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Saccharomyces cerevisiae

Bread: carbon dioxide production

Saccharomyces cerevisiae

Ale beers

Saccharomyces pastorianus

Lager beers

Saccharomyces cerevisiae

Wine

starter culture yeasts.As with other starter cultures, strain selection is based primarily on the desired flavor and sensory attributes of the final product. However, other production traits, including the ability to flocculate, to grow at high sugar concentrations, and to produce adequate ethanol levels, are also relevant and are taken into consideration. The brewing industry is another potential user of yeast starter cultures, although large breweries generally maintain their own proprietary cultures.As for wine, strain selection is based on characteristics relevant to the needs of the particular brewer, and include factors such as floccula-tion, flavor development, and ethanol production rates.

Mold starter cultures

Despite the relatively few products made using fungi, manufacturers of these products often prefer to use mold starter cultures rather than to propagate their own cultures. Thus, fungal starter cultures are available for several types of cheeses, including both the blue-veined types (e.g., Roquefort and Gorgonzola), as well as the white surface mold-ripened cheeses (e.g., Brie and Camembert).The blue mold cultures consist of spore suspensions of Penicil-

lium roqueforti and white mold cheese cultures contain Penicillium camemberti. Fungal cultures are also used in the production of many of the Asian or soy-derived fermented foods (Chapter 12).The main examples include tempeh, made using Rhizopus microsporus subsp. oligosporus, and miso and soy sauce, made using Aspergillus sojae and Aspergillus oryzae. There is also a small but important market for fungal starter cultures used to produce European-style sausages and hams.

Strain identification

Although it may seem obvious that the precise identities of the organisms present in a starter culture should be known, strain identification is not a trivial matter. In fact, microbial system-atics is an evolving discipline that has been significantly influenced by advances in genetics, genomics, and various molecular techniques (Box 3-2). Thus, the movement or reassignment of a given organism from one species to another is not uncommon. In recent years, for example, strains of lactic acid bacteria, brewers' yeasts, and acetic acid-producing bacteria used in vinegar production have all been re-classified into different species or assigned new names altogether.

Box 3—2. Who's Who in Microbiology: Identifying the Organisms in Starter Cultures

In the early days of microbiology, Pasteur, Lister, and their contemporaries could view an organism microscopically, note its appearance, perhaps even describe a few physiological properties, and then assign a name to it (Krieg, 1988).Thus, classification was initially based on a limited number of phenotypic properties or traits. However, this situation changed in the early 1900s as more biochemical, physiological, and genetic characteristics became known and as objective methods for assigning particular organisms to specific groups were adopted.Thus was borne the science of microbial taxonomy and the development of classification schemes for bacteria.

Although classification and nomenclature are generally important for all biologists, how an organism is named and the taxonomical group into which it belongs are especially relevant for starter culture microbiologists. For many years, up until the 1970s, identification of lactic acid bacteria relied mainly on ecological, biochemical, and physiological traits (see below).The first comprehensive description of these bacteria was provided in 1919 by Orla-Jensen, and 20 years later the Sherman scheme was published (Orla-Jensen, 1919 and Sherman, 1937).As an aside, both of these historical reviews make for excellent reading; the Sherman paper was the first review article in the very first issue of the prestigious Bacteriological Reviews journal series.The Sherman scheme, which is still often used today, systematically placed streptococci into four distinct groups: pyogenic, viridan, lactic, and enterococci (Table 1).

Table 1. Sherman scheme for classification of streptococci1.

Growth in presence of:

Division

Representative

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