Starter boil off question

[-Liam-]

This is probably a silly question, but here goes...When making a starter, if I have a boil off of around 300ml and I add this extra amount of water at the beginning, so that my finished boil is at the amount I need. Do I need to add DME for the total volume or the finished volume?
For eg. I need a 1l starter, so I use 1300ml water to finish at 1l. Should I add 100gm or 130gm of DME?

 

[Singletrack]

You want an OG of ~1.040 (maybe a little less) for healthy yeast growth. Yeast don't care what the gravity was before they were added. (You are left with the OG math as an exercise.)

 

[davejanssen]

Yeah, as Singletrack said you want to add DME for the final volume, so 100 g for a final vol of 1 L. You are only boiling away water, not DME. At first I thought you had boiled over 300 mL (lost 200 mL) of your starter and were trying to figure out how to correct it, but that would be a totally different issue.

 

[acidrain]

After boil off, but why do you have boil off?
There is no reason to boil for any length of time. By the time the water comes up to a boil, it's sterile.

 

[-Liam-]

I guess I meant evaporation, as opposed to 'boil off'. So the whole "boil for 15mins" idea is a bit of a sham then?

 

[Singletrack]

I boil for 10 min, and I have a little boil off. I found the volume added by the DME was more than the boil off, so I made an adjustment in the water volume calc so I end with the volume that gives me OG = 1.038 to 1.040. There's probably no good reason to make such adjustments, but I enjoy it so I do it.

Seems prudent to hold at boiling temp for a bit, since most of the energy is used to get to boil, but I couldn't say that it is really necessary.

I boil in my flask, so I don't have to pre-bake it or Star San it or anything beforehand. Maybe that's a good reason to hold at boiling for 10 min.

 

[jahdub]

I just boil about a liter per about 100g of DME. You don't have to be exactly right, you just want to be roughly 1.035-1.040ish