Whether you’re a baking novice or a veteran, chances are you buy more dry active yeast than you’re likely to use at one time. If you’re not storing your leftover yeast carefully, you run the risk of losing some of your yeast’s activity. Proper storage can extend the life of your yeast.
Dry yeast packets are the easiest to store. Just keep them in a cool, dry place – or perhaps the refrigerator if you want them to last a bit longer. However, even a shelf stored packet can last for more than a year, as they only lose about 1-2% of their viable yeast cells per month. Dry yeast is also nice to work with as you don’t usually need a starter – and can just hydrate the yeast 20 minutes before pitching in a little warm water.
Liquid yeast vials or smack packs are an entirely different story. They are more prone to heat and light degradation and also lose significantly more cells as they age. Liquid yeast really must be kept refrigerated – you can’t store it on the shelf. Even then, liquid yeast loses about 20% of its viable yeast cells per month, which means it has a useful shelf life of about six months. Unless the package is brand new, most beers will benefit from a yeast starter when working with liquid yeast – a good starter will ensure you are pitching the ideal number of cells for your batch.