Vol. XIV · Fermentation Plate 1 of 11fermentation, yeast, lactic acid bacteria, moulds, koji, Aspergillus, Acetobacter, fermented foods, microbesM

Microbes as Cooks.

A third of what people eat has been fermented somewhere along the way. The cooks were too small to see.

Fermentation, in the kitchen sense, is any change microbes make to food that we want. Yeasts (Saccharomyces above all) turn sugars into carbon dioxide and alcohol: bread, beer, wine, 米酒. Lactic acid bacteria (Lactobacillus, Leuconostoc, Streptococcus) turn sugars into lactic acid: yogurt, cheese, sauerkraut, kimchi, 泡菜, sourdough, salami. Moulds, chiefly Aspergillus oryzae (koji) in East Asia, make enzymes that break starch into sugar and protein into amino acids: soy sauce, miso, sake, 豆瓣酱. Acetic acid bacteria turn alcohol into vinegar (XIV.8).

Each group has its own range of temperature, acidity and salt, and fermentation is the art of setting conditions so that the wanted microbes win and the others lose: salt and cool for vegetables, warmth for yogurt, sugar and no air for wine. The results preserve food (acid, alcohol, low sugar) and create flavours no cooking can — the tang of yogurt, the depth of soy sauce, the aroma of bread. The workshop compares the groups' preferred temperatures.

Below: the relative activity of four fermenting groups against temperature (teaching cardinal-temperature curves within published ranges), with the conditions each product uses, and the activity of each at the temperature you choose.

What you will learn

Name the main fermenting microbes and their products.
Match each to its temperature.
Explain how conditions select the wanted microbe.
See fermentation as both preservation and flavour.

Workshop

Teaching curves, each scaled to 1 at its optimum: baker's yeast (course yeast model, optimum near 34 °C), mesophilic lactic acid bacteria (optimum 32 °C), yogurt cultures (optimum 43 °C) and koji mould (Aspergillus oryzae, optimum about 32 °C, cardinal-temperature shape). Real strains vary.

Set the conditions, choose the cook

The curves explain much of fermentation practice. Bread dough rises happily at 24–28 °C, where yeast is lively; yogurt is held near 43 °C, where its heat-loving cultures race and few other microbes do well; sauerkraut and kimchi ferment best in the cool, 15–22 °C, slowly enough that the right sequence of lactic bacteria takes hold and the vegetables stay crisp; koji is grown in warm, humid rooms near 30 °C.

Alongside temperature the cook sets salt (which lactic bacteria tolerate better than most spoilers, XIV.3), sugar (food for yeast, and in high amounts a preservative), acid (which the fermenters make and the spoilers cannot bear) and air (yeasts make alcohol without it; acetic bacteria and moulds need it). Get the conditions right and the wanted microbes outgrow the rest; get them wrong and something else wins, which is why the last plate of this volume is about knowing a failure when you see one (XIV.10).

At the stove

Taste side by side: plain yogurt, sauerkraut or kimchi, a slice of sourdough, a drop of soy sauce and a sip of vinegar. Each is a different microbe's work on a different food.

Louis Pasteur showed in the 1850s and 1860s that fermentation is the work of living microbes — yeast for wine and beer, bacteria for lactic and acetic souring — overturning the idea that it was a purely chemical decay.

Terms

Fermentation: Desired changes to food made by microbes.
Lactic acid bacteria: Bacteria that turn sugars into lactic acid.
Koji: Steamed grain or beans grown with the mould Aspergillus oryzae.
Starter culture: A chosen population of microbes added to begin a fermentation.

Yeasts, lactic bacteria, moulds and acetic bacteria cook for us. The cook sets temperature, salt, sugar, acid and air so the wanted microbes win.

Sources

A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.