# Quantitative Characterization of _Geotrichum candidum_ Growth in Milk
## 1 Introduction  
The close connection of _G. candidum_ with milk and dairy products was reported as early as 1850, when [PERSON] isolated and named this microorganism _Oidium lactis_. _G. candidum_ has undergone extensive taxonomic revision since the genus _Geotrichum_ was firstly erected by [PERSON] in 1809, and it is still evolving [1; 2; 3; 4; 5]. Nevertheless, the classification of this species as a filamentous yeast-like fungus is generally accepted [3; 4]. _G. candidum_ is currently known as a ubiquitous microscopic fungus with a worldwide distribution that is commonly found in soil, water, air, silage, grass, plants, fruits, vegetables, raw milk, and dairy products. It is also a commensal organism of the human and animal digestive tract [2; 3; 4; 5; 6; 7]. According to the International Dairy Federation (IDF) and the European Food and Feed Cultures Association (EFFCA), _G. candidum_ is included in an authoritative list of microorganisms with a documented history of safe use in fermented foods [8]. The application of this fungus in the manufacturing of fermented dairy products resulted from its common presence in raw milk, regardless of animal origin (cow, goat, sheep, camel, or buffalo) [2; 9; 10]. However, the concentration of _G. candidum_ in raw milk is generally low (\(<\)10\({}^{2}\) colony-forming units (CFU)\(\cdot\)mL\({}^{-1}\)). In dairy practice, _G. candidum_ is used as an adjunct culture in the production of regional fermented mills (e.g., Vili, Kefir) and a wide range of ripened cheeses, including soft mould-ripened cheeses, soft and semi-hard smear-ripened cheeses, and acid-coagulated cheeses [2; 5; 11; 12; 13; 14]. The growth of this milk fungus can be observed on the cheese's surface around the third day of ripening, and it can reach a density of up to 10\({}^{5}\)-10\({}^{7}\) TFU (thallus-forming units) per gram of cheese [4; 15]. G. candidium contributes to the cheese's flavour, aroma, and appearance. Characteristic organoleptic attributes resulting from the presence of _G. candidium_ in dairy products include mildly cheesy, mouldy, sweaty, putrid, acidic, yeasts, musty, fermented, cidery, and fruity flavours and a velvety, felted, or lightly fluffy appearance, with an overall good hedonic perception [2,16]. Additionally, this species may contribute to the microbial safety of dairy products. The antimicrobial potential of _G. candidium_ against undesirable and pathogenic microorganisms, including _Listeria monocytogenes_, _Staphylococcus aureus_, _Bacillus cereus_, _Escherichia coli_, _Aspergillus ochracus_, and _Mucor_ species has been described [2,4,17,18]. However, another aspect of the relevance of _G. candidium_ in the food industry must also be considered. _G. candidium_ is known as a spoilage organism in certain dairy products, e.g., butter, cream, and cottage cheese. Uncontrolled growth of this species may lead to the development of off-flavours and cheese defects, such as a slippery rind or toad skin defects [5,7]. _G. candidium_ has also been noticed as a machinery mould that can readily grow on all surfaces contacted by food products, and thus, its growth may cause severe economic losses for food producers [5,19]. With respect to its relevance in the food industry, the biochemical activity of _G. candidium_ has already been deeply investigated [5,7,20,21,22]. However, there are only a few studies regarding the quantitative approach to the growth dynamics of _G. candidium_. Detailed knowledge of microbial responses to the environmental conditions enables a better control under microbial behaviour, which is an important part of food quality management. Microbial ecology and growth parameters can be quantitatively defined with the use of predictive microbiological tools. The concept of predictive microbiology is based on mathematical models, that enable an objective evaluation of microbial behaviour (growth, survival, inactivation, etc.) in a dependence on environmental factors [23,24], since the responses to a changing environment are reproducible [25].