Professional coffee roasting | Analysis of water evapotranspiration during coffee roasting

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Spend a little space to supplement the discussion about the degree of evaporation of water during baking at the Atti bug last night.

The following figure shows the water evapotranspiration ratio distribution measured by a 12-minute (720s) baking (LTLT: Low Temperature Long Time slow baking for a long time, it can be assumed to be using 230C hot air).

Source: www.probatburns.com

The black solid line is a raw bean with a moisture content of 8.3%. According to the evapotranspiration ratio (g / s) of total water (free water plus chemical reaction) measured by baking time, we find that the highest evapotranspiration ratio is from 40 seconds to 280 seconds at the beginning of baking. the water evapotranspiration per second is more than 0.02 g / s, and after 400 seconds, the evapotranspiration per second falls below 0.01 g / s. It means that there is not much water in the beans, as emphasized in the text below, Roasted coffee is never 0% water (the moisture content of roasted coffee is never 0%).

The blue dotted line measures the evapotranspiration ratio of the initial water content of raw beans (excluding water generated by chemical reactions), and its trend is the same as the total water evapotranspiration ratio.

The red dotted line measures the evapotranspiration ratio of the water produced by the chemical reaction, which is equivalent to the difference between the total water evapotranspiration and the initial water evapotranspiration, showing a stable state of loss (about 0.006 g / s). That is, the evapotranspiration rate of chemical reaction water is about the same at the beginning of baking or at the end of baking.

The red solid line is pre-dried to a raw bean with a moisture content of only 1.1%. During the baking process of 720 seconds, we found that although it was very dry, as long as it was heated for 80 seconds, its reaction water evapotranspiration reached the ratio of ordinary raw beans, and reached the peak of 0.022 g in 200s, and then quickly dropped to overlap with the reaction water evaporation ratio of ordinary raw beans. After pre-dried beans, the chemical reaction water is produced and evaporated much more during baking, which is an interesting phenomenon, which may be used as a reference for secondary baking techniques.

In addition, according to experience, using hands to feel or actually measure the humidity in the exhaust pipe, we find that the highest humidity is in the stage of explosion, which seems to be different from the measurement in the picture above. I guess, when the beans burst, the volatile gases produced by water vapor and organic matter burst instantly, resulting in a relatively high humidity in the exhaust space at that time. It is different from the basis of using small batch hot air baking to collect and analyze the water evapotranspiration of beans, so the conclusions are different, but in terms of the whole roasting process of coffee beans, there is no doubt that the first stage (dehydration stage) is the stage with the most moisture loss.

In fact, the literature I quoted earlier is this picture. In addition to LTLT, there is also a case of water evapotranspiration in 180 seconds of HTST (bottom left). It can be found that high temperature and fast baking beans leave more water (the total evapotranspiration at the end is still 0.065 g / s).