Constructing recombinant Saccharomyces cerevisiae strains for malic-to-fumaric acid conversion.

Saccharomyces cerevisiae with its robustness and good acid tolerance, is an attractive candidate for use in various industries, including waste-based biorefineries where a high-value organic acid is produced, such as fumaric acid could be beneficial. However, this yeast is not a natural producer of dicarboxylic acids, and genetic engineering of S. cerevisiae strains is required to achieve this outcome. Disruption of the natural FUM1 gene and the recombinant expression of fumarase and malate transporter genes improved the malic acid-to-fumaric acid conversion by engineered S. cerevisiae strains. The efficacy of the strains was significantly influenced by the source of the fumarase gene (yeast versus bacterial), the presence of the XYNSEC signal secretion signal and the available oxygen in synthetic media cultivations. The ΔFUM1Ckr_fum + mae1 and ΔFUM1(ss)Ckr_fum + mae1 strains converted extracellular malic acid into 0.98 and 1.11 g/L fumaric acid under aerobic conditions.

Valorization of apple and grape wastes with malic acid-degrading yeasts.

It is estimated that more than 20% of processed apples and grapes are discarded as waste, which is dominated by pomace rich in malic acid that could be converted to high-value organic acids or other chemicals. A total of 98 yeast strains isolated from apple, grape, and plum wastes were evaluated for their ability to degrade malic acid relative to known yeast strains. Most (94%) of the new isolates degraded malic acid efficiently (> 50%) in the presence and absence of exogenous glucose, whereas only 14% of the known strains could do so, thus confirming the value of exploring (and exploiting) natural biodiversity. The best candidates were evaluated in synthetic media for their ability to convert malic acid to other valuable products under aerobic and oxygen-limited conditions, with two strains that produced ethanol and acetic acid as potential biorefinery products during aerobic cultivations and oxygen-limited fermentations on sterilized apple and grape pomace. Noteworthy was the identification of a Saccharomyces cerevisiae strain that is more efficient in degrading malic acid than other members of the species. This natural strain could be of value in the wine-making industry that often requires pH corrections due to excess malic acid.