Yeasts and Molds as Ancillary Cultures
Yeasts and molds are able to heavily contaminate the environment of dairy farms or production plants. They are often considered as harmful microorganisms for dairy products because they can cause defects (e.g., in yogurt). The common yeasts present in raw milk are Debaryomyces hansenii, Kluyveromyces marxianus var. marxianus, Kluyveromyces marxianus var. lactis, Saccaromyces cerevisiae, Candida lacticondensi,
C. famata, C. versatilis, C. lusitaniae, and Yarrovia lipolytica (formerly Candida lipol- ytica). Molds that can generally be found in raw milk belong to the genera Rhizomucor, Rhizopus, and Aspergillus. Several yeasts can survive and grow in refrigerated conditions, and together with some psychrotrophic bacteria capable to produce proteinase and phospholipase may affect the quality of raw milk stored under refrigerated temperatures (Moatsou et al., 2015).
On the other hand, some species are positively involved in the aromatic and organoleptic characteristics of many fermented dairy products. Some study (Romano et al., 2001) remarks that the significative yeast presence in water buffalo mozzarella can definitely contribute to the finished product organoleptic characteristics, despite the fact of being true environmental contaminants, massively present in the raw whey starters and in the acidified cooling brine. In particular, the lactose-fermenting (Klyveromices marxianus, Klyveromices lactis) and galactose-fermenting (Saccharomyces cerevisiae) yeasts contribute to the formation of CO2, but also of flavoring substances or their precursors such as ethanol, acetaldyde, and ethyl acetate. The above-mentioned yeasts, in association with some species of mesophilic hetero-fermentative lactobacilli, can be utilized as auxiliary cultures in a special patented way (Zambrini et al., 2012): pasteurized milk is cultured to prepare a semi-fermented milk that is added to milk in the cheesevat together with starter lactic acid bacteria, to produce an industrial mozzarella having the same organoleptic characteristics of the traditional Italian fresh mozzarella-in-brine.
Among Kluyveromyces species, Kluyveromyces marxianus has been isolated from a great variety of habitats which results in high metabolic diversity and elevated degree of intraspecific polymorphism. This species has been investigated for several biotechnological applications such as: production of enzymes (fi-galactosidase, fi-glucosidase, inulinase, polygalacturonases), of single cell-protein, of aroma compounds, of ethanol, reduction of lactose contenent in food products and production of bio ingredients from cheese whey as well (Fonseca et al., 2008).
Klyveromices marxianus presents a high thermotolerance, high growth rate, and lower tendency to slow down fermentation when exposed to excess sugar. The enzymes of industrial interest or application are usually inulinase, fi-galactosidase, fi-glucosidase, endopolygalacturonases.
Blue cheese production is perhaps the most well-known case of yeasts and molds application as ancillary cultures. Penicillium spores are added to the milk so that the final product is spotted or veined throughout with blue, blue-gray or blue-brown mold and carries a characteristic and distinctive smell and taste, deriving from the action of the moulds principally on milkfat. The main Italian blue cheese is Gorgonzola: In this case, the acidification of the curd is operated by a yogurt-type starter composed of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. Then an alcoholic fermentation of the curd by Saccharomyces cerevisiae strains takes place with formation of openings due to CO2 production. Finally, the cheese is dry salted and stored in the cool cellars to be cured. The blue veins are the result of the development of Penicillium roqueforti and Penicillium glaucum spores, added to the cheesemilk as well, that takes places only after the ripening cheeses are pierced to let the air get in. Centuries ago, Penicillium roqueforti and Penicillium glaucum spores were naturally present in the caves where the cheese wheels were stored to ripen and the best caves were those that guaranteed a deep contamination of the cheeses with the good molds. Today, all cheese producers use commercially available Penicillium roqueforti cultures, either liquid or dehyderated.
In the European Union, many blue cheeses such as Roquefort, Danablu, Cabrales, Gorgonzola, and Blue Stilton carry a protected designation of origin, meaning they can bear the name only if they have been made in a particular region in a EU member state. Blue cheeses with no protected origin name are designated simply blue cheese.