354 Modern Food Microbiology
Controlled-Atmosphere Packaging or Storage
Although controlled-atmosphere packaging or storage (CAP, CAS) is regarded by some as being
different from MAP, it may be considered a form of MAP. While in a typical MAP the compositions
may change upon storage, in CAP the gas compositions remain unchanged for the duration of the
storage period. While low- and high-O 2 MAP systems may be prepared with high-barrier plastic
films, CAP requires aluminum foil laminates, metal or glass containers, since single plastic film is not
entirely impervious to gases.
Since vacuum, MAP, or CAP methods alter the concentrations of O 2 and CO 2 , albeit in different
ways, the distinction between them is often obscured in studies on the effectiveness and mode of action
of CO 2 . In the remaining sections of this chapter, the inhibitory effects of increased levels of CO 2 on
foodborne microorganisms and food quality are examined without regard to methodology.
PRIMARY EFFECTS OF CO 2 ON MICROORGANISMS
The following facts are well established following prolonged exposure of microorganisms to around
10% and above of CO 2 .
1. The inhibitory activity increases as incubation or storage temperatures decrease. This is due in
part to the greater solubility of CO 2 in water at the lower temperatures, and in part to the additive
effect of a less than optimal growth temperature. At 1 atm, 100 ml of water will absorb 88 ml of
CO 2 at 20
◦ C, but only 36 ml at 60
◦ C.
2. Although concentrations from about 5–100% have been used, 20–30% seems optimal, with no
additional benefits derived from higher levels. This is especially true for fresh meats, where 20% is
about ideal.
30,31 Higher levels can be used for seafoods. To maintain red meat color, they can first
be exposed to carbon monoxide (CO) before CO 2 , or be stored in a 20:80 mixture of CO 2 + O 2 .
3. Inhibition increases as pH is decreased into the acid range. One practical effect of this is that
CO 2 is more effective for fresh red meats than for seafoods. The vacuum packaging of red meats
with pH >6.0 is not effective. The self-life of vacuum-packaged fish is shortened by the growth
of Photobacterium phosphoreum
16 and Shewanella putrefaciens.
1
4. In general, the Gram-negative bacteria are more sensitive to CO 2 inhibition than Gram positives,
with pseudomonads being among the most sensitive and clostridia the most resistant (Table
14–4). Upon prolonged storage of meats, CO 2 affects a rather dramatic shift in biota from
one that is largely Gram negative in fresh products to one that is largely or exclusively Gram
positive. This can be seen in Table 14–5 for smoked pork loins and frankfurter sausage.
6
5. Both lag and logrithmic phases of growth are retarded.
6. CO 2 under pressure is considerably more antimicrobial than when not under pressure, and
pressures of 6–30 megapascal (MPa) can destroy bacteria and fungi under varying conditions
(see High Hydrostatic Pressure in Chapter 19). The destructive action is believed to occur when
pressure is released suddenly.
Mode of Action
As to the mechanism of CO 2 inhibition of microorganisms, two explanations have been offered.
King and Nagel
57 found that CO 2 blocked the metabolism of Pseudomonas aeruginosa and appeared
to affect a mass action on enzymatic decarboxylations. Sears and Eisenberg
79 found that CO 2 affected
Controlled-Atmosphere Packaging or Storage
Although controlled-atmosphere packaging or storage (CAP, CAS) is regarded by some as being
different from MAP, it may be considered a form of MAP. While in a typical MAP the compositions
may change upon storage, in CAP the gas compositions remain unchanged for the duration of the
storage period. While low- and high-O 2 MAP systems may be prepared with high-barrier plastic
films, CAP requires aluminum foil laminates, metal or glass containers, since single plastic film is not
entirely impervious to gases.
Since vacuum, MAP, or CAP methods alter the concentrations of O 2 and CO 2 , albeit in different
ways, the distinction between them is often obscured in studies on the effectiveness and mode of action
of CO 2 . In the remaining sections of this chapter, the inhibitory effects of increased levels of CO 2 on
foodborne microorganisms and food quality are examined without regard to methodology.
PRIMARY EFFECTS OF CO 2 ON MICROORGANISMS
The following facts are well established following prolonged exposure of microorganisms to around
10% and above of CO 2 .
1. The inhibitory activity increases as incubation or storage temperatures decrease. This is due in
part to the greater solubility of CO 2 in water at the lower temperatures, and in part to the additive
effect of a less than optimal growth temperature. At 1 atm, 100 ml of water will absorb 88 ml of
CO 2 at 20
◦ C, but only 36 ml at 60
◦ C.
2. Although concentrations from about 5–100% have been used, 20–30% seems optimal, with no
additional benefits derived from higher levels. This is especially true for fresh meats, where 20% is
about ideal.
30,31 Higher levels can be used for seafoods. To maintain red meat color, they can first
be exposed to carbon monoxide (CO) before CO 2 , or be stored in a 20:80 mixture of CO 2 + O 2 .
3. Inhibition increases as pH is decreased into the acid range. One practical effect of this is that
CO 2 is more effective for fresh red meats than for seafoods. The vacuum packaging of red meats
with pH >6.0 is not effective. The self-life of vacuum-packaged fish is shortened by the growth
of Photobacterium phosphoreum
16 and Shewanella putrefaciens.
1
4. In general, the Gram-negative bacteria are more sensitive to CO 2 inhibition than Gram positives,
with pseudomonads being among the most sensitive and clostridia the most resistant (Table
14–4). Upon prolonged storage of meats, CO 2 affects a rather dramatic shift in biota from
one that is largely Gram negative in fresh products to one that is largely or exclusively Gram
positive. This can be seen in Table 14–5 for smoked pork loins and frankfurter sausage.
6
5. Both lag and logrithmic phases of growth are retarded.
6. CO 2 under pressure is considerably more antimicrobial than when not under pressure, and
pressures of 6–30 megapascal (MPa) can destroy bacteria and fungi under varying conditions
(see High Hydrostatic Pressure in Chapter 19). The destructive action is believed to occur when
pressure is released suddenly.
Mode of Action
As to the mechanism of CO 2 inhibition of microorganisms, two explanations have been offered.
King and Nagel
57 found that CO 2 blocked the metabolism of Pseudomonas aeruginosa and appeared
to affect a mass action on enzymatic decarboxylations. Sears and Eisenberg
79 found that CO 2 affected