The process of fermentation is one that has been with humanity since nearly the beginning in the Neolithic era and has been used in a variety of ways for unique types of foods. The most obvious kinds include bread and alcohol, but there are also plenty of other, more specialized products out there. And there are inherent benefits to fermentation, as certain kinds of it help preserve the foods in question and protect them against decay due to microorganisms and also the introduction of harmful compounds. But they aren’t fully immune to such effects.
Fermentation and Fermentation’s Enemy
For fermented foods that use yeast and involve liquid of some sort, whether done through the fermentation of vegetables, meat, fish, or more, there is often a layer of microorganisms that grow on the surface of the liquid, creating colonies of unknown origin. When some samples have been tested, it has been discovered that these colonies are made up of several strains of yeast from different species and so came to be known as film yeast due to the film they form on the surface. But their alternative name brings up the major concern with their occurrence, spoilage yeast.
Though the officially recognized scientific term for the phenomenon is white colony-forming yeasts (WCFY) and, in addition to spoiling food and softening food, there has also been the possible concern that these yeasts introduce contaminants or toxin proteins into the fermented product. So that was yet another reason to investigate them. While several studies in the past five years have focused on a community level observation of the colonies, none have yet conducted a next generation sequencing method of truly isolating the genomes of the involved yeast species.
A Kimchi Investigation
That is, until now, thanks to South Korea’s World Institute of Kimchi. As would be guessed, their focus was on the fermentation of vegetables in the form of kimchi and how to both characterize and prevent these yeast colonies from forming. They began with samples of these white films from four types of kimchi: cabbage, mustard leaf, young radish, and watery kimchi. Their DNA libraries were focused around the internal transcribed spacer 2 (ITS2) gene region, which has for about a decade been considered a strong candidate as a DNA barcode for certain genomes due to its consistency and conserved nature.
After this identification, the isolated genomes were run through sequencing machines and the strains in question IDed as to which species they belonged to. Overall, 8 species were pinned down and labeled as operational taxonomic units (OTUs), though 3 of them were separated as minor candidates, due to being at less than 1% of the overall colony amount in all of the samples. The rest were then studied in greater detail.
H. uvarum was one of the isolates and a possible concerning one. It is known to alter the composition of wine, in a positive manner, and produce toxins against competing microorganisms. It is however non-pathogenic, as were the other major specimens found, along with them having flavor altering compounds. None of this would be a problem inherently, except for one strain that is known to cause spoilage through gas production. Furthermore, all of the other compound-making strains are unknown in their effect on food preservation, especially when combined in such a manner inside a biofilm.
Now that WCFY components have been identified for the first time, along with the first sequencing of the yeast species Pichia kluyveri and Candida sake, the next step the researchers will take is direct exposure of the yeast colonies to animal models to confirm that there are no toxicity concerns. And, even if that is indeed the case, they will still look into ways to minimize the creation of WCFYs on fermented foodstuffs so as to reduce issues of spoilage and taste changes.
Photo CCs: Gimchi from Wikimedia Commons