Technology

Probat’s research on coffee roasting and degassing timings.

As most roasters know, during the coffee roasting process a series of physical and chemical changes takes places through we which create the highly sought after wide range of aromas. At the same time roasting gases are generated – in some cases up to six litres of gases can come from a single kilogram of coffee – of which the biggest part escapes. Even after roasting, grinding the coffee creates an even stronger degassing due to the cell destruction and enlarged specific surface. The degassing of ground coffee can take place in simple or in special gas-tight silos. The silos are filled with inert gas so as to exclude the aroma-detrimental contact with oxygen as far as possible. Normally three or four silos are used. The first one is filled with freshly ground coffee. The coffee degasses in the second silo, which takes between four and 24 hours, depending on the coffee type. Meanwhile, the degassed coffee can be drawn from the third silo and packaged. Various parameters have an influence on the degassing of coffee. One is the coffee’s degree of grinding. Finer grinding destroys more cells. This causes an increase in the specific surface and a shortening of the diffusion paths that again accelerates degassing. A further parameter is the cell structure that largely depends on the coffee type. The porosity of the cells can also be influenced by pre-processing of the green coffee as well as by the degree of roasting and roasting time. The pressure equalisation, vis-à-vis the ambient atmosphere, the so-called partial pressure gradient, is also an important factor. This process reduces the pressure inside the cell through diffusion until a pressure equalisation with the ambient atmosphere pressure has occurred. To prevent coffee from coming into contact with oxygen and losing aroma, inert gases such as carbon dioxide or nitrogen are used in this process, which simultaneously create pressure equalisation, influencing degassing time. Until now, the effects of each parameter during the ground coffee degassing process could not be precisely determined. The reciprocal effects of the various parameters hardly allowed a reliable conclusion regarding degassing time of ground coffee – for roasting operators and packagers something almost like “uncharted territory” in the coffee world. This was Probat’s exact point of departure with the tests on degassing behaviour of ground coffee and analysis of the effects of the respective parameters. In a degassing unit designed specifically for this purpose up to six different coffee samples can be gauged under laboratory conditions. The benefits here are that the reliable parameters for degassing behaviour reduce imponderabilities at the interface between grinding and packaging and thus help improve quality standards. A constant challenge for roasting operators is having reliable process indicators on the basis of test results and thus optimise their process flows. In the context of a test series the effects, parameters of roasting time, degree of grinding and ambient atmosphere have on the degassing behaviour of various coffee types were explored and analysed. In this way, the degassing behaviour of Vietnam Robusta is influenced by the roasting time. With a short roasting time, initially a sharper increase of the volume of gas occurs as against a longer roasting time, with identical quality and colour of the ground coffee in both cases. The reasons for the increase in the pressure inside the cell structure of the individual bean is because it is higher at shorter roasting times. The amount of gas released increases slightly at shorter roasting times. A reverse effect of roasting time can be measured with certain Brazil coffee varieties. The surprising result – the influence of roasting time can have an opposite effect on the amount of gas released by respective coffee types. The result also documents the fact that general conclusions on the degassing behaviour of coffee are difficult to draw. In this area, special tests of individual coffee types are necessary to optimise the manufacturing process. When applying inert gases to protect aromas, it was discovered that nitrogen, in comparison to carbon-dioxide, tends to increase coffee degassing. This effect is due to the different gas density of nitrogen and carbon-dioxide, or in other words, to the influence of partial pressure. The tests identified the parameters, partial pressure gradient and degree of grinding as significant influencing variables of degassing time. It was found that the finer the grain size setting, the quicker the degassing took place. However, with coffee beans, the amount of gas released is very small within the initial 24 hours due to long diffusion paths and the smaller specific surface. So an interim storage of roasted coffee beans with the aim of pre-degassing does not make much sense. With roasted coffee beans, the only reason for interim storage is to produce a homogenous overall product.

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