Industrial scale roasting companies tend to operate with lower quality coffee. Usually, lower quality coffee has a larger spread of under-ripe, overripe coffee, and defective coffee cherries. (This article examines chemical differences between underripe, and overripe coffee cherry.) From a sensory perspective, roasting darker with poor quality coffee is advisable because it reduces the impact of these defective flavours, trading off sweetness, acidity, and fruitiness with more bitterness as a result. When you are brewing with darker roasted coffee, you may find some sensory advantages in pulling shorter shots. This approach saw the ristretto become a popular beverage in the early days of specialty coffee when dark roasting was the norm.
The solution to the excess of bitterness in dark roasts is either a lot of milk, a shorter beverage weight and shot time, or a combination of both. The shorter beverage weight and shot time, usually at a ratio of between 1:1 and 1:1.67, is how we define a ristretto.
Take the example of a strong ristretto recipe with a 21g dose producing a 30g yield. We can use coffee algebra to estimate what TDS we need if we want to extract the coffee into Earl E. Lockhart’s optimum extraction region of 18–22%. Again we use the TDS equation:
Extraction yield % x dose
____________________ = TDS
The calculation looks like this:
18% x 21g
_________= 12.6% TDS
So the minimum strength to extract a ristretto into the 18–22% range on this ratio is 12.6%
Our experience with brewing ristrettos has shown us there is an extraction ceiling we can’t get past once the brew ratio passes a certain point. We identify this point as the 1:1.67 ratio, where the highest yield possible with this ratio is 18%. If you brew at higher ratios than this, you’ll be unable to navigate into Lockhart’s optimum extraction range at all,