Why Discard Starter: A Mathematical View.

DiscardingYou see this question pop up all the time in sourdough discussions:

“Why do I have to discard? I hate the idea of throwing anything away. Can’t I just keep feeding the starter?”

Although reducing the waste we produce as a species is a good thing, there are times when it’s actually more logical to NOT do so. And feeding a starter is one of those times.

Let’s first see about the starter when you’re just starting it up from scratch.

Early in your starter’s young life, within the first two weeks or so of starting a starter from scratch, there is absolutely no good reason to save the excess “flour soup” you’re cutting back. It has done it’s job and is spent, and unless you’re building excessively large amounts (see the Starter Step by Step series to see the recommended  quantity), it should only be about 2 tablespoons of flour each day, this is not worth worrying about. Even if ditched directly into the trash or compost, it’s still not “wasted” any more than that nice dinner you ate yesterday was “wasted”: it was used to feed your starter, so it had a purpose and served it well. So right off the bat, we need to get out of this idea that bit of flour you used to feed our yeast is wasted.

Aside from the dubious economics this would entail, this “brand new starter” flour soup likely has a fair bit of seriously questionable bacterial activity still going on and even if it was $8 worth of flour as opposed to the mere 3¢ it more likely is, that stuff really NEEDS to be tossed out. Once your starter has matured, is expanding regularly and consistently and has developed a good, pleasant scent, if it’s ready to be used in bread, then it can be saved for other uses. But until that point, THROW IT AWAY (or put it in the compost). And if you could have smelled Wally in his first couple of days, you would most certainly not want to keep that.

But what about the starter once it’s become viable and working well?

Even after your starter has reached actual starter stage – this will take two or three weeks – there’s a simple reason why you need to remove your excess. And it is just that, “excess”. After having developed properly, there’s nothing “bad” about the amount you remove, it’s just as good a starter as what you keep; the left side of the jar is no different than the right side. So if you wanted to, you could give this fine starter excess away to friends who want a ready-made starter or use it in “discard” recipes like pancakes or english muffins or anything else you think the flavour might be a nice addition to. So if it’s just as good as the starter you keep, why not keep ALL of it then?

You really do want to reduce your starter. If you were to just keep adding and not removing any because you “hate to throw anything away”, here’s what you’d end up with in very short order.

One common method of feeding says to remove half your starter and add that much fresh flour/water again; for example, “Remove one cup of starter (from a two cups batch) and replace it with one fresh cup”. It is assumed here that this new cup is made of equal weights of water and flour.

For our project, we’re going to add that 50% starter but, because we’re testing the “don’t want to throw any out” idea, we will NOT be removing/discarding the original half. We also keep this hypothetical starter “out on the counter” and feed it twice daily, morning and evening, and go for one entire week for a total of 14 feeds.

We’ll start on day 1 with a very small amount of starter: 2 tablespoons or 1/8 cup. At each feed, we give it an amount of feed equal to the original starter. This is, in effect, a 1:½:½ feeding ratio (one part old starter given a half part water and a half part flour) which is really skimpy on fresh food. Personally, I’d want to give at least a 1:1:1 ratio – and in fact I feed mine at 1:2:2 which is plenty of food for that amount of old starter. If we followed that plan, the amounts we see below would increase even faster.

And just for fun…

Before you read the rest, take a guess at how much starter you’ll have at the end of one week, starting with just 2 tablespoons and feeding twice a day. Ok, got an amount in your head? Write it down somewhere… No cheating, can’t look at the end or change your mind, now!

Alright, let’s feed our starter!

  • Day 1, Morning Feed: we start with 1/8 cup starter – a mere 2 tablespoons – so we add 1/8 cup of feed and we now have 1/4 cup of fed starter. Whew! Saved from throwing out 1 tablespoon of starer or about 2 teaspoons of flour!
  • Day 1, Evening Feed: 1/4 cup starter + 1/4 cup feed = 1/2 cup total starter. No problem here. Good night, little starter, feed you in the morning!
  • Day 2, Morning Feed: 1/2 cup starter + 1/2 cup feed = 1 cup total starter. Bah, this is easy. Oh wait, our starter is doubling in size so we have to make sure the jar is big enough. So into a quart jar (4 cups) it goes, that should do for a while.
  • Day 2, Evening Feed: 1 cup starter gets 1c feed = 2 cup starter. Hmm. Hope it doesn’t expand past double.
  • Day 3, Morning Feed: 2 cup starter + 2 cup feed = 4c total starter. Well geez, now that quart is full and it hasn’t even expanded yet. OK, into a gallon jug you go.
  • Day 3, Evening Feed:  4 cup starter + 4 cup feed = 8 cup total. Wow, that’s 2 whole quarts. If it expands to double, this gallon jar will just hold it (hopefully).
  • Day 4, Morning Feed: 2 qt starter + 2 qt feed = 1 gallon total starter. Gah! I need a big bucket for this stuff now. And I just added about 4 cups of flour to this thing. And we’re just barely past the middle of week one. Wonder how long that 5 lb bag of flour is gonna last?
  • Day 4, Evening Feed: 1 gal starter + 1 gal feed = 2 gal total. OK, into that big 5 gallon Home Depot pail now. Man, I used 12 cups of flour today alone…
  • Day 5, Morning Feed: 2 gal Starter + 2 gal feed = 4 gal total. Off to the store for more flour…
  • Day 5, Evening Feed: We now have 8 gallons, which need at leasts a 16 gal container. The kiddy pool would do…
  • Day 6, Morning Feed: We’re now up to 16 gallons of starter.
  • Day 6 Evening Feed: Increased the starter to 32 gal and you used 192 cups of flour to do so. At ~4.5 oz of flour per cup, that’s  54 lbs of flour in just this one feed, that’s five 10 lb bags and then a bit more… And you’re not quite at the end of week one yet!
  • Day 7, Morning Feed: we now have 64 gal of starter. We better start planning on making bread soon.
  • Day 7, Evening Feed: 128 gallons of starter. That’s more than your standard bathtub can hold, never mind when it expands. If you want to think about “economy”, you need to ask yourself how much flour you are willing to use to feed this exponentially growing monster, never mind where you’ll be keeping it in a day or two. Maybe the neighbours are away and you can use their pool…

Now let’s keep in mind we began with just two tablespoons. And of the final 128 gallon starter, only about 40 gallons of that is water, the other 80 gallons is flour (water is heavier than flour). That’s SIXTEEN five gallon drums. You will be running to the grocery store and buying out their flour supply in no time. That won’t be cheap.


You can discard a couple of tablespoons of flour at each feed and keep the starter at a reasonable size for your needs, whether that’s keeping it to 50g (about a quarter cup) or a little bigger because you bake a lot each week. But you’d be an industrial bakery pumping out thousands of loaves to be using up 128 gallons of starter. And besides, industrial bakeries don’t make real sourdough anyway, they just want to pump out ‘fluffy’ bread in 4 hours, tops.

Need a larger amount of starter for your baking day? Take one feed’s discard and build that up to that amount. Or collect a couple days’ excess starter and use it in other baking.

And how close were you in your guess? Go ahead, post it in a comment below.

What of the future in our “no discarding” scenario? In three more days, you’d have 8,192 gallons. To hold that (and it’s expansion) you’d need a swimming pool 18′ X 36′ X 4′ deep, about 2600 cubic feet. And the next feed, two of them.

If you were to do your feeding at a better ratio of 1:1:1, you’d have 2,335 gallons by the end of the original 7 days.

The preferable 1:2:2 ratio would get you 193 gallons at the end of day four and 2.9 million gallons by end of day 7.

Now obviously no one would go anywhere near that far or even past a few days before realizing that NOT removing excess starter is insanely expensive, but it may not be obvious how FAST this adds up when you’re looking at your tiny 2 Tablespoons of starter and thinking it’s not worth throwing that extra tablespoon of flour out. Even by about day 4 you’d have got to the point where you’d need to throw out just under 1 gallon of starter to get back to a decent amount, so you’d be tossing at least 7 cups of flour away. Removing 1 tablespoon per feed over that same time is just 1/2 cup of flour.

Which is actually more economical and less wasteful? Which will give you enough extra starter to make pancakes for the family vs pancakes for the entire local school board?

And that, dear friends, is why it really does makes sense to dispose of a little bit of excess starter.

But I STILL don’t want to throw out any starter!

Actually, once you’ve passed the beginning stage, you really don’t have to. There are  ways you can manage your starter that doesn’t end up in just tossing away that excess. I discussed this and other ways to reduce or even totally prevent the need to throw out any starter in this post: The Waste Conundrum.

15 Replies to “Why Discard Starter: A Mathematical View.”

  1. LOL–thank you for this. I try to explain this to newbies all the time–now I’ll just point them to your post.

    I learned this lesson as a kid. My dad challenged me to double the number of pennies I had every day to see how many I’d end up with in a month. I think it took me about 10 days to realize what doubling daily really means. It was a good way to teach the concept because I really got it.

  2. Very funny and very eloquent! I love the idea of the two swimming pools.
    I still hate to throw excess starter out but throw it out I do when I can dream up no other use or just don’t have time for other uses. What can I do? I have no swimming pool and neither do my neighbors…
    Paul, if you sent me your email, I’d like to suggest something.

  3. I cannot stop laughing ! how eloquent you have been. I wish more people will read this and understand that the discard phase is only at the beginning. When I weight my starter “mother” which stand in the fridge, yesterday, I realised it was 500 gr ! and I thought that I was going to reduce it to maximum 300gr or even less as I bake only once a week and use between 120 and 160 gr. starter.
    Your illustration with the pool is deserving the Charlie Chaplin mark !

  4. I bake a Miche loaf every Sunday. Saturday night I add 250gm of Water and 250gm of flour to my Levain ( starter). I leave it out of the fridge a few hours and put it back after things are “bubbling” nicely. Sunday morning I require 500gm of levain for my Miche. I remove this amount from the levain and all is well until the following Saturday when I repeat the process. I do occasionally take the levain out of the fridge during the week and give it a nice stir, but I don’t feed it. This has been working well for one year. Is there anything “wrong” with what Im doing.

    1. Hi gymcon,

      Your process is a great way to save from having excess to worry about and is basically what a professional bakery would do: build enough that you will use in your next bake.

      The downside to what you’re doing is that your starter is constantly just in the “getting back from sleeping in the fridge for a week” mode and you’re therefore not getting the full benefit of the complex flavours you could be enjoying. Getting that extra character in your starter can easily be achieved, however.

      Let’s look at your current process. You say you take your starter out of the fridge Saturday night, add 500 grams (half flour, half water), let it rise overnight then take that 500 back out to use in your miche the next day.

      How to give your starter a better character profile without ending up with excess to handle? Simple. And we’re going to give that starter three feeds, a much better opportunity for it to get back into full action than a single fresh-from-the-fridge feed.

      Take the starter out Friday night. Since we want to end up with 500 g of usable starter, we want to change how much Mother we keep.

      Feed #1: Friday night, Mother starter from the fridge: 20 grams. Feed this with 20 g flour and 20 grams water (1:1:1 ratio). You now have 60 grams.
      Feed #2: Saturday morning: feed that 60 grams the same ratio (1:1:1) so you now have 180 grams total
      Feed #3: Saturday night, feed the 180 grams one last time with 170 grams* each water and flour and you’ll finish up with 520 grams of three-times fed starter on Sunday morning.
      Sunday bake: Take out your 500 grams for the miche and you’ll be back to 20 grams as your Mother starter which now went through three full feeds and has had enough time out on the counter to keep developing it’s character and strength.

      * Note that this method cheats a hair and feeds a couple grams less than the exact 1:1:1 ratio should be in order to get you to your precise final needed amount for your Mother and the miche. This tiny difference won’t be an issue.

      Let us all know how this works out if you decide to give it a go.

      Also, after a couple of months (when your starter has developed further and you really like the flavour) you may want to increase the last feed to 190 grams each flour and water and use the 40 extra grams to dry some back-up. Look at the Drying Your Starter post for more details.

  5. I’m doing my first starter, and did thing the opposite way.
    Instead of keeping only 50g of starter (out of an original 100g) I only removed 50 g every day, and feed a 100g more.

    How is this affecting my starter?
    Its already frothy, yeasty, fruity tangy small, and active.
    What should be my next step.

    should I keep only 50g (plus 100g of the feeding) for my next feeding?

    1. Hi Alejandra,

      I’m not really clear on how you are doing your feedings; you say you are keeping 50 and adding 100 so are you keeping a starter of 150 grams total?

      If it is 50 + 50 + 50 (150g total) you are doing a 1:1:1 ratio. If you are feeding 50 + 100 + 100 (250g total) you are doing a 1:2:2 ratio.
      The main difference between these is that the first, smaller amount will be eaten up faster by your starter pets. This means you will likely need to feed three times a day, perhaps more on warmer days. The larger one, with more food given, means your starter may be able to live with two feeds a day but again, if it is warm, you may see it eat its food faster and will need another feeding.

      A good, active starter will eat through a 1:1:1 ratio rather fast (whether that’s 50g + 50g + 50g or 20g + 20g + 20g or any other combination like it). A larger sized feed like 50g + 100g + 100g or 10g + 20g + 20g means a little less original starter getting a little more food so it can go longer before it colonizes all the new flour, reaches its peak and runs out of fresh food.

      So the main issue you would face is how fast your starter eats up all its food, rises and begins to sink again. If you are home most of the day, then it’s easy to feed it 3 times. If you are out of the house a lot, you may want to think about feeding it a larger amount so you don’t have to babysit it so much. You can end up with a starter of ANY SIZE YOU WANT using either ratios; just use different amounts.

      Say you want to keep 150 grams of starter because that will give you the perfect amount of “extra” for your favourite recipe you use most of the time, the a 1:1:1 ratio needs 50+50+50. A 1:2:2 ratio needs 30+60+60. Both get you 150 grams total.

      There is no specific quantity you “should” keep. It will depend on how much your go-to recipe needs, how often you bake, how active you specific starter culture is, and so forth. I do recommend keeping your starter smaller since A) it will use up less flour during the feeds and B) you may hardly ever have to discard any [not 100% never but pretty close] and if you do, it is so small an amount it won’t be a bother to just get rid of it, where if someone throws away a cup or more of flour every feed (some people do keep their starters pretty darn huge) then tossing that away, even if in the compost, gets to feel very wasteful. And it is, since keeping a tiny starter will still work just fine.

  6. I hate throwing starter away, so I will take out starter and add enough flour and a small amount of honey to make a soft dough and let it rise over night and bake a small bread for breakfast, or use it later for a flat bread for dinner.

  7. I still don’t discard it. I start in a quart jar with 1/2 cup water and 1/2 cup flour, next day add 1/2 cup and 1/2 cup, and so on so forth until it froths. If it starts to get full after about 3-4 days, I cut it down to 1/4 cup and 1/4 cup. By the time the jar is close to full, my starter is ready to make a loaf of bread. Works just fine for me, but maybe in a cooler climate or a less humid area, that may take too long.

    1. Just a note on the process you describe here.

      You mention that you use 1/2 cup water and 1/2 cup of flour, etc., so this means you’re measuring by volume and not weight. What you’re building there is not going to be 100% hydration starter, which in your recipe may be fine. But a lot of recipes DO call for 100% hydration starter which is going to be closer – if you insist on doing volumes and not weight – to almost a full cup flour per 1/2 cup water. Flour is lighter than water, so you’d need more of it to be closer to the same weight as your water.

      1/2 cup of flour is about 60 grams (depending on how it’s scooped), where 1/2 cup of water is about 118 grams, just a touch under 120 grams or almost double the flour weight.

      Just something to keep in mind should you need actual 100% hydration starter in another recipe. But as noted already, if this works for you in your current recipe, and you like the bread you’re getting, then absolutely do keep using it. I would expect this recipe was likely designed specifically with a wetter starter in mind and adjusts the water in the final dough to balance the water in the starter, which is perfectly logical.

      Keep at it and do what works and gets you bread you like which, in the end, is THE most important part of bread baking!

  8. Hi there! I’m a newbie here and not sure where else to turn… on day 1 and 2 I didn’t discard my starter…but on day 3 and 4 I fed my starter and then discarded half (I did it backwards). Is that ok or do you think I’ve ruined it? Everything I read says to discard and then feed…ugh… how could something so simple be so confusing ?. Thank you in advance!

    1. Hi, sorry about the delay in replying. Hope things are going well with the starter.

      So you fed your seater, let’s say it was 50g, I presume then you fed it 100+100g so you had 150g total, then discarded half, leaving 75g at the end.
      You should have halved your original 50g taking you to 25g. Feed that 50 + 50 gets you 125g at the end.

      Otherwise, the starter was fed correctly (double its weight in each flour and water) the only difference being your end starter’s total weigh was a little too small. Plus you added flour that you then threw away the next minute. A small amount but not an earth shatteringly huge problem.

      As long as you’ve switched back to the correct feed process, you’ll be fine except you may want to bump it back up to the 50g “old” starter level before feeding again – so take only 25g out of that 75g small batch and you’ll be back to 50 grams. Carry on from there as usual, it will all be good.

  9. Hi
    I have questions about sourdough starter.
    My sourdough starter not rising, just bubble after 7 days. There is no activity just bubble after feeding. I discard some starter and feed on day 2 and day 5 but nothing happens. I added instant yeast then it rises, so the texture is not the problem cause my sourdough texture is runny and watery like tomato ketchup but raising after instant yeast work. Now I already discard all cause it rise natural. I want start again, so what should I do to make rising natural? Thank you

    1. Hi Ryan,
      I’m not entirely clear what went on with your first try at starter, other than you said you added instant yeast, in which case, what you ended up with was not really sourdough starter.

      How to try and start it up again? I’d say follow the instructions I’ve set out with photos. And give it time. As I say a fair bit, the most important ingredient in sourdough bread making is patience. It’s not an instant process and will take time to get to where you’d like to get it.

      And please be sure to WEIGH your ingredients, not use volume measures. Equal volumes of water and flour is NOT the same as equal weights. So if your starter was “runny” as you say, I’d suspect flour to water ratio was not exactly spot on; ¼ cup of flour weighs a lot less than ¼ cup of water so that final product would not really be the desired 50/50 mix. A cup of flour is about 120 grams; a cup of water is 237 grams, so nearly twice as much by weight.

      That issue would leave you with runny starter and you’d not see much movement, even if it was developing just fine because any rise in it would be gas bubbles that easily float to the top and escape where in a stiffer starter, they’re trapped by the thicker starter and will then visibly rise.

      See my post about digital scale weighing and this one about flour volumes,

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