For this reason, at The Coffee Training Co. I train new baristas that they are back up. Heating and foaming milk is a sensory process where the barista is listening, smelling and looking at the milk and feeling the temperature of the jug whilst heating and foaming.
How does a standard milk thermometer work.
Lets start by understanding how a standard dial milk thermometer works. The common size has 140mm stem that attaches to a needle and dial, and comes with a clip so it can attach to a milk jug. Inside the stem is a small bi-metal coil or spring that connects to a rod and when heated the bi-metal will expand, resulting in the coil undoing and in turn, turning the rod that is attached to needle on the dial. The hotter the coil is the more it will expand and the more the needle will move. On the other hand, if the stem is then cooled the coil will tighten up and contract moving the needle in the opposite direction.
How a standard milk thermometer lose its calibration.
Like any spring or coil, repetitive stretching will result in permanent damage. So heating milk all day, up to 60-70 degrees Celsius will, over time, take it’s toll on the unit. Then throw in the customer who wants an extra hot coffee (yes this still happens and is common) and the coil is stretched further.
Other reasons your milk thermometer can lose its calibration comes down to day to day care. Tapping bubbles from the milk after heating and foaming with the thermometer still attached to the jug will loosen the coil and needle. The unit needs to be removed from the jug prior to this.
Also, washing the thermometer in the dishwasher will damage the unit. The high temperature, normally 85 degrees Celsius plus that a commercial dishwasher operates at will not only over stretch the coil but crack the hard plastic that protects the dial. The result is water getting into the unit and seizing the coil up over time. Best to hand wash and sanatise.
All in all, with care a standard milk thermometer should last 3 to 6 months in a commercial environment.
How to calibrate a standard milk thermometer.
The idea here is to adjust the dial to the position of the needle and coil. Remember the coil has been stretched, so there is no fixing that but it will still expand at the same rate when being heated. So you are adjusting the dial to suit the coil’s new position.
You will need;
- A Glass of 1/2 cold water & 1/2 ice
- Digital probe
- Small pliers or a shifting spanner
Step 2 – When the digital probe reads 0-1 degree Celsius, insert the milk thermometer into the ice slurry. Watch the needle and take note what it reads on the dial. For example; it could come down as cold as 10 degrees Celsius where the digital probe reads 1 degree Celsius. This means the milk thermometer is out 9 degrees and the dial needs to be adjusted.
Step 3 – Remove the milk thermometer from the solution and using the pliers or shifting spanner, hold the lock nut located beneath the dial. Don’t turn this nut, it will completely break the unit.
Step 4 – Still holding the nut with the pliers or shifting spanner, use your thumb and forefinger of your opposite hand to move the dial until the needle reads 0 degrees Celcius.
Step 5 – Reinsert the milk thermometer into the ice water slurry and check it now reads 0 degrees Celcius. If it doesn’t, you may have moved the dial too far or not enough. Simply repeat steps 2 to 4 until correct.
Milk Thermometers can be purchased from our partner, Baristatools.Net who offer free, traceable shipping on all orders placed before 3:30pm aest.
Brent Deller – Trainer @ The Coffee Training Co.