Starch Conversion

96% (?) of the solids in brewing wort are carbohydrates. Proteins make up only 3% and the rest are β-glucans, vitamins and minerals. Of the carbohydrates more than 95% are products of the starch conversion that happens in the mash tun. As a result the starch degradation is the main purpose of mashing. It produces fuel that the yeast needs for fermentation from the insoluble starch found in malt and mash tun adjuncts. The extend to which this conversion happens will determine the strength (gravity or extract content) of the produced wort and its fermentability as well as other quality parameteres.

In mashing starch conversion is preceded by gelatinization of the starches. While this is not necessary for conversion, as plants do exactly this when they germinate, it greatly speeds up the process though the exposure of a lot of substrate (amylose and amylopectin) from the starch granule to the starch converting enzymes (mainly α- and β-amylase).

Gelatinization

Starch granules are insoluble in cold water and will absorb only little water. The form a suspension that quickly settles once agitation stops. As the water is heated (50+ C) more and more water is absorbed and the granules start to swell [Narziss, 2005]. The water absorbed during this process can be up to 30 times the weight of the starch granule. This uptake of water initially happens within the amorphous growth rings. The granule starch to leach amylose and the crystalline layers break open and separate from the starch granule as gelatenous sheets. At this point the crystalline structure is lost and the process becomes irreversible with respect to the shape of the starch granule [Shetty, 2006]. The starch granule has gelatinized.

Figure 1 - Temperature ranges for the gelatinization of various starches [Briggs, 2004]. Starches marked with (*) also benefit from boiling before being used in the mash.

While the temperature range during with gelatinization occurs has been found to be quite narrow for individual starch granules (~ 1C) the temperature range between the gelatinization of the first granules and the complete gelatinization of all granules can be quite large. Figure 1 shows the temperature ranges for gelatinization for a number of starches. It can be seen that not all these starches fully gelatinize at temperatures that are encountered in a saccharification rest. If this is not the case they will have to be gelatenized before that rest though either a cereal mash or the use of pre-gelatinized (e.g. flaked) forms. Another interesting aspect is the different gelatinization temperature ranges that have been determined for large and small barley granules. 90% of the starch in barley are large granules which will be gelatenized at saccharification rest temperatures while the rest are small starch granules which may not fully gelatenize until higher temperatures are reached. This effect can explain some of the efficiency benefits that can be gained from a mash-out or a decoction mash.

Gelatinization is a process that requires free water for the swelling and breaking the hydrogen bonds that hold the crystalline structures in place. If free water is limited due to a high concentration of starch (e.g. overly thick mash conditions) less swelling takes place and a melting of the crystalline section needs to occur [Donald, 2004]. This leads to an increase in the gelatinization temperature. This limitation of free water can also be caused by the presence of sugars other dissolved solids. For corn starch it has been shown that a 25% sucrose solution increase the gelatinization temperature from 70 to 78C [Donald, 2004]. This could have been one of the factors why thick mashes showed a lower efficiency compared to thin mashes in the Mashing Experiments.

As starch starts to gelatenize the viscosity of the liquid will increase. This can be noticed in brewing to some extend but by far less that what is commonly seen in cooking. The reason for that is the presence of enzymes (in particular α-amylase) that will start breaking down the amylose and amylopectin molecules as soon as they become accessible. This process reduces the viscosity of the mash and is therefore called liquification [Kunze, 2007]. Liquifying decoctions through a rest at 70-74C before further heating to a boil for example mitigates the risk of scorching the mash during the decoction boil.

A strong increase in viscosity can however become a problem in cereal mashes. Especially when using rice starch which is known to swell very strongly. This can lead to scorching or even the immobilization of mash agitators [Kunze, 2007]. To counteract that some malt should be added to cereal mashes and a short liquification rest might be held between 75 and 80C (just before all the α-amylase gets denatured) before it is then heated to boiling.

Figure 2 - Plot of VZ 45C analysis values over the gelatinization temperature for 48 different samples of malt.

The gelatinization temperature also depends on growing conditions and crop year [Kunze, 2007]. And Kessler showed that a weak correlation exists between the VZ 45C malt analysis number and the gelatinization temperature [Kessler, 2008]. VZ 45C is the ratio between the extract that can be extracted through mashing at 45C and the amount that can be extracted with a congress mash. This number is given on some malt analysis sheets. Figure 2 shows the data that was publushed in Brauwelt International. According to Weyermann the VZ45 for their malts can be as low as 35. This may result in a gelatinization temperature as high as 65C. While this temperature should not be a problem when using a single saccharification rest, it can become problematic when a maltose rest is held at 63C at which temp the starch will not be fully gelatenized. If this is the case an extended rest at 65C needs to be held in order to achieve the desired fermenntability of the produced wort.

Enzymatic starch breakdown

Enzymes involved

Figure 3 - A large branched dextrin that shows which links can be broken (hydrolyzed) by which of the mash enzymes

Effect of mashing conditions

[Briggs, 2004]:

• gelatinization
• active enzymes
• emzyme temperature and pH optima
• mash parameters affecting conversion

References

[Valclavik] Vickie A. Valclavik, Elizabeth W. Christian, Essentials of Food Science, Third Edition, Springer

[Champe] Pamela C, Champe, Richard A. Harvey, Denise R. Ferrier, Biochemistry, Lippincott's Illustrated Reviews

[Kunze, 2007] Wolfgang Kunze, Technologie Brauer und Maelzer, 9. Auflage, VLB Berlin

[Narziss, 2005] Prof. Dr. agr. Ludwig Narziss, Prof. Dr.-Ing. habil. Werner Back, Abriss der Bierbrauerei, Technische Universitaet Muenchen (Fakultaet fuer Brauwesen, Weihenstephan). WILEY-VCH Verlags GmbH Weinheim Germany, 2005

[Donald, 2004] A. M. Donald, Understanding Starch Stucture and Functionality, Chapter 5 in Starch in Food: Structure, Function and Applications By Ann-Charlotte Eliasson, CRC Press, 2004

[Shetty, 2006] Kalidas Shetty, Food Biotechnology, CRC Press, 2006

[Kessler, 2008] Dr.-Ing. Matthias Kessler, Dr.-Ing. Stefan Kreisz. Dipl.-Ing. Martin Zarnkov. Univ.-Prof. Dr.-Ing. Werner Back, Do Brewers need a starch modification index?, Brauwelt International, 2008/1