Two yeasts, one job, but they behave differently

Walk into any British supermarket and you will find yeast in two guises: small sachets of instant yeast, often labelled fast-action, easy-bake or quick, and tins or sachets of active dry yeast. Both are dried forms of the same single-celled fungus, and both will raise a respectable loaf. The difference lies in how they were dried, how many living cells survive per gram, and therefore how you need to treat them.

Active dry yeast is dried at higher temperatures, which kills a proportion of the cells and leaves the survivors wrapped in a protective layer of dead yeast. The granules are larger and coarser. Instant yeast is dried more gently and milled finer, so it contains a higher concentration of living cells and needs no rehydration before it meets the dough. That single fact explains most of the practical differences below.

Activating active dry yeast properly

Active dry yeast benefits from a short wake-up call. Warm the liquid you are using to somewhere between 38°C and 43°C — lukewarm, so a drop on your wrist feels neither hot nor cold. Anything above about 50°C will start killing the yeast, and by 55°C you have a very expensive way of making warm water.

Sprinkle the yeast over the water, add a pinch of sugar if you like, and leave it for five to ten minutes. You are looking for a creamy, foaming surface and a faintly beery smell. Once it has foamed, tip the whole lot into your bowl and count that liquid as part of the recipe's total. If nothing happens after ten minutes, the yeast is dead — throw it out rather than wasting flour on it.

• Use a clean jug, not a metal bowl, which can hold onto heat unevenly.
• Keep salt away from the yeast at this stage; it slows activation.
• Check the packet, as some modern active dry yeasts are formulated to go straight into dry ingredients.

Using instant yeast straight from the sachet

Instant yeast is the relaxed one. Whisk it through your flour with the salt, add your liquid at any temperature from cool to lukewarm, and get on with kneading. There is no separate proving step, no foaming jug, and it is more tolerant of a slightly cool kitchen or a dough that sits a little longer than planned.

In the UK, most sachets are 7g, which is enough for around 500g of strong bread flour. That is a convenient ratio to remember: one sachet, half a kilo of flour. Instant yeast also dissolves readily in doughs enriched with butter, milk or egg, which is why it suits brioche, milk rolls and hot cross buns so well.

Adjusting quantities when swapping between the two

Recipes rarely warn you, so here is the working rule. By weight, use roughly 25 per cent more active dry yeast than instant. By spoons, 1 teaspoon of instant yeast equals about 1¼ teaspoons of active dry. A 7g sachet of instant yeast therefore needs around 8.5g to 9g of active dry to do the same job.

If you are converting from fresh yeast, multiply the instant weight by two and a half to three — so that same 7g sachet is roughly 18g to 21g of fresh yeast, crumbled into the liquid. Going the other way, fresh to instant, divide by about three. When in doubt, err on the side of slightly less yeast and a slightly longer prove; over-yeasted dough rises fast and then collapses, tasting harsh and yeasty.

Storing yeast so it stays vigorous

Instant yeast is the more perishable of the two because it has no protective coating. Keep unopened sachets in a cool, dark cupboard and use them within a few months. For anything longer, decant into an airtight container and store in the fridge or freezer.

Active dry yeast is happier in the pantry and will hold its strength for longer, especially in a sealed tin. Freezing works for both, extending life by a year or more, but avoid repeated swings in temperature; condensation is what actually kills yeast. Take out what you need, reseal quickly, and let frozen yeast come up to room temperature before adding it to cool dough.

• Always note the best-before date, and test older yeast before trusting it.
• Test by stirring 1 teaspoon into warm water with a pinch of sugar and waiting ten minutes.
• Discard any yeast that has clumped, darkened or smells sharply sour.

Matching your yeast to British flours and proving times

Strong white bread flour here usually sits around 12 to 13 per cent protein, and it rewards a good ten minutes of kneading followed by a warm, unhurried rise. Wholemeal and malted grain flours absorb more water, and their bran particles can slow the rise, so add a splash more liquid and consider a little extra yeast or a longer prove. A heavily chlorinated water supply can also inhibit yeast, so if your loaves are consistently sluggish, try filtered water.

Instant yeast is the better choice for an overnight cold prove in the fridge, where a quarter to a half of the usual amount develops lovely flavour over twelve to eighteen hours. Active dry suits same-day baking, where you want a predictable, steady rise. Whichever you choose, weigh your ingredients, keep your dough warm but never hot, and let the smell and feel of the dough guide you more than the clock.