I just read at wikipeda about it. They speak of the IR absorption. I can´t help you with doing FFT at IR frequency spectrum.
Afaik. you can also use the angle of total reflection to calculate brix. Just put the fluid on a glass with known optical density. from below adjust a laser beam to point to the area where the fluid is on the glass.
Adjust the angle (laser beam to glass) to find the point of total reflection. The angle is in relation with brix. Maybe you find a table with the relationship.
thanks for the prompt reply mate. is there anyway to control the pot value in microcontroller because every time i need to adjust the pot to increase the IR Sensor intensity
As an alternate to adjusting the resistance but keep the voltage constant, you can also change the voltage through an unchanging resistance. The microcontroller can create a voltage by means of D-to-A conversion, or whatever method is least trouble.