One Starch, One Sweet Spot: How Corn Starch Concentration Decides Whether Freeze-Dried Grapes Keep Their Shape

Freeze-drying is supposed to be the gentle giant of food preservation. Fruit is frozen, the surrounding pressure drops, and the ice inside skips the liquid phase entirely, sublimating straight into vapor while leaving behind a light, porous scaffold. For low-sugar produce this works beautifully. But grapes are a different beast. Packed with soluble sugars and organic acids, they form thick, glassy sugar-acid phases during freezing that offer almost no mechanical support. When the ice leaves, the structure around it simply gives up, collapsing into shrunken, fragmented remnants. A new study published in Current Research in Food Science tackles this stubborn problem with a deceptively simple tool: pregelatinized corn starch, added at different concentrations to reconstructed grape matrices, and tracked from the wet purée all the way to the final dried pore architecture.

The research team, led by Xinyu Wei and colleagues at Henan Province institutions in China, worked with Shine Muscat grapes, a popular seedless cultivar with roughly 77 percent moisture content. Rather than drying whole grapes, they built a controlled model system: whole grapes were blended with dispersions of pregelatinized corn starch, or PGS, at mass fractions ranging from 2 to 10 percent, then homogenized, molded into uniform hemispherical cavities, frozen at minus 25 degrees Celsius, and freeze-dried under identical conditions. The choice of PGS was deliberate. Unlike thermally gelatinized starch or multi-hydrocolloid blends, pregelatinized starch hydrates and swells in cold water, meaning it can build viscosity during ordinary room-temperature mixing without any additional heating step. That simplicity matters scientifically: with a single structuring agent, the contribution of that agent can be isolated in a way that is nearly impossible in complex formulations.

The baseline told the researchers why the work was needed. In preliminary trials, formulations without any PGS underwent severe collapse and fragmentation during freeze-drying, producing samples too deformed for meaningful mechanical testing. Even at the lowest tested concentration of 2 percent, deformation was pronounced, and the dried samples failed to retain their molded geometry. As the starch concentration climbed, the picture changed dramatically. Volumetric shrinkage, measured precisely from three-dimensional X-ray micro-computed tomography reconstructions, fell steadily as concentration rose from 2 to 10 percent, with the difference between the 8 and 10 percent formulations no longer statistically significant. The 8 percent formulation, labeled PGS-D8, largely retained its original shape and showed a smooth, uniform surface. Intriguingly, the 10 percent formulation kept its volume just as well but developed slight surface granulation and undulation, an early hint that more starch is not always better.

To understand why, the team turned to rheology, measuring how the wet purées behaved before freezing ever began. All formulations showed shear-thinning, pseudoplastic flow, the signature of a weak gel network being disrupted and aligned under shear. As concentration increased from 2 to 6 percent, low-shear viscosity rose steadily, reflecting progressive hydration, swelling, and weak gelation of the starch particles alongside grape pectin and soluble polysaccharides. But then came a surprise: the 8 percent formulation showed a higher viscosity than the 10 percent one. The explanation lies in water. In a high-sugar matrix, water is already scarce, and at the highest starch loading, competition for available water and particle crowding appear to have limited full hydration. The 8 percent formulation also posted the highest storage modulus, meaning it contributed the greatest elasticity of any tested blend, a property that would prove decisive for what happened during drying.

After freeze-drying, the mechanical testing told a consistent story. Compression hardness increased from 2 to 8 percent and then dipped slightly at 10 percent, mirroring the wet-state viscoelasticity. Force-relaxation experiments, in which samples are compressed to a fixed strain and held for 120 seconds, showed that initial force, residual force, and both equilibrium and relaxing force coefficients were dramatically higher in the 8 and 10 percent samples, with characteristic relaxation times also lengthening. The stretching exponent, which describes how coordinated the relaxation response is, peaked at 8 percent, indicating a narrower and more orderly distribution of relaxation mechanisms. Puncture tests added nuance: the 6 percent formulation actually showed slightly higher local puncture hardness than the 8 percent one, a reminder that a small probe sampling localized load-bearing regions and a large probe compressing the bulk of a sample interrogate fundamentally different spatial scales of the same material.

Molecular-level techniques filled in the mechanistic picture. Fourier transform infrared spectroscopy revealed no new covalent chemistry, but clear signs of physical reorganization: the O-H stretching band around 3400 per centimeter shifted and weakened with increasing starch, the C-H stretching band declined at medium and high concentrations, and the water bending band near 1630 per centimeter shrank, all consistent with starch hydroxyl groups competing for water and reorganizing hydrogen-bond networks among starch, pectin, sugars, and grape polysaccharides. Ratios within the carbohydrate fingerprint region showed a non-monotonic response to concentration, confirming that the molecular packing of the composite phase changed in ways that starch content alone could not explain.

Low-field nuclear magnetic resonance then tracked how protons, and by proxy water, moved within the dried matrices. Three relaxation components emerged in every sample: a highly restricted, matrix-bound population; a moderately mobile bound-water population; and a relatively mobile population associated with weakly constrained water in pore walls and capillary regions. As starch concentration rose, signal shifted away from the most restricted population toward more mobile states. Crucially, the 8 percent formulation showed a high proportion of moderately mobile protons while its most mobile population did not expand further, and its third relaxation peak shifted toward shorter times, indicating tighter restriction of the remaining mobile water. The 10 percent formulation showed the opposite tendency, with more weakly constrained proton environments, consistent with its less uniform internal structure.

The pore architecture data delivered the study’s most striking finding: the response was nonlinear. The 2 percent samples were highly interconnected but severely shrunken, their porosity representing the wreckage left after contraction. Porosity rose through 4 and 6 percent as volume retention improved, yet these samples still carried spatially heterogeneous pore distributions that concentrated loads unevenly. The 8 percent formulation showed the lowest total and open porosity but the highest closed porosity, together with the largest retained volume, a combination indicating a high effective solid fraction partitioned by continuous walls. Layer-by-layer porosity analysis confirmed that the 8 percent samples had the smallest fluctuations along their height, the most uniform pore distribution of any formulation. At 10 percent, total and open porosity climbed again while closed porosity fell, meaning the sample held its shape around a more open, less orderly interior. Scanning electron microscopy corroborated everything: discontinuous pore walls at low concentrations, a continuous and evenly distributed wall network at 8 percent, and clustered particles with sheet-like regions at 10 percent, the fingerprints of particle crowding and polymer self-association.

The broader lesson reaches well beyond grapes. Structural retention in freeze-dried high-sugar foods, the authors conclude, is governed not by the amount of pore space alone but by its connectivity, its spatial distribution, and the continuity of the walls that surround it, all of which trace back to the balance between polymer hydration, wet-state network development, and water availability established before freezing. An 8 percent starch dispersion hit that balance, combining the strongest wet-state elasticity with high compression hardness, reduced pore connectivity, and remarkably uniform pores, while pushing to 10 percent bought no further improvement in compression resistance or pore uniformity. For food engineers designing freeze-dried fruit snacks, restructured blocks, or even 3D-printed purée architectures, the message is that the recipe written in the wet state is the blueprint for the dried one, and that the optimal formulation is a sweet spot, not a ceiling.

Subject of Research: Concentration-dependent structural retention of freeze-dried reconstructed grape matrices formulated with pregelatinized corn starch

Article Title: Concentration-dependent structural retention in freeze-dried reconstructed grape matrices formulated with pregelatinized corn starch: Linking wet-state viscoelasticity to final pore architecture

Article References: Wei, X., Long, T., Liu, R., Li, L., Cao, W., Liu, W., Ren, G., & Duan, X. (2026). Concentration-dependent structural retention in freeze-dried reconstructed grape matrices formulated with pregelatinized corn starch: Linking wet-state viscoelasticity to final pore architecture. Current Research in Food Science, 13, Article 101583. https://doi.org/10.1016/j.crfs.2026.101583

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101583

Keywords: freeze-drying, pregelatinized corn starch, Shine Muscat grapes, pore architecture, rheology, micro-computed tomography, food structure, hydrocolloids, shrinkage, LF-NMR, FTIR, texture analysis