Sugar

Grain size distribution of sugar in the laboratory or directly in the crystallisation reactor

Sugars belong to the group known as carbohydrates and constitute a significant part of the human diet. The economically most important sugar is the industrially manufactured sucrose (table sugar), which is used as a sweetener and fast supply of energy. Sucrose consists mainly of sugar cane or sugar beet. Sucrose is a disaccharide (double sugar), consisting of the joined monosaccharides (simple sugars) glucose (grape sugar, dextrose) and fructose (fruit sugar). In addition to sucrose, glucose and fructose, the disaccharide lactose (milk sugar) – which is extracted from whey – is also an important additive or auxiliary material in the food and pharmaceutical industry. Disaccharides and monosaccharides have a sweet taste, are generally highly soluble in water and are usually marketed as white crystals or crystalline powder with various grain sizes for industrial further processing or direct consumption. The smaller the grain size, the faster they dissolve in aqueous media. Sugars with fine crystals are particularly well suited for mixtures with other powders, whereby a narrow particle size distribution also counteracts segregation.

The grain size analysis is an important criterion for the final product quality and therefore also a target and control indicator in sugar production. The extraction of crystalline sugar is performed within the scope of a crystallisation reaction. Uniform crystal formation is stimulated by the addition of seed crystals. Mixers keep the pulpy mass in motion, and the sugar crystals grow to the desired size in the concentrated solution. The crystals are then filtered or centrifuged, washed and dried.

The diagram shows the results of an in-situ measurement with an OPUS ultrasonic sensor for four consecutively running batch crystallisations of pharmaceutical grade glucose. The growth of the crystal size from an average of approximately 50 microns to nearly 350 microns (x50 value) in batches #1, #3 and #4 is clearly apparent. The volume concentration of the crystalline solid increases from a few percent to around 50 volume percent. Batch #2 has a considerably coarser average final grain size of the sugar crystals of over 450 microns for a significantly longer service life.

The most common form of household sugar is white crystal sugar that has been purified through refining – fine crystals of around 400 microns are typical. Powdered sugar or icing sugar with an average particle size of approximately 50 microns is produced by grinding refined white sugar. Its fine crystals can no longer be sensorily perceived and are used as a basis for fine desserts or for icing.