engleski

Determination of optimal processing particle sizes and physical properties of Stevia rebaudiana (Bertoni) powder

Stevia rebaudiana (Bertoni) glycosides are extensively used as a high intensity sweetener with low calorie value. Glycosides are extracted from leaves to obtain steviosides and rebaudiosides as sweet components. According to EU and Croatian regulations, ground leaf powder is only to be used for extraction of glycosides. Besides solvents and extraction conditions, particle size and physical properties also influence the extraction process. In this research, dry Stevia rebaudiana (Bertoni) leaves were ground to a desired particle size and particle size fractions were separated by sieving. Obtained fractions were subjected to laser diffraction particle sizing in order to determine median diameter, Sauter diameter and particle size span. Fractions were also analysed by a powder rheometer to define powders cohesion and caking properties. A jolting volumeter was used for bulk density determination and reconstitution properties were determined by quick test methods. Experimental data was analysed by Spearman Correlation Coefficients. Sieving analysis resulted in five particle size fractions. Smaller particle size fractions exhibited typical bulk powder behaviour, while larger fractions contained a large amount of fractured leaves, resulting in a tea-like bulk rather than a powder. All the fractions were categorized as easy or free flowing based on cohesion index values. Cake strength values showed a significant increase (p<0.05) with an increase in particle size. Fraction above 800 µm could not be tested rheometrically, due to large amounts of air entrapped between leaves, which interfered with powder rheometer blade. Wettability times ranged from approximately 8 to 10 minutes, while dispersibility times ranged from 11 to 15 seconds. These results confirm the significance of the milling process and the particle size on the physical properties of the stevia powder. They also point to the possible importance of these parameters on the efficiency of extraction procedures, which present an interesting aspect of future research.

Stevia rebaudiana (Bertoni); powder; milling; particle size; physical properties

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