Influence of slice thickness on drying kinetics of tray-dried fresh water chestnut chips

Introduction

Water chestnut (Trapa natans), commonly known as Singhara in India, is a widely consumed aquatic crop valued for its nutritional, medicinal, and economic importance. According to Alam et al. (2021) [1] , the crop thrives in freshwater lakes, ponds, and slow-moving rivers. Rich in carbohydrates, proteins, minerals like iron and calcium, and various vitamins, water chestnuts are consumed fresh, boiled, dried, or as flour. Their high water content, however, makes them highly perishable, necessitating proper preservation methods to extend shelf life and maintain nutritional integrity. This aquatic nut crop is primarily grown as a submerged plant community in tropical and subtropical regions. It grows well in neutral to slightly alkaline water bodies and has adapted to muddy edges (Alam et al., 2021) [1] . The kernels are nutrient-dense, containing approximately 52% carbohydrates, up to 20% protein, 9.4% tannins, and small but valuable amounts of fat and minerals. These nuts are also a good source of calcium, potassium, iron, zinc, and vitamin B, and are traditionally consumed for their cooling properties and therapeutic effects, such as alleviating coughs. Drying is one of the oldest and most widely used methods for food preservation. It reduces the moisture content of food to a level that inhibits the growth of spoilage-causing microorganisms. In the context of water chestnuts, drying also helps in reducing bulk and improving shelf stability. Traditionally, open sun drying is practiced, but it is slow, weatherdependent, and often leads to quality degradation. Modern drying techniques like hot-air tray drying offer better control, faster drying rates, and improved hygienic conditions, making them suitable for commercial processing (Singh et al., 2008) [12] . Several factors influence the drying behavior of food materials, such as drying temperature, air velocity, initial moisture content, and sample geometry. Among these, slice thickness plays a pivotal role in determining the drying rate and final product quality. Thinner slices generally dry faster due to a higher surface area-to-volume ratio, but may lead to increased brittleness or shrinkage. Conversely, thicker slices may retain moisture for longer durations but result in better textural integrity. Therefore, optimizing slice thickness is crucial for balancing drying efficiency with product quality. The present study aims to examine the drying characteristics of fresh water chestnut chips of varying slice thicknesses (2 mm, 3 mm, 4 mm, and 5 mm) under different hot-air tray drying temperatures (60 °C, 80 °C, and 100 °C).

Materials and Methods

Sample Preparation: Fresh water chestnuts were procured locally, peeled, washed, and sliced into uniform chips of 2mm, 3mm, 4mm, and 5mm thickness. Equal weights were used for each drying batch to ensure consistency. Aviara & Igbeka, 2024) [6,2] . At 80°C, the drying rate was optimal for 3mm and 4mm slices, balancing drying speed with quality. These results are similar to what researchers found when drying apple slices-medium temperatures helped dry them faster without losing quality (Jeevarathinam et al., 2023) [5] . The same was seen with tomatoes and papayas, where drying at intermediate temperatures gave the best results for keeping good texture and color (Karathoula & Chouliara, 2014; Pradhan et al., 2018) [8,10] . At 100 °C, brittleness increased sharply in 2mm slices. Drying at very high temperatures caused the slices to become too dry and lose their structure. This is similar to what happened in studies on mango and sweet potato, where thin slices became too crisp or even burnt, making them less appealing to eat (Kamal et al., 2023) [6] . A similar result was found by Karathoula and Chouliara (2014) [8] , who saw that tomato slices dried too quickly at high heat ended up with poor texture.

Graphical Observations

Graphs of drying rate vs. time and moisture content vs. time confirmed that thinner slices dried faster. Drying rate decreased over time for all samples. At 60 °C, the drying process was the slowest across all slice thicknesses. The 2mm slices reached equilibrium moisture content faster than the others, but they exhibited higher brittleness and more surface cracks. The 5mm slices retained their structure better but showed prolonged drying times and inconsistent drying towards the core. The intermediate thicknesses, especially 3mm and 4mm, displayed a balanced drying behavior with moderate drying time and good textural integrity. At 100°C, drying was most efficient in terms of rate and product quality. The 3mm and 4mm slices emerged as the optimal thicknesses under this condition, providing good texture, acceptable drying time, and low moisture content. The 2mm slices, though dried quickest, were overly crisp and fragile, making them less suitable for handling or postprocessing. The 5 mm slices still retained some moisture at the core even after prolonged drying, affecting their suitability for further applications like frying. Drying rate analysis revealed that thinner slices started with a higher drying rate, which declined over time following a falling rate trend typical of biological materials. The initial moisture removal was rapid due to surface evaporation, while the latter phase was governed by internal diffusion. The drying rates were highest at 100 °C for all thicknesses, particularly during the first 60 to 100 minutes of drying. The thicker slices had lower drying rates and longer drying durations due to a lower surface area-to-volume ratio.

Conclusion

Both slice thickness and drying temperature significantly influenced the drying behavior of water chestnut chips. Among all combinations, slices of 3mm and 4mm thickness dried at 100 °C produced the most desirable quality in terms of texture, moisture level, and drying efficiency. These findings can aid in optimizing tray drying conditions for water chestnut processing in commercial applications.

References

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