Cold plasma-induced quality changes in foods and beverages and plasma’s potential use in space exploration
The long duration and extreme environment of space missions have led to significant challenges (weight loss and food aversion) in providing sustenance for astronauts. Traditional treatments that extend the shelf-life of food are often synonymous with reduced sensorial quality. Furthermore, future missions, such as to Mars (in the late 2030s), will increase the need for food with prolonged long shelf-life, of diverse sensory appeal. Cold plasma (non-thermally ionised gas) can reduce the negative impacts of thermal processing methods (e.g., nutritional loss, colour leaching and texture degradation), whilst also reducing microbial growth and increasing shelf-life. Cold plasma consists of free electrons, ions, reactive atomic and molecular species, and ultraviolet radiation that can generate various chemical reactions which can be controlled through various input parameters. However, there has been little research into the sensory impacts of cold plasma treatments, especially the effects on flavour, which this PhD thesis therefore sought to address. The potential for cold plasma (a known food preservation processing tool) to enhance food flavour through intentional volatile aroma modification of foods and how this can impact food development for long-distance space missions was examined. This thesis explored five core research questions on flavour and the use of plasma, on Earth and in extreme environments, via a literature review, a preliminary study, three research articles, and two conceptual designs. The literature review focuses on how cold plasma can modify biomolecular and organoleptic properties, and the research gap into how cold plasma affects food flavour. This addressed the first research question: “What are the trends in current plasma research, and are there potential benefits for applications on Earth or in space?”. This review found that cold plasma has not only been shown to curtail microbial growth, enhancing food safety and shelf-life but also does so without diminishing certain essential organoleptic attributes (e.g., flavour). This dual benefit is especially crucial for long-term space missions where the preservation of both taste and nutritional quality is of the essence. This was followed up by preliminary research on the effects of cold plasma treatment on the key flavour compounds in fresh and freeze-dried strawberries (as potential space food), culminating in publication of a research article focused on optimising and characterising the specific interaction effects of cold plasma, freeze-drying, and extreme environmental effects (“zero-averaged gravity"). An aroma analysis method was also designed for a microgravity environment. This research answered the questions: “Can plasma enhance volatile aromas lost during freeze-drying?” and “What does this mean for preparing freeze-dried space food?”. Freeze-drying, a staple in space food preparation, typically leads to a loss of key volatile aroma compounds (predominantly esters). However, the aromas lost during freeze-drying of strawberries were substantially regained with the application of cold plasma. This discovery hints at the possibility of space food that is both long-lasting and flavourful. Cold plasma was then evaluated as a strategy to enhance esterification and, therefore, the sensory impact of these volatile aroma compounds. The mechanistic interactions between different plasma types and esterification elements were highlighted to instigate and pave the way for future bespoke aroma modifications, hinting at the untapped potential in flavour science. Plasma was also evaluated for its ability to break bonds (i.e., those within glycosylated aroma compounds) to reduce the impact of smoke aromas in wine following vineyard exposure to bushfire smoke, which can negatively impact the wine industry. This study sought to answer the following questions: “Can plasma be used to modify other compounds, i.e., can it assist in the joining or breaking of bonds between reactants?”, “Can the mechanical interactions between plasma and reactants be determined?” and “Could this be useful on Earth or in space?”. Plasma was not just found to preserve flavour, but also potentially catalyse esterification processes, turning less desirable aromas (e.g., hexanoic acid) into more pleasant ones (e.g., methyl hexanoate). This finding could significantly impact gourmet food preparation on Earth and also hints at the vast potential plasma holds in transforming food in space, all without extensive resource requirements. While plasma treatment decreased the concentration of some smoke-derived volatile phenols, it was not powerful enough to cleave glycosylated phenols. To improve plasma’s applicability in space, future projects are being developed including a dynamic microfluidic microplasma system that could potentially be used in microgravity was designed. This addressed two questions: “Can plasma be generated inside a system for liquid applications used in microgravity?” and “How could this fit into what’s onboard the ISS?”. Since flavour enhancement should not stop at solid foods, this research proposed the use of a dynamic DBD-based microfluidic microplasma system that could process liquids in microgravity. With this, there is potential to enhance liquid foods onboard space missions (e.g., in combination with the ISSpresso machine), making them tastier and safer. After improving the aroma of foods typically used for space and designing a plasma system that could be used in space, subsequent research is being developed to make microgravity-suitable spectrophotometer for aroma analysis in microgravity sought to answer the following questions: “What analytical equipment can be used in space to monitor changes in quality?”. Monitoring is crucial to ensuring consistent flavour quality, especially in the challenging environment of space. This study introduced novel methods, such as a compact spectrophotometer setup and an adapted volatile extraction technique that could be suitable for use in microgravity environments, promising real-time flavour analysis, even in space. This thesis shows novelty in flavour modification, flavour in space, plasma-induced esterification, and enhanced analytical techniques for future flavour and space research. By uncovering the myriad of potential applications of cold plasma, this thesis not only shows promise for advancing food quality on Earth, but also sets the foundation for gourmet experiences in space.
Cold plasma, freeze-dried, space food, wine, aroma modification, microgravity, microfluidics
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