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

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Abstract

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.

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Data availability

Important strains and their sequences were submitted to PPRI and GenBank, respectively. Other raw data/strains are available on request.

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Acknowledgments

The authors would like to thank Mrs. Lisa Warburg for the operation of the HPLC instrument.

Funding

This work was financially supported by the South African National Research Foundation (NRF) as part of the activities of the Chair of Energy Research: Biofuels and other clean alternative fuels (NRF grant 86423 awarded to WHvZ).

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Correspondence to Willem Heber van Zyl.

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12223_2021_850_MOESM1_ESM.pptx

Supplementary file1: Fig. S1 Malic acid degradation by newly isolated strains obtained from (a) apple pomace, (b) grape pomace and (c) plum samples, as well as (d) culture collection strains after 48 h of growth in YMC (malic acid only) and YMCG (malic acid + glucose) broth (with the exception of isolate 26 for which only YMC broth data was obtained). Each bar represents the average malic acid degradation per strain based on triplicate results after standardization against the reference strain, C. utilis YMV1153. Error bars indicate standard errors Fig. S2 Malic acid degradation capabilities of the 28 selected strains (indicated by their numerical designations) in (a) YMC broth and (b) YMCG broth when cultivated for 24 h and 48 h. Reference strain C. utilis YMV1153 indicated as (+). Each bar represents the average malic acid degradation per strain based on triplicate results strandardized against the (+) reference strain. Error bars indicate standard errors Fig. S3 GMI-assay depicting the color changes observed for the 28 strains after 70 h of growth. A color shift towards blue indicated the increase in pH from 3.3 towards 5.2 (conversion of malic acid to pyruvic acid). Triplicate strain cultures are labelled with their respective isolate numbers (as established in Table 1 and 3) (PPTX 417 KB)

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Steyn, A., Viljoen-Bloom, M. & van Zyl, W.H. Valorization of apple and grape wastes with malic acid-degrading yeasts. Folia Microbiol 66, 341–354 (2021). https://doi.org/10.1007/s12223-021-00850-8

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  • DOI: https://doi.org/10.1007/s12223-021-00850-8

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