Stratification of dissolved solids?

Yes, I know that it's technically not possible for a true solution to have stratification of the dissolved solids, as the essence of dissolution is a regular and equal amount of the dissolved solid throughout the liquid, regardless of where in the liquid a sample is drawn, but empirical data leads me to suspect that with hot wort, something is going on. I constantly get low hydrometer readings out of the boil kettle before boiling unless I seriously agitate the wort before taking a hydrometer sample. It can sometimes bump up my hydrometer reading by 5 or 6 points. Why is that?

Are temperature differentials to blame? Can hotter wort carry more suspended sugar than cooler wort? And, given the volume of wort, the shape of the container, and its approximate temperature is it possible to take a sample from the top of the brew kettle and correct for the effect of uneven distribution of sugars throughout the liquid?

I only ask because I keep forgetting to agitate before taking my SG, and then I get all bent out of shape when I realize the gravity is 5 points too low, and then it dawns on me during the boil that I never agitated, and then I take a sample during the boil. Most recently I cracked a class measuring cup doing that, but it proved that, after the boil had been underway for about 15 minutes, my SG was 6 points higher.



Before anyone goes there, I cooled all samples to 59F, the calibration temp for my hydrometer, and the same hydrometer was used for both readings. Yes, I am sure I took the sample correctly. Also, it was indeed an all-grain recipe, I know this is kind of a known issue with extracts.

Is stratification of dissolved sugars a possibility in wort, perhaps owing to temperature stratification or some other phenomenon? Or is my hydrometer making this **** up and laughing at me behind my back?



I'd love to hear informed opinions and some people with similar or different experiences.

Temperature differentials and concentration: cold wort of higher concentration is denser than hot wort of lower concentration. It is, especially where one is looking at the wort just collected from a sparge, important to thoroughly stir before taking a hydrometer sample. Or wait until the boil starts. That mixes things up pretty well.

Unless you mix things well, i'm not surprised that you'd have different concentrations of sugars at various levels, especially since your first runnings are going to be densest and also go into the kettle first, so they start at the bottom. You hit the nail on the head when you said if it was a "true solution" it wouldn't have any stratification. Until you mix it well, its not a true solution.

As for temp differences, yes, solubility is a function of temperature. For solids, solubility goes up as temp goes up, for gases, it goes down. But I'm guessing that incomplete mixing is all you're seeing here.

The amount of sugar you can dissolve in water is usually far more than you put into your wort. You can dissolve 4.2 grams of sucrose per ml of water at 90 degrees C [1][2] and it would be even higher at boiling. I couldn't find figures for maltose but I would have to imagine they would be pretty similar. If you put 7 lb of malt extract in a 3 gallon partial boil of wort, you would get .28 g/ml of maltose. This is far far below the saturation point.

How rapidly is your wort boiling. The following is rampant speculation but bear with me. In a boiling liquid you have small bubbles form on the bottom of the pot. These bubbles break free of the pot and grow as they merge with other bubbles and the wort next to them changes to steam. When the water changes to steam it leaves behind the malt and other stuff in solution. The wort right next to the bubble will be denser until it mixes with the rest of the wort. Fluid dynamics are complex and chaotic. I would expect all that dense wort to mix as quickly as it is created but really who knows. </rampant speculation>

What i have not considered is the solubility of the myriad other chemicals that occur in the wort and how those might affect things. I would assume that they are in small enough concentrations to not throw things off as badly as you are seeing but who knows.

sources
[1]http://en.wikipedia.org/wiki/Sucrose#Physical_and_chemical_properties
[2]http://chestofbooks.com/food/science/Experimental-Cookery/The-Solubility-Of-The-Sugars.html