Cultured whey is a by-product from cultured cheese making and yogurt straining. It is teeming with live Lactobacillus cultures. If you're not a cheese maker (yet!) you can extract your own whey from homemade or store-bought plain, additive-free yogurt.
Pour the yogurt into a muslin bag, cloth or thin tea towel over a large bowl. Pull the ends of the towel up and secure with string or a rubber band. Tie the towel to a cabinet handle above the bowl. Leave it to drain for 2-12 hours depending on how much whey you need.
Pour the liquid in the bowl (this is the whey!) into a glass jar and store in the fridge for up to 1 month. What's left in the towel is a thick Greek yogurt (2 hour strain) or a creamy cheese (12 hour strain). Store in the fridge for up to 2 weeks.
Image: Rebecca Siegel