This is a question I was not able to answer in a recent BBR interview: Do the starches, that are present in wort but are not converted yet, contribute to the extract content that is measured with a refractometer?
So I set out to test this in a quick experiment. I mixed 6.5 g corn starch with 116 g water and heated that in the microwave until it boiled. The result was a gooey mass of gelatinized starch. When I tested this paste with the refractometer the line was very blurry around about 5 Brix. The total starch content was 5.3 % by weight. Based on that it seems that gelatinized starch does change the refractive index of a solution.
flitering the cold mash to obtain amylase enzymes but no additional starch
To get this solution more liquid and a clearer refractometer reading I had to add amylase enzymes. The easiest way to get them for a brewer is to make a cold malt extract. This means making a cold water mash and filtering it. The resulting wort had a extract content of about 1.6 Plato and I added 76g of this wort to the gooey starch mass which lowered its temp to ~60 C. Almost instantly the viscosity dropped and the solution became liquid.
If you do the math the resulting solution contains has about 3.8 % w/w solids. Immediately after mixing the solution tested at 3.2 Brix but 6 min later it tested at 3.6 Brix, which is about 3.7-3.8 Plato and close enough to the expected extract content while still showing a positive, albeit reduced, iodine reaction. I did not wait for the iodine reaction to completely disappear.
iodine test results after different rest times
This experiment shows that gelatinized starch also contributes to the mash extract that is measured with a refractometer.
The difference to an actual mash is that in an actual mash not all the starch is pulverized and immediately mixed with all the mash water as it was in this experiment. A lot of starch is inside grit particles and it takes time for this starch to be released into the sweet wort or to be converted inside the grit particles and for the resulting sugars and dextrins to diffuse into the wort. The starch present in the wort is converted within 20-30 min but there is a continued release of sugars into the wort which causes the increase in mash extract content that can be seen even after the iodine test is negative.
Have you read “Brewing” by Lewis and Young? It has a nice graph of extract and for eight mashes at different temperatures (although it only shows relative temperatures), and the authors mention that gelatinization – not alpha amylase – is responsible for extract. Thanks for validating it!
No, this is a book I have not read yet. But good to know that there are also literature sources that state this. Most other literature sources I have read credit a-amylase for the creation of extract. This might be true for grit pieces where the starch never leaves the but is converted to more soluble sugars and dexrtrins which then diffuse into the sweet wort.