Lesson 2 - Mashing Biochemistry
This lesson deals with the biochemical aspects of mashing. We will meet the main substrates and the enzymes that break them down. We will learn what enzymes are, how they operate and how they are affected by the environment of the mash. Finally, we will cover how starch is gelatinised, liquefied and saccharified to make a wort with the desired sugar spectrum.
α-Glucosidase
α-Glucosidase (also called maltase) is an exoenzyme that has a minor impact on saccharification and no impact on liquefaction.
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It can only catalyse the hydrolysis of α-(1,4) glycosidic bonds.
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It catalyses the hydrolysis of starch, polysaccharides and maltose starting only from the non-reducing end and produces only glucose.
The impact of α-glucosidase in mashing is comparatively minor, with wort typically containing around 10% glucose and 50% maltose.
As β-amylase and α-amylase cannot catalyse the hydrolysis of the α-(1,6) bonds nor the α-(1,4) bonds close to them, we can be (and often are) left with polysaccharides known as limit dextrins. Fortunately, there is a barley enzyme that can help reduce these in number!
Limit Dextrinase
In order for the amylases to catalyse the hydrolysis of the α-(1,4) bonds adjacent to the α-(1,6) branch points, we must hydrolyse the α-(1,6) bonds. Enter limit dextrinase. Limit dextrinase is also known as a de-branching enzyme as it catalyses the hydrolysis of the α-(1,6) bonds where the chains of glucose residues branch off. The action of limit dextrinase thus provides more non-reducing ends for the action of β-amylase.
Until relatively recently, limit dextrinase was thought to play a very minor role in starch hydrolysis in the mash. Evaluation of the heat stability of the enzyme was undertaken using a purified enzyme that exhibited greater heat sensitivity than the enzyme in standard mashing conditions. We now know that our earlier view was wrong and at standard saccharification temperatures, limit dextrinase is significant in determining the fermentability of wort by providing more non-reducing ends for the action of β-amylase.
Figure: Enzymatic hydrolysis of starch
The starch degrading enzymes work synergistically to cause the breakdown of starch with the action of ɑ-amylase and limit dextrinase, increasing the concentration of substrates of β-amylase and α-glucosidase.