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Food Protection with Modified
Atmospheres
Hypobaric (Low Pressure) Storage
 Foods are stored in:
 low pressure air
 low temperature
 high humidity
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The hypobaric (Low Pressure) state results in:
 Reduced concentrations of O2
 Reduced fat oxidation.
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Atmospheres have been found to be effective :
 10 mm Hg for meats and seafoods;
 10–80 mm Hg for fruits and vegetables
(1 atm = 760 mm Hg).
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Vacuum Packaging
By this method, air is evacuated from gasimpermeable pouches followed by sealing.
Upon storage of a vacuum-packaged food product,
an increase in CO2 occurs as a result of both tissue
respiration and microbial respiration where O2 is
consumed and CO2 is released in equal volumes.
In the case of meats, up to 10–20% CO2 may develop
within four hours and the concentration may
ultimately reach 30% from respiratory activities of the
aerobic biota.
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vacuum packaging minimizes :
 aerobic spoilage organisms
 product shrinkage
 fat oxidation
 discoloration
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Modified Atmosphere Packaging (MAP)
Overall, MAP is a hyperbaric process that consists
of altering the chamber or package atmosphere by
flushing with varying mixtures of CO2, N2, and/or
O2.
The initial gas concentration cannot be readjusted
during storage.
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Two types of MAP are recognized:
1. In high-O2 MAP, up to 70% of O2 along with
about 20–30% CO2 and 0–20% N2 may be used.
 Growth of aerobes is slowed but not suppressed
by the moderate concentration of CO2.
 This method is suitable for the packaging of red
meats, as the high level of O2 will aid in
maintenance of the red color.
2. In a low-O2 MAP system, O2 levels may be as
high as 10% while CO2 is maintained in the 20–
30% range with N2 added as necessary.
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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-O2 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.
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PRIMARY EFFECTS OF CO2 ON MICROORGANISMS
The following facts are well established following
prolonged exposure of microorganisms to
around10% and above of CO2.
1. The inhibitory activity increases as incubation
or storage temperatures decrease.
 This is due in part to the:

greater solubility of CO2 in water at the
lower temperatures,
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in part to the additive effect of a less than
optimal growth temperature.
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.
 Higher levels can be used for seafoods.
 To maintain red meat color, they can be stored
in a 20:80 mixture of CO2 + O2.
3. Inhibition increases as pH is decreased into the
acid range.
 One practical effect of this is that CO2 is more
effective for fresh red meats than for seafoods.
4. In general, the Gram-negative bacteria are
more sensitive to CO2 inhibition than Gram
positives, with pseudomonads being among the
most sensitive and clostridia the most resistant.
 Upon prolonged storage of meats, CO2 affects
a rather dramatic shift in biota from Gram
negative to Gram positive.
5. Both lag and logrithmic phases of growth are
delayed.
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Mode of Action of CO2
 1affect a mass action on enzymatic
decarboxylations
 2-CO2 is dissolved in the form of carbonic acid
(and thus can cause changes in cell permeability)
 3- is an arginine antagonist (like Diacetyl)
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Food Products
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Fresh and Processed Meats
In general, the self-life of red meats at
−1◦C can be extended for up to 2 months if
packaged in 75% O2 + 25% CO2.
The high level of oxygen ensures that the
red-meat color is maintained.
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Poultry
Overall, the generally higher initial pH of fresh
poultry meat is primarily responsible for this
product’s not having the MAP self-life of products
such as fresh beef.
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Seafoods
Whereas the practical upper limit of CO2 for red
meats is around 20%, higher concentrations can be
used with fish because they contain lower levels of
myoglobin.
The concern over the use of MAP for fishery
products has to do with the fact that
nonproteolytic botulism strains are found in
waters and they can grow at temperatures <4◦C
coupled with the fact that the pH of seafoods in
general is higher and more favorable to growth by
pathogens.
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THE SAFETY OF MAP FOODS
Clostridium botulinum
As a general rule, foods that are to be subjected to
MAP should possess one or more of the following
antibotulinal hurdles:
 1. have a water activity (aw) < 0.93
 2. have a pH of 4.6 or less
 3. cured with NaCl or NO2
 4. contain high levels of nonpathogens (for raw
meat, poultry, and the like)
 5. maintained in frozen state
 6. maintained at (4.4◦C) or below
 7. have a definitive self-life (e.g., not to exceed
10 days)
Listeria monocytogenes
 The fact that this bacterium can grow in the
refrigerator temperature range raises concerns
about its presence and potential for growth in MAP
foods.
 L. monocytogenes numbers were reduced by the
antilisterial effects of the lactobacillus due
apparently to its production of lactic acid.
 Regarding the behavior of this organism on
vacuum-packaged beef, it has been shown that
critical factors are:
 Storage temperature (must be less than 4 0C)
 pH (must be acidic)
 Type of tissue (grew more on fat than lean beef)
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Other Pathogens
When cooked bologna-type sausage was vacuum
packaged, the growth of Yersinia enterocolitica
and salmonellae was restricted but not that of
Staphylococcus aureus.
SPOILAGE OF MAP AND VACUUM-PACKAGED MEATS
From the research of many groups, it is clear that
when vacuum-packaged meats undergo long-term
refrigerator
spoilage,
often
the
predominant
organisms are:
 lactobacilli
 Brochothrix thermosphacta.
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Volatile Components of Vacuum-Packaged Meats
and Poultry
1- Short-chain fatty acids:
 produced by both lactobacilli and Brochothrix
thermosphacta
 produce sharp off-odors.
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2-acetoin (more important) and diacetyl:
 have been found to be the most significant
relative to spoiled meat odors.
 3-di-methyl di-sulfide or di-methyl tri-sulfide, or
methyl mercaptan.
 Some
organisms (except B. thermosphacta)
produced them.
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The volatile compounds produced by B.
thermosphacta may be expected to vary between
products
with
high
and
low
glucose
concentrations.
The addition of 2% glucose to raw ground beef has
been shown to decrease pH and delay off-odor.
It would seem to be a way to shift the volatile
components from short-chain fatty acids to
acetoin (that derive from glucose.)