Control by Modified Atmosphere (or Reducing O-R Potential) ◾ 485
objectives
The objectives of MAP are to control or reduce the growth of undesirable microorganisms in food. The
technique also helps retard enzymatic and respiratory activities of fresh foods. The growth of aerobes
(molds, yeasts, and aerobic bacteria) is prevented in products that are either vacuum packaged or flushed
with 100% CO 2 , 100% N 2 , or a mixture of CO 2 and N 2 . However, under these conditions, anaerobic
and facultative anaerobic bacteria can grow unless other techniques are used to control their growth.
Mechanism of Action
The antimicrobial action in MAP foods can be produced by changes in the redox potential (Eh)
and CO 2 concentrations based on the methods used. Aerobes and anaerobes, depending on the
microbial species and reducing or oxidizing state of food, have different Eh requirements for
growth, whereas facultative anaerobes grow over a wide Eh range. Vacuum packaging and gas
flushing, especially with CO 2 or N 2 or their mixture and no O 2 , discourage growth of aerobes but
encourage growth of facultative anaerobes and anaerobes. However, even under these conditions
of packaging, tissue oxygen and dissolved and trapped oxygen can allow the initial growth of aerobes to produce CO 2 even by the proteolytic microbes. In addition, natural reducing components
in foods, such as the –SH group in protein-rich foods, and ascorbic acid and reducing sugars in
fruits and vegetable products, can alter the Eh of a food and encourage growth of anaerobes and
facultative anaerobes. Thus, it is not possible to control microbial growth just by changing the Eh.
Other methods, in addition to modification of the environment, are necessary for effective preservation of foods. However, by controlling the growth of aerobic bacteria, many of which have short
generation times, the shelf life of a product is greatly extended.
When CO 2 is used in high concentrations (20%–100%), alone or in combination with N 2
or O 2 , or both, the shelf life of MAP foods is also extended. Several mechanisms by which CO 2
increases the lag and exponential phases of microorganisms have been proposed, such as rapid
cellular penetration of CO 2 and alteration in cell permeability, solubilization of CO 2 to carbonic
acid (H 2 CO 3 ) in the cell with the reduction of the pH inside the cells, and interference of CO 2
with several enzymatic and biochemical pathways, which, in turn, slow the microbial growth rate.
The inhibitory effect of CO 2 on microbial growth occurs at the 10% level and increases with the
increase in concentration. Too high a concentration can inhibit growth of facultative spoilage
bacteria and stimulate growth of Clostridium botulinum. 1,5
influencing Factors
Nature of Process
These factors include efficiency of the vacuum, permeability of the packaging materials to O 2 , and
the composition of gas (in the gas-flushing method) used. 6 A high-efficiency vacuum can effectively
control the growth of aerobes by removing O 2 from the products, except probably trapped and dissolved O 2 . Similarly, packaging films that prevent or considerably reduce permeation of O 2 during
storage effectively control the growth of aerobes. Minute leaks in the film can adversely affect the
protective effect of vacuum packaging or gas-flush packaging. The development of polymeric film
has helped greatly reduce the O 2 permeability. In gas flushing, CO 2 and N 2 , and in the case of fresh
objectives
The objectives of MAP are to control or reduce the growth of undesirable microorganisms in food. The
technique also helps retard enzymatic and respiratory activities of fresh foods. The growth of aerobes
(molds, yeasts, and aerobic bacteria) is prevented in products that are either vacuum packaged or flushed
with 100% CO 2 , 100% N 2 , or a mixture of CO 2 and N 2 . However, under these conditions, anaerobic
and facultative anaerobic bacteria can grow unless other techniques are used to control their growth.
Mechanism of Action
The antimicrobial action in MAP foods can be produced by changes in the redox potential (Eh)
and CO 2 concentrations based on the methods used. Aerobes and anaerobes, depending on the
microbial species and reducing or oxidizing state of food, have different Eh requirements for
growth, whereas facultative anaerobes grow over a wide Eh range. Vacuum packaging and gas
flushing, especially with CO 2 or N 2 or their mixture and no O 2 , discourage growth of aerobes but
encourage growth of facultative anaerobes and anaerobes. However, even under these conditions
of packaging, tissue oxygen and dissolved and trapped oxygen can allow the initial growth of aerobes to produce CO 2 even by the proteolytic microbes. In addition, natural reducing components
in foods, such as the –SH group in protein-rich foods, and ascorbic acid and reducing sugars in
fruits and vegetable products, can alter the Eh of a food and encourage growth of anaerobes and
facultative anaerobes. Thus, it is not possible to control microbial growth just by changing the Eh.
Other methods, in addition to modification of the environment, are necessary for effective preservation of foods. However, by controlling the growth of aerobic bacteria, many of which have short
generation times, the shelf life of a product is greatly extended.
When CO 2 is used in high concentrations (20%–100%), alone or in combination with N 2
or O 2 , or both, the shelf life of MAP foods is also extended. Several mechanisms by which CO 2
increases the lag and exponential phases of microorganisms have been proposed, such as rapid
cellular penetration of CO 2 and alteration in cell permeability, solubilization of CO 2 to carbonic
acid (H 2 CO 3 ) in the cell with the reduction of the pH inside the cells, and interference of CO 2
with several enzymatic and biochemical pathways, which, in turn, slow the microbial growth rate.
The inhibitory effect of CO 2 on microbial growth occurs at the 10% level and increases with the
increase in concentration. Too high a concentration can inhibit growth of facultative spoilage
bacteria and stimulate growth of Clostridium botulinum. 1,5
influencing Factors
Nature of Process
These factors include efficiency of the vacuum, permeability of the packaging materials to O 2 , and
the composition of gas (in the gas-flushing method) used. 6 A high-efficiency vacuum can effectively
control the growth of aerobes by removing O 2 from the products, except probably trapped and dissolved O 2 . Similarly, packaging films that prevent or considerably reduce permeation of O 2 during
storage effectively control the growth of aerobes. Minute leaks in the film can adversely affect the
protective effect of vacuum packaging or gas-flush packaging. The development of polymeric film
has helped greatly reduce the O 2 permeability. In gas flushing, CO 2 and N 2 , and in the case of fresh