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August 22, 2007
Brix is a unit of measurement named after Adolph Brix. It is used commonly by food technologists to measure the amount of sugar dissolved in water. It can be calculated by dividing the dissolved solids by the sum of the dissolved solids plus the water, all multiplied by 100. That is, Bx = (Ds x 100)/(Ds + w), where Ds is the weight of dissolved solids and w is weight of water. (Note that suspended solids do not enter into the equation for calculating Brix.)
Brix worked with the fact that dissolved solids in water cause light to refract (bend), with greater refraction being caused by higher concentrations of dissolved solids. Refraction is measured with a refractometer, and most refractometers are calibrated to read in degrees Brix (Bx).
An appreciation of dissolved solids is vital in understanding what can be achieved in a screw press. This is detailed in Pressing News #169, Material Balance.
To test Brix, we took two 100 gram samples, one each of sugar and of salt, and placed them, respectively, in 900 grams of water. We stirred, and in both cases we ended up with one kilo samples of clear liquid that looked like water but certainly did not taste that way. We dried samples of these two liquids in an oven, weighing before and after drying, and both calculated out at 10% solids, 90% moisture.
When we put a drop of the sugar water on the lens of a refractometer, it measured 10º Bx. We tried a drop of the salt water on the refractometer, and we were surprised it also read 10º Bx. Who would have guessed that inorganic salt molecules would have the same effect as organic sugar molecules? Thus we learned that refractometers measure total dissolved solids, not just dissolved sugar.
Clearly, if we pour these liquid samples into a screw press, regardless of any screen option we can think of, we will never form a bit of press cake. A screw press cannot separate dissolved solids.
Screw presses are used where suspended solids (sometimes called insoluble solids) are present. The suspended solids are most commonly organic fibers. Frequently the material to be fed to a screw press has both dissolved solids and suspended solids. A common example is orange peel which typically has 80% water (moisture) and measures 10º Bx. This works out to 9% dissolved solids (mostly sugars) and 11% suspended solids.
Another example is onion, which is typically 91.5% water and measures 7º Bx. Onion has only 1.5% suspended solids, so relatively small amounts of press cake are produced when onions are run through a screw press.
When waste orange peel or onion is run through a screw press, the press liquor is going to be loaded with BOD. That is, it will be about 9% and 7% dissolved solids, respectively, in addition to any suspended solids that were forced through the screen of the screw press. This can represent a very large load on the wastewater treatment plant. Many waste dewatering projects are abandoned after considering the need to dispose of such press liquor.
PS If your kid is hunting for a project for the school science fair, give him this one. We will lend a refractometer.
Issue 190