A kinetic study on the thermal inactivation of barley malt α-amylase and β-amylase during the mashing process.

To obtain an efficient conversion of starch into fermentable sugars and dextrins during the brewing process, mashing time-temperature profiles need to promote starch gelatinisation and enzyme activity while avoiding thermal inactivation of the amylases. This study focused on the second part of this balance by investigating the thermal stability of α-amylase and β-amylase of Planet barley malt throughout mashing. Thermal inactivation in wort was modelled for both enzymes resulting in the estimation of thermal inactivation kinetic parameters such as rate constant of thermal inactivation k (the rate of thermal inactivation of an enzyme at a constant temperature), activation energy for thermal inactivation E, decimal reduction time D (the time needed to inactivate 90% of the enzyme activity at a given temperature) and the z-value. First-order inactivation was observed for α-amylase. For β-amylase, fractional conversion inactivation occurred with a residual fraction of 13% of the β-amylase activity that remained after prolonged heating at 72.5 °C. The β-amylase protein population hence seems to consist of thermolabile and thermostable isoforms. The kinetic parameters for thermal inactivation of the enzymes were used to predict their residual activities throughout a laboratory-scale mashing process. The predicted residual activities met the experimentally determined residual enzyme activities closely, except for β-amylase at temperatures higher than 72.5 °C. The results obtained in this work allow designing new mashing processes or tailoring existing processes towards variability in the input material, barley malt, without the need for trial-and-error experiments.

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