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Numerous traditionally aged cheeses are surface ripened and develop a biofilm, known as the cheese rind, on their surfaces. The rind of such cheeses comprises a complex community of bacterial and fungal species that are jointly responsible for the typical characteristics of the various cheese varieties. Surface ripening starts directly after brining with the rapid colonization of the cheese surface by yeasts. The initially dominant yeasts are acid and salt‐tolerant and are capable of metabolizing the lactate produced by the starter lactic acid bacteria and of producing NH3 from amino acids. Both processes cause the pH of the cheese surface to rise dramatically. This so‐called deacidification process enables the establishment of a salt‐tolerant, Gram‐positive bacterial community that is less acid‐tolerant. Over the past decade, knowledge of yeast diversity in cheeses has increased considerably. The yeast species with the highest prevalence on surface‐ripened cheeses are Debaryomyces hansenii and Geotrichum candidum, but up to 30 species can be found. In the cheese core, only lactose‐fermenting yeasts, such as Kluyveromyces marxianus, are expected to grow. Yeasts are recognized as having an indispensable impact on the development of cheese flavour and texture because of their deacidifying, proteolytic, and/or lipolytic activity. Yeasts are used not only in the production of surface‐ripened cheeses but also as adjunct cultures in the vat milk in order to modify ripening behaviour and flavour of the cheese. However, yeasts may also be responsible for spoilage of cheese, causing early blowing, off‐flavour, brown discolouration, and other visible alterations of cheese.

Change History: September 2019 The title of the chapter has been adapted from Propionibacterium spp. to Propionibacterium spp. and Acidipropionibacterium spp., due to adaptions in the classification. The authorship has been expanded. Overall, the term "Emmental" has been replaced by the generic term "Swiss-type cheese". MT Fröhlich-Wyder and A Thierry revised the 'Introduction' section. U von Ah updated the 'Classification and Identification' section, described the new genus Acidipropionibacterium and Propionibacteriaceae family, added Table 1 as a new Table and updated Table 2 (from the Table 1 of the previous version). A Thierry and SM Deutsch revised and enriched the 'Morphology, Envelopes and Growth Conditions' section. M Turgay largely extended and updated the section 'Genetics', renamed into 'Genetic properties', from new complete genome sequence data, added Table 3 as a new Table, and updated Table 4 (from Table 2 of the previous version), replaced Table 3 by an extended section 'Genetic engineering' and added a section 'Immunity'. S Irmler restructured and enriched the section 'Metabolism' with short information on the biosynthesis of vitamin B12, trehalose, and pigment and additional information on the conversion of aspartate into succinate and modified Figure 1. G Jan updated and edited the section 'Adaptation to Stress'. MT Fröhlich-Wyder updated the section 'Use as Ripening Cultures in Cheese'. G Jan and H Falentin updated and edited the section 'Relation to Nutrition as Probiotics and Producers of Nutraceuticals'. HP Bachmann expanded the section 'Antimicrobial activities' into a section entitled 'Use as Protective Cultures' and updated Table 5 (from the Table 4 of the previous version). This is an update of A.

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