In a previous article (https://keigancoffee.com/blogs/research/wash-raw-coffee-beans) , I speculated that "washing raw beans with water removes dirt from them, making the coffee taste better." However, there was insufficient data to reach this conclusion, so I have changed the speculation and added supplementary information to the research methodology.

[Survey Methodology]

Survey Date

May 2021 - June 2021

Survey subjects:

20 men and women who regularly drink coffee

Coffee beans used in the survey

①Brazil No.2 Washed Medium Roast (no water washing)
②Brazil No.2 Washed Medium Roast (washed)
③Brazil No.2 Washed City Roast (no water washing)
④Brazil No.2 Washed City Roast (washed)
* Wash the raw beans with tap water as you would rice, then dry them thoroughly using a fan.

Roasting method

Roaster: Fuji Royal semi-hot air type
Input amount: 250g
Bottom: 100℃
Temperature change inside the kiln: 1°C increase every 6 seconds up to the first crack, then 1°C increase every 12 seconds

<<Investigation process>>

1. We asked the subjects to taste coffee made from beans ① and ② that had been roasted one week ago (we did not tell the subjects which coffee was made from washed raw beans), and had them rate the coffee out of 10 points and also ask for their impressions of the taste.
   (1 point: not tasty, won't buy next time - 10 points: tasty, will buy next time)
   
   
2. We asked the subjects to taste coffee made from beans ③ and ④ that had been roasted one week ago (we did not tell the subjects which coffee was made from washed raw beans), and had them rate it out of 10 and also ask for their impressions of the taste.
   (1 point: not tasty, won't buy next time - 10 points: tasty, will buy next time)
   
   
3. Conduct a paired t-test on 1 and 2, respectively.
   
   

4. An observation will be made based on the results of the t-test (paired) and the impressions of the survey participants.

[Survey results]

Null hypothesis

For medium roasts, there is no difference in the score between coffee whose green beans are not washed before roasting and coffee whose green beans are washed before roasting.

The calculated t value is -4.33, which is smaller than -2.861 (t at 1% significance level with 19 degrees of freedom is 2.861), and falls within the rejection region at the 1% significance level. Therefore, the above null hypothesis is rejected.

《Conclusion》

For medium roasts, there is a difference in the evaluation points between coffee whose green beans are not washed before roasting and coffee whose green beans are washed before roasting.

Null hypothesis

In the case of city roast, there is no difference in the evaluation points between coffee whose green beans are not washed before roasting and coffee whose green beans are washed before roasting.

The calculated t value is -5.29, which is smaller than -2.861 (t at 1% significance level is 2.861 with 19 degrees of freedom), and falls within the rejection region at the 1% significance level. Therefore, the above null hypothesis is rejected.

《Conclusion》

In the case of city roast, there is a difference in the evaluation points between coffee whose green beans are not washed before roasting and coffee whose green beans are washed before roasting.

[Considerations and Summary]

For both medium and city roasts, coffee whose green beans were washed with water before roasting had an average evaluation score that was more than one point higher, and a t-test was used to prove that there was a difference in these averages at a 1% significance level (this difference was not due to chance).

In addition, coffee beans that are washed with water before roasting are
・Clear aftertaste ・I would like to buy it again

Coffee that is not washed before roasting
・Leaves an unpleasant bitter taste on the tongue ・Earthy (compared to water-washed coffee)
There were many negative comments such as:

In this survey, the coffee that was washed with water was rated as having a higher taste.

・Our roasting profile was suitable for washed raw beans.
- Brazil No.2 Washed raw beans were suitable for washing with water.

There are various possibilities, such as the above, so it cannot be said that rinsing raw beans with water will necessarily improve their taste.

However, I believe that roasters who sell specialty grade, expensive coffee beans should conduct this type of survey repeatedly to understand whether their main customers want coffee made from washed or unwashed beans.

Before releasing a new batch of coffee beans, our store conducts a simplified version of this type of survey (sometimes we use a smaller sample size or the decision is made only by me, the store owner) to determine each time whether or not the green beans should be washed with water.

[Supplementary image]