When thinking about the malted barley we use in our beer, there are three fixed points to consider. We have to keep the amount of potential alcohol the same from batch to batch. We have to keep the colour the same, since people tend to notice things like changes in colour. And we have to keep the flavour the same. The amount of potential alcohol is determined by the yeast we use and the amount of sugar we get out of the malt. Maltsters supply spec sheets listing the LDK (litre degrees per kilo), essentially how many ‘sugar points’ yielded by each kilo of malt. A kilo of malt with an LDK value of 300˚, for example, would yield 1 litre of wort with a specific gravity of 1.300 or 10 litres of wort with a specific gravity of 1.030 (i.e. 1 litre with an excess gravity of 300, what I’m calling ‘sugar points’, or 10 litres with an excess gravity of 30 (30 sugar points). If the LDK values change, then it is a simple matter to increase or reduce the weight of malt accordingly to keep the original gravity, and there the potential alcohol content, constant.
Maltsters also provide values for the colour of wort made from the different malts. As with strength, colour adjustments can be made by increasing or reducing the proportions of the different malts as necessary to accommodate changes in the colour from batch to batch.
Unfortunately, there is no objective reckoning for taste. I don’t even know if taste follows strength or colour. If the recipe calls for a certain proportion of the sugar points to come from a certain type of malt, then because the brewer just has to take the LDK and colour of the malt as it comes, the mass of malt and therefore its effect on the colour of the final beer will be determined. On the other hand, the recipe could be turned on its head, and we could specify what proportions of the overall colour of the beer come from what malts. Just as before given the colour specifications, the weight of malt will be determined, and therefore, how much sugar will be obtained and indirectly the strength. In other words, if the malt varies, then keeping colour fixed may mean a change of strength and vice versa.
I don’t really know how to solve this problem. I can see why large breweries would want to dictate to the maltster what malt specifications they will accept. It would be so much easier to keep a consistent product if you could rely on colour and potential strength not changing.
I think I’m going to sort of assume flavour follows colour. I don’t know how safe an assumption this is, but at least some of the compounds responsible for the colour of darker coloured malts are also responsible for some of the malt flavours (Maillard reaction products). I’ll try to have the coloured malts contribute as much colour as they previously did, but I’m not going to try to keep the colour contribution of the base malt the same. Instead, I’ll just adjust the base malt however I have to adjust it once the coloured malts are calculated to keep the original strength the same. There’s no real principles behind this; it’s a compromise to meet competing desiderata.