Rheology and Encrusting: Why the Flow Behavior of Ingredients Matters So Much

What Is Rheology?

Rheology studies how materials flow and deform when force is applied. Water flows easily and steel barely deforms; both extremes are easy to predict. The hardest materials are the ones in between, such as dough, bean paste, meat filling, and mochi: they flow under force but spring back a little when released.

Encrusting machines and rheology are closely connected. Japan's RHEON takes its company name from rheology. Its official company history states that founder Torahiko Hayashi developed the encrusting machine on the basis of rheology, and when the Japan Society of Mechanical Engineers listed the Model 105 encrusting machine as a Mechanical Engineering Heritage item, it also noted that the machine was built on a forming theory grounded in applied rheology.

Dough Is a "Solid That Flows"

Academic research on wheat dough found early on that dough behaves both like an elastic solid and like a viscous liquid. A 2012 review of dough rheology models by Hosseinalipour et al. of the Iran University of Science and Technology notes that once water, flour, and small amounts of ingredients such as salt and yeast are mixed, they form a viscoelastic dough, the key to which is the three-dimensional network formed by gluten. It also notes that each ingredient and the water content can greatly change the dough's rheological properties, making its behavior nonlinear.

In everyday terms:

• Viscosity determines "how easy it is to push": high-viscosity materials need more force to extrude.
• Elasticity determines "whether it shrinks back": highly elastic dough pulls back after being stretched, so cuts tend to open up.
• Nonlinearity means "it behaves differently when pushed fast or slow": the same dough may flow smoothly when pushed slowly, but stiffen or break when pushed quickly.

How Viscosity and Elasticity Affect Encrusting

During co-extrusion, the dough and filling must travel down through the nozzle together, ideally flowing at similar speeds; see Co-Extrusion Principle.

• Dough too firm, filling too soft: The dough cannot be pushed, the filling rushes out first and drifts to one side, leaving one side thick and the other thin.
• Dough too soft, filling too firm: The firm filling tears through the soft dough or pushes the seal open.
• Dough too elastic: The dough shrinks back after the shutter seals it, so the cut is hard to close properly. RHEON developed the CN601 specifically for "highly elastic fermented dough," allowing the feed ratio between the hopper cylinder and the flow regulator to be adjusted to suit the material.
• Filling too sticky: It tends to stick to the shutter blades, requiring dusting or a non-stick design; see Dusting Flour and Anti-Stick Measures.

When the industry says "the dough and filling should be equally soft," it is really saying the rheological properties of the two materials should be close.

Why Does the Same Machine Fail When the Ingredients Change?

This is the situation customers run into most: everything works during the trial, but with their own recipe the dough tears, the filling goes off-center, and the weight is inconsistent. Usually the machine is not at fault; the rheological properties of the ingredients have changed:

• Water content: 2%–3% more water makes dough much softer; slightly less sugar in bean paste changes its firmness.
• Temperature: Dough and bean paste change noticeably in viscosity as they warm up or after chilling, and fermented dough keeps changing the longer it sits.
• Types of oil, sugar, and starch in the recipe: Even among red bean fillings, a high-oil one is slick and a high-sugar one is sticky.
• Degree of mixing: The more fully developed the gluten, the more elastic the dough.
• Resting time: Extensibility differs greatly before and after the dough relaxes.

So when the ingredients change, the feed ratio, extrusion speed, and shutter timing need to be readjusted. Newer machines can store recipes precisely so that the best conditions for each ingredient can be recorded; see Control Systems.

Applying Rheology on the Production Floor

You do not need to buy a rheometer to use rheological concepts to manage production:

1. Fix the recipe and process: Standardize water content, mixing time, and resting time.
2. Control material temperature: Record the temperature at loading, and save separate conditions for summer and winter.
3. Test small first: At the start of each batch, make a dozen or so pieces to confirm there is no tearing or off-center filling before full production.
4. Adjust dough and filling together: Do not adjust just one side; the closer they are in firmness, the easier they are to encrust.

Dong-Yih Machinery's DY-201 Automatic Steamed Bun and Mantou Machine is designed around "a traditional technique that does not break down the dough structure," which is exactly about handling the rheology of dough. To find out whether your ingredients are suited to machine production, call us at 06-257-4644 and bring your ingredients to discuss.

FAQ

Q: What is rheology?
A: The study of how materials flow and deform under force. "Solids that flow," such as dough, bean paste, and meat filling, are typical subjects of study.

Q: Is the name RHEON related to rheology?
A: Yes. The official company history explains that founder Torahiko Hayashi developed the encrusting machine on the basis of rheology (レオロジー), and the company name comes from that science.

Q: Why did the trial go fine, but my recipe doesn't work?
A: Your recipe's water content, oil-to-sugar balance, temperature, and mixing and resting times have changed the viscosity and elasticity of the ingredients, so the feeding and shutter conditions need to be readjusted.

Q: How soft should the dough and filling be for good encrusting?
A: There is no fixed number; the principle is that the two should be similarly firm. Find the best conditions through test runs with your own ingredients.