I'd like to be able to see at a glance what type of fluid is contained within a barrel without having to open an interface. Barrel labels would be a great addition for this purpose. In order to simply convey the contents I'd suggest using icons on these labels.

• An apple for cider
• Wheat for ale
• Grapes for wine
• ... you get the gist of it by now.

When looking into barrel labels further, it seems in a real life situation the label and brand is usually etched into the lid and/or side of the barrel:

https://www.google.com/search?q=barrel+labels&client=firefox-b-ab&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiV-N7ZjsffAhVOb1AKHWcVBkQQ_AUIDigB&biw=1680&bih=936

More thoughts on this one ... we could just extend an Item Frame and have it offset to look like it fits on the end of the given fermentation barrel.

Or add another slot, like the tap that reflects the given slot item contents on both end of the barrel.

I prefer the latter, Item frames take up space in front of the block that might not be workable.