A kinetic study on the thermal inactivation of barley malt α-amylase and ß-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 ß-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 kT (the rate of thermal inactivation of an enzyme at a constant temperature), activation energy for thermal inactivation Ea, decimal reduction time DT (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 ß-amylase, fractional conversion inactivation occurred with a residual fraction of 13% of the ß-amylase activity that remained after prolonged heating at 72.5 °C. The ß-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 ß-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.

Starch hydrolysis during mashing: A study of the activity and thermal inactivation kinetics of barley malt α-amylase and ß-amylase.

Hydrolysis of starch is key in several industrial processes, including brewing. Here, the activity and inactivation kinetics of amylases throughout barley malt mashing are investigated, as a prerequisite for rational optimisation of this process. Varietal differences were observed in the activity of α- and ß-amylases as a function of temperature for six barley and malt varieties. These differences were not reflected in the resulting wort composition after mashing, using three isothermal phases of 30 min at 45 °C, 62 °C and 72 °C with intermediate heating by 1 °C/min. Thermal inactivation kinetics parameters determined for α- and ß-amylases of an industrially relevant malt variety in a diluted system showed that enzymes were inactivated at lower temperatures than expected. The obtained kinetic parameters could predict α-amylase, but not ß-amylase inactivation in real mashing conditions, suggesting that ß-amylase stability is enhanced during mashing by components present or formed in the mash.