InterPore2025

Investigating Drying Dynamics and Shrinkage in Biological Materials Using NMR Techniques: Potatoes as a Case Study

20 May 2025, 12:50

15m

Oral Presentation (MS20) Biophysics of living porous media MS20

Speaker

Yuliang ZOU

Description

The drying of biological materials, such as vegetables and fruits, is a critical process in food preservation, enhancing shelf life, maintaining nutritional value, and reducing transportation costs. Potatoes, as a staple crop and a primary source of carbohydrates globally, are particularly significant in this context. However, drying processes for biological materials are often complex, involving coupled heat and mass transfer, non-linear shrinkage, and evolving pore structures. A deeper understanding of these phenomena is essential for optimizing drying techniques, minimizing energy consumption, and improving product quality.

In this study, we employ Nuclear Magnetic Resonance (NMR) techniques with home-made sequence [1] to investigate the drying dynamics and shrinkage behavior of potatoes. A NMR CPMG sequence is adopted to distinguish and track water states, which enables us to precisely distinguish three types water according to their different transversal relaxation times (T2): intracellular free water, cell wall water, and bound water adsorbed in starch granules. As the drying rate on boundary may impact the drying mechanism [2, 3], one-dimensional drying experiments on cylindrical potato samples have been conducted. Under slow drying conditions, the drying rate remains constant while most of the water is extracted, then it starts decreasing. We show that during the constant rate period the free water is first extracted, then the cell wall water; the decreasing rate period corresponds exactly to the extraction of bound water. Under fast drying conditions, the drying rate continuously decreases, and T2 signals corresponding to intracellular and cell wall free water diminished simultaneously, with bound water drying occurring only after the disappearance of all free water and bound water. Interestingly, the drying rate decreased continuously from the beginning.

These results yield the complex interplay between water transport mechanisms and structural changes. Our findings suggest that intracellular free water may first transition to cell wall free water for effective transport and diffusion. Additionally, isotropic drying shrinkage was observed during initial drying, maintaining sample saturation. Anisotropic shrinkage appears once longitudinal shrinkage reached a threshold. As drying progressed, voids pore emerged, facilitating water vapor diffusion, indicating a shift from liquid (bound) water transport to combined liquid (or bound water)-vapor transport in later stages. A similar phenomenon of bound water-vapor transport was observed in our previous work [4]. Based on our experimental observations, we propose a comprehensive physical model to describe the coupled processes of drying-induced shrinkage, water transport, and vapor diffusion. This model provides a robust framework for advancing drying technologies in food science and material engineering.

The implications of this study extend beyond potatoes to other fruits and vegetables, offering insights into the optimization of drying processes across a range of agricultural products. Improved understanding of these dynamics can lead to energy-efficient drying techniques, reduced food waste, and better preservation of quality and nutritional content. These outcomes are particularly critical in the context of global food security and sustainability.