Ultrasound Assisted Hydration as a Novel Method for In Situ Annealing of Pinto Bean Starch

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Starch, a vital component of many everyday foods like beans, rice, and bread, plays a crucial role in determining texture, stability, and nutritional value. However, its natural structure often limits its performance in cooking and industrial applications.

A groundbreaking study published in Carbohydrate Polymers in 2025 explores how ultrasound technology, combined with heat, can reshape the starch inside whole pinto beans. This research, led by scientists from Monash University and Yangzhou University, challenges traditional views on how ultrasound affects food at the molecular level.

By focusing on in-situ starch—starch embedded within intact plant cells—the study reveals that the heat generated during ultrasound processing, rather than the physical forces of sound waves, drives significant changes in starch properties.

How Ultrasound Technology Alters Starch in Pinto Beans

Starch, a carbohydrate found in plants, is composed of two types of glucose-based molecules: amylose and amylopectin. Amylose consists of linear chains of glucose units linked by α-1,4 glycosidic bonds, while amylopectin is a highly branched molecule with additional α-1,6 glycosidic bonds at branching points.

These molecules are packed into semi-crystalline granules in their natural state, creating a structure that is compact and resistant to water absorption. When heated in water, starch undergoes gelatinization—a process where granules swell, absorb water, and lose their crystalline structure.

This transformation is essential for cooking, as it gives foods like pasta or sauces their desirable texture. However, native starch often lacks the stability required for industrial uses, such as thickening soups or preventing ice crystals in frozen foods.

To improve starch’s functionality, scientists use methods like debranching (breaking the α-1,6 bonds in amylopectin to create simpler molecules) or annealing (heating starch in water to rearrange its structure into a more orderly form). These modifications enhance properties like heat resistance, digestibility, and gel strength.

Recently, ultrasound technology has emerged as an eco-friendly alternative for starch modification. Ultrasound uses high-frequency sound waves (≥20 kHz) to create acoustic cavitation in liquids.

During cavitation, sound waves generate microscopic bubbles that grow and violently collapse, producing intense localized heat (up to 5,000°C for microseconds) and physical forces like microjets (high-speed liquid streams) and shockwaves.

While earlier studies focused on how these physical forces alter isolated starch, this research investigates their effects on in-situ starch—starch protected within the cellular structure of whole pinto beans.

Ultrasound and Heat Effects on Pinto Bean Starch Analyzed

To understand whether ultrasound modifies starch through heat, physical forces, or both, the researchers designed four distinct hydration treatments for pinto beans (Phaseolus vulgaris). Each treatment mimicked real-world processing conditions:

1. Normal Hydration (N): Beans were soaked in water at a constant 25°C for four hours. This served as the baseline to study natural starch behavior.
2. Temperature-Controlled Ultrasound (TC): Beans underwent one hour of ultrasound processing at 20 kHz frequency (50% amplitude) with a cooling system to keep water temperature below 20°C. This minimized heat generation to isolate ultrasound’s physical effects. Afterward, beans were hydrated at 25°C for three hours.
3. Temperature-Uncontrolled Ultrasound (TuC): Ultrasound was applied without cooling, allowing water temperature to rise naturally due to energy dissipation during cavitation. Post-sonication, beans were hydrated at 25°C for three hours.
4. High-Temperature Treatment (HT): Beans were hydrated under the same temperature profile as TuC but without ultrasound. This isolated the effects of heat alone.

After hydration, starch was extracted using a multi-step process involving grinding, sieving, and centrifugation. The extracted starch was analyzed using advanced techniques:

• ¹H Nuclear Magnetic Resonance (NMR): A method that uses magnetic fields to study molecular structure, particularly the branching of starch molecules.
• Size-Exclusion Chromatography (SEC): Separates starch molecules by size to analyze chain length distribution.
• Differential Scanning Calorimetry (DSC): Measures heat absorption during gelatinization to assess thermal stability.
• X-Ray Diffraction (XRD): Reveals long-range crystalline structure by analyzing how X-rays scatter off starch granules.
• Scanning Electron Microscopy (SEM): Provides high-resolution images of starch granule surfaces to detect physical damage.
• Rapid Viscosity Analyzer (RVA): Simulates cooking conditions to study starch’s thickening and breakdown behavior.

Heat Transforms Starch Structure and Functionality in Beans

The study’s most striking discovery was that heat, not ultrasound’s physical forces, played the dominant role in modifying starch.

For instance, Temperature-Uncontrolled Ultrasound (TuC) and High-Temperature Hydration (HT) produced nearly identical results, despite only TuC involving ultrasound. This similarity highlighted that heat generated during cavitation was the primary driver of change.

At the molecular level, starch from TuC and HT treatments showed minor debranching—breaking of the branched amylopectin structure. The degree of branching (DB), calculated using ¹H NMR, dropped from 2.0% in normal starch to 1.9% in TuC and HT starch.

Size-exclusion chromatography (SEC) further revealed a 25% increase in long-chain amylose (molecules with over 1,600 glucose units) in these samples.

These changes suggest that heat caused subtle rearrangements in starch’s molecular architecture, making it more linear and stable. In contrast, Temperature-Controlled Ultrasound (TC) had no significant effect, proving that sound waves alone couldn’t alter starch without heat.

Enhanced Heat Resistance Through Starch Crystal Rearrangement

When starch is heated in water, its helical structures unwind and reorganize into denser, more orderly crystals—a process called annealing. Differential Scanning Calorimetry (DSC) showed that TuC and HT starches required 8°C higher temperatures to gelatinize compared to normal starch.

For example, the peak gelatinization temperature  rose from 66.3°C in normal starch to 74.1°C in TuC starch. This increase signals stronger, more heat-resistant crystals. X-Ray Diffraction (XRD) patterns supported this finding, showing sharper peaks at 18°—a hallmark of tightly packed amylopectin crystals—in TuC and HT starches.

Small-Angle X-Ray Scattering (SAXS), which examines structures at the nanometer scale, further confirmed that TuC and HT starches had higher peak intensities, indicating denser crystalline packing. Despite these changes, the overall crystal structure (C-type pattern) remained intact, ruling out the formation of entirely new crystals.

Instead, existing crystals became more orderly and heat-stable. These structural shifts explain why TuC starch performed better under high-temperature cooking conditions.

Pinto Bean Cell Walls Protect Starch During Ultrasound

Scanning Electron Microscopy (SEM) images provided visual evidence of how pinto bean cell walls protected starch granules. All treatments produced starch granules with smooth, unbroken surfaces, even under intense ultrasound.

This finding contrasts with studies on isolated starch, where ultrasound often causes pitting (small holes) or cracking due to microjet impacts. The thick cell walls of pinto beans likely absorbed and dissipated ultrasound’s physical forces, shielding the starch inside.

However, TuC-treated starch showed more debris—likely fragments of broken cell walls—due to heat-driven tissue breakdown. This debris did not affect starch functionality but highlighted how heat could indirectly alter the surrounding plant matrix.

TuC Starch Performs Best in High-Temperature Cooking

The study tested starch’s cooking behavior using a Rapid Viscosity Analyzer (RVA), a device that simulates real-world conditions like boiling or baking. At 95°C, normal starch (N) reached a peak viscosity of 2,209 centipoise (cP), a measure of thickness.

However, it lost 1,691 cP during cooking (termed breakdown), indicating poor heat stability. TuC starch, however, peaked at just 1,443 cP and lost only 933 cP, demonstrating superior resistance to breakdown.

At 140°C, TuC starch required 20°C higher temperatures to start thickening compared to normal starch. These results make TuC starch ideal for products like canned foods, instant noodles, or baked goods, where high heat stability is critical.

Statistical Proof of Starch Benefits for Food Industry

All results were validated using rigorous statistical methods, including one-way ANOVA (a test to compare group means) and Tukey’s post hoc test (to identify specific differences between groups). For example, the gelatinization temperatures of TuC (74.1°C) and HT (73.4°C) were statistically identical but significantly higher than normal starch.

Similarly, Fourier-Transform Infrared (FTIR) Spectroscopy, which analyzes molecular vibrations, revealed small but significant differences in short-range molecular order between treatments. These findings underscore the reliability of the conclusions and their relevance to food science.

For the food industry, this research offers practical benefits. Letting temperatures rise during ultrasound processing could reduce energy costs by eliminating the need for cooling systems.

Additionally, annealed starch—produced through heat-driven restructuring—could replace chemically modified starches in “clean label” products (foods with minimal artificial ingredients).

Nutritionally, slower-digesting TuC starch might help lower the glycemic index (a measure of how quickly a food raises blood sugar) of foods, benefiting individuals with diabetes.

Future Research on Ultrasound for Legume Starch Modification

While the study answers critical questions, it also raises new ones. For instance, how do ultrasound parameters like frequency or power affect heat generation?

Can this method be scaled for industrial bean processing? The researchers suggest future studies could explore combining ultrasound with enzymes (proteins that speed up chemical reactions) for targeted starch modification or applying these techniques to other legumes like lentils or chickpeas.

Conclusion

This study reshapes our understanding of ultrasound-assisted food processing. By demonstrating that heat—not sound waves—drives starch modification, it paves the way for energy-efficient, scalable methods to create functional starches.

For consumers, this could mean longer-lasting sauces, healthier baked goods, and beans that retain their texture in ready-to-eat meals. As the food industry seeks sustainable solutions, this research highlights the power of simplicity—harnessing heat—to revolutionize an essential ingredient.