By Dr. Vinayan Nair
The product quality and shelf life of packaged foods can be affected by the appropriate selection of packaging materials. Packaging can become a shelf life limiting factor in its own right. This may be as a result of migration of tainting compounds from the packaging into the food or the migration of food components into the packaging. Thus inappropriate packaging materials can have adverse effects on product quality and shelf life.
Microbiological safety of the product as well as quality issues has to be considered, while selecting a packaging material. This may be based on microbial numbers, chemical specifications or sensory assessment.
In most cases it is likely to be a combination of these. Increasing temperature is the most common means of accelerating shelf life, but other parameters such as humidity, exposure to light etc known to affect product stability can be used; often product specific and based on a considerable knowledge of the product. The danger with this approach is that chemical reactions or microbiological growth is initiated, that would not take place under normal storage conditions.
Factors affecting Quality and Safety of a Food Product
For many foods, the product shelf life is limited by specific features that can be predicted at the time of product development. This is either on the basis of experience with similar products or observations of them or from a consideration of various intrinsic factors such as pH, water activity, oxygen availability etc and extrinsic factors such as time-temperature control during processing, temperature/relative humidity during storage and distribution etc, affecting the product.
Selection of raw materials is important for controlling intrinsic factors since subsequent processing can’t compensate for poor-quality raw materials. Product packaging can have significant effects on many of these extrinsic factors, and many developments in packaging materials have been driven by the need to reduce the impact of these environmental factors and extend shelf life.
In some instances, the packaging alone may be effective in extending shelf life. The interactions of intrinsic and extrinsic factors affect the likelihood of the occurrence of reactions or processes that affect the shelf life. These shelf life-limiting reactions or processes can be classified as chemical/biochemical, microbiological and physical. The effects of these factors are not
always detrimental and in some instances, they are essential for the development of the desired characteristics of a product.
Under suitable conditions, most microorganisms will grow or multiply. Bacteria multiply by dividing to produce two organisms from one, their numbers increased exponentially. Under ideal conditions, some bacteria may grow and divide every 20 min, so one bacterial cell may increase to 16 million cells in 8 hrs. Under adverse conditions, this doubling or generation time is prevented or extended – a feature that is exploited when developing food products and processes to achieve the desired shelf life.
During growth in foods, microorganisms will consume nutrients from the food and produce metabolic by-products such as gases or acids. They may release extra-cellular enzymes (e.g. amylases, lipases, proteases) that affect the texture, flavour, odour and appearance of the product. Some of these enzymes will continue to exist after the death of the microorganisms that produced them, continuing to cause product spoilage.
When only a few organisms are present, the consequences of growth may not be evident, but when numbers increase their presence is evident from the formation of visible colonies, the production of slime or an increase in the turbidity of liquids, and from the effects that gas production, acidification and the off-odours caused by secondary metabolites have on the acceptability of the food product.
The relationship between microbial numbers and food spoilage is not always clear. Whether or not high numbers of microorganisms result in spoilage is dependent upon the type (genus/species) of microorganism and numbers present and its stage of growth or activity, and the intrinsic and extrinsic factors of the food in which it is present. The key to achieving the desired shelf life of a product is to understand which microorganisms are likely to be the ones that will give rise to product spoilage, and what conditions can be used to either kill or reduce the rate of growth and multiplication. This requires careful selection or manipulation of the intrinsic and extrinsic factors of food.
The presence of food poisoning organisms (pathogens) is not necessarily evident from changes in the food, and may only be apparent from the effects they produce, ranging from mild sickness to death. With many human pathogens, the greater the number of cells consumed, the greater the chance of infection and the shorter the incubation period before the onset of disease. Therefore, destruction, inhibition or at least control of growth is essential. For some invasive pathogens e.g. viruses, Campylobacter, the infectious dose is low and growth in the food may not be necessary. From the point of view of product shelf life, the first question must always be ‘is the product safe?’ Once this safety is assured, quality and commercial aspects can be considered.
Using knowledge of the initial levels of microorganisms and the conditions that destroy them or reduce their growth rate, food products are developed and designed by use of the best combination of intrinsic and extrinsic factors.
Significance of Packaging in Maintaining Food Safety
Heat processing which kills microorganisms is a widely used means of achieving safe products and extending shelf life. The amount of heat treatment required depends on the characteristics of the most harmful microorganism present, the nature of the food in terms of its viscosity, the pH of the food, the shape of the pack and the shelf life required. However, the heat process also changes the texture, taste and appearance of the product. This has prompted the move to minimally processed foods where a number of factors are combined to achieve the desired shelf life, e.g. a mild heat treatment, antioxidant action and controlled atmosphere packaging each restricting microbial growth, such that their combined effect allows the product to retain its sensory and nutritional properties.
In canning, low acid foods are filled into containers that are hermetically sealed and sterilised, typically at 115.5–121°C or above, to ensure all pathogens, especially Clostridium botulinum, are destroyed. The critical factor is the thermal treatment, which is the integration of product temperature with time throughout the heating cycle at given process temperature and initial product temperature, required for the coldest part of the product to receive the required minimum cook. The size and shape of the container is also important.
Similarly, for sous-vide processing, foods are cooked in a vacuum in sealed evacuated heat-stable pouches or thermoformed trays. In aseptic processing, the barrier that the packaging poses to heat transfer is removed completely – the product and packaging being sterilized separately and then brought together under clean (aseptic) conditions.
Where heat processing has been used to achieve sterility, the use of packaging to maintain sterility throughout the subsequent life becomes a key factor to achieving the desired product shelf life – both the packaging and the pack seals must provide a barrier to the ingress of microorganisms. Low temperatures might inhibit the growth of an organism and affects its rate of growth. Some microorganisms are adapted to grow at chill temperatures, hence the composition of organisms in the natural microflora will change. For example, in fresh milk, the dominant microflora is Gram-positive cocci and bacilli that may spoil the product by souring if stored at warm temperatures. At chill temperatures, the microflora becomes dominated by psychrotrophic Gram-negative bacilli (most commonly Pseudomonas spp.). When temperature is used as the key limiting factor to control the rate at which shelf life-limiting processes proceed from a microbiological point of view, the role of packaging is less significant to shelf life because regardless of the packaging if the temperature is not maintained, spoilage will proceed. Storage at frozen temperatures will stop microbial growth and can kill some microorganisms, but is not necessarily a lethal process. Where vacuum packaging or modified atmospheres are the key shelf life-limiting factors controlling microbiological growth, the packaging is a critical factor in achieving the desired shelf life.
Pseudomonas species, the major spoilage group in chilled proteinaceous foods, require the presence of oxygen to grow. The use of vacuum packaging or modified atmospheres excluding oxygen will prevent the growth of this type of bacteria. Whilst other organisms can grow in the absence of oxygen, they generally grow more slowly and so the time to microbial spoilage is increased. Carbon dioxide at 20–60 percent has bacteriostatic and fungistatic properties and will retard the growth of mold and aerobic bacteria by increasing the lag phase and generation time of susceptible microorganisms. Several factors influence the antimicrobial effect of carbon dioxide, especially the microbial load, gas concentration, temperature and permeability of the packaging film. The antimicrobial effect is enhanced at lower temperatures because carbon dioxide is more soluble in water at lower temperatures, forming carbonic acid, so good temperature control is essential to obtain the maximum potential benefits of modified atmosphere packing and vacuum packaging. However, the effect of carbon dioxide is not universal – it has little effect on yeast cells and the growth of lactic acid bacteria is improved in the presence of carbon dioxide and lower oxygen levels. Nitrogen is an inert gas that has no antimicrobial effect. It is generally used to prevent package collapse in products that absorb carbon dioxide and is used to replace oxygen in products that are susceptible to the growth of aerobic microorganisms.
Packaging materials designed to have antimicrobial activity provide a hurdle for microbial growth but seldom act alone as the key shelf life-limiting factor. Antimicrobial activity can be obtained in two ways. Preservative-releasing or migrating systems contain a preservative intended for migration into the food. Non-migrating systems contain or produce a compound that has antimicrobial activity when the target organism comes into contact. For both systems, the antimicrobial substance can be incorporated into the packaging material or applied to the surface. Maximum contact is required between food and packaging to ensure adequate protection. Therefore, it is particularly suitable for vacuum-packed foods. A number of antimicrobial packaging materials are commercially available and their activities and effectiveness have been reviewed. One example of this technology is Microban by Microban Products Co., UK, which incorporates the biocide triclosan into almost any type of plastic in a way that it is still free to migrate to the surface to act against developing bacteria.
The desire to extend product shelf life will continue to stimulate the development of new processing and packaging innovations. Selection of the most appropriate product packaging requires a knowledge and understanding of the food chemistry and microbiology of the product, the environmental conditions that it will encounter from production to consumption and how this affects interactions between the packaging and the food.
* VINAYA R NAIR (+919961423671, (firstname.lastname@example.org) is a Quality-oriented professional with M. Phil in Microbiology, with MBA in Pathology Lab management, offering extensive experience of nearly 13 years’ in Quality Management & Assurance, Lab Management (Microbiology & Chemistry) & Hospitality Services Trained Food Safety/Occupational Health & Safety Managing Professional with exposure in ISO 17025:2005 (NABL), ISO 22000:2005 (FSMS Standards), Food Safety Level IV, HACCP Level III, NEBOSH IGC, NEBOSH HSW, IOSH Proficient in Automated Microbiology Analysis of food samples as per US FDA BAM, IS, AFNOR Procedures.