Food Packaging Technology 3(2+1)

Lesson- 15 Vacuum, Gas and Shrink packaging

15.1. Vacuum Packaging:

Vacuum packaging is the simplest and common means of modifying the internal gaseous atmosphere in a pack. Vacuum packaging is a form of modified atmosphere packaging in which food is placed in a gas-impermeable package, most of the oxygen around the food is removed, and the package is hermetically sealed.

Vacuum packaging inhibits the growth of aerobic microorganism including food spoilage bacteria and molds that would normally deteriorate the quality of products. The removal of oxygen can also prevent degradation or oxidative processes that limit product shelf-life, for example, oxidative rancidity in fats and oils, or color deterioration in raw meats. An added advantage for frozen foods is that the sealing of the food within a skin-tight package prevents dehydration and evaporative water loss from the surface of the food, and can minimize the effects of “freezer burn” (excessive dehydration loss from the product surface) and post-thaw exudate (drip loss) that often limit the quality shelf-life of frozen foods. Reduced oxygen packaging (ROP), which provides an environment that contains little or no oxygen, offers particular advantages but also raises many microbiological concerns.  environment that contains little or no oxygen favor the growth of anaerobic microorganism such as facultative patho­genic bacteria (e.g., Listeria, Salmonella, Escherichia coli, Yersinia, and Staphylococcus) and anaerobic bacteria such as nonproteolytic (psychrotropic) Clostridium botulinum type E that can grow at chill temperature (>3.3°C) [11]. Freezing is an excellent.

 

Table 15.1: Self life of different food commodities in different storage conditions

S.N.	Food	Storage condition	Normal shelf life	Vacuum shelf life
1.	Large cuts of meat: beef, poultry, lamb and pork	Freezer	6 months	2-3 years
2.	Fish	Freezer	6 months	2 years
3.	Coffee beans	Room temperature	4 weeks	16 months
4.	Coffee beans	Freezer	6-9 months	2-3 years
5.	Berries: strawberries, raspberries, blackberries	Refrigerator	1-3 days	1 week
6.	Berries: cranberries, huckleberries, blueberries	Refrigerator	3-6 days	2 weeks
7.	Cheese	Refrigerator	1-2 weeks	4-8 months
8.	Cookies, crackers	Room temperature (periodically opening)	1-2 weeks	3-6 weeks
9.	Flour, sugar, rice	Room temperature	6 months	1-2 years
10.	Lettuce	Refrigerator	3-6 days	2 weeks
11.	Nuts	Room temperature	6 months	2 years
12.	Oils with no preservatives, like safflower, canola, corn oil	Room temperature	5-6 months	1-1.5 years
13.	Wine	Refrigerator	1-3 weeks	2-4 months

Ref: Table adapted by Tilia Inc. from Dr. G.K.York, Dept. of Food Science & Tech, U of California, Davis.

 means of long-term food preservation, but it is also an excellent means of preserving microorganisms already present in the food at the outset.

As we can see from table 5.1 vacuum packaging technology is well suited for frozen food products, owing to which it can enhance the shelf life and overall quality of food for a longer period of time. It is now widely used in many kinds of foods, particularly in ready meals. A successful vacuum packaging combined with freezing technology is influenced by the nature of food, the nature of package, and the freezing process. The vacuum packaging requires a high-barrier material to keep almost no oxygen for food products inside package. Vacuum packaging consists of multilayer films with a heat-seal layer and a high-barrier layer. The future materials with high-barrier properties can be developed by using nanotechnology. The nanocomposites film with small amount of organoclay (2%–5%) and nanocoating by plasma technology can enhance the barrier properties and still maintain the transparency of material, and is cost effective.

15.2. Gas packaging:

Gas packaging is a form of packaging involving the removal of air from the pack and its replacement with a single gas or mixture of gasses. Gas packaging can be achieved in two fundamental ways. These are the replacement of air with a gas or mixture mechanically or by generating the atmosphere within the package either passively as in the case of fruits or vegetables or actively by using atmosphere modifiers such as oxygen absorbents. There are two techniques for mechanical air replacement in a package

  15.2.1 Gas flushing

This method employs a continuous gas stream that flushes air out from the package prior to sealing. This method is less effective at flushing air out of the pack, and this result in residual oxygen levels of 2–5%. Gas flushing is therefore not suited for oxygen-sensitive food products. Generally, gas flushing machines have a simple and rapid operation and therefore a high packing rate. The gas flush process is usually performed on a form-fill-seal machine. Gas is injected into a package to replace the air. This dilutes the air in the head space surrounding the food product. When most of the air has been replaced the package is sealed. There is a limit to the efficiency of the system since replacement of the air in the package is accomplished by dilution. Typical residual oxygen levels in gas flush technique is not suitable for packaging very oxygen sensitive food. The great advantage of the gas flush process is speed. It is a continuous operation.

15.2.2 Compensated vacuum

This method uses a two-stage process:

15.2.2.1 The evacuation stage

A vacuum is pulled on the pack to remove air. Generally, it is not possible to achieve a full vacuum, since reduced pressures will result in water to boil, at which point the vacuum cannot be improved.

15.2.2.2 Gas flushing stage

The pack is flushed with the modified gas mix. The evacuation of air from the pack results in lower residual oxygen levels than that achieved by gas flushing, and therefore this method is better suited for packing oxygen-sensitive products. The two-stage process employed by the compensated vacuum method results in a lower packaging rate than that possible with gas flushing.

15.3. Shrink Packaging

Shrink film is used as the basic material and heat forms an important part of the operation in shrink packaging or commonly known as shrink wrapping. Shrink wrapping is done in following four stages:                                

• wrapping (sleeve wrapping or over-wrapping)
• sealing (necessary only for over-wrapping)
• shrinking (with application of hot air), and
• cooling

Shrink wrapping is mainly used for unitization, but some time same is also being used as primary packaging system. This system is also used for bulk packaging as well as retail packaging of foods. Shrink wrapping is now being used for some major applications like sleeving for labelling on various containers or sealing, besides wrapping. But material selection plays an important role, because without the correct material, proper shrink wrapping may not be possible.

15.3.1 Plastics used for Shrink Wrapping

Shrink wrapping can be quite complex in their structure. Most of the packaging films that are used for shrink wrapping are from the polyolefin range. These are materials produced from oil based chemicals by what is called a polymerization process, which basically means getting the right molecules and atoms to club together in a way that is required or desirable for a particular application.

The most commonly used plastic materials for shrink packaging  are

15.3.1.1 Polypropylene

Polypropylene is comparatively less used in shrink and stretch wrapping, because it is slightly harder than the other commonly used materials. It has a higher melting temperature and is less stable when shrinking. However, many over-wrapping

 machines use polypropylene and some can be put through a shrink tunnel to give a slight tightening effect.

 15.3.1.2 Poly vinyl chloride

PVC is a dense material. As most polymers are sold by weight and there has been ecological pressure in Europe and America against its use, sometime use of PVC is restricted. However, it is still considered to be a common material in India, when clarity is an important selection criterion, particularly for consumer packaging.

 15.3.1.3 Polyethylene

Polyethylene is the most commonly used material for shrink and stretch wrapping because it is relatively cheap and can be produced in a range of different densities and modified with additives to perform many functions. The vast majority of shrink film is LDPE and some of the more sophisticated films have blends of LLDPE as well. Sometimes a little quantity of HDPE material is also added. For selection of plastic material, besides type of plastics, the yield of the film is also important to be considered from the economy point of view.

Table 15.2: Performance Comparison of Shrink Films

S.N.	Film type	Advantage	Possible problems
1.	Polyethylene (low density)	1) Strong heat seals (2) Low temperature     shrinks. 3) Medium shrink force for broad application 4) Lowest cost	1) Narrow shrink temperature range. 2) Low stiffness 3) Poor optical property 4) Sealing wire contamination
2.	Polypropylene	1) Good optical   appearance 2) High stiffness 3) High shrink force 4) No heat sealing fumes 5) Good durability	1) High shrink temperature 2) High shrink force, not suitable for delicate or fragile product. 3) Brittle seals 4) High sealing temperature
3.	Co-polymers	1) Strong heat seals 2) Good optical appearance 3) High shrink force 4) No heat sealing   vapours	1) High shrink force, not suitable for fragile products 2) Higher shrink temperature 3) Higher heat seal temperature 4) Lower film slip-may give machine problems
4.	Poly Vinyl Chloride	1) Lowest shrink temperature 2) Wide shrink temperature range 3) Excellent optical appearance 4) Controlled stiffness by plasticizer content control 5) Lowest shrink force for wrapping fragile products	1) Weakest heat seals 2) Least durable after plasticizer loss 3) Toxic and corrosive gas emission  from heat sealing, therefore  good ventilation required 4) Durability problem at low      temperature 5) Low shrink force inhibits use as a multiple –unit bundling film 6) Low film slip causes machine wrapping difficulties
5.	Multilayer Co-extrusion	1) Excellent optical appearance 2) Good machineability 3) Low shrink temperature	1) In co-extruded films, one ply co-extrusion compensates for the deficiencies of the other. As a result, they are superior films with no significant performance shortcomings. 2) The wide variability in layer composition and number of layer makes performance analysis difficult.

 

For shrink film, the next important factors considered are the shrinkage and the slip of the film. Shrinkage means the percentage shrink in the machine direction, i.e. along the reel of the film, and also in the transverse direction i.e. across the reel of the film. The slip can be of different types – high, medium or low depending upon how much slippery property in the film is required from the operational point of view. Usually, low slip is desirable. For shrink wrapping small packs at high speed, particularly for consumer products or display purposes, PVC or specially modified Polyolefin may be used. The “high shrink films” are crystal clear but generally expensive.