I typically use a wort sample for my liquid yeast starter by draining off a portion, cooling it, then use a hydrometer to check the gravity. If it's above 1.030, I dilute with filtered water then mix in the yeast.
While the rest of the wort boils, the yeast energizes. When I underpitch, I will use .5l of diluted wort and one packet of yeast.
When the wort is cooled and the starter has whitish foam visible, I will pour the yeast in and be on my way.![]()
Why 1.030?
It's a diluted wort with less sugar.
Yeast will need a medium that has nutrients but doesn't quite overwhelm their ability to re-energize themselves to reproduce. A heavy wort is loaded with sugar and will stress the yeast. Weak yeast can re-hydrate themselves, then go after the sugars. In a liquid packet the yeast will reproduce at different rates. It's like Darwin's Law in a beaker.
Dry yeast can be re-hydrated with water or pitched "as is". An 11.5 gram packet will have more cells compared to a liquid packet, so you don't have to worry as much about under-pitching a weak yeast.
That's about it.![]()