Brewing and hitting the sweet spot.

If you’ve ever looked at Brewing Control Chart or had it tattooed on your body and have always been confused by it, or never bothered to look at it too closely, I am willing to try and parse it all out for you. You want to hit that sweet spot in the zone, otherwise the coffee which just successfully passed through the great chain of custody was just murdered in the brew.

The vintner has complete quality control up to the time of bottling - it is pretty easy to pour wine into a cup. Coffee requires that the consumer use a bit of their own craftmaship during preparation. There are a number of variables that can ultimately ruin a cup of coffee - temperature, weight, extraction time, etc.

Luckily, much of guess work in preparing a good cup of coffee has already been investigated by the Coffee Brewing Center (CBC) during the 1960's. Led by Dr. Earl Lockhart, his research has become the body of knowledge in truly understanding the physics and science behind coffee brewing.  By measuring the soluble coffee flavoring content in brewed coffee relative to brew formula, the CBC was able to graphically illustrate "solubles yield" given the coffee to water ratio. This work has provided the industry with the framework in which to discuss and compare coffee quality.

It really comes down to three questions:

1) Yield = How much did you get from the grounds?
2) Strength = How much ended up in the cup?
3) Ratio = How did you get there?How the chart works

Here, 'strength' means how much of the coffee beverage is actually coffee, so 1.25 on the scale above indicates that 1.25% of what you're drinking are coffee solids dissolved in the water. 'Extraction' means what fraction of the original dry ground coffee has ended up in your cup. For example, if you start with 5 oz of ground coffee, and 1 oz dissolves during brewing, then the extraction is 1/5 or 20%. The red diagonal lines show how much coffee you started with -- for example, the line labeled 3.75oz (106 grams) means you put that much ground coffee into the brew basket. On this particular chart, the brewing formula always assumes 1/2 gal (1.9 liters) of hot water with each coffee weight.

Remember that the brew chart applies to the actual amount of water you pour over the coffee grounds. If you want to end up with 64oz of brewed beverage, you'll need to start with more water -- about 70 oz in this case. To maintain the 4oz/half gallon ratio, you would need to increase the weight of coffee to (70/64) * 4oz = 4.37 oz.

A complete brew analysis looks also at the temperature of the brew water and how long the water is in contact with the coffee, and includes assessments of how fine or coarse the coffee is ground, the bed depth of ground coffee in the brew basket, and how well the spray head is wets the grounds.  

For the algebraically inclined, the calculations assume that about 1.5oz of water per oz of ground coffee will remain in the basket after brewing. So if you start with 4 oz of ground coffee, pour 1/2 gallon of hot water over it, and measure the strength to be 1300 ppm, then the extraction is just:

{[64oz water - (4oz coffee x 1.5oz water/oz coffee)] x .0013}/4oz coffee ~ 19% extraction




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