Here you will find answers to some of the common questions that arise when people start baking sourdough bread. Some of these answers refer to Bread Matters by Andrew Whitley which has extensive advice on starting sourdoughs with wheat, rye, spelt, chick peas or rice as well as suggested baking schedules and recipes. If you can’t find an answer here, try asking on the BMFN Forum.
Your starter may have bubbled a little but now it just looks flat and a layer of unsavoury-looking liquid is beginning to appear on the surface. You don’t see how this is going to raise your bread if you use it. Or perhaps you’ve made a loaf with it and it didn’t rise much.
What to do
The following steps (which can be repeated if necessary) will dilute excessive acidity and provide nutrients for the yeasts in your starter:
1. Take just 10 grams (or a flat teaspoon) of your starter and put it in a fairly small glass, ceramic or plastic bowl or jar.
2. Add 30 grams of rye flour (wholemeal if possible, because the branny bits are much richer in wild yeasts and bacteria than the white flour) and 60 grams of warm water (at about 40°C). Warmth is really important to the development of both the yeasts and the bacteria that occur naturally in the starter and the flour.
3. Mix this all up with your fingers. Your hands have important beneficial bacteria on them and, provided you observe normal personal hygiene measures, you will probably add useful elements into the sourdough just by mixing by hand. The bacterium Lactobacillus sanfranciscensis, the organism at the heart of the famous San Francisco sourdough bread, is, unlike most sourdough bacteria, not found in flour and is presumed to get into sourdough on the hands of bakers.
4. Cover and leave in a warm place (but not over 35°C and not in direct contact with e.g. a radiator) for 16-20 hours. It should be beginning to bubble a little at this point. If it is, you can proceed to use it to make a ‘production sourdough’, as described in the recipe that accompanied the starter.
5. If nothing is happening, it may be that the acid-forming bacteria are over-active and are getting away before the yeasts can show signs of working. The remedy for this is to repeat stages 1-4. This will dilute any accumulated acidity again and should provide a neutral platform for the yeasts to grow normally.
The starter seems to be fermenting OK, but when you make a Production Sourdough and then the Final Dough, it doesn’t work. So you left your loaf to rise (‘prove’) for a long time but it didn’t rise very much, if at all. You finally decided to bake it and, though the flavour’s wonderful, the texture is dense and very sticky.
What to do
Before suggesting reasons why the above may be happening, here’s a check on what a Starter and a Production Sourdough should look like.
If this hasn’t been refreshed for a while, it will look inactive (i.e. few if any bubbles on the surface) and some grey-brown liquid may have risen to the surface. This is quite normal. Don’t assume that your starter is dead. It isn’t. It’s just resting.
This is the term for the dough formed by adding a small amount of old starter to some fresh flour and water, a process known as refreshment. The aim is to enable the yeasts and bacteria in the starter to multiply so that there will be enough of them to flavour and raise (aerate) the final dough. What a viable Production Sourdough looks like depends on the flour(s) on which it is based and the amount of water in the mix.
Rye starters are almost always made very wet and sloppy; wheat starters can be made like this but are usually more like a soft dough in consistency, i.e. definitely not pourable. Several (10-16) hours after refreshment, a rye starter should show signs of bubbling on the surface. A rye Production Sourdough will usually froth up after a few hours in a warm place. Eventually, the bubbling (which is the evidence of the yeasts working) subsides and the dough relapses back to its former height, leaving a ‘tide mark’ on the sides of the jar or bowl it is in. The bacteria keep on working, creating more acidity which begins to inhibit the yeasts. Eventually the acidity becomes so great that the bacteria themselves go into a kind of dormancy.
Wheat starters: refreshment times are typically shorter for wheat (four hours at room temperature is a rule of thumb) but this depends on dough temperature and consistency. Stiff doughs rise more slowly, as do cooler ones. Unless it is very liquid, a wheat Production Sourdough will increase in volume rather like an ordinary dough. For a typical flour mix, it might double in volume before gently collapsing down on itself as the yeasts stop producing gas in the increasingly acid environment.
So… if your Production Sourdough (PS) seems inactive and/or your Final Dough did not rise very much, this could be caused by
If you are sure that you have a vigorous PS, the problem must be occurring in the final dough. Logical trouble-shooting steps that a baker would take include:
The Bread Matters Sourdough Starter is 100% rye and the recipe in the packet is for a pure rye bread. But you would prefer to make a wheat bread.
What to do
No problem. There is a recipe here. You refresh your rye starter with wheat flour to make a Production Sourdough that is mostly wheat. If you then use only wheat flour in the final dough, you’ll have a wheat sourdough bread with just a hint of rye flavour, but little of the stickier texture of rye.
What happens to the starter? After all, you’ve mixed a rye starter with wheat flour, so any residue of the PS that you keep with be a hybrid wheat-rye starter. You have two options:
If you don’t want to keep a hybrid starter going, then simply adjust the amount of PS that you produce and use it all up in the final dough.
If you do want to keep this hybrid wheat-rye starter, put it in a separate tub in the fridge (and label it). If you subsequently refresh it again with wheat flour, it will quickly become a virtually 100% wheat sourdough. Similarly, if you were to refresh it a couple of times with spelt flour, it would take on the character of a spelt sourdough, i.e. the species and strains of yeasts and bacteria would reflect the particular conditions created by your choice of flour.If you want to keep a pure rye starter going (say because you want to be able to make a guaranteed wheat-free loaf from time to time), bear these points in mind:
Don’t use up all your rye starter in initiating a hybrid wheat-rye one. Keep a little back in your dedicated rye starter tub.
You may therefore need to do an ‘interim’ refreshment to bulk up the remains of your rye starter so that you have enough next time you plan to use it to make bread. Follow the basic instructions that came with the original starter or as given in ‘Why won’t my starter eventually run out?’ below. Adjust the amounts to ensure that you don’t end up with too much starter.
You make bread regularly using a rye Production Sourdough as specified in the Bread Matters book. Normally this ferments quite quickly but sometimes, for no apparent reason, it doesn’t and the resulting bread takes ages to rise and tastes very acidic. What’s going on?
The main variables are temperature and flour quality. It is unlikely to be the latter unless your flour is very old, in which case some degradation of the bioactive components may have taken place. Or it is sometimes the case that a new batch of rye flour has a different enzyme activity: there were some cases of this after the bad harvest of 2012 and several people reported that their rye sourdoughs were fermenting unusually quickly.
So that leaves temperature. If your sourdough gets very warm (say 30°C or more), it will ferment quickly and might be overfermenting. When this happens, the yeasts in the flour use up the available nutrients quickly and stop working – so you see no froth or bubbles. At the same time, the elevated temperature makes the lactic acid bacteria work faster. They produce acids which lower the pH of the dough (i.e. make it more acid), further inhibiting the action of the yeasts. If you use a production sourdough in this condition, the yeasts will be less active than normal, which means that they will take longer to produce enough gas to raise the bread. But, at the same time, the increased acidity of the production sourdough will make the final dough more acidic and this inhibits the action of the yeasts, allowing more time for the final dough to become more acidic…and so on, in a vicious circle.
What can you do? First, try to maintain some consistency in the temperature of your production sourdough. If the weather is very warm, put it in a cooler place. Monitor the temperature if you can and keep it below 30°C. Second, if in the morning you notice that the production sourdough appears to be rather ‘dead’, i.e. there are few bubbles and perhaps even a layer of liquid on the top, try using only half of the normal amount in the final dough. You will need to readjust your fresh flour and water quantities to achieve the same yield of loaves. The point of this is to dilute the production sourdough to a larger extent than normal so that there is less acidity to inhibit the yeasts.
Starters, usually wheat and usually ones that have been kept in a not-very-cool place for a while, begin to smell rather ‘chemical’.
What to do
This a fairly common occurrence with wheat starters, especially in summer: they start smelling of ‘pear drops’, i.e. a bit chemical. The smell is actually acetone. Under certain conditions, the lactic acid bacteria in the sourdough produce copious amounts of acetic acid which gives the familiar vinegar smell. Another couple of chemical steps and this can turn into acetone. It can be a bit alarming to sniff your sourdough and get the aroma of nail varnish remover, but it is nothing to worry about. As soon as you dilute the sourdough by refreshing it with flour and water, the smell goes.
If you want to delay the onset of the acetone effect, you can stiffen up your starter by adding flour to the pot of it that you keep in the fridge. A stiffer sour will work more slowly, but, in the long run, will produce more acetic than lactic acid.
If you don’t ‘feed’ a starter, how does it keep going?
What to do
[See also pages 165-166 of Bread Matters.]
Sourdough is (and always was) a self-perpetuating dough leavening system. In the days before baker’s yeast – or shops, for that matter – if you ran out of starter you’d have no bread, or pretty heavy loaves at best.
There are three stages to a basic sourdough baking system:
1. The Starter – the small quantity of dough that contains a reservoir of live yeasts and bacteria. Between uses, this is kept in the fridge. It doesn’t need ‘feeding’. It can last, unused, in the fridge for years.
2. The Production Sourdough – a mixture of a small amount of starter and much larger amounts of flour and water. Over several hours, the yeasts and bacteria in the starter, augmented by more of the same from the fresh flour, metabolise sugars in the flour into CO2 gas (the yeasts) and lactic, acetic and other acids (the bacteria). The gas raises the dough, the acids give it flavour (and a whole lot more).
3. The Final Dough is composed of most – but not all – of the Production Sourdough plus flour, water, salt and any enrichments that the recipe may call for (fat, seeds, nuts, herbs, spices, fruits etc).
The idea is that you take some old starter, multiply it up to several times its size, let this ferment, then use most of it to make bread, keeping the residue to go back into your starter pot, ready to start the whole process again.
Take plain rye bread as an example:
1. ‘Refresh’ the starter and thereby make a ‘Production Sourdough’
Old Starter 50 g
Wholemeal (‘dark’) rye flour 150 g
Warm water 300 g
Total Production Sourdough 500 g
2. Make the Final Dough
Production Sourdough 440 g
Dark or light rye flour 330 g
Salt 5 g
Water 200 g
Total 975 g
By using only 440 g of the refreshed production sourdough, you will be left with a notional 60 g to go back into your starter pot for next time. Thus the process goes on. In practice, you lose a little on the sides of bowls etc, which is why the recipe for the production sourdough requires only 50 g of starter, not 60 g.
Each time you ‘refresh’ your starter to make a production sourdough, you make about 10% more than you need for the final dough and that extra sourdough goes back into your starter pot in the fridge for next time. And so the process is self-perpetuating and your starter never runs out.
If you want to make a lot more bread one day and don’t have enough starter in the fridge, you can do a little ‘intermediate’ refreshment to bulk up your starter. If you like precision, you can do the maths and work out exactly how much starter you need to refresh to arrive at the right amount for your purpose (see ‘I’m making a big batch of sourdough bread. Do I need to alter the proportions of Production Sourdough to Final Dough?’).
The important thing to note is that your starter pot will, over time, contain the residues of many different batches of refreshed sourdough (i.e the left-over bits that are not used in the final dough). This is fine. Just mix the newer with the old and leave it all in the fridge (in a reasonably well sealed plastic container, not a glass jar with a metal strap) until you next want to make bread. No need to throw any away. No need to worry about ‘feeding’. You probably have better things to do with your life than fuss unnecessarily over a starter.
NB. The ingredient proportions and timings given above are for a pure rye bread. They are a bit different for wheat breads. But the principles are the same.
Temperature has a big influence on the speed at which yeasts and bacteria ferment. But they don’t need to be kept warm all the time. Here are some pointers:
You should try to keep your starter warm during the four-day initial build-up stage because it gets going better when it’s not too cool. It will work at cooler temperatures only more slowly. Sometimes novice bakers can mistake this slowness for inactivity – or they just get bored waiting for something to happen! A cheap plant propagator from the garden centre can make a good sourdough incubator. But a word of warning: put a thick mat (wood or cork or similar) on the heated surface of the propagator and then put your starter on that. Direct contact with the heat mat may ‘cook’ (and therefore kill) some or all of your starter.
After your sourdough is established and you have used it, i.e. ‘refreshed’ it and then made a final dough with it, you can safely keep it in the fridge in a sealed container (not necessarily hermetically sealed but well closed so that cross-contamination from other fridge items cannot happen). It stays there until you next want to make bread, whereupon you refresh it again to re-activate the yeasts and bacteria.
You don’t need to ‘feed’ a starter. Those who talk about ‘feeding’ leavens and sourdoughs misconstrue the biology of yeasts and bacteria. These organisms are not like pets or children in requiring three meals a day. They don’t die once their immediate energy source is exhausted, but tend to go into a dormant state.
That said, prolonged storage of a leaven will result in it going more acid and it may benefit from an ‘intermediate’ refreshment to dilute the acidity and re-balance the ratio of yeasts and lactic acid bacteria. If you do an intermediate refreshment, remember to do the maths beforehand so that you don’t end up with too much refreshed leaven when you come to make the final dough.
Yes, you can use the wheat leaven (starter) to start a spelt sourdough. Just use spelt flour when you refresh the leaven and after two refreshments you will have an almost wholly spelt leaven.
A word of caution, though: spelt flour ferments more quickly than wheat, so you may need to both lower the temperature and reduce the refreshment time of your spelt production sourdoughs.
High temperatures and humidity can play havoc with sourdough starters and breads.
Yeasts and bacteria work faster the warmer it gets – up to about 35°C when their activity starts to suffer due to excess heat. In general, the warmer it is, the quicker a sourdough will ferment. There are three ways to deal with this problem:
1. Use cold water to refresh your sourdough and keep it in a cool place, possibly in the fridge for some of the time. It’s difficult to be prescriptive about time and temperature because situations vary, but try to keep the dough below 27°C.
2. Reduce the duration of any refreshment stage. For example, if a normal refreshment of a wheat sourdough is 4 hours, in very warm weather this might be reduced to 2½ hours.
3. Make both the starter and your production sourdoughs stiffer, i.e. reduce the water content. This will slow down the fermentation, but will tend eventually to produce more acetic acid, giving a more vinegary taste to your bread. To limit this effect, do not chill your dough, for instance by refrigerating it for a long proof.
You can use one or all of these levers to adjust your method to warm weather. The humidity of the tropics means that you may need to use a bit less water than the recipe says. But most recipes prescribe too little water anyway, so this is unlikely to be a big issue.
Under certain conditions, moulds can form on a sourdough starter, even in the fridge. Why? What should I do if this happens?
Sourdoughs are set up by nature to be safe and healthy by default, providing simple good practice is followed. However, in the first day or two of a starter’s life, when the lactic acid bacteria (LAB) are still ‘finding their feet’ and have not produced sufficient selective anti-bacterial and anti-fungal compounds to sterilise the mixture, it is possible for moulds to get a foothold, usually as a result of some cross-contamination (a waft or splash of something else in the kitchen) or something lurking in the flour you are using.
In an older starter that is being kept in a tub (perhaps with an incomplete seal), moulds sometime creep in where there is a lot of ‘headspace’ above the starter itself, i.e. it isn’t occupying much of the tub. This space can allow enough oxygen for any adventitious moulds that may settle on the sides of the tub or even on the surface of the sourdough to multiply, unimpeded by the acidity of the main bulk of starter that would almost certainly inhibit any such growth.
Black moulds are unusual and are likely to be from ‘dirty’ flour or over-relaxed kitchen hygiene. White moulds are more common. Indeed, yeasts and moulds are biologically pretty similar and it is sometimes hard to distinguish between a white mould and the natural efflorescence of yeasts on the surface of an active sourdough starter.
With black moulds, caution is certainly advisable, especially knowing what can easily grow on (for instance) cooked rice left in a warm kitchen. So if there is any degree of contamination, it is best to throw the starter away, clean the tub well and begin again. If there is just the odd spot, remove it carefully, stir up the sourdough (with a clean spoon) and give it a ‘mini-refreshment’ by adding some fresh flour and water. If the black spots reappear, it would be best to start again – perhaps with a different source of flour.
With white moulds, either simply stir the moulds into the body of the mix and, if necessary, transfer to a smaller tub with less space between starter and lid, or skim off the majority of the mould before proceeding as just described.
In either case, a couple of refreshments with fresh flour should allow the good microbes to re-establish themselves in your sourdough eco-system. And one final thing: firmer sourdoughs (i.e. those with a lower water content) don’t ferment as quickly but they don’t decompose so quickly either and are somewhat less prone to getting layers of mould on their surface.
The (rye) loaf looks OK from the outside but a couple of slices in and there it is – a hole, separating the top crust from the rest of the crumb.
This is a problem that isn’t confined to rye breads or sourdoughs, but the answers below are mostly relevant to these breads. Here is a list of possible factors contributing to what is sometimes called the ‘handbag’ effect: pick up a slice of your problem loaf by its top crust and you’ll get the picture.
1. The dough is too wet – weak rye gluten is unable to hold the gas-filled structure together at the most vulnerable part of the loaf, i.e. the top. Remedy: reduce water in final dough.
2. Over-acid production sourdough. If the lactic acid bacteria (LAB) produce too many protein-degrading (proteolytic) acids, the final dough may begin to break down during proof (when the dough is rising in the tin). This may cause a structural weakness in the vulnerable area at the top of the loaf. Remedy: shorten the production sourdough fermentation time and/or reduce the proportion of production sourdough in the final dough (some maths will be required to adjust the amounts of fresh flour and water so that the loaf isn’t too small for the tin).
3. Over-proof (rising too long and/or too much dough in the tin). The structure begins to bubble and collapse at the top and falls away from under the top crust which forms a ‘bridge’ that may look OK but hides a crumbly centre.
4. Under-proof (not being left long enough to rise in the tin). Sounds paradoxical in view of 3 above, but an underproved loaf will try to expand in the oven and, if it is unable to break through the crust, may produce a distorted internal structure. This may be the explanation if the loaf has risen rather unevenly with a ‘dome’ effect in the middle of the top, suggesting that the dough wanted to stretch further but couldn’t (see 5). Remedy: prove for longer. Expect the dough to rise to nearly twice its original height before it is ready to be baked (assuming it was fairly flat and evenly distributed in an almost straight-sided tin).
5. Skinning of the dough during proof. If the dough surface has dried out during proof it cannot expand as much as it wants and the gas bubbles immediately under the top crust will rupture and subsequently collapse. Remedy: prove in a warm, humid atmosphere, e.g. a polythene bag that is inflated to stop it collapsing onto the dough. If the dough surface looks a bit dry and leathery during proof, mist or gently brush it with warm water to soften the crust.
6. Weak flour. Rye grain quality varies like wheat. Odd batches can have elevated levels of (naturally-occurring) enzymes which can produce an overactive fermentation and degradation of the dough structure. This may cause a weakening of the loaf structure under the crust. Remedy: try a different source of rye flour.
7. Trapped water. If you trap water in the middle of the dough during moulding with wet hands, it is possible that this will ‘blow up’ into a hole as the trapped water vaporises during baking. However, the chances of this always occurring just under the crust are small.
8. Tin too wide. Rye gluten (yes, it does have some, but not the stretchy type like wheat) is not very strong, and it can’t easily support the structure of dough in a wide tin. Many tins sold in kitchen shops are a bit low and wide for rye bread. Excuse the commercial, but you can get excellent tins for rye bread at the Bread Matters online shop
Is there a trick to working with very moist doughs, especially spelt?
Spelt is a close relative of wheat, but there are differences, some of which seem to explain its greater digestibility, especially for people with ‘non-coeliac gluten sensitivity’. These are the main points to note:
1. Spelt doesn’t usually have as strong a gluten network as ordinary wheat. So it tends not to hold its structure very well, in both tins and after tipping out of proving baskets if you are baking on a stone etc. The solution is to reduce the proof time, almost to the point where you put the loaf into the oven significantly under-proved. This means that the dough will not be quite so collapsible at the moment of going into the oven or being tipped out of the basket. The corollary is that the dough must be fairly soft, so that the expansion that takes place during baking can happen without extreme splitting of the loaf surface and the retention of a solid core of underbaked dough in the middle.
2. Spelt, in most bakers’ experience, ferments much more quickly than ordinary wheat, in both yeasted and sourdough forms. Therefore you have to be on the watch to make sure that you don’t overprove it anyway, a common source of difficulty at baking time.
3. Handling doughs that are very tacky on the surface can be difficult. If you are going to bake the loaf in a tin, I would recommend handling it with wet hands and on a wet table before putting it in the tin. Water is the best lubricant for dough because it creates a slippery smooth surface for a few seconds before eventually becoming absorbed into the dough and going sticky again. If you are proving in a basket, you’ll need to pick the dough piece up in a lump and gently dip it in a bowl of flour to coat the surface of the dough that is going to come into contact with the basket (which you will have pre-floured too). If you cannot pick up the dough piece at all, it is too soft and you should add just enough flour to make it possible to handle as one lump. But, having said that, the softer the dough, the better the eating and keeping quality will be.
4. There is as much variation between batches of spelt flour than there is with wheat (if not more). Since spelt has not been aggressively hybridised by plant breeders, it has been perpetuated largely by farmers saving their own seed. So you can never be quite sure what you are going to get. As with all cereals, the quality will depend on variety, climate, weather during the growing season and post-harvest treatment and storage (e.g. moisture content). If your problems persist, try getting some spelt flour from a different source: it may be better.
5. Finally, a little lateral thinking. If your dough gets away from you and you end up with a pancake (aka the Frisbee effect), don’t despair. The bread is the same as it would have been in a bolder loaf, just a flatter shape. So try cutting it at a steep angle instead of the normal 90°. With a bit of care, you can cut respectable slices from even a pretty flat loaf, making toast and sandwiches that don’t completely betray your earlier baking problems.
Apart from rather too ‘rustic’ an appearance, craggy crusts can snag on the knife and make cutting even slices difficult.
Spelt has a less elastic type of gluten than ordinary wheat and its structure ‘breaks down’ more quickly during fermentation, especially with sourdough. So what you are seeing is, to some extent, in the nature of spelt. To lessen this effect, try shortening the refreshment time of your production sourdough (say to three hours rather than four), keep the dough temperature down a bit and shorten the final proof, even at the risk of putting the bread in the oven rather sooner than you think is right. You could also ‘slash’ the loaf just before you put it in the oven a little deeper than usual to try to guide the natural splits that appear on most crusts during baking.
It’s worth remembering that this ‘breaking down’ of spelt’s protein is probably one of the main reasons why spelt is often easier to digest than common wheat. If the gluten bonds are already weakened or broken before the bread is baked and eaten, the human gastro-intestinal tract has less work to do and there is less chance of inappropriate fermentation in the lower gut (leading to bloating etc). Industrial enzymes in most factory bread are designed to have exactly the opposite effect on gluten, i.e. to strengthen it and prevent it from breaking down too soon. It doesn’t take a bio-chemist to see which is likely to be easier on the stomach.
Yes. You should make the Production Sourdough with quite cool water, leave it in your kitchen for an hour or two and then put it in the fridge. You can put it in the fridge immediately, but it’s better to give the yeasts a chance to get going a bit before slowing them right down by chilling them to 5°C or so which should be your fridge temperature. The following evening, make your final dough with quite warm water to compensate for the chilling effect of the cold leaven.
You may need to experiment a bit with water temperatures etc, in both the leaven and the final dough. Ideally, your leaven should have risen to near its maximum height in the fridge but should not have fallen back at all. If it has, it may mean that it will be rather acidic. This isn’t a problem as far as flavour is concerned, but it may inhibit the yeasts a bit and lead to a slower final proof (and a late bedtime). If you think of temperature as one of your main control levers, you can learn to perform adjustments that take account of your schedule, the weather and any other variables.
When scaling up yeasted bread doughs, it is logical to reduce the proportion of yeast. But does the same apply to sourdough breads, where the Production Sourdough (PS) is the leavening agent or ‘yeast’ of the main dough?
No. Keep the ratios of PS to final dough the same whatever the size of dough. This isn’t to say, however, that all recipes have the same percentage of PS – far from it. But here we’re talking about one particular recipe: you want to make a batch that’s, say, four times the size of your normal batch. If it were a yeasted dough, it would be desirable to reduce the percentage of yeast for the following reasons.
The key issue here is actually temperature. Very small doughs, i.e. below 3 kg in weight, tend to lose heat in an average British kitchen unless placed deliberately in a very warm place. However, large doughs cannot so easily dissipate the heat generated by both the energy of mixing and the process of fermentation because they have relatively less surface area to weight. Commercial bakers often used cold water or even ice in summer to keep large doughs cool enough after mixing. Since most bakery books tend to ignore the benefits of long fermentation, it has become customary to increase the amount of baker’s yeast recommended for small dough quantities in order to compensate for the gradual loss of heat that usually occurs in domestic baking. The dough would rise perfectly well with less yeast if given time, but people have been encouraged to be impatient and to assume that if the dough hasn’t ‘doubled in size’ after an hour or so, something is wrong.
The yeasts in a natural (sourdough) fermentation will, like any yeast, work slower at lower temperatures. But because they are fewer in number (by orders of magnitude) than the yeasts added from a highly concentrated source (i.e. a packet), the speed difference when the dough gets a bit warmer is fairly modest. There is therefore no real need to vary the amount of production leaven or sourdough as you scale up your doughs. But do keep an eye out for the temperature issue.
In fact the proportion of production leaven or sourdough in the final dough can be varied considerably with only modest effects on the character of the bread. But here is an interesting (apparent) paradox. You’d have thought that the more PS in your final dough, the sourer the bread would be. But the opposite appears to be true, certainly if your final dough has around 60% PS in it as opposed to the more normal 30% or thereabouts. The reason appears to be this: it is the lactic acid bacteria (LAB) that produce the acids (acetic and lactic, mainly) that create the sour flavour. LAB work more slowly than yeasts. More PS means more yeasts to raise the rest of the dough. The dough is therefore ready for the oven a bit sooner, and before the LAB have had time to produce as much acid as they would have over a longer period. Result: milder flavoured bread.