Yeasts are generally considered as food contaminants resulting in excessive food waste and indicating poor hygienic conditions in the food industry. Although there are worldwide numerous reports on the occurrence of yeasts in dairy products, literature provides only limited data on the diversity of yeasts in white-brined cheese (WBC) and the production environment.

Yeasts easily disseminate in the dairy manufacture through different sources such as the starter cultures, brine, floor, cheese vats, air, and personnel. HACCP, GMP, and ISO are some examples of principles implemented for prevention and control of yeastcontaminants in dairy industries.

Yeast species such asDebaryomyces hansenii,Geotrichum candidum,Kluyveromycesmarxianus,Kluyveromyces lactis,Rhodotorula mucilaginosa, Trichosporon spp., andYarrowia lipolytica are dominant in WBCs produced in Mediterrenean and Middle East, such as Feta, Domiati, Beyaz Peynir etc. Deterioration of cheese products has been reported when yeasts occur in levels of 5 – 6 log CFU/g. Proteolytic and lipolytic activity of yeasts result in release of free amino and fatty acids responsible for yeasty, bitter, and fruity off-flavours. Metabolic activities such as carbohydrates fermentation and tyrosine catabolism leads to CO2 production and discoloration of cheese, respectively.

In the first study, the aim was to enumerate, taxonomically characterize, and identify yeast contamination in the production area of WBC. A total of 26 sites, including milk pasteurization, curd processing, and packaging were examined for yeast contaminants. Samples of whey, curd, and air were detected with yeasts counts up to 6.27 log CFU/mL, 5.44 log CFU/g, and 1.02 log CFU/m3, respectively. Following the genotypic classification and identification yeasts species Candida intermedia, K. marxianus, and Pichia kudriavzevii dominated. However, less frequently Candida spp ., Cutaneotrichosporon spp., Papiliotrema sp., Rhodotorula sp., Vanrija sp., and Wickerhamiella sp. were identified.

Afterwards, the potential succession of yeast contaminants in two types of WBC for 52 weeks shelf-life was studied. Yeasts counts were high enough (7 log CFU/g), especially in lower incubation temperature (5 °C). A total of 132 representative isolates were further identified as Candida zeylanoides, D. hansenii, Candida parapsilosis, Kazachstania bulderi, K. lactis, Pichia fermentans, P. kudriavzevii, R. mucilaginosa, Torulaspora delbrueckii, and Wickerhamomyces anomalus. Interestingly, higher heterogeneity observed in lower yeast counts indicating yeast-yeast interactions.

Antagonistic interactions have long been studied between microorganisms. Killer toxins or mycocins are the antimicrobial compounds produced by yeasts having lethal effect against bacteria and fungi. Purified mycocins have been determined to act in a narrow pH and temperature assortment. However, the exact modes of action, mechanisms, and responsible genetic determinants need further research and clarification.

Moreover, in the last part of this thesis, yeast-yeast antagonic interactions characterized and the responsible genetic determinants identified. A panel of 84 yeasts isolated from WBC and its production facilities were evaluated for their killer activity and production of the respective killer toxins. D. hansenii, K. lactis, and W. anomalus were the ones with the highest fungicidal effect against a variety of yeast species. The proteinaceous nature of killer toxins was verified and the corresponding genes were obtained for D. hansenii and K. lactis using Oxford Nanopore Technologies.

To summarize, this PhD study shed light on the knowledge of spoilage yeast taxonomy and occurrence in WBC and the dairy production environment. It will further contribute to the control and quality management of cheese at dairies, and the proper implementation of hygienic and sanitation practices. The results of yeast-yeast interactions were further highlighted the importance of antimicrobial compounds produced by yeasts and their potential use in food industry in terms of a more natural preservation method.