Unclear Bun Pleats and Collapse After Steaming

Whether a steamed bun looks good comes down to two things: whether the pleats on top are clear, and whether it stands tall after steaming. These two complaints often come together, but their causes aren't quite the same. Pleats are mostly determined at forming; collapse mostly happens during proofing and steaming.

Pleats Not Distinct

• Dough too soft: The pleats pressed in during forming can't hold, and get stretched flat during proofing.
• Over-proofing: The final proof runs too long, and the bun expands, stretching the pleats open and smoothing them out.
• Dough surface too sticky: The pleats stick to each other, so the layers don't show.
• Gluten too strong: The dough shrinks back after forming, making the pleats shallower.
• Too much filling: The top is pushed round and the pleats are pulled flat.
• Residue on the forming parts: If dough is stuck on the part of the machine that forms the pleats, the pleats come out incomplete.

RHEON's model lineup includes an optional mechanism specifically for forming pleats on Chinese steamed buns, which shows that pleat forming is a distinct challenge in automating bun production, and different models produce different pleat results.

Collapse After Steaming: Machine or Proofing?

Most of the time it's a proofing and steaming problem, and less often the machine itself:

• Over-proofing: The gluten is stretched to its limit; during steaming the gas keeps expanding and the structure can't hold, so it collapses.
• Under-proofing: The crumb is dense and doesn't expand enough, so the skin shrivels after steaming.
• Insufficient gluten strength: Low-protein flour or under-mixing means the gluten network can't support the volume. One study found that when steamed over high heat, mantou made with both medium- and high-gluten flour could shrivel after being taken off the heat, so heat level is also a factor.
• Steam and heat: If the steam supply is insufficient or unstable, the buns don't set quickly enough; water dripping from the steamer onto the buns also causes dents.
• Opening the lid too soon: Opening the lid right after steaming lets cold air shrink the surface rapidly, often causing wrinkled skin and collapse.
• Watery filling: A wet filling softens the bottom and sides so they sag.

How to tell: if buns made from the same batch of dough collapse in some cases but not others, it's usually an uneven proofing environment (uneven temperature and humidity in the proofer, different positions) or different positions in the steamer; if all of them collapse, start by checking the dough recipe and proofing time. Only if the buns are already poorly shaped right after forming is it more likely to be the machine. For the proofing process, see Running Fermented Dough on Machines; for flour selection, see Flour Types and Machine Forming.

Adjustments You Can Make Yourself First

1. Fix dough temperature and proofing time, and keep the proofer's temperature and humidity stable and recorded.
2. Shorten or lengthen the final proof, making a tray of each for comparison, to find the conditions that give the tallest buns.
3. Wait a moment after steaming before opening the lid to avoid a sudden temperature drop.
4. Check whether the steamer lid drips.
5. Control dusting and dough firmness so pleats are clear. See Dough Temperature and Resting Time.

Checking the Machine

Before checking anything by hand, press stop, turn off the power, and unplug the machine or cut the main switch, and make sure the machine has come to a complete stop. In factories with multiple workers, lock the switch and hang a "Under maintenance, do not energize" tag. Article 57 of Taiwan's Occupational Safety and Health Facilities Rules (職業安全衛生設施規則) requires that when machinery is being cleaned, oiled, inspected, repaired, or adjusted, operation and feeding must be stopped, and measures such as locking out and posting signs must be taken to prevent accidental start-up.

Once the power is confirmed off, clean residue from the forming parts according to the operating manual and confirm the parts are reinstalled in their original positions.

Dong-Yih Machinery's DY-201 Automatic Steamed Bun and Mantou Machine is designed around a traditional technique that doesn't damage the dough structure. It produces 2,000–12,000 pieces per hour at 2.0KW and can make meat buns, vegetable buns, red bean buns, xiaolongbao, mantou, silver thread rolls (銀絲捲), and more.

FAQ

Q: Why aren't my bun pleats distinct?
A: Commonly the dough is too soft, over-proofed, too sticky on the surface, too strong in gluten, or there's too much filling; there may also be residue on the forming parts.

Q: Is buns collapsing after steaming a machine problem?
A: Usually it's over- or under-proofing, insufficient gluten, steam and heat, or opening the lid too soon; only if the shape is already poor at forming should you look to the machine.

Q: Why do some buns in the same batch collapse and others don't?
A: Usually uneven temperature and humidity in the proofer or different steaming positions; check the proofing and steaming environment first.

Q: What if proofing and recipe have been adjusted but forming is still poor?
A: Stop the machine, cut the power, and clean the forming parts; if it still doesn't improve, note the machine model and symptoms and contact the manufacturer or a service provider. Dong-Yih Machinery: 06-257-4644.