In coffee enthusiast circles, this is a classic emergency question: “What if I bought the wrong filter paper/ran out of it? Can I use a substitute?”
While the answer is yes, from a fluid dynamics perspective, this “cross-use” can significantly alter the flavor of your coffee. Here’s an in-depth technical analysis compiled by Deer Filter Lab:
- Physical Conflict: Structural Void
The flat-bottom basket is designed to evenly support the coffee grounds with its flat bottom.
Failure Risk: When you place a No. 2 or No. 4 cone filter paper into the flat-bottom basket, an air gap forms between the tip of the filter paper and the bottom of the basket.
Consequence: As hot water is poured in, the slurry weight of the coffee grounds concentrates entirely at the suspended cone tip. Without physical support, the filter paper is highly susceptible to structural breach during extraction, causing the coffee grounds to fall directly into the pot.
- Extraction Disaster: Channing Flat-bottomed machines typically have a wide, multi-hole spray head designed to evenly cover a flat coffee bed.
Hydrodynamic Analysis: The conical filter paper forces water flow from the periphery to the center. This leads to:
Over-extraction: The center accumulates the most coffee grounds and receives the most water, resulting in bitterness.
Under-extraction: Coffee grounds adhering to the sides may not be adequately wetted.
TDS Fluctuations: According to tests, this irregular coffee bed results in extremely uneven TDS (Total Dissolved Solids) distribution, often manifesting in a chaotic flavor profile of “bitter and acidic” in cupping.
- Emergency Hack Guide: How to Fold Properly
If you’re in an emergency where you absolutely must drink it, follow Deer Lab’s recommended folding method to minimize flavor loss:
Base Fold: Do not insert directly. Fold the pointed bottom of the conical filter paper upwards by about 1.5cm-2cm to create a flat base.
Lateral Reinforcement: Fold the original edge strips of the filter paper in the opposite direction to increase the stiffness of the sidewalls.
Pre-wetting: After placing the filter paper in the basket, be sure to thoroughly wet it with hot water. Use the surface tension of the water to make the filter paper adhere as closely as possible to the sidewalls of the basket, eliminating excess air wrinkles.
- Conclusion: Temporary Fix, Not a Permanent Solution
While folding can achieve brewing, for optimal Extraction Equilibrium, it is strongly recommended to use the correct specifications.
If you are using a Moccamaster or Bonavita: Please insist on using Deer No. 4 fan-shaped filter paper.
If you are using a Hario or Kalita: Ensure that your filter paper perfectly matches the geometry of the filter cup.
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