Yoghurt

Yoghurt is an acidic fermented milk product, which normally has acetaldehyde in the final aroma. It is made of milk inoculated with thermophilic lactic acid starter cultures. During fermentation about 35% of the lactose is hydrolyzed to glucose and galactose. The glucose is metabolized to lactic acid while galactose remains in the environment of the yoghurt (Goodenough and Klein 1976). Therefore, galactose fermenting yeast strains in particular are known to cause spoilage of yoghurt (Caggia et al. 2001; Giudici et al. 1996).

Yoghurt is often added nuts, honey, preserved or dried fruit, containing sucrose and is a source of yeast infections. Yeast spoilage of yoghurt is seen as excessive gas production followed by swelling of the package, unpleasant yeasty odor and taste, changes in texture and color, and formation of visible yeast colonies (Caggia et al. 2001; Fleet and Yeasts 1998).

With intervals, studies of yeast in retail yoghurt have been reported from different countries. It is not unusual to detect yeast in yoghurt in numbers of 103 cells/g, and yeast count up to 108 cells/g has been reported. The contamination rate of yeast in yogurt were in the order of 103 for 20% of the examined samples in United Kingdom (Davis 1975) and Canada (Arnott et al. 1974).

The most common yeast species reported in yoghurt are C. famata/D. hanseniil, Pichia anaomala, C. versalitilis, S. cerevisiae, and K. lactis. These yeast are able to assimilate and ferment several of the main carbohydrates in plain and fruit yoghurt. They are also able to assimilate lactate and citrate. They grow at low pH-values and temperatures below 10°C. Further several are able to produce esterases and lipases, proteinases and peptidases hydrolyzing the milk fat and protein. Therefore the occurrence of yeast even in low numbers in yogurt will limit the shelf-life (Fleet 1990).

The occurrence and growth of yeast in yoghurt are closely related to poor hygiene and sanitation (Fleet and Mian 1987). As emphasized by Fleet (1990); Jakobsen and Narvhus (1996) spoilage should be prevented through implementation of general principles of good manufacturing practice. To extend the shelf-life of yoghurt the use of preservatives are permitted in some countries. However, yeast can be very tolerant to some preservatives. As an example C. famata and K. maxianus are still able to multiply in the presence of sorbate and benzoate at concentrations of 500 mg/l (Fleet and Mian 1987).

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