Brewing Yeasts

The brewing industry has a long tradition for the use of starter cultures of brewing yeast based on single cell cultures. Worldwide, up to a thousand different brewing yeast cultures have been described. The brewing yeast strains vary in their technological properties, including aroma production, rate and degree of attenuation, flocculation, oxygen requirement, and reproduction (84). During brewing fermentation, maltose is the most dominant carbon source but sucrose, glucose, fructose, and maltotriose will also be present and utilized. Brewing yeast strains have been shown to vary in the ability to utilize maltose, and genotypic variations in the number of maltose transporter genes have been reported (85).

Two types of Saccharomyces yeasts are involved in beer fermentation: ale yeasts (also known as top-fermenting yeasts) and lager yeasts (also known as bottom-fermenting yeasts) (84). Ale yeasts have, since the last century, been classified as S. cerevisiae, whereas lager yeasts have been known under a variety of names such as Saccharomyces carlsbergensis, S. uvarum, and S. cerevisiae. The development of molecular typing techniques has revealed several genetic differences between ale and lager brewing yeasts (86-90), and according to recent classifications, lager yeasts are now considered to belong to Saccharomyces pastorianus (91) even though they often are still referred to as Saccharomyces carlsbergensis (92). However, some confusion still exists regarding the phylogenetic relationship between lager yeasts and other yeast within the genus Saccharomyces. It appears to be generally accepted that lager yeasts are alloploid and contain parts of two divergent genomes (93,94), one from S. cerevisiae and one from another Saccharomyces species, most likely S. bayanus (88,95,96)

Table 9 Examples of Yeast Species Used as Starter Cultures or Occurring Spontaneously at High Numbers in Fermented Products

Fermented foods and beverages Yeast speciesa Products

Beer

Wine

Indigenous fermented beverages

Distilled alcohol

Bread

Cheese

Fermented milk

Meat products

Saccharomyces cerevisiaeb Saccharomyces pastorianusb Saccharomyces cerevisiaeb Saccharomyces bayanusb Candida spp. Hanseniaspora spp. Kloeckera spp. Metschnikowia spp. Pichia spp.

Saccharomyces cerevisiaeb Candida spp. Galactomyces geotrichum

(G. candidum) Hanseniaspora uvarum

(K. apiculata) Kluyveromyces africanus Pichia spp. Rhodoturula spp. Saccharomyces spp. Schizosaccharomyces pombe Schizosaccharomyces japonicus Torulaspora delbrueckii

(C. colliculosa) Saccharomyces cerevisiaeb Schizosaccharomyces pombe Saccharomyces cerevisiaeb Saccharomyces exiguous (C. holmii) Debaryomyces hansenii (C. famata)b Galactomyces geotrichum

(G. candidum)b Saccharomyces cerevisiaeb Candida zeylanoides Yarrowia lipolytica (C. lipolytica) Kluyveromyces lactis (C. spherica) Kluyveromyces marxianus (C. kefyr) Galactomyces geotrichum (G. candidum)b Candida spp.

Kluyveromyces marxianus (C. kefyr) Saccharomyces unisporus Saccharomyces spp. Torulaspora spp.

Debaryomyces hansenii (C. famata)b Candida zeylanoides Debaryomyces polymorphus Pichia guilliermondii (C. guilliermondii) Pichia membranifaciens (C. valida) Cryptococcus spp.

Ale, stout, porter, Pilsner Red and white wine, sherry,

Kafir beer, plantain beer, palm wine, sugar cane wine, sake

Whisky, rum, aquavit

Wheat bread, rye bread

Surface-ripened cheeses, Camembert, Gorgonzola, and other blue-veined cheeses

Viili, kefir, indigenous sour milk

Sausages, cured ham, bacon a Anamorph form is given in parenthesis. b Commercial starter cultures are available.

or a specific strain of Saccharomyces monacensis (92,97), which according to recent taxo-nomic keys, now also belongs to S. pastorianus (91).

Starter cultures of S. cerevisiae have also been reported to be used in the production of South African sorghum beer (98), and in addition to its use as an industrial starter culture, S. cerevisiae has been isolated from a variety of different indigenous spontaneously fermented beers or beer-like beverages (75).

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