Identification and evaluation of thermotolerance of yeasts from milk in natura exposed to high temperature and slow and fast pasteurization

Abstract

Milk is considered one of the basic raw materials of animal origin; it must present hygienic quality and physical-chemical properties suitable for processing and human consumption. Thus, the ingestion of milk in natura when not properly treated can be characterized as an opportunistic route of transmission of possible microbial pathogens, which can offer risks to public health. The present study aimed the yeast identification, to analyze the thermo-resistance of yeasts isolated from fresh milk, and to trace the susceptibility profile of the isolates to antifungal agents. For this, 23 samples of fresh milk type B, collected by manual or mechanical milking, were stored in collective refrigeration tanks of farms located in the Metropolitan Region of Natal and nearby, State of Rio Grande do Norte (RN), Brazil. Twenty samples of fresh milk commercially traded in the city of Ceará-Mirim RN were also analyzed. The yeasts were quantified by count of colony-forming units (CFU). All isolated species were treated by slow pasteurization (62-64 °C for 30 min) and fast (72-75 °C for 20 s), as well as by boiling (100 °C). Fifty yeast strains were obtained, and the species were identified as Candida tropicalis (28%), Candida parapsilosis (14%), Candida albicans (12%), Candida glabrata (10%), Candida krusei (10%), Kluyveromyces marxianus (10%), Candida guilliermondii (8%), Candida rugosa (2%), Candida orthopsilosis (2%), Pichia manshurica (2%), and Kodamaea ohmeri (2%). Five isolates showed resistance to the antifungal agents tested. Among all the isolates submitted to heat treatment, 80% were resistant to fast pasteurization and 60% to boiling, but none of them resisted the slow pasteurization. The milk collected through mechanical milking and stored in collective cooling tanks, presented higher rates of yeast contamination, compared to milk samples collected by manual milking and kept under the same storage conditions.

Keywords: Boiling; Candida; Milk; Pasteurization; Yeasts.