What happens to barley during malting?
How do β-glucans break down during malting?
Green plants use chlorophyll pigments to convert radiant energy into potential chemical energy. Prior to the operation of the chlorophyll system, the early stages of germination of the barleycorn, the embryo obtains its food supplies from the reserve that is the endosperm. Once the barleycorn has absorbed the necessary steeping moisture and oxygen, the embryo transitions from a dormant state to a state of active growth. At the start of the malting process, the endosperm is made up of stable high molecular weight compounds such as starch, proteins, organophosphates, lipids, and other substances. A rootlet is pushed from the base of the barleycorn with several additional rootlets (De Clerck, 1957). During this “chitting” process, the acrospire pierces the testa but not the glume, forcing the root to grow between the two along the dorsal side of the corn. The most important changes occur in the endosperm; the seedling starts to produce gibberellic acid and when it reaches the aleurone layer, the formation of new enzymes occurs; creating starch degrading enzymes, cytolytic enzymes, proteolytic enzymes, lipases, and phosphatases (Kunze, 2014). The collective term for the polysaccharides derived from unbranched chains of β-D-glucopyranose are β-glucans and they are the primary component of the endosperm cell walls. The endosperm’s cell walls consist of a lamella formed of protein which has a coating of β-glucan, which in turn is layered with pentosan. During the germination phase of malting, the middle lamella layer is degraded which allows the pentosans and organic acids to be broken down by xylanases. The hemicelluloses, that are the main structural component of the cell walls, are broken down when contacted with water and the enzyme cytase. The cytolytic enzymes include: endo-β-glucanase, exo-β-glucanase, β-glucan-solubilase, and endo-xy-lanase. This step is followed by the glucanases converting the β-glucans which reacts with the structure of the aleurone layer cells. The initial stage is the solubilisation of the β-glucan primarily by β-glucan-solubilase with assistance from endo-β(1-3)glucanase and endo-β(1-4)glucanase. Once soluble, the β-glucans are further degraded by endo- β(1-3;1-4)-glucanase to tri- and tetrasaccharides. The final enzymatic reaction is to be further degraded to glucose by β-glucosidases (Bryce, 2016). The degradation of the layers occurs in a set sequential order following exposure from the conversion of the previous layer. These layers must be degraded for modification of the endosperm to occur; the largest change in modified corns is within the first 24-48 hours. Approximately 80% of the β-glucans present in grains are degraded by malting, but this quantity can change with barley cultivar and growth origin (Wang, 2004). If outside stresses such as high heats during the maturation process interfere with this process, then cell residues of intact β-glucans remain in the barley malt along with pentosan and proteins and can influence the wort filtration ability of the mash in the lauter tun (Runavot, 2011). Additionally, if lower hydration levels are used, then the diffusion and hydrolysis of β-glucans may be reduced; however, this impact can be diminished by increasing the germination period (Runavot, 2011).
How are proteins affected by malting?
The crude protein and total nitrogen (TN) are values used for measuring the quality of malt. While there is a rough approximation for the relation between protein and total nitrogen by a factor of 6.25% (Nie, 2010), the total nitrogen can be measured by the Kjeldahl method (Briggs, 1998). The total soluble nitrogen (TSN) consists of free amino acids and peptides, it comes as a percentage of the total nitrogen and can also be measured by the Kjeldahl method. Free amino nitrogen (FAN) is a measure of the soluble nitrogen that has been broken down by the proteolytic enzymes in free amino acids and short peptides (Briggs, 1998). All proteins have similar structures and are polymers of amino acids linked by peptide bonds to create polypeptides, it is the bonding between the different polypeptides that creates the different types of proteins.
For a brewer, free amino acids play a critical role in fermentation as they are the main source of nitrogen for yeast in the form of Free Amino Nitrogen (FAN). They are introduced into the wort using malt and are a necessary measure of healthy fermentation. Typical barley mashes contain up to nineteen different amino acids and they are consumed by the yeast at different rates (Oliver, 2012). For the healthiest fermentation, the α-amino content should be greater than 100mg/L with an ideal concentration of 150-200mg/L (Oliver, 2012). Since there are sixteen different transport systems identified in yeast, a balance of the different types of amino acids should be supplied to ensure micronutrient uptake is as efficient as possible. In the absence of all-malt mashes or less than ideal mashes, commercial yeast nutrient blends provide a substitute to the mixture of amino acids in addition to a source of zinc (Oliver, 2012).
While malt modification depends on the hydration levels, temperature, and duration of the germination process, the enzymes present can affect the overall structure of the barley and can change depending on variety and growing region (Agu, 2001). The Kolbach Index is an indicator of the modification of the malt and measures the degree of solubilization of proteins occurring during malting. The free amino acid, amino acid, composition, and the Kolbach Index combined can measure the degradation of the proteins (Agu, 2001).
The different proteins have different properties and can be divided based on their solubility properties. The largest groups are the albumins and globulins, which are soluble in salt solutions. The next group are hordeins, which are insoluble in salt solutions but can be dissolved in warm aqueous alcohol solutions. Hordeins are the main storage protein in barley and are further separated into groups classified as A, B, C, and D. The major groups in barley are the B and C hordeins and are highly heterogeneous. These non-uniform characteristics differ on a variety of barley and allow for the stain to be identified by electrophoresis (Kunze, 2014). Type D hordeins are a gelling protein, along with Type B hordeins, can form a polymer with di-sulphite bridges under oxidative conditions that form during lautering and is known to slow wort separation (Pöyri, 2002). The final group are residual proteins that cannot be extracted by either saline nor an alcohol solution, they are named glutelins (Kunze, 2014).
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