Measuring moisture and water activity in green coffee samples
Coffee, the liquid deliciousness that binds us all together. Whether it is from Africa, South America or Brazil – we all love our cup of joe, especially when it’s fresh! A fresh brew, fresh roast, or even better fresh harvest.
But what makes a coffee fresh? And what makes a coffee not fresh? Maybe you have heard about woodiness or deterioration in coffee. But what does it mean? And where does it come from?
I have pondered, paced and given it very thorough thought, and here are my findings so far.
Shelf-life: woody coffees and past crop.
Moisture in coffee, is important to check.
Moisture content indicates the amount the beans have already been dried and how much drying is still required. This will impact the weight loss of the green beans during storage and roasting, all of which decide the quality of the finished product.
I have also learned that moisture in the coffee predicts how long it will last as greens. And it is generally accepted that the drying phase is an important process after the coffee is harvested.
Drying the coffee at lower temperatures and over longer time is less aggressive for the green coffee and less likely to cause cellular damage, hence is better for the quality. As we know, coffee is dried a little differently in different countries but a general norm these days is 15-20 days.
So if there is water inside the green bean, this water can escape the bean.
This means that quality can escape from green coffee even as it rests on a shelf, through moisture being lost and moisture gained due to damage done in the drying process. Which may affect the quality in the cup.
Meaning that if the moisture in the coffee is too high it will very quickly taste woody, past crop or just boring. Lacking sweetness and structure.
Sooo, does that mean that all coffee that is older than 6 months taste bad?
I would say no, because I have had coffees from Ethiopia and Kenya that were 14 months old and still tasted amazing!
We monitor moisture content in our coffees in order to try to predict how long they will last. Generally, the range one uses is below 9% and above 11.5%.
Whenever we have coffee sample in the lab, we can have it in 3 stages:
Offer sample– these are samples that exporters or farmers send, for us to buy. These samples can be anything from type samples, samples from parchment that still require milling, sorting and grading to coffees that are export ready.
Pre-shipment sample – These are samples we get before the coffee gets shipped but after all preparation is complete, hulling, sorting and grading. We use PSS for the last approval stage. That means, before the exporter can actually ship the coffee, we test it in the lab, by doing both physical and sensorial tests.
Arrival sample – These are samples we receive after the coffee as arrived at our warehouse.
At all three stages we test the coffees:
- By cupping
- Checking moisture content
- Checking water activity
These are basically the only ways we can check if the coffees are ok and to try to figure out the longevity of the coffees. And whether there is a strange flavor in the cups, too high or low moisture content, or to high / low water activity.
And maybe there is a lucky few of you who has been introduced to the newest fad: Water activity.
What is water activity?
Well, it took me some time to understand, but here is what I have so far:
Water activity is basically the measure of the energy status of the water left in the coffee. Water activity is based on a scale from 0-1. The formula:
where p is water vapor pressure above the product surface and ps is the water vapor pressure above the surface of pure water at the product temperature. Water activity is currently used for measuring the shelf-life of a lot of supermarket products (like orange juice).
I have learnt that the water activity should be reading between 0.40 and 0.70. But the golden range is around 0.50-0.60. The tricky thing is, some people are even saying that: the higher the values, the more flavourful roast. Especially for those who roast darker.
However fungus and bacteria can live in beans with a water activity over 0.70.
Are you confused yet ?
I’ll wrap it all up for you know:
- Moisture in coffee is compounds that are bound up in the inside of the green bean.
- We measure it in percentage % and the general accepted levels are between 9% and 11.5%
- When it is above or below this range, the coffee usually has a problem.
- When the coffee is too low in moisture, it will be harder to roast, because there is so little water that needs to escape the beans.
- When the coffee is too high in moisture, it may taste woody quicker, be more difficult to roast and have strange stripes across the beans.
- And moisture is also something that may escape the beans while it is resting on the shelf. Meaning, the longer the coffee stays on the shelf, the dryer it will get. Leaving you with a lesser quality.
- Water activity is still a quite new thing in coffee, but it is fairly easy.
- Water activity is basically the measure of the energy status of the water left in the coffee. Water activity is based on a scale from 0-1.
- If the number is too high, fungus and bacteria may start to develop. And in some way, water activity and moisture is probably linked together.
But, I am still in the testing phase. I’ll come back with an updated blogpost on my findings on correlations between moisture and water activity. Stay tuned, and thank you for reading.