Why does my homemade mustard thicken only some of the time?
Could the problem possibly be due to the variation in the amount of starch in the mustard used from batch to batch of your own making, so its thickening quality varies each time? Or could it be stale stock?

According to their website, Colman's mustard is made from 60% locally sourced yellow mustard seed and quality controlled, so it is fairly constant from year to year and has been since 1814. At least we presume it has remained so the whole time since Unilever took over in 1995 and was the case beforehand.

Working on that presumption, using the other brands or types of mustard (such as black, brown or oriental) could be the issue for you. Or have Unilever decreased the English yellow mustard content to 60% so there have been inconsistencies since 1995? I guess you would have to ask them directly to eliminate that as a possible cause. 

According to Canadien researchers, yellow mustard seed contains 20% to 30% protein, 24% to 35% oil, 6% to 12% lipids and 12% to 18% carbohydrates, but according to this source the starch content is fairly minimal at only 0.6 g per ounce. Although that seems a slight amount and small variation range respectively, it may still influence your mustard making. If this proves to be the problem, you could add and cook off plain flour to cure "thin" mustard or increase the mustard powder content.

I hear what you say about the cost of good imported English mustard. Have you considered sourcing whole English yellow mustard seeds and grinding them yourself? I was considering that myself, after I looked everywhere for Colman's this summer, but 113 g / 4 oz containers are the biggest on supermarket shelves and even then they are only rarely in the larger stores. I hate the tiny (relatively expensive) tins, so it looks like I'll have to buy via the internet or visit the Colman's shop in Norwich to finally get a pound sized tin. Still a visit to a museum is always interesting. Especially when food-related. I'll be needing a bulk supply, with winter coming soon...



«John Hassall (1868–1948): Colman’s Mustard Advert, 1899»