Hydroxy fatty acids: Structures and antifungal activities in foods

• Author / Creator

  Nuanyi Liang

• Spoilage and mycotoxin production by mycelial fungi threatens global food security. Climate change and increasingly complex food value chains may exacerbate these concerns. Alternative or complementary antifungal agents for food and agricultural applications may alleviate problems associated with fungi. Hydroxy fatty acids (HFAs) are novel antifungal agents, however, due to the diversity in the molecular structures of hydroxy fatty acids from various sources, their structure-function relationships, modes of action, and potential applications, have not been fully investigated. In this work, several HFAs were extracted from two types of lactobacilli fermented cultures and four types of plant seed oils. HFAs were purified using solid-phase extraction (SPE) or high-speed counter-current chromatography (HSCCC). The purified HFAs were then characterized by LC-APPI-MS/MS and LC-ESI-MS/MS. Eight types of HFAs were tested to challenge the growth of common food-related fungi, including filamentous fungi and yeasts (Chapter 3-5). Structure-antifungal activity relationships demonstrated that the location of the hydroxy groups within a HFA determines its antifungal activity. Mono-HFAs with hydroxy group located at the middle of the fatty acid chain (C9-13) were found to have high anti-mold activities. Remarkably, filamentous fungi, but not yeasts, were sensitive to HFA with a hydroxy group in C9-13 location. To investigate the difference between HFA-resistant yeasts and -sensitive molds, the content of a cellular membrane fluidity modifier, ergosterol, was quantified. With one exception, the HFA-resistant yeasts tend to have high sterol content (Chapter 5). This structure-function relationship elucidated the application of HFA in preventing mold growth while allowing yeast viability and activities. This relationship also indicates that HFA mainly targets fungal strains with a relatively low ergosterol content, but other resistant mechanisms may also be involved.
  A representative HFA was applied in sourdough bread to determine their efficacy in situ (Chapter 6). The anti-mold effect of ricinoleic acid was weaker in flaxseed-flour sourdough bread when compared to wheat-flour sourdough bread. In addition, collaborative work in plants indicated synergistic antifungal effects of coriolic acid with other plant components. These indicate that the antifungal activities of HFA can be affected by the matrixes of food and plants synergistically or antagonistically.
  Due to the bacterial capacity to convert unsaturated fatty acids into HFA, the bacterial production of HFA and fatty acid metabolism in situ were also evaluated in a fermented sausage model with well-controlled microbiota (Chapter 7). HFAs were detected in the model sausages, but antifungal HFAs were not produced at levels that exert antifungal activity. This supports the compatibility of fermented sausage with the use of mold as ripening agents. Unpublished experimental data also documented that the antifungal activity of lactic acid bacteria in model dairy systems was not dependent on their ability to convert fatty acids to HFA.
  Results presented in this thesis contribute to understanding the structure-function relationship of HFAs, their mode of action, and their application in situ. This can further enable the development of lipid-based antifungal approaches suitable for application in food and agriculture, which in turn has the potential to reduce food waste and so help to address the increasing global food and agricultural demands.

• Subjects / Keywords

  • plant protection
  • fermented sausage
  • filamentous fungi
  • LC-MS/MS
  • high-speed counter-current chromatography (HSCCC)
  • hydroxy fatty acid
  • yeast
  • lactobacilli
  • food protection
  • antifungal
  • spoilage fungi
  • fermentation
  • purification
  • bread spoilage
  • food security
  • plant seed oil

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-wmf4-rs17

• License

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