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Accelerated Quality-Retentive Radio Frequency Drying of Squid with Pretreatment

This study demonstrates that pulsed electric field (PEF) pretreatment at 20 pulses significantly enhances the drying efficiency and preserves the quality of squid undergoing hot-air-assisted radio frequency drying by creating a porous microstructure, outperforming both brining and ethanol dehydration methods.

Original authors: Zitian Xia, Feilong Zhang, Yuqi Ge, Yongyang Xu, Yinzhe Jin, Wenzheng Shi, Juming Tang, Fanbin Kong, José V. García-Pérez, Yang Jiao

Published 2026-08-06

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CC BY 4.0

Original authors: Zitian Xia, Feilong Zhang, Yuqi Ge, Yongyang Xu, Yinzhe Jin, Wenzheng Shi, Juming Tang, Fanbin Kong, José V. García-Pérez, Yang Jiao

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

Imagine you are trying to dry a wet sponge, but this sponge is made of muscle, packed tight with water, and you want to dry it without cooking it into a tough, brown brick. This is the daily struggle of food scientists working in the world of food preservation. They are constantly looking for ways to remove water from perishable foods like fish and squid to stop them from rotting, but they face a tricky balancing act. If you dry them too slowly with hot air, the food can get mushy or lose its flavor. If you dry them too fast with intense heat, the outside burns while the inside stays wet, or the texture turns into leather.

To solve this, scientists use a technique called Radio Frequency (RF) drying. Think of this like a microwave oven's cousin that heats food from the inside out by making water molecules wiggle and generate heat. It's usually faster and more even than blowing hot air. However, some foods, like squid, have a dense, tight skin and muscle structure that acts like a fortress, trapping moisture inside and blocking the energy from working efficiently. To break down this fortress, scientists often use pretreatments—special steps taken before the main drying process to make the water easier to remove. These can include soaking the food in salty water (osmotic dehydration), using alcohol to pull water out, or zapping it with Pulsed Electric Fields (PEF), which are short, sharp bursts of electricity that poke tiny holes in the food's cells to let water escape. The big question is: which of these "keys" unlocks the best dried squid?

In this study, a team of researchers from Shanghai Ocean University and their international colleagues decided to put three different pretreatment methods to the test on squid. They wanted to see which method would make the Radio Frequency drying process faster while keeping the squid looking, tasting, and feeling as close to fresh as possible. They compared brining (soaking in salt water), ethanol dehydration (soaking in alcohol, sometimes helped by sound waves), and Pulsed Electric Field (PEF) treatment.

The results painted a clear picture. The brining method, where squid was soaked in salty water for four hours, turned out to be a bit of a trap. While it removed some water initially, it left a hard "salt crust" on the surface of the squid. This crust acted like a shield, blocking the Radio Frequency energy from heating the squid effectively. Once the moisture dropped below a certain point (1.56 g/g), the drying process basically stopped, leaving the squid too wet to be fully preserved.

The ethanol method was faster than the salt water, but it came with its own problems. The alcohol caused the squid's tissues to shrink and tighten up, almost like a deflated balloon. This made it harder for the water to move out during the actual drying, slowing the process down compared to doing nothing at all. It also made the squid less able to rehydrate later, meaning it wouldn't soak up water well if you tried to cook it again.

The winner of the experiment was the Pulsed Electric Field (PEF) treatment. By giving the squid exactly 20 pulses of electricity, the researchers created tiny, uniform pores in the squid's surface without damaging the structure. This was like opening a thousand tiny doors in the fortress walls. As a result, the PEF-treated squid dried the fastest, reaching a drying rate of 1.12 g/(g·h), which was higher than the untreated control group.

Beyond just speed, the PEF method was the champion of quality. The dried squid kept its color much better, with a total color difference (ΔE) of only 20.95 ± 1.85, compared to 33.37 ± 6.75 for the untreated group. It also had the lowest levels of lipid oxidation (a measure of rancidity), with a TBARS value of 0.53 mg MDA/kg, meaning it stayed fresher and less likely to go bad. When the researchers looked at the squid under a microscope, the PEF-treated samples had a nice, porous structure, whereas the others were either covered in salt, shrunken, or uneven.

In short, the paper suggests that while soaking squid in salt or alcohol might seem like a good idea to speed things up, they actually create barriers that slow down the process or ruin the texture. The PEF pretreatment, however, acts as the perfect helper. It pokes just enough holes to let the water escape quickly without wrecking the squid's delicate structure, resulting in a product that dries faster, looks better, and stays fresher. The researchers concluded that this method offers the best overall performance for making high-quality dried squid, though they noted that more work is needed to fully understand how these changes affect the flavor and nutrients in the long run.

Technical Summary: Accelerated Quality-Retentive Radio Frequency Drying of Squid with Pretreatment

Problem Statement
Squid is a highly perishable seafood with significant economic value, particularly in Asian markets where dried squid is a staple. While drying is an effective preservation method, conventional hot-air drying suffers from low thermal efficiency and quality degradation due to the dense microstructure of squid muscle, which restricts moisture migration and creates opposing heat and mass transfer fluxes. Although Hot-Air-assisted Radio Frequency (HA-RF) drying offers volumetric heating that aligns heat and mass transfer, the inherent compactness of squid tissue can still limit RF energy coupling and moisture diffusion. Consequently, there is a need to identify pretreatment strategies that can optimize HA-RF drying kinetics while preserving the sensory and nutritional quality of the final product.

Methodology
This study evaluated three distinct pretreatment strategies to enhance the HA-RF drying of squid:

Brining-induced Osmotic Dehydration: Samples were immersed in a 200 g/L NaCl solution at 25°C for up to 6 hours to determine mass transfer kinetics (water loss and solid gain).
Ultrasound-assisted Ethanol Dehydration: Samples were treated with absolute ethanol in an ultrasonic water bath (40 kHz, 150 W) at 25°C for up to 30 minutes to assess rapid dewatering capabilities.
Pulsed Electric Field (PEF): A custom-built device applied electric fields (15 kV/cm) with varying pulse counts (10 to 50) to induce electroporation.

Following pretreatment optimization, squid samples underwent HA-RF drying (60°C hot air, 1 m/s). The study monitored drying kinetics, temperature profiles, and dielectric properties (specifically the loss tangent, $\tan \delta$ ). Quality attributes were assessed through microstructural analysis (SEM), color measurement ( $L^*, a^*, b^*$ , and $\Delta E$ ), rehydration ratio, and lipid oxidation levels (TBARS). Drying curves were fitted to five mathematical models (Lewis, Henderson and Pabis, Logarithmic, Page, and Jena) to characterize the drying behavior.

Key Results

Pretreatment Optimization:
- Brining: Mass transfer stabilized after 4 hours. However, this method resulted in a dense surface salt crust that significantly hindered further drying.
- Ethanol: Ultrasound-assisted ethanol treatment reached equilibrium within 30 minutes. While effective at water removal, it caused tissue shrinkage and protein denaturation.
- PEF: Treatment with 20 pulses created uniform micropores on the surface without excessive structural damage, effectively increasing membrane permeability.
Drying Kinetics:
- Brined Samples: Drying effectively ceased when moisture content dropped below 1.56 g/g. The formation of a salt crust reduced the dielectric loss tangent ( $\tan \delta$ ) by approximately 50%, limiting RF energy absorption and preventing the sample from reaching the target moisture content.
- Ethanol-treated Samples: The drying rate (0.84 g/(g·h)) was lower than the untreated control (0.98 g/(g·h)), likely due to ethanol-induced protein contraction reducing internal diffusion pathways.
- PEF-treated Samples: This group achieved the highest drying rate (1.12 g/(g·h)) and the shortest total drying time (5 hours). The Page model provided the best fit for the drying kinetics across all groups.
Quality Attributes:
- Color: PEF-treated samples exhibited the smallest total color difference ( $\Delta E = 20.95 \pm 1.85$ ) and the highest lightness ( $L^*$ ), indicating superior color retention compared to the control ( $\Delta E = 33.37$ ) and ethanol groups.
- Rehydration: PEF-treated samples demonstrated the highest rehydration capacity, whereas ethanol-treated samples showed the lowest, attributed to irreversible tissue collapse.
- Lipid Oxidation: PEF treatment resulted in the lowest TBARS values (0.53 mg MDA/kg), suggesting effective suppression of lipid oxidation, potentially through the inactivation of endogenous oxidative enzymes.
- Microstructure: SEM analysis revealed that PEF-treated dried squid possessed a porous, uniform microstructure, whereas brined samples had a dense crust and ethanol-treated samples showed significant shrinkage.

Significance and Claims
The paper concludes that Pulsed Electric Field (PEF) pretreatment (specifically 20 pulses) offers the optimum overall performance for accelerating HA-RF drying of squid while preserving product quality. Unlike brining, which creates a barrier to drying, or ethanol, which compromises texture and rehydration, PEF enhances cell membrane permeability to facilitate moisture diffusion without inducing excessive structural damage.

The study claims that PEF pretreatment successfully balances drying efficiency with quality retention, evidenced by:

Increased drying rates and reduced processing time.
Superior preservation of color and reduction in lipid oxidation.
Improved rehydration capacity and a more favorable porous microstructure.

The authors posit that these findings provide a theoretical basis and technical guidance for optimizing efficient, high-quality squid drying processes. They note that while the current study focuses on drying kinetics and physical quality, future work should investigate the impacts of these pretreatments on flavor compounds, specific protein denaturation mechanisms, and nutrient retention. Additionally, the paper suggests that quantitative modeling of cell-level structural changes and the exploration of synergistic effects with other physical field pretreatments are necessary next steps.

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