Applications for Pico Products

Espresso Coffee Grinder Testing

This application shows how a TC-08 thermocouple data logger is used in the food industry to help make the perfect Espresso.

The background

Fresh coffee has to be treated carefully to ensure that the flavour profile designed into the blend is not impaired or altered by the methods of packing and storage or how it is ultimately ground by the user.

The Coffee Roaster for whom I work takes great care to roast coffee to a very precise temperature / time profile. It is then carefully cooled, again to a precise temperature / time profile, and packed in light–proof, gas–tight multi–layer valve bags that will allow CO² out but will not allow air to enter.

For the ultimate taste and crema (the light brown cream on top of the black coffee that is typical of all Espresso based drinks) the coffee must be absolutely fresh and must be freshly ground in a good quality grinder that does not abuse the coffee by heating it or crushing it.

Contrary to what you might think coffee grinders should not crush the coffee beans but grind them by a slicing action into a controlled range of particle sizes. This is done using two flat, or conical, grinding blades with several different sizes of cutting edges machined on to them in a radial pattern. One blade is static while the other is connected to the motor shaft. Temperature and humidity also affect how the coffee beans will grind.

The problem

Commercial coffee grinders often have a high duty cycle, especially first thing in the trading day when the ground coffee chamber is completely empty (unfortunately most grinders are designed for around a 30% duty cycle). At this time the grinder must produce up to 1 kilo of ground coffee in a short space of time while imparting the absolute minimum heat into the ground coffee. As these machines usually have a relatively small casing size yet contain a powerful electric motor turning at about 1,200 rpm there is a big risk of heat being conducted from the motor up into the grinding chamber. Heat is also generated by the grinding action itself. The purpose of testing the coffee grinder is to identify and minimise all unnecessary heat. It is hoped that from the test results the “maximum run time” can be calculated for the grinder.

Test setup

The actual grinding chamber of a typical coffee grinder is very small and there is little free space. To get a temperature probe in to this space without interfering with the flow of coffee, and thus distorting the results, proved to be a challenge. Ultimately a “fine wire, exposed junction” K–type thermocouple was used. This was bonded into the grinding chamber outlet with a small dab of epoxy resin to provide insulation from any heat conducted up through the metal of the grinder casing while leaving the junction exposed to the ground coffee. The probe wire was fed out of the chamber via the ground coffee outlet chute.

A second probe was attached to the side of the motor stator to detect the heat rise in the actual motor. Here a “fine wire, exposed junction” K–type thermocouple bonded to a flexible magnetic pad was used. This kept the probe in contact with the soft iron of the motor stator.

A third probe was attached to the metal of the grinder casing adjacent to the point where the motor was bolted on to the grinder body just below the grinding chamber to show the conducted heat from the motor.

Finally a hypodermic type probe was inserted in to the base of the coffee bean hopper to monitor the temperature of the beans prior to entering the grinding chamber.

Figure 1 below shows a schematic of the coffee grinder and the location of the thermocouples.

To replicate “Coffee Shop” conditions a temperature controlled room was used. This was set to 28 degrees Celsius and an average humidity level.

Using the USB TC-08 connected to a PC the data was gathered over a period of 10 minutes with readings taken every 500 ms. The test was conducted three times and the results compared. The results from the three tests were so close that they were not averaged and just one set was used.

Figure 1: Schematic of a coffee grinder

Conclusion

The majority of the rise in motor casing temperature was not passed on to the grinding chamber within the 10 minute period; however, there was a notable rise of about 12 degrees Celsius in the temperature of the ground coffee during the period.

This is not acceptable, however referring to the graph (Figure 2) it will be seen that the rise can be limited to 4 degrees Celsius if the run time is limited to just over 4.5 minutes. As this is equivalent to just under 1 kg of coffee being ground this should be sufficient to maintain the provision of ground coffee to the associated Espresso coffee machine for long enough to allow the grinder’s 30% duty cycle to be adhered to.

Thus the test has shown that by controlling the run time of the grinder the taste profile of the coffee can be maintained.

A small electronic timer will now be developed to stop the grinder 4.5 minutes after grinding begins.

 

Figure 2: Coffee grinder temperature