What Yogurt Should You Use as a Starter?

Making yogurt means adding live bacterial cultures to milk. These cultures turn the milk’s sugar into acid, and the acid causes milk proteins to coagulate. So the yogurt used as a starter has just one job: to carry live cultures. That gives us a direct criterion: what matters is whether the cultures inside are alive and active, not the yogurt’s taste, brand, or price. An expensive yogurt is not necessarily a better starter. When choosing a starter, checking the ingredients label matters more than choosing a brand.

That is why products with additives are not used as starters. Fruit-flavored and sweetened yogurts interfere with fermentation because of their sugar and flavorings. Yogurts thickened with starch, gelatin, or milk powder are also unsuitable; their thickness comes from additives rather than cultures, and they do not produce the same result when added to milk. Choose a product by reading the ingredients list on the label. A statement on the package that it contains live cultures also helps with the choice.

Freshness is the second criterion. Cultures weaken over time; milk fermented with yogurt close to its expiration date may take a long time to set or may not set at all. Yogurt forgotten at the back of the refrigerator is not a good starter for the same reason. Never use frozen yogurt, because freezing damages the cultures. The most reliable starter is plain yogurt bought or made a few days earlier. Taking the starter out of the refrigerator and letting it sit for a while also reduces the temperature difference. For the same reason, keep the bowl reserved for the starter on an inside refrigerator shelf, not in the door.

Use less starter than you might think. Adding too much does not make yogurt firmer; an overcrowded culture consumes the milk sugar quickly, resulting in sour, loose yogurt. A few 15 ml measures are often enough for one liter of milk. Do not add the starter directly to the milk; first loosen it in a small bowl with a little warm milk, then add it to the pot. Make sure no lumps remain; lumpy starter will not disperse evenly through the milk.

You can also use yogurt from your own batch as a starter; this is common practice. Set aside a bowl from each batch to use as the starter for the next one. However, this cycle cannot continue forever; after a few transfers, the balance of the cultures shifts, and the yogurt becomes sourer and loses thickness. At that point, start over with fresh yogurt. Keep the bowl reserved for starter covered and do not dip a spoon into it. Cover the reserved starter and store it on an inside refrigerator shelf. Keeping track of which transfer you are on also makes it easier to estimate when to replace the culture.