Variation in Density and Moisture Diffusion Coefficient of Shrimp Affected by Shrinkage During Drying

Introduction
Drying agricultural and aquatic products is a widespread practice globally, aimed at increasing shelf life and preventing microbial growth. During the drying process, heat and mass transfer are dominant phenomena, which are closely linked to the moisture diffusion coefficient. Drying is a thermal process where heat and moisture transfer occur simultaneously, with moisture being reduced as the sample heats up. This process concludes when significant temperature changes, and moisture transfer stops once the moisture content reaches a stable point. The drying process can lead to product shrinkage and pore formation, which can affect the quality and efficiency of drying. Shrinkage occurring alongside moisture release, can impact moisture transfer during drying. In meat processing, muscle shrinkage influences the energy consumption and tenderness of meat products, a key quality trait. Shrinkage and structural changes also contribute to a reduction in food volume during drying. Without considering shrinkage in drying models, results may lack accuracy. Mathematical modeling is a powerful tool for studying moisture transfer phenomena and understanding drying processes. This study examines the effect of shrimp shrinkage on the moisture diffusion coefficient during hot air convection drying. The aim is to assess the impact of shrinkage on moisture diffusion in both linear and nonlinear models and select the most accurate relationship for practical applications.
Materials and Methods
In this study, 212 Vannamei shrimp (Litopenaeus vannamei) which were packed in a standard packaging size for export, with an average weight of approximately 3 kg per box, were purchased from Marjan-e-Bushehr Company. The frozen shrimp boxes were transferred to a laboratory and stored in freezer for experiments. Required samples from batch were randomly chosen and kept for 12 hours at laboratory temperature before testing and skinned. The moisture content of the shrimp was determined by drying 20 shrimp at 105°C for 24 hours. The shrimp were dried in a convective hot air dryer at temperatures of 40, 50, and 60 °C with air velocity of 1 and 2 m/s. Samples were prepared in two forms: intact shrimp and cuboid shape samples (15×17×30 mm). During the drying process, parameters such as mass, volume, thickness, and image data were collected at intervals. The drying continued until the moisture content of the shrimp reached 17 % (d.b.). The shrinkage of dried samples based on volume and surface area were calculated, and mathematical models were developed to describe the relationship between moisture content and shrinkage. These models, evaluated using MATLAB 2020, were used to calculate the moisture diffusion coefficient (D_e) for both intact and cuboid shrimp at different drying duration times. The results were analyzed using various error indices to validate the models' accuracy.
Results and Discussion
The results present the average bulk density values, ranging from 704 to 3374 kg/m³, indicating a significant effect of drying treatments on shrimp volume changes. Previous studies report a bulk density range of 1042 to 1093 kg/m³ for shrimp at 20°C. Drying weakens molecular bonds in shrimp’s protein structures, leading to shrinkage, which ultimately changes its volume and density. Factors like sample geometry, drying method, temperature, and air velocity also influence bulk density. During drying, a consistent trend was observed across all treatments, bulk density increased towards the end as volume reduction slowed. This is due to the decreased moisture content, while shrinkage continues to increase. Analysis showed that polynomial models fit the bulk density data better than linear or exponential ones. Additionally, drying time was strongly affected by temperature, with each degree increase reducing drying time by approximately 45 minutes. The study also explored the use of image processing for assessing shrinkage, which provided more accurate volume measurements compared to surface area-based methods. Moreover, the study suggests that for accurate models of moisture-shrinkage relations, intact samples and volume-based shrinkage measurements using toluene are preferable. However, image processing, being non-destructive, could serve as a suitable alternative. Lastly, moisture diffusion coefficients were affected by the drying conditions, with higher temperatures and air speeds leading to faster drying, as confirmed by other studies. The study also concluded that sample size impacts drying rates, with smaller samples drying faster.
Conclusion
It was found that the true density of shrimp increased due to greater volume reduction than mass change at the end of the drying process. Drying time was influenced by air temperature, velocity, and sample size (shape). The best models subjected to moisture-shrinkage data were Polynomial D2 and D3. Moisture diffusion coefficient was then calculated based on D3 shrinkage model, and it was found that this coefficient is inversely affected by the shrimp shrinkage.