Although yeasts play a minor role in dairy fermentations, several fermented milk products with a natural yeast-containing microflora exist. A distinctive feature of these products is that, in addition to the lactic acid fermentation induced by lactic acid bacteria, a slight alcoholic fermentation due to yeasts takes place (41,42). The best-known examples offermentation of milk by a combination of yeasts and lactic acid bacteria are kefir and kumiss, both originating from countries of Eastern Europe and Asia. The predominant species of yeast in kefir and kumiss are Kluyveromyces marxianus, Candida kefir, Saccharomyces cerevisiae, and Saccharomyces delbrueckii. The heterogeneity of the species found can be explained by the different techniques of kefir grain cultivation. Yeasts play an important role in promoting symbiosis among the microorganisms present, CO2 formation, and development of the characteristic taste and aroma (43).

Yeasts are also involved in the ripening of surface-ripened cheeses. Their primary role is to lower the acidity of the cheese surface, which allows the development of a secondary microbial flora (44,45). Positive interactions between yeasts and starter organisms are well documented for semi-soft cheeses, such as Limburger and Tilsit (46). The yeasts metabolize lactate, which causes an increase of the pH, and may secrete factors that promote the growth of Brevibacterium linens, which is essential for the ripening of these cheeses (45). Yeasts are also believed to promote the development of Penicillium roqueforti in blue cheeses. They may contribute to the open structure of these cheeses by formation of gas (47). On the surface of Camembert, the dominating yeast flora inhibit the growth of

Figure 3 Effect of addition of the thermophilic adjunct culture I233 on the flavor development of a Gouda cheese after 3 months of ripening. The application of this adjunct culture results in a strong decrease in the bitter score during sensory evaluations. Starters A and B are known for the production of bitter-tasting peptides during cheese ripening.

the spoilage molds. The most prominent yeasts in cheese are Debaromyces hansenii, Kluyveromyces marxianus, Kluyveromyces lactis, Yarrowia lipolytica, Saccharomyces cere-visiae, and Zygosaccharomyces rouxii (48,49).

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