How To Tell If An Egg Is Bad Float Test Reliability

Eggs are a staple in kitchens around the world, prized for their versatility and nutritional value. But because they’re perishable, knowing whether an egg is still safe to eat is essential. One of the most widely shared methods for checking egg freshness is the float test—dropping an egg into water to see if it sinks or floats. While this method is popular, its reliability is often misunderstood. Understanding how and why the float test works—and where it falls short—is key to making informed decisions about food safety.

How the Egg Float Test Works

The float test is based on a simple principle: as eggs age, moisture evaporates through thousands of tiny pores in the shell, and air replaces it, enlarging the air cell at the broader end of the egg. Over time, this increases buoyancy. A fresh egg will sink and lie flat on the bottom of a glass of water. As it ages, it begins to tilt or stand upright. If it floats to the surface, it’s likely quite old.

This change in density explains why older eggs float. However, floating does not automatically mean the egg is spoiled. It only indicates age. Conversely, a sinking egg isn’t always guaranteed to be fresh or safe—it could still harbor bacteria like Salmonella even if it’s relatively young.

Tip: Use room-temperature water for the float test to avoid thermal shock that might crack the shell.

What the Float Test Can and Cannot Tell You

The float test is useful as a preliminary indicator of age but fails to assess microbial spoilage. An egg can be old enough to float yet still be free of harmful bacteria. On the flip side, an egg contaminated with pathogens may appear perfectly normal and sink like a fresh one.

Dr. Catherine Donnelly, a food microbiology expert at the University of Vermont, notes:

“While physical changes in eggs over time are predictable, pathogen growth is not. Relying solely on buoyancy ignores the invisible risks.” — Dr. Catherine Donnelly, Food Safety Researcher

In other words, the float test should never be used as the sole determinant of whether an egg is safe to consume.

Limitations of the Float Test

The widespread popularity of the float test has led many home cooks to treat it as a definitive freshness check. However, several factors undermine its reliability:

• It doesn't detect bacterial contamination. Eggs can be infected with Salmonella enteritidis without any visible signs. The bacteria can grow inside the egg even when stored properly.
• Storage conditions affect results. Eggs stored in humid environments may lose less moisture, delaying air cell expansion and skewing float test outcomes.
• Shell thickness varies. Some breeds of chickens lay eggs with thicker shells that slow evaporation, affecting how quickly the air cell grows.
• No flavor or texture insight. Even if an egg passes the float test, prolonged storage can degrade yolk integrity and cause off-flavors.

Because of these limitations, relying exclusively on the float test can lead to either unnecessary waste (discarding edible but aged eggs) or health risks (consuming contaminated ones).

Real-World Example: A Home Cook’s Mistake

Sarah, a home baker in Portland, regularly uses the float test to screen eggs before baking. She once discarded a carton of floating eggs, assuming they were spoiled. Later, she learned from a local farmer that her backyard flock laid eggs with naturally thinner shells, causing them to age faster in terms of air cell size—but the eggs were still nutritionally sound and safe when cooked thoroughly.

Conversely, months later, Sarah used a batch of sinking eggs in homemade mayonnaise. Several family members fell ill. Testing confirmed Salmonella contamination—despite all eggs having passed the float test. This experience taught her that appearance and buoyancy alone aren’t enough to guarantee safety.

Better Methods to Assess Egg Freshness and Safety

To get a more accurate picture of an egg’s condition, combine multiple evaluation techniques. Here’s a step-by-step guide to assessing eggs beyond the float test:

1. Check the expiration or Julian date. Most commercial eggs are stamped with a packing date (Julian date, 1–365) and a sell-by date. Eggs are typically good for 3–5 weeks past the pack date when refrigerated.
2. Inspect the shell. Look for cracks, sliminess, or powdery spots (indicative of mold). Any damage increases contamination risk.
3. Smell the egg after cracking. A foul, sulfurous, or rotten odor is a clear sign of spoilage due to bacterial decomposition.
4. Observe the yolk and white. In a fresh egg, the yolk should be firm and centered, surrounded by thick, gel-like egg white. Older eggs have flatter yolks and runnier whites—but not necessarily unsafe ones.
5. Cook thoroughly when in doubt. High heat kills most pathogens. Avoid raw or undercooked uses (e.g., Caesar dressing, soft-boiled) with older eggs.

Tip: Crack eggs into a separate bowl before adding to a recipe. This prevents contaminating the entire dish if one egg is bad.

Comparison Table: Evaluation Methods vs. Accuracy

Method	What It Detects	Reliability	Best Used For
Float Test	Egg age via air cell size	Moderate – indicates aging, not spoilage	Quick sorting of older eggs
Sniff Test (after cracking)	Bacterial decomposition	High – strong odors indicate spoilage	Final safety check before use
Visual Inspection (cracked)	Yolk shape, white consistency	Moderate – shows freshness level	Assessing quality for presentation
Expiration Date Check	Time since packaging	High – if stored properly	Initial screening
Candling (in dark with light source)	Internal defects, blood spots, air cell size	Moderate to high – requires practice	Farmers, hatcheries, enthusiasts

When Is a Floating Egg Still Safe to Eat?

An egg that floats isn’t automatically dangerous. If it has been refrigerated consistently and is within 3–5 weeks of the pack date, it may still be safe—especially if used in thoroughly cooked dishes. To verify:

• Crack it into a clean bowl.
• Look for unusual colors (pink, green, black discoloration in the white).
• Smell it immediately—rotten eggs emit a strong sulfur odor.
• If there’s no smell and the appearance is normal, cook it well (above 160°F / 71°C) and consume promptly.

Note that hard-boiling older eggs is actually easier because the enlarged air cell makes peeling simpler. So floating eggs can be ideal for deviled eggs or egg salad—as long as they pass the sniff and visual tests.

Do’s and Don’ts of Egg Freshness Testing

Do’s	Don’ts
Store eggs in the coldest part of the fridge, not the door	Leave eggs out at room temperature for more than 2 hours
Use older eggs for hard-boiling and baking	Use floating eggs for raw preparations like Hollandaise
Wash hands and surfaces after handling raw eggs	Assume a clean shell means a safe interior
Label home-stored eggs with dates	Keep eggs in original carton indefinitely without checking dates

FAQ: Common Questions About Egg Freshness

Can I eat an egg that floats but smells fine?

Yes, if it smells normal and looks intact when cracked open. Cook it thoroughly and avoid using it raw. Floating indicates age, not necessarily spoilage.

Does refrigeration stop the aging process in eggs?

No, but it slows it significantly. Moisture loss and air cell growth continue slowly even in cold storage. Refrigeration primarily inhibits bacterial growth, extending safety far beyond what’s possible at room temperature.

Why do some countries not refrigerate eggs, while others do?

In the U.S., commercial eggs are washed, which removes the natural protective cuticle. This makes refrigeration essential to prevent bacterial entry. In many European countries, eggs are not washed and retain their cuticle, allowing safe room-temperature storage for a few weeks. Always follow local guidelines based on processing standards.

Conclusion: Rely on More Than Just Buoyancy

The float test has a place in the kitchen as a quick way to sort older eggs from fresher ones. But treating it as a definitive test for spoilage is misleading and potentially risky. True egg safety depends on proper storage, timely use, and sensory evaluation after cracking. No single method is foolproof, but combining date checks, smell tests, and visual inspection gives you the best chance of avoiding foodborne illness while minimizing waste.

Understanding the science behind egg aging empowers smarter decisions. Whether you're meal prepping, baking, or cooking for a crowd, don’t let a floating egg scare you unnecessarily—but don’t ignore the invisible threats either. Prioritize hygiene, trust your senses, and cook with confidence.

💬 Have you ever had a surprise from an egg that looked fine but wasn’t? Share your story or tip in the comments—your experience could help others stay safe!