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 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 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. 13.1 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 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 (w o ) and fat (f o ) con- tents 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 vari- ability in specific heats between different bones. For minced beef: oo KfwT=-++() - 10 400 4 49 0 147 1 74 3 .. . For whole beef: oo =--+ - 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 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. 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 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. 13.2 Freezing, thawing and tempering 13.2.1 Ice content It is well known that, below its initial freezing point, meat becomes more frozen the lower the temperature. This is due mainly to the fact that freez- ing results in an increase in the concentration of the tissue fluids and con- sequently a lower temperature is required for further freezing to occur. About 10% of the water content does not appear to freeze even at absolute zero, and it is generally assumed to be too tightly bound to protein, while the remaining 90% of the water content is freezable.Although there is some disagreement between the various investigators about the amount of ice in lean meat at different temperatures, the work of Riedel (1957) is perhaps the most authentic. Figure 13.1 (after Riedel, 1957) shows the percentage of the freezable water that is frozen at different temperatures. Fikiin (1996) has reviewed Eastern European methods of predicting ice content. It can be seen that freezing commences at ca. -1.5 °C and although about half of the freezable water is frozen by -2 °C, freezing is not entirely com- plete even at -30°C. 13.2.2 Heat extraction Figure 13.2(a) (after Riedel, 1957) shows the heat extraction required in cooling lean meat from 0 °C to temperatures down to -40°C. On the commencement of freezing the heat extraction increases steeply owing to the high latent heat of freezing. Thereafter the heat extraction increases less and less steeply as the formation of ice diminishes, as in Fig. 13.1. For example, in cooling from -1 to -5 °C the required heat extraction is 193 - 5 = 188kJkg -1 , i.e. an average of 47 kJ kg -1 °C -1 , whereas between -30 and -40 °C, where freezing is virtually complete, only 1.9 kJkg -1 °C -1 is required. If such calculations were made based on the water content, as is done in certain refrigeration books, erroneous results can arise, caused mainly by the fact that not all of the water content becomes frozen as is Thermophysical properties of meat 277 Table 13.5 Published enthalpy values of meat Temperature Enthalpy (kJ kg -1 ) Temperature Enthalpy (kJ kg -1 ) (°C) Pork Beef Lamb (°C) Pork Beef Lamb 40 0 0 0 20 -66.0 -61.6 -62.7 35 -12.4 -14.2 -16.5 15 -82.1 -76.6 -78.2 30 -24.8 -29.4 -32.6 10 -95.7 -91.3 -94.5 25 -41.1 -46.0 -47.7 5 -109.0 -106.3 -110.5 0 -122.8 -122.7 -127.3 Source: Lindsay and Lovatt, 1994. assumed. If, for example, the heat extraction required in cooling lean meat (74% water) between -1 and -5 °C was calculated in such a manner, a value of 254kJkg -1 would be obtained, compared with 188kJ kg -1 from Fig. 13.2(a). Figure 13.2(b), (c), (d) and (e) shows the heat extraction required in freezing lamb loin cuts and carcasses (after Fleming, 1969). The mean specific heat of fat in the main meat freezing region (-1 to -20 °C, for instance), though very variable, is roughly 3kJ kg -1 °C -1 , com- pared with ca. 7kJkg -1 °C -1 for bone and 13 kJ kg -1 °C -1 for lean. Thus, the heat extraction required in freezing different meats depends mainly on the quantity of lean. 13.2.3 Thermal conductivity The thermal conductivity of lean meat varies with temperature as shown in Fig. 13.3 (after Lentz, 1961). The thermal conductivity of ice is some four times that of water and thus the conductivity of lean meat increases with increasing ice content. The thermal conductivity of lean meat also depends on the continuity of the ice to the flow of heat – the more continuous the ice structure, the greater the conductivity. Thermal conductivity in a direction parallel to the muscle fibres is some 8–30% greater than perpendicular to the muscle fibres (Hill et al., 1967; Lentz, 1961). This is due to the fact that ice crystals are parallel to the muscle fibres and thus present a more continuous path for heat flow in this direction. Ice structure also varies with freezing conditions. Slow freezing produces large extracellular columns of ice of greater conti- nuity than the small intracellular ice crystals produced by fast freezing. The mean thermal conductivity of fat is ca. 0.25Wm -1 °C -1 , which is only about one sixth that of frozen lean. The thermal conductivity of bone varies 278 Meat refrigeration 100 90 80 70 60 50 40 30 20 10 0– 10– 20– 30 Temperature (°C) F roz en (%) Fig. 13.1 Percentage of the freezable water that is frozen (source: Morley, 1974). throughout its structure, being similar to that of fat in its inner region (spongy bone): 0.33 W m -1 °C -1 (Morley, 1966), whereas it is about double this in its outer region (compact bone): 0.64 W m -1 °C -1 at -30 °C (Morley, 1974). Beef liver has a similar thermal conductivity to lean meat 0.9 W m -1 °C -1 Thermophysical properties of meat 279 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 (a) (b) (c) (d) (e) – 40 – 30 – 20 – 10 Temperature (°C) Heat extraction (kJ kg –1 ) Fig. 13.2 Heat extraction required in cooling meat below 0 °C (a) Lean meat (74% water, 4% fat), (b) lean loin cut (64.9% water, 11.7% fat, bone in), (c) moderately lean carcass (60% water, 22% fat), (d) moderately fat loin cut (52.5% water, 28.4% fat, bone in), (e) fat loin cut (44.4% water, 39.4% fat,bone in) (source: Morley, 1974). 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 – 10– 20– 30 Temperature (°C) Ther mal conductivity (W m – 1 ° C – 1 ) Fig. 13.3 Variation of the thermal conductivity of lean meat with temperature (source: Morley, 1974). at -2 °C and rising to ca. 1.3Wm -1 °C -1 at -30°C (Barrera and Zaritzky, 1983). 13.2.4 Density A knowledge of the density of meat components is important in heat con- duction analysis since density (r) appears in the general transient heat con- duction equation. Table 13.6 shows the mean specific gravities during chilling of lean meats, fats and bones. It can be seen that the density of bone is much greater than that of lean and fat and that there is a fairly large variation in density between differ- ent bones. 13.3 Mathematical models Computer programs are now available such as COSTTHERM and FoodProp that will accurately predict the thermal properties of food from their compositional properties. In general a knowledge of the initial freez- ing point of the product is required to obtain accurate data in the freezing range. Programs are under development that will automatically predict the initial freezing point. 13.4 Conclusions 1 The thermal properties of meat are both a function of its composition and its temperature. 2 At temperatures above -1.5°C: ? the thermal conductivity of lean meat is roughly two and a half times that of fat; ? the specific heat of fat is also very variable with temperature. 3 At temperatures below -1.5°C: 280 Meat refrigeration Table 13.6 Specific gravity Mean specific gravity Lean meats (also liver) 1.07 Fats 0.92 Bones beef, fresh: humerus, femur 1.33 tibia 1.41 radius 1.44 cannon bones 1.56 Source: Morley, 1972a. ? the thermal properties are a function of the ice content; ? the thermal conductivity of lean meat is approximately three times that of the unfrozen material. 4 Because of the latent heat of freezing the enthalpy change between -1.5 and -5 °C is very high for lean meat. 13.5 References baghe-khandan m s, okos m r and sweat v e (1982), The thermal conductivity of beef as affected by temperature and composition, Trans Am Soc Agric Eng, 1118–1122. barrera m and zaritzky n e (1983), Thermal conductivity of frozen beef liver, J Food Sci, 48 1779–1782. fikiin k a (1996), Ice content prediction methods during food freezing: A survey of the Eastern European literature. 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