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The hypothesis that maltose would increase the thermostability of barley malt β-amylase activity during isothermal mashing was tested. Malts of the two-row cultivar Harrington and the six-row cultivar Morex were incubated in maltose concentrations of 0–500 mM at 68, 73, and 78°C and compared with isothermal mashing at 63°C. Incubations with additions of mannitol from 0 to 500 mM at the same temperatures were used to compare with incubations containing maltose. Incubations were sampled every 30 min for 120 min, and β-amylase activities were determined. In the absence of exogenous sugars, extracts of Harrington had higher initial β-amylase activities than did extracts of Morex at 63, 68, and 73°C but not at 78°C, for which the reverse was observed. In the absence of exogenous sugars, extracts of both cultivars had the highest activities at 63°C, and both exhibited maximal responses to sugar additions with 500 mM maltose (889% increase for Morex and 541% for Harrington) compared with 0 mM controls at 63°C. Activities in extracts of both cultivars were significantly influenced by maltose additions starting at 50–100 mM and as early as 30 min at 63°C, and these effects were pronounced at and above 200 mM maltose. The maximum % survival of β-amylase at 120 min compared with 0 min for both cultivars occurred at 63°C (85–93% for Morex and 90–91% for Harrington) when incubations were with 400 and 500 mM maltose. A comparison of β-amylase thermostability between the cultivars was calculated as the % activity detected under the most favorable condition (0 min at 63°C) and that detected under the least favorable condition (120 min at 78°C). Our study showed that β-amylase in Harrington extracts was more thermostable than that in Morex extracts (10.1 and 5.9%, respectively). β-Amylase in both Harrington and Morex extracts showed some increases in activity and thermostability when mannitol was included in incubations. Increases were most pronounced at 63°C with 500 mM mannitol. However, these effects were not as great or as consistent as seen with maltose additions to the incubations. The extent of the maximum response to both maltose and mannitol on the thermostability of β-amylase from Morex malts was 60% greater than that of the β amylase from Harrington malts. Keywords: β-Amylase, Barley, Compatible solute, Maltose, Mashing, Osmolyte, Thermostability
