Part 3 Process contr
Part 3 Process control
13 Thermophysical properties of meat In chilling, freezing, thawing and tempering processes heat has either to be introduced or to be extracted from the meat to change its temperature The rate at which heat can be removed or introduced into the surface of meat is essentially a function of the process being used, for example air blast, plate, immersion, and so on. However, the rate at which heat can flow from within the meat to its surface is a function of the thermophysical prop- erties of the meat. If we continue to refrigerate meat in the form of car casses, quarters or primals, heat flow within, rather than from, the meat will always limit our ability to achieve rapid uniform rates of temperature We are interested in the thermal conductivity, which governs heat flow, and the specific heat, which is a measure of the amount of heat to be removed. Since the specific heat of meat is not constant with temperature it is often better to use the difference in enthalpy between the tempera tures of interest to provide a value for the energy change required Meat is not a homogeneous product and in a carcass the three main com- ponents- fat, lean muscle and bone- have very different properties. In frozen meat the ice content dominates the thermal properties. The basic structure of this chapter is based on the publications of morley (1972a, 1974). Comprehensive reviews of the thermal properties of food can be found in Morley(1972b), Polley et al.(1980), Miles et aL. (1983)and Rahman(1995). Few publications provide data on enthalpy, heat capacity and thermal conductivity of meat over the total temperature range -40 to +30 C that can be encountered in the refrigeration of meat. Two par- ticular publications that do provide such data are, Tocci et al.(1997)on boneless mutton and Lind (1990)on minced lean meat
13 Thermophysical properties of meat In chilling, freezing, thawing and tempering processes heat has either to be introduced or to be extracted from the meat to change its temperature. The rate at which heat can be removed or introduced into the surface of meat is essentially a function of the process being used, for example air blast, plate, immersion, and so on. However, the rate at which heat can flow from within the meat to its surface is a function of the thermophysical properties of the meat. If we continue to refrigerate meat in the form of carcasses, quarters or primals, heat flow within, rather than from, the meat will always limit our ability to achieve rapid uniform rates of temperature change. We are interested in the thermal conductivity, which governs heat flow, and the specific heat, which is a measure of the amount of heat to be removed. Since the specific heat of meat is not constant with temperature it is often better to use the difference in enthalpy between the temperatures of interest to provide a value for the energy change required. Meat is not a homogeneous product and in a carcass the three main components – fat, lean muscle and bone – have very different properties. In frozen meat the ice content dominates the thermal properties. The basic structure of this chapter is based on the publications of Morley (1972a, 1974). Comprehensive reviews of the thermal properties of food can be found in Morley (1972b), Polley et al. (1980), Miles et al. (1983) and Rahman (1995). Few publications provide data on enthalpy, heat capacity and thermal conductivity of meat over the total temperature range -40 to +30 °C that can be encountered in the refrigeration of meat.Two particular publications that do provide such data are, Tocci et al. (1997) on boneless mutton and Lind (1990) on minced lean meat
274 Meat refrigeration Table 13.1 Mean thermal conductivities in chilling Mean thermal ariation with type onductivity(wm-°C) ean m 0.49 (also kidney and liver) +0.02 ere Bone +0.02 compact bone spongy bone marrow Source: Morley. 1972a. 13.1 Chilling 13.1.1 Thermal conductivity Table 13. 1 shows the mean thermal conductivities during chilling of lean meats, fats and bones, together with the total variation amongst the differ ent samples considered. Thermal conductivity is given in watts per metre m It can be seen that the thermal conductivity of lean meat is roughly two and a half times that of fat. Rendering fat reduces its thermal conductivity owing to the ensuing loss of water, which has a relatively high thermal conductivity of 0.60Wm-oC-. The thermal conductivity of bone varies throughout its structure. Hard, outer compact bone has a similar thermal conductivity to that of lean meat, whereas inner spongy bone and marrow, having high fat contents, are similar in thermal conductivity to fat Beef liver has a similar thermal conductivity to lean meat, 0.49WmC, over the chilling temperature range from 30 to 0C(Barrera and Zaritzky, 1983) Little data are available on the thermal conductivity of meat in the cooking temperature range For predictive purposes Baghe-Khandan et al (1982) developed models based on the initial water(w) and fat (.)con tents at 30C to predict thermal conductivities at temperatures(T) up to 90C and heating rates of <0.5Cmin For whole beef: K=10-(732-4.326-3.56w+0.6367)[13.1 For minced beef.:K=10-3(400-4.496+0.147+1.747)[13.2 13.1.2 Specifie The specific heats of different types of meat are given in Table 13. 2. The pecific heats of fats are given in Table 13.3, and Table 13. 4 shows the vari- ability in specific heats between different bones
13.1 Chilling 13.1.1 Thermal conductivity Table 13.1 shows the mean thermal conductivities during chilling of lean meats, fats and bones, together with the total variation amongst the different samples considered. Thermal conductivity is given in watts per metre per °C (Wm-1 °C-1 ). It can be seen that the thermal conductivity of lean meat is roughly two and a half times that of fat. Rendering fat reduces its thermal conductivity owing to the ensuing loss of water, which has a relatively high thermal conductivity of 0.60 W m-1 °C-1 . The thermal conductivity of bone varies throughout its structure. Hard, outer compact bone has a similar thermal conductivity to that of lean meat, whereas inner spongy bone and marrow, having high fat contents, are similar in thermal conductivity to fat. Beef liver has a similar thermal conductivity to lean meat, 0.49 W m-1 °C-1 , over the chilling temperature range from 30 to 0 °C (Barrera and Zaritzky, 1983). Little data are available on the thermal conductivity of meat in the cooking temperature range. For predictive purposes Baghe-Khandan et al. (1982) developed models based on the initial water (wo) and fat (fo) contents at 30 °C to predict thermal conductivities at temperatures (T) up to 90 °C and heating rates of <0.5 °Cmin-1 . [13.1] [13.2] 13.1.2 Specific heat The specific heats of different types of meat are given in Table 13.2. The specific heats of fats are given in Table 13.3, and Table 13.4 shows the variability in specific heats between different bones. For minced beef: K f wT = -+ + ( ) o o - 10 400 4 49 0 147 1 74 3 .. . For whole beef: K fw T = -- + ( ) o o - 10 732 4 32 3 56 0 636 3 .. . 274 Meat refrigeration Table 13.1 Mean thermal conductivities in chilling Mean thermal Variation with type conductivity (W m-1 °C-1 ) Lean meat 0.49 +0.05 (also kidney and liver) Fats +0.02 Natural 0.21 Rendered 0.15 Bone +0.02 compact bone 0.56 spongy bone 0.26 marrow 0.22 Source: Morley, 1972a
Thermophysical properties of meat 275 Table 13.2 Specific heat of meat Temperature eef, lean(74.5% water) 0-10 Beef, lean(0% water) 0-10 1.3-14 Beef (74.5-78.5%water) 0-30 Beef, lean(72% water) 0-100 3.43 eef, fatty (51% wate 0-100 0-100 Veal(77.5% water, 4.4% fat) 3.683.60 Veal(63% water Pork, lean(73.3% water) 0-18 Pork, lean(57% water) 0-100 Pork, fatty(39% water) 0-100 2.60 Pork(76.8% water) 0-30 Ham (52% water) 4.5-24 3.8-3.5 Bacon(50% water) 0-100 Bacon, back(69% water amb, loin(64.9% water. 11.7% fa amb, loin(52.5% water, 28.4% fat) amb, loin(44.4% water, 39.4%fat) 3.10-3.52 amb, loin(52.3% water, 30.4% fat) 3.14 amb, forequarter (54.3% water, 25. 1% fat) amb, leg(57.8% water, 20.4% fat 3.18 Lamb, rack(50.5% water, 29.2% fat) amb, flap(49.9% water, 30.2% fat) 2.89 Mutton(70% water) 3.39 Chicken, lean(73% water) 0-100 3.39 Source: Morley. 1972b Table 13.3 Specific heat of fats Temperature range Specific heat Beef (7.7% water Beef, kidney(rendered) 5-25 4.06-3.89 Beef, loin(rendered 5-25 49-3.60 Beef, hind shin(rendered) 4.5-25 5.53-3.35 (1%water 4.694.31 hard fat(rendered, 0.2% water) 5-25 5.783.73 soft fat(rendered, 3.0% water) 3.944.40 Pork, American lard(0.1% water) 0-21 4.80-3.34 Pork, lard(water free) 2-60 5.53-2.0 acon back(8.6% wate 0-18.5 Bacon, back (7.3% water) 0-17 Chicken(11.4% water) 0-15 4.44 Source: Morley, 1972a
Thermophysical properties of meat 275 Table 13.2 Specific heat of meat Type Temperature Specific heat range (°C) (kJ kg-1 °C-1 ) Beef, lean (74.5% water) 0–10 3.6 Beef, lean (0% water) 0–10 1.3–1.4 Beef (74.5–78.5% water) 0–30 3.81 Beef, lean (72% water) 0–100 3.43 Beef, fatty (51% water) 0–100 2.89 Beef, ground 0–100 3.52 Veal (77.5% water, 4.4% fat) 0–32 3.68–3.60 Veal (63% water) 0–100 3.22 Pork, lean (73.3% water) 0–18 3.52 Pork, lean (57% water) 0–100 3.06 Pork, fatty (39% water) 0–100 2.60 Pork (76.8% water) 0–30 3.81 Ham (52% water) 4.5–24 3.8–3.5 Bacon (50% water) 0–100 2.01 Bacon, back (69% water) 0–18 3.39 Lamb, loin (64.9% water, 11.7% fat) 0–32 3.39 Lamb, loin (52.5% water, 28.4% fat) 0–32 2.93 Lamb, loin (44.4% water, 39.4% fat) 0–32 3.10–3.52 Lamb, loin (52.3% water, 30.4% fat) 0–32 3.14 Lamb, forequarter (54.3% water, 25.1% fat) 0–32 3.06 Lamb, leg (57.8% water, 20.4% fat) 0–32 3.18 Lamb, rack (50.5% water, 29.2% fat) 0–32 3.01 Lamb, flap (49.9% water, 30.2% fat) 0–32 2.89 Mutton (70% water) 0–100 3.39 Chicken, lean (73% water) 0–100 3.39 Source: Morley, 1972b. Table 13.3 Specific heat of fats Type Temperature range Specific heat (°C) (kJ kg-1 °C-1 ) Beef (7.7% water) 0–17 3.59 Beef, kidney (rendered) 5–25 4.06–3.89 Beef, loin (rendered) 5–25 7.49–3.60 Beef, hind shin (rendered) 4.5–25 5.53–3.35 Pork (3.1% water) 0–30 4.69–4.31 Pork, hard fat (rendered, 0.2% water) 5–25 5.78–3.73 Pork, soft fat (rendered, 3.0% water) 4–26 3.94–4.40 Pork, American lard (0.1% water) 0–21 4.80–3.34 Pork, lard (water free) 2–60 5.53–2.09 Bacon, back (8.6% water) 0–18.5 3.38 Bacon, back (7.3% water) 0–17 3.95 Chicken (11.4% water) 0–15 4.44 Source: Morley, 1972a
276 Meat refrigeration Table 13.4 Specific heat of bones Temperature range(°C) kJkg°) Beef (32% water) Pork(34% water Pork(35.4%water 2.39 Pork(bone from che 5-1 Pork(bone from chops) 5-38.5 Pork(rib 31. 5% water) Pork(knuckle joint) Chicken(35.6% water) Source: Morley. 1972b It can be seen that there is quite a small variation in the specific heat of different types of lean meat, whereas there is a relatively large variation in the specific heats of different fats. The specific heat of fat also varies greatly with temperature. This is due to latent heat associated with phase changes. The temperatures at which these occur depend on the type of fat. Studies by Morley and Fursey(1988) have shown that the values of specific heat and enthalpy change in fats measured during cooling differ from those measured during subsequent heating. This suggested that further fat solid ification occurred during storage. Using thermal data obtained in inappro- priate conditions could lead to errors in prediction of temperature changes The variability in the specific heat of fats with temperature should result in corresponding, though smaller, variations in the specific heats of cuts and ses,although no detailed investigations have been undertaken to show this. The effect of carcass composition variations on the mean specific heat in chilling can be estimated. The result is a total variation of about +0.o5 from the specific heat of an average beef, pork or lamb carcass. There appears to be little difference between the specific heats of typical beef, pork and lamb carcasses. Many specific heat tables for foods (e.g. ASHRAE Guide and Data Books)are based on Siebel's formula of 1892, i.e. calculated from the water content,assuming the solid content has a specific heat of 0. 2 btu/bF. This can obviously result in considerable error, as for example in estimating the mean specific heat in chilling a typical beef, pork or lamb carcass Siebels formula gives a value that is about 35% too low 13.1.3 Enthalpies Published enthalpy values for meat are shown in Table 13.5. Further data for lean pork, pork sausage meat, beef sausage meat, beef mince, beef fat and pork kidney fat over the temperature range -40 to +40C can be found in Lindsay and Lovatt(1994)
It can be seen that there is quite a small variation in the specific heat of different types of lean meat, whereas there is a relatively large variation in the specific heats of different fats. The specific heat of fat also varies greatly with temperature. This is due to latent heat associated with phase changes. The temperatures at which these occur depend on the type of fat. Studies by Morley and Fursey (1988) have shown that the values of specific heat and enthalpy change in fats measured during cooling differ from those measured during subsequent heating. This suggested that further fat solidification occurred during storage. Using thermal data obtained in inappropriate conditions could lead to errors in prediction of temperature changes. The variability in the specific heat of fats with temperature should result in corresponding, though smaller, variations in the specific heats of cuts and carcasses, although no detailed investigations have been undertaken to show this. The effect of carcass composition variations on the mean specific heat in chilling can be estimated. The result is a total variation of about ±0.05 from the specific heat of an average beef, pork or lamb carcass. There appears to be little difference between the specific heats of typical beef, pork and lamb carcasses. Many specific heat tables for foods (e.g. ASHRAE Guide and Data Books) are based on Siebel’s formula of 1892, i.e. calculated from the water content, assuming the solid content has a specific heat of 0.2 btu/lb °F. This can obviously result in considerable error, as for example in estimating the mean specific heat in chilling a typical beef, pork or lamb carcass. Siebel’s formula gives a value that is about 35% too low. 13.1.3 Enthalpies Published enthalpy values for meat are shown in Table 13.5. Further data for lean pork, pork sausage meat, beef sausage meat, beef mince, beef fat and pork kidney fat over the temperature range -40 to +40 °C can be found in Lindsay and Lovatt (1994). 276 Meat refrigeration Table 13.4 Specific heat of bones Type Temperature Specific heat range (°C) (kJ kg-1 °C-1 ) Beef (32% water) 0–18 2.46 Pork (34% water) 0–20 2.85 Pork (35.4% water) 0–19 2.39 Pork (bone from chops) 5–15 2.40 Pork (bone from chops) 5–38.5 2.75 Pork (rib 31.5% water) 5–15 2.21 Pork (knuckle joint) 5–15 2.23 Chicken (35.6% water) 0–21 2.92 Source: Morley, 1972b