changing hydration

Easy. Hydration % always refers to the proportion of water to flour, and flour is always 100% (ie: we're talking bakers' percentages here.) So 100% hydration means flour = 100% and water = 100% = same weight of each.

70% hydration means flour = 100% and water = 70% of flour by weight. So, if you have 50gm of flour, you mix 35gm (70% of 50gm) of water with it to get a hydration of 70%.

If your starter is currently 100% hydration and you want to make it 70%, just do a build starting with a small amount of starter (eg: 20gm), and add, say, 30gm of flour and 70% of 30gm (21gm) of water. Then next feed, 60gm flour + 70% of 60gm (42gm) water. Keep going until you have enough starter for your recipe.

I usually calculate it so that I end up with a bit more than the recipe weight of starter in 2 feeds, but this will depend on how active your starter is and how long since you've fed it. As long as you start your build with a small amount of starter, it takes only one or two feeds to build it to the hydration you're after.

Clear as mud? Hopefully, a bit clearer!

Cheers!
Ross

Thank you, Clare, for asking this question.

And a BIG Thank you, Ross, for answering it so clearly.

Hi Clare,

Take say 200 gms of your 100% hydration starter and add 40 gms of flour and no water. Mix and you get 240 gms of 70% hydration starter. Clear or you need calculations?

Gamya

Hi Clare,

Take say 200 gms of your 100% hydration starter and add 40 gms of flour and no water. Mix and you get 240 gms of 70% hydration starter. Clear or you need calculations?

Gamya

 If you add 40gms to the starter as you're suggesting, you're really only ending up with a 100% hydration starter with flour added. Not exactly the same as a 70% hydration starter.

If you leave the starter for another 9 hours (or whatever) after feeding it only 40gms of flour, that 40gm feed will be used up in no time and the starter will be well in decline by the time it should be peaking at the end of the feed cycle, and thus not in great shape for leavening dough.

If you don't wait for the feed cycle to run its course to peak ripeness, then you might as well just be adding 40 gms extra flour to the recipe to compensate for the 100% hydration starter (ie: adjusting the hydration of the dough, rather than the starter). I assume that's what you are suggesting, in effect. Indeed, that will work, but it's not the same as creating a 70% hydration starter, which has different properties from a 100% hydration starter - properties which can affect the bread in various ways.

eg: I was discussing different hydration starters with Shiao-Ping some time ago, and she was of the view based on her experience that lower hydration starters give a better rise in some breads. I think crumb quality and texture can also be affected by the hydration of the starter. Maybe even flavour...certainly pate fermentee markedly affects the flavour of bread, as is evident when you use it instead of starter in a recipe you know well.

It's astonishing, isn't it,  just how many variables are at play in bread baking - and they all have some effect on the finished product. All part of the mystique that draws us into this fascinating process...

Hi Ross,

As you add 40 gms flour to 200 gms of 100% hydration starter, the resulting 240 gms will not become 70% hydration starter immediately but now it has the correct amount of flour to make 70 % hydration starter after fermentation. This 40 gms flour will be left to ferment for sometime in the sour company of the initial starter till it merges with it completely in mass and character and loses it's identity of flour. Now the final fermented starter mass is made up from 140 gms flour and 100 gms water  i.e 100/140 = 70.14% hydration.

Gamya

Fair enough, too.