Coffee machine with milk frothing system

Coffee machine comprising:

• a boiler for heating water and supplying steam;
• a steam wand suitable for dispensing a pressurized air-steam mixture;
• said steam wand being suitable for being immersed in a jug containing milk to inject said air-steam mixture into the milk so as to froth and heat the milk in the jug;
• a temperature probe arranged in the steam wand to detect a temperature of the milk in the jug;
• a steam valve arranged between the boiler and the steam wand to open/close a steam flow from the boiler to the steam wand;
• a compressor to provide compressed air;
• an air valve arranged between the compressor and the steam wand and configured to adjust an air flow from the compressor to the steam wand;
• said air valve being a proportional valve with an adjustable shutter according to an opening degree of the air valve;
• a user interface configured so that the user can enter a final temperature (Tf) and a frothing degree (Gf) of the milk;
• and
• a control unit connected to said temperature probe to receive the detected temperature (T), to said user interface to receive said final temperature (Tf) and said frothing degree (Gf) of the milk, and to said steam valve and air valve in order to control them,
• wherein said control unit comprises:
• an interpolator configured to interpolate the temperatures (T) detected by the temperature probe so as to obtain a temperature curve over time and calculate an inclination (α) of said temperature curve;
• and
• an algorithm configured to receive said final temperature (Tf) of the milk, said frothing degree (Gf) of the milk, and said inclination (α) of the temperature curve and accordingly calculate an injection time (ti) to close said steam valve and air valve and a quantity of air (Q) to adjust said shutter of the air valve.