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Gluten Free Home Brewing Blog

How To Calculate Rice Hulls For Proper Circulation & Filtration

By GFHB  -  January 2nd, 2018

Rice hulls provide an important function during the mash and sparge process by providing proper circulation in the mash and filtration during the sparge. In November we revised our recommendation from adding 20-25% rice hulls to a grain bill, to just 10-15%. At minimum, you want to use enough rice hulls to prevent a stuck mash, and we have found that 10-15% provides sufficient circulation and filtration. That does not mean you need to shy away from using a bunch of rice malt in your grain bill! Several of our all-grain recipe kits have over 30% rice hulls in the grain bill. Malts like buckwheat, corn and millet have very minimal hulls and will benefit from the addition of rice hulls. However, if you use a few pounds of hulled rice malt in your grain bill, the odds are the rice malt is providing sufficient rice hulls for the other malts.

There are two ways to calculate the rice hulls in your grain bill. We now know the grain to hull ratio of rice malt is approximately 62.5% grain to 37.5% hull. But how do you figure out how much of your grain bill consists of rice hulls? One way is to work backwards starting with your total grain bill and then calculating how much of it is rice hulls. Here is an example of a grain bill for a 1 gallon batch we brewed yesterday:

1.25 LB Naked Pale Rice Malt
1.25 LB Naked Biscuit Rice Malt
1 LB Amber Rice Malt
9.6 OZ Crystal Rice Malt

The dehulled or "naked" rice malts do not contain hulls. So that leaves us with the Amber and Crystal rice malts.

1 LB (or 16 OZ) Amber Rice Malt - 37.5% = 6 OZ
9.6 OZ Crystal Rice Malt - 37.5% = 3.6 OZ

So that means the grain bill contains 9.6 OZ of rice hulls. A 3.5 LB grain bill needs at least 5.6 OZ of rice hulls (10-15%) to ensure proper filtration and circulation. So we are covered for this recipe!

Another way is to write your grain bill assuming all the malt you are using does not contain rice hulls, then adding up the rice hulls for the malt that does. Here is the same example grain bill using this method:

1.25 LB Naked Pale Rice Malt
1.25 LB Naked Biscuit Rice Malt
10 OZ Amber Rice Malt
6 OZ Crystal Rice Malt

Now lets figure out how much rice hulls the Amber and Crystal rice malts contain.

10 OZ Amber Rice Malt X 60% = 6 OZ
6 OZ Crystal Rice Malt X 60% = 3.6 OZ

So that means the math works out the same, and we've got this recipe covered!

We prefer the second method because it allows us to know how much malt, with no rice hulls, that we are working with for any given recipe. Generally we find that even if a grain bill is a little heavy on malts other than rice malt, the rice malt present in the recipe helps provide much of the needed rice hulls.

1.21.18 Update: We test brewed a few high percentage "naked" rice malt grain bills in a row and experienced a stuck mash each time at various times during the mash using only 10% rice hulls. Our camera does not do a very good job at photographing really small bits of spent grain, but we can describe our observations and offer a simple solution. If you have ever cooked rice before you are familiar with what cooked rice looks like. When mashing with milled rice malt, you get partially "cooked" bits of rice. As compared to millet and buckwheat malt that releases the soft powdery starches inside the hull of the grain. Therefore our recommendation when brewing with a high percentage "naked" rice malt  grain bill is to add 15% rice hulls to the grain bill to ensure circulation and filtration. We have brewed countless beers with hulled rice malt and never had a stuck mash because hulled rice malt naturally contains 37.5% hull. So when do you use 10% or 15% rice hulls in a grain bill when using rice malt? We think you can safely use 10% rice hulls when rice malt is about 1/4 or less of your grain bill. But without conducting numerous test batches we will have to guess where that threshold is when you need to increase to 15% rice hulls. Our best guess at this time would be when using more than 1/4 rice malt in your grain bill, up the rice hulls to 15%.


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