For such products, the factor most likely to limit shelf life is the growth of mould. Another factor reducing quality is staling, which is caused as the starch changes its structure during storage and the bread becomes dry. Modified atmosphere packaging (MAP) is the method that has received most attention in recent years.
Oxygen is necessary for the growth of most types of mould, but some species can grow in oxygen-free conditions. In general, even for oxygen loving mould fungi like the Penicillium and Aspergillus species, the O2 concentration must be reduced below 2 0.5% in order to achieve any significant reduction in the speed of growth. Many studies have shown that mould can survive and grow in O2 concentrations as low as 1 per cent, even in combination with an increased concentration of carbon dioxide. The amount of oxygen in the pack therefore has a great influence on shelf life. The most important factors for controlling the oxygen level in the pack are the amount of residual oxygen and the integrity of the packaging (O2 barrier and effectiveness of seal).
Carbon dioxide generally has a better inhibiting effect against mould than against yeast and bacteria. The effect of the CO2 increases with increased concentration and the recommendation is to use as high a concentration of CO2 as possible. Carbon dioxide is dissolved in the water phase of the products and a high percentage of CO2 can give an effect similar to a vacuum. It is therefore normal to use nitrogen as an inert gas to counteract this vacuum effect. Some researchers also maintain that CO2 reduces the ageing process in bread, but others have not observed this effect.
Grain mixtures
Dry extruded oat is used as an ingredient of many breakfast cereals. This product is easily subject to rancidity. Nofima Mat has also investigated how light and oxygen affects the rancidity of the oats. Packaging material and packaging solution play a vital role in shelf life.
Trials carried out by Nofima Mat have shown that storage in the dark at room temperature gives a relatively long shelf life for extruded oat. There is currently a trend towards the use of transparent packaging, such as plastic bags for muesli products. Given a combination of copious oxygen and normal ceiling lighting, the extruded oat becomes rancid after only 30 days or less. However the oat was more resistant to light exposure when the access to oxygen was reduced. By packing the oat in a nitrogen atmosphere in a pack with very good oxygen barrier qualities, the product did not become rancid even after almost 300 days storage with lighting throughout.
