EP0918022A2

EP0918022A2 - Packaging of plant parts

Info

Publication number: EP0918022A2
Application number: EP98308953A
Authority: EP
Inventors: Cyril c/o Rexam Flexibles Viking Ltd. Twist; Barry Stuart Waters
Original assignee: Rexam Flexibles Viking Ltd
Current assignee: Rexam Flexibles Viking Ltd
Priority date: 1997-10-31
Filing date: 1998-11-02
Publication date: 1999-05-26
Also published as: GB9723100D0; EP0918022A3

Abstract

Plant parts, particularly potatoes, are packed in enclosures such as plastic bags designed so that the respiration of the plant parts generates a stable modified atmosphere which enhances the storage lifetime. Nylon film allows the escape of water vapour. Limited communication with the atmosphere, e.g. using non-gastight seals and/or micropores, enables oxygen levels to reach a dynamic equilibrium. In potatoes, oxygen level are desirably 2-14%, more preferably 3-8% by volume. This suppresses greening and sprouting. Excessively low oxygen levels can allow anaerobic organisms and processes, leading to spoilage.

Description

The present invention is concerned with methods and means for the packaging of plant parts, and with packages thereof. It is primarily concerned with edible parts, particularly fruit and vegetables (including mushrooms), especially potatoes. It may be applied to immature ('early' or 'new') potatoes, or to maincrop potatoes.
Fresh fruit and vegetables are living things, with natural processes occurring inside them. Respiration causes the uptake of oxygen and the production of carbon dioxide and water. There are also various spoilage processes. For example it is well known that potato tubers exposed to light produce chlorophyll and turn green. The green colour is generally unacceptable to consumers, and conditions which induce greening are also known to be associated with increased levels of glycoalkaloids which are known to be toxic and also cause bitter flavours. As a result of this phenomenon, food distributors, such as supermarkets, will generally mark the potato packaging with a "use by" date (or a "display until" date or a "best before" date) that is as little as 3 - 7 days after the potatoes have been packed. Even within this time, light induced greening may still cause considerable wastage both at point of sale and after purchase (in the home).
There has been a great deal of research into providing ways to extend the shelf life of potatoes. Much of this research has been directed towards producing a packaging that provides a barrier to the harmful light that induces greening. However this is undesirable from the consumer's point of view, because such packaging is generally more or less opaque which makes it difficulty to see the contents.
Although greening is the major issue to be addressed, the storage life of potatoes can also be affected by fungal or bacterial rots as well as the premature sprouting of potatoes reaching the end of their dormancy period. There are particular problems with relatively large weights of potatoes (eg 1 kg or more),due to the large amounts of water vapour that may be produced, and the relatively high respiratory gas exchange leading to a large uptake of oxygen and production of carbon dioxide. The purchaser will probably expect a substantial weight of potatoes to keep for at least a week and perhaps two or more weeks even though they may be stored at room temperature and in the light. In contrast, a pack of prepared salad may weigh of the order of 100g so that its capacity for generating respiration products is small. Furthermore it will generally be stored in a refrigerator, ie at a reduced temperature (and in the dark), and be consumed within a few days of purchase.
We have found a way to reduce the rate at which potatoes turn green. Broadly we have found that potatoes turn green at a much slower rate if they are stored in an atmosphere having a reduced oxygen content, preferably under 14% by volume. To avoid undesirable anaerobic processes the oxygen level should not fall below 2% by volume. Thus the preferred range of composition is:

OXYGEN - 2 vol.% to 14 vol.%

CO₂ - 5 vol.& to 25 vol.%

OTHER GASES - 73 vol.% to 80 vol.%
More preferably the atmosphere has substantially the following composition:

OXYGEN - 3 vol.% to 8 vol.%

CO₂ - 3 vol.& to 18 vol.%

OTHER GASES - 79 vol.% to 84 vol.%
The other gases will generally be predominantly nitrogen, and usually some water vapour. It is important to avoid excessively low oxygen levels, as this can allow the occurrence of processes which may lead to increased susceptibility to microbial spoilage, tainting of the product, and potential food safety hazards.
In the above-specified composition ranges, it is probably the oxygen content that is important, with the carbon dioxide being probably of lesser, or no, significance.
We have also found that it is highly desirable for the package atmosphere to have a relative humidity which is not too high and for there to be no condensation. This may then operate in synergy with the gaseous atmosphere to enhance pack life. Such a package can help prevent greening whilst at the same time minimising internal condensation and its negative effect on other spoilage mechanisms such as sprouting and wet and other rots.
If we were concerned with the packaging of non-living or 'stable' materials, it would be a simple matter to provide a sealed package containing a chosen atmosphere. But in a sealed package of living plant material such as potatoes, for example, respiration would cause the oxygen content to fall and the carbon dioxide content to rise. Moisture levels external to the potatoes would also rise, owing both to respiration and simple transpiration/evaporation, typically leading to condensation. The conditions of wetness and low oxygen would put the potatoes at risk of a variety of harmful processes.
Conventionally, the packaging of potatoes in moderate amounts (e.g. 1-10 kg) uses polyethylene bags. These are cheap and strong. However the material allows little diffusion of water vapour. Therefore such bags commonly have punched apertures, e.g. of 6mm diameter. Nevertheless it is common to see condensation inside such bags, and potatoes in such bags are prone to the ills mentioned above - greening, wet rot and so on.
According to the present invention we provide a package of plant parts (preferably potatoes) comprising the plant parts and packaging therefor, said packaging defining an enclosure for said plant parts and comprising plastic film material having a moisture vapour-transmission rate (MVTR)in excess of 70 mg/day.m² (and preferably in excess of 100 mg/day.m²; said enclosure being provided with restricted atmosphere communication means such that if said enclosure initially contains atmospheric air, then respiration of said plant parts causes a depletion of oxygen therein and this is partially counteracted by ingress of oxygen through said communication means, leading to a modified atmosphere with an oxygen content which remains in the range 2-14% by volume for at least 7 days.
All MVTR values are measured at 38° and 100% RH. Generally said plastic film material delimits at least 15%, preferably at least 25% and often essentially 100% of the enclosure so that the enclosure as a whole has good moisture transmission properties.
The conditions produced and maintained within such a package are found to suppress greening and wet rot problem for potatoes so stored. Sprouting was also suppressed. Furthermore there is no substantial formation of glycoalkaloids during storage times of 10 days or more. Other types of plant parts may also benefit from this form of packaging, with a self-generated modified atmosphere. For example preliminary studies indicates its utility for mushrooms.
It is to be appreciated that such packages can be implemented by a number of means.
The packaging itself will usually comprise a bag made of a film material, or a carton or tray usually of a rigid material at least partially enclosed by a film material. We have found that it is possible to provide packaging such that after potatoes have been placed in the packaging, an equilibrated modified atmosphere with composition in the required range will develop automatically over a period of time (usually 1 - 3days).
This requires that the packaging allows the ingress of oxygen and can transmit water vapour to the outside.
We find that suitable water vapour transmission can be effected by transmission through the film material if it has a moisture vapour transmission rate of at least 100 and more preferably at least 200g/day.m² (measured at 38° and 100% RH), eg 100-1000 g/day.m², preferably 200-800 g/day.m².
The film may have an oxygen transmission rate of 8-260 cm³/day.m² (at 23°C and 0%RH) though this is less critical since additional means for oxygen ingress may also be provided.
Carbon dioxide transmission rates of 37-780 cm²/day.m² (at 23°C and 0%RH) may be suitable.
It will be appreciated that there will be a need to vary the transmission properties of the film and the exact composition of the atmosphere, depending upon a number of variables including the potato variety, the pack weight, the pack construction, the film thickness and holding temperature. In some cases account will need to be taken of permeability through pack features such as the closure mechanism, and in other cases it may be advantageous to increase permeability by inducing micropores into the pack by laser or other means, so as to facilitate gaseous exchange. A conventional form of closure involves a loop of tape adhered around a neck region of a bag. This does not produce a hermetic seal. We have found that the use of an automatic bagging machine produces seals with a consistent degree of "leakiness". Of course this may vary with different grades or compositions of film material. Suitable machines are produced by NEWTEC A/S, e.g. Polybagging machine model G45 or Automatic Computerised Polybagger model VB40CA
Potatoes are usually packed in portions of 0.5 - 10 kg and apart from the transmission properties the packaging film should have sufficient physical strength to contain and support weights of this magnitude. This will be particularly important in areas where the pack construction requires that it is sealed to itself or to other films or materials.
To meet all of these requirements the film is preferably a film comprising polyamide (nylon), and suitably one which has been extruded by the blown or cast film process, and which may be mono or bi-axially oriented. The polyamide can be a homopolymer such as nylon 6 or co-polymer such as nylon 6.66, and can be blended, laminated, or coextruded with other suitable polymers which could include other polyamides, EVA, ionomer (e.g. Surlyn (trademark)), polyester, polycarbonate, styrene/butadiene resins, and polyolefin polymers including copolymers and metallocene resins. Generally blends are preferred to laminates or coextrusions for maintaining good moisture vapour transmission properties. Blends may contain up to 40% by weight of the other polymer(s), preferably up to 30%, more preferably up to 25%.
Whilst it is preferred that the film is transparent and glossy in order to facilitate the viewing of the package contents, this is not essential. In circumstances where transparency is not required, any of these films can be laminated to paper and still provide good anti-greening performance.
Furthermore, it has been shown by analysis that extending greening-free shelf life by using this invention does not result in increased glycoalkaloid formation by the tubers. Levels remain much lower than the accepted threshold values for causing bitter flavours.
The invention will now be illustrated with reference to some experiments. In these, filling and (where applicable) taping of bags was carried out using a Newtec bagging machine.

Experiment 1

Estima potatoes (a maincrop variety which is prone to greening) were packed in 2.5 kg portions in bags formed from 25µm cast nylon. Two bags (H1 and H2) were heat sealed and three bags (T1 - T3) had their necks closed by taping. A further 2.5kg portion was packed in a conventional LLDPE bag with 6mm apertures as control (C). The bags were stored under illumination. Oxygen and carbon dioxide levels inside the nylon bags were measured after 4 and 7 days. The results are shown in Table 1.

atmospheric modification
Bag	CO₂(4d)	O₂ (4d)	CO₂ (7d)	O₂ (7d)
T1	12.1	7.6	12.6	9.9
T2	10.8	9.8	11.0	11.9
T3	8.5	13.2	9.4	13.9
mean T	10.5	10.2	11.0	11.9
H1	14.5	4.3	18.1	4.6
H2	18.8	1.1	25.9	1.0
Mean H	16.7	2.7	2.2	2.8
All figures are percentages by volume

Since atmospheric air contains 21% oxygen and 0.03% carbon dioxide, it is clear that all of the nylon bags developed a substantially modified atmosphere ("MA"). The taped bags in particular had reached fairly constant MA compositions by 4 days, which had changed little by 7 days. The sealed bags showed much lower oxygen levels, with one bag (H2) developing undesirably `anaerobic' conditions.
The contents of all bags were inspected for greening after 4, 7 and 10 days. The results are presented in Table 2.

% of greening

Bag Type 4d 7d 10d Comments

T 3 17 20 all slight

H 0 0 0

C 30 60 90 Some serious greening

Experiment 2

In a similar experiment, potatoes were analysed for glycoalkaloid contents, in peel and in whole tubers. The starting values (t=0) were 64 and 9 mg/kg of peel and of whole tubers respectively. In the control (C) potatoes these values rose to 95 and 10 at t = 11 days. The values found for packs embodying the invention ranged from 70 and 6 mg to 127 and 15 mg. The "127 and 15" bag had an oxygen content of 8.3% at 6 days. At 11 days 70% of its tubers showed some slight greening. NB 15 mg/kg of whole tubers is a very low value: values of 110 mg/kg are needed for perceptible bitterness, and toxicity requires in excess of 200 mg/kg.

Experiment 3

This experiment used potatoes which had passed their dormancy period and hence were prone to sprouting.
2.5kg portions of Estima potatoes were packed into nylon bags (P) with 6 laser perforations (ca 300µm diameter) per bag, or unperforated (U). A control (C) portion was packed in a conventional LLDPE bag with 6mm apertures. Bags were closed by taping, and then stored for 1 day in darkness, 5 days under 1000 lux fluorescent lighting for 16 hours/day, and 5 days with 8 hours/day subdued lighting. This three-part regime is intended to imitate unusually hostile conditions of distribution, supermarket storage and home storage. After 3 days the average oxygen contents (by volume) were 4.3% (U) and 12.2% (P). Tubers were examined for sprouting after 6 and 11 days. The results are shown in Table 3.

Sprouting

Bag 6 days 11 days

U none some small sprouts in 2/10 bags

P some sprouts <5mm sprouts about 10mm

C about 10mm copious sprouting

This shows that the MA conditions which inhibit greening also inhibit sprouting.

Experiment 4

This experiment used new potatoes (Jersey Royal). Such immature tubers, lacking a `set' skin, are particularly prone to spoilage by soft-rotting fungi and bacteria.
Bags and storage conditions were as in Experiment 3.
After 4 days the U bags contained 3.9 - 7.1% oxygen (mean: 5.8%) while the P bags contained 9.7% - 11.8% oxygen (mean: 10.5%).
Greening : the control bag showed severe greening after 6 days. The U bags showed no greening after 10 days. the P bags showed slight greening of 15% of tubers after 6 days, rising to 50% with slight greening after 10 days.
Rotting : The control and P bags showed no rotting after 6 days, and some rotting (0-6% of contents of each bag) after 10 days. The U bags included 2 with some rotting after 6 days, and all 4 bags had 11-35% rotted tubers after 10 days.
This shows that there is a delicate balance to be struck for immature potato tubers without set skins. This single experiment did demonstrate that much improved storage for about 6 days can be achieved using bags (P) that produce and maintain a slightly modified atmosphere.

Experiment 5

There can be problems in processing nylon films, e.g. producing bags with side welds. This experiment was to establish whether a blend of nylon and another polymer is still suitable for potato packing.
King Edward potatoes, which are very susceptible to greening, were packed in 2.5kg portions in bags made of 20µm Nylon 6/Nylon 6.66 blend("NN") (water vapour transmission 680g/m².day); and in bags made from a 25µm film of a blend of 80% Nylon and 20% of another polymer(by weight) ("XN") (wvtr:400g/m².day). A control sample (C) was packed in a punch-perforated LLDPE bag, as before. Bags were taped and stored as in Experiment 3.
Atmospheres were analysed after 4 days. They were similar in the NN and XN bags, with oxygen level means of 10-12%.
Throughout the experiment, the control bag showed misting whereas the NN and XN bags showed none.
The control bag showed greening after 4-5 days, the NN and SN bags showed none after 7 days. No evidence of soft rotting was seen.

Claims

A package of plant parts comprising plant parts and packaging therefor, said packaging defining an enclosure for said plant parts and comprising plastic film material having a moisture vapour transmission rate in excess of 70 mg/day.m²; said enclosure being provided with restricted atmospheric communication means such that if said enclosure initially contains atmospheric air, then respiration of said plant parts causes a depletion of oxygen therein and this is partially counteracted by ingress of oxygen through said communication means, leading to a modified atmosphere with an oxygen content which remains in the range 2-14% by volume for at least 7 days.
A package according to claim 1 wherein said plastic film is composed of a nylon or blend of nylons, optionally blended with up to 25% by weight of another plastic film material.
A package according to claim 1 or claim 2 wherein said restricted atmosphere communication means comprises one or more micropores in the film material.
A package according to any preceding claim wherein said enclosure is a bag having a taped neck and wherein said taped neck is not hermetically closed and thus provides at least in part said communication means.
A package according to any preceding claim wherein said oxygen content range is 3-8% by volume.
A package according to claim 5 wherein said modified atmosphere contains, by volume, 3-8% oxygen, 8-18% carbon dioxide and 79-84% of other gases.
A package according to any preceding claim wherein said plant parts are potatoes.
A package according to any preceding claim wherein said plant parts weigh 1-10 kg.
A package according to claim 8 wherein said enclosure is a bag formed of said plastic film material which can be picked up so that the film material supports the weight of the plant parts.
A package according to any preceding claim wherein said film material is clear and untinted, and the plant parts can be viewed through it.
A bag for the storage of 1-10 kg of potatoes, the bag when filled and closed constituting a package according to any preceding claim the bag being composed of a plastic film as defined in claim 2, and being adapted to be closed by taping of a neck region.
A method of storing plant parts comprising enclosing them in an enclosure to produce a package according to any of claims 1 - 10.
A method according to claim 12 wherein said enclosure is a bag into which the plant parts are placed, whereupon the bag is closed by taping.
A method according to claim 12 or claim 13 including a step of producing micropores in the film material.