For dairy products, yeasts are mainly used in cheese production but may also be involved in the production of fermented milk. Yeasts are primarily used as single-starter cultures but many products are still produced by back-slopping or spontaneous fermentation. A mixture between the used starter culture and a dominant indigenous flora is also seen in many dairy products. Yeasts are in most cases used as secondary starter cultures in order to enhance the aroma production or to facilitate the growth of other microorganisms. In general, the functions of yeasts during cheese production and their influence on the cheese quality are poorly investigated (114).
The occurrence and functions of yeasts have been especially studied for the production of surface-ripened cheeses such as Brick, Limburger, Port Salut, Taleggio, Tilsitter, Trappist, and Danish Danbo cheese. The surface smear of these cheeses is found to consist of a mixed flora comprising both yeasts and bacteria. For cheese such as Danish Danbo, the osmotolerant yeast Debaryomyces hansenii has been found almost exclusively (76), whereas in other types of surface-ripened cheeses, yeasts such as Candida zeylanoides, Yarrowia lipolytica, Kluyveromyces lactis, and others have been found (115-117). The yeasts initiate the ripening process by degradation of lactate, thereby increasing the pH on the cheese surface and allowing the growth of a more acid-sensitive bacterial flora comprising, among others, Brevibacterium linens (118). It should be mentioned that the species D. hansenii has been divided into two varieties, D. hansenii var. hansenii and D. hansenii var. fabryii (119); apparently, the predominant variety seen in cheese production is D. hansenii var. hansenii, and isolates of this variety have been introduced as starter culture (76,118). K. lactis has further been found to occur in high numbers in soft cheeses such as Camembert (20). S. cerevisiae has also been used as a starter culture in the production of Gorgonzola, especially, but it apparently also occurs spontaneously, together with other yeasts, as an integral part of the microflora of both blue-veined cheeses and some types of soft cheeses (20,120). Saccharomyces cerevisiae has been reported to stimulate mycelial growth and conidia formation of Penicillium roqueforti as well as influencing aroma formation and having some proteolytic activity (120,121).
Even though previously considered as a mold, Galactomyces geotrichum (Geotrichum candidum) is now thought to be as a yeast species (122) and will be considered as such in the following. G. geotrichum is associated with milk and the dairy environment. It is known as a starter culture for several types of mold-ripened cheeses (e.g., Camembert), surface-ripened cheeses, and cheeses such as the French St. Albray. On the other hand, G. geo-trichum has also been shown to be a potential spoilage organism. G. geotrichum is sometimes used in combination with other microorganisms such as Penicillium camemberti or Brevibacterium linens for the production of surface-ripened cheeses.
Even though not used as a starter culture in the dairy industry, Yarrowia lipolytica is often found in soft, blue-veined, and surface-ripened cheeses. Y. lipolytica is characterized by having a quite pronounced lipolytic and proteolytic activity (123) that might be difficult to control if used as a starter culture for cheese production. Furthermore, Y. lipolytica is known to produce brownish pigments in cheeses.
For fermented milk, yeasts within the genera Candida, Galactomyces, Kluyveromyces, Saccharomyces, and Torulaspora are generally used (124). G. geotrichum (G. candidum) is used as commercial starter culture in the production of viili, a Scandinavian fermented milk product, and Saccharomyces unisporus and Kluyveromyces marxianus (Candida kefyr) are used as commercial starter cultures in the production of milky kefir. S. cerevisiae has also been reported to be involved in the fermentation of a number of indigenous African fermented milk products known under names such as amasi, nono, and rob (125-127).
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