Refractometer readings for glucose and fructose solutions

[sikaha]

Assume I have two 100 ml solutions. One has 20 grams of fructose, and the other has 20 grams of glucose. When I use a refractometer to measure the Brix degrees, do I need to convert the output to different values using some conversion table, or is this output correct? I ask this because the refractometer typically measures sucrose.

My readings are:

20g fructose in 100 ml solution: 20.2 bx

20g glucose in 100 ml solution: 18.7 bx

20g sucrose in 100 ml solution: 20 bx

In other words, can I say that 20g of fructose is sweeter somehow?

 

[rburrelli]

Refractometers don’t measure sweetness. How are you making your assumptions?

 

[sikaha]

I might be wrong, but don't we expect a liquid with 20 °bx to be perceived sweeter than a liquid with 15 °bx (Based on the Brix definition, which says 1 °bx refers to 1g of sucrose in 100ml solution).

 

[IslandLizard]

In other words, can I say that 20g of fructose is sweeter somehow?

Sweeter than what?
For example, sweeter than 18 grams of fructose in the same amount of water? Yes!

And no, as @rburrelli already said:

Refractometers don’t measure sweetness. How are you making your assumptions?

Refractometers measure light refraction of solutions:

My readings are:

20g fructose in 100 ml solution: 20.2 bx

20g glucose in 100 ml solution: 18.7 bx

20g sucrose in 100 ml solution: 20 bx

So, fructose has a (slightly, 1%) higher refractive index than sucrose, and a quite a bit (8.0%) higher than glucose.

 

[dmtaylor]

Assume I have two 100 ml solutions. One has 20 grams of fructose, and the other has 20 grams of glucose. When I use a refractometer to measure the Brix degrees, do I need to convert the output to different values using some conversion table, or is this output correct? I ask this because the refractometer typically measures sucrose.

My readings are:

20g fructose in 100 ml solution: 20.2 bx

20g glucose in 100 ml solution: 18.7 bx

20g sucrose in 100 ml solution: 20 bx

In other words, can I say that 20g of fructose is sweeter somehow?

You can only conclude that aqueous solutions of fructose, glucose, and sucrose bend light to a slightly different extent. Nothing to do with sweetness at all.

 

[sikaha]

Thank you all— @rburrelli, @dmtaylor, @IslandLizard.

 

[IslandLizard]

I might be wrong, but don't we expect a liquid with 20 °bx to be perceived sweeter than a liquid with 15 °bx (Based on the Brix definition, which says 1 °bx refers to 1g of sucrose in 100ml solution).

If they're solutions with the same sugar compound, yes.
But not when they're different sugars, some sugars taste sweeter than others, so less is needed to taste just as sweet.

https://en.wikipedia.org/wiki/Sweetness

 

[Elysium82]

You can only conclude that aqueous solutions of fructose, glucose, and sucrose bend light to a slightly different extent. Nothing to do with sweetness at all.

Hi. What does this mean in terms of wort then........when it comes to converting the brix value of different types of beers? Surely a stout and an IPA have very refractive properties, right?

UPDATE: Never mind. I have just found the answer to my question here (Refractometers - Brew Your Own) in the "wort calibration" section.