EFFECT OF ALPHA-AMYLASE AND XYLANASE ACTIVITIES ON PROPERTIES OF FROZEN DOUGH PREPARED FROM FLOURS OF DIFFERENT WHEAT VARIETIES

摘要

This study was to evaluate alpha-amylase and xylanase activities in flours of different wheat varieties. Various physicochemical characteristics of flours were studied. Thermal properties of prepared doughs were measured from its freezing curves. Alpha amylase and xylanase activities were assessed for doughs during 70 days of frozen storage at -18±2ºC. Residual enzyme activities of pan breads made from frozen doughs were determined after 72 h of baking. Data revealed that, the variations in the proximate chemical composition among tested wheat flours could be attributed to the difference of wheat varieties and efficiency of milling process. Damaged starch (DS, ) was significantly (p≤0.05) higher in Shandaweel 1 flour (F1) and the mixture of F1 and F2 with 1:1 ratio (w/w) (F4) than in Sakha 94 flour (F2) and the local wheat flour from Amoun for milling Company (F3). Using of compressed yeast in dough preparation results in a considerable decline of freezing point which may be attributed to the difference in the amount of free water. The end freezing point ranged from -16ºC to -9ºC. Flours were significantly (p≤0.05) differed in their enzyme activities where, F4 had the highest α-amylase activity followed by F2, F3 and F1, respectively. Whereas, xylanase activity was higher in F1 than in other tested flours. Dough samples had higher enzyme activity comparing to flour samples that were made from. A significant (p≤0.05) increase in α-amylase activity was recorded by extending the frozen storage period of dough up to 70 days at -18±2ºC; while it declined significantly (P≤0.05) after bread baking as a result of the inactivation effect of baking temperature. Xylanase activity significantly (p≤0.05) increased during the first 14 days of dough frozen storage then decreased gradually till the end of frozen storage period owing to the increase of pH values. Also, xylanase activity has the same behavior as α-amylase for pan bread. There was a strong relationship between the storage time of dough and enzyme activities. The longer the frozen storage time of dough at -18ºC, the higher the decrease of bread residual enzyme activities. However, enzyme activities in the doughs and pan breads strongly depended on the flour and yeast types. Pan breads prepared from doughs with a high alpha-amylase activity had better specific volumes.