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How and why? |
The good and bad points of packaging with a gas cocktail |
Packaging in modified atmosphere has certainly helped the fresh pasta "boom". As everybody
is well aware of, packaged fresh pasta offers greater consumer benefit than the unpackaged
form; we can be sure it will be fresh up until the very last moment of its sell-by date. Packaging
of fresh pasta with modified atmosphere (MAP) permits product preservation for long periods
of time, it inhibits the growth of micro-organisms such as bacteria and mould and also slows
down general enzyme and biochemical processes. Hence a modified atmosphere prolongs the
commercial life of fresh pasta.
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Modified atmosphere packaging supersedes the old concept of "package = product protection".
Actually, the functions of modified atmosphere processes are far wider and more complex than
this: providing product presentation, portioning, moisture maintenance at a constant
temperature, protection from external contamination, inhibition of the growth of microfloras,
slowing down of enzyme functions and biochemical processes within the product. |
So, the question is, is modified atmosphere packaging for fresh pasta really the best solution to
all the commercial problems that this extremely delicate product presents? At first glance it
would seem to be so, indeed many manufacturers and distributors are firmly convinced of its
efficiency, but the truth of the matter is quite different and modified atmosphere packaging
must be considered for what it really is ... a modern, very effective way of stabilising
freshness in certain foods, perishables in particular, and not a method of indefinite preservation.
Manufacturers and distributors of fresh pasta must be made aware of its limits. |
Now let’s take a more detailed look at what modified atmosphere is, and what the effects are on
microbiological stability of the product.
Packaging in modified atmosphere consists of extracting air from the internal area of the
package and substituting it with another gas, generally a mixture of carbon dioxide and
nitrogen. This operation is carried out by specialised packing units, most of which use
thermoformable materials. The machine sucks air out of a series of perforations in a selected
part of the package - subsequently discarded during the forming phase - and then substitutes it
with the desired gas combination. In order to do this, certain conditions are required. In the first
place it is essential that the utmost care is taken that all air is extracted and new gases evenly
inserted and that the package is properly sealed in order to avoid changes of modified
atmosphere taking place inside the actual package. Secondly, the material used should also
provide a gas barrier, both on entry and exit of the package thus creating very low permeability.
Should the package undergo heat treatment through hot air or microwaves, it is clear that the
material used should be suitable for such a treatment. |
Next we can examine the effect of the modified atmosphere on microrganisms. Carbon dioxide
(CO2) has an inhibitory effect that varies from microbe to microbe and, for the same species,
depending on the concentration (in air CO2 normally has a concentration of less than 0.1%),
the moisture of the product (or rather "free water" Aw) and the pH (acidity), as well as the
temperature at which the product is preserved. |
The higher the concentration of CO2 in proportion to the air (at the same pressure), the higher
the inhibitory effect will be, further enhanced with low pH and low temperatures. |
Carbon dioxide tends to dissolve in the water of the product, the combination of which forms
carbonic acid (H2CO3) with a reaction that is enhanced by low temperatures. For many moist
products (e.g. gnocchi) it is best that the concentration of CO2 is not excessive (<50%), since
too much carbonic acid causes the package to contract (vacuum effect), and this begins to be
seen 25-30 days after packaging. This is why the concentration of CO2 is normally maintained
at 30% as for fresh pasta with a lower moisture level than gnocchi (by law not to exceed 30%),
although there are some exceptions depending upon the shelf-life required.
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Micro-organisms inhibited by carbon dioxide are above all moulds, yeasts and aerobic bacteria,
particularly Pseudomonas and Bacillus, mainly responsible for the degeneration of the product.
On the other hand, dairy bacteria are in general much less inhibited, whereas Gram Positive
bacteria, most of which are anaerobe, such as clostridia, are not affected at all. Modified
atmosphere does not present any inhibitory effects on the most common and/or dangerous
pathogens such as Clostridium botulinum, Clostridium perfrigens, Salmonella (MAP is
effective only with high concentrations of CO2) and Listeria. In this case the protection of the
product must be obtained with other synergetic factors (temperature, pH, Aw).
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CO2 acts on bacteria during its dissolution rather than through the pH variations determined by
the formation of carbonic acid.
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According to various sources, Carbon Dioxide influences the respiratory action of microbe cells
by blocking the synthesis of enzymes that the micro-organisms need to create the chemical
reaction necessary for this biological process. It is important to remember that this inhibitory
reaction diminishes with an increase of temperature, and so in any case modified atmosphere
packaged pasta must be refrigerated, preferably between +3 and +5 degrees.
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The effects of Nitrogen (N2) on micro-organisms are diverse. It is generally considered as a
mere support gas, but it has also been proved that it is possible to use it to block certain germ spores such as Bacillus cereus, as well as to reduce the proteolitic and lipolitic enzyme activities. In fact Nitrogen has inhibitory effects against protease enzymes (proteolitic enzymes), lipase (enzymes that dry out fatty acids), decarbolissase (enzymes that release Carbon Dioxide from organic compounds).
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In MAP limited percentages of oxygen (02) may be present, the concentration of which will
increase over time due to the inevitable barriers to the gas represented by the packaging materials.
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To summarize:
MAP increases its efficiency in prolonging the shelf-life of fresh pasta provided that synergetic conditions are used to advantage (temperature, pH and Aw).
In case of products with high water content (Aw) and raised pH levels, MAP is of little or no use at all. Under these conditions, or when there is anaerobic contamination of the pasta (i.e. bacteria that multiply in an oxygen poor environment) or in the presence of facultative anaerobe (i.e. bacteria that multiply with or without oxygen), MAP can even be disadvantageous.
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One should also bear in mind that the order of importance of the factors regarding the microbe
stability of fresh foodstuffs are as follows: pH, Aw ( water availability for micro-organisms),
temperature, oxide reduction potential (oxide content) and pasteurisation.
These are the factors required for a correct synergy to achieve increased MAP stabilisation
efficiency.
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Modified atmosphere undergoes composition changes for various reasons. Due to Carbon
Dioxide reactions with the product and due to the inevitable permeability of materials used, or
because of imperfect sealing of the package (incorrect welding or imperfect closure of carton
lids or thermoformable bags).
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