MALT AND MALTING

Malt and malting

MALT AND MALTING

  • Malting involves the controlled partial germination of barley grain.
  • This modifies the hard vitreous grain into a friable (easily crushed) form containing a lot of readily degradable starch and generates hydrolytic enzymes, particularly amylases, b-glucanases and proteases.
  • Barley grains have approximately 65% starch, located within the endosperm region.
  • Endosperm cells are filled with starch granules embedded in a protein matrix, and their walls are composed of a mixed linkage b-glucan (b-1,3; 1,4), hemicellulosic pentosans and protein.
  • The starch granules cannot be accessed until the cell walls have been breached and the protein matrix has been at least partially degraded.
  • Hence, the requirement for adequate levels of b glucanases and proteases.
  • Malting begins by soaking or steeping the barley in water for 2 days at 10–16°C, in order to increase the moisture content to around 45% (w/w).
  • Periodically, the water is temporarily drained off and aeration is provided, thus preventing anaerobic conditions that can cause embryo damage.
  • The water used for steeping is often reused to save on costs of both water and effluent treatment.
  • After steeping, the barley is partially germinated for 3–5 days at 16–19°C.
  • Traditionally, this simply involved spreading the grain on malting floors to a depth of 10–20cm.

Structure of a barley grain

  • However, various mechanized systems are now operated, which have grain beds of about 1m in depth.
  • These are aerated with moist cool air and turned mechanically every 8–12 h to aid respiration by the grain and prevent the build-up of heat, otherwise the embryo may become damaged.
  • Grain modification can be promoted in several ways.
  • Abrasion (the controlled damage of the husk before steeping) improves access of water and any additives.
  • The application of the plant hormone gibberellic acid, at levels of 0.1–0.5 mg/kg barley, may be made to augment the natural embryo-produced hormone that stimulates de novo synthesis of certain hydrolytic enzymes, including a-amylase.
  • Cellulase supplements, such as enzymes from Trichoderma reesei, at levels of 24–48mg/kg barley, also speed germination by aiding endosperm cell wall breakage.
  • Suppression of unnecessary root and shoot growth can be achieved by adding bromate at levels of 100–200 mg/kg barley.
  •  When sufficiently modified, the malt is ‘kilned’ via a two-stage process.
  • First it is dried at 50–60°C and then ‘cured’ at 80–110°C.
  • Kilning takes about two days and has several functions.
  • It arrests embryo growth and enzyme activity, while minimizing enzyme denaturation, and develops flavour and colour (melanoidin compounds).
  • Pale lager malts that require little colour development are subjected to mild conditions.
  • Consequently, they retain lots of enzyme activity than do coloured malts.
  • Highly coloured malts required for flavouring and colouring dark beers have low enzyme activity.
  • At a final moisture content of 2–3% (w/w), the malt is biologically stable for several months.

MALT AND MALTING

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