HOW TO GRADE GREEN COFFEE
Coffee beans are an agricultural product and like any agricultural product, you get variance in the size, colour, and shape. Each country that exports coffee has its own classification system to outline the different grades (types) of coffee grown there. They do this by coming up with a set of criteria to define the different types of coffee produced.
This set of criteria is usually applied to a 300g sample of coffee – although, this can vary, for Colombia coffees for example, a 500g sample will be used. Let’s use a microlot from Brazil as a case study. Assume that there are three bags of this coffee. If we take 100g samples from each bag to create a sample of 300g, this sample can then be graded and acts as a representative of the coffee as a whole.
Once you have the sample in front of you, the grading process will begin. The main things to look out for when grading coffee are; the quantity of defects/foreign matter, the size of the bean, and the moisture content.
Each classification will divide a sample of coffee into grades according to the number of defected beans allowed. A defected bean is one that we would not consider to be a regular bean e.g. a bean that is broken or insect-ridden. A Grade 1 Sumatran coffee will allow up to 11 defects, so if you find 12 defects in the sample, it will fall in to the Grade 2 category.
Defects can affect both the appearance of the green bean (its uniformity) and the final cup profile. If, for example, 20% of your coffee sample is comprised of broken beans, this will make the coffee hard to roast and it will roast unevenly. One black bean or sour bean can ruin the flavour of a cup of coffee. Therefore, if you’re serious about the quality of your coffee, it is fundamental to be aware of the importance of defects and how many are permitted in each grade.
If you’re buying an Ethiopian Djimma Grade 5, for example, you know that numerous defects are permitted in that grade and that is reflected in the price. If, on the other hand, you are buying an Ethiopian Yirgacheffe Grade 2, you would expect to find very few defects.
Depending on the quality of the coffee that you are grading, the classification might mention if the coffee will be free from foreign matter (anything that is not from the coffee cherry e.g. cherry-sticks, stones, nuts etc.) or how much foreign matter to expect. Specialty coffee will usually be free from foreign matter but in some lower grades there will be a tolerance that will allow for some to be present in the sample. Some commercial coffees are stored in dirty warehouses or next to other products, so it is not uncommon to find kernels of corn or sweepings from the floor in the lower grades.
Screen (SCR) Size:
This measures the size of the coffee beans. If a bean is sold at a certain size, SCR 18 for example, around 0-10% (the exact figure depends on the classification) of the sample might be smaller than the size outlined. The most common tolerance is 5% below the size stated. The typical way of checking your SCR size is by pouring your sample through a screen with holes of a particular size. If, for example, you pour 100g through a screen that has holes of SCR 18 and 3g fall through the holes, this means that 3% of your coffee is below SCR 18. When you buy coffee at a certain SCR size, such as SCR 18, you’re paying more to get coffee that is above SCR 18. If you get a delivery and 30% of the coffee is below SCR 18, you’re not getting your money’s worth.
SCR size is particularly important for roasters who are looking to pre-blend coffees before roasting, because if you have coffee beans that vary significantly in size, this might make it very difficult to get an even roast.
Even if you aren’t pre-blending, it is very important to have an even SCR size because, if you have serious SCR size differentiation in your coffee (e.g. 50% SCR 18 and 50% SCR 14), this will make it hard to roast, as the smaller beans will roast quicker than the larger ones. While the larger beans will have roasted sufficiently, the smaller ones could roast too long and become burnt. The same can be said for roasting potatoes of different sizes.
The moisture level of green coffee is very important. All green beans contain a certain amount of water, which has a great impact on the cup. Roughly speaking, you want a level between 9% and 12%, but as the moisture level is constantly changing (the amount and speed of change depends on the conditions you store your coffee in), it is better to be around 10-11%. This gives you a buffer so that if the moisture level goes up or down slightly, you’re still in your target zone. Certain studies show that on average, coffees taste better when it has a moisture level between 10 and 11%.
To sum up, the act of grading coffee is a way of analysing the physical quality of the coffee. It is the first part in a two-part process of getting to know a coffee and finding out its attributes. The second part is cupping (‘cupping’ is the verb for tasting in the coffee industry), which is a way of assessing the flavours and taints of a coffee.
If you liked this article, you might be interested in reading about green coffee sacs and how they affect the quality of your coffee.