Guinness Stout is made from: water, barley, roast malt extract, hops, and yeast. The reason Guinness has it's dark colouring is due to the barley being roasted.
The biggest technological change for Guinness happened during 1959. Guinness began using nitrogen, which ultimately changed the texture and flavour of Guinness entirely. of Nitrogen bubbles are far smaller than CO2, which gives a creamier and smoother consistency over a CO2 taste.