Next to tasting and analysing our roasts every day on the bar, and every week in our quality control meetings, we measure the following parameters on a monthly basis:

Relative Density
Moisture Content
Water Activity
Ultra Violet (UV) Luminosity
Defect Count
Scoring

While Green Grading we work within very tight margins - only very small changes are accepted in a coffee we are working with. This way we can pick up signs of ageing early on, allowing us to act quickly and remove any low quality green beans that could affect flavours.

This is part two in our series on why we take our green bean quality grading very seriously. If you haven't read it already, we suggest reading part one first. You can find it here.

Weight (g) ÷ Volume (mL) = Density (g/mL)

RELATIVE DENSITY

The relative density of a green coffee is expressed in gr/ml. This measurement is ‘relative’ because it considers the air space between the beans when calculating - important because the size of the bean will always influence the relative density. This is very useful for a roaster as it provides a lot of information on how to approach the roast profile. Higher density beans need more heat at the start of the roast, and usually crack harder. There is no ‘acceptable’ or ‘ideal’ range for the relative density, because it should simply never change. If it does, you may have a very big moisture problem.

RELATIVE DENSITY

Weight (g) ÷ Volume (mL) = Density (g/mL)

The relative density of a green coffee is expressed in gr/ml. This measurement is ‘relative’ because it considers the air space between the beans when calculating - important because the size of the bean will always influence the relative density. This is very useful for a roaster as it provides a lot of information on how to approach the roast profile. Higher density beans need more heat at the start of the roast, and usually crack harder. The density of the coffee should never change. If it does, you may have a very big moisture problem.

MOISTURE CONTENT

This is the percentage of water present in the coffee, which tells you how well the coffee was dried. Our acceptable range of moisture content in coffee is 9.0-11.5%. Any higher and the risk of mould or other fungal infections is high. Any lower and the coffee will lose flavour quickly, and show signs of ageing earlier.

The moisture content also has an influence on the roast style. Too much moisture means the beginning of the roast needs a lot of energy, but once the moisture is gone the built up energy makes the remainder of the roast difficult to control. Too little moisture and the coffee can scorch easily, or it will not be able to maintain enough energy to keep the momentum up for the rest of the roast.

A sudden change in moisture content can be a sign of bad storage or ageing. When the moisture percentage rises it is usually due to high moisture levels in the air of the storage space, and mould or fungus can infest the green coffee. The ideal storage facility has a stable temperature of 18-20 degrees celsius, and stable humidity of 40-50%.

A moisture meter for measuring moisture content

MOISTURE CONTENT

A moisture meter for measuring moisture content

This is the percentage of water present in the coffee, which tells you how well the coffee was dried. Our acceptable range of moisture content in coffee is 9.0-11.5%. Any higher and the risk of mould or other fungal infections is high. Any lower and the coffee will lose flavour quickly, and show signs of ageing earlier.

The moisture content also has an influence on the roast style. Too much moisture means the beginning of the roast needs a lot of energy, but once the moisture is gone the built up energy makes the remainder of the roast difficult to control. Too little moisture and the coffee can scorch easily, or it will not be able to maintain enough energy to keep the momentum up for the rest of the roast.

A sudden change in moisture content can be a sign of bad storage or ageing. When the moisture percentage rises it is often due to high moisture levels in the air of the storage space, and mould or fungus infests the green coffee. The ideal storage facility is a stable of 18-20 degrees celsius, with a stable humidity of 40-50%.

A water activity meter (the coffee beans sit under the black part on the right)

WATER ACTIVITY

The water activity is the energy of the water present in an organism. This sounds complex, but to understand it in relation to our green grading report we only need to know it is measured on a scale of 0 to 1 - with 1 being pure water, and our acceptable range being from 0.5-0.65. Anything lower can be a sign of an old crop, or too long a drying period. Anything higher can be caused by bad or uneven drying, or moisture absorption after drying (e.g. in the warehouse).

This reading should also never change. A change in water activity usually precedes a change in moisture content, and can serve as an early warning sign of bad storage conditions or ageing. We know from our own observations that, before a coffee dies, the water activity suddenly goes up for a short period. Which is a last warning sign - but it should never come to that in our case.

WATER ACTIVITY

A water activity meter (the coffee beans sit under the black part on the right)

The water activity is the energy of the water present in an organism. This sounds complex, but to understand it in relation to our green grading report we only need to know it is measured on a scale of 0 to 1 - with 1 being pure water, and our acceptable range being from 0.5-0.65. Anything lower can be a sign of an old crop, or too long a drying period. Anything higher can be caused by bad or uneven drying, or moisture absorption after drying (e.g. in the warehouse).

This reading should also never change. A change in water activity usually precedes a change in moisture content, and can serve as an early warning sign of bad storage conditions or ageing. We know from our own observations that, before a coffee dies, the water activity suddenly goes up for a short period. Which is a last warning sign - but it should never come to that in our case.

ULTRA VIOLET (UV) TEST

Our final measurement involves looking at the coffee under a UV light. Beans that are high in moisture, unripe, or defective will light up brighter than the rest. The UV light can even identify defects that aren’t visible yet, like phenolic beans or fungi. These types of defects can spread in transit and eventually affect an entire 60 or 70kg bag of green beans, which is why this step is incredibly important.
UV luminosity usually correlates with our other measurements. For example, if UV is high and moisture and water activity both low, we know to re-calibrate our equipment and check again. If the results still differ, this could be an early sign of moisture problems.

Spot the defect

ULTRA VIOLET (UV) TEST

Spot the defect

Our final measurement involves looking at the coffee under a UV light. Beans that are high in moisture, unripe, or defective will light up brighter than the rest. Phenolic beans or fungus also does that.
UV luminosity usually correlates with our other measurements. For example, if UV is high and moisture and water activity both low, we know to re-calibrate our equipment and check again. If the results still differ, this could be an early sign of moisture problems.

An assortment of defects including parchment, insect damage, and full black

DEFECT COUNT

Primary and secondary defects are clearly defined by the Specialty Coffee Association. No primary defects are allowed. Secondary defects, such as broken beans, minor colour deviations etc, are only allowed in very small quantities: 5 defects per 350gr of green beans. Those do not have a significant impact on the cup scoring. We run monthly defect checks to ensure consistent sorting of our green materials.

DEFECT COUNT

An assortment of defects including parchment, insect damage, and full black

Primary and secondary defects are clearly defined by the Specialty Coffee Association. No primary defects are allowed. Secondary defects, such as broken beans, minor colour deviations etc, are only allowed in very small quantities: 5 defects per 350gr of green beans. Those do not have a significant impact on the cup scoring. We run monthly defect checks to ensure consistent sorting of our green materials.

SCORING

Cupping and scoring of coffees is done at different points throughout the process. We do this at farm level, in the exporters lab, when we receive pre-ship samples, and again when we have received the bulk order (the samples we take from the bulk order are called “landed samples”). From then on we score our coffees once a month to ensure consistent qualities. Coffee is, of course, organic matter so with resting and time it does change slightly, drying out as it ages. We are constantly fine-tuning our roast profiles for cup consistency, to ensure we get the best out of our beans.

We also host cupping sessions every Sunday at 12:00 in our roastery

SCORING

We also host cupping sessions every Sunday at 12:00 in our roastery

Cupping and scoring of coffees is done at different points throughout the process. We do this at farm level, in the exporters lab, when we receive pre-ship samples, and again when we have received the bulk order (the samples we take from the bulk order are called “landed samples”). From then on we score our coffees once a month to ensure consistent qualities. Coffee is, of course, organic matter so with resting and time it does change slightly, drying out as it ages. We are constantly fine-tuning our roast profiles for cup consistency, to ensure we get the best out of our beans.

We store all of this in a database called Cropster - this allows us to follow the performance of each coffee and learn from it every season.



While all this might sound very geeky, we find that by staying on top of our green bean qualities when can be confident in applying THE BARN quality label to each bag of coffee that leaves our roastery. Our staff is immensely proud to be working in such a high quality environment, and our coffee producers feel exactly the same. With our green grading results we are able to provide them with valuable information that can help improve their process, and by presenting their coffees in the best possible way we become one of their partners of choice. We love growing with them and we love being able to include you, the people who enjoy the coffee, as we grow by sharing this learned knowledge. We are so grateful to be able to connect the farm communities to our coffee communities.