Malting

Malting

Introduction

•Approximately 96% of the malt produced in the world is used as the primary raw material, along with water, to make beer.

•Malt is extremely important in Scotland for whisky production, but it only represents 3% of the world’s malt use.

•Malt is also used as an essential ingredient in the food industry as natural flavouring, breakfast cereals, malted milk drinks, or malt vinegar.

•Although barley, wheat, and sorghum can be malted, barley malt is preferred for brewing.

•After harvesting, barley is prepared for storage during its period of dormancy, before conversion to malt, under conditions minimizing the risk of infestation from insects, fungal attack, etc.

•To achieve this, it is necessary to dry the wet or green grain from the fields for long-term storage, down to a moisture content of 12%.

•Grain with moisture levels between 10% and 12% can be stored for long periods.

Pre-cleaning

•Barley is pre-cleaned before passing to the drier or storage silos.

•Pre-cleaning consists of a rough sieving process combined with or followed by aspiration of the grain stream with a flow of air.

•In this stage, items such as leaves, straw, stones, clods of earth, snail shells, barley awns, and other light material are removed.

Cleaning

•It has to remove impurities of about the same size as barley, such as weed, seeds of grains of other cereals, as well as broken grains or undersized grains.

•The light material is removed with the help of cleaners and separators.

•De-stoners, which separate the stones from the grain using their density difference, can be optionally installed.

Grading

•Sieving equipment, barley for malting is graded by kernel size (width) into different categories, for example, grain sizes of more than 2.5 mm, 2.2 to 2.5 mm, and less than 2.2 mm.

•Broken kernels and grain of less than 2.2 mm width, removed as screenings, may be collected in a silo for eventual use as animal feed.

•All these technologies are fitted with a dust extraction system to minimize the risk of dust explosions.

•Barley that has been processed ready for malting is stored in large-capacity circular steel bins, giving maximum versatility in the separation of varieties.

Steps involved in malting

• Steeping 
• Germination 
• Kilning

Steeping

•Steeping initiates malting and is a crucial step in producing quality malt. During steeping, barley takes up water and swells by one-third.

•Respiration increases slowly at first and then more rapidly, causing the grain in steep to accumulate CO2 and heat up; this is aggravated by the action of microorganisms associated with the barley.

•The steeping stage consists of alternating periods when the grain is immersed in water, referred to as the “underwater periods,” and periods with the water drained from the grain referred to as the “air rest periods.”

•This combination is required to promote and maintain the efficiency of germination.

•Water content of barley should be increased to approximately 42% to 46% during the steeping process, in a typical temperature range from 15°C to 20°C.

•When barley absorbs water, the embryo becomes active and uses the oxygen dissolved in the steeping water for respiratory purposes.

•During the first period underwater, the dissolved oxygen in the water is rapidly adsorbed by the organic material and microflora in the grain.

•In case of prolonged failure of the oxygen supply, respiration with the release of CO2 may induce fermentation with the production of ethanol.

•A lack of oxygen at any stage will stop the growth and enzyme production.

•Prolonged anaerobiosis is progressively damaging and finally lethal to the grain.

•In addition to supplying water and oxygen to the barley, steeping is of considerable value in removing dirt from the grain.

•The film of dust and many of the microorganisms adhering to the grain are washed away by the steep water, and this cleaning action can be increased if special equipment is used to agitate the barley during steeping.

•Steep water leaches husk components that may impede germination and any musty smell that may be associated with the barley.

Germination

The germination process is characterized by the growth of the embryo of the grain, manifested by the rootlets growth and increase in length of the shoot (acrospire), with the concomitant modification of the contents of the endosperm.

•The growth of the acrospires should be limited between two-thirds and three-quarters of the length of the grain for pale malts and between three-quarters and the full length of the grain for dark malts.

•Germination rate is controlled by regulating the moisture content and temperature of the grain.

•Traditionally, the steeped grain spends between 4 and 6 days in humid and ventilated conditions.

•The temperature of germinating grain is controlled to between 14°C and 20°C by a flow of air through the bed; the germinating grain is humidified by atomizing water jets, which may be cooled by refrigeration.

•Ensure grain bed homogeneity throughout kernel germination, in a gentle revolving of barley along the germination period.

•A significant increase in the quantities of some of the hydrolytic enzymes present in the grain is one of the essential changes of modification.

•The alteration of the starchy endosperm begins due to the partial degradation of reserve substances (cell walls, gums, proteins, and starch.

•The hydrolytic enzymes that catalyze modification are derived mainly from the scutellum and aleurone layer.

•In the initial stages of germination, the scutellum releases many hydrolytic enzymes that begin to degrade the cell walls of the crushed cell layer, protein, and starch granules of the endosperm.

•In addition to enzymes, the scutellum releases gibberellins, which stimulate the production and release of numerous hydrolytic enzymes by the aleurone layer.

•α-Amylase is absent from the hydrated aleurone, and its de novo synthesis and release are independently triggered by the arrival of the hormone.

•It is estimated that in malt, approximately 85% of this enzyme originates from the aleurone layer and 15% from the scutellum layer.

•β-Amylase, which exists in the original barley in an inactive form in the starchy endosperm, is liberated, and the degradation of the components proceeds.

•Proteases and carboxypeptidases accumulate in the starchy endosperm during germination.

•β-Glucansolubilase is a carboxypeptidase that converts the insoluble cell wall protein matrix to high molecular weight soluble but viscous β-glucans, which can create problems during wort separation and beer filtration.

•Those are broken down further to low molecular weight glucans and glucose by the endo-β-glucanases and β-glucosidases. These enzymes develop slowly in the malting process and are slightly sensitive to gibberellic acid.

•Gibberellic acid is an example of a germination regulator which, like potassium bromate, can be added to the grain to greatly accelerate the modification rate.

•At this stage, the material is called green malt.

•The metabolism of the growing grain is an exothermic process.

Kilning

Kilning has the characteristics of a typical industrial drying process occurring in two distinct stages.

•First, moisture is removed from the green malt, reducing it from approximately 44% to 12%. With an upward airflow, this process takes about 12 h to pass through the bed for a single-deck kiln and 24 h for a double-deck kiln. This phase of drying is rapid and is referred to as the “free-drying” or “withering” phase.

•The second phase of drying, in which the malt is dried from 12% to 4%, occurs in a much slower process, which is commonly referred to as the “falling rate” phase.

•At the end of the drying process, the kiln temperatures may be raised for 1 or 2 h in the “curing” stage, followed by a cooling period to achieve a temperature suitable for discharge and storage.

•Kilning not only dries the malt, preventing further growth and modification.

•It also removes undesirable flavors, partially or wholly destroys some hydrolytic and other enzymes, and develops flavor and color in the final product.

•Moreover, malt is kilned to produce a friable, readily milled, stable product that may be stored for long periods and from which roots may easily be removed.

•The composition of malt changes during kilning so that normally, the enzyme activity declines, whereas its color, aroma, flavor, and extractable polyphenol content increase.

•Some enzymes, for example, α-glucosidase, are appreciably inactivated even at 45°C, which is well below kilning temperatures.

•At 80°C, the activities of enzymes, such as β-glucanase and β-amylase, are significantly reduced.

•α-Amylase is the most stable enzyme, and its activity may even increase during kilning.

•The degree of enzyme destruction at any temperature is greater when the malt is wet.

•Highly enzymatic malts are kilned at lower temperatures and in a rapid airflow to ensure that the grain is cooled by evaporation.

•In roasted malts, on the contrary, enzyme destruction is complete.

•Two isoforms of lipoxygenase(LOX-1 and LOX-2) are known to induce the oxidation of linoleic acid into the highly flavor-active aldehyde, E-2-nonenal, which is responsible for the cardboard flavor of aged beers.

•Both enzymes are extremely heat-sensitive and are extensively lost during most kilning regimes, with LOX-1 being more resistant.

•The presence of sulfur compounds, such as dimethyl sulfide (DMS), is responsible for an unpleasant “cooked vegetable” taste. During kilning, the DMS precursor, which is formed during grain germination, is converted to free DMS, which is volatilized and lost.

•When kilning is complete, the heat is switched off, and the grain is allowed to cool before it is stripped from the kiln in a stream of air at ambient temperatures.

•On its way to steel or concrete hopper-bottomed storage silos, the malt is “dressed” to remove dried rootlets, or culms, which are highly hygroscopic, and must be stored out of contact with the air.

•They have a much lower economic value than the malt and are used for feeding cattle.

•Approximately 3% to 5% of the original barley is recovered as culms in the traditional malting processes.

•To break and remove rootlets, the cooled malt is agitated with beaters and

screened by passing through a revolving perforated drum that retains the

kernels and allows the culms to pass out through the perforations.

•The malt is, at the same time, aspirated with dry air to cool it and to remove the dust.

 

References

Kruma, Z., Tomsone, L., Ķince, T., Galoburda, R., Senhofa, S., Sabovics, M., ... & Sturite, I. (2016). Effects of germination on total phenolic compounds and radical scavenging activity in hull-less spring cereals and triticale. Agronomy Research, 14(S2), 1372-1383.