Shelf-life and Compatibility Studies
The term “Shelf-life” is generally understood to be the duration of that period between packing a product and using it, for which the quality of the product remains acceptable to the product user. An attempt to predict this period from data on the product, the package, the distribution and storage conditions is appropriate, where, alternative packaging materials are available which contribute positively (but to different levels) to the extension of shelf-life of the packed food product.
Shelf-life prediction is appropriate or required when the package is permeable or semi- permeable to atmospheric agents like water-vapour or/and oxygen. In the case of
impermeable containers such as tinplate containers and glass jars, it is not necessary to predict shelf-life. However, in such cases it is to be assumed that the seams and the closures of the packs are perfect. Metal containers, glass bottles, aluminium foil are used mainly for their being an absolute barrier against moisture vapour and gases. However, compatibility of metal containers with specific food items need to be ascertained and wherever necessary, suitable lacquer coatings may need to be provided to achieve product- package compatibility. The lining materials/wads of the closures/caps in glass jars also should be compatible with the packed food product.
Though polymeric packaging materials are not absolute barriers against moisture vapour and gases, they have been found to be increasingly useful due to various advantages like light weight, easy to carry, easy to transport, handle and stock.
The most important function of the package is to contain the
product and providde protection against changes in quality caused by adverse effects of surrounding environment. The selected packaging material has to be compatible with the product to be packed and should provide specific protection to maintain shelf-life i.e. quality preservation as well as economic considerations and competitive packaging. All these are taken into consideration in design and selection of a packaging system.
Shelf-life determinations help in:
- Selection of a package for a new product, which could
be optimum i.e. provide the desired shelf-life period at the most economic cost.
- Selection of an alternative package for an already marketed product, either to extend the shelf-life or to reduce the cost by using newer materials.
- Government requirement for open dating, declaration of “Best before date” assures the consumer of wholesome, nutritious, safe food.
Food Degradation Factors
For shelf-life determination studies, it is important to understand the mechanisms of food deterioration or degradation, and the factors for the same. From the time a food product is manufactured and packed, the process of degradation commences. Protective plastic packaging slows-down some of the reactions due to the action of light, moisture, atmospheric oxygen, etc. There are many identified food degradation mechanisms, the major ones being, the gradual loss of colour, texture, flavour and nutrients. Such deteriorative changes could be due to:
Storage studies can also be carried out at cyclic conditions. For the purpose of creating the storage conditions, humidity cabinets or environmental walk-in-chambers are used at IIP.
During the exposure period, the packages are withdrawn at fixed intervals of time to assess the quality of the product as per the product quality attributes laid down earlier. This is continued till the product becomes commercially unacceptable i.e. degradation occurs of the primary product quality attribute. To assess the product quality, organoleptic testing or sensory evaluation is also required to be carried out. The details of sensory evaluation are discussed later in the chapter.
When the packages are withdrawn, besides assessment of the product, checks are also made on the packaging material/package to observe for any softening/delamination/cracking, opening of seals, discolouration, surface stickiness etc. These checks help to establish the product-package compatibility, which is of prime importance for any packed product.
The results obtained on the assessment of the product quality and the package are tabulated and analysed for overall acceptability. Based on the analysis, shelf-life of the product in a particular package is determined. The ultimate selection of the packaging material or method would be governed by the marketing requirements and the economics of the packaging system.
Sensory Evaluation of Foods for Shelf-life Determination
A number of quality assurance procedures are used to examine and maintain the quality of a food product at different stages starting from receipt of the raw materials up to the finished product. These tests are physical, chemical, microbiological and sensory. Amongst all these methods, sensory evaluation is of paramount importance. The sensory quality has to be included in product evaluation since it is the only integrated multi-dimensional measurement. The sensory evaluation procedures have been studied in considerable details with the result that this scientific discipline has come to be recognised as fairly objective in nature (Larmond, 1987). The inherent variability of sensory evaluation by human subjects can be generally overcome through appropriate selection and training procedures, coupled with application of statistical methods so as to take full advantage of the high sensitivity of human sense organs that even today surpass the most sophisticated instrumental means for flavour, texture and colour examination (Ogden, 1993).
A successful implementation of sensory evaluation programme for shelf-life determination requires proper laboratory facilities, trained sensory panels and adoption of appropriate sensory methods. card comprising of different attributes, often the total score is taken as the “overall acceptability” of the product.
Different sensory tests coupled with relevant statistical methodologies make sensory evaluation an important tool for shelf-life determination.
Several successful attempts have been made to develop shelf-life prediction models for various food products, each relying heavily on a sound sensory evaluation technique.
A study conducted by IIP on shelf-life of “Rabri” using 9-point Hedonic Scale method for Sensory Evaluation is summarised at the end of the chapter.
Shelf-life Prediction by Formulae
Prediction of shelf-life of moisture sensitive products is carried out by using equations/ formulae. These are based on various theories by different people. The most simple one is described here. The prediction of shelf-life based on formulae, requires the determination of the moisture absorption isotherm of the food product.
Moisture Absorption Isotherm
The water content of a foodstuff or other moisture-sensitive product, and the relative humidity (or water activity) with which it is in equilibrium, are linked by a characteristic curve for the product. If the product is placed in an atmosphere with which it is not in equilibrium, its
Moisture Absorption Isotherm
moisture content will alter to bring it to equilibrium. The final moisture content usually differs for a given relative humidity, depending on whether the product has lost or gained moisture to reach equilibrium.
The experimental technique to obtain the water isotherm has been standardized following the COST 90 project of the European Cooperation in Scientific and Technical Research (Jowett, 1984). Saturated salt solutions are used in temperature-controlled enclosures to provide air of known relative humidity. Quantities of the product are exposed in these
enclosures until weight equilibrium is established.