## I keep a 100% starter, but Reinhart wants 125%...

So, there it is. I keep a 100% hydration starter. For this multi-step San Fran sourdough recipe of his I've been following and liking, it takes me like, 3-4 days to do.. which is just long enough, I figure. So, i keep a 100% starter, because the math is easy. I have a variable amount of starter on hand at any time, from 10 grams to 90 grams at any time, depending on all sorts of things. Right now, I have 75 grams to spare, and I'm looking to build the 125% hydration starter right now, ferment it for 4 hours or so and then fridge it overnight like this recipe calls for.. but some times, I only have 30 grams, and need to build it up.. I do a 1.5lb loaf a t a time, so I need 172 grams of 125% starter here to kick off the process.. for this recipe. So, knowing I have 75 grams of 100% right now, and 172-75 = 97 grams of extra stuff I need, how do I best convert this to 125-ish percent hydration? This math should be easy, but I can't find the answer anywhere here on the site or in any of my books.. and I'm not smart enough to figure it out on my own. I know if I just split the 97/2 I'll keep it at 100% hydration, and I know I *could* just sorta wing it and add a bit more water.. and I know the 100% hydration of such a large percentage of the starter I plan to add this 97grams of material into is going to throw off the books a lot anyway, but.. really, how do I do it? :)

If you need a total of 172g starter at 125% hydration, divide 172 by 225 (100 'units' of flour and 125 of water). You get 0.764g - that's the weight of each unit in the starter.

Now multiply this weight by the number of units of flour and water that will constitute the starter:

0.764 x 100 = 76.4g total weight of flour that you need.

0.764 x 125 = 95.5g total weight of water that you need.

At the moment you have 37.5g each of flour and water. From here it's simple: 76.4 - 37.5 = 38.9; 95.5 - 37.5 = 58.

So add 39g flour and 58g water to your 75g starter to get 172g at 125% hydration.

You are my newest hero. I will sing your name in praise during the remainder of my starter mixing adventures for the day, and perhaps even for tomorrows, as well. Thank you!

No problem! There's a really good explanation of this at the Wild Yeast blog if you want to reference it in the future: http://www.wildyeastblog.com/2008/04/13/bakers-percentage-3/

but i always think of it as 5 to 4 - 5 parts water, 4 parts flour. I maintain my starter, but whenever i need to convert it to 125%, i just mix a teaspoon of starter with 50g water and 40g flour, a couple of feedings, and there you go

Like HokeyPokey, I think of 125% hydration as a 5:4 ratio (5 ÷ 4 = 125%) and use 9 (= 5 + 4) as my water/flour base part.

Taking 172g and dividing by 9, gives a part factor of 19.111...

So 172g at 125% hydration is:

5 x 19.111... = 95.555... grams of water

4 x 19.111... = 76.444... grams of flour

Converting from 100% hydration is then very simple. Since 100% hydration is 1/2 flour and 1/2 water, take the smaller number (76.444g of flour) and double it to give 152.888... of your 100% starter, which now represents:

4 x 19.111... parts water = 76.444...

4 x 19.111... parts flour = 76.444...

Now just add one part of 19.111... grams of water to create the 5:4 ratio.

Of course, depending on the resolution of your scale, you will have to round a couple of those numbers off.

I usually convert the hydration of a starter a recipe calls during the elaboration feeding prior to mixing.

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For your example, I might do the following ~6-8 hours before I would be mixing the dough:

As a general guide, I like to double the starter in the elaboration phase, so I would divide the final starter amount by 2 to get 86g. I now want to change that number to the nearest one divisible by 9 to arrive at my feeding amount. Just so happens that 90g is close and very easy to work with.

90 ÷ 9 = 10 (part factor)

90g of 125% starter =

5 x 10 = 50g grams of water

4 x 10 = 40g grams of flour

Hopefully the following makes sense:

172g (final starter amount) - 90g (elaboration feeding amount) = 82g (beginning starter amount)

82g ÷ 9 = 9.111... (part factor)

5 x 9.111... = 45.5 (water in 82g of 125% starter)

4 x 9.111... = 36.5 (flour in 82g of 125% starter)

2 x 36.5 = 73g (amount of 100% starter)

73g (100% starter) + 9g of water = 82g (125% starter)

82g (125% starter) + 50g (water) + 40g (flour) = 172g (final amount of 125% starter)

So my elaboration/conversion feeding would consist of 73g (100% starter) + 59g (50 + 9 water) + 40g (flour)

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The fun part of that all that is if you needed to convert to 172g of starter at 80% hydration, which is a ratio of 4:5 (4 ÷ 5 = 80%). So you could just swap the words 'water' and 'flour' and substitute '80%' for '125%' and the calculations would be the same.

I keep several starters, my San Fran and WW at 80% (4:5 ratio) and my Rye at 125% (5:4 ratio). I have become quite adept at multiplying and dividing by 9.

How much difference does a 100% starter vs. a 125% starter make? What's the percentage of starter in the recipe, and couldn't a 100% starter be used, with an adjustment to the quantity of water and flour to compensate for the reduced quantity of water in the starter (5g or so, maybe)? OK, maybe that doesn't work, but if so, how does that affect the recipe?