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Wort Boiling

Following extraction of the carbohydrates, proteins, and yeast nutrients from the mash, the clear wort must be conditioned by boiling the wort in the kettle. The purpose of wort boiling is to stabilize the wort and extract the desirable components from the hops. The principal biochemical changes that occur during wort boiling are as follow:

• sterilization
• destruction of enzymes
• protein precipitation
• color development
• isomerization
• dissipation of volatile constituents
• concentration
• oxidation

Traditionally, kettle times lasted between 90 and 120 minutes, with a minimum of 10% evaporation per hour. However, today kettle times for an all-grain beer last from 60 to 90 minutes, with a 5 to 8% evaporation rate. Boil times of 2 hours or longer are usually reserved for special beers, such as strong Scottish ales. In order to save time, most brewers begin applying heat as soon as the wort covers the bottom of the kettle to minimize charring (or scorching), and to prevent damage to the kettle. Some systems may require that the kettle be more than halfway full before applying heat. Care must be taken when using direct gas fire since the first runnings are easily caramelized. If steam jackets are used, heating may be started as soon as several inches of wort are in the kettle by shutting off the side jacket.

Following the boil, the next step is to separate the hop debris and the trubaceous matter (hot break) from the boiled wort before cooling.

Whole hop separation systems (such as hop back, hop separators, and hop jacks) are usually placed in-line after the kettle before the hot break separation vessels such as coolships, settling tanks, whirlpools, and centrifuges. If whole hops are used, the amount of spent hops will be between 0.7 and 1.4 kg/hl wet weight.

The hop back is only used for removing whole hops or as a holding vessel for finishing hops. Some brewers have been known to put whole hops into the hop back to impart a hoppiness to the beer and to form a filter bed.

Wort from the kettle is run into the hop back and strained by the slotted base. The wort in the earliest stages is recycled from under the plates through the filter bed of hops until the wort runs clear. As the spent hop material accumulates, it progressively improves the straining action so that hot trub is retained. Wort flow through the hop back is controlled by adjusting the valve on the positive side of the wort pump, which applies back pressure. An in-line sight glass is very useful for checking clarity. The filter bed is later rinsed to recover the wort in the bed. After the wort flows through the hop back, it is transferred to a settling tank, a whirlpool, or a centrifuge and then pumped through a heat exchanger for cooling.

Hop backs are less in use nowadays partly due to lower hopping rates and the extensive use of pellets and extracts. Hop backs also suffer from high labor costs and effluent loading, and they require disposing of the spent hops. In general, hop backs are not suited for large breweries and are more likely to be used in small-scale operations. Because of its disadvantages, the hop back has been replaced in some instances by the hop separator.

The hop separator or strainer is only employed where whole hops are used and by large-scale breweries. The hop separator is an apparatus – a primary screen or slotted plates – through which the bulk of the wort flows. The hops retained on the screen are then forced or tumbled onto another set of screens from which they are removed by means of a chain belt or a worm conveyor, with residual wort passing through the screen. In some cases water is sprayed over the hops to wash out absorbed wort. All of the wort passing through the screen falls into a small holding tank. There it is continuously pumped to the settling tank or directly to the whirlpool or centrifuge before being transferred to the heat exchanger.