Evaluating Starter Culture Performance
Objectively, the work of a SC is to carry out the desired fermentation, promptly and consistently, and to generate the expected flavour and texture properties relevant to the specific food product. Furthermore, particular requirements for a given SC, however, depend on the application for that culture. As industries expect particular product specifications, the specific traits and properties expressed by SC have also become quite demanding. For example, LAB SC used in dairy fermentations are now selected based not just on lactic acid production rates, but also on flavour and texture producing properties, salt sensitivity, compatibility with other strains, phage resistance and viability during production and shelf life (Hutkins 2006). As another example, traits desirable for brewing SC of Saccharomyces cerevisiae (e.g. flocculation and ethanol tolerance) are quite different from those important for bakers’ yeasts SC of Saccharomyces cerevisiae.
For the measurement of the activity and evaluation of the performance, several tests are routinely carried out to assess properties of SC. The term activity refers to the ability of SC to produce desirable changes in fermented products and is a consequence of many factors, such as physiological state of cultures. Usually, activity measurements are confined to the ability of SC, for example, to acidify the milk (LAB), whereas for yeast cultures rates of CO2 evolution may be appropriate.
Acidification activity (Fonseca et al. 2000) is considered the classic way to determine SC activity for LAB measuring the pH in the matrix at different time intervals. The capacity of a culture to adapt to a new environment and the biomass production may all be useful determinants. Activities other than cited above, such as proteolysis (Gatti et al. 2008), lipolysis and specific enzymes production, can also be determined by other means. Another critical test used specifically for dairy SC involves the determination of bacteriophage resistance (Leroy and De Vuyst 2004).
Performance characteristics for the yeast SC depend on the specific product. The ability of brewing yeasts to ferment all available fermentable sugars (attenuation) is a critical property that must be measured. For bakers’ yeasts, the measure of growing phases and the production of carbon dioxide in the dough (gassing rate) is necessary to evaluate activity. In wine manufacture, wine yeasts are evaluated based on the results for the growth at low temperatures, as well as for the resistance to sulfiting agents. For some applications, wine yeast should also have high ethanol tolerance and osmotolerance.
One absolute requirement for multiple SC strains is that each microorganism must be compatible with all the others within a particular culture. For example, LAB are well-known producers of bacteriocins, hydrogen peroxide and others antimicrobial components that can inhibit other microorganisms. The same manner, some yeasts produce inhibitory proteinaceous compounds called “killer” factors that are active against members of the same species or closely related species, therefore influencing how mixed SC strain are prepared. The activities of these toxins are analogous to the activities of bacteriocins in bacterial strains (Lowes et al. 2000). El Baz and Shetaia (2005) described four different assays (methylene blue, thiazolyl blue tetrazolium bromide, bromo cresol purple and plate count agar) for the evaluation of killer toxin activity of six isolates of yeast, Kluyveromyces lactis, Candida silvatica, Candida tropicalis, Rhodotorula mucilaginosa, Rhodotorula lactosa and Lipomyces tetrasporus, in the assessment of the viability of Saccharomyces cerevicae sensitive strain.