Info

Steam

Condensate n

Steam

Fig. 2.11 A mash converter.

Hot mash water

Transfer pump

Fig. 2.11 A mash converter.

separate vessel where it was taken to boiling and then returned to the main mash, leading to an increase in temperature. This is a rather simplified version of the process, which traditionally involved several steps of progressive temperature increase.

Temperature-programmed mashing. Although there are some adherents to the decoction-mashing protocol, most brewers nowadays employ the related but simpler temperature-programmed mashing. Again, the mashing is commenced at a relatively low temperature, but subsequent increases in temperature are effected in a single vessel (Fig. 2.11) by employing steam-heated jackets around the vessel to raise the temperature of the contents, which are thoroughly mixed to ensure even heat transfer. Mashing may commence at 45-50°C, followed by a temperature rise of 1°C.min-1 until the conversion temperature (63-68°C) is reached. The mash will be held for perhaps 50 min to 1 h, before raising the temperature again to the sparging temperature (76-78° C). High temperatures are employed at the end of the process to arrest enzymic activity, to facilitate solubilisation of materials and to reduce viscosity, thereby allowing more rapid liquid-solid separation.

The decision whether to use an adjunct or not is made on the basis of cost (does it represent a cost advantageous source of extract, compared to malted barley?) and quality (does the adjunct provide a quality benefit, in respect of flavour, foam or colour?). Liquid adjuncts (sugars /syrups) are added in the wort boiling stage (discussed later). A series of solid adjuncts may be added at the mashing stage because they depend on the enzymes from malt to digest their component macromolecules. Solid adjuncts may be based on other cereals as well as barley: wheat, corn (maize), rice, oats, rye and sorghum.

Adjuncts

Table 2.2 Gelatinisation temperatures of starches from different cereals.

Gelatinisation

Source

temperature (°C)

Barley

61-62

Corn

70-80

Oats

55-60

Rice

70-80

Rye

60-65

Sorghum

70-80

Wheat

52-54

In turn, these adjuncts can be in different forms: raw cereal (barley, wheat); raw grits (corn, rice, sorghum); flaked (corn, rice, barley, oats); micronised or torrefied (corn, barley, wheat); flour/starch (corn, wheat, sorghum) and malted (apart from barley this includes wheat, oats, rye, and sorghum).

A key aspect of solid adjuncts is the gelatinisation temperature of the starch (Table 2.2). A higher gelatinisation temperature for corn, rice and sorghum means that these cereals need treatment at higher temperatures than do barley, oats, rye or wheat. If the cereal is in the form of grits (produced by the dry milling of cereal in order to remove outer layers and the oil-rich germ), then it needs to be 'cooked' in the brewhouse. Alternatively, the cereal can be pre-processed by intense heat treatment in a micronisation or flaking operation. In the former process, the whole grain is passed by conveyor under an intense heat source (260°C), resulting in a 'popping' of the kernels (cf. puffed breakfast cereals). In flaking, grits are gelatinised by steam and then rolled between steam-heated rollers. Flakes are not required to be milled in the brewhouse, but micronised cereals are.

Cereal cookers employed for dealing with grits are made of stainless steel and incorporate an agitator and steam jackets. The adjunct is delivered from a hopper and the adjunct will be mixed with water at a rate of perhaps 15 kg per hL of water. The adjunct will be mixed with 10-20% of malt as a source of enzymes. The precise temperature employed in the cooker will depend on the adjunct and the preferences of the brewer. Following cooking, the adjunct mash is likely to be taken to boiling and then mixed with the main mash (at its mashing-in temperature), with the resultant effect being the temperature rise to conversion for the malt starch (cf. decoction mashing). This is sometimes called 'Double mashing'.

Wort separation

Traditionally, recovering wort from the residual grains in the brewery is perhaps the most skilled part of brewing. Not only is the aim to produce a wort with as much extract as possible, but many brewers prefer to do this such that the wort is 'bright', that is, not containing many insoluble particles which may present difficulties later. All this needs to take place within a time window, for the mashing vessel must be emptied in readiness for the next brew.

Irrespective of the system employed for mash separation (traditional infusion mash tun, lauter tun, or mash filter), the science dictating rate of liquid recovery is the same and is defined by Darcy's equation:

. , „ Pressure x bed permeability x filtration area

Bed depth x wort viscosity

And so the wort will be recovered more quickly if the device used to separate the wort has a large surface area, is shallow and if a high pressure can be employed to force the liquid through. The liquid should be of as low viscosity as possible, as less viscous liquids flow more readily. Also the bed of solids should be as permeable as possible. Perhaps the best analogy here is to sand and clay. Sand comprises relatively large particles around which a liquid will flow readily. To pass through the much smaller particles of clay, though, water has to take a much more circuitous route and it is held up. The particle sizes in a bed of grains depend on certain factors, such as the fineness of the original milling and the extent to which the husk survived milling (discussed earlier). Furthermore, a layer (teig or oberteig) collects on the surface of a mash, this being a complex of certain macromolecules, including oxidatively cross-linked proteins, lipids and cell-wall polysaccharides, and this layer has a very fine size distribution analogous to clay. (The oxidative cross-linking of the proteins is exactly akin to that involved in bread dough - see Fig. 12.3). However, particle size also depends on the temperature, and it is known that at the higher temperatures used for wort separation (e.g. 78°C), there is an agglomeration of very fine particles into larger ones past which wort will flow more quickly.

Lauter tun

Generally this is a straight-sided round vessel with a slotted or wedged wire base and run-off pipes through which the wort is recovered (Fig. 2.12). Within the vessel there are arms that can be rotated about a central axis. These arms carry vertical knives that are used as appropriate to slice through the grains bed and facilitate run-off of the wort. Water can be sparged onto the grain to ensure collection of all the desired soluble material. The spent grains are shipped off site to be used as cattle food.

Mash filters

Increasingly, modern breweries use mash filters. These operate by using plates of polypropylene to filter the liquid wort from the residual grains (Fig. 2.13). Accordingly, the grains serve no purpose as a filter medium and their particle sizes are irrelevant. The high pressures that can be used in the squeezing of the plates together overcome the reduced permeability due to smaller particle

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