The sour tang of a crusty loaf, the fizzy bite of kefir, the earthy depth of a slice of tempeh—these are not accidents of geography. Each of these foods owes its character to a specific community of microbes, and to what those microbes eat and excrete. A sourdough starter, a kefir grain, and a tempeh mold are, in effect, three different tiny workforces with three different job descriptions. Understanding who they are and what they produce is the key to understanding why the world's fermented foods taste the way they do.
Sourdough: The Tang of Lactic Acid Bacteria and Yeast
A sourdough starter is a living culture of yeasts and bacteria working in tandem. As an eLife digest on sourdough starter microbiomes explains, these microbes ferment the carbohydrates in flour and produce carbon dioxide, the gas that makes dough rise before baking. The same community also churns out acids and enzymes that shape the bread's flavor, texture, and shelf life.
Two acids do most of the flavor work. Lactic acid lends a mild, rounded sourness, while acetic acid—vinegar's own compound—contributes a sharper, more puckering note. The balance between them is part of why one starter tastes gentle and another tastes assertive. The result of all this microbial activity is the profile bakers prize: a distinctly sour, tangy flavor and an open, airy crumb with a satisfying chew.
What makes sourdough durable across generations of bakers is a technique called backslopping: rather than starting from scratch each time, a baker keeps the culture alive and transfers a portion of it into fresh flour and water, portioning and sharing the starter indefinitely. The starter, in other words, is less an ingredient than a continuous lineage.
Kefir: The Effervescent Symbiosis of Grains
Kefir begins with something that looks like a small, rubbery cauliflower: the kefir grain. These grains are not a single organism but a symbiotic consortium of bacteria and yeasts embedded together in a matrix that can be transferred from batch to batch of milk. Drop the grains into fresh milk and the consortium ferments the sugars inside it.
Which microbes dominate? A review of fermented foods research, summarizing a culture-independent analysis of Italian kefir grains, found that the majority of the microorganisms present were Lactobacillus kefiranofaciens ssp. kefirgranum, Kluyveromyces marxianus, and Kazachstania exigua—a pairing of lactic acid bacteria with yeasts that mirrors sourdough's structure, but in a dairy medium.
The consortium's outputs are what set kefir apart from yogurt. Alongside lactic sourness, kefir microbes produce acetic acid, ethanol, and carbon dioxide, which is why kefir carries its signature sour, tangy flavor and a slight fizz on the tongue. Stanford nutrition educators note that dairy ferments like kefir and yogurt typically list their live bacterial species on the label, precisely because their communities are defined enough to name.
