Shelf Life Testing:
Unless a food has undergone a commercial sterilisation process (e.g. canning) or has a water activity which will not permit microbial growth (e.g. sugar, breakfast cereals), the rate of growth of spoilage microorganisms is likely to be the major factor determining shelf life.
This rate is determined by a number of factors including:
- Food properties (e.g. pH, total acidity, water activity, presence of preservatives either natural or added)
- Environmental factors (temperature, relative humidity, gaseous atmosphere)
- Any process designed to kill or retard growth of microorganisms (thermal processing, freezing, packaging)
- The type of microflora present on the food, and the initial population.
Moisture and water vapour transfer
Not only is water (measured as water activity) a critical factor which determines which, if any, microorganisms will grow in a food, many foods are sensitive to loss or gain of water.
This in turn can be affected by the choice of packaging and in many instances will determine which packaging is used.
Many biscuits and savoury snacks including nuts suffer in quality as a result of moisture gain. Some baked foods such as cakes may suffer from moisture loss.
Chemical or biochemical changes
Numerous possible reactions which could limit shelf life fall into this category.
The most important are oxidation, non-enzymic browning, enzymic browning and, in some cases, food and packaging interaction.
Oxidation of fats and oils leads to the development of rancidity marked by off odour and flavour.
This may limit the shelf life of fats and oils but can also limit the shelf life of many other foods containing fats and oils. Examples of foods stored at ambient temperatures which can develop rancid off flavours are nuts, potato crisps and biscuits.
Storage of these foods in high oxygen atmospheres can sometimes be used to accelerate shelf life studies but atmospheric oxygen is not the only initiator of oxidative spoilage.
Many frozen foods can also have their shelf life limited by fat oxidation.
While freezing arrests microbial activity, chemical reactions proceed at a much reduced rate even at recommended storage temperatures. Examples of frozen foods whose storage life is limited by oxidation include fish and meats.
A number of different vitamins are sensitive to oxygen including vitamin C (ascorbic acid) and vitamin B (thiamine).
When vitamins are added to fortified foods such as breakfast cereals or sports drinks and a label declaration made, then shelf life determinations will have to take account of any vitamin degradation which will occur with time in addition to any other changes in quality.