Behind the Scenes | What We've Learned About Green Coffee Storage

Coffee beans are the seeds of the fruit of trees from the genus CoffeaCoffea Arabica (usually referred to simply as Arabica) is the variety that most kiwis will be familiar with, though many roasters also use small amounts of Robusta coffee, which comes from Coffea Canephora Plants.

The process of getting the coffee from the tree to our roaster is a long and complex one and will be covered in due course over several blog posts. For now, all that is important for me to mention is that what we receive in sacks, ready to roast, is referred to as Green Coffee, or Green Beans. It usually comes in 60kg jute sacks, lined with plastic Grain Pro bags, and can be stored for 6-12 months before any changes in flavour start to appear. How long exactly it lasts depends on a variety of factors (Processing method, bean moisture content, proper use of hermetically sealed liners, time spent in transit, exposure to sunlight, and many others).

Back in 2017, we couldn't find a huge amount of information on the effects of temperature on green bean storage, and by the end of 2018, we had ourselves a purpose-built temperature-controlled storeroom for our coffee. We took the opportunity to experiment.

The two reasons we had for building a storeroom were to find out to what degree storage temperatures affected flavour, and to find the ideal temperature to charge the coffee into the roaster.

Over the year prior to us deciding to build a temperature-controlled storeroom we measured temperatures inside coffee sacks as low as 7°C in winter, and as high as 31°C in summer. The following year we kept sacks of the same washed coffee from PNG stored inside and outside our storeroom for a period of one year, cupping samples monthly and recording our scores and tasting notes.  Surprisingly, we discovered that we lost less sweetness storing the coffee outside of the storeroom where the highest bean temperature we recorded in summer was 30°C and the lowest winter reading was 7°C, as opposed to keeping them at 18°C year-round. In the end the PNG, which we had initially scored 82.75, scored 82.0 after staying out in ambient temperatures, and 81.5 after living in our storeroom at 18°C.

This was an interesting discovery as it appeared that positive impact of cool winter temperatures outweighed the negative impact of summer's highs. It’s important to note that we received the coffee in September, so the really warm temperatures occurred in the first half of the experiment, when the coffee was still relatively fresh. It’s a little difficult to compare results with a different coffee from a different country harvested at a different time as there are many variables at play, but we did notice a slightly more significant impact when we did the same experiment with a semi-washed Sumatran coffee which arrived the following May.

We’ve been asked why we didn’t choose to store the coffee at a lower temperature. Surely the coffee would have lasted better at, say, 10°C? We’ve discovered a few things along the way which explain why that would not have worked.

The first, and possibly most obvious reason is that we simply could not build a room big enough to store tens of tonnes of coffee! We only have room in our whole warehouse to store around a third of the coffee we roast in a year, at any one time. Space was absolutely a limitation. If we're receiving coffee that has been stored in a green bean dealer's warehouse, we can assume it's been stored at ambient temperature until it reaches us. In addition to this, cooling an entire warehouse to 15-20°C below the outside temperature would have been a serious undertaking, and would have used a serious amount of power.

But the most important factor is that 10°C is simply too cold to begin a roast.  

In my previous post I briefly mentioned the impact of green bean temperature on the progression of the early stages of the roast. We’ve come to realize that this is the single most important variable in ambient conditions in terms of impact on roasting efficiency. Far more crucial than factors like ambient temperature, humidity, and even (within certain limits) bean moisture content.

The two main control points we have at the beginning of a roast are the temperature of the air inside the drum, and the temperature of the metal drum itself. The problem for most coffee roasters is that throughout the year the difference in temperature of the green beans between summer and winter is significant. When the green coffee is too cold when it enters the drum, too high a charge temperature is required, which causes problems with scorching or tipping (when the soft end of the coffee bean, where the seed would have germinated from, becomes scorched).

Now that we know what we know, we use our storeroom as more of a “pre-warming” room, where the coffee will be kept for up to a month at a comfy 20°C, ensuring that we start every single roast with the green coffee at exactly the same temperature. This appears to be an ideal way to minimize the impact of the warmer summer months, while maximize the benefit of winter’s cooler temperatures, all the while getting the best flavour out of our coffee. It’s also a crucial part in the process of producing the most consistent, delicious flavour possible, every season of the year.