Kombucha Biofilm Elasticity

ISEF Category: Physics and Astronomy

Subcategory: Biological Physics  ·  Difficulty: Intermediate  ·  Setup: Home Setup  ·  Time: 1 to 2 Months

The Hook

A jelly-like layer can act more like a spring than a slime. That layer is a SCOBY pellicle, the biofilm that forms on kombucha. If you change the sugar or tea mix, you may change how stiff it gets. You can test that with a simple hanging-weight setup and your phone camera.

What Is It?

A SCOBY pellicle is a biofilm, which means a slimy but organized layer of living material and fibers. In kombucha, yeast and bacteria build a sheet that floats on top of the drink. That sheet is not just a weird kitchen object. It has mechanical properties, including stiffness and stretchiness.

You can think of it like a net made of tiny threads. If the threads are packed tightly, the sheet resists stretching more. If the network is looser, it stretches more easily. Young's modulus is the number physicists use to describe that stiffness. A higher modulus means the material is harder to stretch. A lower modulus means it gives way more easily.

This project asks how growth conditions change those mechanics. You grow pellicles under different sugar and tea conditions, then measure how they deform under load. You can compare your measurements to a simple fiber-network model, which treats the biofilm like a web of connected fibers instead of a solid plastic sheet.

Why This Is a Good Topic

This is a strong science fair topic because you can change one growth variable at a time and measure a clear physical outcome. You get real data from a living material, not just a yes-or-no result. The topic connects to food science, biomaterials, and how nature builds lightweight structures. You can learn sample prep, calibration, strain measurement, graphing, and model fitting without needing a university lab.

Research Questions

• How does sugar concentration in the growth medium affect the Young's modulus of kombucha SCOBY pellicles?
• What is the effect of tea type on the stiffness of the pellicle?
• Does pellicle thickness change the measured strain response under the same hanging load?
• To what extent does fermentation time change the elastic modulus of the biofilm?
• Which growth condition produces the largest difference between measured stiffness and the fiber-network model?
• How does repeated loading affect the elastic recovery of the pellicle?

Basic Materials

• Kombucha starter culture with live SCOBY pellicle.
• Black tea, green tea, or other chosen tea base.
• Granulated sugar.
• Glass jars with lids or breathable cloth covers.
• Digital kitchen scale with 0.1 g accuracy.
• Ruler or calipers.
• Small hanging weights or washers with known mass.
• Binder clips or a simple frame for holding the sample.
• Phone camera with slow-motion or high-resolution video mode.
• Bright desk lamp or fixed light source.
• Graph paper or spreadsheet software.
• Gloves and clean containers for safe handling.

Advanced Materials

• Load cell with data logger.
• Force gauge.
• Digital calipers or micrometer.
• Camera tripod or fixed phone mount.
• ImageJ for strain tracking.
• Materials testing frame or custom tensile setup.
• Environmental monitor for temperature and humidity.
• Reference polymer film or gelatin sample for comparison.
• Statistical software for curve fitting and model comparison.

Software & Tools

• ImageJ: Tracks marker spacing or sample deformation frame by frame from phone video.
• Python: Fits stress-strain curves and compares your data to a fiber-network model.
• Google Sheets: Organizes repeated trials and makes quick plots of modulus by condition.
• GeoGebra: Helps you sketch simple model relationships and check curve shapes.
• NIH PubMed: Finds review articles on biofilms, bacterial cellulose, and mechanical testing.

Experiment Steps

1. Define the one growth variable you will change first, such as sugar level or tea type.
2. Plan how you will keep sample size, fermentation conditions, and harvest timing as consistent as possible.
3. Design a strain measurement method that gives you a repeatable length change from video or photos.
4. Build a stress-strain calculation plan before you collect data, so you know how force becomes modulus.
5. Choose a simple fiber-network model and decide which measured features you will compare against it.
6. Set up controls and repeats that let you separate real material differences from sample-to-sample noise.

Common Pitfalls

• Using pellicles with uneven thickness, which makes stiffness look different when the sample is just thicker in one spot.
• Letting samples dry out before testing, which raises the apparent modulus and hides the growth-condition effect.
• Measuring strain from a moving phone camera, which blurs the marker positions and weakens your data.
• Comparing pellicles of different ages without controlling fermentation time, which mixes age effects with sugar or tea effects.
• Pulling samples past the elastic range, which creates permanent deformation and breaks the model comparison.

What Makes This Competitive

A competitive version of this project uses careful controls and real material modeling. You would not just say one sample felt stiffer than another. You would quantify uncertainty, repeat tests across multiple pellicles, and compare your data to a theoretical prediction. Strong entries also look for one extra layer, such as anisotropy, thickness effects, or a better way to separate elastic response from damage.

Project Variations

• Test pellicles made with different tea blends, then compare how caffeine and polyphenol-rich teas affect stiffness.
• Compare fresh pellicles with dried and rehydrated pellicles to see how water content changes the measured modulus.
• Use image analysis to compare fiber alignment or surface texture with the mechanical data from each sample.

Learn More

• PubMed: Search for review articles on bacterial cellulose, biofilms, and tensile mechanics.
• NASA NTRS: Search for lightweight fiber network models and materials testing papers that use the same mechanics language.
• MIT OpenCourseWare: Look for introductory materials science and mechanics lectures on stress, strain, and Young's modulus.
• USGS Water Science School: Use this for clear background on water content, moisture, and material behavior in wet samples.
• ImageJ Documentation: Learn how to measure distances and motion from video frames for strain tracking.