Note: Descriptions are shown in the official language in which they were submitted.
W092/~276 2 0 8 7 5 9 9 PCT/GB91/01457
NITROGEN ATMOSPHERES
This invention relates to nitrogen atmospheres, in
particular to nitrogen in admixture with ozone.
Because of its low level of chemical reactivity nitrogen is
widely employed in inert atmospheres, for e~ample in
protective or purging atmospheres in the food, chemical and
petrochemical industries. Typical applications include
blanketing or purging of storage vessels such as grain
silos, oil storage tanks and hazardous waste containers.
Other important applications arise with foodstuffs and
beverages, including for example modified atmosphere
packaging (MAP) to prolong the shelf life of a food product. -
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In several fields it has however been increasingly found
that while nitrogen atmospheres are effective in controlling
the primary problems such as a risk of chemical explosion or
the spoilage of foods by oxidation they are less effective
in controlling more insidious risks such as the build-up of
mould in a grain silo or the growth of anaerobic bacteria in
packaged foods. These other risks have therefore hitherto
been reduced by other forms of treatment. In the example of
grain silos the mould growth has been prevented by the
addition of insecticides.
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2087~9~ ~
In the fresh vegetable and salads food industry there iscommon use of sterilizing techniques using relatively high
concentrations of chlorine in water to eliminate the
bacteria which give rise to food spoilage. Such
sterilization is however not popular with the consumer since
it tends to affect the taste of the food. Moreover the
relatively high volumes that are required to achieve
complete sterilization of food or other products may permit
residual chlorine to escape into the environment.
we have now found that the addition of trace amounts of
ozone to nitrogen-based atmospheres creates a treatment
medium which is highly effective in storage atmospheres,
both for general inerting and in inhibiting spoilage
processes such as the growth of bacteria.
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Thus according to the present invention there is provided a -
nitrogen-based atmosphere which comprises, by volume,
1 to 500 vpm ozone, 0.05 to 10% oxygen and at least 90% of a
nitrogen-containing inert gas. ;-
The invention also provides a process which comprises ~-
contacting the material with a nitrogen-based atmosphere
containing, by volume, 1 to 500 ppm ozone, 0.05 to 10%
oxygen and at least 90% of a nitrogen-containing inert gas.
The invention further provides apparatus which comprises
means for supplying a nitrogen-based atmosphere containing,
by volume, 1 to 500 vpm ozone, 0.05 to 10% oxygen and at
least 90% of a nitrogen-containing inert gas.
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W O 92/04276 2 ~ 8 7 ~ 9 9 PC~r/GB91/01457
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The atmospheres, processes and apparatus according to the
invention can advantageously be used for a wide variety of
applications and products, including foods and beverages,
chemical, medical and and pharmaceutical products. They are
especially suitable for blanketing and purging of storage
vessels such as grain silos and for use in vegetable or
fruit stores (e.g. for apples or bananas). Surprisingly it
has been found that volumes of ozone well below the levels
required for full steriliz3tion achieve the required
effects, in particular in the inhibition of spoilage
mechanisms. Thus in modified atmosphere packaging the
atmospheres of the invention substantially increase the
shelf life of the product while not adversely affecting its
taste and flavour. Additionally since the breakdown product
of the o. ne is simply oxygen there is no ha_mful residual
product t be released into the ambient atmosphere.
The invention is beneficial in permitting the use of low
oxygen atmospheres, which would otherwise tend to encourage
anaerobic activity, in situations where anaerobic activity
is to be strictly prevented.
The reason for the etfectiveness of the low proportions of
ozone according to the invention appears to be that the
ozone volumes, while small, have highly disruptive effect
on spoilage mechanisms. Typical of these mechanisms are
rapid bacterial or mould growth on a material surface, cell
autolysis through enzyme action and premature ripening
through the spontaneous generation of ripening hormones.
In many instances the spoilage is displayed as a
discolouration of the product or the generation of
unpleasant odours. According to the invention the
mechanisms causing these e'fects are disrupted or
substantially prevented.
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W092/~276 PCT/GB91/014
2087~99 4 ~ ~
we have found that even if the ozone volumes are
insufficient to achieve complete destruction of bacteria,
enzymes or mould-producing organisms and even if they do not
penetrate into the interior of a solid material, they
display a considerable inhibiting eff ect, especially at the
contact surface(s). Since much of the spoilage tends to
occur at or near the surface the ozone is thus applied to
portions of the material at which it has its most
significant effect. Another benefit is that the ozone
reacts with ripening hormones such as ethylene at the very
surfaces which emit such hormones and thus inhibits the
ripening process. This benefit is especially marked in the -
case of banana storage, in which removal of the ethylene
generated by the initially-ripening bananas in a store
prevents this ethylene from ripening the other bananas and
thus permits all the bananas in the store to be offered to
the consumer in a "ready to ripen" condition.
Similar advantages to the above are obtained in employing
the atmospheres to sparge liquids, in particular to sparge
beverages such as water, soft drinks and alcoholic drinks
(carbonated or non-carbonated). It is generally preferred
to conduct the sparging prior to any carbonation of the
liquid and prior to the introduction of ingredients such as
flavouring concentrates. In the case of liquid sparging the
benefits of the invention appear to result from the intimate
contact between the ozone and the liquid throughout a high
proportion of the liquid volume.
In the medical and pharmaceutical fields, the atmospheres
find application in maintaining sterile conditions achieved
in a preceding full sterilisation step. For example the
packaging of wound dressings, prepared under sterile
conditions, can be achieved under the low-dosage ozone
atmospheres of the invention without loss of sterility.
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W;092/04276 2 0 8 7 5 9 9 PCT/GB91/01457
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A side benefit of the use of atmospheres according to the
invention in many of its applications is the suppression of
unpleasant odours. For example in the bulk storage of
organic materials the avoidance of associated odours from
anaerobic activity provides a more acceptable ambient
atmosphere in the vicinity of the store.
As indicated above, the invention is particularly well
suited to the provision of nitrogen/ozone mixtures for
modified atmosphere packaging. The MAP package is provided
with a closure film, typically a laminated film, comprising
such polymers as cellulose, polypropylene, polyvinylidene
dichloride and polyamines. The film encloses and seals in
the protective atmosphere and is generally reguired to
maintain substantially the same atmosphere through the life
of the product. In some instances the film is chosen to
allow some permeation of gases, such as carbon dioxide or
oxygen, in order to maintain the preferred proportions of -
constituent gases. The particular gas compositions and the
closure films are selected to suit the type of food
product. They are generally characterized by a low oxygen
content, possibly together with a small content of carbon
dioxide. In some instances the film is chosen to allow a
small amount of permeation of gases, such as carbon dioxide
or oxygen, with the objective of maintaining the preferred
proportions of constituent gases within the package.
Within a sealed film envelope there exists a risk of undue
growth of anaerobic bacteria in the oxygen-depleted
atmosphere, leading to rapid spoilage of the packed
product. The invent: ?n guards against such spoilage by
substantially delaying the onset of any significant
anaerobic activity.
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~o87~99 6 ~
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The invention is especially suitable for MAP storage of
foodstuffs with a high surface area, for example
beansprouts, chopped salads produce, mushrooms and cabbage.
It is also well suited to packaging of fish, which tends to
be susceptible to anaerobic spoilage.
The presence of the ozone is also beneficial in ensuring
that the surfaces of the closure film and containers for the
foodstuff are maintained in a hygienlc condition.
The absence of moisture has also been found to be
advantageous when using atmospheres according to the
invention, especially for food storage. Accordingly it is
preferred in many embodiments of the invention that the
atmosphere contains substantially no water vapour (e.g. less
than 500 vpm). A significant catalytic effect is displayed
by water vapour in many chemical and biological procedures,
including spoilage mechanisms. In order to ensure a
hygienically clean product it has hitherto been customary to `
include a step of washing with chlorinated water, often
immediately prior to packaging. The washing however leaves
a residual moisture content which is difficult to remove and
which increases the rate of spoilage within the package.
Since the atmosphere of the invention has an inherent
sterilising effect it is possible in many instances to
employ a preparation procedure which avoids the use of
chlorinated water altogether but still gives a hygienically
clean product.
In general it is preferred that the substantially all of the
inert gas content of the inhibiting atmosphere is nitrogen
(possibly together with argon and other rare gases), but
there are some applications (especially in modified
atmosphere packaging) in which up to 50% of the nitrogen can
be replaced by another gas, such as carbon dioxide, which is
substantially inert under the conditions of use.
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~092/04276 2 0 8 7 5 9 9 PCT/GB91/01457
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Compared with the relative nitrogen:oxygen volumes in air
the atmospheres of the invention should be of depleted
oxygen content, specifically an oxygen content of not more
than 10% by volume. This volume is below the combustible
limit for almost all combustible materials, and thus
generally ensures the inert character Jf the atmosphere.
The complete absence of oxygen is not required and in many
instances, again particularly in modified atmosphere
packaging, must be avoided. Indeed, the complete absence
of oxygen encourages undue growth of anaerobic bacteria,
which are mostly harmful in their effects on the materials
to be treated, and would increase the volume of ozone
required to maintain the desired conditions. In general
the lower limit of oxygen content is 0.05% by volume. For
many foodstuffs, especially vegetables, the oxygen content
is preferably in the range 2.0 to 6.0% by volume.
Another key factor in delaying spoilage mechanisms is the
storage or treatment temperature. For foodstuffs this is
advantageously in the range l to 7C, often preferably about
2 to 4C. It is therefore desirable in many applications to
employ a combination of the atmospheres of the invention and
the preferred low temperatures for the material concerned.
Freezing is generally not required when employing
atmospheres according to the inv?ntion. Its absence makes
for a ~'fresh~ product with more consumer appeal, since there
is no risk of frost damage to the structure of the product,
and reduces the amount of energy required for the storage.
In terms of ozone content it is found that the desired
inhibiting effect is obtained with ozone levels in the
atmosphere of l to 500 vpm by volume. The generally
preferred limits for ozone content are 2 to 200 vpm, most
preferably 5 to lO0 vpm.
W092/04276 PCT/GB91/0145L~
~Q87~99 ~:
A further key factor in treatment of materials with a gas -
having a sterilising effect is the duration of contact
between the gas and the material. In general the required
period of treatment is inversely proportional to the
concentration of the sterilising;component. Storage
atmospheres can therefore employ low ozone concentrations,
typically l to lO vpm, whereas treatments involving short
contact times require ozone concentrations towards the upper
end of the above limits discussed above.
According to the invention the ozone is produced by passing
an oxygen-containing stream through an ozonizer. The
ozonizer applies an electrical charge which converts some of
the oxygen into ozone. Although it may be possible to
adjust the electrical input to the ozonizer, and thereby to
adjust the ozone content of the product stream, in general
it is preferred for the purposes of the invention to operate
the ozonizer at a fixed electrical setting and to adjust the
volume and content of the product stream by adjusting other
parameters such as the proportion of a feed stream to be
passed through the ozonizer or the oxygen content of the
~eed stream. According to the invention it is also
preferred to use an oxygen-depleted feed stream to the
ozonizer, i.e. a feed stream containing not more than 10% by
volume of oxygen. This gives the advantage of producing a
product stream which is already low in ozone content and
which can be further diluted if required. Hitherto the
objective in operating an ozonizer has been to maximise the
output of ozone and thus to employ an o~ygen-enriched
feedstream wherever possible.
Thus in one embodiment the invention provides a process for
inhibiting bacterial growth in a medium, which comprises
ozonizing a feed stream containing nitrogen and not more
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W092/04276 - ! PCT/GB91/014~7
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than 10% oxygen by volume to form a nitrogen-based
atmosphere containing, by volume, 1 to 500 vpm ozone, 0.05
to 10% oxygen and at least 90% of a nitrogen-containing
inert gas and contacting the medium with the said
nitrogen-based atmosphere .
In a further embodiment of the invention the gas mixture to
be ozonized is part of a larger gas stream which is first
divided into a portion to be ozonized and a portion which
by-passes the ozonizer. The two portions are recombined
downsteam of the ozonizer. By appropriate selection of the
respective volumes of portions it is possible to give
precise control of the ozone content in the combined stream
without adjustm~t of the ozonizer which can thus be
operated throug..Jut at its most effective setting.
Further control of the ozone output from a given ozonizer
can be provided by the proportion of oxygen in the feed
stream. Thus in a typical example with a gas feed volume o~
3.8 Nm3/h to the ozonizer an oxygen content of 2.0~ (the
balance being nitrogen) gave a product with 26ppm ozone, an
oxygen content of 3.5% gave a product with 31ppm ozone and
an oxygen content of 5.0% gave a product with 36ppm ozone
The nitrogen-containing g~ to be ozonized will generally
most conveniently be a ni.rogen-enr ched stream from an air
separator. For many duties, including modi-ied atmospheres
for food packaging, the air separator is mc conveniently a
membrane separator. This can produce a nitrogen stream with
an oxygen content up to 10% by volume which can be used
directly as the feed stream to the ozonizer. Alternatively
it is possible to ozonize the oxygen-rich stream from the
membrane separator and to dilute the thus-ozonized product
with inert gas to the required proportions.
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W092/04276 2 ~ 8 7 ~ 9 9 1 o PCT/GB91/014~ ~
A membrane-generated nitrogen stream has the further
advantage that it delivers a moisture-free feed stream to
the ozonizer, which thus provides directly the preferred
moisture-free atmosphere of the invention.
A further advantage of atmospheres according to the
invention when produced from a dry feed stream is that they
have little or no content of nitrogen oxides (NOX),
thereby assisting in meeting the environmental standards for
NOX levels in working atmospheres and in contact with
foodstuffs.
If desired, the feed stream to the ozonizer can be
pre-cooled so that the atmosphere is obtained directly at a
preferred low temperature.
For additional flexibility of production the system can
include one or both of a feed reservoir upstream of the
ozonizer and a product reservoir downstream of the ozonizer.
Several different versions of apparatus according to the
invention are illustrated in the accompanying drawings,
Figures l to 5, which show schematic views of several forms
of apparatus for introducing a small and controlled ozone
concentration into a nitrogen/oxygen mixture. The
illustrated versions are intended for use in a food
packaging line.
In each of the illustrated versions a compressor C (Cl, C2,
etc) supplies compressed air, via a water separator/filter
unit W to an air separator S, from which an
oxygen-containing stream is fed to an ozonizer G powered by
an electrical supply E.
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2087~99
~092/04276 PCT/GB91/01457
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In the version shown in Figure l the feed stream is passed
through a membrane drier Dl to ensure a dry air feed to a
membrane air separation unit Sl. Flow line Fl conveys
nitrogen-enriched non-permeate gas having a higher
nitrogen:oxygen ratio than is required for the packaging
gas. Flow line F2 conveys an oxyg~n-rich dry permeate gas
stream through the ozonizer Gl. The stream leaving the
ozonizer Gl has contents of oxygen and ozone higher than
required for the packaging gas and is therefore diluted by
recombination with the gas stream from line Fl. Adjustment
of the volume and ozone content of the packaging gas is
provided by a control valve Vl in line Fl, which adjusts the
flow rate through the separator Sl, and a control valve V2
which permits venting of part of the oxygen-rich stream in
line F2.
In the version shown in Figure 2, the oxygen-rich stream
from the separator S2 is simply vented through outlet T2.
Part of the nitrogen-enriched non-permeate gas is passed via
line F9 through the ozonizer G2 and remainder is passed via
line F3 directly to the product supply point P2. The
nitrogen:oxygen ratio in the non-permeate output of the
separator S2 is set at the desired final ratio by adjustment
of control valve V3, and the respective proprtions of the
said output passing through lines F3 and F4 are adjusted by
control valve V4. The figure 2 version offers a :
particularly simple configuration of process and apparatus ;
for achieving the desired atmosphere and thus represents â
particularly preferred version of process and apparatus
according to the invention.
The apparatus shown in Figures l and 2 is best suited to
applications in which the nitrogen/ozone product volume
matches the demand flowrate of the packaging line. In many
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W092t~276 12 PCT/~B91/014~
08r~ ~9 9
instances however, this match is not possi~le. Figures 3
and 4 therefore illustrate apparatus in which the ozone
generation is substantially separate from the main gas f
supply.
In the apparatus shown in both Figures 3 and g a
nitrogen-rich stream from an air separator S3 is fed to a
buffer reservoir Rl. This stream has a nitrogen:oxygen
ratio above that required for the packaging duty. The
Figure 3 version also has a separate, smaller, ozone
generating system with a membrane air separator S4 supplying
an oxygen-rich stream to the ozonizer G3. A membrane drier
D3 is provided upstream of the separator S4 and a recycle
line F6 returns nitrogen-rich gas to the drier D3. The
ozonized output is conveyed via line F8 for blending with -:~.
the main stream drawn from the reservoir Rl. The flow rate
through the product outlet point P3 is adjusted by control
valve V5 in line F7.
The Figure 4 version similarly has a separate, smaller,
ozone generating system with a membrane air separator S5
supplying an oxygen rich stream to the ozonizer G4 and again
has a membrane drier D4 upstream of the separator S5. In
this version however the nitrogen-rich non-permeate is
simply vented through control valve V8. The ozonized output
is conveyed via line FlO for blending with the main stream
drawn from the reservoir Rl. The flow through the product
outlet point P4 is adjusted by control valve V7 in line F9
and the ozone supply is adjusted by the control valve V8.
The Figure 3 version produces a dry and highly
oxygen-enriched feed gas for the ozonizer G3. The Figure 3
version produces dry but moderately oxygen-enriched feed gas
for the ozonizer G4.
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~ 92/04276 2 0 8 7 5 9 9 ` PCT/GB91tO1457
The version shown in Figure 5 has components C6, W, S4, V9
and R2 generally similar to those Figures 3 and 4 for
producing a nitrogen-rich gas stream. In the Figure 4
version however the supply to the ozonizer G5 is also drawn
from the reservoir R2, the proportion of stored gas passing
through the ozonizer being controlled by valve VlO. Like
those in Figures 3 and 4 this system is suitable for use
where the gas output does not match the pattern of the
packaging requirements.
Examples
The invention is further described in the following
non-limiting examples of the use of a nitrogen/oxygen/ozone
mixture in processing and packaging of fresh produce.
Example l.
A batch of newly-picked lettuces was stored overnight in a
refrigerator at 2C, removed from storage and chopped into
strands of typically 35 mm x 0.4 mm. The chopped lettuce
was then placed in polystyrene packaging trays, in batches
weighing 250 g per tray, and put into a packaging chamber
containing a controlled atmosphere. A packaging atmosphere
was produced in apparatus of the type illustrated in Figure
2 above and had a composition, by volume, of carbon dioxide
lO. %, oxygen 5.0%, ozone 25 vpm, ~ er vapour nil, balance
nitrogen. Each tray was then covere~ by a transparent film
of extruded polyvinylidene dichloride and the film was heat
sealed to the tray. Ten such sealed packages were placed in
a refrigerator maintained at 2C.
Each package was examined once daily for signs of spoilage
of the lettuce, in particular for browning of the leaves. -
No spoilage signs were observed until day 12 when eight of
the packages were found to have a brown discolouration at
some of the edges of the chopped leaves.
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~087 ~9 14 PCT/GB91tO14~
Control Example.
As a control a further ten packages were stored in an '`
ambient atmosphere at a temperature of 2C and also examined
once daily. Brown discolouration of the leaf edges was
detected in seven of the packages on the fifth day and in
the remaining three packages on the sixth day. The
treatment according to the example represented more than a
doubling of the shelf life of the product.
Example 2.
A batch of newly-picked lettuces was stored overnight in a
refrigerator at 2C, removed from storage and chopped into
pieces typically 50 mm x 50 mm after removal of damaged
outer leaves. The chopped pieces were exposed to a
pre-cooled atmosphere at 2 to ~C containing, by volume,
carbon.dioxide 10%, oxygen 5%, ozone 5 vpm, water vapour
nil, balance nitrogen, in a tunnel conveyor with the product
and gas running countercurrent for a treatment time of 5
minutes. The treated pieces of lettuce were then placed in
polystyrene packaging trays in batches weighing 50 gm per
tray and put into a packaging chamber containing a
controlled atmosphere of composition the same as used in
Example l. The trays were then each covered by the
polyvinylidene dichloride film, sealed and stored in the
same manner as described in Example l.
Each package was examined once daily for signs of spoilage
of the lettuce, in particular for browning of the leaves.
No spoilage signs were observed until day 17 when 6 of the
packages were found to have some browning at the edges of
some chopped leaves.
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