Whey and whey by-products as drivers of yeast microbial diversity

Whey is largely discarded by cheese manufacturing plants and in the least transformed to ricotta cheese and whey concentrates. Whey-derived yeasts have recently been considered for both spoilage activity and biotechnological potential of these products. Nevertheless, their ability to hydrolyze lactose and whey proteins is less studied. In this work yeast isolates (106), randomly collected from different samples of whey, ricotta, de-proteinized whey (scotta) and fermented ricotta cheese were identified and screened for glucose, galactose and lactose assimilation and lipase and proteolytic activities. Most of isolates belonged to Kazachstania servazzii (45%) and Kluyveromyces spp. (28%), whereas the remaining genus isolates were in descending order represented by Candida (11%), Trichosporon (7%), Pichia (6%), Yarrowia (2%) and Debaryomyces (1%). Interestingly, Kazachstania servazzii predominated in all whey-based products, in contrast to microbial whey composition that varied more. Most of yeast isolates highly metabolized glucose and galactose, whereas few (17 out of 73) fermented lactose. In particular, lactose was mostly metabolized by Kluyveromyces marxianus and K. lactis but not by all K. servazzii isolates. However, three lactose negative isolates of K. marxianus (RFY9, RFY30 and RFY34) were found for the first time. Lipolytic activity were registered only for Candida spp. Yarrowia lipolytica and Trichosporon spp. isolates and for K. marxianus RFY95. In addition, few K. marxianus and K. servazzii showed proteolytic activity towards milk total proteins, but only K. marxianus RF15 was able to hydrolyze whey proteins. These results suggest that the dominant yeast species of whey and whey by-products encompass isolates with different sugar preferences. In fact, unlike the most represented K. servazzii, most of K. marxianus isolates was well adapted to whey environment because of their biochemical features. On the other hand, the dominance of K. servazzii could be supported by lactose-fermenting species such as K. marxianus or lactic acid bacteria.