Accurate detection of neotame hydrates and their transformation using terahertz spectroscopy

Neotame as a new non-nutritive sweetener is widely used in food. It has several hydrate forms which can convert into each other at different environmental conditions. In this work, terahertz time domain spectroscopy (THzTDS) is used to quickly differentiate its two main hydrates and to study their transformation. Neotame monohydrate has obvious terahertz absorption peaks at 0.89, 1.45 and 1.73 THz, whereas neotame anhydrate shows no apparent features in the frequency range of 0.5-3.0 THz due to their different crystalline states. The transformation from monohydrate to anhydrate upon heating is monitored according to the normalized THz absorption peak areas of the monohydrate around 1.45 THz at different heating temperatures and heating times and the activation energy of the transformation is derived to be 67.6 +/- 2.0 kJ/mol with a deviation of about 1.7 % from the traditional difference scanning calorimetry (DSC) measurement. Unlike the DSC method, THz spectroscopy method derives the thermodynamic parameters of the transformation without heating the sample until it dissolves. These results indicate that THz-TDS can provide an accurate method to detect molecular hydrates and to study their transformations in a nondestructive manner.