Question about iodine test for starch conversion

[golson3]

Whenever I pull a sample from the mash for an iodine test, I always seem to get some grain particles in there, which gives me a false negative for full conversion. How do you all pull samples and test without having particles muddle your test?

 

[RM-MN]

Your wort should never test positive for starch. The starch that isn't converted will be in the grain particles. Unless your grains are milled very fine you likely will always test positive for starch. Longer mash times will convert more of this starch but possibly not all of it.

 

[jwelch1103]

I will tell you how I test for starch conversion, not saying it is the best method but it pretty much eliminates grain particles interfering with test interpretation and it has worked for me for at least a couple dozen batches.

I use a piece of white sidewalk chalk. Just like the chalk teachers used on the blackboard except it is much thicker, around 1-1.25" in diameter. First I form a nice flat surface on both ends (I can do two tests if needed then). When I am ready to test I use a thermometer probe to take a drop or two of wort and put it on the flat area on the chalk stick. The probe only holds a drop or two of wort and once it is on the chalk any minuscule grain particles are visible and I either remove it(which I did early on) or ignore any color change at that pin point (which is what I do now). Usually no particles "ride" with the wort but one or two occasionally make it onto the chalk.

The wort spreads a little and is absorbed into the chalk. Having the iodine tincture and an eye dropper at the ready I place a drop of the tincture on top of the wort and voila! If I see any darkening then conversion is not complete. If a second test is called for I use the opposite end.

After the chalk has dried I once again abrade the ends of the chalk on a smooth, hard flat surface.

 

[eric19312]

@jwelch1103 that sounds like a pretty good technique however I am not sure the grains darkening is really a false positive. I think it is showing that there is still starch available to be converted, only the starch hasn't yet been released into the wort. Yes you could stop here and lauter and not have to worry about unconverted starch in your wort. But if you are leaving starch behind in your husks then you are not reaching 100% conversion efficiency and ultimately brewhouse efficiency will suffer.

If you have a refractometer there is an easy way to test for conversion which depends on knowing the water to grist ratio in your mash and calculating expected specific gravity at 100% conversion. As you approach what you think is the end of the mash start testing the mash wort every 10 minutes until you reach expected gravity at 100% conversion. I find when I use this technique I usually end up doing a longer than expected mash (90 minutes at 151F might be typical on my system) and end up with 5-10% bump in brewhouse efficiency. Possibly this longer mash has other impacts on the beer (increased fermentability or potential for astringency both seem possible in theory) so I tend to use this strategy on bigger beers where my mash tun capacity is being challenged and for smaller beers just take the hit on efficiency and use a bit more grain.

 

[jwelch1103]

@jwelch1103 that sounds like a pretty good technique however I am not sure the grains darkening is really a false positive. I think it is showing that there is still starch available to be converted, only the starch hasn't yet been released into the wort. Yes you could stop here and lauter and not have to worry about unconverted starch in your wort. But if you are leaving starch behind in your husks then you are not reaching 100% conversion efficiency and ultimately brewhouse efficiency will suffer.

If you have a refractometer there is an easy way to test for conversion which depends on knowing the water to grist ratio in your mash and calculating expected specific gravity at 100% conversion. As you approach what you think is the end of the mash start testing the mash wort every 10 minutes until you reach expected gravity at 100% conversion. I find when I use this technique I usually end up doing a longer than expected mash (90 minutes at 151F might be typical on my system) and end up with 5-10% bump in brewhouse efficiency. Possibly this longer mash has other impacts on the beer (increased fermentability or potential for astringency both seem possible in theory) so I tend to use this strategy on bigger beers where my mash tun capacity is being challenged and for smaller beers just take the hit on efficiency and use a bit more grain.

That sounds like a very good idea. I have a refractometer and just started a mash literally 3-4 minutes ago. At the risk of sounding stupid or lazy, can you explain how I could calculate the expected gravity for my grain bill? I'd like to try that technique.

 

[eric19312]

here is the table (under conversion efficiency heading)

http://www.braukaiser.com/wiki/index.php?title=Understanding_Efficiency

For example, if you are mashing at 1.24 qt/lb you are looking to hit 1.096 SG in the mash tun wort before sparging. Need to be careful if you are using Beersmith with the way it calculates mash thickness. Best to check the math yourself and do the division, then look at the table and extrapolate.

 

[brandonlovesbeer]

Watch this video.
PS: a couple pieces of grain won’t change the results of the test.

 

[jwelch1103]

here is the table (under conversion efficiency heading)

http://www.braukaiser.com/wiki/index.php?title=Understanding_Efficiency

For example, if you are mashing at 1.24 qt/lb you are looking to hit 1.096 SG in the mash tun wort before sparging. Need to be careful if you are using Beersmith with the way it calculates mash thickness. Best to check the math yourself and do the division, then look at the table and extrapolate.

Thanks, I will look into this.

 

[eric19312]

Thanks, I will look into this.

So I gave the technique a try yesterday. Brewing a DIPA which was a stretch for my mash tun. Planned to mash at 148F for 90 minutes to get a highly fermentable wort. My grist was 38.2 lbs and my strike water was 11.75 gallons. This gave me 1.23 qt/lb. Looking at Kaiser's chart my target would be just a little over 22.8 Plato. I aimed for 23 Brix on the refractometer.

Going into the mash tun I noticed my crush looked a little light but the strike water was ready so I proceeded with what turned into a long brew day. At 90 min I was only at about 19.5 or about 85% converted so I stuck it out. At 110 min I decided to increase the heat to 158F (I use a direct fire recirculating mash tun). This did help somewhat and I finally settled for 22 Brix or 95.6% conversion at the end of 3 hours. After fly sparging for an hour I ended up with 20 gallons at pre boil gravity of 1.059 for mash efficiency of 83.5%.

Mashing at 148F I was prepared for a longer than usual mash but I think the main issue was probably the crush. You can see the very fast conversion even at 148 at the beginning. In first 35 minutes I'd converted nearly 70% of the total available starch. I expect these were the well crushed grains whereas the rest of the time was spent trying to mobilize the starches in the less well crushed grain.

In any case it was interesting example of the technique. I didn't do an iodine test but my guess is that quite early on in this mash I might have seen a negative iodine test if I looked only at the clear wort.

 

[TLaffey]

When I do an iodine test I don't see evidence of starch, but my efficiency hasn't been very good either. So this looks like something I want to try. But I wonder, with that long a mash, could you end up with lower than expected FG--and higher than expected ABV?

 

[eric19312]

When I do an iodine test I don't see evidence of starch, but my efficiency hasn't been very good either. So this looks like something I want to try. But I wonder, with that long a mash, could you end up with lower than expected FG--and higher than expected ABV?

Was aiming for highly fermentable wort. This is a DIPA. I was aiming for 8% ABV with FG around 1.010. I have 5% table sugar in the kettle. My OG was 1.069 (had been shooting for 1.072) so will need 85% attenuation or so to get there,

 

[Denny]

Whenever I pull a sample from the mash for an iodine test, I always seem to get some grain particles in there, which gives me a false negative for full conversion. How do you all pull samples and test without having particles muddle your test?

Which is one reason I say the test is worthless.

 

[ancientmariner52]

I can't agree with Denny. If the iodine can reach the starch, then the enzymes can reach the starch, so conversion isn't complete.

I use a little disposable coffee stirrer to take samples for iodine and refractometer tests. So cheap you can toss them after each use. Very little grain can get in.

 

[jwelch1103]

So I gave the technique a try yesterday....

OK, now I think I understand your point. Had you used an iodine test you may have stopped mashing at say 90 minutes and end up leaving a lot of convertible starch/sugars behind. Is that it?

 

[eric19312]

OK, now I think I understand your point. Had you used an iodine test you may have stopped mashing at say 90 minutes and end up leaving a lot of convertible starch/sugars behind. Is that it?

Yes exactly