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Extraction of Sugar by Diffusion

Removing soil from beets
The first step when the beets enter the factory is to remove any soil which is clinging to the beet. This is accomplished by spraying the beets with high-pressure water sprays as well as passing the beet through a series of cylindrical tumble washers and rock catchers which will catch and rocks or large dirt clumps that could damage equipment and would dull the slicing knives.

The next step in the “diffusion” process is slicing. In order to extract the soluble materials from the insoluble materials in the sugarbeet we must first expose a maximum of surface area of the sugarbeets to the extracting solution (hot water). This is done by slicing the sugarbeets into thin rectangular strips called “cossettes”, which resemble shoestring potatoes. This is done in large drum slicers, a type of industrial grater, set with several pairs of rotating opposed groove knives.

Slicing beets into Cossettes
The cossettes are then conveyed across a weightometer, which feeds back information that controls the rate of slicer activity, and into the cossette mixer. Here the cossettes are scalded with hot juice to sterilize them and to break down the cell walls in preparation for extraction of the juice, and then pumped to two diffusers. The two diffusers used at SMBSC, both counter current mixing devices, are called tower diffusers, each rated at 8,000 metric tons capacity per day (combined capacity of 17,600 short tons). Hot water is introduced at one end of the diffusers and the cossettes at the other. The cossettes are propelled upward by a large, vertical screw conveyor.


The purpose of the counter current design is for the cossettes to continuously come in contact with water which has a lower sugar content than that left in the cossettes as the cossettes continue to lose sugar. What actually happens is a complex process involving osmosis, diffusion, and active transport through and between denatured and ruptured beet cells and beet cell membranes, and the concentration gradients of the water flowing in the opposite direction. What is important is that out of the opposite end of the diffuser from which they entered, come nearly sugarless cossettes. Out of the end from which the cossettes entered, and opposite where the hot water entered, emerges the sugar extracted from the cossettes contained in a “raw juice” which also contains many other soluble beet materials in addition to the sugar, all dissolved in the hot water.

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