Interview: Curro Polo — Kombucha: A Word on Metamorphosis
Curro Polo is currently pursuing a PhD under the Basque Culinary Center program, with the academic guidance of Harvard University The research project will take place at Ama Brewery, exploring the fascinating world of microbiology applied to beverages. We’d met at the 2024 KBI conference in Reno, Nevada, where Curro and I were both Kombucha Kup judges. At that time he was finishing up his Masters in Gastronomy and Culinary Arts at Harvard. He presented his dissertation in June, 2024.
Curro’s Master’s thesis, Kombucha: A Word on Metamorphosis, provides a comprehensive and meticulous exploration of kombucha, moving beyond its health-focused image to investigate its intricate microbiology, flavor development, and gastronomic potential. The research encompasses kombucha’s economic, social, and scientific aspects, and combines a thorough literature review with experimental analysis of the fermentation process, particularly focusing on the impact of yeast and bacteria on the volatile and non-volatile compounds produced during prolonged anaerobic bottle fermentation. The research explores the impact of different tea substrates and the introduction of a specific yeast strain on the final flavor profile of the beverage.
Research Objectives and Methodology
The primary objective of the thesis is to explore kombucha as a gastronomic product and its potential in the low/no-alcohol market through bottle aging. Secondary objectives include:
- Providing a holistic view of kombucha.
- Characterizing the initial fermentation stage through continuous data logging.
- Analyzing volatile compounds during secondary fermentation and bottle aging.
- Investigating the relationship between phenolic VOCs, substrate, and the addition of a Brettanomyces bruxellensis strain.
Themes
- Kombucha as a Metamorphic Gastronomic Product: The central theme revolves around understanding kombucha not just as a beverage but as a dynamic gastronomic product undergoing “metamorphosis” through fermentation. This involves a shift in focus from solely health benefits to its complex flavor profile and potential within the low/no-alcohol market.
- Historical and Cultural Context: The thesis briefly touches upon the historical origins of kombucha and the human appreciation for sourness. It suggests a potential evolutionary link to this preference: “If this trait is genetic, it could explain the human predisposition towards beverages with a significant level of acidity.”
- The Importance of Microbiology: Microbiology is the central axis of the research. The thesis emphasizes the multifaceted nature of microbiology, including morphology, metabolism, and growth, and their impact on fermented beverages. Key historical figures in microbiology, such as Leeuwenhoek and Pasteur, are mentioned, highlighting the shift from a macro to a micro understanding of food production. Pasteur’s work on controlled fermentation and the subsequent standardization of brewing processes are noted.
- A Symbiotic Ecosystem: A significant portion of the thesis is dedicated to the Symbiotic Community of Yeast and Bacteria (SCOBY) that drives kombucha fermentation.
- Role of Yeast: Yeast is identified as fundamental, responsible for converting sucrose into glucose and fructose, producing ethanol and carbon dioxide, and generating secondary metabolites that significantly influence kombucha’s flavor. The concept of yeast domestication and the differences between wild and domesticated yeast reproduction are explained. The diversity of yeast species found in SCOBY across various studies is compiled. The thesis also explores why yeast ferments, attributing it to a survival mechanism: “Yeasts secrete ethanol to decrease competition in the environment where they are found, meaning fermentation is a survival mechanism.” The process of glycolysis and cellular respiration in yeast is also outlined.
- Role of Bacteria: Acetic Acid Bacteria (AAB) are described as strictly aerobic, gram-negative microorganisms crucial for converting alcohols and sugars into organic acids, notably acetic acid. Genera like Acetobacter, Gluconobacter, Komagataeibacter, and Gluconacetobacter are highlighted as prevalent in kombucha. The thesis details various organic acids found in kombucha and their sensory attributes. The formation of the cellulose-based biofilm by AAB is discussed as a protective mechanism promoting microbial cooperation.
- The Complexity of Kombucha Flavor: Flavor is defined as a multisensory perception involving taste and smell. The significant role of olfactory receptor genes in humans underscores the evolutionary importance of flavor. The thesis emphasizes that flavor is the brain’s interpretation of sensory inputs. “What is known as Flavor involves a complex network of interdependent brain connections: the consumption context, attributes perceived by sight, molecules detected by taste receptors, texture perceived by the tongue, and volatile molecules captured by the nose, both retro-nasally and ortho-nasally, are part of the sensory experience. Another fundamental component added to this is memory.”
- Aldehydes: These are mentioned as important flavor precursors, particularly in the early stages of fermentation, contributing notes like sweet, fruity, malty, almond, and green.
- Acids: The sour flavor of kombucha is attributed to organic acids produced during fermentation. The thesis explains the concepts of pH and titratable acidity, highlighting that pH alone is not a complete representation of perceived sourness due to weak acids’ gradual dissociation. “Using the pH scale is an affordable way to track fermentation; however, it is not purely representative of the amount of sourness perceived by the tongue.”
- Esters: These volatile compounds, formed from alcohols and carboxylic acids, are crucial for the fruity, candied, and perfume-like aromas of kombucha. Their production is linked to yeast metabolism and specific enzymes. Acetate and ethyl esters are identified as the main groups, with examples like ethyl octanoate (sour apple) and ethyl hexanoate (red apple) provided in a table.
- Phenols and Brettanomyces: Phenolic compounds, particularly 4-Ethylguaiacol (4-EG) and 4-Ethylphenol (4-EP), mainly produced by the yeast Brettanomyces, are discussed as contributors to “funk” or “brett” character, described with terms like leather, horse stable, and smoky. Brettanomyces (anamorph) / Dekkera (teleomorph) is noted for its prevalence in kombucha consortia and its ability to thrive after S. cerevisiae fermentation. Discovered in 1904 and being the first microorganism ever patented, Brettanomyces was historically considered a spoilage organism but is now being explored for its potential to create novel flavor profiles.
In Conclusion
The thesis poses a significant question: “What is the maximum expression of the minimal shared in kombucha in microbiological terms?” This implies an inquiry into the core, essential microbiological elements that define kombucha and the full potential of these elements to shape the beverage. In essence, the question asks: Despite the vast microbial diversity in kombucha, what are the absolute essential microorganisms that make it kombucha, and what is the fullest spectrum of flavors, aromas, and chemical complexity that can be achieved by focusing primarily on these core microbes?
Polo suggests that answering this question is crucial for several reasons:
- Standardization: Understanding the “minimal shared” could pave the way for more standardized kombucha production, moving away from reliance on the unpredictable nature of diverse wild cultures. This is particularly relevant as kombucha transitions from domestic to industrial production.
- Predictability and Control: Identifying the core microorganisms and their potential could allow producers to better predict and control the final characteristics of their kombucha.
- Scientific Understanding: Defining the “minimal shared” in microbiological terms would enhance the academic understanding of what constitutes kombucha as a fermented beverage.
- Innovation: By focusing on the core microbial elements, researchers and producers can explore new production methods and push the boundaries of kombucha’s organoleptic complexity. The author’s personal reflection about the Gallery of the Louvre painting further illustrates this point. Just as a gallery comprises various artworks, kombucha contains diverse microorganisms. Identifying the “Mona Lisa” of the kombucha microbiome – the essential elements – and understanding their full artistic potential (their “maximum expression”) is key to truly appreciating and potentially standardizing the “gallery” of kombucha.
This summary was produced with the help of NotebookLM. You can download the complete 90-page thesis Kombucha: A Word on Metamorphosis for free.
Interview
Curro shares insights into his Master’s thesis, updates on his Doctorate, and research into pét-nat teas in the Basque Region of Spain with the AMA Brewery.
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Great article – I’m just starting to read through the thesis and I’m sad to say that the very first part of the book completely misreads the information in Beth Ann Petro’s book and ignores the decades of research that we at Kombucha Kamp conducted to validate and verify the myths and legends of Kombucha’s origins. I’m more certain the scientific studies will be approached with more rigor, just disappointing to see that these outdated myths about the origins of Kombucha continue to circulate.
There is indeed more to the history of kombucha than in the brief section at the start of Curro’s dissertation. We’ve posted a very thorough three-part report by Tadeusz Zagrabinski, the founder of Bärbucha Kombucha in Berlin, Germany. Part One discusses what he calls the “Period of Myths & Legends” in the ancient world. Part Two discusses the period 1913 till about 1995.The series concludes with a look at California’s influence and commercial production today.
First of all, thank you for your comment. As mentioned in the introduction, tracing the origins of kombucha is a challenging task. I suspect this is, among other reasons, because there is still no clear definition of what kombucha truly is.
During the research process, I had the opportunity to interview Beth Ann Pietro, and I remember her pointing out that, to this day, there is no solid literary or cultural evidence regarding the origins of the product. It’s a fascinating topic.
It’s also interesting to draw parallels with other beverages that, in contrast, have benefited from extensive literary documentation by their makers—allowing us to pinpoint their origins more precisely. One example that comes to mind is the wine industry in France, which has managed to support the origins of its grape-based drinks through a robust legal and historical framework.
Nevertheless, the aim of this work is not to offer definitive answers but to open up new questions. And I must confess, I find something quite beautiful in the idea that we may never truly know the origins of kombucha. Perhaps it’s fitting to leave the door open to a bit of mystery.
Once again, thank you for your kind words and for all the research work you carried out—it was incredibly valuable for my own investigation.