HYDROTHERMAL TREATMENT OF STARCH IN PRESE NCE OF ALPHA-AMYLASE - PART4 - CHANGES OF PROPERTIES OF POTATO STARCH BY HYDROTHERMAL ENZYMATIC TREATMENT

Aim of this investigation was to alter functional properties without c hanging the granular structure of potato starch by hydrothermal enzyma tic modification under conditions of excess of water (annealing). Pota to starch can be modified hydrothermally without addition of alpha-amy lase but annealing needs a number of days. In order to accelerate this procedure, higher temperatures have been used favouring the formation of an irreversible swollen starch portion which can be degraded enzym atically by bacterial alpha-amylase. But, the resulting spectrum of ca rbohydrates comprising apparently 50% of maltose protects the granula of potato starch only at higher concentrations. Hydrolytic degradation by bacterial alpha-amylase during annealing causes therefore strong d amage of potato starch granula. This agrees with results of electron m icroscopical investigations on starch granula. which show large caviti es after the modification procedure. Furthermore, high temperatures al ter reactions of gelatinization and gelation behaviour in a characteri stic manner, for instance by a strong decrease of the hot past consist ency at 96 degrees C. Hydrothermal enzymatic modification of potato st arch 24 hours long at temperatures up to 70 degrees C causes a dramati c decrease of the Brabender consistency in comparison to untreated sta rch. The consistency at the beginning of the keeping phase at 96 degre es C amounts only to 150 BE and increases to 390 BE at the end of the cooling phase at 50 degrees C. However, the material does not swell fu lly. The difference in the DSC swelling temperature amounts to apparen tly 13 K. The gain of modified starch is increased from 35% up to 48% without changes in properties, if the temperature in the final keeping phase of the annealing procedure is not higher than 66 degrees C. The results confirm that the used annealing conditions in presence of alp ha-amylase are not simply transferable to a hydrothermal enzymatic tre atment of potato starch. To change functional proper ties of starches of different botanical origin distinctly one has to adopt the conditio ns of annealing to the specific structure of the corresponding starch. A hydrothermal enzymatic modification of potato starch is inappropria te for achieving starch preparations with an increased set back.