During roasting, the pressure inside the coffee bean can reach 25 bars, which is 2 times more than in the tires of Olympic track bikes. This effect in it creates moisture in the form of water vapor. What else is known about how water evaporates during roasting and what evaporation front means for the process?
Methods for measuring moisture in bean
Measuring the moisture level in coffee beans during roasting is a labor intensive process. Researchers use 2 approaches, and each of them has drawbacks.
Measurement of moisture levels in samples of coffee beans at various stages of roasting. Before such a measurement, the coffee must first be cooled down. At this time, it may continue to release moisture or absorb it from the air. Even the most accurate method of measuring the moisture level directly in the beans – in a dehydration oven – is subject to error. Before this, the coffee sample must be ground, and during this time gases and moisture can be released from it. This also affects the measurement result.
Measurement of the moisture level of evaporating gases. This is a technically difficult process, since their moisture level is determined in the ventilation pipes of the roaster. But in them, the moisture evaporating from the coffee beans is mixed with that formed as a result of other chemical reactions during roasting.
James Davison, a chemical engineer and founder of Williamstown Roasters, used the second approach to measure coffee bean moisture. According to his results, the total amount of water released is about 16% of the original weight of green coffee. Most, in his opinion, should have been formed directly during roasting.
This means that it is difficult to accurately determine the moisture content of coffee during roasting, and attempts often yield inconsistent results. However, most evidence suggests that moisture evaporates throughout the entire roasting process, not just during the drying phase at the very beginning. In this case, the moisture content decreases unevenly throughout the bean.
1. Core. It contains water and high pressure steam. 2. The outer layer of grain through which steam escapes. 3. Evaporation front
What is evaporation front?
At the beginning of roasting, the temperature of the bean rises most rapidly near its surface. The slowest heat spreads to the inner layers. There is a layer separating them between the dry part on the outside of the coffee and the wet part on the inside of the bean. Moisture evaporates from it first of all and exits in the form of water vapor. This layer is called the evaporation front.
When the temperature of the bean rises above 100 °C, the pressure inside it begins to rise. This is explained by the fact that water vapor accumulates faster than it can escape. In this case, the pressure inside the bean can increase tenfold. This makes it difficult for the water to evaporate. Similar to how the boiler in an espresso machine holds both steam and water at 120°C or more. The pressure inside the boiler prevents evaporation, which allows the temperature to rise above the normal boiling point.
In the outer layers of the bean, water vapor can escape through its pores - then the pressure drops, and the remaining water evaporates. In the inner layers, the steam cannot escape, because it is under pressure from all sides.
The result is that the outer part of the bean becomes dry, but the center still contains a lot of moisture. During the roasting process, the evaporation front moves inward towards the center of the bean as more and more steam escapes from the coffee.
Connection between the evaporation front and the first crack
Turning water into steam requires a lot of energy, it is called the latent heat of vaporization. This knowledge is the key to understanding how moisture evaporation affects the reaction of bean when heated in a roaster.
If you heat the water evenly, its temperature rises gradually. However, something strange happens at 100°C. You keep heating the water, but its temperature stops rising, it stays at 100°C. The energy expended now goes to turning water into steam.
This means that at the evaporation front, most of the energy that the bean absorbs is spent on evaporating the water in it, and not on raising the temperature. At the same time, in the outer layer, all this heat can start the chemical reactions of roasting and raise the temperature of the coffee.
The effect of an evaporation front is that heat raises the temperature of the outer layer faster than the inner layer.
The evaporation front prevents the heating of the core. This leads to a sharp temperature gradient between the outer and inner parts of the grain.
This phenomenon also has another important effect. As the temperature of the bean rises, at some point they gradually pass from an elastic state into a brittle state - the so-called glass transition occurs. The glass transition temperature depends on the amount of moisture: the more water, the lower it is.
This gives an interesting effect: at a certain temperature, the core of the bean undergoes vitrification. But since it contains more moisture, it remains elastic for a while and expands with increasing steam pressure. At the same time, the outer layer of the bean is more fragile, because it is drier, and it resists the expansion of the core. As the temperature of the coffee rises, tension builds up: the inside of the bean tries to expand, but is held back by the fragile outer layer. This may be the cause of the first crack.
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