Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY MODIFIED ATMOSPHERE (OR REDUCING O–R POTENTIAL) 493
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.
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III. MECHANISM OF ANTIMICROBIAL 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
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 proteinrich
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 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 to 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 2CO 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 increase with the increase in concentration. Too high a concentration
can inhibit growth of facultative spoilage bacteria and stimulate growth of Clostridium
botulinum. 2–6
A. Nature of Process
IV. INFLUENCING FACTORS
These factors include efficiency of vacuum, permeability of packaging materials to
O 2, and the composition of gas (in the gas-flushing method) used. 2–6 High 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