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Investigating the microbiological safety of plant-based food distillates and hydrosols

Primary Supervisor: Dr Amir Anvarian, School of Chemical Engineering

Secondary supervisor: Dr Taghi Miri

PhD project title: Investigating the microbiological safety of plant-based food distillates and hydrosols

University of Registration: University of Birmingham

Project outline:

    The primary aim of this study is to determine the microbiological safety of plant-based food distillates and hydrosols, also known as floral, herbal or essential waters. These include floral waters such as rose and orange blossom waters as well as hydrosols of spices and herbs like peppermint, thyme, rosemary and oregano. Hydrosols have historically been used either as natural flavourings in foods or as topical and/or oral traditional medicines throughout the world and increasingly in developed countries among the health-conscious population.

    Once produced, most hydrosols are not subjected to any heat treatment in order to maintain their organoleptic quality and to prevent the degradation of their valuable aromatic volatile compounds. Moreover, hydrosols are traditionally produced once a year, during the harvest season of the flower or the herb and then bottled and stored in a cool and dark place for up to a year until the following season. Due to their mild acidic to neutral pH, most hydrosols could potentially support the growth of mycotoxin-producing fungi and pathogenic bacteria. Interestingly, despite their extensive worldwide use and potential for supporting microbial growth, including pathogenic enteric bacteria, very few studies have been conducted on their microbiological safety and shelf-life. This study will aim to investigate not only the microbiological quality of the products but also the processing conditions needed to achieve microbiologically safe products while maintaining optimal organoleptic quality.

    The main objectives of the study are as follows:

    • Isolation, enumeration, and molecular detection of microorganisms in hydrosols. This will include aerobic microorganisms, yeasts and moulds, coliforms and Enterobacteriaceae including Escherichia coli, Pseudomonas, Listeria spp., Staphylococcus aureus and anaerobes especially Clostridium spp. In addition to traditional plating techniques which will be used in order to determine the culturability of the cells, the cellular viability will also be determined using flow cytometric and staining techniques. This will enable us to investigate the potential induction and resuscitation of viable but non-culturable (VBNC) cells in food hydrosols. The natural microbiota of the product will also be determined using sequencing techniques.
    • Determining the occurrence and concentration of mycotoxins in the final product throughout its shelf-life.
    • Application of novel food processing techniques. Although most hydrosols contain potentially antimicrobial essential oils and/or volatile compounds, nonetheless, some studies have indicated that their concentration might not be sufficient to prevent the growth of pathogenic microorganisms. And even though traditional techniques such as pasteurisation and sterilisation can render the product safe, these techniques can have adverse effects on the product’s organoleptic qualities. This study will investigate the potential for using mild novel food processing techniques such as pulse field electric processing, ohmic heating, and ultra-sonication to produce high quality yet microbiologically stable hydrosols.
    • Conducting objective and subjective sensory analyses of hydrosols in order to investigate the impact of minimal and non-thermal processing on organoleptic properties of the final product. This will include using objective instrumental techniques such as GC-MS and HPLC in order to determine the volatile compound and essential oil content of the product, both of which play an critical role in the organoleptic quality of the product. Subjective sensory profiling of the product will also be conducted using analytical and affective tests using trained panels and consumers respectively.


    BBSRC Strategic Research Priority: Sustainable Agriculture and Food: Microbial Food Safety

      Techniques that will be undertaken during the project:

      • Plating techniques
      • Flow cytometry
      • 16S rRNA Sequencing
      • Pulse Field Gel Electrophoresis (PFGE)
      • Whole Genome Sequencing (WGS)
      • High-performance liquid chromatography (HPLC)
      • Gas chromatography–mass spectrometry (GC-MS)
      • Pulse Field Electric (PEF) Processing
      • Ohmic Heating
      • Ultra-sonication

      Contact: Dr Amir Anvarian, University of Birmingham