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The solubility of carbon dioxide in meat
Meat Science 22 (1988) 65-71
The Solubility of Carbon Dioxide in Meat C. O. Gill Meat Industry Research Institute of New Zealand (Inc.), PO Box 617, Hamilton, New Zealand (Received 3 November 1987; revised version received 14 January 1987; accepted 17 January 1987)
ABSTRACT The solubility o f CO 2 in muscle tissue of pH 5"5 at O°C was approximately 960 ml at STP/kg of tissue. The solubility increased with increasing tissue pH by 360 ml/kg for each pH unit. The solubility decreased with increasing temperature by 19 ml/kg for each °C rise. Solubilities in beef pork and lamb muscle tissue were comparable. The solubility of CO 2 in fat tissues initially increased as the temperature was raised above O°C, but then declined at higher temperatures, with the temperature of peak solubility and the solubility curves being markedly different for fat tissue from the three species.
INTRODUCTION Carbon dioxide inhibits the growth of a wide range of microorganisms (Enfors & Molin, 1980). This inhibitory effect is exploited in modified atmosphere packaging to extend the storage life of chilled meats. In aerobic systems, such as are used for display packs, atmospheres containing 20 to 30% CO2 are used, as increasing the CO2 concentration beyond that level has little additional inhibitory effect on Pseudomonasdominated spoilage floras (Gill & Tan, 1980). In anaerobic systems, used to extend the storage life before meat is prepared for display, atmospheres of 100% CO2 may be used, as the inhibitory effects of CO2 on both the lactobacilli and enterobacteria components of anoxic floras increase with increasing CO2 concentration (Gill & Penney, 1986). 65 Meat Science0309-1740/88/$03-50 © 1988ElsevierAppliedSciencePublishersLtd, England. Printed in Great Britain
66
C. O. Gill
Carbon dioxide is highly soluble in both water and oils. Therefore, when CO 2 is applied to meat in a rigid pack, the gas will be absorbed by the muscle and fat tissues until equilibrium is attained. At equilibrium the partial pressure of CO2 will be less than that of the original gas mixture, and the total gas pressure also will be less than that at which the gas mixture was initially applied. Similar considerations will apply with a flexible packaging system. When gas mixtures are used, the less soluble nitrogen and unrespired components of the gas mixture will maintain a volume of gas at atmospheric pressure around the meat that will be less than the volume initially added. When CO2 alone is added, the pack will collapse around the meat as gas is absorbed, and the partial pressure of CO2 within the pack will be less than atmospheric unless CO2 is added in excess of the quantity required to saturate the meat at atmospheric pressure. Clearly, if reproducible effects are to be achieved with meat packaged under CO2, the absorptive capacity of the meat for CO2 must be taken into account. Although CO2 solubilities in muscle tissue may be estimated from data on CO2 solubilities in water and salt solutions (Weiss, 1974), these estimates would be only approximate as the specific solutes in muscle tissue and variations in muscle tissue composition will affect CO2 solubility (Edsall & Wyman, 1958). Moreover, few data are available on the solubility of CO2 in the triglycerides that are the major components of fat tissue. This work was therefore undertaken to allow more accurate estimation of the solubility of CO 2 in meats packaged under atmospheres containing that gas.
MATERIALS A N D METHODS
Determination of CO2 solubility in tissues The solubility of CO2 in tissue was determined by saturating the tissue with CO2, then absorbing the gas evolved from the tissue in standard 0.05u Ba(OH)2 solution and titrating the residual Ba(OH) 2 with 0 " l i HC1 using phenolphthalein as the indicator. For each determination, three pieces of tissue of approximately 1, 1.5 and 2 g were accurately weighed and placed in petri dishes. The petri dishes were placed in an anaerobic jar standing in a water bath, the temperature of which was controlled to -I-0.1°C of the required temperature. The anaerobic jar was lined with moist filter paper to prevent desiccation of the samples. The jar was evacuated, flushed with CO2, re-evacuated, then filled with CO2 at atmospheric pressure. After 3 to 5 h the gas pressure within the jar was
The solubility of carbon dioxide in meat
67
equilibrated to the ambient atmospheric pressure. The samples were held under CO2 overnight. Absorbed CO2 was evolved from the tissues in an apparatus composed of two 100 ml Buchner flasks, the side arms of which were joined by a minimal length of pressure tubing. One flask contained 3 ml of standard Ba(OH)2 solution, the other 5 ml of 6% perchloric acid or, for fat tissue, 5 ml of a 1:1 mixture of 12% perchloric acid and ethanol. Both flasks were closed with neoprene bungs. On removal from the CO2 atmosphere, each tissue sample was immediately transferred to a pair of flasks. The sample was placed in the flask containing perchloric acid solution, then the bung was immediately replaced. The acidified samples were left overnight at room temperature to allow for completed absorption of the evolved CO 2 by the Ba(OH) 2 solution. Each Ba(OH)2 solution was then washed from the flask with 20 ml of distilled water, filtered, and the filter washed with 10ml of distilled water. The collected filtrate was titrated against standard HC1. A control pair of flasks to which no tissue had been added was included in each determination. For each determination, the values for the volumes of Ba(OH)2 neutralized by CO 2 for each of the three samples, and the value for the control, were plotted. This plot was used to estimate the CO2 evolved from 1 g of tissue saturated with CO2 at atmospheric pressure. The effect on these values of variation of the ambient atmospheric pressure was neglected. Gas volumes were calculated on the basis that: 1 ml 0"IN Ba(OH)2 - 2 - 2 0 m g CO2 - 1.12ml CO2 at STP Selection and storage of muscle tissues Post rigor muscle tissues free of fat cover were obtained from the longissimus dorsi muscles of sheep, pig and cattle carcasses 48 h after slaughter. The pH of each muscle was determined by homogenizing samples of about 2 g in 10 ml of distilled water, then measuring the pH of the homogenate with a glass electrode. For estimation of the effect of temperature on CO2 solubility, a single muscle was obtained for each species. The muscles were wrapped in cling film and stored in an anaerobic jar under CO2 at - 1°C. Tissue samples were cut from these muscles as required over a period of 2 weeks. The effect of muscle pH on CO 2 solubility was determined using beef muscle held at 0°C. Portions of beef muscles of appropriate pH values were obtained as required, and discarded after samples had been cut from them.
68
C. O. Gill
Selection and storage of fat tissues Pieces of cover fat tissues from a single lamb, pig and cow carcass were stored under C02 to provide the sources of samples as described for the muscle tissues. RESULTS The solubility of CO2 in beef muscle tissue at 0°C increased linearly with increasing tissue pH by approximately 360 ml/kg for each pH unit rise. The solubility of CO2 in single samples of lamb and pork muscle tissues at 0°C was in good agreement with the data for CO2 solubility in beef muscle tissue (Fig. 1). The solubility of CO2 in muscle tissue from the three species decreased with increasing temperature. Plotting CO2 solubility against temperature for each species gave three shallow, concave curves that were approximately parallel (Fig. 2). For approximate calculation, the curves can be assumed to be linear in the temperature range to which chilled meat could be exposed during commercial handling and storage (-1.5 to + 10°C). Within that temperature range, CO2 solubility would decrease by approximately 19 ml/kg for each I°C rise in temperature. The plots of CO2 solubility against temperature for fat tissue were complex curves, distinctive for the tissue from each species (Fig. 3). At - 1°C, the volume of CO2 dissolved was in the order lamb > beef> pork. The solubility of CO2 in lamb fat rose rapidly with temperature, at an initial approximate rate of increase of 80ml/kg for each °C rise. The rate of increase declined rapidly above 3°C, a maximum value for dissolved CO2 being attained at about 10°C. The solubility of CO2 in pork fat increased with temperature at an initial approximate rate of 25 ml/kg for each °C rise. The rate of increase was enhanced above 10°C, a maximum value for CO2 solubility being attained at about 14°C. The solubility of CO2 in beef fat 1400
1200
'~
1000
(.0
(.2 Muscle pH
e'.e
Fig. 1. Effect of the pH of muscle tissue on the 7.0 solubility of C02 in the tissue. Beef (©); pork (O); lamb (D).
69
The solubility o f carbon dioxide in meat 1400
1400
q
I[
111O0
a~ 1ooo
a~ tooo
t4 ~4
eo,
400
.
-5
.
.
0
.
.
'
6
10
.
. 18
.
.
.
20
i
25
i
i
30
,
400
38
-6
.
.
o
.
.
5
'
~
~o
,
i
,5
,
i
~o
,
.
,
.
a
so
.
Tamp (*C)
romp (*c)
Fig. 2. The effect of temperature on CO 2 solubility in muscle tissue. Beef, pH 5.42 (O); pork pH 5-75 (0); lamb, pH 6.20 (l'q).
Fig. 3. The effect of temperature on CO 2 solubility in fat tissue. Beef (O); pork (0); lamb (I-q).
showed only little increase with temperature between - 1 and 10°C, but then increased more rapidly to reach a maximum value at about 22°C.
DISCUSSION The solubility of C O 2 in aqueous solution will increase with solution pH above pH 5-0, as an increasing fraction of the dissolved CO2 will be present as HCO~-. The relationship between HCO~- formation and pH is described by the Henderson-Hasselbalch equation (Edsall & Wyman, 1958). Consequently, if muscle tissue pH were unaffected by the dissolution of CO 2, a plot of solubility against tissue pH would give a logarithmic curve. However, dissolution of CO2 in muscle tissue produces a fall in pH, despite the buffering capacity of the tissue (Bendall, 1972), to give the observed linear relationship between the initial pH of the tissue and the amount of CO2 that dissolves in that tissue. The decrease in CO2 solubility in muscle tissue with increasing temperature was to be expected from the general behaviour of gases in aqueous solution. The effect of temperature on CO2 solubility in water is described by a polynomial equation. Increasing salt concentration reduces the solubility of CO 2 and the curvature of solubility vs temperature plots become less pronounced (Li & Tsui, 1971). The observed change with temperature o f C O 2 solubility in muscle tissue conforms to this pattern. The species from which muscle tissue originated was apparently not a significant factor for CO2 solubility.
70
c. o. Gill
The few available data on the solubility of gases in fats have been largely obtained for temperatures above 30°C. These data indicate that the solubility coefficients of gases are not substantially affected by differences in fat composition, and that although the solubility coefficients of most gases increase with temperature, those o f C O 2 decrease (Formo, 1979). This simple description is obviously not applicable to the solubility of CO 2 in fat tissues at temperatures between - 1 and 30°C. Both the evident effect of differences in fat composition, and the initial increase in CO2 solubility with increasing temperature, probably reflect the varied and complex phase changes of the different triglyceride mixtures in the fat tissues that occur in that temperature range (Timms, 1984). The more consistent solubility in fats reported for temperatures substantially above 30°C is presumably due to the fats being wholly liquid at those temperatures. The solubility of CO2 in meat is affected by the muscle tissue pH, the temperature, and the proportion and composition of the fat that is present. The amount absorbed will be reduced by the presence of endogenously generated CO2. The rate of dissolution will be affected by the size and shape of meat pieces, the composition of the exposed surfaces and the boundary conditions that exist at those surfaces. The rate of dissolution will decline with the decreasing difference in partial pressure between the atmosphere and the surface layers o f the meat. Obviously, the multiplicity of factors affecting CO 2 dissolution will generally preclude accurate calculation of the rate and final quantity of CO 2 absorbed by any particular piece of meat. However, the data presented in this paper should allow useful calculation of CO 2 volumes to be made for practical purposes where meats are packaged under atmospheres containing CO 2. For such calculations it would be necessary to make realistic assumptions with regard to meat composition and in-pack conditions to ensure that packaging objectives with respect to CO2 were obtained in the great majority of packs. For example, for lamb packaged for prolonged storage under an atmosphere of 100% CO2, it is desirable to maintain the CO2 at atmospheric pressure. The effect of normal variation of the ambient atmospheric pressure can be neglected as it will be small in comparison with other factors. As the amount of CO2 absorbed by meat pieces of different composition will be highly variable, it would be appropriate to ensure that sufficient CO2 is added to saturate those meat pieces which have the greatest capacity for CO2 at the lowest temperature at which the meat is likely to be held. For lamb, it could be assumed that the pH of some muscles would be high, say, 6.5, that fat cover could be negligible and that the optimum storage temperature of - 1-5°C would be experienced. These assumptions would give the quantity of CO2 to be added to packs as 1250ml at STP per kg of meat. For other circumstances, assumptions that were similarly appropriate would be required when making calculations of this type.
The solubility of carbon dioxide in meat
71
ACKNOWLEDGEMENT I thank Miss K. Fitzgerald for technical assistance.
REFERENCES Bendall, J. R. (1972). In: The Structure and Function of Muscles, Vol. 2, part 2 (2nd edn), ed. G. H. Bourne, Academic Press, New York, p. 243. Edsall, J. T. & Wyman, J. (1958). In Biophysical Chemistry, Academic Press, New York, p. 550. Enfors, S. O. & Molin, G. (1980). J. Appl. Bacteriol., 48, 409. Formo, M. W. (1979). In Bailey's Industrial OilandFat Products, Vol. 1 (4th edn), ed. D. Swern, John Wiley, New York, p. 215. Gill, C. O. & Penney, N. (1986). Meat Sci., 18, 41. Gill, C. O. & Tan, K. H. (1980). Appi. Environ. Microbiol., 39, 317. Li, Y. H. & Tsui, T. F. (1971). J. Geophys. Res., 76, 4203. Timms, R. E. (1984). Prog. Lipid Res., 23, 1. Weiss, R. F. (1974). Marine Chem., 2, 203.
The Solubility of Carbon Dioxide in Meat C. O. Gill Meat Industry Research Institute of New Zealand (Inc.), PO Box 617, Hamilton, New Zealand (Received 3 November 1987; revised version received 14 January 1987; accepted 17 January 1987)
ABSTRACT The solubility o f CO 2 in muscle tissue of pH 5"5 at O°C was approximately 960 ml at STP/kg of tissue. The solubility increased with increasing tissue pH by 360 ml/kg for each pH unit. The solubility decreased with increasing temperature by 19 ml/kg for each °C rise. Solubilities in beef pork and lamb muscle tissue were comparable. The solubility of CO 2 in fat tissues initially increased as the temperature was raised above O°C, but then declined at higher temperatures, with the temperature of peak solubility and the solubility curves being markedly different for fat tissue from the three species.
INTRODUCTION Carbon dioxide inhibits the growth of a wide range of microorganisms (Enfors & Molin, 1980). This inhibitory effect is exploited in modified atmosphere packaging to extend the storage life of chilled meats. In aerobic systems, such as are used for display packs, atmospheres containing 20 to 30% CO2 are used, as increasing the CO2 concentration beyond that level has little additional inhibitory effect on Pseudomonasdominated spoilage floras (Gill & Tan, 1980). In anaerobic systems, used to extend the storage life before meat is prepared for display, atmospheres of 100% CO2 may be used, as the inhibitory effects of CO2 on both the lactobacilli and enterobacteria components of anoxic floras increase with increasing CO2 concentration (Gill & Penney, 1986). 65 Meat Science0309-1740/88/$03-50 © 1988ElsevierAppliedSciencePublishersLtd, England. Printed in Great Britain
66
C. O. Gill
Carbon dioxide is highly soluble in both water and oils. Therefore, when CO 2 is applied to meat in a rigid pack, the gas will be absorbed by the muscle and fat tissues until equilibrium is attained. At equilibrium the partial pressure of CO2 will be less than that of the original gas mixture, and the total gas pressure also will be less than that at which the gas mixture was initially applied. Similar considerations will apply with a flexible packaging system. When gas mixtures are used, the less soluble nitrogen and unrespired components of the gas mixture will maintain a volume of gas at atmospheric pressure around the meat that will be less than the volume initially added. When CO2 alone is added, the pack will collapse around the meat as gas is absorbed, and the partial pressure of CO2 within the pack will be less than atmospheric unless CO2 is added in excess of the quantity required to saturate the meat at atmospheric pressure. Clearly, if reproducible effects are to be achieved with meat packaged under CO2, the absorptive capacity of the meat for CO2 must be taken into account. Although CO2 solubilities in muscle tissue may be estimated from data on CO2 solubilities in water and salt solutions (Weiss, 1974), these estimates would be only approximate as the specific solutes in muscle tissue and variations in muscle tissue composition will affect CO2 solubility (Edsall & Wyman, 1958). Moreover, few data are available on the solubility of CO2 in the triglycerides that are the major components of fat tissue. This work was therefore undertaken to allow more accurate estimation of the solubility of CO 2 in meats packaged under atmospheres containing that gas.
MATERIALS A N D METHODS
Determination of CO2 solubility in tissues The solubility of CO2 in tissue was determined by saturating the tissue with CO2, then absorbing the gas evolved from the tissue in standard 0.05u Ba(OH)2 solution and titrating the residual Ba(OH) 2 with 0 " l i HC1 using phenolphthalein as the indicator. For each determination, three pieces of tissue of approximately 1, 1.5 and 2 g were accurately weighed and placed in petri dishes. The petri dishes were placed in an anaerobic jar standing in a water bath, the temperature of which was controlled to -I-0.1°C of the required temperature. The anaerobic jar was lined with moist filter paper to prevent desiccation of the samples. The jar was evacuated, flushed with CO2, re-evacuated, then filled with CO2 at atmospheric pressure. After 3 to 5 h the gas pressure within the jar was
The solubility of carbon dioxide in meat
67
equilibrated to the ambient atmospheric pressure. The samples were held under CO2 overnight. Absorbed CO2 was evolved from the tissues in an apparatus composed of two 100 ml Buchner flasks, the side arms of which were joined by a minimal length of pressure tubing. One flask contained 3 ml of standard Ba(OH)2 solution, the other 5 ml of 6% perchloric acid or, for fat tissue, 5 ml of a 1:1 mixture of 12% perchloric acid and ethanol. Both flasks were closed with neoprene bungs. On removal from the CO2 atmosphere, each tissue sample was immediately transferred to a pair of flasks. The sample was placed in the flask containing perchloric acid solution, then the bung was immediately replaced. The acidified samples were left overnight at room temperature to allow for completed absorption of the evolved CO 2 by the Ba(OH) 2 solution. Each Ba(OH)2 solution was then washed from the flask with 20 ml of distilled water, filtered, and the filter washed with 10ml of distilled water. The collected filtrate was titrated against standard HC1. A control pair of flasks to which no tissue had been added was included in each determination. For each determination, the values for the volumes of Ba(OH)2 neutralized by CO 2 for each of the three samples, and the value for the control, were plotted. This plot was used to estimate the CO2 evolved from 1 g of tissue saturated with CO2 at atmospheric pressure. The effect on these values of variation of the ambient atmospheric pressure was neglected. Gas volumes were calculated on the basis that: 1 ml 0"IN Ba(OH)2 - 2 - 2 0 m g CO2 - 1.12ml CO2 at STP Selection and storage of muscle tissues Post rigor muscle tissues free of fat cover were obtained from the longissimus dorsi muscles of sheep, pig and cattle carcasses 48 h after slaughter. The pH of each muscle was determined by homogenizing samples of about 2 g in 10 ml of distilled water, then measuring the pH of the homogenate with a glass electrode. For estimation of the effect of temperature on CO2 solubility, a single muscle was obtained for each species. The muscles were wrapped in cling film and stored in an anaerobic jar under CO2 at - 1°C. Tissue samples were cut from these muscles as required over a period of 2 weeks. The effect of muscle pH on CO 2 solubility was determined using beef muscle held at 0°C. Portions of beef muscles of appropriate pH values were obtained as required, and discarded after samples had been cut from them.
68
C. O. Gill
Selection and storage of fat tissues Pieces of cover fat tissues from a single lamb, pig and cow carcass were stored under C02 to provide the sources of samples as described for the muscle tissues. RESULTS The solubility of CO2 in beef muscle tissue at 0°C increased linearly with increasing tissue pH by approximately 360 ml/kg for each pH unit rise. The solubility of CO2 in single samples of lamb and pork muscle tissues at 0°C was in good agreement with the data for CO2 solubility in beef muscle tissue (Fig. 1). The solubility of CO2 in muscle tissue from the three species decreased with increasing temperature. Plotting CO2 solubility against temperature for each species gave three shallow, concave curves that were approximately parallel (Fig. 2). For approximate calculation, the curves can be assumed to be linear in the temperature range to which chilled meat could be exposed during commercial handling and storage (-1.5 to + 10°C). Within that temperature range, CO2 solubility would decrease by approximately 19 ml/kg for each I°C rise in temperature. The plots of CO2 solubility against temperature for fat tissue were complex curves, distinctive for the tissue from each species (Fig. 3). At - 1°C, the volume of CO2 dissolved was in the order lamb > beef> pork. The solubility of CO2 in lamb fat rose rapidly with temperature, at an initial approximate rate of increase of 80ml/kg for each °C rise. The rate of increase declined rapidly above 3°C, a maximum value for dissolved CO2 being attained at about 10°C. The solubility of CO2 in pork fat increased with temperature at an initial approximate rate of 25 ml/kg for each °C rise. The rate of increase was enhanced above 10°C, a maximum value for CO2 solubility being attained at about 14°C. The solubility of CO2 in beef fat 1400
1200
'~
1000
(.0
(.2 Muscle pH
e'.e
Fig. 1. Effect of the pH of muscle tissue on the 7.0 solubility of C02 in the tissue. Beef (©); pork (O); lamb (D).
69
The solubility o f carbon dioxide in meat 1400
1400
q
I[
111O0
a~ 1ooo
a~ tooo
t4 ~4
eo,
400
.
-5
.
.
0
.
.
'
6
10
.
. 18
.
.
.
20
i
25
i
i
30
,
400
38
-6
.
.
o
.
.
5
'
~
~o
,
i
,5
,
i
~o
,
.
,
.
a
so
.
Tamp (*C)
romp (*c)
Fig. 2. The effect of temperature on CO 2 solubility in muscle tissue. Beef, pH 5.42 (O); pork pH 5-75 (0); lamb, pH 6.20 (l'q).
Fig. 3. The effect of temperature on CO 2 solubility in fat tissue. Beef (O); pork (0); lamb (I-q).
showed only little increase with temperature between - 1 and 10°C, but then increased more rapidly to reach a maximum value at about 22°C.
DISCUSSION The solubility of C O 2 in aqueous solution will increase with solution pH above pH 5-0, as an increasing fraction of the dissolved CO2 will be present as HCO~-. The relationship between HCO~- formation and pH is described by the Henderson-Hasselbalch equation (Edsall & Wyman, 1958). Consequently, if muscle tissue pH were unaffected by the dissolution of CO 2, a plot of solubility against tissue pH would give a logarithmic curve. However, dissolution of CO2 in muscle tissue produces a fall in pH, despite the buffering capacity of the tissue (Bendall, 1972), to give the observed linear relationship between the initial pH of the tissue and the amount of CO2 that dissolves in that tissue. The decrease in CO2 solubility in muscle tissue with increasing temperature was to be expected from the general behaviour of gases in aqueous solution. The effect of temperature on CO2 solubility in water is described by a polynomial equation. Increasing salt concentration reduces the solubility of CO 2 and the curvature of solubility vs temperature plots become less pronounced (Li & Tsui, 1971). The observed change with temperature o f C O 2 solubility in muscle tissue conforms to this pattern. The species from which muscle tissue originated was apparently not a significant factor for CO2 solubility.
70
c. o. Gill
The few available data on the solubility of gases in fats have been largely obtained for temperatures above 30°C. These data indicate that the solubility coefficients of gases are not substantially affected by differences in fat composition, and that although the solubility coefficients of most gases increase with temperature, those o f C O 2 decrease (Formo, 1979). This simple description is obviously not applicable to the solubility of CO 2 in fat tissues at temperatures between - 1 and 30°C. Both the evident effect of differences in fat composition, and the initial increase in CO2 solubility with increasing temperature, probably reflect the varied and complex phase changes of the different triglyceride mixtures in the fat tissues that occur in that temperature range (Timms, 1984). The more consistent solubility in fats reported for temperatures substantially above 30°C is presumably due to the fats being wholly liquid at those temperatures. The solubility of CO2 in meat is affected by the muscle tissue pH, the temperature, and the proportion and composition of the fat that is present. The amount absorbed will be reduced by the presence of endogenously generated CO2. The rate of dissolution will be affected by the size and shape of meat pieces, the composition of the exposed surfaces and the boundary conditions that exist at those surfaces. The rate of dissolution will decline with the decreasing difference in partial pressure between the atmosphere and the surface layers o f the meat. Obviously, the multiplicity of factors affecting CO 2 dissolution will generally preclude accurate calculation of the rate and final quantity of CO 2 absorbed by any particular piece of meat. However, the data presented in this paper should allow useful calculation of CO 2 volumes to be made for practical purposes where meats are packaged under atmospheres containing CO 2. For such calculations it would be necessary to make realistic assumptions with regard to meat composition and in-pack conditions to ensure that packaging objectives with respect to CO2 were obtained in the great majority of packs. For example, for lamb packaged for prolonged storage under an atmosphere of 100% CO2, it is desirable to maintain the CO2 at atmospheric pressure. The effect of normal variation of the ambient atmospheric pressure can be neglected as it will be small in comparison with other factors. As the amount of CO2 absorbed by meat pieces of different composition will be highly variable, it would be appropriate to ensure that sufficient CO2 is added to saturate those meat pieces which have the greatest capacity for CO2 at the lowest temperature at which the meat is likely to be held. For lamb, it could be assumed that the pH of some muscles would be high, say, 6.5, that fat cover could be negligible and that the optimum storage temperature of - 1-5°C would be experienced. These assumptions would give the quantity of CO2 to be added to packs as 1250ml at STP per kg of meat. For other circumstances, assumptions that were similarly appropriate would be required when making calculations of this type.
The solubility of carbon dioxide in meat
71
ACKNOWLEDGEMENT I thank Miss K. Fitzgerald for technical assistance.
REFERENCES Bendall, J. R. (1972). In: The Structure and Function of Muscles, Vol. 2, part 2 (2nd edn), ed. G. H. Bourne, Academic Press, New York, p. 243. Edsall, J. T. & Wyman, J. (1958). In Biophysical Chemistry, Academic Press, New York, p. 550. Enfors, S. O. & Molin, G. (1980). J. Appl. Bacteriol., 48, 409. Formo, M. W. (1979). In Bailey's Industrial OilandFat Products, Vol. 1 (4th edn), ed. D. Swern, John Wiley, New York, p. 215. Gill, C. O. & Penney, N. (1986). Meat Sci., 18, 41. Gill, C. O. & Tan, K. H. (1980). Appi. Environ. Microbiol., 39, 317. Li, Y. H. & Tsui, T. F. (1971). J. Geophys. Res., 76, 4203. Timms, R. E. (1984). Prog. Lipid Res., 23, 1. Weiss, R. F. (1974). Marine Chem., 2, 203.