The shelf life of sour milk products is determined by product-specific micro-organisms and by contaminants. The metabolism of the lactic acid bacteria involved in fermentation is reduced dramatically by the reduction in temperature at the end of production. However metabolism does not come to a complete stop, as the lactic acid bacteria continue to acidify, even at a storage temperature of 4 to 5 °C. The intensity of this after-acidification during yoghurt storage depends on the initial pH value and storage temperature. During storage the quantity of L(+)- and D(-) lactic acid also changes in parallel to the increase in lactic acid content. Streptococcus thermophilus forms nothing but L(+) and Lactobacillus bulgaricus only D(-) lactic acid. Since the streptococci can tolerate only relatively low lactic acid concentrations, the lactic acid is largely formed by the lactobacilli during after-acidification. This means that the amount of D(-) lactic acid is increased during after-acidification.
In parallel to the increase in acidity and the associated change in the lactic acid bacteria flora, the yoghurt undergoes changes in taste and viscosity caused by proteolysis of the lactobacillae. The taste changes are found mainly in “natural products”. Yoghurt can easily become bitter as it ages.
The multiplication of foreign bacteria during the storage of sour milk products causes actual spoilage. Foreign bacterial flora are considered to be any bacteria in the milk product which do not belong to the culture organisms inoculated. They get into the product by surviving pasteurisation or by subsequent recontamination. Only those foreign bacteria able to multiply in fermented milk have an adverse effect on product quality. The most dangerous contaminants are doubtless yeast and mould fungi. Contamination takes place via air, ingredients or packaging material. Yeasts and moulds multiply most rapidly at relatively high storage temperatures. The consequences are inflated packaging and unwanted changes in smell, taste and appearance. Spoilage due to contamination can be averted only by spotless production and factory hygiene, top quality additives and ingredients, and enclosed, if possible aseptic, production lines.