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Influence of Ultrasound Pretreatment on the Infrared Drying Kinetics and Quality Attributes of Mallow Leaves

This study demonstrates that pretreating mallow leaves with ultrasound significantly enhances infrared drying efficiency by increasing effective moisture diffusivity and reducing activation energy and specific energy consumption, while also improving the final product's color and rehydration quality.

Original authors: Abolfazl Akhoundzadeh Yamchi, Maziar Feizolahzadeh, Adel Hosainpour, Adel Rezvanivand Fanaei, Ariga Moradians

Published 2026-09-04

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CC BY 4.0

Original authors: Abolfazl Akhoundzadeh Yamchi, Maziar Feizolahzadeh, Adel Hosainpour, Adel Rezvanivand Fanaei, Ariga Moradians

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Preserving the bounty of the harvest is one of humanity's oldest challenges. Fresh herbs and vegetables are rich in nutrients and flavor, but they are also fragile; their high water content makes them prone to rapid spoilage, limiting how long they can be stored or how far they can travel. To extend their life, people have long turned to drying, a process that removes water to halt decay. However, traditional drying often relies on intense heat, which can act like a double-edged sword. While it removes moisture, the heat can also degrade the very things that make the plant valuable: its vibrant green color, its delicate medicinal compounds, and its ability to reabsorb water later when needed. Scientists are now exploring ways to dry these sensitive plants faster and more gently, looking for methods that speed up the removal of water without subjecting the plant to damaging temperatures for too long. One promising approach involves using sound waves before the drying begins, a technique that subtly changes the plant's internal structure to help water escape more easily.

In a recent study, researchers at Urmia University in Iran investigated this approach using mallow leaves, a common medicinal herb known for its anti-inflammatory properties and high content of vitamins and antioxidants. The team wanted to see if treating the fresh leaves with ultrasound—a type of high-frequency sound—before drying them with infrared heat would improve the final product. They set up a controlled experiment where they took fresh mallow leaves and subjected them to ultrasound for different lengths of time: none at all, five minutes, or ten minutes. After this treatment, the leaves were placed in an infrared dryer, which uses radiant heat similar to the warmth felt from the sun, at three different temperatures: 40, 45, and 50 degrees Celsius. The goal was to dry the leaves until they reached a specific low moisture level, while carefully tracking how long it took, how much energy was used, and how the quality of the leaves changed.

The results revealed a clear and beneficial pattern. The combination of ultrasound pretreatment and infrared heat significantly sped up the drying process. The leaves that received the longest ultrasound treatment and were dried at the highest temperature finished the process much faster than those that received no sound treatment or were dried at lower temperatures. This acceleration happened because the ultrasound waves created tiny channels and disruptions within the leaf's cellular structure, essentially opening up pathways for the water to move to the surface more quickly. Because the water could escape faster, the leaves spent less time exposed to heat, which is crucial for preserving their quality. The researchers measured the energy required to remove each kilogram of water and found that the most efficient method—using the highest temperature and the longest ultrasound treatment—consumed the least amount of energy, specifically 33.81 kilowatt-hours per kilogram.

Beyond speed and energy, the study showed that this method helped maintain the visual and functional quality of the mallow leaves. Drying at lower temperatures combined with ultrasound treatment resulted in leaves that retained more of their natural green color and suffered less browning, a common sign of degradation in dried herbs. The leaves also proved better at reabsorbing water when soaked later, a key indicator of how well the plant's structure survived the drying process. The leaves treated with ultrasound for ten minutes and dried at 50 degrees Celsius showed the best ability to rehydrate, suggesting that the sound waves helped create a more porous structure that could easily take up water again. The researchers also looked at the thermodynamics of the process, which describes the energy changes involved in moving water out of the plant. They found that the ultrasound treatment lowered the energy barrier required to remove the water, making the process more efficient.

The study concluded that using ultrasound before infrared drying is a highly effective strategy for preserving medicinal herbs like mallow. It offers a way to dry the leaves quickly and with less energy while protecting their color, nutritional value, and ability to be reconstituted later. The researchers identified an optimal set of conditions for this process: drying at approximately 49 degrees Celsius after a ten-minute ultrasound treatment. This approach provides a practical, scalable solution for food processors and herbal manufacturers who need to produce high-quality dried products without the excessive energy costs or quality loss associated with traditional drying methods. By understanding how sound waves can gently prepare plant tissues for drying, this research opens the door to more sustainable and efficient ways of preserving the world's valuable botanical resources.

Technical Summary: Influence of Ultrasound Pretreatment on the Infrared Drying Kinetics and Quality Attributes of Mallow Leaves

Problem Statement
Fresh mallow (Malva sylvestris) leaves are highly perishable due to high moisture content, leading to significant postharvest losses (25–30%) and limiting their marketability. While drying is a primary preservation method, conventional techniques often expose plant tissues to prolonged thermal stress, degrading heat-sensitive bioactive compounds and compromising sensory qualities such as color and rehydration capacity. Although ultrasound-assisted drying has shown promise in other contexts, there is a scarcity of research specifically addressing the combined effects of ultrasound pretreatment and infrared (IR) drying on mallow leaves, particularly regarding drying kinetics, thermodynamic parameters, and specific energy consumption (SEC).

Methodology
The study investigated the effects of ultrasound pretreatment duration (0, 5, and 10 minutes) and infrared drying temperatures (40, 45, and 50°C) on the drying behavior and quality of mallow leaves.

Experimental Setup: Fresh leaves were pretreated with an airborne ultrasonic system (20 kHz, 20 kW) and subsequently dried in a laboratory-scale IR dryer equipped with four 250 W near-infrared halogen lamps.
Process Control: Drying continued until the moisture content reached 10% (wet basis). Moisture content was monitored at 20-minute intervals.
Quality Assessment: Key attributes included color parameters (L*, a*, b*, chroma, total color difference $\Delta E$ , and browning index), rehydration ratio (RR) at 25°C and 65°C, and specific energy consumption (SEC).
Modeling and Analysis: Effective moisture diffusivity ( $D_{eff}$ ) was calculated using Fick's second law. Activation energy ( $E_a$ ) and thermodynamic properties (enthalpy $\Delta H$ , entropy $\Delta S$ , and Gibbs free energy $\Delta G$ ) were derived using the Arrhenius relationship and standard thermodynamic equations. Statistical analysis was performed using two-way ANOVA and Response Surface Methodology (RSM) for optimization.

Key Results

Drying Kinetics: Both increasing drying temperature and extending ultrasound pretreatment time significantly reduced total drying time. The synergistic effect of higher IR temperatures and longer sonication accelerated moisture removal by enhancing internal mass transfer.
Moisture Diffusivity and Activation Energy: The effective moisture diffusivity ( $D_{eff}$ ) increased with higher temperatures and longer ultrasound exposure, ranging from $2.39 \times 10^{-7}$ to $8.68 \times 10^{-7} \text{ m}^2\text{s}^{-1}$ . Conversely, the activation energy ( $E_a$ ) decreased from 86.32 to 73.73 kJ mol $^{-1}$ as ultrasound pretreatment time increased, indicating that ultrasound reduced the energy barrier for moisture migration.
Quality Attributes:
- Color: Color degradation (increased $\Delta E$ and browning index) was more pronounced at higher temperatures and shorter ultrasound times. The best color preservation (highest chroma, lowest $\Delta E$ and browning) was achieved at 40°C with 10 minutes of ultrasound.
- Rehydration: The highest rehydration ratio was observed in samples dried at 50°C with 10 minutes of ultrasound pretreatment and rehydrated at 65°C, suggesting that ultrasound-induced microstructural changes (microchannels) facilitated water absorption.
Thermodynamics: Enthalpy ( $\Delta H$ ) and entropy ( $\Delta S$ ) decreased with increasing temperature and ultrasound time, while Gibbs free energy ( $\Delta G$ ) increased. The consistently positive $\Delta G$ values confirmed that the drying process is non-spontaneous and requires continuous external energy.
Energy Efficiency: The lowest specific energy consumption (SEC = 33.81 kWh kg $^{-1}$ ) was recorded at the highest drying temperature (50°C) combined with the longest ultrasound pretreatment (10 min).

Optimization
Using Response Surface Methodology, the study identified optimal processing conditions balancing drying time, energy consumption, and quality. The predicted optimum was an infrared drying temperature of 49.27°C combined with a 10-minute ultrasound pretreatment. Under these conditions, the model predicted a drying time of 61.88 minutes, an SEC of 33.81 kWh kg $^{-1}$ , and maximized rehydration capacity and effective diffusivity.

Significance and Claims
The authors claim that this study provides a quantitative linkage between pretreatment intensity, drying behavior, energy demand, and thermodynamic evolution in mallow leaves. The primary contribution is demonstrating that ultrasound pretreatment prior to infrared drying significantly improves both the efficiency and quality of dried mallow leaves. Specifically, the integration of these technologies:

Accelerates Drying: Reduces drying time and specific energy consumption by creating microchannels and disrupting cell walls, thereby lowering internal mass transfer resistance.
Preserves Quality: Mitigates thermal degradation of pigments and bioactive compounds, offering a scalable solution for producing high-quality, minimally processed medicinal herbs.
Offers Practical Guidelines: The identified optimal conditions provide a feasible operating window for industry to enhance product shelf life and meet consumer demand for healthy, energy-efficient dried products.

The paper concludes that ultrasound-assisted infrared drying is a promising, energy-efficient strategy for processing heat-sensitive medicinal herbs, though it notes that further investigation is needed to validate scalability at pilot or industrial levels.

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