Yeast Preservative Growth Curves
ISEF Category: Animal Sciences
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Subcategory: Cellular Studies · Difficulty: Intermediate · Setup: School Lab · Time: 1 to 2 Months
The Hook
A tiny packet of yeast can stand in for a much bigger biology question. Food preservatives like sodium benzoate and potassium sorbate can slow cell growth, and you can track that shift with a smartphone instead of a pricey lab reader. That makes this a clean way to study dose-response with a real food science angle.
What Is It?
This project asks whether common food preservatives slow the growth of baker's yeast, S. cerevisiae. Yeast is a single-celled eukaryote, which means it has a nucleus and other cell parts like your cells do. When preservatives work, the culture stays less cloudy because fewer cells are multiplying.
Turbidity means cloudiness. Think of it like looking through water with more and more sand stirred in. A smartphone camera can track that cloudiness over time, then you can turn the photos into numbers and compare how each preservative changes the growth curve.
Why This Is a Good Topic
This is a strong science fair topic because you can measure it, graph it, and compare treatments with a real dose-response model. It connects to food safety, shelf life, and how cells respond to chemical stress. You can build it with common materials, but you still get room for careful controls, repeat trials, and real data analysis.
Research Questions
- How does sodium benzoate concentration change the lag time of yeast growth?
- How does potassium sorbate concentration change the maximum growth rate of yeast?
- What is the effect of pH on the inhibitory strength of sodium benzoate in yeast cultures?
- To what extent do sodium benzoate and potassium sorbate differ in their dose-response curves at the same concentration?
- Which smartphone image feature, brightness or contrast, gives the most stable turbidity readout for yeast growth?
- Does starting yeast density change the apparent preservative sensitivity of yeast?
Basic Materials
- Active dry yeast.
- Sodium benzoate.
- Potassium sorbate.
- Distilled water.
- Sucrose or a simple yeast growth medium.
- Identical clear cups, culture tubes, or cuvettes.
- Smartphone with manual exposure control.
- Fixed stand or tripod.
- Desk lamp with constant brightness.
- White background or light box.
- Digital kitchen scale with 0.1 g accuracy.
- Spreadsheet software.
Advanced Materials
- Incubator or temperature-controlled shaker.
- Spectrophotometer or microplate reader.
- Sterile 96-well plates or sterile culture tubes.
- Micropipettes and sterile tips.
- Laminar flow hood or clean bench access.
- pH meter.
- Analytical balance.
- Hemocytometer or cell counter.
- Autoclave or sterile filtration setup.
- Reference standards for calibration.
- Computer for curve fitting and statistics.
Software & Tools
- ImageJ: Measures image brightness and compares turbidity across samples.
- Python: Fits growth curves and calculates dose-response metrics.
- RStudio Desktop: Runs statistical tests and plots preservative effects.
- Google Sheets: Organizes replicate data and makes quick charts.
Experiment Steps
- Define the response variable you will compare, such as lag time, growth rate, or final turbidity.
- Fix the imaging setup so every sample is photographed from the same distance, angle, and background.
- Choose the preservative range, the control group, and the number of replicates for each condition.
- Build a calibration plan that turns photo brightness into a numeric turbidity measure.
- Decide how you will fit the curves and compare treatments with the same statistical method.
Common Pitfalls
- Letting the smartphone move between photos, which changes brightness and makes turbidity look like growth changes.
- Mixing yeast inconsistently, which makes starting cell density differ across treatments.
- Testing benzoate and sorbate at different pH values without controlling pH, which confounds preservative strength.
- Using cloudy media or colored containers, which hides real changes in yeast turbidity.
- Reading only the final cloudiness, which misses lag phase and growth rate differences.
What Makes This Competitive
A stronger entry goes beyond asking whether the yeast grows and measures the full dose-response curve. If you control pH, starting density, and lighting, then compare sodium benzoate and potassium sorbate with a fitted growth model, your data gets much harder to dismiss. Validation against a school spectrophotometer, or repeated calibration checks, would make the measurement side much stronger.
Project Variations
- Compare the same preservatives in apple juice, tea, or broth to see how the food matrix changes yeast growth.
- Test whether pH-adjusted benzoate and sorbate behave differently at the same nominal dose.
- Validate the smartphone method against a school spectrophotometer or light sensor to check how close the readings match.
Learn More
- PubMed: Search review articles on yeast growth inhibition, food preservatives, and dose-response curves.
- NCBI Bookshelf: Find free textbook chapters on cell biology, growth kinetics, and experimental design.
- FDA Food Additives and Ingredients: Read the agency pages on sodium benzoate and potassium sorbate in foods.
- OpenStax Biology 2e: Review free chapters on cells, metabolism, and growth models.
- MIT OpenCourseWare: Search introductory biology and data analysis materials for graphing and curve fitting.
- PubChem: Look up chemical properties, pH behavior, and safety basics for each preservative.