Chapter 6: Sourdough and Wild Fermentation

Summary

This chapter dives deep into the living ecosystem inside a sourdough starter — a community of wild yeast and lactic acid bacteria that bakers have cultivated for thousands of years. Students capture wild yeast from their environment, learn to feed and maintain a starter using the 1:1:1 ratio, and explore how temperature, hydration, and time shape the balance of lactic and acetic acid in the final loaf. The hands-on sourdough lab is a multi-week class project that bridges microbiology, chemistry, and baking.

Concepts Covered

This chapter covers the following 9 concepts from the learning graph:

1. Wild Yeast Capture
2. Sourdough Starter Ecosystem
3. Sourdough Lactic Acid Bacteria
4. Sourdough Feeding Ratio
5. Sourdough Starter Float Test
6. Sourdough Temperature Effects
7. Lactic Acid Production
8. Acetic Acid Production
9. Bulk Fermentation

Prerequisites

This chapter builds on concepts from:

• Chapter 2: The Molecules of Food
• Chapter 4: Food Microbiology
• Chapter 5: Baking Science

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Welcome to the Wildest Chapter in This Book!

Science is delicious — and sourdough is living proof! I, Zyme, am literally a yeast cell of the type you'll be capturing in this chapter. Humans have been harvesting wild yeast from the air and flour for at least 6,000 years. Now you're going to do it too. Let's bubble up some answers!

What Makes Sourdough Different?

Most modern bread is leavened with commercial yeast — a single pure strain of Saccharomyces cerevisiae that has been cultivated, dried, and packaged for consistent, fast results. Open a packet of active dry yeast, add it to warm water, and within an hour your dough is rising. Reliable, predictable, and fast.

Sourdough is completely different. Instead of adding a single strain of commercial yeast, you use a sourdough starter — a jar of flour and water that contains a living community of wild yeast and bacteria captured from your local environment. A sourdough starter is essentially a small, managed ecosystem that you feed, observe, and bake with over weeks, months, and even years.

Sourdough takes longer (12–24 hours versus 2–3 hours for commercial yeast), is more complex to manage, and produces completely different flavors. That complexity is exactly what makes it one of the most scientifically interesting foods in the world.

Wild Yeast Capture: Finding Your Microbial Community

Wild yeast and lactic acid bacteria are everywhere — on the surface of grains, on fruit skins, on flower petals, and floating in the air. The process of wild yeast capture involves creating conditions that favor the growth of these naturally occurring microorganisms.

To start a sourdough starter from scratch, you need only two ingredients: flour and water. Whole-wheat or rye flour is best at first because the outer bran of the wheat grain harbors large populations of wild yeast and lactic acid bacteria.

The basic procedure: 1. Mix equal weights of whole-wheat flour and unchlorinated water in a clean jar 2. Cover loosely (allows gas exchange, prevents contamination) 3. Leave at room temperature (68–75°F / 20–24°C) for 24 hours 4. Discard half and add fresh flour and water (this is called "feeding") 5. Repeat daily for 7–14 days, observing daily changes

In the first few days, you may see no activity, or you may see bubbles and an unpleasant smell as unwanted bacteria (like Leuconostoc species) initially dominate. By days 4–7, wild yeast and heterofermentative lactic acid bacteria gradually take over because the acids they produce lower the pH, creating conditions that favor them but kill off the less acid-tolerant early colonizers. By days 10–14, a healthy starter bubbles vigorously, rises and falls predictably after each feeding, and smells pleasantly tangy and yeasty.

The Sourdough Starter Ecosystem

A mature sourdough starter ecosystem contains at least two types of microorganisms working in a stable partnership:

Wild Yeast — primarily Saccharomyces cerevisiae and related wild species like Kazachstania humilis (Candida humilis). These yeast perform alcoholic fermentation — converting sugars to CO₂ and ethanol. The CO₂ is what leavens the bread. In sourdough, wild yeast strains are often more acid-tolerant than commercial yeast, allowing them to coexist with the bacteria.

Lactic Acid Bacteria (LAB) — primarily heterofermentative LAB from genera like Lactobacillus, Levilactobacillus, and Fructilactobacillus. These bacteria produce both lactic acid and acetic acid (and some CO₂), contributing directly to the sour flavor of sourdough bread.

The relationship between wild yeast and LAB in a sourdough starter is an example of mutualism: the yeast produce CO₂ for leavening and consume oxygen, creating the anaerobic conditions that favor LAB. The LAB produce acids that lower pH, creating a selective environment that favors acid-tolerant wild yeast over unwanted bacteria.

Zyme Thinks: How Do Yeast and Bacteria Share a Jar?

In a commercial yeast packet, you get one pure strain. In a sourdough starter, you get dozens of microbial species competing and cooperating at the same time. The yeast tend to dominate numerically during the active rise phase; the bacteria are often 100× more numerous overall. Each feeding resets the competition by providing fresh food (flour), and the community re-establishes its balance within a few hours. I find this deeply beautiful.

Sourdough Lactic Acid Bacteria: Two Pathways, Two Flavors

The sourdough lactic acid bacteria (LAB) in a healthy starter are mostly heterofermentative — meaning they ferment one molecule of glucose and produce multiple products, including lactic acid, acetic acid, CO₂, and ethanol. This is different from homofermentative LAB (like those in yogurt), which produce only lactic acid.

The two organic acids produced by sourdough LAB have very different flavor profiles:

• Lactic acid — mild, creamy, slightly sour (like yogurt or buttermilk). Associated with warm, wet conditions.
• Acetic acid — sharp, vinegary, pungent (like wine vinegar). Associated with cool, dry conditions.

The balance between lactic and acetic acid in your starter — and therefore your bread — is controlled primarily by temperature and hydration, which we will explore in detail in the next section.

Sourdough Feeding Ratio: The 1:1:1 Protocol

The sourdough feeding ratio describes how much flour and water you add relative to the amount of existing starter. The standard protocol is 1:1:1 — equal parts (by weight) of existing starter : fresh flour : fresh water.

For example, if you have 50g of mature starter, you discard enough to leave 50g, then add 50g of flour and 50g of water. This brings your total to 150g of refreshed starter.

Why discard before feeding? Two reasons:

1. Acid control — as the starter ferments, it accumulates lactic and acetic acid that lower the pH progressively. If you never discard, the pH drops so low that it inhibits even the acid-tolerant wild yeast
2. Volume management — without discarding, the starter would double in size with each feeding and quickly overflow any container

The feeding schedule depends on temperature:

• At room temperature (70–75°F): feed once per day for a stable, moderately active starter
• At warmer temperatures (80–85°F): feed twice per day; microbes are more active
• In the refrigerator (38–40°F): feed once per week (yeast and bacteria go dormant but survive)

The Float Test: Reading Your Starter's Readiness

Before baking, you need to verify that your starter is active enough to leaven bread. The sourdough starter float test is a simple readiness check: drop a small spoonful (about 1 teaspoon) of starter into a glass of water. If it floats, the starter is ready to use.

Why does a ready starter float? An active starter is full of CO₂ gas bubbles produced by yeast fermentation. These bubbles make the starter less dense than water. A starter that sinks has not yet reached peak fermentation activity — the yeast haven't produced enough gas yet, meaning the starter won't have the leavening power needed to make bread rise.

A starter passes the float test at its peak — the moment after it has risen to its maximum height following a feeding but before it begins to collapse. The rise-and-fall cycle looks like this:

1. Immediately after feeding: flat, dense — sinks (fails float test)
2. Hours 2–4: begins to rise as yeast ferment, producing CO₂
3. Hours 4–8 (warm conditions): reaches peak — domed top, maximum rise — floats!
4. Hours 8–12: begins to fall as food runs out, acid builds up
5. Hours 12–24: collapsed back to near original volume — sinks again (time to feed)

Diagram: Sourdough Rise-and-Fall Cycle Simulation

Sourdough Starter Activity Tracker MicroSim

Type: microsim sim-id: sourdough-rise-fall-cycle
Library: p5.js
Status: Specified

Learning Objective: Students will identify (L1 — Remember) when a sourdough starter is at peak activity and apply (L3 — Apply) that knowledge to time their baking correctly.

Canvas size: 740 × 480 px, responsive.

Layout: Three sections stacked vertically.

Top section (200 px) — Jar Visualizer: A cross-section of a glass jar (200 px wide) showing the starter level as a colored liquid. The level rises and falls in real time matching the simulation. A small floating spoonful next to the jar simulates the float test — it floats when starter is at peak activity.

Middle section — Controls: - Temperature slider: 60°F – 90°F (warmer = faster cycle) - Flour type selector: Whole wheat (faster, more microbial activity) vs. White flour (slower) - "Feed Starter" button — resets the cycle

Bottom section (240 px) — Activity Graph: - X-axis: Time since last feeding (0–24 hours) - Y-axis: Starter height (arbitrary units, 0–100%) - Animated line showing the rise-and-fall curve - Colored zones: red = too early (fails float test), green = peak window (passes float test), yellow = falling (still usable but declining) - Yeast population and acid level bars update alongside the graph

Tooltips: Clicking any point on the curve shows: simulated starter height, yeast activity level, pH estimate, and float test result.

Responsive: Redraws on window resize.

Sourdough Temperature Effects: Controlling Sour Flavor

One of the most powerful tools a sourdough baker has is temperature control. The same starter — the same microbial community — will produce dramatically different flavors depending on the temperature at which it ferments.

Here is the key relationship to understand before looking at the details:

• Warm fermentation (75–80°F / 24–27°C): favors lactic acid bacteria that produce more lactic acid → mild, creamy sourness, similar to yogurt
• Cool fermentation (55–65°F / 13–18°C): favors lactic acid bacteria that produce more acetic acid → sharp, vinegary sourness

Temperature affects this balance because different metabolic pathways and different bacterial species have different optimal temperature ranges. At higher temperatures, the heterofermentative LAB tend to run the lactic acid pathway faster. At lower temperatures, the acetic acid pathway is relatively more active.

Hydration interacts with temperature to further control acid balance:

• Wetter starter (high hydration) → more aqueous environment → more lactic acid production
• Stiffer starter (low hydration) → less water available → more acetic acid production

Master sourdough bakers exploit these variables deliberately. A San Francisco–style sourdough (sharp and tangy) is typically fermented cool and stiff. A mild French pain de campagne is typically fermented warm and wet.

The table below summarizes how temperature and hydration shape sourdough flavor:

Condition	Temperature	Hydration	Dominant Acid	Flavor Profile
Mild sourdough	75–80°F warm	Wet (80%+ hydration)	Lactic acid	Creamy, mild sour
Sharp sourdough	55–65°F cool	Stiff (65–70% hydration)	Acetic acid	Vinegary, sharp
Overnight retard	Refrigerator 38°F	Either	Acetic acid	Strong sour, complex

Lactic Acid Production: The Chemistry of Mild Tang

Lactic acid production in sourdough comes from heterofermentative LAB metabolizing the maltose released when flour starch is broken down by amylase enzymes (present in the flour itself). The bacteria convert maltose to glucose, then ferment glucose via the phosphoketolase pathway, producing a mixture of lactic acid, ethanol, CO₂, and (under some conditions) acetic acid.

Lactic acid (\(CH_3CHOHCOOH\)) is a mild, non-volatile organic acid. It tastes sour but not harsh, and it doesn't evaporate into the air — so you can taste it in the finished bread but not smell it strongly when the bread is baking.

Lactic acid has a crucial role beyond flavor: it is the primary reason sourdough bread keeps longer than commercial yeast bread. At pH 3.5–4.0 (a typical sourdough loaf's internal pH), mold growth is severely inhibited. This is why properly fermented sourdough bread can remain mold-free for 4–7 days at room temperature, compared to 2–3 days for commercial yeast bread.

Acetic Acid Production: The Chemistry of Sharp Sour

Acetic acid production in sourdough comes from the same heterofermentative LAB following a slightly different metabolic path. When oxygen is limited and maltose is the primary carbon source, some LAB produce acetic acid (CH₃COOH) rather than lactic acid and ethanol.

Acetic acid is a volatile acid — it evaporates at cooking temperatures, and you can smell it strongly. It's the same compound that gives vinegar its sharp smell. In bread, acetic acid contributes:

• A sharp, pungent sourness on the nose and palate
• A chewy, slightly dense crumb structure (acids affect gluten)
• Excellent preservation (acetic acid is an even stronger antimicrobial than lactic acid)

Bulk Fermentation: When Time Is an Ingredient

Bulk fermentation is the first rise of sourdough bread dough, after you've mixed the flour, water, salt, and starter together but before you shape the loaf. During bulk fermentation, the wild yeast and LAB in the starter do most of their important work: producing CO₂ (which inflates the dough), developing flavor acids, and beginning to break down gluten through enzymatic activity (which, paradoxically, makes the dough more extensible and easier to shape).

A typical bulk fermentation timeline at 75°F: - Hours 0–1: Yeast begin activating; little visible change - Hours 1–3: Dough becomes noticeably airy and jiggly; bubbles visible at the surface - Hours 3–5: Dough volume increases 50–75%; dough feels light and passes the poke test (indentation slowly springs back) - Hours 5+: If pushed too far, the dough becomes over-fermented — sticky, very acidic, with collapsed gluten structure

Stretch-and-fold is a technique used during bulk fermentation instead of kneading. Every 30–45 minutes, the baker gently stretches the dough to one side and folds it over itself. This builds gluten strength progressively while allowing fermentation to continue — a gentler approach than kneading that works well with wet sourdough doughs.

Zyme Encourages You Through the Waiting

Sourdough requires patience — and that's the hardest part. Your starter might take 10 days to become active. Your dough might need 12 hours in the refrigerator overnight. But here's the secret: most of that time you're not doing anything. The microbes are doing the work. Your job is to observe, record, and learn to read the subtle signs — the dome of the starter, the jiggly bounce of the dough, the smell shifting from sharp to yeasty-tangy. That's real science.

The Sourdough Lab Project: What to Observe Each Day

If you're running the sourdough starter lab project over two weeks, here's what to record in your lab journal:

• Rise height — measure in centimeters using a rubber band on the jar to mark the starting level
• Time to peak — how many hours after feeding does the starter reach its highest point?
• Aroma stage — describe the smell: sour/vinegary (days 1–3), putrid/unpleasant (days 2–4 as early bacteria dominate), then shifting to yeasty and pleasantly tangy (days 7+)
• Texture — sticky and thin early on; bubbly and somewhat elastic when mature
• Float test result — record pass or fail on day 7 and day 14
• Temperature — note the ambient temperature in the room (this directly affects fermentation rate)

Signs of a healthy, mature starter:

• Reliably doubles in volume within 4–8 hours of feeding at room temperature
• Has a domed top at peak (not flat)
• Smells pleasantly tangy and yeasty (like beer and yogurt together)
• Passes the float test consistently
• Has a web-like, airy internal structure when you drag a spoon through it

Signs of trouble:

• Pink or orange streaks → contamination by unwanted bacteria; discard and restart
• No activity after day 10 → try warmer location, use more whole-grain flour, use filtered water
• Liquid layer on top (called "hooch") → normal! This is alcohol from starving yeast; stir it in or pour it off and feed immediately

Key Takeaways

• Sourdough starter is a living ecosystem of wild yeast and heterofermentative lactic acid bacteria, self-selected from the natural environment
• Wild yeast capture takes 10–14 days of daily feedings; early unpleasant smells are normal as the microbial community establishes itself
• The 1:1:1 feeding ratio maintains starter health by providing fresh nutrients and controlling acid accumulation
• The float test confirms a starter is at peak activity and ready to leaven bread
• Temperature and hydration are the primary controls over flavor: warm + wet → lactic acid (mild); cool + stiff → acetic acid (sharp)
• Bulk fermentation is where yeast and bacteria transform dough — developing flavor, building gas, and softening gluten
• Sourdough keeps longer than commercial yeast bread because lactic and acetic acids inhibit mold growth

Zyme is Bubbling With Pride!

You have just learned to understand and manage a living microbial ecosystem — something bakers have been doing for 6,000 years without knowing the science behind it. Now you know the science AND the craft. Your starter is not just a jar of goo. It's a community of organisms that you are nurturing, feeding, and guiding toward producing one of the most complex and ancient foods in human history. Time to rise to the occasion — science is delicious!

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