A Study of Infrared Heating Technology for Tomato Peeling: Process Characterization and Modeling

The conventional hot lye and steam peeling methods used in the tomato processing industry are energy and water intensive operations. Moreover, these peeling methods also lead to serious wastewater salinity and disposal problems. A sustainable peeling alternative has long been pursued to reduce the usage of chemicals and water while maintaining or enhancing the product quality. Infrared (IR) radiation has the characteristic of rapid surface heating, offering the potential to develop a novel peeling method. For the first time, this research investigated the feasibility of using IR for peeling tomatoes, elucidated the underlying IR peeling mechanism, and characterized the key engineering parameters associated with IR heating.;To study the feasibility, four potential peeling techniques, including IR peeling, lye-IR peeling, enzymatic peeling, and enzyme-IR peeling, were evaluated based on metrics of interest and compared with conventional lye peeling. IR peeling yielded the most promising results compared to the other peeling methods, reducing peeling loss by 9% and producing a firmer product with a similar ease of peeling and processing time compared to lye peeling. Because no chemicals and water are required in the process, it was termed as IR dry-peeling. To further study this technique, comparable evaluations between regular lye and IR dry-peeling methods were performed using multiple cultivars in different seasons. The results for IR peeling indicated lower peeling loss (8.3%–13.2%), thinner peel thickness (0.39–0.91 mm), and slightly firmer texture of peeled products (1.05–2.01 N) as compared to the regular lye peeling (i.e. 13.2%–15.8% of peeling loss, 0.85–1.06 mm of thickness, 0.96–1.40 N of firmness) while achieving the same degree of peelability (<0.015 cm2/g) and ease of peeling (>4.0).;To better understand the IR heating effect on skin separation, which involves peel loosening and cracking phenomena, both experimental and theoretical analyses were performed. Substantial biomechanical changes in IR heated skins were characterized as reduced adhesive energy, increased dynamic moduli, and shifted transition temperature by means of textural analysis and dynamic mechanical analysis. With use of Light Microscopy and Scanning Electron Microscopy, it was observed that peel loosening appeared to result from reorganization of extracellular cuticles, thermal expansion of cell walls, and collapse of several cellular layers. Crack behavior of tomato skin was studied within a framework of elastic thin shell theory. Mechanical stress analyses together with experimentally measured failure stress of tomato skin were integrated to interpret the occurrence of peel cracking due to IR heating. In order to achieve a sufficient skin separation under IR heating, promoting rapid and uniform heating on the tomato surface is essential.;The variation in size and irregular shape of processing tomatoes introduce additional challenges to achieve uniform heating and design optimal IR emitter configurations. To address this challenge, a three-dimensional geometric model of processing tomatoes was developed to capture the important morphological features of tomatoes and accurately describe the variability in shape and size. Modeled tomatoes with realistic shapes and different sizes were further employed to predict the temperature distributions on the surface and within tomatoes during IR heating. The IR heating process was postulated as a mathematically gray-diffuse radiation problem based on the enclosure theory. A predictive heat transfer model was developed and solved by using a finite element scheme. The predicted temperatures agreed well with experimental data (r2>0.9). Simulation results illustrated that IR heating induced a dramatic temperature increase on the tomato surface which extended to 0.6 mm beneath (>90°C) during a 60 s heating period, whereas interior temperature at the tomato center remained low (<30°C). Sensitivity analysis suggested that strategies to enhance IR heating performance can be implemented through varying emissive power, adjusting the distance between emitters, and presorting tomatoes according to size.;Through this research, a significant breakthrough of using IR radiation to effectively peel tomatoes without using chemicals and water was achieved. The innovative IR dry-peeling method overcomes limitations of conventional methods and would bring numerous benefits, including eliminated chemical contamination, huge savings in water and water-related energy consumption, higher quality products, and recovery of tomato peels as a value-added byproduct. This dissertation characterizes the fundamentals of IR dry-peeling through integrated experimental and modeling approaches, and provides important scientific information to guide the development and design of commercial IR dry-peeling systems.