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Granted, there have been a lot of technical improvement […]
Granted, there have been a lot of technical improvements since the 1600s, but in terms of fundamental ingredients, there are really two critical differences:
1) Yeast: Micro-organisms were simply unknown until @Antonie van Leeuwenhoek discovered them in the 1700s , and (more importantly) the role of yeast wasn't understood until @Louis Pasteur proved that it plays the central role in transforming sugar into alcohol. Much of modern brewing focuses on the selection and cultivation of yeast, but brewers in the 1600s would have had no concept of that.
2) Modified malts: The other important difference is in the development of modified malts -- basically, the degree to which the grains have already begun to sprout, and create the sugars that make beer.
As a general rule, modern malts are already "fully modified", which Fermenters Manufacturersmeans that by the time you buy them, they've already been coaxed into an almost ideal, uniform state where the vast majority of their starches have already been converted into sugars, and all you have to do as a brewer is finish the process and extract them.
Historically, though, the malted grains supplied to brewers were not only much more starchy, but they were also much less uniform. That means that the mashing process had to be much more involved, and much more carefullyWholesale Fermenter Suppliers controlled than it usually does now. Modern brewers are usually able to mash their grains at a single temperature (between 140F - 158F) for a fixed period of time, and be done with it. Brewers in the 1600s would have had to start at a very low temperature (around 100F), and then bring the mash up through a more complex series of temperature plateaus, in order to carefully control a series of complex chemical reactions. (Not that they understood exactly what they were doing -- it had all been worked out through trial and error over centuries.