One of the most important discoveries we made while developing and refining the recipes in Modernist Bread is that yeast is among the most resilient life-forms we’ve ever encountered (and we encounter many in our lab, which we share with a bunch of biologists). As it turns out, freezing temperatures do not kill all the yeast and lactic acid bacteria in a preferment. Some die, but most remain dormant while frozen. The key is to know how to “wake it up” properly and to feed it well so it comes back strong and ready to leaven.
There are a lot of great reasons to try freezing your sourdough starter. Using a frozen preferment affords an almost instant starter; even with the added thawing and feeding time required, it provides a significant time savings over starting one from scratch. Having a preferment ready to go is convenient—you can freeze it in portions and just thaw what you need—and frees you from a feeding schedule. There’s no need to worry about entrusting someone with your starter when you go on vacation.
Our experiments demonstrated that a frozen levain will perform well for up to 2 weeks after freezing it. Eventually the ice crystals in the frozen preferment grow big enough to damage the yeasts and bacteria, rendering them useless for leavening. If you have levain that has been frozen for more than 2 weeks, you can still use it in combination with commercial yeast. The less-active levain will still provide your bread with complex flavor, and the yeast makes the dough rise.
Tips for Freezing Levain
Working with frozen levain is simple, although freezing your starter involves more than throwing it in a jar and stashing it in the freezer. Here are a few easy tips to get you started.
1. Freeze your preferment immediately after you make it. Freezing a ripe preferment won’t give the yeast the nutrients it needs because there will be little food left.
2. Our experiments demonstrated that a frozen levain will perform well for up to 2 weeks after freezing it. If you have levain that has been frozen for more than 2 weeks, you can still use it in combination with commercial yeast for an instant sourdough flavor. We utilize this technique for the Second-Chance Sourdough recipe in Modernist Bread.
3. Divide the preferment into whatever weight you would typically use for a specific dough. Stiff levain can be portioned directly into zip-top bags. You may want to add 10 g to the amount that you are freezing because ultimately some will stubbornly remain in the bag. Lay the bags flat on a sheet pan to freeze them.
4. For liquid levain, portion the preferment into an ice cube tray and use an offset spatula to even out the tops of the cubes. We use a piping bag to inject it deeply into the tray as possible, eliminating air pockets. Once it has frozen into cubes, remove them from the tray, and put them in a zip-top plastic bag in the freezer.
5. When you’re planning to make fresh bread with your levain, just thaw what you need. Take the portion out of the freezer about a day before you need it and let it thaw at room temperature (21 °C / 70 °F). When it’s ready, the bag will inflate as carbon dioxide bubbles form in the preferment. If you froze your starter into cubes, pull out however many cubes you need for your recipe, put them in a bowl, and cover them with plastic wrap. After making our dough, we like cold-proofing our levain in refrigeration for 24-36 hours to help develop the flavor.