Why Does Bread Rise?

Yeast is a single-celled fungus. Given flour, water and warmth it feeds on simple sugars and produces carbon dioxide gas and small amounts of alcohol as waste products.

That gas has to go somewhere. In a properly mixed dough it collects in thousands of tiny pockets, and as more gas is produced those pockets expand and the whole dough swells. Most of the alcohol burns off in the oven, though it contributes to the aroma of baking bread.

Yeast alone would not be enough. Flour contains two proteins, glutenin and gliadin, which combine with water to form gluten. Kneading lines these proteins up into long linked strands.

The result is a network that is both stretchy and strong, so it can expand around a growing bubble without tearing. This is why bread flour with a higher protein content gives an airier loaf, and why cake flour with less protein produces something softer and tighter.

Kneading develops that gluten network by working the dough, though time can do a lot of the same job. Long slow rises in the fridge produce good structure with far less effort.

The first rise lets the yeast multiply and build flavour. Knocking the dough back redistributes the yeast and the gas, and the second rise gives a finer, more even crumb. Rushing either stage is the most common cause of a disappointing loaf.

Bread does most of its final rising in the first few minutes of baking, an effect bakers call oven spring. Several things happen at once:

• •The gas already trapped in the dough expands as it heats
• •The yeast has a final burst of activity before the heat kills it
• •Water in the dough turns to steam, adding more pressure
• •Gluten and starch then set, locking the risen structure in place

Soda bread and most cakes rise without any yeast, using a chemical reaction instead. Baking soda is an alkali that releases carbon dioxide when it meets an acid such as buttermilk, yogurt or lemon juice.

Baking powder combines the soda with a powdered acid, so it only needs moisture and heat to work. These reactions are fast, which is why soda bread goes into the oven immediately while yeasted bread needs hours. Sourdough sits in between, using wild yeast and bacteria that work slowly and add a sour tang.

A sourdough starter is not just wild yeast. It is a stable partnership between yeasts and lactic acid bacteria, and the bacteria are what produce the sour flavour and the distinctive keeping quality.

The bacteria also lower the pH of the dough, which strengthens the gluten network and slows the staling process, so sourdough keeps noticeably longer than a standard yeasted loaf. The trade off is time. A sourdough rise can take twelve hours or more, because the wild organisms work far more slowly than commercial yeast bred for speed.

Many people assume bread goes stale simply because it dries out, but the main cause is a change in its starch. During baking, starch granules absorb water and swell, becoming soft and disordered. As the loaf cools and sits, the starch molecules slowly realign into tighter, more crystal-like arrangements and squeeze water out, a process called retrogradation. The crumb becomes firm and crumbly even if the bread is wrapped well enough to lose very little moisture overall. Meanwhile, water moving outward makes a crisp crust go leathery.

This explains some kitchen habits that seem odd at first. Retrogradation happens fastest at cool temperatures just above freezing, so storing bread in the fridge usually makes it stale more quickly than leaving it at room temperature. Freezing slows the process almost to a stop, which is why frozen bread keeps well. Warming a stale loaf in the oven can temporarily reverse some of the change, making it soft again, but the effect wears off quickly as the starch cools and realigns once more.

Knowing when a dough has risen enough is a skill bakers practise with a simple poke test. Flour a fingertip and press it gently about a centimetre into the risen dough. If the dent springs back immediately, the dough is under-proofed and needs more time. If it fills back slowly and only partly, the dough is usually ready to bake. If the dent stays put or the dough sighs and sinks, it has over-proofed: the gluten has stretched so thin that bubbles merge and leak, and the loaf may collapse in the oven or taste sour and beery.

Salt and temperature are the two easiest controls. Salt tightens the gluten network and slows yeast down, which is why dough without salt rises quickly but tastes flat and spreads, and why many recipes avoid tipping salt directly onto a heap of dried yeast. Warmth speeds fermentation, but liquid hot enough to feel uncomfortable on your wrist can damage or kill yeast before it starts. Cold has the opposite effect. Many bakers deliberately rise dough slowly overnight in the fridge, because the extra hours give enzymes and microbes time to develop a fuller flavour.

Not every bread relies on gluten. Rye flour contains proteins related to wheat's, but they do not form a strong, stretchy network, so rye dough cannot inflate in the same way. Instead it depends on gums called pentosans, which soak up large amounts of water and form a sticky gel that traps some gas. That is why traditional rye breads from northern and eastern Europe are dense, moist and close-textured. Rye bakers usually use a sourdough method, because the acidity holds back rye's active starch-digesting enzymes, which would otherwise make the crumb gummy.

Gluten-free bakers face the same problem with no gluten at all, using flours such as rice, maize or buckwheat. They borrow binders that imitate gluten's job. Xanthan gum, made by bacterial fermentation, and psyllium husk, from the seeds of a small plant in the plantain family, both thicken the mixture and help form films around gas bubbles. Eggs add proteins that set firm in the oven. Gluten-free doughs are usually much wetter, closer to a thick batter, and they are often baked in tins because they cannot hold their shape unsupported while the bubbles grow.

Flour contains surprisingly little free sugar, often only around one or two percent, so yeast would quickly run out of food if that were all it had. Most of its fuel is made in the dough itself. Flour carries natural enzymes called amylases, and once water is added they begin cutting starch into maltose, a sugar made of two glucose units, which yeast can then break down and ferment. The enzymes work best on starch granules that were damaged during milling, because intact granules are tightly packed and hard to reach. Flour with too little of this enzyme activity tends to produce a slow rise and a pale loaf.

Some bakers add a small amount of diastatic malt powder, made from sprouted and dried barley that is rich in these enzymes, to boost the supply of sugar. Too much enzyme causes the opposite problem, a sticky, gummy crumb, because too much starch has been broken down. Leftover sugar matters at the end too. Whatever the yeast has not consumed by the time the loaf goes into the oven helps the crust brown, which is why dough left to ferment far too long, with little sugar remaining, can bake into a pale loaf with a disappointing crust, however well it has risen.

Bread rises because yeast produces carbon dioxide and a gluten network traps it, then oven heat expands the gas one final time before the structure sets, which is why both good kneading and enough resting time matter so much.