Molecular rotational relaxation time of water is believed to be an important parameter to evaluate the deterioration rate of biomaterial. In this work, we proposed and verified a method of using Fourier‐transform near infrared spectroscopy, which has advantages of non‐invasion, low‐cost, high spatial resolution as well as being time efficient, to predict the relaxation time of water in biomaterial. The Langevin equation of rotational Brownian motion of water dipole with double‐fold cosine potential barrier, solution of which gives the relaxation time as a function of potential barrier height, was applied to water–saccharide systems. We assumed that this potential barrier height is comparable to the increase of hydrogen bond strength of water in saccharide solution. The hydrogen bond strength values of pure water and saccharide solutions were calculated from the measured NIR absorbance band of water at 4500 –5500 cm−1. The relaxation times yielded by this method are in good agreement with those measured by dielectric spectroscopy. Practical Applications Water plays important role in the investigation of long‐term preservation of biomaterials, since it is the medium of many chemical reactions which eventually lead to the deterioration. In principle, both cryopreservation and dry‐preservation involve the reduction of the mobility of water molecules to reduce chemical reaction rates. The rotational relaxation time as an indicator of molecular mobility of water was proved directly related to the protein deterioration rate. Compared with conventional methods of measuring water relaxation time such as dielectric spectroscopy or magnetic resonance, FT‐IR system has many advantages: low cost, non‐invasion, time efficient, and can provide spatial resolution up to μm level with infrared microscope. Therefore, our method may provide convenience to monitor crop growth and evaluate the quality of food in storage.

Deterioration of biomaterial is subject to the interaction between biomolecules and water, which is often of special interest to the realization of long-term preservation.In previous study, we reported the relationship between dielectric rotational relaxation time and the ν2 + ν3 NIR combination band of water as well as the theory behind it using saccharide solutions as examples and proposed a method accordingly which takes advantages of the infrared microscope to measure the dielectric relaxation time of water in biomaterials. In this work, we examined protein, polypeptide, sugar alcohol and some shelf-stable vegetables to further discuss its applicability and limit. We also proved that this method can be used for measuring water diffusion coefficient and showed example of how the diffusion coefficient can be used to predict water distribution during drying process at a significantly enhanced spatial resolution relative to other techniques.