Conservation of water in catering by innovative cold water thawing method

Transcription

1 Conservation of water in catering by innovative cold water thawing method Michael K.H. Leung Department of Mechanical Engineering The University of Hong Kong Abstract Cold water thawing method is commonly used in commercial kitchens. The running tap water provides a suitable medium to transfer heat from water to frozen food at a low temperature to achieve effective and rapid defrosting effect. However, the continuously running water causes deficiency in both environmental and economic aspects. In this study, engineering techniques are systematically applied to enhance the cold water method to operate in an environmental manner. Theoretical analysis, computational fluid dynamics (CFD) modelling and experimental works were conducted to investigate the fluid dynamics and heat transfer behaviours. The findings were employed to redesign the hardware device and to develop guidelines for innovative environmental cold water thawing practice. Besides the environmental benefits gained, this study clearly demonstrated the importance of engineering and its extensive contribution to other professions. Key Words Cold water thawing; Water conservation, Fluid dynamics, Phase-change heat transfer Introduction For highest-quality cuisine, fresh meat, vegetable and fruit are usually refrigerated and stored in the freezer of a restaurant to preserve the freshness until cooking is ready. Frozen food is normally stored at -18 o C for preservation. Thawing is necessary before any subsequent food processing or cooking. There are different frozen food thawing methods available, including cold water thawing (Chourot et al. 1997), refrigerator thawing (Anderson et al. 004), microwave thawing (Zeng and Faghri 1994, Basak and Ayappa 00) and high-pressure thawing (Denys et al. 000). Among them, cold water thawing is the most favourable defrosting method used by the chefs, especially when the frozen food is required to thaw within a short period of time. Although cold water thawing has long been used commonly in food processing, catering, as well as household cooking, most users do not fully understand the thawing mechanisms and characteristics, leading to poor time control, excessive consumption of water and unnecessary waste water discharge. The present cold water thawing practice is to dump the frozen food into a pail of water continuously overflowed by running tap water. The thawing process is neither efficient nor environmental-friendly. The overflowed water discharged into the wastewater grease trap causes an increase in the grease trap loading. The stratification effect inside the pail also implies poor defrosting effect and inefficient use of our precious water. 1

2 In this study, the environmental problem in catering was tackled by an engineering approach. The fluid dynamics and heat transfer involved in cold water thawing were analyzed by theoretical analysis, computational fluid dynamics (CFD) modelling and experimental testing. The findings were successfully employed to develop appropriate hardware and guidelines for efficient use of cold water thawing method to achieve both economic and environmental benefits. Physical Phenomena In a cold water thawing process for frozen meat, the two major modes of heat transfer are convection and conduction, as illustrated in Fig. 1. The surface of meat gains heat from the surrounding water by convection and the heat is conducted into the body. The meat temperature profiles during the thawing process are described as follows (see Fig. 1(b)): (1) Meat surface temperature quickly increases and approaches the water temperature. () Meat centre temperature increases to about the melting point (0 o C) and stays for a certain period of time. (3) Once the centre temperature exceeds the melting point, the meat is completely thawed. The centre temperature then increases abruptly. (4) Meat eventually reaches the equilibrium at the surrounding water temperature. When the water flow rate is increased to a certain extent, further increase will not yield a noticeable reduction in the thawing time. It is because conduction, which is not directly affected by the water flow, becomes the mode that really restricts the overall heat transfer. Exceeding water flow rate does not speed up the thawing process. Conversely, the consequences are inefficient use of water and high water and sewage treatment charges. (a) Moving phase-change interface (b) Temperature profiles Fig. 1 Transient phase-change heat transfer in thawing of frozen food

3 Theoretical Analysis A theoretical solution was obtained for the transient phase-change heat transfer problem in thawing of frozen food. In the physical model, a sphere originally at a uniform temperature below the phase-change temperature is suddenly immersed in a fluid at a temperature above the phase-change temperature. As the body temperature increases, the phase-change interface will be first formed on the surface. Subsequently, the interface will absorb the latent heat and move towards the centre until the whole body undergoes complete phase change. In this moving phase-change interface heat transfer problem, it is assumed that the thermal properties of the material are constants independent of temperature. The body temperature T only varies with the radial coordinate r and time t. The temperature T(r,t) presented in the following derivation is measured in excess to the temperature of the surrounding fluid in order to satisfy the homogeneous boundary condition for a convective heat transfer surface. The heat-conduction equation in the spherical coordinate system can be written as t) ( t) 1 T ( r t) 1 T ρl ri, r + δ ( r ri () t ) =, (1) r r r k t α t where ρ, k, α, and L are the density, thermal conductivity, thermal diffusivity, and latent heat, respectively. The second term on the left-hand-side of equation (1) represents the heat generation (or absorption) by the moving phase-change interface. The movement of the interface is specified by the radial coordinate r I, which is a function of time t. The interface condition is that the material changes phase at the phase-change temperature T m, ( r ( t), t) = Tm T I. () In the initial condition, the body temperature is uniform at T f which is below the phase-change temperature, t) = Tf T, at t = 0. (3) The heat transfer on the surface by means of convection is applied for the boundary condition, t) T k + ht = r t) 0, at r = R o, (4) where h is the convective heat transfer coefficient ad R o is the radius of the body. One advantage of adopting the above convective heat transfer boundary condition is that the theoretical solution to be obtained is readily used to determine an approximate solution for a constant-temperature boundary condition by setting h to a high value. 3

4 By the use of Green's function (Ozisik 1993), the theoretical solution to the above nonhomogeneous, time-dependent problem can be generally written as a sum of a homogeneous part T h (r,t) and a nonhomogeneous part T n (r,t), T t) = T t) + T t) = h R o r' = 0 r' G n t t r',τ) T dr' + dτ r' Gt r',τ) ρlδ( r' r ( τ) ) τ = 0 f α k τ = 0 R o r' = 0 I dri dτ () τ d r'. (5) The homogeneous part of the solution can be solved with the heat generation term omitted in the governing equation (1) yielding, R o T f αβ t β + K m m Th t) = e sin( β ) ( ) mr ( + ) r'sin βmr' dr', (6) r m= 1 Ro βm K + K r' = 0 where the coefficient K is expressed by h 1 K = (7) k R o and the eigenvalues β m are the positive roots of the eigenfunction, ( R ) + R K 0 β mr o cot β m o o =. (8) By solving the integral in equation (6) analytically, the homogeneous part of the solution can be further simplified to T h t) Tf = r m= 1 e α ( β R ) β R cos( β R ) β t β + K sin m m m o m o m o sin( β ) mr, (9) R o ( βm + K ) + K βm By comparing equations (5) and (9), the desired Green's function can be identified as αβ ( ) K m t τ β + G( r, t r', τ ) = e sin( βmr) sin( βmr' ) (10) rr' R + K m= 1 o m ( β + K ) m Therefore, the nonhomogeneous part of equation (5) becomes 4

5 t ( β ) mr αβ ( t τ ) e sin( β r ( τ )) r ( τ ) αρl β ( ) ( ) = = m + K sin dr m I Tn r, t = k m I I dτ. (11) m 1 R o β m + K + K r dτ τ 0 ( τ ) A one-step Newton-Raphson method was specially designed to solve for the position of the moving interface to satisfy the interface condition. CFD Modelling Regarding the effect of running water in cold water thawing, computational fluid dynamics (CFD) modelling was employed to analyze the water flow and convective heat transfer on the food surface. The main objectives of this study were to characterize the mechanisms of cold water thawing and, accordingly, to design an efficient thawing device. A steady-state CFD modelling was implemented to analyze the thermal stratification effect for preliminary design of cold water thawing device configuration. The commercial CFD software FLUENT 6.1 was used to facilitate the analysis. A k-ε turbulence model was selected as the rough surface of food behaved as a turbulence promoter. Both conventional design with water inlet on top (Fig. (a)) and modified design with water inlet on bottom (Fig. (b)) were studied. Further CFD analysis of the new design (Fig. 3) with small frozen food specimens was conducted. (a) Conventional practice (b) Modified design Fig. CFD models for study of thermal stratification effect 5

6 Fig. 3 Design of cold water thawing device Experimental Study In the experimental study, cold water thawing of frozen food was tested by the apparatus shown in Fig. 4. The specimen was prepared by a piece of pork cut to round shape sized 0.05 m in diameter. The estimated properties of the pork specimen are summarized in Table 1. Two thermocouples were inserted to measure the temperature at the center and 0.01 m away from the center. The specimen was stored in a freezer until it reached a uniform temperature of -18oC. The frozen specimen was then immersed in a stream of running water which was at a mean temperature of 0.4oC. The varying body temperature was monitored until the specimen was thawed. For the tests of multiple specimens (Fig. 4(b)), one thermocouple was installed in each selected specimen. (a) Test of single specimen (b) Test of multiple specimens Fig. 4 Experimental apparatus 6

7 Table 1. Properties of specimen and test conditions Pork specimen (lean) Density, ρ (kg m -3 ) 1,433 Thermal conductivity, k (W m -1 K -1 ) Heat capacity below melting point,,19 Cp T<0 oc (J kg -1 K -1 ) Heat capacity above melting point, 3,601 Cp T>0 oc (J kg -1 K -1 ) Latent heat, L (J kg -1 ) 41,904 Melting point ( o C) 0 Specimen diameter (mm) 50 Initial temperature ( o C) -18 Cold water thawing operations Volumetric flow rate (L min -1 ) 0.5 Apparent speed of water flow (mm s -1 ) 0.15 Convective heat transfer coefficient, h (W 819 m - K -1 ) Mean temperature of water inlet ( o C) 0.4 Note: Obtained by measurements of the pork specimen. Refer to ASHRAE (00) for material properties of pork. Results and Discussions The predicted results based on the theoretical solution (Eqs. (5), (9) and (11)) are plotted in Fig. 5, along with the experimental measurements and conventional finite difference numerical results. The reasonable agreement shows the good accuracy of the theoretical model. (a) r = 0 m (centre) (b) r = 0.01 m Fig. 5 Theoretical predictions and experimental measurements in tests of single specimen 7

8 Regarding the CFD analysis, as shown in Fig. 6, with the water inlet located on top, the large temperature variation with cold water staying on the bottom clearly revealed the thermal stratification developed in the flow field. As the lower part of the meat gained heat from the low-temperature water, the overall convective heat transfer was low. The magnitude of stratification became worst if the inlet flow rate decreased. Relocating the water inlet on the bottom could result in a better mixed condition. Hence, the convective heat transfer was enhanced. (a) Conventional practice (b) Modified design Fig. 6 Steady-state temperature fields resulting from water inlet on top and bottom The temperature measurements are plotted with the CFD numerical modelling results in Fig. 7. A reasonable agreement was obtained. The discrepancy was probably due to the fact that the input thermal properties used in the modelling slightly deviated from the actual values. The frozen pork did not actually melt at a single melting point of 0 o C, but rather over a temperature range from -3 to 0 o C. For this reason, the middle segments of the measured temperature profiles near the melting point exhibited a minor slope, rather than a flat line as generated by the numerical model. Nevertheless, the assumption of constant properties yielded reasonable numerical prediction. 8

9 Fig. 7 CFD modelling results and experimental measurements of cold water thawing of frozen pork Environmental Cold Water Thawing Guidelines The validated theoretical model and computational CFD model were further used as design tools to help optimize the hardware and operational practice of cold water thawing method. The detailed recommendations are documented in the booklet entitled Environmental-Friendly Cold Water Thawing Practice (Leung et al. 004). The simple apparatus shown in Fig. 8 and operating procedures are specially designed for effective cold water thawing used in a restaurant environment. The parts include 0-litre container (70 mm diameter and 350 mm height), fixed PVC pipe, flexible tube, and stainless steel rack positioned in the upper part of the container. The upward water flow with meat placed at a higher position can minimize the stratification problem and, thus, improve the convective heat transfer. The recommended method can save 90% water consumption, water and sewage treatment charges. Fig. 8 New design of cold water thawing apparatus 9

10 Environmental cold water thawing can be properly carried out by the following steps: 1. Fill up the container with tap water.. Take about kg of frozen meat from freezer at -18 o C; refer to Tables to 1 for approximate number of pieces of meat. 3. Spread the pieces evenly on the rack; make sure all pieces are fully immersed in the water. 4. Adjust running water at about 0.5 litre per minute (corresponding speed of water flow of 9 mm per minute inside the container). 5. Refer to Table for required thawing time. Similar data have been obtained for chicken and beef of various sizes. Table. Timetables for Recommended Cold Water Thawing Method Pork Relationship between body temperature and process time in thawing of frozen pork chops based on water supply at 4 o C Pork chops Approximate quantity: - 30 pieces - Total weight kg Centre Temperature ( o C) Mid-Layer Temperature ( o C) Time Taken (min) Time for complete thawing of frozen pork chops at different water supply temperature Water Supply Temperature ( o C) Corresponding Outdoor Air Temperature ( o C) Thawing Time (min) Conclusion In this study, the fluid flow and heat transfer involved in cold water thawing were analyzed by theoretical and numerical CFD models. The predicted results agreed reasonably well with the experimental measurements. Using the theoretical and numerical models as design tools, a simple cold water thawing device was designed to reduce the thermal stratification and, thus, improve the thawing efficiency. The recommended apparent speed of water flow was 0.15 mm s -1 resulting in nearly the shortest thawing time. Any higher flow rate 10

11 would only cause a marginal reduction in the thawing time because the conductive thermal resistance, rather than the convective thermal resistance, became the dominating factor in the overall heat transfer. One may implement the recommendations or refer to the numerical and experimental results presented in this paper to improve the performance of cold water thawing method. Acknowledgements The author would like to thank Environment and Conservation Fund and Woo Wheelock Green Fund for the financial sponsorship to support this research (EFC Project 0/00). References Anderson, B.A., Sun, S., Erdogdu, F., and Singh, R.P. (004). Thawing and freezing of selected meat products in household refrigerators. Int. J. Refrigeration, 7, pp Basak, T. and Ayappa, K.G.. (00). Role of length scales on microwave thawing dynamics in D cylinders. Int. J. Heat and Mass Transfer, 45, pp Chourot, J.-M., Boillereaux, L., Havet, M., and Bail, A.L. (1997). Numerical modeling of high pressure thawing: application to water thawing. J. Food Engineering, 34, pp Leung, M.K.H., Leung, D.Y.C., Lam, G.C.K., Ching, M.W.H., Environmental-Friendly Cold Water Thawing Practice, The University of Hong Kong, ISBN , 004. Ozisik, M.N., 1993, Heat Conduction, nd ed., Wiley, New York, pp. 14-3, Zeng, X. and Faghri, A., (1994). Experimental and numerical study of microwave thawing heat transfer for food materials. ASME J. Heat Transfer, 116, pp