Lesson 8 - Mashing End of Unit SAQ

*** Describe what happens to a fragment of barley endosperm containing a starch granule attached to a piece of storage protein and cell wall during a triple decoction mash. Include the physical and biochemical processes that occur.

Response

To answer this question, we can take the triple decoction process step by step and outline the fate of our endosperm fragment.

1. The mash mixer is filled with warm water, and whilst agitating, the grist is mashed in via a simple hydrator at ambient temperature. At this stage, the water to grist ratio is around 2:1-2.5:1.

At this step, any water-soluble material in the fragment will dissolve, and the temperature of the mash will rise slightly due to the slaking heat (heat of hydration). Any gaps in the structure of the fragment will fill with water. The temperature of the water will be too low for significant water uptake to occur in the fragment.

2. Hot water at around 95°C is added to the mash mixer either from the mash kettle or from a hot water tank. This raises the mash to around 35°C and achieves the target water to grist ratio of between 1.3:1 and 1.54:1. Once the mash is homogenous, the agitator is stopped, and the solid material in the mixer drops to the bottom, forming a thick mash.

The fragment will start taking up water more readily now, and hydration will be more extensive. Pentosanases, barley β-glucanase, and endo-β-(1,4)-glucanase will start to catalyse the hydrolysis of the cell wall material exposed to the water, with the products of the hydrolysis dissolving in the water. The fragment will be denser than the water, so the particle will sink towards the base of the mixer and hence form part of the thick mash.

3. This thick mash is pumped from the bottom of the mash mixer into the bottom of the mash kettle. One-third of the volume in the mixer is transferred at this point. In the mash kettle, the thick mash is heated to 65°C and held at this temperature for 20 minutes.

Cytolysis will continue in the mash kettle after transfer until the heat applied denatures the cytolytic enzymes. As the temperature rises, proteolytic enzymes will start to operate at increased rates, rapidly catalysing the hydrolysis of the storage protein until they pass their optima. β-Glucan solubilase will become more active as the temperature rises and will remain active during the 65°C rest releasing chains of β-glucose residues into the mash and increasing viscosity. The heat will cause the barley starch granules to gelatinise, and α-amylase and β-amylase in the mash will start to liquefy and saccharify the starch in solution. It is unlikely that 20 minutes will be sufficient for the breakdown of all the starch in the fragment, especially any parts encased in protein and β-glucan. The higher temperature will mean that cytolysis and proteolysis will also be incomplete.

4. The thick mash is then heated to boiling over 30 minutes and boiled for between 15 and 45 minutes before being returned to the main mash.

The boiling of the thick mash helps gelatinise and liquefy the starch in the fragment and break down the protein and cell wall material to an extent. This makes them more soluble and ripe for hydrolysis. The boil will denature any enzymes in solution so that no significant hydrolysis will occur.

5. The boiled thick mash is returned to the max mixer, increasing the temperature to approximately 50°C.

The temperature is low enough to avoid denaturing the proteolytic and cytolytic enzymes and promotes proteolysis so the hydrolysis of the pentosans, β-glucans, and proteins in the fragment can be catalysed by the enzymes in the thin mash. The β-glucans and pentosans are converted to fermentable material and dextrins while the proteins are changed into FAN. The remaining protein matrix and β-glucans are degraded to enable amylases to access the starch granules.

6. Once the mash is fully mixed and the temperature is even, the agitator is stopped and the mash allowed to settle again. A portion of the thick mash is then pumped to the mash kettle and brought to the boil over a 30-minute period, whilst the remaining mash in the mixer sustain the 50°C protein rest.

The undegraded starch in the fragment means that it will still form part of the thick mash, so it will be pumped over to be boiled again. The boil will halt any proteolysis and cytolysis but may help to physically break down any remaining proteins and cell wall material in the fragment. It is likely that unless the fragment is very large, almost all of the proteins and cell wall material will have been removed from the starch granule by now.

7. Following the ramp-up to boil, the mash is boiled for 30 minutes before being pumped back to the main mash. This raises the temperature to 65°C, to gelatinise and saccharify any starch.

On re-joining the thin mash, the starch, which should be well prepared for hydrolysis, will be degraded by the action of α and β-amylase. At 65°C, β-amylase will be close to the optimum temperature, so working rapidly while α-amylase is operating sub-optimally. However, it is likely that there will be sufficient α-amylase activity to keep β-amylase supplied with substrates. There should also be some limit dextrinase activity helping further saccharify the α-glucans. For our endosperm fragment, this step in the mash is the end of the line. Although it has gone, its breakdown products live on to be transformed into alcohol, CO₂, body, foam stabilising material and yeast cell structure.