First thing we would like to say is the physical dimensions of the electrodes you are using are very high. Ideally the dimensions of the buttons we recommend are 1.5cm * 1.5cm (in diameter max). Due to the the large size of buttons the parasitic capacitance of the sensors will be very high. For the operation of MBR3 sensors, the parasitic capacitance of the sensors should be less than 45pF.
Also the parasitic capacitance of the sensors will depend majorly up on the environment factors like temperature and humidity.
The smart sense algorithm inside the MBR3 will calibrate the sensors on every power up. Due to this you may have to calibrate the sensors again when there is power off.
In your case please check the parasitic capacitance of the sensors in various conditions like outside and inside refrigerator with worst case frost conditions. The parasitic capacitance in all the cases should be less than 45 pF. Please ensure that the electrodes are not touching the ground. Also please do not touch the copper plate directly.
However, MBR3 is primarily designed for detecting human touch or proximity. For this kind of applications, we recommend you to go with programmable device called PSoC especially PSoC 4000S/4100S/4100S plus families. These devices have latest CapSense IP which can allow the parasitic capacitance of the sensors upto 200pF. You can buy a very cheap stamp board of these devices and start testing your application.
Please go through following links for kit related information: