I am not sure of the compositions of Milk powder but it is possible. I know for sure that >50% is a great deal of residues in any sample.

Basically, residual weight is a sum of the ash contents and the fixed carbon content.

I predict that ash contents in milk powder may not exceed 10%.

This specific milk powder could have high contents of fixed carbon; accruing from its combustion to form char.

Obviously you have performed the analysis under an inert atmosphere. If you run the TGA under O2 atmosphere the residual weight will be zero.

Please mention the purge gas type with flow rate/pressure, heating ramp, temperature range, i.e., entire program of TGA is necessary to answer.

Dear sir,

temp range: 40-700 °C @ 20 °C/min and Nitrogen was used as a purge gas @ 20 mL/min

The principal constituents of milk are water, fat, proteins, carbohydrates (milk sugar) and minerals (in the form of salts). Among the constituents, organic compounds contain mainly carbon, oxygen and hydrogen and inorganic compounds (minerals) contain mainly other atoms (metals and non-metals). In entirely nitrogen (inert) atmosphere organic compounds initially be degraded followed by removal of oxygen gas, hydrogen gas and steam. At the same time the inorganic compounds also be decomposed and non-metal part will go out of the media by forming gas and the metal part will combine with oxygen, removed from organic materials and form metal oxides. So the high percentage of (>50%) residual weight signifies the carbonaceous materials and inorganic metal oxides.

Mamta... The thermal analysis of most organic material result in the formation of residual carbon if the heating is carried out under nitrogen atmosphere. As mentioned by Nematollahzadeh above the oxidative atmosphere will cause burning of such residual carbon and there will remain any inorganic salts or oxides. The residual material percentage is a function of the maximum hating temperature. Your 50% residual material may indicate incomplete degradation of milk. There will be clear reduction in the residual weight even to much lower than 25% if you proceed with heating.... Good luck