Yeast: The Foundation of Bread
Yeast is the most miraculous part of bread baking. To understand why bread works, you first need to understand what yeast is. As harnessed for bread baking, common yeast (or saccharomyces cerevisiae) is a single-celled organism which produces energy the same way humans do: cellular respiration. Yeast takes in glucose (from flour) and oxygen (from the air) and produces carbon dioxide. If there is no oxygen present, yeast will undergo alcoholic fermentation, converting starches into ethanol and carbon dioxide. The carbon dioxide is what fills bread with air; the ethanol gives bread flavor and its alcoholic scent.
Proofing
Proofing, or fermenting, is the period in which yeast and bacteria consume sugars and produce carbon dioxide, alcohols, and acids. Proofing depends on several factors, but the key ones are temperature and dough content. A dough with more simple sugars will rise faster than one with plain wheat, simply because the starches (or sugars) found in wheat are more complex. Heat causes microorganisms to speed up and work faster, and cold temperatures cause microorganisms to become more dormant. Hot and cold ferments have their benefits and drawbacks: hot fermentation makes bread rise faster, but allows for less flavor development as the most complex starches are not broken down as much.
Cold fermentation can be very slow, but it also produces better flavor and can give bakers some flexibility. Like any population, yeast and bacteria grow at an exponential rate until they reach carrying capacity, at which point more alcohol and acid are produced and microorganisms begin to die.
Usually, bread undergoes two proofs: a bulk fermentation and a final rise. Bulk fermentation is a longer rise where yeast reproduce and produce carbon dioxide—however, the disbursement of yeast and gas is often uneven. Before shaping, bread is usually punched down and kneaded slightly, both to redistribute the gases and give the dough extra strength. Sourdough does not require punching down; since it has a long rise and is folded throughout its bulk fermentation, the yeast and air are evenly dispersed.
What is a sourdough starter?
A sourdough starter is a colony of bacteria and yeast. If you mix flour and water together and wait, over time, wild microorganisms found in the air will feed on the starches and produce gas, sugar, and alcohol. The “funk” associated with a starter does not come from the microorganisms themselves, but rather from the gases and liquids they create. Like any organism, yeast and bacteria cannot be active and reproduce without feeding. A good starter will act predictably; mine, for example, takes around eight hours to double in size after a feeding.
I choose to feed my starter with a mixture of all-purpose flour and whole wheat flour. The all-purpose is a neutral and inexpensive flour, but the whole wheat flour provides more complex carbohydrates, which invite more diverse microflora. I recently made a starter using just AP flour; the aroma, compared to a starter fed with whole wheat flour, was much less nuanced. Just like animals have different diets, microorganisms have different food preferences. Most bakers use 100% hydration (or equal weights flour and water) for their starters—these are called “liquid starters,” and they are popular because they are easier to mix. However, some bakers prefer stiffer starters.
The Circle of Life
The microorganisms within a starter do not necessarily have symbiotic relationships. At the beginning of a starter’s life, thousands of types of yeasts and bacteria reside within a starter, all vying for the same food (flour). The main types of bacteria in sourdough starters are lactic acid bacteria and acetic acid bacteria—both create energy in a different manner from yeast. Bacteria feed on sugars, proteins, and other nutrients, and produce acids and carbon dioxide. Acetic acid and lactic acid in large quantities can inhibit yeast growth—this is why a starter becomes unusable after fermenting for too long.
Some of the bacteria also have unique defense mechanisms—usually, they will produce chemicals which are toxic to other bacteria or yeast. When creating a starter, a fight to the death occurs as tiny organisms kill each other for a scarce supply of food. A young starter will usually smell bad due to the array of chemicals and alcohols produced by the bacteria and yeast. After several weeks, just a few strains of bacteria and yeast will remain. Though they may compete with each other over food, they will kill any “bad” bacteria which enter a starter. A mature starter will produce lots of gas, a sign that it is digesting flour. Though a starter which has been alive for a year may be more flavorful than one alive for a month, after several years, a starter’s flavor will not change much.
Photo: Modernist Cuisine
Observing and Maintaining Your Starter
The key indicators of a starter’s health are its growth, aroma, and viscosity. A starter’s growth is an indicator of how much bacteria and yeast are present; if lots of microorganisms are present, they will produce more gas, and the starter will be very bubbly. A “young” starter is one which has just been fed, and is beginning to grow. It will smell very faintly of yeast, and there may not be any sour aroma. A “mature” starter will have doubled in size, and will smell very yeasty. When my starter is mature, it smells like overripe fruit and cider vinegar. Usually, I bake with a very bubbly mature starter that smells somewhat sour.
A “ripe” starter has reached its peak of growth; it will smell very yeasty and sour, and it will appear frothy and liquidy. When a starter is very watery, or “proteolytic,” it means the gluten network has been weakened by enzymes and acids. If your starter is very thin, it is probably too ripe to bake with and should be fed again. Most stages of starter are fine to bake with, but be wary of baking with very young or very ripe starters; the former will take a long time to ferment, and the latter will produce a very tangy flavor.
In order to manage a starter, you actually have to discard much of it when feeding. When the population grows, it demands more food. With more food available, the population grows even more. If you were to keep feeding a starter without discarding any, it would grow exponentially large. Think of feeding a starter like making a cocktail: an ounce of liquor may be very strong by itself, but add other non-alcoholic ingredients, and the drink becomes weaker. Feeding a starter is essentially taking a small, very populous sample of the original starter and adding it to a much larger supply of food.
Bakers can feed starters in different ratios depending on when they plan to bake bread. Usually, professional bakers will feed their starters twice a day, because they are constantly baking. If you’re not baking bread regularly, you can certainly feed your starter once a week and keep it in the fridge. If I plan to make sourdough the next morning, I will feed my starter the night before with a ratio of 1:5:5 (starter:water:flour). It takes about 12 hours for my starter to achieve the level of maturity I prefer for bread, so when I wake up, the starter is perfectly risen. That said, every person’s starter (and schedule) is different—the best way to tell how your starter reacts to feedings is to feed it different amounts of flour and see how it rises.
A Note About Water
I used to obsess about the water I used to bake. Some bakers only work with non-chlorinated water; however, the scientists at Modernist Cuisine proved that filtration level of water has no effect on fermentation.
What about Instant or Dry-Active Yeast
There are some people who will publicly decry dried yeast; I am not one of those people. Dry yeast is incredibly helpful, mostly because it takes no maintenance to preserve, it lasts a long time, and it acts predictably. The main drawback is that it lacks the nuanced flavor and texture of sourdough, as it reacts with starches differently and does not produce any acid; however, you can add starter to breads to supplement the flavor. In fact, for recipes such as pizza, bagels, and enriched doughs, I recommend using a mixture of dry and fresh yeast, as the doughs are much easier to work with and have a greater rise.
Can You “Kill” A Starter?
Technically, you can kill a starter—however, it is very difficult. Yeast and bacteria can go into stasis, meaning they lie dormant and do not need nutrients. Dormant yeast has been found in ancient Egyptian tombs, so your starter can probably go for a few weeks without dying in the fridge. However, starters can be finicky. If you change their feeding schedule, or feed them different types of flour, they may react differently. As a rule of thumb, if you change a starter’s diet or feeding plan, you should feed it daily for a week before keeping it in the fridge.
If your starter begins to grow mold, however, the best option is to discard the starter. You may be able to remove the mold, but the very presence of the invasive fungus suggests that the yeast and bacteria are inactive. If your starter seems inactive, try feeding it several times before baking bread. Only once your starter rises predictably is it ready for baking.
Making Your Own Starter
Each starter is a reflection of the person who makes it—this is the beauty of bread-baking. The air quality of your house, the flour you use, your feeding schedule, all impact the way your starter will turn out.
To make your own starter, you will need:
All-purpose flour
Whole wheat flour
Water
In a small mason jar, mix 50g water, 25g AP flour, and 25g whole wheat flour. Loosely cover with a lid and let rest at room temperature for 48 hours. After two days, the starter should be bubbly and pungent. If it is not yet showing signs of life, let it rest for another day at room temperature.
Transfer 25g of starter to a small bowl. Add 50g of water and whisk to combine. Add 25g AP flour and 25g whole wheat flour, and stir to combine. Discard the starter from the mason jar, add the freshly mixed starter to the jar, loosely cover, and let rest at room temperature for 24 hours.
Repeat the feeding step every day for two weeks, using the ratio 1:2:2 of starter:water:flour. After two weeks, your starter should rise and fall predictably, and smell more pleasant as it matures. Once your starter is fully matured, you can use it to bake bread. To preserve it, keep it in the refrigerator and feed it once per week or whenever you bake bread with the ratio 1:5:5, starter:water:flour.
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