Yeast have been isolated from all kinds of dairy products (Fleet 1990; Fleet and Mian 1987; Jakobsen and Narvhus 1996; Rohm et al. 1992; Tilbury et al. 1974). Yeast are used as important starter cultures in food products such as bread, beer, wine, traditional fermented foods (Fleet 1990; Fleet and Mian 1987; Jakobsen and Narvhus 1996) and seems to be the most important micro-organism exploited by man. The role of yeast as micro-organisms, which could have a positive influence on the quality of dairy products, especially cheese, has not been widely accepted (Fleet 1990; Jakobsen and Narvhus 1996). Instead, yeasts have often been seen as spoilage organisms originating from poor hygiene and sanitation (Fleet and Mian 1987; Jakobsen and Narvhus 1996). The nutritional requirement of yeast, the ability to grow at low temperature, low pH, low moisture content, and high salt or sugar concentrations together with their enzymatic activity make yeast a natural part of the microbiota in dairy products. In 1995 the first symposium on "Yeasts in the dairy industry: Positive and Negative Aspects" was organized and held by the International Dairy Federation (IDF) expert group F47—Yeast. As reflected in the published literature the interest in yeast in the dairy industry has increased since and the knowledge of the positive and negative role played by yeast in dairy products has become more detailed. The key areas have been: (a) study of important technological characteristics of yeast, e.g., proteolytic and lipolytic activity, aroma formation and fermentation and/or assimilation of residual sugars and lactate and citrate and pigment formation especially related to different kind of cheese; (b) properties affecting both the negative role of yeast as spoilage organism causing quality defects as well as the potential of yeast as highly controlled starter cultures; (c) yeast classification and taxonomy based upon molecular methods. The present review will deal with the positive and negative aspects of yeast in the dairy industry.

A variety of yeast species has been isolated from milk and dairy products (Fleet 1990; Fleet and Mian 1987; Gadaga et al. 2001; Rohm et al. 1992; Roostita and Fleet 1996a,b; van den Tempel and Jakobsen 1998; Tilbury et al. 1974; Tudor and Board 1993; Westall 1998) but a few main species are frequently detected in dairy products. These yeast are representatives of the genea Debaryomyces, Klyveromyces, Yarrowia, Candida, and Saccharomyces (Viljoen 2001). The yeast occur both in the teleomorphic and the anamorphic state in dairy products, depending on the yeasts' ability to produce ascospores. Some of the most common yeast related to dairy products mentioned first by their teleomorphic then by their anamorphic state are: Debaryomyces hansenii/Candida famata, Galactomyces geotrichum/Geotrichum candidum, Yarrowia lipolytic/Candida lipolytica, Klyveromyces marxianus/Candida Kefyr, Klyveromyces lactis var. lactis/ Candida spherical, and Saccharomyces cerevisiae/Candida robusta.

To describe yeast occurring in dairy products and select and maintain yeast as a starter culture, identification at subspecies level is a necessity. The taxonomy of yeast is mainly based on the criteria described in the third and fourth edition of the taxonomical study "The Yeasts" edited by Kreger van Rij (1984) and Kurtzman and Fell (1998), respectively. The criteria includes micro- and macro-morphology in standard media, assimilation, and fermentation profiles of standard carbohydrates, growth at different temperatures and under defined osmotic pressure, development of pseudo or true mycelium, formation of ascospores as well as other phenotypic criteria. It takes time and experience to conduct a correct classical identification. Therefore, easier identification systems like API-ZYM systems (Biomerieux, Macy l'Etoile, France) are widely used. The API-test is based on assimilation profiles of the yeast. The API-kit's were originally made to offer a fast identification of yeast of medical importance. Several investigations have found food-related yeasts that were not included in the reading key of the kit (Deak and Beuchat 1988). The enlarged version of the test, API-kit 32C, has been found to be reliable in assisting in yeast identification and acceptable for a rapid identification of yeasts associated to cheese, as long as it is used in combination with the classical methods, especially when a reliable determination of the sexual form is required, for the identification (van den Tempel 2000). Other methods based on criteria such as profiles of volatile metabolites of the yeast (Magan et al. 2001; Westall 1998), analysis of volatile profiles of yeast by an electronic nose (Magan et al. 2001) and identification based on Fourier-transform of the specific infrared spectrum of the biochemical compounds of the dried yeast cell (Kiimmerle et al. 1998) have been reported as methods for fast identification at species level.

The number of yeast genera and yeast species has increased 100% or more since 1970 where the first edition of "The Yeasts" was published. At the same time yeast species described as belonging to different genera or species have been proven to belong to the same species. This shows the complexity of the yeast taxonomy but also the growing interest in yeast classification. One of the major reasons for the changes in yeast classification and taxonomy is the use of molecular methods including analyses of whole chromosomes and as with other micro-organisms, the classification of yeast is based more and more on genotypic rather than phenotypic criteria for yeast isolated from dairy products. Several methods have been reported for taxonomical use, for e.g., molecular techniques using SDS-gelelectrophoresis of whole-cell protein pattern, pulse field gelelectrophoresis (PFGE) (Montrocher et al. 1998; Esteve-Zarzoso et al. 1999; Gente et al. 2002; Jespersen and Kiihle 2000), restriction fragment length polymorphism analysis of mitochondrial DNA (DNA-RFLP) (Petersen et al. 2002; Romano et al. 1996; van den Tempel and Jakobsen 2000), internal genomic spacer sequences (IGS), 18S rRNA analysis (Cappa and Cocconcelli 2001) and polymerase chain reaction—restriction fragment length polymorphism (PCR-RFLP) analysis of the intergenic transcribed spacer region (ITS) (Caggia et al. 2001; Petersen et al. 2001). However, for many of the yeast genera related to dairy products the molecular methods are not yet fully developed or sensitive enough to separate strains on species or subspecies level.

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