Directional freezing turns whole legumes into fibrous foods without isolates or additives

Researchers have demonstrated a method for turning whole legumes into structured, anisotropic foods using directional freezing, potentially offering a simpler route to plant-based textures without protein isolates, fractionation or additives.

Published in npj Science of Food, the study, Freeze structuring unlocks minimal-processing strategies for legume texturization, shows that suspensions made from whole chickpeas, lentils, peas, soybeans, lupins and several types of beans can be transformed into self-supporting gels with aligned internal structures by controlling the direction in which they freeze.

The researchers, Andrea Bach, Elin Perler, Lenja S. Lemcke and Patrick A. Rühs, found that the process can substantially increase hardness, cohesiveness and chewiness compared with refrigeration or conventional freezing. At higher chickpea concentrations, the resulting textures approached the chewiness of mozzarella.

• Perhaps more significantly for commercial food production, the method works without separating legumes into protein, starch and fiber fractions. The researchers said it can also be implemented using standard freezing equipment, including domestic freezers.
• Directional freezing created aligned, fibrous structures from whole chickpeas, peas, lentils, soybeans, lupins and beans without protein isolation or additives.
• Directionally frozen chickpea gels were around 1.5 to three times harder than refrigerated or conventionally frozen samples across the concentrations tested.

At 20% solids, chickpea gels reached chewiness of 8.7 to 10.0 N, close to the approximately 9.7 N measured for mozzarella.

The work addresses a familiar problem in plant-based food development: creating desirable structure without first heavily processing the raw material.

High-moisture extrusion can produce controlled fibrous structures, but commonly uses protein isolates or concentrates and requires capital-intensive equipment and carefully controlled processing conditions. Dry extrusion can handle less refined ingredients but can struggle to reproduce comparable fibrous textures.

Directional freezing takes a different approach.

The researchers first hydrated and mixed legumes into a suspension before applying heat treatment. This causes starch gelatinization and protein denaturation, creating an initial gel. The gel is then frozen using a temperature gradient so that solidification progresses primarily in one direction.

As the ice crystals advance, they exclude legume components from the growing ice phase. Proteins, starches and other components therefore become concentrated and redistributed in aligned regions between the crystals.

Once thawed, the ice returns to water, but the reorganized material remains as a hydrated lamellar scaffold. This leaves the food with a directional, or anisotropic, internal structure rather than the relatively uniform structure produced through refrigeration.

The researchers compared refrigerated, conventionally frozen and directionally frozen chickpea gels. Microscopy showed that refrigerated samples remained largely homogeneous, while conventional freezing generated some local alignment with random orientations.

Directional freezing produced the clearest alignment, with material concentrated along lamellae created by advancing ice crystals.

That structural difference also translated into substantial changes in mechanical performance.

At a chickpea concentration of 12.5%, directionally frozen samples produced an elastic modulus of approximately 5 × 10⁴ Pa, compared with around 10⁴ Pa for refrigerated gels.

Above 10% solids, frozen gels were approximately twice as elastic as refrigerated samples, with directional freezing producing the strongest response as concentration increased.

Hardness provided another clear distinction. Directionally frozen chickpea gels were approximately 1.5 to three times harder than refrigerated and conventionally frozen gels across the concentrations studied.

The direction of compression also affected their mechanical behavior. At concentrations between 15% and 20%, samples compressed parallel to the freezing direction were around 1.1 to 1.2 times harder than those compressed perpendicular to it, providing further evidence that directional freezing had created an aligned internal architecture.

The researchers also examined cohesiveness, springiness and chewiness.

Directionally frozen gels showed greater cohesiveness than either refrigerated or conventionally frozen samples, while maintaining recovery of approximately 82% to 84% across most concentrations.

Average chewiness reached 4.4 N for directionally frozen chickpea gels, compared with 1.0 N for refrigerated gels and 1.1 N for conventionally frozen samples.

By changing the solids concentration, however, the researchers were able to produce a much wider range of textures.

Directionally frozen chickpea gels ranged from 0.9 to 10.0 N in chewiness across formulations containing 12.5% to 20% solids. At the lower concentrations, values of 0.9 to 2.9 N were comparable with silken tofu, measured at approximately 0.85 N.

At 20% solids, chewiness increased to between 8.7 and 10.0 N. For comparison, mozzarella tested under the same conditions measured approximately 9.7 N, Brie around 5.7 N and firm tofu approximately 20.2 N.

The findings suggest that the same basic process could potentially be adjusted to produce foods ranging from softer gels to substantially firmer, cheese-like structures.

The approach was not confined to chickpeas.

Researchers applied directional freezing to legumes covering 96% of global production, including red lentils, green peas, black beans, mung beans, black-eyed peas and soybeans. Black and yellow lentils, kidney beans, navy beans and lupins were also successfully structured.

All of the legumes tested at 12.5% solids developed anisotropic structures, although their mechanical properties differed considerably.

Red lentils, black lentils and mung beans produced firm, stable gels with little syneresis, while black bean gels were softer. Soybeans produced weaker gels at 12.5% solids but still developed pronounced lamellar alignment.

Increasing soybean solids to 15% and 17.5% improved texture while retaining the anisotropic structure, with mechanical strength increasing further at 20%.

Composition appears to be an important factor. Legumes with relatively greater proportions of non-fiber carbohydrates, including starch, generally formed stronger gels. Soybeans, with a much higher protein-to-non-fiber-carbohydrate ratio, produced weaker structures, which the researchers linked partly to lower starch content and higher lipid levels.

That finding could allow manufacturers to select or combine legumes according to the texture required rather than relying on a single protein ingredient.

There is also a potential processing advantage in keeping the entire legume intact.

The researchers contrasted freeze structuring with processes requiring fractionation. In tofu manufacturing, for example, they noted that approximately 85% of dietary fiber, 16% of lipids and 23% of protein from whole soybeans are removed into okara and whey.

Extrusion-based foods also frequently begin with fractionated protein and starch ingredients.

Directional freezing instead reorganizes components already present in the legume. No protein isolation or subsequent recombination is required, meaning fiber and other components of the original raw material remain in the finished matrix.

The researchers said the technique could be implemented at household level using standard freezers, while small and medium-sized food producers could use commonly available freezing technology. Experiments showed similar directional structures could be created using a simple insulated mold placed inside a household freezer.

There are still questions to resolve before the process can be assessed as a commercial alternative to established texturization technologies. The researchers identified protein gelation, starch gelatinization, pH, digestibility and nutrient bioaccessibility among areas requiring further investigation. Comparative assessments would also be needed to quantify any sustainability advantages over existing processing methods.

But the study demonstrates that sophisticated texture does not necessarily require sophisticated raw materials. By controlling something as established as the direction of freezing, whole legumes themselves can provide much of the structure needed to create a texturized food.

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