is traditionally a vessel between a mash separation device (e.g., a mash tun, lauter tun, or mash filter) and the kettle. The vessel is essential to maintain wort flow and save time in the brewhouse when more than one full kettle is required from a single mash. When the required volume of wort is reached in the kettle, it is brought to a boil; the wort that continues to run from the separation device is then run into the underback. This wort is held at 70°C–80°C and is run to the kettle once the previous contents have been boiled and sent to the whirlpool or fermenter. In the British parti-gyle system of brewing (where the wort from a single mash is run off to separate vessels in stages), the first wort run into the underback is often called the “first worts” and the next are called “second worts.” See parti-gyle. The size of the underback can vary; if the size of a kettle is half the volume that can be extracted from a full parti-gyled brew, the underback will be large enough to take the rest of the wort from that brew. In modern breweries, underbacks are often referred to as “wort holding tanks” and can be anything up to the full brew volume. In most breweries outside the UK, the underback receives the wort from a second mash, while the wort from a previous mash is occupying the kettle. This saves considerable time because the brewery can be collecting one wort while boiling another. Care needs to be taken to ensure that the wort holding vessels are regularly cleaned because holding hot wort can encourage themophilic bacteria to produce undesirable nitroso-compounds called ATNCs (Apparent Total Nitroso Compounds) that can be carried through to finished beer.
