internal temperature decreased the product yield, which
is consistent with the moisture loss in the product. A
reasonable correlation was obtained between the product yield and the internal temperature. At a relative
humidity of 50%, the correlation coefficient was 0.838,
and the constants a, b, and c were −0.0138, 1.308, and
59.6, respectively. At a relative humidity of 95%, the
correlation coefficient was 0.784, and the constants a, b,
and c were −0.0113, 1.203, and 59.9, respectively. Because
the product yield was obtained by evaluating the mass
changes before and after cooking, the relative low correlation coefficients were due to the dripping losses, meat
stuck on the belt, and free water condensed on the
meat surface.
Soluble Proteins
Standard least squares regression was used to relate
the soluble proteins to the oven air and patty tempera-
FIGURE 5. Product yield of chicken patties cooked at an oven air
temperature of 149 C and relative humidities of 50 and 95%, respectively.
At RH = 50%, product yield (%) = −0.0138 (patty temperature)2 + 1.308
(patty temperature) + 59.6, R2 = 0.838; and at RH = 95%, product yield
(%) = −0.0113 (patty temperature)2 + 1.203 (patty temperature) + 59.9,
R2 = 0.784.
tures. The parameter estimates are listed in Table 2. The
total soluble proteins in the cooked patties were related
to the oven air and product temperatures. Increasing
oven air temperatures and product temperatures significantly reduced the total soluble proteins in the
cooked patties. For the chicken patties cooked in an air
convection oven, the total soluble proteins decreased
with increasing product internal temperature from 50 to
80 C (Figure 6). At an oven air temperature of 163 C,
the total soluble proteins decreased from 27 mg/g (wet
basis) to 2 mg/g (wet basis) with increasing the product
temperature from 50 to 80 C. At an oven air temperature
of 218 C, the total soluble proteins decreased from 8
mg/g (wet basis) to less than 0.05 mg/g (wet basis) with
increasing the product temperature from 50 to 80 C.
A reduction in the soluble proteins was also observed
in a water bath cooking of the chicken patties (Murphy
FIGURE 6. Standard least squares regression model for soluble proteins in the chicken patties cooked at an air temperature of 163 to 218
C and an air velocity of 9.9 m3/min, without steam present. Soluble
proteins (mg/g) = 211.2967 + 0.1532 (oven air temperature) − 0.0023
(oven air temperature)2 − 5.9236 (patty temperature) + 0.02684 (patty
temperature)2 + 0.0097 (oven air temperature × patty temperature), R2
= 0.8948.
513
THERMAL PROCESSING OF CHICKEN PATTY IN A PILOT OVEN
1
TABLE 2. Parameter estimates for the standard least square regression model for soluble proteins
Parameter
Parameter estimates
Standard error
Intercept
Ta
211.2967
0.1532
33.9636
0.2971
−0.0023
−5.9236
Ta2
Tp
Tp2
Ta × Tp
0.02684
0.0097
t-value
df
P-value
6.38
0.52
1
1
<0.0001
0.6071
0.0007
0.4288
−3.07
−13.81
1
1
0.0028
<0.0001
0.0027
0.0012
9.74
7.85
1
1
<0.0001
<0.0001
Ta = oven air temperature; Tp = patty center temperature. Replicates = 3. R2 = 0.8948.
1
and Marks, 2000). In a study of chicken breast meat by
Murphy et al. (1998), several endothermic transitions
were observed in a temperature range of 55 to 80 C.
Murphy et al. (1998) found that increasing cooking temperature resulted in fragmentation of muscle proteins
with high molecular weight (> 40 kDa). Similarly, disintegration, shortening, and unfolding of protein polypeptide chains have also been observed in heated beef samples (Dube et al., 1972; Hearne et al., 1978) and pork
samples (Zayas and Naebanij, 1986).
In this study, at the same oven air temperature, the
total soluble proteins decreased over 10-fold with increasing product internal temperature from 50 to 80 C.
The effect of oven air temperature on the soluble proteins
in the chicken patties was also investigated during air
convection cooking. The total soluble proteins decreased
with increasing oven air temperature from 163 to 218 C.
At a final product temperature of 50 C, the total soluble
proteins decreased approximately 70% with increasing
the oven air temperature from 163 to 218 C. At a final
product temperature of 80 C, the total soluble proteins
decreased nearly 100% with increasing the oven air temperature from 163 to 218 C.
While increasing the oven air temperature of a thermal
process, the surface temperature of a patty increased,
and the temperature gradient between the patty surface
and the patty center increased. Therefore, a higher average temperature was achieved for the patty cooked at a
higher oven air temperature. Because the whole patty
was ground and used in the evaluation of the soluble
proteins, the data obtained in this study represented
an average change in soluble proteins throughout the
cooked patty. A higher average patty temperature correlated to a lower amount of total soluble proteins. Therefore, increasing oven air temperature decreased the total
soluble proteins in a cooked patty.
This result agrees with the previous published information (Veeramuthu et al., 1998; Wang et al., 1996). Bouton and Harris (1972) indicated that heat-induced
changes in protein solubility related to changes in the
water-holding capacity of meat. Because protein
changed with heating, water content within the meat
myofibrils in the narrow channels between the filaments
changed as meat shrank within the tissue matrix (Bertola
et al., 1994), resulting in moisture loss with heating and,
consequently, reduction of product yield. The results
from this study indicated that the soluble proteins might
be used as an indicator for the degree of cooking.
In summary, the information concerning the heat flux
and effective heat transfer coefficients was useful for
modeling a commercial air convection cooking system.
Increasing the humidity increased the heat transfer coefficient and consequently reduced cooking time. The
moisture change, product yield, and soluble proteins
in a cooked chicken patty were influenced by product
temperature and cooking conditions. Reducing internal
temperature and increasing humidity increased the
product yield. This result can be used for commercial
poultry processing in correlating the product yield with
product temperature and processing conditions.
ACKNOWLEDGMENTS
This research was partially supported by the USDA
Food Safety Consortium research program and the research grant from U.S. Poultry and Egg Association. The
chicken meat was graciously provided by Tyson Foods,
Inc., Springdale, AR.
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