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Ultrasound-Assisted Convective Drying of Grape Pomace: Mathematical Modeling, Effective Moisture Diffusivity, and Rehydration Characteristics

This study demonstrates that a 15-minute ultrasonic pretreatment significantly reduces the convective drying time of grape pomace by 27% and increases its effective moisture diffusivity by 37% without compromising rehydration, with the Page model identified as the most accurate predictor of the drying kinetics.

Original authors: Amir Hosseinvand, Fizza Saleem, Elena. G. Kovaleva

Published 2026-07-06

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Amir Hosseinvand, Fizza Saleem, Elena. G. Kovaleva

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

The Big Picture: Turning Wine Waste into a Shelf-Stable Product

Imagine the wine industry as a massive party. After the grapes are crushed to make wine, you are left with a huge pile of "leftovers"—the skins, seeds, and stems. This is called grape pomace.

The problem is that this pile is like a wet sponge sitting in the sun. It's full of water and sugar, which means it rots and smells bad very quickly. To save it, the researchers wanted to dry it out so it could be stored and used later (like turning it into a powder for food or cosmetics).

However, drying a wet sponge takes a long time. The researchers asked: "Can we use sound waves (ultrasound) to speed up the drying process, like shaking a wet sponge to get the water out faster?"

The Experiment: The "Sound Bath" for Grapes

The team took grape pomace and gave it a "sound bath" before putting it in a hot oven. They used an ultrasonic machine (the kind dentists use for cleaning teeth, but scaled up) to blast the grapes with sound waves for different amounts of time:

0 minutes: No sound (the control group).
5 minutes: A quick sound bath.
10 minutes: A medium sound bath.
15 minutes: A long sound bath.

After the sound bath, they put the grapes in a hot air dryer and timed how long it took to get them completely dry.

The Results: Shaking the Sponge Works

The results were clear: The more sound they used, the faster the grapes dried.

The Time Saved: Without any sound, the grapes took 465 minutes (almost 8 hours) to dry. With a 15-minute sound bath, they only took 340 minutes (about 5.5 hours). That's a 27% reduction in time.
The "Why": Think of the grape pomace as a dense, wet sponge with tiny, tight cells holding the water inside. The ultrasound waves act like tiny, invisible hammers. They create bubbles in the water that pop (a process called cavitation). These popping bubbles create tiny cracks and channels in the grape skins and seeds.
The Result: Instead of water slowly sweating out of the grape, it now has "highways" to escape. The researchers measured this as the Effective Moisture Diffusivity (a fancy way of saying "how fast water moves"). This speed increased by 37% with the sound treatment.

The Math: Predicting the Drying

The researchers didn't just watch the grapes dry; they tried to predict how they would dry using six different mathematical formulas (like trying to guess the shape of a falling leaf using different equations).

They tested models named after scientists like Newton, Page, and Henderson.
The Winner: The "Page Model" was the best at predicting exactly how much water was left in the grapes at any given moment. It was the most accurate map for the journey from wet to dry.

The Aftermath: Can It Get Wet Again?

Once the grapes were dried, the researchers wanted to see if they could soak them back up in water (rehydration). This is important because if you turn the grapes into a powder, you might want to add water to them later.

The Finding: Soaking the grapes back up didn't change much based on how long they were blasted with sound. Whether they had 5 minutes or 15 minutes of ultrasound, they only got about 6% better at soaking up water.
The Takeaway: The ultrasound helped them dry faster, but it didn't dramatically change how they drink water later.

The Final Verdict

The study concludes that using 15 minutes of ultrasound before drying grape pomace is the "sweet spot."

It's an eco-friendly way to:

Save time: You get dry product 27% faster.
Save energy: Less time in the oven means less electricity used.
Improve efficiency: The water moves out of the grapes 37% faster.

The researchers recommend this method as a smart, green way to handle the massive amounts of waste the wine industry creates, turning a perishable mess into a stable, useful product.

Technical Summary: Ultrasound-Assisted Convective Drying of Grape Pomace

Problem Statement
The global wine industry generates approximately 8.49 million metric tons of grape pomace annually, a by-product comprising skins, seeds, and stalks. This material is highly perishable due to its high initial moisture content (60–87% wet basis) and residual sugars, leading to rapid microbial spoilage. Historically, management has relied on linear disposal methods (landfilling, incineration), which result in greenhouse gas emissions and the loss of valuable biomass. While convective hot air drying offers a viable method to stabilize pomace for use in functional foods and nutraceuticals, the process is time-consuming. Furthermore, there is a lack of systematic research regarding the impact of ultrasound pretreatment on the drying kinetics of red grape pomace, nor have thin-layer mathematical models been previously evaluated for this specific application.

Methodology
The study utilized Merlot grape pomace sourced from a Russian wine-processing factory. The experimental design involved the following steps:

Pretreatment: Whole grape pomace samples were subjected to ultrasound pretreatment in a 40 kHz, 150 W ultrasonic bath for durations of 0 (control), 5, 10, and 15 minutes.
Drying Process: Treated samples were dried in a hot-air convection oven at 45°C until equilibrium moisture was reached. Weight loss was recorded every 15 minutes.
Diffusivity Calculation: The effective moisture diffusivity coefficient ( $D_{eff}$ ) was calculated using Fick's Second Law of diffusion, assuming spherical particle geometry and no external resistance.
Kinetic Modeling: Six mathematical models (Newton, Page, Modified Page, Henderson & Pabis, Logarithmic, and Two Term) were fitted to the experimental moisture ratio data using nonlinear regression. Model performance was evaluated using statistical indicators: coefficient of determination ( $R^2$ ), root mean square error (RMSE), chi-square ( $\chi^2$ ), and modeling efficiency (EF).
Rehydration: The rehydration ratio (RR) was determined by immersing dried samples in distilled water at 45°C for 30 minutes.

Key Results

Drying Time Reduction: Ultrasound pretreatment significantly reduced the total drying time. Increasing the sonication time from 0 to 15 minutes decreased the drying duration from 465 minutes to 340 minutes, representing a 27% reduction.
Effective Moisture Diffusivity: The effective moisture diffusivity coefficient increased significantly ( $p < 0.05$ ) with longer sonication times. The coefficient rose from $3.26 \times 10^{-10} \, m^2/s$ (0 min) to $4.52 \times 10^{-10} \, m^2/s$ (15 min), an average increase of 37%.
Mathematical Modeling: Among the six models tested, the Page model provided the best fit for the drying kinetics of ultrasound-treated grape pomace. It demonstrated the highest correlation coefficients ( $R^2$ ranging from 0.9984 to 0.9986) and the lowest error values (RMSE between 0.0096 and 0.0156) across all treatments.
Rehydration Characteristics: While ultrasound pretreatment accelerated drying, its effect on rehydration was minimal. Increasing pretreatment time from 5 to 15 minutes resulted in only a 6% increase in the rehydration ratio, with no statistically significant difference observed between varying application times.

Significance and Claims
The authors claim that this study fills critical knowledge gaps by systematically quantifying the effects of ultrasound pretreatment on the drying behavior of red grape pomace and identifying the most accurate mathematical model for this process. The research demonstrates that a 15-minute ultrasound pretreatment is an effective, eco-friendly method to enhance mass transfer capabilities and significantly decrease overall drying duration without compromising the material's ability to rehydrate. By validating the Page model for this specific application, the study provides an objective basis for the industrial valorization of grape pomace, supporting its transformation into stable, shelf-stable products for the circular economy. The study concludes that the proposed method reduces drying time by 27% and increases the moisture diffusion coefficient by 37%, offering a practical optimization for convective drying processes.