Starch Nanocrystals and Plant Proteins Team Up to Keep Food Foams Alive for Hours

Aqueous foams are everywhere in the food world, from the cloud of whipped cream on a dessert to the dense froth of a mousse or the head on a carbonated drink. They define texture, deliver low-calorie volume, and create a sense of indulgence that consumers crave, particularly as plant-based and reduced-calorie diets gain ground. Yet anyone who has watched a cappuccino deflate knows the fundamental problem: most aqueous foams are fleeting. Their lifetimes are typically measured in minutes, sometimes only seconds, because the thin liquid films separating bubbles are constantly fighting gravity, capillary pressure, and interfacial rupture. A new study published in the Journal of Agriculture and Food Research reports a plant-derived solution that stretches foam lifetime from under half an hour to more than twelve hours, an approximately twenty-three-fold improvement, using nothing more exotic than rice protein, soy protein, and starch nanocrystals.

The research team, led by Yong Liang, Wei Wang, Kai Huang, Zhengxing Chen, and Tao Wang, began from a well-known obstacle. Rice proteins are nutritionally attractive because they offer a balanced profile of essential amino acids and low allergenicity, but as foam stabilizers they are notoriously difficult. They are largely insoluble in water and prone to severe aggregation, which prevents them from diffusing quickly to the air-water interface and forming a coherent film. Foams stabilized by plant protein particles alone often collapse within seconds or minutes, because these proteins tend to sit more comfortably in the air phase than in the liquid phase, leaving the foam vulnerable to rapid water drainage. Enhancing the water retention capacity of the interfacial film therefore became the central design goal of the work.

To solve the solubility problem, the researchers co-assembled rice proteins with soy protein isolates using a pathway of alkali solvation followed by acid precipitation. Both proteins were first unfolded at high pH, allowed to interact, and then refolded together into uniform, water-dispersible nanoparticles. Transmission electron microscopy showed that while rice proteins alone formed aggregates of roughly thirty-nanometer elementary particles, the co-assembled rice-soy protein nanoparticles, prepared at a one-to-one mass ratio, emerged as monodisperse particles of about forty nanometers. Zeta potential measurements revealed something unexpected: the complexes carried stronger surface charges than either protein alone, while their surface hydrophobicity was lower than that of either parent protein. This indicated that the hydrophobic groups of both proteins had been buried inside the co-assembled structures while the charged groups were exposed outward, a rearrangement driven by hydrophobic interactions, electrostatic attraction, and hydrogen bonding.

These nanoparticles already improved foaming. Soy protein isolates alone could generate foams filling seventy-five percent of the initial dispersion volume, but only five percent survived thirty minutes. The co-assembled rice-soy nanoparticles at optimal ratios reached one hundred percent foaming ability and pushed thirty-minute stability to roughly fifteen percent at higher concentrations, plateauing at fifty percent survival once the interface became saturated. That was progress, but more than eighty-five percent of the foam still vanished within half an hour. The team then turned to starch nanocrystals, rigid platelets produced by hydrolyzing waxy corn starch in sulfuric acid for seven days. These crystals carry abundant hydroxyl groups on their surfaces, making them strongly hydrophilic and mechanically robust, but they also aggregate heavily through hydrogen bonding between their faces.

When the protein nanoparticles and starch nanocrystals were simply mixed together, the two components associated into hybrid complexes the researchers call RSP@SNCs. Atomic force microscopy revealed that the aggregated starch crystals disaggregated and reappeared as sheet-like hybrid nanoparticles of one hundred to two hundred nanometers, a morphology distinct from either component. Intrinsic fluorescence spectroscopy provided quantitative evidence of close association: as starch nanocrystal concentration increased, the fluorescence of the protein’s aromatic amino acid residues was progressively quenched, following an apparent Stern-Volmer-type relationship with a mass-based quenching coefficient of 1.896 milliliters per milligram and a correlation coefficient of 0.9677. Fourier transform infrared spectroscopy showed no new absorption bands or significant shifts, confirming that the binding was non-covalent.

The molecular glue holding the complexes together turned out to be hydrogen bonding. In a clever chemical blocking experiment, the team repeated the complexation in the presence of sodium chloride, sodium dodecyl sulfate, or thiourea, which selectively disrupt ionic forces, hydrophobic attractions, and hydrogen bonding respectively. Thiourea dramatically weakened the protein-starch interactions and restored fluorescence, while the salt and surfactant had only minor effects. Because the starch nanocrystals self-aggregate primarily through hydrogen bonds between their hydroxyl groups, competition from protein-starch hydrogen bonding appears to reduce the surface energy of the crystals, suppressing their self-aggregation and yielding dispersed hybrids with balanced amphiphilic character, exactly what is needed to anchor at an air-water interface.

The foaming results were striking. Adding starch nanocrystals doubled the initial foam formation of the protein nanoparticles at the highest tested ratio of 1.2, while optical microscopy showed that bubbles became smaller and more homogeneous as the starch proportion rose. Confocal laser scanning microscopy, with proteins and starch labeled in different fluorescent colors, revealed that both components were jointly trapped at the air-water interface, forming a thick physical barrier around each bubble. In foams stabilized by protein alone, bubbles approached a millimeter in size with thin, patchy coverage; with the hybrids at a ratio of 0.8, bubble sizes dropped to hundreds of micrometers and the liquid-filled spaces between adjacent bubbles widened, reducing direct bubble contact and hindering coalescence.

The stabilization mechanism operates on three coupled fronts. First, the interfacial film itself: contact angle measurements showed starch nanocrystals are highly hydrophilic at forty degrees while rice-soy protein nanoparticles are more hydrophobic at seventy-nine degrees, and the hybrids strike a balance that wets both phases, lowers interfacial tension, and satisfies the Gibbs stability criterion against coarsening. Second, the bulk viscosity: shear rheology demonstrated that viscosity of the stabilizer dispersions increased with starch content, and cryo-scanning electron microscopy performed at minus 145 degrees Celsius revealed ordered hybrid structures arranged throughout the continuous phase, not just at the interface. These ordered microstructures elevated system viscosity, slowing bubble disproportionation and liquid drainage. Notably, the dispersions showed shear-thinning behavior, meaning that during the violence of whipping the mixture temporarily thins, allowing particles to race to the interface, after which viscosity rebuilds to lock the structure in place. Third, water retention: the hydrophilic starch crystals trap water in the interstitial spaces between bubbles, deterring the gravity-driven drainage that kills most foams.

The practical implications extend well beyond the laboratory. The authors point to applications in milky tea, lattes, and other foamed beverages where stability over hours rather than minutes would transform product quality, particularly for delivery and takeaway markets. Because every ingredient is edible and plant-derived, the system sidesteps the limitation of surfactant crystals, which can produce extraordinarily stable foams but are not food-grade. The work also demonstrates a broader design principle: rather than searching for a single miracle stabilizer, combining a diffusion-competent protein nanoparticle with a mechanically strong, hydrophilic nanocrystal produces synergy that neither component achieves alone. As demand grows for plant-based, low-calorie foods with indulgent textures, engineering the air-water interface with hybrid biocolloids may become a standard tool in the food engineer’s repertoire, turning one of the most fragile structures in cuisine into one of the most durable.

Subject of Research: Stabilization of aqueous food foams using rice and soy protein nanoparticles reinforced with starch nanocrystals

Article Title: Starch nanocrystals reinforce rice/soy protein-structured air-water interface to stabilize aqueous foams

Article References: Liang, Y., Wang, W., Huang, K., Chen, Z., & Wang, T. (2026). Starch nanocrystals reinforce rice/soy protein-structured air-water interface to stabilize aqueous foams. Journal of Agriculture and Food Research, Article 103352. https://doi.org/10.1016/j.jafr.2026.103352

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103352

Keywords: aqueous foams, starch nanocrystals, rice protein, soy protein isolate, food colloids, air-water interface, hydrogen bonding, foam stability, plant-based foods, Pickering stabilization, interfacial tension, food engineering