Food Packaging Technology 3(2+1)

Lesson- 1. Factors affecting shelf life of food

1.1 Introduction

Packaging is an essential part of processing and distributing foods. Whereas preservation is the major role of packaging, there are several other functions for packaging, each of which must be understood by the food manufacturer. Packaging must protect against a variety of assaults including microorganisms, insects and rodents. Environmental factors such as oxygen and water vapor will spoil foods if they are allowed to enter packages freely.

Packaging can become a shelf life limiting factor in its own right. For example, 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. Different groups within the food chain, i.e. consumers, retailers, distributors, manufacturers and growers, proffer subtly different perspectives of shelf life, reflecting the aspect of greatest importance and significance to them. For consumers, it is imperative that products are safe and the quality meets their expectations. Consumers will often actively seek the product on the shelf with the longest remaining shelf life as this is considered to be indicative of freshness.

1.2 Shelf life

The quality of most foods and beverages decreases with storage or holding time. The shelf life of a product is best determined as a part of the product development cycle. The Institute of Food Technologists (IFT) in the United States has defined shelf life as “the period between the manufacture and the retail purchase of a food produ-ct, during which time the product is in a state of satisfactory quality in terms of nutritional value, taste, texture and appearance”. The Institute of Food Science and Technology (IFST) in the United Kingdom has defined shelf life as “the period of time during which the food product will remain safe; be certain to retain desired sensory, chemical, physical, microbiological and functional characteristics; and comply with any label declaration of nutritional data when stored under the recommended conditions” .

The date of minimum durability is defined as the date until which the food retains its specific properties when properly stored. It must be indicated by the words “Best before” followed by the date (or a reference to where the date is given on the labeling). Depending on how long the food can keep, the date can be expressed by the day and the month, the month and the year, or the year alone.

1.3 Factors affecting shelf life

1.3.1 Product characteristics

Product characteristics including formulation and processing parameters i.e. intrinsic factors. Intrinsic factors are the properties resulting from the make-up of the final product and include the following:

• Water activity (a_w)
• PH/total acidity
• Natural micro flora and surviving microbiological counts in final product
• Availability of oxygen
• Reduction potential (E_h)
• Natural biochemistry/chemistry of the product
• Added preservatives (e.g. salt, spices, antioxidants)
• Product formulation

 1.3.2 Environmental factors

 Environment to which the product is exposed during distribution and storage i.e. extrinsic factors. Extrinsic factors are a result of the environment that the product encounters during life and include the following:

1.3.2.1 Temperature

Temperature is a key factor in determining the rates of deteriorative reactions, and in certain situations the packaging material can affect the temperature of the food. For packages that are stored in refrigerated display cabinets, most of the cooling takes place by conduction and convection. Simultaneously, there is a heat input by radiation from the fluorescent lamps used for lighting. Under these conditions, aluminum foil offers real advantages because of its high reflectivity and high conductivity.

1.3.2.2 Relative humidity

The RH of the ambient environment is important and can influence the water activity (a_w) of the food unless the package provides an excellent barrier to water vapor. Many flexible plastic packaging materials provide good moisture barriers, but none is completely impermeable.

1.3.2.3 Gas atmosphere

The presence and concentration of gases in the environment surrounding the food have a considerable influence on the growth of microorganisms, and the atmosphere inside the package is often modified. The simplest way of modifying the atmosphere is vacuum packaging, that is, removal of air (and thus O₂) from a package prior to sealing; it can have a beneficial effect by preventing the growth of aerobic microorganisms. Flushing the inside of the package with a gas such as CO₂ or N₂ before sealing is the basis of modified atmosphere packaging (MAP). For example, increased concentrations of gases such as CO₂ are used to retard microbial growth and thus extend the shelf life of foods. MAP is increasing in importance, especially with the packaging of fresh fruits and vegetables, fresh foods, and bakery products.

Atmospheric O₂ generally has a detrimental effect on the nutritive quality of foods, and it is therefore desirable to maintain many types of foods at a low O₂ tension, or at least prevent a continuous supply of O₂ into the package. Lipid oxidation results in the formation of hydroperoxides, peroxides, and epoxides, which will, in turn, oxidize or otherwise react with carotenoids, tocopherols, and ascorbic acid to cause loss of vitamin activity.

With the exception of respiring fruits and vegetables and some fresh foods, changes in the gas atmosphere of packaged foods depend largely on the nature of the package. Adequately sealed metal and glass containers effectively prevent the interchange of gases between the food and the atmosphere. With flexible packaging, however, the diffusion of gases depends not only on the effectiveness of the closure but also on the permeability of the packaging material, which depends primarily on the physicochemical structure of the barrier.

 1.3.2.4 Light

Many deteriorative changes in the nutritional quality of foods are initiated or accelerated by light. Light is, essentially, an electromagnetic vibration in the wavelength range between 4000 and 7000 A, the wavelength of ultraviolet (UV) light ranges between 2000 and 4000 A. The catalytic effects of light are most pronounced in the lower wavelengths of the visible spectrum and in the UV spectrum. The intensity of light and the length of exposure are significant factors in the production of discoloration and flavor defects in packaged foods.

There have been many studies demonstrating the effect of packaging materials with different light-screening properties on the rates of deteriorative reactions in foods. Among the most commonly studied foods has been fluid milk, the extent of off-flavor development being related to the exposure interval, strength of light, and amount of milk surface exposed.

 1.3.3 Enzymic reactions

In food packaging technology, knowledge of enzyme action is essential to a fuller understanding of the implications of different forms of packaging. The importance of enzymes to the food processor is often determined by the conditions prevailing within and outside the food. Control of these conditions is necessary to control enzymic activity during food processing and storage. The major factors useful in controlling enzyme activity are temperature, a_w, pH, chemicals that can inhibit enzyme action, alteration of substrates, alteration of products, and preprocessing control.

Three of these factors are particularly relevant in a packaging context. The first is temperature i.e.  the ability of a package to maintain a low product temperature and thus retard enzyme action will often increase product shelf life. The second important factor is a_w, because the rate of enzyme activity is dependent on the amount of water available, low levels of water can severely restrict enzymic activities and even alter the pattern of activity. Finally, alteration of substrate (in particular, the ingress of O₂ into a package) is important in many O₂ dependent reactions that are catalyzed by enzymes, for example, enzymic browning due to oxidation of phenols in fruits and vegetables.

 1.3.4 Chemical reactions

Many of the chemical reactions that occur in foods can lead to deterioration in food quality (both nutritional and sensory) or the impairment of food safety. Such reaction classes can involve different reactants or substrates, depending on the specific food and the particular conditions for processing or storage. The rates of these chemical reactions are dependent on a variety of factors amenable to control by packaging, including light, O₂ concentration, temperature, and a_w. Therefore, the package can, in certain circumstances, play a major role in controlling these factors, and thus indirectly the rate of the deteriorative chemical reactions.

The two major chemical changes that occur during the processing and storage of foods and lead to deterioration in sensory quality are lipid oxidation and nonenzymic browning (NEB). Chemical reactions are also responsible for changes in the color and flavor of foods during processing and storage.

  1.3.4.1 Lipid oxidation

Autoxidation is the reaction of molecular O₂ by a free radical mechanism with hydrocarbons and other compounds. The reaction of free radicals with O₂ is extremely rapid, and many mechanisms for initiation of free radical reactions have been described. The crucial role that autoxidation plays in the development of undesirable flavors and aromas in foods is well documented, and autoxidation is a major cause of food deterioration.

 1.3.4.2 Nonenzymic browning

Nonenzymic browning (NEB) is one of the major deteriorative chemical reactions that occur during storage of dried and concentrated foods. The NEB or Maillard, reaction can be divided into following three stages.

 (1) Early maillard reactions involving a simple condensation between an aldehyde (usually a reducing sugar) and an amine (usually a protein or amino acid) without browning.
 (2) Advanced maillard reactions that lead to the formation of volatile or soluble substances
 (3) Final maillard reactions leading to insoluble brown polymers.

 

1.3.4.3 Color changes

Acceptability of color in a given food is influenced by many factors, including cultural, geographical and sociological aspects of the population. However, regardless of these many factors, certain food groups are acceptable only if they fall within a certain color range. The color of many foods is due to the presence of natural pigments such as chlorophylls, anthocyanins, carotenoids, flavonoids, and myoglobin.

  1.3.4.4 Flavor changes

In fruits and vegetables, enzymically generated compounds derived from long-chain fatty acids play an extremely important role in the formation of characteristic flavors. In addition, these types of reactions can lead to important off-flavors. Enzyme-induced oxidative breakdown of unsaturated fatty acids occurs extensively in plant tissues, and this yields characteristic aromas associated with some ripening fruits and disrupted tissues.

Aldehydes and ketones are the main volatiles from autoxidation, and these compounds can cause painty, fatty, metallic, papery, and candle like flavors in foods when their concentrations are sufficiently high. However, many of the desirable flavors of cooked and processed foods derive from modest concentrations of these compounds. The permeability of packaging materials is of importance in retaining desirable volatile components within packages and in preventing undesirable components entering the package from the ambient atmosphere.

 1.3.4.5 Nutritional changes

The four major factors that influence nutrient degradation and can be controlled to varying extents by packaging are light, O₂ concentration, temperature, and a_w. However, because of the diverse nature of the various nutrients as well as the chemical heterogeneity within each class of compounds and the complex interactions of these variables, generalizations about nutrient degradation in foods are unhelpful.

 1.3.5 Physical changes

The physical properties of foods can be defined as those properties that lend themselves to description and quantification by physical rather than chemical means and include geometrical, thermal, optical, mechanical, rheological, electrical, and hydrodynamic properties. Geometrical properties encompass the parameters of size, shape, volume, density, and surface area as related to homogeneous food units, as well as geometrical texture characteristics. Although many of these physical properties are important and must be considered in the design and operation of a successful packaging system, in the present context the focus is on undesirable physical changes in packaged foods.

 1.3.6 Microbiological changes

Microorganisms can make both desirable and undesirable changes to the quality of foods, depending on whether they are introduced as an essential part of the food preservation process or arise adventitiously and subsequently grow to produce food spoilage. Every microorganism has a limiting a_w value below which it will not grow, form spores, or produce toxic metabolites. Water activity can influence each of the four main growths cycle phases by its effect on the germination time, the length of the lag phase and the growth rate phase, the size of the stationary population, and the subsequent death rate. Whether a microorganism survives or dies in a low a_w environment is influenced by intrinsic factors that are also responsible for its growth at higher a_w. These factors include water-binding properties, nutritive potential, pH, E_h, and the presence of antimicrobial compounds. Microbial growth and survival are not entirely ascribed to reduce a_w but are also attributable to the nature of the solute. Key extrinsic factors relating to a_w that influence microbial deterioration in foods include temperature, O₂, and chemical treatments. These factors can combine in a complex way to encourage or discourage microbial growth.