The Transformation of Barley: What Happens During Malting?

How Do β-Glucans Break Down During Malting?

Malting is a critical step in brewing, where the barleycorn undergoes biochemical transformations essential for brewing quality. Among the many changes, the breakdown of β-glucans plays a key role in making the barley suitable for brewing. β-glucans, which are polysaccharides derived from unbranched chains of β-D-glucopyranose, are the primary components of the barley endosperm cell walls. Understanding how they degrade during malting is essential for optimizing the process and ensuring efficient mash filtration.

The Role of Germination in β-Glucan Breakdown

Once barley is steeped and has absorbed the necessary moisture and oxygen, the germination phase begins. During this phase, the barleycorn’s endosperm—initially composed of starch, proteins, lipids, and other high-molecular-weight compounds—undergoes significant modification. The embryo transitions from dormancy to active growth, leading to the production of gibberellic acid, which initiates the synthesis of key enzymes like cytolytic enzymes. These enzymes, including endo-β-glucanase, exo-β-glucanase, and β-glucan-solubilase, are responsible for breaking down the β-glucans within the cell walls.

As germination progresses, the middle lamella layer of the endosperm cell walls is degraded, facilitating the breakdown of hemicelluloses by enzymes like cytase. This process solubilizes β-glucans, allowing further enzymatic degradation. The primary enzyme responsible for solubilizing β-glucans is β-glucan-solubilase, assisted by endo-β(1-3) and endo-β(1-4) glucanases. Once the β-glucans are solubilized, they are further degraded into tri- and tetrasaccharides by endo-β(1-3;1-4)-glucanase. Finally, these saccharides are broken down into glucose by β-glucosidases (Bryce, 2016).

Sequential Degradation and Its Impact on Malting

The breakdown of β-glucans occurs in a specific sequence, as the enzymatic reactions progress from one layer of the endosperm to the next. Approximately 80% of the β-glucans present in the barley are degraded during the malting process, though this percentage can vary based on the barley cultivar and its growth conditions (Wang, 2004). If external stresses, such as high temperatures during maturation, affect the grain, intact β-glucans, pentosans, and proteins may remain in the malt. This can negatively impact the wort filtration process, leading to issues in the lauter tun (Runavot, 2011).

Influence of Hydration and Germination Duration

The hydration level and germination duration during malting also influence β-glucan degradation. Lower hydration levels may reduce the diffusion and hydrolysis of β-glucans, though this can be mitigated by extending the germination period (Runavot, 2011). Proper control of these factors ensures optimal modification of the endosperm, leading to more efficient brewing processes and higher-quality malt.

Figure 1. Enzymatic Degradation of β-Glucan (Edi, 2014)

How Are Proteins Affected by Malting?

Proteins play a crucial role in the malting process, influencing not only the quality of the malt but also key aspects of brewing, such as fermentation performance, foam stability, and the final beer's haze potential. During malting, the breakdown of proteins is essential for releasing nitrogenous compounds that are vital for yeast nutrition, making it a critical factor in producing high-quality malt.

Protein Content and Measurement in Malt

Two important metrics used to evaluate malt quality are crude protein and total nitrogen (TN). Crude protein content gives a rough approximation of the total protein in barley, but total nitrogen provides a more precise measure, often calculated using a conversion factor of 6.25% (Nie, 2010). The Kjeldahl method is commonly used to measure total nitrogen, offering a more accurate representation of the nitrogenous compounds in malt (Briggs, 1998).

Additionally, total soluble nitrogen (TSN), which includes free amino acids and peptides, is another important metric. TSN reflects the degree to which proteins have been broken down into soluble forms during malting and is also measured using the Kjeldahl method. Free Amino Nitrogen (FAN) is a critical component of TSN, as it represents the nitrogen available in the form of free amino acids and short peptides, which are essential for yeast nutrition (Briggs, 1998). FAN levels directly impact the fermentation process, making this measurement crucial for brewers seeking to optimize yeast health.

The Role of Proteins in Fermentation

For brewers, the breakdown of proteins during malting plays a vital role in providing yeast with the nitrogen required for healthy fermentation. The main source of this nitrogen comes from FAN, which consists of amino acids and small peptides that yeast can readily assimilate. Typical barley mashes contain up to nineteen different amino acids, each absorbed by yeast at different rates (Oliver, 2012). This diversity is important because yeast relies on multiple transport systems—sixteen have been identified—to take up different types of amino acids (Oliver, 2012).

To ensure optimal yeast performance, FAN levels in wort should be greater than 100 mg/L, with the ideal range falling between 150-200 mg/L (Oliver, 2012). If the malt quality or mash conditions are less than ideal, commercial yeast nutrient blends can be added to supplement the amino acid profile and include essential micronutrients like zinc, which further supports yeast metabolism and fermentation efficiency.

Protein Modification During Malting

Protein modification in barley occurs during the germination phase of malting, where proteolytic enzymes are activated to break down large protein molecules into smaller components like amino acids and peptides. This breakdown is influenced by key factors such as moisture content, temperature, and germination duration, as well as the specific enzymes present in the barley. The level of protein modification is a critical indicator of malt quality, as it directly impacts fermentation performance and the clarity of the final beer.

The Kolbach Index is one of the key metrics used to measure the degree of protein modification during malting. It represents the ratio of soluble nitrogen (TSN) to total nitrogen (TN), expressed as a percentage (Agu, 2001). A higher Kolbach Index indicates better protein solubilization and modification, which typically correlates with better yeast nutrition and smoother fermentation. However, over-modification can lead to excessive protein breakdown, which may negatively affect beer foam stability or body.

Types of Barley Proteins

Barley proteins are divided into several categories based on their solubility properties, and each category plays a different role in the malting and brewing process. The two largest groups are albumins and globulins, both of which are soluble in salt solutions. These proteins are primarily responsible for enzymatic activity and metabolic functions during the malting process. Although they contribute to the overall nitrogen content of the malt, they represent a smaller fraction of the total protein content.

The largest portion of barley protein consists of hordeins, which are insoluble in salt solutions but can dissolve in warm aqueous alcohol solutions. Hordeins are the main storage proteins in barley and are further divided into four subgroups: A, B, C, and D. The B and C hordeins are the most abundant and play a critical role in defining the brewing performance of the malt. These proteins are highly heterogeneous, meaning they differ significantly across barley varieties, making them useful for identifying specific cultivars through electrophoresis (Kunze, 2014).

Type D hordeins, along with B hordeins, are known as gelling proteins. Under oxidative conditions, such as those that occur during lautering, these proteins can form polymers with disulfide bridges. This gelling effect can slow down wort separation, leading to potential issues with wort clarity and filtration (Pöyri, 2002).

Finally, the last major category of proteins in barley is glutelins, which are insoluble in both saline and alcohol solutions. These residual proteins are often considered more difficult to break down and remain largely intact through the malting and brewing processes (Kunze, 2014).

Impact on Brewing

The modification of proteins during malting is essential for brewing, as it not only affects yeast nutrition through FAN but also impacts other important brewing parameters. The balance between protein solubilization and preservation is critical. Over-modification of proteins can lead to poor foam stability and reduced body in the final beer, while under-modification can result in inadequate FAN levels and potential fermentation problems.

Furthermore, the composition of different protein types, particularly hordeins, can influence wort clarity, filtration efficiency, and beer flavor. Managing these factors through careful control of malting conditions and barley selection helps brewers produce high-quality, consistent malt that meets the needs of various beer styles.

References

Agu, R. C., and Palmer, G. H. (2001) The Journal of The Institute of Brewing, vol. 107, no. 2, 93-98

Briggs, D. (1998) Malts and Malting. London: Blackie Academic & Professional, 169-184.

Bryce, J., and Hill, A. (2016) Cereals, Malting & Mashing. Edinburgh, UK: Heriot-Watt University, 197-199.

De Clerck, J. (1957) A Textbook of Brewing. V 1. London: Chapman Hall, 140-149.

Edi, A. (2014) Malt Enzymes The Key To Successful Brewing. MBAC Ontario Technical Conference.

Kunze, W., Manger, H., and Pratt, J. (2014) Technology Brewing & Malting, 5th ed. Berlin: VLB, 144-155.

Nie, C., Wang, C., Zhou, G., Dou, F., and Huang, M. (2010) Effects of malting conditions on the amino acid compositions of final malt, African Journal of Biotechnology, vol. 9, no. 53, 9018-9025.

Oliver, G. (2012) The Oxford Companion To Beer. New York: Oxford University Press, 48.

Pöyri, S., Mikola, M., Sontag-Strohm, T., Kaukovirta-Norja, A., and Home, S. (2002) The Formation and Hydrolysis of Barley Malt Gel-Protein Under Different Mashing Conditions, The Institute & Guild of Brewing, vol. 108, no. 2, 261-267.

Runavot, J., Bakan, B., Geneix, N., Saulnier, L., Moco, K., Guillon, F., Corbineau, F., Boivin, P., and Marion, D., (2011) Impact of Low Hydration of Barley Grain on β-Glucan Degradation and Lipid Transfer Protein (LTP1) Modifications During the Malting Process, Journal of Agricultural and Food Chemistry, vol. 59, no. 15, 8256-8264.

Wang, J., Zhang, G., Chen, J. and Wu, F. (2004) The changes of β-glucan content and β-glucanase activity in barley before and after malting and their relationships to malt qualities. Food Chemistry, 86(2), 223-228.