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Experiment #158: Freezing Gesha Village Coffee For Roasting

Does freezing green coffee impact roasting and it's resulting flavours? 

We decided to try it for ourselves; with Gesha Village coffee. It might be a bit extreme, but the higher complexity and subtle flavours of this coffee will allow us to better gauge and taste if there is any true impact from the freezing. 

From George Howell to Christopher Hendon, the subject of freezing green coffee has become increasingly popular in the last few years. Whether freezing in a roasted or unroasted state, this topic still has much scientific research waiting to be carried out. 

Coffee Used: Gesha Village Auction Lot GVA.20
Source: Oma
Variety: Gesha 1931
Process: Natural
Altitude: 1931 - 2040masl

The coffees were separated into 2 batches, one left in our greens storage room kept at 24°C while the other was stored in a freezer at -18°C. Both batches were kept vacuum sealed and stored in multiple small quantities to allow undisturbed access when retrieving samples. Frozen coffees were taken out of the freezer and left to thaw in the vacuum sealed bag prior to any roasting/analysis.


Moisture Content over time


Plot 1


Surprisingly, there wasn't really a significant difference in moisture content between the untreated coffee and the frozen/thawed sample. The 0.1 - 0.2% difference isn't significant enough to cause any major changes to the coffees' taste or roasting profile.


Density over time

Density


The treated sample had a lower density by about 3g/L which is a small amount. It could possibly be correlated to the drop in moisture content percentage. But overall this drop is quite insignificant in terms of regular density readings.

We had some comments from industry friends that freezing could potentially affect the molecular/bond structure within the bean cells and the effect wouldn't necessarily show in these readings. Hopefully the roast profile and cupping results would shed more light on this matter.


Roast Profile Used

Below is the roast curve of the coffees roasted across the 3 months. There was a total of 4 roasts; 22 July (2 Days after Arrival), 19 Aug, 23 Sep, 21 Oct. All roasts profiles such as gas settings, airflow, drum speed, batch size and pre-heating routines have been controlled to be exactly the same. We have abbreviated the results and shown the roast comparison from the 19 Aug roasts as all 4 trails were almost identical in their comparisons.

The roast profile was kept simple, with 50% gas settings for the entire roast up till about a minute after FC where gas was reduced to 25%. Airflow was increased by the same amount at the same milestones throughout both roasts. All other variables were kept constant.

Gesha Roast Profile

The curve in the background is the untreated sample and the profile at the front is the frozen/thawed sample. As you can see from the graph, the two samples are mostly similar in their reactions to the heat until about a minute and half before First Crack where the ROR dips (Frozen/Thawed Sample) for no particular reason without any changes to the gas. This does not occur to the untreated sample, so there is without a doubt something happening here internally that we are unaware of.


Roast & Cupping Analysis

All roasts were cupped one day post-roast and scored accordingly in 5 main areas: Aroma, Sweetness, Acidity, Mouthfeel and Flavour. We then conducted a second round of cupping 14 days post-roast (except for the final 21 Oct Roast).

Roast Date: 22 Jul 2018
Weight Loss: 12.6%
Untreated Color: 60.4 (Whole Bean), 83.3 (Ground)
Frozen/Thawed Color: 60.2 (Whole Bean), 82.5 (Ground)

Roast Date: 19 Aug 2018
Weight Loss: 12.6%
Untreated Color: 59.9 (Whole Bean), 82.6 (Ground)
Frozen/Thawed Color: 60.8 (Whole Bean), 82.9 (Ground)

Roast Date: 23 Sep 2018
Weight Loss: 12.6%
Untreated Color: 60.0 (Whole Bean), 83.0 (Ground)
Frozen/Thawed Color: 60.2 (Whole Bean), 83.2 (Ground)

Roast Date: 21 Oct 2018
Weight Loss: 12.6%
Untreated Color: 60.3 (Whole Bean), 82.9 (Ground)
Frozen/Thawed Color: 60.5 (Whole Bean), 83.2 (Ground)

Round 1 Cupping Summary: Very similar scores in all categories, only the acidity was different. The untreated sample had a brighter and more vibrant acidity while the Frozen/Thawed Sample had a more muted acidity. Based on cupping scoring, one might score the Frozen/Thawed sample 0.5 points lower on acidity but on a personal level, I'd comment that the muted acidity might make if more suitable for espresso whilst the higher acidity would be more suitable for filter brewing. They were both still good quality, just different in intensity.

Round 2 Cupping Summary: Surprisingly, round 2 had a very different result. In almost every category except Aroma, the frozen/thawed coffee scored 0.5 points higher than the untreated sample. The untreated coffee definitely had a higher quality aroma but lost out in sweetness, acidity, mouthfeel and flavour. I can't explain why, but that was the consensus around the cupping table.


Conclusion

So what can we conclude from this 4-month experiment? We know that the freeze/thaw cycle certainly had an influence on the cupping scores. Even though the bean structure showed no significant changes by regular measurement standards such as moisture content and density, I am sure that we are only scratching the surface when it comes to assessing the properties of the bean and deeper insights are needed to really understand how and why the coffees turned out the way they did. The roast curves showed the coffee beans reacting to the same roast profiles differently during First Crack. Future research could involve different kinds of equipment to analyze the deeper physical structures and chemical makeups of the bean. If anybody has any suggestions, do comment below and let us know!

Overall, is freezing beneficial green coffee beneficial? Maybe. The freeze/thaw cycle has some influence over the final cup and as Christopher Hendon mentions in his interview here, it helps to preserve the coffee for extended periods of time. George Howell has been freezing his coffee for a long time now and experienced its many benefits. However, the study conducted by Chris Kornman as seen in this article here showed freezing caused more of a detrimental effect to the beans. Our own findings both support and contradict his findings as our initial cuppings did show "detriment" to the acidity of the coffee but the results changed significantly after the coffees had rested.

Enjoyed our experiment? Taste it for yourself!
The Gesha Village will be available next week, get your orders in here: Gesha Village Auction Lot GVA.20.
Our V60 brew recipe for this coffee can also be found here: Gesha V60 Brew Recipe


Update:


We've continued this experiment for an additional month and found the frozen/thawed coffee to have deteriorated significantly. These coffees cupped with an unpleasant bitterness in the finish that lingered on for a long time. The original flavours and acidity are still present, only with the additional bitterness at the end.

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