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FOCUS : JSMTV
Modeling heat transfer in a jet
of hot air to decontaminate meat products
A. Kondjoyan1 and M. Havet2
1 – INTRODUCTION
Thermal decontamination of the surface of meat products is an old idea which finds a renew of interest due to the demand of consumers for safer pro- ducts and because of their repulsion for the use of chemicals and irradiation.
Existing scientific papers and prototypes develop in the USA prove that heat treatments are efficient to decrease significantly the microbial contamination at the surface of meat products [1]. But these treatments are difficult to apply and the exact amount of bacteria which are killed remains debated [2]. This amount depends both on biological factors (type and physiological state of the bacteria, substrate…) and on the evolution of the physical conditions (temperature, water activity) at the surface of the product during heat treatments. An European pro- ject (“BUGDEATH” project, 5th European Framework, leads by FRPERC – Uni- versity of Bristol) aims at producing a model which predicts the effect of thermal treatments on the surface decontamination of food products. This model shall be usable by industrials to decide which time-temperature treatment they shall carry out for their application. Thus this model shall be simple, it shall run fast and require: (1) to establish the death kinetics of bacteria under well defined conditions (temperature, humidity type of bacteria and product) and (2) to be able to predict accurately the surface temperature kinetics during heat treat- ments. INRA-SRV and ENITIAA (Nantes) are in charge of the thermal modeling aspects. Heat treatment is based on the use of jets of hot air or steam and fol- lowed by an air chilling stage. Samples are slices of food products placed parallel or cross to the jet. The development of the heat transfer model and its sensitivity to different parameters have already been described in another paper [3]. Present work tests model efficiency to describe surface temperature kine- tics during the fast heating of poultry samples subjected to a jet of hot air.
1. INRA, Station de Recherches sur la Viande, 63122 St Genès Champanelle, France.
2. ENITIAA, rue de la Géraudière, BP 82225, 44322 Nantes cedex 3, France.
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2 – MODEL
The problem was taken as a one dimensional heat transfer, that is that the slice of product was considered as a flat plate of infinite length and width. This approach is justified by the physical phenomena and by the requirement to pro- duce a simple model usable by engineers and industrials. The evolution with time t of the temperature T in the thickness of the product can be simply described by the following differential equation:
Transfers on the bottom side of the product slice (placed on a support) were calculated by:
Tinf is the temperature of the product bottom surface, Tsupport the tempera- ture of the support and hinf a coefficient which describes the exchanges by con- duction between the support and the product.
The boundary condition on the product upside surface has to take into account the exchanges by convection, radiation and evaporation/condensa- tion. The walls surrounding the product are supposed to be black bodies which can be at a different temperature Trad than the air. During heating the surface temperature cannot be higher than Tmax which is either Tair or Trad depending on which of these two temperatures is the greatest. During a coupled heat-water exchange, the mass transfer coefficient can be calcula- ted from the heat transfer coefficient using Lewis relation. Thus the upside boundary condition becomes:
In meat chilling models Trad≈ Tair (this is usually not the case during decon- tamination), term (I) is h and terms (II) et (III) can be considered as two other transfer coefficients hrad et hevap which vary in time and which describe the sur- face transfers by radiation and condensation/evaporation respectively. In chil- ling models the sum of h, hrad , hevap is named classically: « effective transfer coefficient” [4].
The equations (1), (2) and (3) were discretised by a finite difference method using the second order numerical scheme of Crank-Nicolson which has the advantage of being numerically stable and accurate (implementation under matlab 5.2). The effective transfer coefficient value was recalculated at the end of each time step using the actualised values of the different variables.
( )
2 2
t x
t , x D T t
) t , x ( T
∂∂
∂ =
∂ (1)
( )
∂∂Tx inf=hinf(Tsupport-Tinf)λ (2)
( )
( )
( max sup)sup max
sup ws T 0.67 Td
eau atm p air sup max
sup4 rad4
sup max
air sup
sup T T
T T
P a HP ) Le M (
P M hC T T
T T T T
T hT x
T -
- + -
- + - -
= -
∂∂ εσ ∆
λ
(I) (II) (III)
(3)
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The accuracy of the numerical solution depends on the thickness of the nume- rical mesh and on the duration of time steps. A “self-fitting” time-step procedure was implemented in the model to speed-up the calculations while maintaining the accuracy of the results. Thanks to the simplicity of the model and to its “self-fit- ting” procedure, calculations are very fast. The temperature kinetic at the surface of a product during a decontamination treatment can be predicted in a few ten- seconds on a PC computer.
3 – EXPERIMENTAL SET AND PROTOCOL
Experiments were performed using little slices of Teflon and poultry meat which surface dimensions were 1.0 cm ✕ 1.0 cm and which thickness varies between 1.5 cm and 3.0 cm. These little samples were placed in a support made of Teflon and surrounded by little cubes also made of Teflon such as the overall pieces (samples and cubes) formed a block of 5 cm ✕ 5 cm ✕ 3 cm. This block was located at 15 cm from the outlet of a hot gun which produced a jet of hot air such that the 5 cm x 5 cm surface was perpendicular to the hot jet direc- tion. Some parts of the block were painted in black mat or covered with alumi- nium strips. Two thermocouples were located, one at the outlet of the hot gun and the other 1cm above the surface of the sample. Another thermocouple was located at about 1 mm under the surface of one of the Teflon sample. Thermo- couples were connected to a data logging system linked to a computer. An infrared camera and its acquiring system were used to follow the temperatures at the surface of samples. Before an experiment, the hot gun was warmed up during a few minutes until the characteristics of the jet flow were steady. Then the samples were subjected to the jet of air for 20 to 30 minutes. During this period thermocouples temperatures and infrared pictures were recorded every minute. At the end of the experiment the 200 infrared pictures were analysed and treated to obtain the temperature kinetics measured at different locations at the surface of the sample (use of matlab 5.2 « image processing toolbox »).
Calibration of the thermocouples and of the response of the infrared camera ensured that temperatures were measured with an accuracy of +/– 0.5˚C. Inside a jet flow, the distributions of the air velocity and air temperature are very hete- rogeneous. Thus spatial differences in jet air temperature and velocity were carefully characterized before and after each experiment.
4 – RESULTS
Surface temperature kinetics measured on samples of Teflon and of meat poultry (covered by skin) are given in figure 1. Results show how important is the effect of radiation coming from the hot gun walls on the heating of Teflon and poultry. When the sample emissivity is small as for aluminium the surface
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temperature of the sample at the end of the experiment is close to the air tem- perature. For higher emissivities the final temperature is in between the air temperature and the gun walls temperature. Emissity of white (porous) Teflon is 0.97 and thus close to that of black Teflon (0.99). The fact that the surface temperature is higher on white Teflon than on black Teflon seems paradoxal but is explained by the local difference in the air jet velocity and temperature (cf. caption fig. 2). The temperature kinetic measured on poultry skin is very similar to the kinetic obtained on Teflon. Thus the effect of water evaporation on the heat transfer at the surface of poultry seems very limited. The tempera- ture measured 1mm under the surface of white Teflon is very different from its surface temperature, thus: (i) infrared sensors are needed to measure surface temperature and (ii) thin numerical meshes are needed to calculate surface temperature kinetics.
Kinetics predicted by the model using the measured air temperature and velocity above each sample are given in figure 2. Transfer coefficient values introduced into the model come from measurements performed previously in a wind tunnel under the same flow conditions. Thermal properties (conductiv- ity and diffusivity) values are those given by literature for Teflon at 23˚C. The product emissivity is supposed to be 0.20 for aluminium and 0.99 in the other cases. Globally calculated results agree with experimental ones. However pre- dictions are very dependent on food product properties (λ, ε), which shall be measured accurately (and separately). View factors have also to be introduced into the calculations of radiation to be able to describe other practical situa- tions. Moreover poultry skin can be damaged, due to lipids fusion, as soon as the product surface temperature is greater than 65-70˚C. This aspect shall be taken into account in future studies.
Figure 1
Temperatures measured on samples of Teflon and poultry meat subjected to a jet of hot air which average velocity and temperature were 5 m/s and 66°C respectively.
The temperature of the hot gun walls was 175°C 40
50 60 70 80 90 100
0 500 1000 1500 2000 2500 T(°C)
Time(s) Tair
Alu
Black Teflon White Teflon
Poultry meat
1 mm below the surface of white Teflon
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Figure 2
Comparison between the experimental values (Fig.1) and those calculated with:
(1) U = 4.0 m/s, Tu =20%, Tair = 65°C, ε = 0.2; (2) U = 4.0 m/s, Tu =20%, Tair = 65°C, ε = 0.99; (3) U = 5.0 m/s, Tu =20%, Tair = 75°C, ε = 0.99;
(4) U = 1.0 m/s, Tu =20%, Tair = 75°C, ε = 0.99.
CONCLUSION
Model is efficient to describe experimental results obtained during the heating of poultry skin. The measuring of product thermophysical properties and additional sets of experiments are needed to complete the validation.
REFERENCES 40 50 60 70 80 90 100
0 500 1000 1500 2000 2500
Alu
Black Teflon White Teflon
Poultry
1 2
Time (s) 4
3 T surface (°C)
1. KOCHEVAR S. L., SOFOS J. N., BOLIN R.
R., REAGAN J. O., SMITH G. C., 1997.
Steam vacuuming as a pre-evisceration intervention to decontamination beef car- casses. Journal of Food Protection, 60(2), 107-113.
2. JAMES C., JAMES S., 1997. Meat Decontamination the state of the art. Bris- tol, UK: MAFF Advanced Fellowship in Food Process Engineering; 140 p.
3. KONDJOYAN A., HAVET M., JAMES S., 2002. Détermination des coefficients de
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