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Economical roasting thanks to a sensor inside a coffee bean

PTB develops a new technique for measuring the thermal conductivity and thermal diffusivity of small bodies and hereby helps industry save energy


 Coffee is the most popular beverage in Germany - even more popular than beer. And not only in Germany. In 2007, approximately 7.7 million tons of coffee were cultivated, dried, roasted and traded worldwide. Just for drying and roasting, the food industry in Germany spends almost one billion KWh of energy each year. As energy costs are at present very high, it is important to know as exactly as possible how long the beans must - at the most - be dried and when they can - at the earliest - be withdrawn from the furnace, leaving a fragrance of freshly roasted coffee. To achieve this, the thermal transport properties of single coffee beans must, however, be known: How good and, above all, how fast do they transport the offered heat into their interior, and how fast does the humidity escape? To support industry in the economical handling of energy, the Physikalisch-Technische Bundesanstalt (PTB) has successfully investigated the problem of measuring small test pieces. The novel technique determines both of the transport properties "thermal conductivity" and "thermal diffusivity" precisely and fast - even for a single coffee bean.

Kaffeerösterei Münchhausen: Röstung auf dem Kühlsieb. Foto: Jürgen Howaldt Das Foto stammt aus Wikimedia und ist unter den dort üblichen Bedingungen (Nennung des Titels und des Fotografen) frei verfügbar: http://commons.wikimedia.org/wiki/File:KaffeeMuenchhausen-05e.jpg

Up to now, it has been impossible to measure a single bean. Instead, pressed coffee grounds are investigated, a procedure which unfortunately furnishes only rough estimates for the structure-dependent transport properties of beans. Not only the coffee bean is considered to be "too small to be measured", but the same applies also to other foods and luxury food, as, for example, to nuts.

The novel technique from PTB solves the above problem. As a sensor, a commercially available surface temperature detector of 50 m in thickness and a surface area of only 3.1 mm x 3.0 mm, is sufficient. For analysis, the sensor is sandwiched between the two halves of a bean and already after two or three minutes, the result is obtained. As the thermal conductivity of a bean also depends considerably on its humidity, this property can also be determined. The sensor tolerates temperatures of up to 180 °C. This enables it to constantly measure current values for the transport quantities mentioned - even during the drying or roasting process. Thus, the energy-relevant parameters of the two processing steps are at any time available for control and optimization of the entire process.

With the new procedure, the thermal conductivity of a sample having a volume of only 100 mm3 and, if required, also its moisture content, can be determined within a few minutes and with only one single measurement. In addition, the procedure allows the temperature diffusivity and the volumetric specific heat to be measured. A conventional - commercially available - sensor is sufficient for such measurements, which can then be performed at temperatures between -50 °C and 180 °C.

Contact person:
Dr. Ulf Hammerschmidt, Working Group 1.74 Thermal Conductivity, Heat Transport Department
Phone: +49(0) 531 592-3211,
e-mail: ulf.hammerschmidt(at)ptb.de