Characterization of yeasts isolated from artisanal short-ripened cows' cheeses produced in Galicia (NW Spain).

A total of 143 presumptive yeast isolates were obtained from the predominant microflora of 21 short-ripened starter-free raw cow's milk cheeses made in Galicia (NW Spain), and the following 68 isolates were identified by both genotyping and sequencing methods: Yarrowia lipolytica (21 isolates), Kluyveromyces lactis (18), Debaryomyces hansenii (11), Pichia guilliermondii (11), Pichia fermentans (4) and Saccharomyces cerevisiae (3). Of these, Y. lipolytica and K. lactis displayed the strongest extracellular proteolytic activity on skim milk agar, and none of the D. hansenii isolates showed any activity on this medium. Y. lipolytica also displayed the highest lipolytic activity on Tween 80 and on tributyrin. This species, which was characterized by production of butanoic acid, free fatty acid esters and sulfur compounds in pasteurized whole milk, was responsible for rancid and cheesy flavors. K. lactis mainly produced acetaldehyde, ethanol, branched chain aldehydes and alcohols, and acetic acid esters, which were responsible for alcoholic, fruity and acetic notes. The volatile profiles of D. hansenii were rather limited and characterized by high levels of methyl ketones. Most of the yeast isolates were described as tryptamine producers, although low concentrations of histamine were produced by five Y. lipolytica and two P. fermentans isolates. We conclude that selected Y. lipolytica strains could be used as adjunct cultures in the manufacture of Arzúa-Ulloa and Tetilla cheeses, and selected K. lactis strains could be used as co-starters in the manufacture of acid curd Cebreiro cheese, thus contributing to the sensory quality and typicality of the cheeses.

Determination of the growth stages of Lactobacillus paracasei subsp. paracasei M3 from Bulgarian yellow cheese by electroconductivity.

The growth curve and the bacteriocin production by Lactobacillus paracasei subsp. paracasei M3-a strain isolated from Bulgarian yellow cheese, was studied using electroconductivity measurements in the culture medium (MRS). The bacteriocin produced by this bacterium inhibits strains belonging to the Candida species as well as Bacillus subtilis ATCC 6633. The use of the direct impedance technique coupled with statistical treatment of the results for the strain's growth curve allowed the detection of the growth phase-related bacteriocin production. Definition of the average particle size and aggregation estimations for the cells of the bacteriocin-producing strain was also possible. The comparison of results of impedance measurements with results obtained by classic microbiological methods and microscopic observations confirms the good correlation between these methods. In addition, the advantages of an impedance method such as rapidity, simplicity and higher sensitivity are highlighted.