Sourdough Flavour: Acids & Aromas in Every Loaf

Sourdough flavour comes from a precise balance of organic acids—mainly lactic and acetic—plus hundreds of volatile aroma compounds produced by wild yeast and bacteria during fermentation. What you taste as “sourdough tang” is largely the acids, while the nutty, fruity, or buttery notes come from aromatic molecules that evaporate into your nose as you chew. This article explains how those compounds form, what shifts the balance, and how to steer your loaf toward mild or bold flavour.

Formula

The calculator gives two relative scores—acidity and aroma complexity—based on three inputs you control:

• Fermentation time (t) in hours: total from mixing to baking, including bulk ferment and cold proof.
• Average dough temperature (temp) in °C: warmer speeds up all microbial activity.
• Whole-grain flour percentage (pct): whole wheat or rye adds minerals and enzymes that boost both acids and aromas.

Acidity index = (t / 10) × (1 + (temp − 20) / 15) × (1 + pct / 100)

Aroma complexity = (t / 12) × (1 + (temp − 18) / 20) × (1 + pct / 50)

These are relative scores, not laboratory measurements. They rank your fermentation choices against a baseline (18 hours at 20 °C with 0% whole grain gives an acidity of 1.8 and aroma of 1.5). Use them to compare one recipe to another, not to predict exact chemical concentrations.

Worked Example

Using the default values: t = 18 hours, temp = 24 °C, pct = 20%.

Acidity:
(18 / 10) = 1.8
(1 + (24 − 20) / 15) = 1 + 0.2667 = 1.2667
(1 + 20 / 100) = 1.2
1.8 × 1.2667 × 1.2 = 2.736 → rounds to 2.7

Aroma complexity:
(18 / 12) = 1.5
(1 + (24 − 18) / 20) = 1 + 0.3 = 1.3
(1 + 20 / 50) = 1.4
1.5 × 1.3 × 1.4 = 2.73 → rounds to 2.7

So a typical 18-hour bulk at 24 °C with 20% whole wheat scores 2.7 on both scales. Push the same dough to 30 hours at 26 °C with 50% whole wheat, and the acidity jumps to 4.9 while aroma reaches 4.6—a noticeably tangier, more complex loaf.

The Two Main Acids: Lactic and Acetic

Sourdough’s signature tang comes from two organic acids produced by lactic acid bacteria (LAB) living in the starter:

• Lactic acid gives a smooth, yogurt-like sourness. It is produced by many LAB species, including Lactobacillus sanfranciscensis, and dominates in warm, wet, young doughs.
• Acetic acid gives a sharper, vinegar-like bite. It is produced when bacteria metabolize sugars in the presence of oxygen, and it increases with longer, cooler fermentations and stiffer doughs.

The ratio between them matters more than the total amount. A 3:1 lactic-to-acetic ratio tastes mild and creamy; a 1:1 ratio tastes aggressively sour. Most bakers aim for somewhere between 2:1 and 4:1, depending on the style.

Where the Aromas Come From

Beyond acids, sourdough contains hundreds of volatile compounds that contribute to its aroma. Key groups include:

• Alcohols (e.g., phenylethanol): floral, rose-like notes from yeast activity.
• Esters (e.g., ethyl acetate): fruity, sweet smells formed when acids react with alcohols.
• Aldehydes and ketones: buttery, nutty, or grassy notes from fat oxidation and amino acid breakdown.
• Pyrazines: toasted, roasted aromas that develop mainly in the crust during baking.

Many of these compounds are produced by the same microbes that make acids. Yeast contributes ethanol and esters; bacteria produce organic acids and some aldehydes. The Maillard reaction and caramelization during baking then add dozens of new aroma molecules, which is why a fresh crust smells so different from the raw dough.

How Fermentation Time and Temperature Shape Flavour

Time and temperature are the two levers you control most directly.

• Longer fermentation (18–30 hours) increases both acid production and aroma complexity, but too long (beyond 36–48 hours at warm temps) can make the dough overly sour and weaken the gluten structure.
• Cooler temperatures (4–15 °C) favor acetic acid over lactic acid, giving a sharper tang. Warmer temperatures (25–30 °C) favor lactic acid, giving a milder, yogurt-like sourness.
• Stiff starters (50–60% hydration) also promote acetic acid, while liquid starters (100% hydration) promote lactic acid.

A common misconception is that a longer proof always means more sour. In practice, the type of acid matters: a 24-hour cold proof at 4 °C will taste more acetic (sharper) than a 12-hour warm proof at 28 °C, even if the total acid concentration is similar.

How to Choose Your Target Flavour

Decide what you want before you mix, then adjust these variables:

• Mild, buttery sourdough: use a liquid starter (100% hydration), ferment at 25–28 °C for 8–12 hours, and use mostly white flour.
• Classic tangy sourdough: use a stiff starter (60% hydration), bulk ferment at 22–24 °C for 14–18 hours, then cold proof at 4 °C for 12–24 hours.
• Bold, vinegar-like sourdough: extend cold proof to 24–36 hours, add 10–20% rye flour, and keep the dough on the stiffer side.

Whole-grain flours add more minerals and enzymes that feed the bacteria, so they naturally boost both acids and aroma compounds. Rye in particular is rich in pentosans that hold water and promote acetic acid production.

Common Problems and Fixes

• Bread isn’t sour enough: lengthen the cold proof, use a stiffer starter, or add a small amount of rye flour (5–10%).
• Bread is too sour, almost harsh: shorten the fermentation, raise the temperature (to favor lactic acid), or use a more liquid starter.
• Bread tastes flat or just like flour: your starter may be too young or weak. Feed it regularly for a few days before baking, and ensure it doubles reliably.
• Off-flavours (bitter, soapy, or chemical): often from over-fermentation, chlorinated tap water, or a starter that has been neglected. Use filtered water and refresh the starter more often.

If your bread tastes sour but lacks aroma, the problem is usually in the crust: bake at a higher temperature (230–250 °C) with steam for the first 15 minutes to maximize Maillard reactions.

Safety and Storage

Sourdough is safe to eat when fully baked to an internal temperature of at least 93 °C (200 °F). Use a digital thermometer to check the centre of the loaf. After baking, cool completely on a wire rack for at least 2 hours before slicing—this prevents a gummy crumb and lets moisture redistribute.

Store sourdough at room temperature in a paper bag or bread box for up to 3 days. Do not refrigerate whole loaves; the starch retrogrades faster and makes the bread stale. For longer storage, slice and freeze in an airtight bag for up to 3 months. Reheat slices directly from frozen in a toaster or oven.

If your starter develops pink or orange streaks, a strong alcohol smell, or visible mould, discard it and start fresh. Those signs indicate contamination by unwanted microbes.

Frequently Asked Questions

Why does my sourdough taste more sour on day 2?

Acetic acid continues to form slowly even after baking, especially if the loaf is stored in a plastic bag. The acids also become more perceptible as the bread stales and loses moisture. This is normal, but you can slow it by storing in paper rather than plastic.

Can I make sourdough without a starter?

No—sourdough by definition requires a live culture of yeast and bacteria. You can create your own starter from flour and water in about 5–7 days, or use a commercial sourdough starter culture. Commercial yeast alone will not produce the same acids and aromas.

Does adding more starter make bread more sour?

Not necessarily. A higher inoculation rate speeds up fermentation, but it also shortens the time available for acid production. A smaller amount of starter (10–15%) with a longer fermentation often produces more flavour than a large amount with a quick proof.

Why does my sourdough taste like vinegar?

That’s acetic acid dominating. It usually happens with long, cold fermentations and stiff doughs. To reduce it, shorten the cold proof, increase dough hydration, or raise the fermentation temperature slightly.

Is sourdough healthier than regular bread?

Sourdough fermentation reduces phytates, which can improve mineral absorption, and lowers the glycemic index compared to some commercial breads. However, it is still bread and contains similar calories and carbohydrates. The main benefits are digestive for some people and better flavour, not a radical health difference.

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in t | Fermentation time (bulk + cold) | hours | 18 | 0 | 72
in temp | Average dough temperature | °C | 24 | 0 | 40
in pct | Whole-grain flour in recipe | % | 20 | 0 | 100
out acid | Relative acidity index | - | (t / 10) * (1 + (temp - 20) / 15) * (1 + pct / 100) | 1
out aroma | Relative aroma complexity | - | (t / 12) * (1 + (temp - 18) / 20) * (1 + pct / 50) | 1