Lina Hu

Dried fruits, vegetables, and mushrooms can be stored and transported without refrigeration, significantly extending their shelf life. However, improper drying methods can adversely affect the texture, color, sensory qualities, and nutritional value. Air-dried products often become tough and rehydrate poorly, while freeze-dried products generally rehydrate well but tend to have an excessively soft texture. With existing drying technologies, maintaining the original quality of food remains a significant challenge, making it difficult for dried products to match the quality of fresh ones (Chapter 1).

In this thesis, we explore whether serially combining different drying techniques can improve the quality retention of rehydrated mushrooms. We also seek to identify the critical factors influencing rehydration performance by studying microstructural evolution during the drying. Additionally, we have developed a mechanistic model of mushroom drying to better understand the heat and mass transfer processes that shape microstructure formation.

In Chapter 2 we took shiitake mushroom as the case study, and studied various combinations of drying techniques such as instant controlled pressure drop drying (DIC) and freeze drying (FD) for porosity enhancement that improves the rehydration performance. These treatments were applied either before or after an additional drying step at low (35 °C) or high (65 °C) temperatures. Based on the principal component analysis (PCA) on quality indicators including relative volume, rehydration rate, dry matter loss and sensory scores, the DIC treatment before hot air drying at 35 °C was shown to be the most promising combination for improving the rehydration quality.

To validate the hypothesis from Chapter 2, we conducted a follow-up study in Chapter 3. Focusing on the microstructural development induced by DIC, we compared overall porosity and connectivity among three groups: hot air dried samples at 35°C (HA35), DIC treated samples, and samples subjected to DIC combined with HA35. Our findings confirmed that the critical temperature range for cell membrane damage lies between 30°C and 40°C. Importantly, DIC pretreatment creates a porous microstructure with increased porosity, provided the product temperature remains below the critical threshold of 40°C during the subsequent drying.

To develop a theoretical model that accurately represents the physical process of mushroom drying, we investigated the moisture sorption isotherms (MSIs) of mushrooms in Chapter 4, through both experimental measurement and theoretical modeling. Our analysis revealed variations in MSIs and hysteresis among mushrooms dried using different methods. We explored potential causes of these variations.

Based on the Flory-Huggins (FH) theory, our findings suggest that protein denaturation does not significantly influence moisture sorption or hysteresis. Cell membrane integrity primarily affects the moisture sorption and water-holding capacity at high relative humidity. Instead, we attribute the observed differences in MSI and hysteresis to viscoelastic relaxation. This conclusion is supported by dynamic vapor sorption (DVS) experiments, which revealed temperature- and moisture-dependent viscoelastic behavior in mushrooms.

In light of these findings, we propose incorporating elastic stress into the driving force of drying, consistent with the Flory-Rehner theory. To accurately describe the stress evolution in viscoelastic media during drying, we suggest adopting a separate model, such as the generalized Maxwell model.

In Chapter 5 we introduce a mechanistic and multiphase model for simulating the hot air drying of shiitake mushrooms. The model integrates several key physical phenomena, including shrinkage, water activity prediction based on Flory-Huggins theory, viscoelastic relaxation described by an extended Maxwell model, and coupled heat and mass transfer in a porous medium. It captures the distinctive drying behavior of mushrooms, such as prolonged internal evaporative cooling and a sharp temperature rise toward the end of drying process. The model was validated against experimental data across multiple drying temperatures, and it was then applied to interpret low-field NMR results, which verified the presence of two thermodynamic phases with the two distinct water populations corresponding to different cellular compartments. By integrating model predictions with electrical conductivity and NMR data, critical thresholds for the cell membrane integrity were identified—specifically, when the product temperature exceeds 40°C or moisture content falls below 3% (w.b.). These findings provide a valuable tool for further optimization of the drying processes to maintain membrane integrity, thereby improving the rehydration quality.

In Chapter 6 we reflect on our findings in a broader context. We discuss DIC treatments in the context of current literature of serial and parallel combined drying. Furthermore, state the found conditions for the preservation of cell membrane integrity for shiitake mushrooms are likely applicable to other biological materials. The state diagram of mushrooms can guide the design of optimized drying protocols. We also acknowledge the developed model's current limitation in mechanistically describing large deformations and shrinkage, which is due to the complexity of soft materials and the lack of cellular-level material properties. Future models should incorporate momentum balance to couple moisture transport, stress, and shrinkage, potentially integrating poromechanics and diffusion-deformation theories. Besides, the potential of T₂-NMR as a non-destructive tool for monitoring water dynamics and microstructure during drying is discussed. The NMR/MRI methods can validate multiphysics models, quantify spatial changes, and enable real-time quality monitoring in industrial drying processes. Finally, future work is recommended to focus on the complete DIC/HA process, generalizing the findings to other fruits and vegetables with the considerations of the instinct material properties like Tg and initial porosity, and developing simulation-driven optimization frameworks for multi-stage drying.

In conclusion, this thesis advances the understanding of serial combined drying for shiitake mushrooms and provides a foundational modeling framework for cellular food materials. By addressing the identified research gaps, future work can accelerate the development of quality-sensitive drying technologies for a wider range of fruits and vegetables.