This chapter examines in some detail the different functions of packaging. Emphasis is placed on understanding the properties of the product and the hazards it will face in the supply chain, including the demands of the consumer. There are different and sometimes conflicting requirements and expectations at each stage of the life of the product, and these must be understood before appropriate packaging solutions can be sought.
It is difficult to imagine life without packaging, and yet in many cases the consumer barely notices it is there, let alone thinks about what functions it is performing. The delivery of basic commodities such as coffee, tea, sugar and jam from factory to consumer relies on efficient use of packaging, and highly perishable foods such as fresh meat and fish, ready-prepared meals and salads would not be so readily available without the packaging which plays a key role in preventing spoilage, thus extending shelf life and minimising wastage. Electronic equipment such as televisions, personal computers and DVD players, along with domestic appliances such as irons, kettles, toasters and microwave ovens all rely on packaging to protect them against the potential damage encountered in the distribution chain. The growth in Internet shopping brings a need for packaging to be robust enough to survive the mail order delivery system, and to readily identify the product throughout the handling processes.
But packaging has to fulfil more than these functions of containment, preservation, protection and identification. It also influences the convenience in use of a product, and, just as importantly, it is instrumental in selling the product, by attracting the consumer.
Each of these functions will be discussed in the rest of this chapter. many of the points being made will apply not only to consumer goods, but also to all other ‘products’ such as raw materials and ingredients for food, pharmaceuticals and cosmetics, components for the automotive and aeronautical industries, and packaging materials being shipped to their point of use. The functions of containment, protection and identification are just as important for the safe and efficient movement of such materials, and examples will be given where appropriate.
In all cases, to understand the functions of the pack and to ensure that they are adequately met, it is essential to define the product in terms of its nature, critical properties and value. This should always be the starting point; packaging does not have a ‘life’ of its own and without a product it has no reason to exist. The packaging designer/specifier needs to work closely alongside the product developer to gain a full understanding of the product and what can cause it to deteriorate to the point of being unacceptable. Only then can appropriate packaging be specified and sourced.
Properly designed, constructed and sealed packs provide complete containment for the contents, preventing unsightly or dangerous leakage, or loss of parts. This containment must be assured throughout the expected life of the product, including the numerous handling stages from the end of the packaging line to the final consumer use. Containment also means keeping a number of different or the same items packed together, and this applies to primary, secondary and tertiary packs. Examples include:
If leakage or loss occurs, the consequences can vary from a minor inconvenience to a major and potentially catastrophic incident. A leaking bottle of bath foam is likely to result in a failure of the selling function, as no one wants the leaking pack on the shelf, and it may also have affected neighbouring packs so that they are also unsaleable. It may also fail to inform, if the type matter has become obliterated, in which case it may be illegal, and this also applies if it is underweight due to loss of contents. If, instead of a relatively innocuous bath product, the product is an aggressive or corrosive liquid, such as some household and industrial cleaning chemicals, the implications of leakage can be much more serious. If a vital part is lost, for example an instruction leaflet, the product may be rendered unusable.
A key step in evaluating the risk of leakage is to identify the factors which can affect the efficient containment of the product. Consider first the potential leakage points of a pack, including not just the obvious opening points, e.g. a screw threaded cap, but all other points where the pack could fail. Then consider how and why failure could occur at each of these points. Some examples to stimulate thought are given in Table 3.1.
|Pack type||Potential failure points/ mode of failure||Typical possible causes|
|Cartons||Glue seams split||Wrong adhesive for board and conditions of use; poor control of adhesive application conditions|
|Tuck-in flaps work loose, tear||Wrong board weight for the weight of the product; poor cutting and creasing of the carton|
|Bottles/caps||Leakage at neck, misalignment of cap||Dimensional inaccuracies in bottle and/or cap finish; wrong cap application force; wrong wadding material|
|Leakage at mould part line and/or injection point (plastic)||Poor control of moulding conditions|
|Sachets||Leakage in seal areas||Wrong sealing layer; poor control of sealing conditions; product in seal area|
|Leakage in body of sachet||Puncture by product or by external means|
|Tubes||Leakage at neck, misalignment of cap||Dimensional inaccuracies in tube and/or cap finish; wrong cap application force; wrong dimension of orifice|
|Leakage at base of tube||Wrong sealing layer; poor control of sealing conditions; product in seal area|
Once the potential leakage-causing factors are identified, it is the role of the packaging technologist to ensure that the required performance characteristics are designed-in at the development stage, carefully specified on component and process specifications, and that checks are in place to ensure specifications are followed. Leak testing should be carried out during development, making sure that the likely conditions of use are taken into account. For example, a product intended to be carried around in the handbag requires a different test protocol from one which will be stored in one place for most of its useful life. Leak testing can be carried out by a variety of methods, including simple inversion of packs or by applying pressure to seals, in both cases observing the effects over time.
Protection means the prevention/reduction of physical damage to the product, during all stages of its life. This includes manufacture and packaging operations, storage and handling in warehouses, transport to the merchant, distributor or store for sale, display, and moving to the final usage point. It also includes storage and use of the product, e.g. in a kitchen, garage or garden, and any other handling operations which the final user may be reasonably expected to carry out on the product.
Damage can occur at any of these handling stages, although most physical damage happens in the warehousing and distribution environment, due to dropping (from pallets, and during order picking and transit), jolting, vibration (in vehicles), compression (when stacked in warehouses) or puncturing (often due to use of poor quality pallets). Damage can also result from environmental factors such as dust, dirt, birds, insects and rodents. See Table 3.2 for a list of typical hazards, their causes and effects.
|Shock||Falls from conveyors, pallets, vehicles, possibly due to poor stacking; shunts due to irregular movement along conveyors; drops due to manual handling; impacts in transit due to driving over poor road surfaces||Breakage; deformation|
|Vibration||Vibration occurs naturally in all types of transport. In road transport the effects are enhanced over the rear axle of the vehicle, and by any imbalance in the load. Irregular road surfaces also increase vibration||Breakage; scuffing; product separation and/ or settlement; loosening of screw caps; garments falling from hangers|
|Compression – static||Stacking in storage, made worse by damp conditions||Breakage; crushing; load collapse|
|Compression – dynamic||Clamp truck pressure; severe vibration during transport||Breakage, crushing, stack resonance|
|Puncture||Poor quality pallets, bad handling practices||Breakage; product spoilage; load collapse|
|Changes in relative humidity||Loads left outside; goods stored in damp warehouses, or where climatic conditions are not controlled; goods shipped via and to different climates||Product spoilage, e.g. corrosion; packaging failure, e.g. damp corrugated board cases|
|Changes in temperature||As above||Product spoilage; drying out of paper/board materials;|
|Exposure to light||Retail display||Fading of product and/or pack; product spoilage, e.g. rancidity|
|Insects, rodents, birds, dust, dirt||Goods stored in warehouses not cleaned or treated for pest control, or where doors/windows are left open or badly fitting||Product spoilage due to poor hygiene; contamination of product and pack|
|Pilferage and tampering||Goods exposed to uncontrolled personnel access; display on shelf||Loss of products; damaged packs and products; contamination; counterfeit products|
Protection against physical damage is important for all products, not just the obviously fragile such as glass and ceramics, or electronic components, but also for plastics, textiles, paper and board items, etc., whilst the unacceptability of damaged product may be obvious, aesthetic damage to the packaging, such as scuffing of labels/cartons, or scratches on plastic tubs, may also be causes of rejection.
To understand the level of protection required for a product, a simple equation to bear in mind is that the inherent ruggedness of the product plus the protection provided by the pack must be equal to the likely hazards encountered in the journey from factory to consumer and ultimate use of the product (Fiedler, 1995). The key steps to follow to decide what type of packaging will provide the product with the appropriate level of protection are:
Only when armed with the relevant information from each of these three steps will it be possible to propose possible packaging materials and pack formats which will match the defined characteristics. These proposals must then be tested in conditions which simulate ‘real life’ before packaging specifications can be agreed and the proposals implemented and monitored.
When developing solutions, remember it is the combined performance of all levels of packaging which is important. There will potentially be several different solutions, perhaps using a relatively weak primary pack which relies on its secondary pack for physical strength. Or it may be possible to design a sturdy primary pack which dispenses with the need for any secondary packaging, and is stacked directly onto a pallet. Therefore it is essential to consider not only the primary packaging when carrying out a development programme, but to combine this with an investigation of possible secondary/tertiary options. This combined approach will almost certainly result in the most cost effective solution.
As already stated, understanding the product is the key to any packaging development process and product development or research departments are likely to be the source of this information. The data may have to be developed from ‘scratch’ by carrying out actual tests, or there may be similar products with proven performance which can be used for guidance.
The aim is to define the product type (physical form) and how ‘rugged’ it is in terms of what conditions it can withstand before damage occurs, for example what level of shock, what vibration frequency, what top load, what range of temperature and humidity?
The product value will also have to be considered as it will decide the level of protection which can be afforded, and it will have a bearing on its attraction to the pilferer and/or the counterfeiter. Also at this point, any applicable legislation must be noted, e.g. legislation concerning hazardous goods.
How the product is destined to be stored, moved, displayed and sold needs to be identified and clearly understood, as each of these stages has its own inherent hazards. This includes all operations, both internally within the premises and externally when the goods are outside of the direct control of the producer. One of the results of globalisation of manufacturing amongst large producers is that the distribution chain has become more complex, with multiple handling and exposure to a range of different climatic conditions. The more control which can be exercised, the more the risk of damage can be reduced and the less protection will be required from the packaging. Conversely, if control is poor, packaging requirements – and costs – will be high.
The common hazards will now be considered in more detail. In this section, refer again to Table 3.2 for possible causes and effects of each hazard, and also to Table 3.3 for ways of minimising the effects of each hazard. Examples will be given of possible protective packaging materials, but it is not the intention to cover materials in detail in this chapter, as they are covered in other chapters in this text.
Shock is defined as an impact brought about by a sudden and substantial change in velocity, and is usually encountered when an item lands on a stationary surface such as a floor, or it can happen during horizontal impact such as when items knock against each other on a conveyor. Key points known about the shock hazard are:
• Not surprisingly, the more manual handling there is, for example in a mail order environment, the greater the possibility of shock damage occurring. Most of these drops will be from a height of around 1 metre, i.e. hand height.
• As the weight of a pack increases, and personal injury becomes an important consideration, the human being will handle the pack carefully and will eventually resort to mechanical handling. Palletised loads, moved by forklift truck are much less likely to be damaged than primary or secondary packs.
• Damage to the packaging around a product is most likely when the pack is dropped on a corner or edge, but damage to the contents is greatest (and unseen until the pack is opened) when the pack is dropped flat onto one of its faces.
The most effective way of minimising the effect of the shock hazard is to cushion the impact. Figure 3.1 shows the shock pulses for a cushioned and an uncushioned drop. In the cushioned drop, the cushioning attenuates (weakens) the initial shock at the pack surface so that the product’s response takes place over a longer period of time. The areas under the curves represent energy.
Some options for cushioning materials are listed in Table 3.3. The more resilient the material, the better its cushioning properties, so polymeric foams are more effective than corrugated board. The choice must always be made by going back to that fundamental point: what level of protection does the product need? Other factors to be considered are availability, possible contamination, surface abrasion, environmental impact and, of course, total cost.
Manufacturers of specialist cushioning materials produce dynamic cushioning curves which allow the user to calculate the thickness of material needed. This is important in the packaging of electronic goods and household appliances and further information can be found in the sources of information and advice at the end of this chapter.
Vibration refers to oscillation or movement about a fixed point. The distance moved is the amplitude and the number of oscillations per second is the frequency, measured in hertz (Hz). It is impossible to avoid vibration, as it is associated with all modes of transport. Frequencies below 30 Hz are most commonly encountered in road transport, and are of most concern. The combined effects of different frequencies can be considerable, as shown in Fig. 3.2. The greatest effect is typically experienced over the rear wheels.
Vibration resonance describes a condition in which a vibrational input is amplified, such that the output is out of all proportion to the input, sometimes resulting in severe damage. Often the only way to avoid this is to redesign the product to eliminate critical resonance points. Occasionally, loads go into a stack resonance condition, where each container goes into resonance with the previous one until the entire stack is bouncing (see Fig. 3.3). This condition can result in destruction of the product and pack. The dynamic load on the bottom container can be several orders of magnitude greater than the actual weight resting on it, which means that a corrugated case specified to withstand a calculated top load will now be totally inadequate. Also, the top container is subjected to extremes of repetitive shock and vibrations of considerable amplitude. Since this top layer is essentially weightless for short periods of time, small side movements will cause it to move. In a stretch-wrapped load this movement can skew the entire pallet load to one side.
Vibrational effects should be designed-out at the development stage, by ensuring that packs are tightly wrapped and accurately dimensioned so that movement is limited, and by removing obvious contact points. As these are likely to be low-cost or even no-cost options, they should take precedence over adding features such as scuff-resistant coatings.
Like vibration, compression is an unavoidable hazard in the distribution environment, as all products are stacked in warehouses and on vehicles. The challenge for the packaging technologist is to understand the likely compressive forces and specify appropriate packaging to limit product damage. Typically, the warehouse condition is one of static loading over time (static compression) but dynamic compression, encountered when using clamp trucks, during rail shunting and in stack resonance must also be considered. Dynamic compression describes a condition where the compressive load is applied at a rapid rate. Compression testing in the laboratory (e.g. test method ISO 12048:1994) is completed in a short period of time, i.e. it is a measure of dynamic compression values but the results can be correlated to indicate probable long-term warehouse stacking performance, which is likely to be of greater interest. This correlation for corrugated board is shown in Fig. 3.4. The initial part of the curve shows what the case will bear under dynamic or short-term load application. A container with a compression strength of 100 kg load clamped at an 85 kg load would be in danger of failing after about 10 minutes. The same container would fail in about 10 days if loaded to 60 kg. To last 100 days, the stack load should not exceed 55% of the dynamic compression value.
Not surprisingly, the humidity conditions encountered during warehousing and distribution can dramatically reduce the performance of corrugated board cases due to the hygroscopic nature of paper. For example, a change in relative humidity from 40% to 90% can result in a loss of about 50% of the case stacking strength. Thus corrugated cases destined for very humid conditions will have to be specified with sufficient stacking strength to allow for this inevitable loss.
• Pallet construction. Pallets with close-boarded decks (i.e. no gaps between the boards) allow pack weight to be evenly spread across the deck. Open-boarded pallets use less timber, but small packs can be compressed against the edges of the boards, causing excessive damage. Where palletised loads are double-stacked, reversible pallets (i.e. top and bottom decks are identical) allow even spreading of the weight of the top pallet across the top area of the lower pallet, thus avoiding excessive compressive forces in this area. Single-sided pallets without load-spreading perimeter boards do not allow this load spreading and can result in high damage levels (see Fig. 3.5). The greatest load per unit area is not on the bottom case, but on the top case of the lower pallet.
• Pallet stacking. Allowing cases to overhang the edge of a pallet leaves the load-bearing walls of the case suspended in mid-air, with an associated loss of available compression strength, as indicated in Table 3.4. Probably more controversial is the choice of column or interlock stacking pattern. Figure 3.6 shows the distribution of load-bearing ability around the perimeter of a case, clearly indicating that this is greatest at the corners. Therefore, the best possible use of container load-bearing ability is when cases are stacked directly on top of each other in a vertical column. However, this is the least stable stacking pattern and therefore cases are commonly interlocked (brick-style as shown in Fig. 3.5), although this format has a lower stacking strength.
|Degree of overhang||Percentage loss (range)|
|25 mm on one side||14–34|
|50 mm on one side||22–43|
|25 mm on one end||4–28|
|50 mm on one end||9–46|
|25 mm on one side and one end||27–43|
|50 mm on one side and one end||34–46|
Finally, the case contents, i.e. the product in its primary pack, will also influence compression strength. Rigid bottles contribute to the overall strength of the secondary pack, although when using the 0201 style case (FEFCO), in which the short flaps do not meet, not all bottles contribute, as demonstrated in Fig. 3.7. A way to address this is to use an all-flaps meeting style, such as 0204. Flexible primary packs such as bags and stand-up pouches make little or no contribution to case strength and thus the case specification will need to be more robust than for rigid primary packs. The degree of rigidity of the primary packs is important, especially when using apparently rigid containers such as plastic bottles and the known ‘creep’ characteristics of the plastic must be taken into account. Bottles which are expected to contribute to load-bearing should be designed avoiding sharp corners and edges, since these features act as stress concentrators and promote cracking. Circular cross sections, large neck areas to spread the load and shallow angles are all preferable, although of course such features may not meet other functions, such as ease of use and brand image.
Typically, whatever the load-bearing packaging component chosen, it should be designed to have a compression test value three to seven times greater than the maximum stacking load expected during storage. This is frequently referred to as the stacking factor or the safety factor. The maximum stacking load must take account of whether or not pallets are double or triple stacked, i.e. block-stacked on top of each other, without using racking or any other means of supporting the weight. The shorter the distribution cycle and the less handling there will be, the lower the safety factor. Table 3.5 gives guidance for starting points, across a range of different conditions, but actual factors should be calculated for each application.
Puncture refers to the piercing of a pack, which invariably results in product leakage and/or damage. It can be caused by external agents or by the product itself. External agents include piercing by the forks of a forklift truck, with the associated catastrophic effects or, less dramatic but nevertheless still unacceptable, piercing by nails or splinters in wooden pallets. In the former case the effects can be avoided by training; indeed, it is not possible to cost effectively protect goods against forklift truck damage by packaging specification, and training out bad forklift practice is the only feasible option. With regard to damage caused by unsuitable pallets, this emphasises the importance of treating pallets just like any other packaging component and applying appropriate specifications and inspection standards.
This section will concentrate on the direct effects of changes of temperature and humidity on the packaging around the product, rather than on the product contained therein. Product spoilage will be discussed more fully in Section 3.3, the preservation function of packaging. The two hazards are discussed together here because of their interdependence; the higher the air temperature the more moisture it can hold without condensation occurring. The dew point is the temperature at which the air is saturated with water vapour; below this, water condenses out of the moist air in the form of droplets.
Temperature and humidity changes will be encountered anywhere in the supply chain and are usually, but not always, due to climatic changes. The more varied the climatic conditions to which the pack is exposed, the more severe the hazard and its effects, so shipping goods between continents is obviously more hazardous than working solely in a domestic market. However, temperature and humidity changes can also occur due to poor control of storage conditions regardless of the climate, e.g. warehouse doors/windows left open, heating left on/off, or excessive use of gas-powered forklift trucks, and this can occur anywhere, even in a relatively short supply chain.
The effects of these changes are most apparent on cellulose-based packaging, i.e. paper and board. (Metals are also affected and unless suitably treated will corrode in high humidity – see Chapter 8 for more detail.) Cellulose fibres exposed to moist air will expand across their width as they absorb the moisture, hence sheets of board will vary in dimensions according to the humidity level, and such variations can affect carton cutting and creasing accuracy. Sheet flatness can also be affected by moisture, as the different layers absorb moisture differentially, resulting in curl (See Chapter 10). Paper and board packaging components should always be allowed to acclimatise in the conditions in which they are going to be used; bringing folding cartons from a cold warehouse into a warm packaging area and expecting good performance in terms of automatic make up, folding and gluing is unrealistic, and will result in high wastage and downtime.
Almost every property of paper and board is affected by temperature and humidity, but perhaps the most significant point to consider in the protection function of packaging is that as the moisture level increases, strength characteristics can decrease very rapidly. As already mentioned, a change in relative humidity from 40% to 90% can result in a loss of about 50% of the case stacking strength. Hence corrugated board cases specified to provide a level of resistance to compression during pallet stacking in one set of humidity conditions, will fail if the humidity increases significantly. Figure 3.8 gives a chart to estimate the compression strength of corrugated board at different moisture levels. To use the chart, draw a line from the compression strength to the board moisture content (see Chapter 10). Mark the position where the line crosses the pivot line. Now project a line from the new moisture content through the pivot line mark and read off the new compression strength.
As well as taking care over storage conditions, the main way of providing protection against the effects of moisture on paper and board is to use coatings to protect the cellulose base material. With regard to protection against extremes of temperature, other than the inclusion of insulating materials such as polymeric foams, this can only be done by careful management of the conditions throughout the supply chain.
Light is most likely to present a hazard when packed products are on display and exposed to retail lighting, when typical effects will be change of colour, usually fading. To the consumer a faded carton may indicate an ‘old’ product and thus this is likely to be discarded in favour of an apparently newer option. Colourfastness of substrates and printing inks should be checked during the development stages, and susceptible items should be packed in opaque primary and/or secondary packaging, or UV filters can be incorporated into packaging materials. Exposure of some products to light can result in chemical changes, and these will be covered in the section on the preservation function.
These hazards can be encountered at any stage and their effects vary from an aesthetically unpleasing dusty pack to a serious product infestation with potential damage to human health. Prevention is the only viable approach to these hazards, operating good standards of hygiene throughout the whole supply chain. This applies just as much to the manufacture, storage and delivery of packaging components to the packer/filler as it does to the product handling stages in the chain. See Chapter 21 on hazard and risk management for further information on this aspect.
Pilferage and tampering by humans, which is most likely to occur in the selling environment, also comes into the category of physical damage, and concern about this has created a requirement to consider the use of tamper evident and, in some cases, anti-counterfeit packaging. Tamper evidence is defined by the US Food and Drug Administration as: ‘Having an indicator or barrier to entry which, if breached or missing, can reasonably be expected to provide visible evidence to consumers that tampering has occurred’. For a tamper evident pack to be effective, the consumer must recognise that tampering has taken place, i.e. the tamper-evident feature needs to be obvious. Typical tamper-evident devices currently in use include:
None of these would present the determined, malicious tamperer with an insuperable task. The skilled tamperer will not confine his/her attempts at entry to the closure, and all other potential points of access should be considered when assessing the risk of tampering.
Counterfeiting is a serious problem in products such as pharmaceuticals, spirits, tobacco and perfume, and brand owners are constantly striving to stay ahead of the counterfeiter. Anti-counterfeit measures include building in a high level of complexity to the pack, such that it will be difficult to mimic, e.g. unique, custom-designed shapes for containers and closures, holograms on labels, printing inks which show up only in special conditions, e.g. UV light, and embedded micro-chips. The spirits sector also uses a range of special non-refillable fitments inserted in the neck of the glass bottle. The flow of product is maintained but the bottle cannot be refilled once empty.
A useful exercise is to map the journey of the packed product from production to consumption. Note the number and type of product movements and storage conditions, and against each one, list the likely hazards (i.e. shock, vibration, etc., as studied in Section 3.3.3) and their causes and effects. This will identify where the hazards occur and what types of hazards are most prevalent. See Table 3.6 for a list of typical questions to consider, and refer to Chapter 21 on hazard and risk management.
Understanding and, wherever possible, quantifying these hazards is vital in ensuring the packed product can perform satisfactorily throughout its expected shelf life. This can be done by a combination of observation, use of known data (e.g. frequency levels for different modes of transport) and measurement. By quantifying the hazards in this way, the uncertainty surrounding events taking place in the environment can be removed and packaging specifications can be tailored to meet real, rather than imaginary hazards.
Shock, vibration and climatic hazards can be measured in situ, by including data loggers in specific loads. These can be purchased or hired and used to record and measure actual events and the time at which each event occurs. The data obtained can be analysed to show the extent of damage-causing hazards experienced during a journey. Due to the cost involved, this approach is usually only employed when high levels of damage have been experienced, or in the early packaging development stages for high value, fragile items.
Testing should be carried out as part of the development programme and time for this will need to be built in to the development schedule (see Chapter 18). Transit testing is commonly done using actual conditions, or it may be simulated by using calibrated laboratory equipment and the known conditions identified in the process of mapping the journey. The latter is the more expensive option, although it may be more reliable and thus avoid costly mistakes. When using actual conditions, sample packs are made up, sent out on a ‘typical’ journey and assessed at the end. This may be done at various stages, first of all trying out a small number of packs and then building up to full pallet loads, or full vehicle loads. Provided the process is managed well, this testing can provide valuable ‘real-life’ information. Points to consider include:
• If the packs are palletised, the trial should continue to the end point, with secondary packs being transferred to mixed pallets if this is what will happen in reality. Primary packs should then be treated as they are during display, selection and use by the final consumer, e.g. taken home in a shopping carrier bag.
Preservation means the prevention/reduction of changes due to biological and chemical hazards, which would lead to product spoilage. The objective of preservation is to extend the shelf life of a product. This section applies mainly, but not exclusively, to the food, drink, pharmaceutical and cosmetic industries.
When considering the preservation function of packaging, it is important to recognise that whilst packaging can and does contribute to shelf life, it cannot overcome inherent product problems; if the product is unsafe at the point of packing, it is likely to remain unsafe inside the pack. Also, if temperature is a key factor in maintaining preservation, e.g. chilling or freezing, the packaging has only a ‘supporting’ role to play; if the temperature of the packed product is allowed to rise to the point where deterioration occurs, the pack will not compensate for this failure to manage storage conditions.
Within the limitations mentioned above, to determine the optimum packaging required to extend shelf life, we need to define the product in terms of what will cause it to deteriorate, i.e. what is the spoilage mechanism. We then need to understand what process (if any) will be used to prevent/delay spoilage and the extent to which this will affect the packaging used, and therefore determine its key properties. Only when we have a packaging specification which defines the required properties can we begin to investigate possible solutions.
Defining the spoilage mechanism of a product is part of the research and development stage of the product, and this is an example of how product and pack development personnel must work closely together. Product development specialists should be able to provide the information needed to define the product in terms which allow the packaging technologist to specify key packaging attributes. This section provides only a broad introduction to spoilage mechanisms, which is no substitute for the detailed knowledge expected of the food, cosmetic or other product scientist.
Product spoilage, and therefore shelf life is determined by microbiological, physical or chemical factors, depending on the product, the process, the packaging and the storage conditions (Blackburn, 2000). Broadly, spoilage due to microbiological spoilage is referred to as biotic spoilage, and that due to physical and/or chemical factors is known as abiotic spoilage.
Biotic spoilage is caused by microorganisms (bacteria, moulds, yeasts) which may render a product unacceptable in appearance, taste, smell and effectiveness, or be toxic and cause sickness. Different organisms have preferred conditions for growth and adverse conditions in which they will not propagate, and this is the basis of product preservation systems. The conditions to be considered are as follows:
Acidity. Microorganisms have an optimum pH level at which they will grow. In general, moulds and yeasts grow best in acidic environments and bacteria grow best in neutral to slightly alkaline conditions, although there are exceptions to this.
• Presence of oxygen. Some microorganisms need oxygen to propagate and are known as aerobes, while others cannot propagate in the presence of oxygen and are known as anaerobes. Some can propagate in either oxygen or oxygenless environments. In general, moulds and yeasts need oxygen to propagate, although some yeasts grow in anaerobic conditions.
• gain of volatiles which make a product taste odd, e.g. chocolate stored next to highly fragranced soaps, without an adequate barrier in the packaging will quickly pick up the volatiles in the soap and taste soapy. Unacceptable volatiles can also be picked up from printing inks and adhesives, due to high levels of retained solvent.
The basic principle of product preservation processes is to address the cause(s) of spoilage, and then to use appropriate packaging and storage conditions to maintain the product in its desired state. Referring back to the causes of biotic spoilage listed in the previous section, it can be seen that these can be addressed by:
• reducing the water activity (Aw) in a product. Aw is a measure of the amount of available water in the product and lowering this limits microorganism growth. Methods of reducing Aw include drying (which removes water) and the addition of salt or sugar (which ‘ties up’ the free water).
• varying the oxygen level, which can be done by vacuum packaging (where the product is packed and then the air is evacuated from the pack before sealing) or by changing the gaseous mixture around the product (modified atmosphere packaging – MAP; discussed further in Chapter 20). The use of oxygen scavengers to reduce the free oxygen available may also be considered under this heading.
See Table 3.7 for a summary of these preservation methods, along with considerations for packaging requirements.
Using the same approach to reduce/prevent abiotic spoilage, it can be seen that the key property required of the packaging is an appropriate barrier to the ingress/ loss of the damage-causing factor, e.g. moisture, air and light (see Table 3.8). To specify the correct level of barrier, it is necessary to know the extent to which the factor can be tolerated before spoilage takes place, e.g. knowing the extent to which a biscuit can absorb moisture before it becomes unpalatable allows the packaging technologist to design the optimum pack, which will give the required shelf life, but not be overspecified such that resources are wasted.
Provided the critical values of a food product are known, i.e. at what level the environmental spoilage-causing factor of gain or loss of moisture vapour, gas (or light) becomes unacceptable, it is possible to calculate shelf life based on the relevant barrier properties of the packaging material. Conversely, knowing the desired shelf life can dictate the barrier specification of the packaging material. Assuming correct storage conditions are maintained, especially for chilled and MAP products, it is the pack’s barrier to gain or loss with respect to these environmental factors which will determine shelf life. Knowing the spoilage mechanism of the product is thus the first step in predicting packaging requirements. A useful list of deterioration indices for different classes of foods is given in Table 3.9. Note that many foods are sensitive to more than one spoilage factor.
Source: Paine (1992).
The following is a very simple approach to calculating packaging barrier requirements, which can be used as an indicator. It is demonstrated in relation to determining the required moisture barrier, although it can be applied to gas barriers.
First of all the maximum amount of moisture allowable in the product contained in the pack, before spoilage starts to occur must be known. Assume this to be W grams. Next we need to measure the surface area of the pack as carefully as possible, allowing for changes due to handling, especially for flexible packs. Assume this to be A square metres. If the desired shelf life is T days, then we must look for a pack with a moisture vapour transmission rate (MVTR) of less than:
When considering barrier properties such as MVTR, packaging materials can be divided into two categories: those such as glass and metal, which have an absolute barrier, i.e. they are impermeable, and the permeable materials such as paper and plastics. MVTR is a measure of how quickly moisture permeates through the packaging material and is not the only consideration; if a pack is not correctly sealed, even an impermeable glass bottle will result in a permeable pack, hence the need for careful control of sealing parameters during filling, and in-process checks on the packaging line.
MVTR data for some of the common packaging plastics are given in Chapter 13 and these can be used as an initial guide to packaging material selection and to inform storage trials, thus providing a basis on which to start comparing potential alternatives and gain a preliminary idea of packaging material costs. Note that permeation is inversely proportional to thickness of the barrier and hence a 50 μ layer will be twice as good a barrier as a 25 μ layer, and that the temperature and humidity conditions to which the product is likely to be exposed in the supply chain are a vital factor in calculating the required barrier. It is essential to specify these and check that the data being quoted are applicable to the conditions expected.
The data given in Chapter 13 are generic and transmission rates for specific grades within each type of plastic are quoted by individual manufacturers and can usually be obtained from data sheets. Actual measurements of moisture, oxygen and light barriers can be carried out using laboratory testing, and references to independent laboratories carrying out such work are given at the end of this chapter. It should also be noted that the quoted data are for sheet materials in pristine condition and any creases or folds introduced during the packaging and handling processes will reduce the barrier performance. This is why the calculated barrier levels should be regarded as minimum values.
Important factors which have an effect on shelf life and barrier requirements are the size and geometry of the pack. Because moisture vapour and gas transmission rates are related to pack surface area, the smaller the pack (i.e. the higher the pack surface area to product ratio), the greater the permeation through the pack and the higher the barrier required per gram of product. This is important when developing different pack sizes and shapes and it cannot be assumed that a material which provides an acceptable barrier for one size already on the market will be suitable for another size or shape variant.
The way in which a pack is handled (and the product used) is determined by the design of the pack itself. Packaging designers have the opportunity to build in features to make handling easy, convenient and safe. If it is both intuitive and ergonomically sound how the pack should be picked up, opened and unpacked, potential damage to the contents and personnel will be minimised. This applies just as much to secondary packaging as it does to the consumer-facing primary pack.
Good pack design will determine how easy/difficult it is to dispense the desired amount of product and this is especially important for potentially ‘difficult’ products such as nail enamel, shoe polish, syrups, motor oil, viscous adhesives, paint, etc. If the pack offers clean and safe delivery, with no mess or loss of product, it can provide the all-important competitive edge in a crowded market. Just some examples of consumer convenience are:
• the stability (affected by material and shape) of a container is critical in determining the running speed – lightweight plastic bottles are less stable than glass and tall/ narrow shapes are less stable than short/squat shapes;
• containers which do not stand vertically for filling usually require pucks (specially made holders in which each container is supported while on the filling line), which in turn add to cost and development time;
Packaging provides the ideal (and often the only) means of delivering valuable information to anyone engaged in handling the product, as well as the final consumer. There is a need for information on the identity of the product, its weight/volume, destination and handling and possibly unpacking/repacking instructions, and such information must be easy to locate and understand. For important instructions, consideration should be given to the use of pictograms, to overcome any language barriers. Information on secondary and tertiary packaging must also show information such as product, number in pack, product codes and bar codes, for ease of recording stock.
For the consumer there is an increasing need to provide legal, promotional and usage data for a product, whether it is a bar of chocolate, a tube of toothpaste or an expensive perfume or skincare product. The information required is further increased when products are sold into several different geographical markets, where multilingual copy is needed. Legal information required includes product name, use of the product (if not obvious), weight/volume, manufacturer’s or seller’s (e.g. Tesco) name and address, expiry date, ingredients list, batch code (for traceability) and any relevant warning statements.
With regard to promotional and usage information, along with advertising, packaging provides the seller with a means of informing the consumer about the uses and benefits of the product, designed to encourage purchase. Once that purchase is made, it is the packaging alone which is expected to convey the definitive information on the product. This may necessitate the use of a leaflet, which must be signalled to the user (e.g. by a statement on the carton, such as ‘See leaflet for instructions’).
Information is required in the form of printed copy, perhaps supported by illustrations, and in electronically readable form, such as bar codes and matrix codes. Whatever the form, 100% legibility every time is the goal, which means using an appropriate size of type and colour contrast, and ensuring that the printing process used is adequate for the accuracy demanded. Increasingly, the provision of information for the visually-impaired is required, using raised characters as in Braille.
Today, most shopping is done in large supermarkets and this applies not only to food and drink, but also to cosmetics, garden chemicals, hardware, textiles, electrical equipment, etc. The role of the specialised, trained sales assistant to help one make a choice has declined and in the generally impersonal world of the retail store, it is packaging which carries out the selling role – the ‘silent salesman’. The manufacturer/seller must use the packaging, usually seen before the product, as the means of attracting the would-be purchaser. This is done by a combination of the colour, graphics, shape and size of the pack, which must combine to provide novelty, or a familiar chord of recognition, usually backed up by advertising depicting the pack and portraying the desired product image. Think about brands with which you are familiar, and the features used by the brand owner to ensure that you instantly recognise their product. For more information, see Chapter 6 on packaging and marketing and Chapter 18 on pack design and development.
As pointed out in Chapter 1, packaging has the potential to deliver cost effective solutions by reducing wastage and costly (and perhaps irreparable) damage to the product. The development and use of correctly specified packaging materials and formats, related directly to the demands of the goods, will ensure that excessive costs are not incurred unnecessarily. Good packaging solutions also have the potential to reduce the total cost of moving goods, by providing safe and convenient handling systems.
The cost of packaging must be related to the product’s value, image and end use. For most manufacturers this means the minimum spend commensurate with meeting all the required functions of packaging. This is not the same as the lowest cost option. If a product fails to attract the attention of the consumer, due to poor packaging presentation, the packaging is not meeting its required functions.
Finally, the relative importance of each of the functions of packaging varies with the product. For example, protection against physical damage will be very important for a television set, but the selling function of the packaging will be less important, as most people buy a TV after having seen a display model in the store. To the consumer, the packaging is simply a convenient means of getting the item home safely. On the other hand, the packaging of a breakfast cereal or pet food on the supermarket shelf, amongst 25 other varieties of similar products, has a very important selling function to fulfil.
• Soroka, W. (2010) Fundamentals of Packaging Technology Institute of Packaging Professionals, Naperville, IL (www.iopp.org).
• Instrumented Sensor Technology: www.isthq.com
• Dallas Instruments: www.dallasinstruments.com
• Pira International: www.pira.co.uk
Note: Sources used in the preparation of this chapter also include teaching and learning materials written and used by the author in the delivery of courses to a number of organisations, such as the Packaging Society, Loughborough University, University of Warwick, University of Bath and London College of Fashion (University of the Arts London).