Malting

This page is in progress
Please check back later for additional changes

There are 3 stages of malting:

1. Steeping - increase in water content
2. Germination - germination and development of sprouts and rootlets
3. Kilning - drying

Steeping During steeping, water is absorbed by the raw barley grains and germination begins, resulting in an increase of the moisture content from approximately 12% up to 42% to 46%, along with higher concentrations of reducing sugars and amino acids. These compounds are important precursors of thermally generated flavors during heat treatment and contribute to the development of some thermally preformed volatile compounds and their precursors. A typical steeping process consists of an initial water stage for 6 to 16 h (under water period) with a consequent rise of the moisture content to 33% to 37%. Air is then removed trough the grain bed in order to remove moisture films in grains and carbon dioxide produced during respiration. Grains are allowed to rest for 12 to 24 h (air rest period) and embryos are exposed to oxygen. Afterwards, grains are immersed in water for more 10 to 20 h and reach the required moisture content (Briggs and others 1982). --see article for photos!

Germination The germination stage leads to the production of green malt, which is characterized by high moisture content and high enzyme activity. Enzymes are activated through germination and inactivated in the last stage of thermal processing (Channell and others 2010). Enzymes are responsible for the hydrolysis of the cell walls, proteins and starch. The steeped grains are kept 4 to 6 d under humid and ventilated conditions in a controlled temperature between 14 and 20 °C by a flow of air through the bed. This can lead to some loss of moisture that is compensated by spraying water onto the green malt during the first days of germination. The germination step is controlled by regulating the growth of rootlets that are expected to grow to a length of between 1.5 and 2 times the original length of the grain. The formation of α- and β-amylase, and degradation of proteins and β-glucans are also essential to control and regulate the germination process. The breakdown of β-glucans in cell walls, mediated by β-glucan solubilase and endo-β-glucanase, is very important to achieve fast lautering and to improve filterability of beer. Since it is an enzymatic step the degradation of β-glucans to smaller water-soluble molecules, such as glucose, is favored by high moisture contents and a temperature of around 19 °C (Briggs and others 1981, 1982, 2004).

Kilning and roasting The final step is a heat treatment of grains, also called kilning or roasting. The thermal processing steps have the greatest impact upon color and flavor of malt, depending on the time course, temperature, and moisture content. The steps aim at the reduction of the moisture content of green malt and to a condition that ensures stability during transportation and storage (approximately 5%). The removal of water prevents further growth and modification of the grains. Moreover, enzymes are inactivated and preserved. During the first phase of kilning, malt is exposed to air at 25 °C and moisture is removed from the grain, from approximately 44% to 12%. This phase is referred to as the “whitering” or “free-drying” phase. During the second phase of drying, malt is dried from 12% to 4%, and it is a much slower process, commonly referred to as the “falling rate” phase. At the end of the drying process the temperature is increased (“curing” stage). This is followed by a cooling period to ensure an optimum temperature for discharge and storage (Briggs and others 1982). The thermal processing steps (kilning or roasting) have the greatest impact upon final beer color and flavor, depending on the time course, temperature, and moisture content (Yahya and others 2014). Malts are not all equal and their chemical composition largely depends on the time and temperatures of the process (kilning and roasting) (Briggs and others 1982, 2004). Pale malts are the main ingredients used for beer production and are mildly heated at temperatures from 70 to 95 °C. Usually, these malts are dried in conventional kilns at temperatures less than 100 °C to reach moisture contents around 4% to 5% and to ensure stability during storage and transportation. Dark malts can be classified into color brew malts, caramel malts, and roasted malts. Color brew malts are obtained using temperatures up to 105 °C, while caramel malts and roasted malts are produced by roasting green malt (germinated but not kilned) or pale malt (kilned) up to 160 °C and 220 to 250 °C, respectively.

Malting modifies raw grain to make it suitable for mashing. The malting process is divided into 3 stages: steeping, germination and kilning. This process takes a fairly flavorless, unfriable (difficult to breakdown during milling) seed in which any fermentable material is sealed in a matrix of proteins and gums and transforms it into an easily milled, readily converted grain with, for want of a better word, a malty aroma. During malting, the storage materials in the grain are unlocked by the biological systems which the grain uses to grow into a barley plant. The grain is then dried to enable stability ease of transportation. Most craft brewers a start with malt as opposed to buying barley and malting it themselves. Malting is typically a specialty endeavor due to the capital investment required to set up a small malting operation and concerns over quality/consistency of the final product.

Malt kilning comprises five steps: start up (heating up of both grain and kiln, establishing air flow through the grain bed), free drying (removal of free water, air-on temperature of 50 to 60 °C), intermediate drying (increase of air-on temperature), removal of bound water and curing (grain moisture content decreases to 5% to 8%) (Inns, Buggey, Booer, Nursten, & Ames, 2007). Temperature profiles during kilning depend on the type of malt produced.

The three biochemical basic processes taking place during malting are cytolysis, proteolysis, and amylolysis, which are indicated by b-glucan, FAN, and extract concentration, respectively.

During the first stage (steeping) of barley malting, the grains are soaked in water for 1–3 days at 10–15ºC to increase the humidity of the cereal. Afterwards, the steeping water is drained away and the grain is spread out to allow germination for 4–7 days with periodical aeration. During this phase proteases and the starch-degrading enzymes are accumulated. At this step proteases also contribute to starch degradation releasing bound beta-amylase from starch and releasing the inactive limit dextrinase forms from their inhibitors. Final drying (kilning) provides brown coloring and flavor by the Maillard reaction. In the case of malt storage, kilning also allows malt stabilization by stopping the enzymatic reactions and inhibiting an undesirable microbial growth by decreasing the water activity. This malt is still not readily fermentable by S. cerevisiae. Since hydrolysis must be continued in the next stage of brewing, kilning is a critical step because an intensive heating treatment can lead to enzymes deactivation.

Alpha-Amylase, alpha-glucosidase, and bound limit dextrinase are synthesized de novo during seed germination, while beta-amylase and free limit dextrinase are activated by endogenous proteases. In addition to the effect of the genotype on enzymes expression, the main factors that affect both expression and activation of these enzymes during germination are temperature, water/solid ratio, oxygen availability, and concentration of gibberellic acid (GA3)—the hormone that stimulates the synthesis of alpha-amylase and other hydrolases.

Alpha-Amylase synthesis is synergically improved with the increase in temperature from 20–25ºC to 30–35ºC and the addition of exogenous GA3. Beta-amylase, which is synthesized during the development of the grain and accumulated in an inactive starch-bound form, is released from starch and activated by a proteolytic process. Anoxic conditions during germination prevent the production of the endoprotease involved in beta-amylase activation. Oxygen deficit also inhibits alpha-amylase synthesis. Limit dextrinase in barley is partially synthesized following germination under the influence of GA3 and the hydration conditions. Continuous humectation during germination produces higher levels of limit dextrinase than addition of water at the beginning.

Malt is produced by germinating cereal grains, usually barley, for a limited period of time. It is then dried in order to stop the physical germination process and accompanying biochemical processes of enzyme modification. The aim of malting is to activate and produce enzymes able to degrade endosperm cell wall components (predominantly 1,3 and 1,4-β-glucan) and storage proteins. This action allows starch granules to be released from the endosperm protein matrix. Malting is also important to develop the desirable color and flavor of malt. The production of malt entails three processes: steeping, germination and kilning. Many variations of these processes exist, and are tailored to the specific barley specifications. A typical malting process can take six days.

Malting is the controlled germination of grains with the following goals:

• Enzyme development - Unmalted grain does not contain the necessary enzymes to turn the grain into beer.
• Cytolysis - The cell walls must be broken down do the enzymes can access the starch
• Modification - Malting behind the process of degrading protein and starch
• Maillard products - These products are responsible for the color and special flavors from different malts.

Several brewing problems are associated with microbial infections of malts. Off-flavors may occur and there is always the concern that mycotoxins may be present on poor malts. Particular attention has been paid to the possible presence of aflatoxins, ochratoxin, zearalenone, deoxynivalenol, fuminosins and citrinin, which can be produced by a range of fungi infecting barley. Some, such as citrinin, do not survive the brewing process, but others, such as deoxynivalenol, can survive into beer. Fungi also produce factors that cause gushing (over-foaming) in beers. The solution seems to be to avoid making malts from cereals that are heavily infected with fungi. High levels of bacteria on malt can also give problems. Bacteria multiply very greatly during malting, especially on the substances leached from split grains. Malts made with heavily infested barleys have, on mashing, given rise to very slow wort filtrations, possibly due to microbe-produced polysaccharides clogging the grain bed. Other malts have given worts having persistent hazes due to suspended dead bacteria, about 0.6 m in diameter (Walker et al., 1997). Another problem caused by microbial infestations of malts are the wild, and unpredictable fluctuations in the pH of worts (e.g. pH 5.45–6.06). Multiplication of lactic acid bacteria on the growing malt and particularly in the initial stages of kilning high-moisture green malts, is largely to blame. The malting process must be designed to minimize these problems.

Aerobic germination conditions increase malt limit dextrinase levels. This is likely the reason that analyses of small-scale micromalt batches have shown higher levels of limit dextrinase. However, it's been shown that the level of malt limit dextrinase doesn't correlate well with its activity in the mash. Limit dextrinase activity is increased by germinating grains under anaerobic conditions, probably due to increased activity of cysteine proteases, which degrade the limit dextrinase inhibitor. Whether anaerobic conditions can be obtained with a home malting process is unclear.

Endoproteases are essential for grain modification. They degrade the storage proteins especially hordein and glutelin. In addition, endoproteases process and activate functionally important proteins such as β-amylase.

As malting begins, these internal lipid stores are the primary energy source supporting metabolic function and growth. As such, a ~23% decrease in total lipid content has been observed in finished malt as compared with the raw barley; the decrease was driven by reduced triacylglyceride and phospholipid levels.

Lipid-degrading enzyme levels and activity increase during germination, such as lipoxygenases (LOXs). LOX-1 is already present in the barley kernel, but LOX-2 is synthesized during germination.

Lipase activity greatly increases during malting.

The activity of both LOX isoenzymes increases during germination and decreases during kilning.

The low moisture content of malt helps protect it from microbial spoilage and staling from oxidation and enzymatic activity.

Malting barley and the malting process can have impact on beer instability owing to the presence of pro-oxidant and antioxidant activities. cites Boivin P (2001) Pro-and anti-oxidant enzymatic activity in malt. Cerevisia 26:109–115

Dur­ing soaking and germinating, the TPC in barley malt has been found to decrease due to dissolution, and the AOX decreased. The contents of vitamin E and C increased significantly, which can be explained by the synthesis of vitamin E and C during grain germination. During the withering and kilning stages, the content of vitamin C decreases. Previous studies have also suggested that the amount of vitamin C in food decreases during heating (Mistry & Kennedy, 2003). Vitamin E has a certain thermal stability, but its content depends on the kilning tem­perature. Higher kilning temperature leads to greater loss of vitamin E. An 8 h kilning of the germinated barley causes the content of vitamin E decrease notably (Dabina-Bicka et al., 2011, pp. 121–126).

In conclusion, TPC and contents of melanoidins and vitamins C and E in malt have been found to be higher than those in barley (Dabina-Bicka et al., 2011, pp. 121–126; Dabina–Bicka et al., 2010, pp. 111–115; Dvořáková, Guido, Skulilová, Moreira, & Barros, 2008; Li, 2017; Zheng, 2017). After steeping, germination, withering, removal of roots and shoots and kilning, 34% of selenium in wheat malt was lost (Rodrigo et al., 2015). The technological operation involved in the malting pro­ cess has a great effect on antioxidants and their AOX in malt. It is a fact that heat treatment not only increases the content of free radicals in beer but also produces a large number of free radical scavenging antioxidants.

See Fix chapter 1 pp. 36-45

See Kunze chapter 2 and section 8.4.3

• 

  Home kilning of malt. from TMB

• Psota, V., a Sladarsky, V. U. P., Cmelik, R., Sachambula, L., & a Sladarsky, V. U. P. (2011). The effect of malting conditions on dextrin contents in beer production intermediates. Kvasny Prumysl, 57, 253–259 (Czech Republic).
• https://www.homebrewtalk.com/threads/malting-my-own.695514/
• Maillard, M. N., & Berset, C. (1995). Evolution of antioxidant activity during kilning: Role of insoluble bound phenolic acids of barley and malt. Journal of Agriculture and Food Chemistry, 43, 1789–1793.
• https://brewingbeerthehardway.wordpress.com/
• https://www.homebrewtalk.com/threads/growing-barley.701351/
• Lu, J., Zhao, H., Chen, J. 2007. Evaluation of phenolic compounds and antioxidant activity during malting. Journal of Agriculture and Food Chemistry, vol. 55, no. 26, p. 10994-11001.