If you've ever experienced a weak and watery brew when using a very dark roasted coffee, the hot bloom method may help. The large amount of surface oils on dark roasted coffee, due to the extended roasting process, can make it difficult for water to fully wet the coffee bed during the brew cycle. There's nothing more frustrating than seeing sections of the coffee bed that are completely dry.
We've found that the "hot bloom" method can help you get a fully saturated coffee bed and an even extraction when using these dark roasts. You're simply pouring about 2 ounces of hot water (just off boiling) onto the coffee bed. This allows the subsequent dripping cold water to fully saturate the coffee bed and properly extract a cold brew.
Not sure if you need to do a hot bloom with your coffee? Simply sprinkle a teaspoon of the ground coffee onto the surface of some cold water. If a dry pile just sits on the surface, you need to do a hot bloom. If after 30 seconds, the coffee grounds begin to sink and are completely wetted, there's no need for the hot bloom.
In the photo(left), I've sprinkled a very dark roast onto two cups of water. One contains hot water and the other cold. This illustrates how the hot water has the ability to saturate the oily surface of the coffee grounds while simply using cold water does not.
Why this works...
Water's surface tension is at play here. Water molecules have cohesive forces that bind them to one another. For the water molecules on the surface, these cohesive forces are stronger. Rather than seep into small spaces, the water has a tendency to bridge the gaps as a result of surface tension. Any surface oils on the ground coffee further exacerbate this issue. As water increases in temperature, its surface tension decreases significantly. This allows the water to flow into tiny crevices rather than bridging the gaps leaving them dry.
"Is it still cold brew if you're hot blooming?"
From the testing we've done, we haven't noticed any significant changes in flavor when doing the hot bloom (still the smooth, low acidity brew that we've come to love). Since we're using such a small amount of hot water, it's really just saturating the bed. It's still the cold water and a whole lot of time that's doing the majority of cold brew extraction.
If you ever experience dry grounds, give it a try and let us know how it works for you!