In the video above Baker Dr Vanessa Kimbell gives you an overview of what a sourdough starter is – with particular references to make a sourdough boule.
The Science and Art of Sourdough Starters: A Deep Dive into Microbial Alchemy
A sourdough starter is at the heart of sourdough baking and I’ve been using my starter I was 11 year old. Sourdough starter is a living, breathing ecosystem that embodies the transformative power of fermentation. This microbial community, housed in a simple mixture of flour and water, is responsible for the rise, flavour, and digestibility of sourdough bread. To understand the intricacies of a sourdough starter is to appreciate the complex interplay of microorganisms that work in harmony to produce the perfect loaf, in other words you, as the baker, are responsible for the way it behaves and performs. You can read more here about how to create your own stater.
The Microbial Community of a Sourdough Starter
It is a relationship, and what fascinates me is how we are related to the microbes in the starter, some of these species can be found in soil systems, in our bodies and in the food we ferment. A sourdough starter is essentially a culture of wild yeasts and lactic acid bacteria (LAB) that coexist in a symbiotic relationship. The yeasts are primarily responsible for producing carbon dioxide (CO2), which leavens the dough, while the LAB produce lactic acid and acetic acid, contributing to the distinctive sour flavour and improving the bread’s nutritional profile.
Yeasts: The Leavening Agents
The yeast species most commonly found in sourdough starters belong to the genus Saccharomyces and Candida. The dominant species, Saccharomyces cerevisiae, is the same yeast used in commercial baking and brewing. However, sourdough starters often contain other species like Candida milleri, Kazachstania exigua, and Saccharomyces exiguus. These wild yeasts are naturally present on the grains, in the environment, and even on the baker’s hands.
Yeasts are facultative anaerobes, meaning they can produce energy through aerobic respiration when oxygen is present and switch to fermentation when it is not. During fermentation, yeasts consume sugars in the flour, primarily glucose and fructose, and produce CO2 and ethanol as by-products. The CO2 is trapped in the dough’s gluten network, causing the dough to rise.
Lactic Acid Bacteria: The Fermentation Powerhouse
Lactic acid bacteria are the other key players in a sourdough starter. These bacteria can be divided into two main types based on their metabolic pathways: homofermentative and heterofermentative LAB.
Homofermentative LAB, such as Lactobacillus plantarum and Lactobacillus acidophilus, primarily produce lactic acid as the sole end product of glucose fermentation. This process is relatively simple and results in a cleaner, less tangy sourness in the bread.
Heterofermentative LAB, such as Lactobacillus brevis and Lactobacillus fermentum, have a more complex metabolic pathway. They produce not only lactic acid but also acetic acid, ethanol, and CO2. The presence of acetic acid contributes to a sharper, more complex sour flavour and increases the bread’s shelf life due to its preservative properties.
The balance between these two types of LAB in a starter is influenced by factors such as hydration, temperature, and the flour used. A higher hydration level and warmer temperatures tend to favour homofermentative LAB, resulting in a milder flavour. In contrast, lower hydration and cooler temperatures encourage heterofermentative LAB, producing a more tangy and aromatic bread.
Capturing and Cultivating a Sourdough Starter
Creating a sourdough starter is an exercise in patience and observation. It begins with a simple mixture of flour and water, left to sit at room temperature. Over several days, wild yeasts and bacteria from the environment begin to colonise the mixture, feeding on the sugars in the flour. Regular feedings of fresh flour and water help to build up the microbial population, and within a week or so, the starter becomes active, bubbling with life and ready to leaven bread.
The microbial composition of a starter is influenced by many factors, including the type of flour used, the local environment, and the care taken by the baker. Each starter is unique, reflecting the terroir of its origins and the hands that tend it. This is why bakers often form a deep personal connection with their starters, viewing them as living members of the family.
The Longevity and Robustness of Starters
I’ve noticed a trend with bakers to dismiss the uniqueness of older starters, especially those who have a tendency to neglect their starter or consider the celebration of older starters as a form of elitism. However there are a few things to consider here, not least the evidence that older starers are more robust.
Older, more established starters tend to be more robust and resilient than younger ones. This is because, over time, the microbial community within the starter stabilises, reaching a balance that allows it to thrive even under less-than-ideal conditions. This stability is why some starters can be passed down through generations, their age contributing to their complexity and strength, but what I find remarkable is that some of the bacteria originates on human hands. One conversation I particularly remember was with Karl at the Sourdough Library with some unusual bacteria and yeast found a 100 year old starer that they had not seen before. What was curious was that the microbes may well have originated from hands that had never been sterilised – pre sanitation, which made us pause for a moment and wonder how our hygiene habits may have resulted in loss of the microbial communities that we evolved with for the previous millennia. I digress…
Back to my own starter, for example, has been with me since I began baking in Southwest France at the age of 11. Now over 40 years old, it has travelled with me through various stages of my life, adapting to different flours, environments, and routines. This long history has endowed it with a unique character and a remarkable resilience, and it includes a trip to DEFRA that culminated in a change in the UK legislation on sourdough.
The Influence of Neglect and Care
A sourdough starter is a living entity that requires regular care and feeding. Neglecting a starter can lead to the development of “funky” flavours and smells, as the balance of yeasts and bacteria shifts. A starter that has been left unfed for too long may develop a layer of liquid on top, known as “hooch,” which is a sign that the yeasts are running out of food. This hooch can be stirred back into the starter, but the starter may also develop off-flavours that can affect the bread.
However, even a neglected starter can often be revived with a few regular feedings, as the resilient microbial community begins to bounce back. The robustness of older starters, like mine, means they can recover from neglect more easily than younger ones.
The Sourdough Library and My Global Experience
My work with sourdough starters has not been limited to my own kitchen. I have had the privilege of collaborating with the Sourdough Library in Brussels, an institution dedicated to preserving and studying sourdough starters from around the world. This unique library houses over 130 starters, each with its own history, microbial composition, and characteristics.
Through this work, I have had the opportunity to handle and bake with starters from diverse regions, each influenced by different climates, flours, and baking traditions. This experience has deepened my understanding of how various factors affect the behaviour and flavour of a starter.
Additionally, I have conducted experiments comparing the microbial communities in my starter with those in my gut microbiome. Despite the close relationship between the microbiome and diet, I did not find any direct crossovers between the two. This underscores the unique and specialised nature of the microbial community within a sourdough starter, which, while influenced by its environment, remains distinct from the human microbiome.
The Science Behind Sourdough’s Transformation
I love understanding the science behind Sourdough and the microbes, but regardless of the specific yeasts and bacteria that dominate a starter, all sourdough cultures work through the same basic principles of fermentation. The microbes within the starter consume sugars and produce CO2, lactic acid, and acetic acid. These by-products are responsible for the key transformations that occur during sourdough baking:
- Leavening: The CO2 produced by the yeasts causes the dough to rise, creating the open crumb structure characteristic of sourdough bread.
- Flavour Development: The lactic and acetic acids contribute to the bread’s sourness, with the balance between the two acids determining the exact flavour profile.
- Nutritional Enhancement: Fermentation also breaks down phytic acid in the flour, which can inhibit the absorption of minerals like iron and zinc. This process makes sourdough bread more nutritious and easier to digest than non-fermented bread.
- Gluten Modification: The acids and enzymes produced during fermentation partially break down gluten, making the bread more digestible for those with gluten sensitivities (though not suitable for those with celiac disease).
This alchemical transformation is what makes sourdough bread so special. The interaction between the starter’s microbes and the flour results in a product that is not only delicious but also healthier and more digestible than many other types of bread.
A personal relationship
For me personally I feel a connected to my starter. It is alive and a sourdough starter is much more than just a leavening agent; it is a complex, dynamic ecosystem that embodies the art and science of fermentation. From the wild yeasts and bacteria that make up its microbial community to the acids and gases they produce, every aspect of a starter’s behaviour is a testament to the wonders of nature and the skill of the baker.
Whether you are nurturing a starter like mine, that has been passed down through generations, or capturing wild microbes in a pot for the first time, understanding the science behind sourdough will enhance your appreciation for this ancient method of bread-making. As my own experience with my 40-year-old starter shows, a well-cared-for starter is a robust and resilient companion in the kitchen, capable of producing extraordinary bread while telling the story of its journey through time and place.
In the end, the magic of sourdough lies in its ability to bring together science, tradition, and community in the humble act of baking bread. Whether you are a novice or an experienced baker, each loaf is a reflection of the microbial life within your starter and the care you have invested in it. As you continue to bake, remember that you are part of a tradition that stretches back thousands of years, and each loaf you bake contributes to the ongoing story of bread and humanity.
