Part V — Fermentation and Microbiology

A jar of cucumbers sits on the kitchen counter. There is salt water in the jar. There are no instructions on the lid. The jar has been there for ten days. Inside, no human is doing anything. Outside, no human can see anything happening. And yet, in those ten days, the cucumbers will have transformed — softer, sour, fizzing slightly when the lid lifts, smelling like a different food than the one that went in.

Nothing visible has changed. Everything has changed.

This is fermentation, and it is the oldest technology humans have for making food without cooking it. Older than the oven. Older than the pot. Possibly older than fire as a culinary tool. Every culture on earth, working independently, with no chemical theory and no microscope, discovered the same trick: under the right conditions, food can be left alone in a way that makes it better. The conditions vary. The microbes vary. The results vary. The principle is universal.

That principle — that you can use living organisms as part of your cooking, even though you cannot see them — is the subject of Part V. Five chapters. The most quietly miraculous part of the book.

The scope of what humans figured out without science

Sit with this for a moment. Long before anyone had ever seen a microbe, before anyone knew what yeast was, before the words bacterium or enzyme existed in any language —

In Egypt, bakers were leavening bread with a wet flour-and-water culture as far back as 5,000 years ago, the same culture-keeping principle that San Francisco sourdough bakers use today. In Mesopotamia, brewers were making beer at roughly the same time. In China, fermented soybean preparations, including the ancestors of soy sauce and miso, go back at least 2,500 years. In Korea, the practice of making kimchi — the spectrum of fermented vegetable preparations centered around but not limited to baechu kimchi (napa cabbage) — has 1,500-plus years of documentation, with the modern chile-pepper version emerging after the Columbian exchange brought Capsicum to Asia. In the Caucasus, dairy was being inoculated with what we now call kefir grains. In Mesoamerica, the Aztec and Maya were fermenting cacao pulp around the seed (a step we will spend a chapter on, because it is the reason chocolate tastes like chocolate). In the Andes, chicha fermentation has thousands of years of practice. In India and across South Asia, idli and dosa batters were being soured by wild bacterial-yeast consortia for at least a millennium. In Sub-Saharan Africa, ogi, injera, and dozens of other grain ferments have anchored daily food for centuries. In Northern Europe, sauerkraut. In Japan, miso, sake, natto. In Indonesia, tempeh. In Mexico, pulque.

This is a partial list. It is partial in every sense — partial geographically, partial historically, partial in every culture that doesn't appear here only because there is not room. The full list is essentially every culture that ever existed.

What this list means is that fermentation is not a niche technique brought into kitchens recently by chefs. It is one of the central technologies of human food, present in every cuisine, predating chemistry by thousands of years. The science we are about to discuss, in Chapters 30 through 34, is what those traditions independently discovered. The lactic-acid bacteria in your sauerkraut and the lactic-acid bacteria in kimchi are close cousins. The yeast in sourdough and the yeast in beer are the same species (and sometimes the same strain). The mold on the surface of dry-aged beef and the mold on the surface of camembert are doing related work.

When the science catches up to a tradition, the tradition does not need the science. But the science can help us understand why the tradition works — which lets us troubleshoot it, scale it, share it, and protect it. That is what we are here for.

What lives in this part

Chapter 30. What Is Fermentation? The biology and chemistry of controlled decomposition. Three categories of organism do most of the work: yeasts (which mostly produce alcohol and CO₂), bacteria (which mostly produce acid), and molds (some of which produce remarkable things, and some of which we want to keep out). The salt-water trick that selects for the right ones. The pH drop that kills competition. The reason a controlled ferment is reliably safe and a forgotten leftover is not.

Chapter 31. Bread and Beer. The two oldest biotechnologies on earth and probably the deepest-rooted relatives. Both rely on Saccharomyces cerevisiae — the same species, often the same strain — running glycolysis to convert sugar into ethanol and CO₂. Bread keeps the gas, vents the alcohol. Beer keeps the alcohol, vents the gas. One yeast. Two civilizations.

Chapter 32. Cheese, Yogurt, and Cultured Foods. Bacterial fermentation of milk. Lactic-acid bacteria turn lactose (milk sugar) into lactic acid, which drops the pH, which makes the proteins clump, which gives you everything from yogurt to fresh cheese to aged Parmigiano-Reggiano. The longer aged cheeses are slow-motion microbiology projects, with bacteria, molds, and enzymes working in succession over months and years.

Chapter 33. Pickles, Sauerkraut, Kimchi, Miso. Lacto-fermentation of vegetables and grains across the world's traditions. The salt concentration that lets Lactobacillus and its cousins outcompete the spoilage organisms. The pH endpoint that locks the food in stability. Cultural variants — and an honest discussion of how to learn from a tradition you are not part of without flattening or appropriating it. Chef Aroon's grandmother, who made fish sauce and pickled greens her whole life in northern Thailand, used to say that the jar tells you when it is ready. The jar does. The science explains why.

Chapter 34. Coffee, Tea, and Chocolate Fermentation. Three of the most widely consumed foods on earth, and almost no one knows that all three involve fermentation. Coffee cherries get pulp-fermented before drying (wet process) or dried with the pulp on (dry process), and the bacterial activity during this step is much of what makes coffee taste like coffee. Cacao seeds ferment inside the pulp of the fruit for several days before they are dried — and that fermentation produces the precursors that, on roasting, become chocolate. Black tea is technically oxidized rather than fermented, but the word stuck, and we will explain why.

How to read this part

Slowly. Several of these chapters describe processes that take days or weeks in real time. If you can, set up one ferment alongside your reading. A small jar of sauerkraut is the cheapest, simplest, most informative experiment in this book. You will read about the science while the science is doing itself in your kitchen. There is no substitute for that.

Danny, who works weekends at a restaurant whose entire walk-in is colonized by ferments, will tell you that fermentation is the part of cooking that taught him the most patience. You cannot rush it. You can only set the conditions and step away. The microbes are doing the work. Your job is to give them the right room.

Turn the page. Chapter 30: what fermentation actually is, and why it ever worked.

Chapters in This Part