How the Air Cell in an Egg Grows Over Time and What It Tells You

Every egg contains an air pocket at its blunt end, sandwiched between the inner and outer shell membranes. This air cell is not a defect or a sign of poor quality: it forms naturally as the freshly laid egg cools and its contents contract. But the air cell does not stay the same size. It grows continuously as long as the egg exists, because moisture and gas slowly escape through the shell's pores. The USDA uses air cell depth to grade egg freshness: Grade AA eggs have the smallest air cells, Grade B the largest. Understanding the air cell explains the float test, USDA grading, and some aspects of hard-boiling and incubation. ## How the Air Cell Forms When a hen lays an egg, its internal temperature is approximately 41 degrees Celsius (106 degrees Fahrenheit). As the egg cools to ambient temperature, the contents shrink due to thermal contraction. The two shell membranes, which were in contact at the blunt end inside the shell, separate as the contents pull away, creating a small pocket. Room air fills this pocket through the pores, establishing the air cell. In a freshly laid egg, the air cell is typically 2 to 3 millimeters deep. The air cell is always at the blunt end because that end has a higher concentration of pores and because the contents settle downward (the yolk is denser than the white), leaving the blunt end as the last place content fills. Gravity and pore distribution work together. ## Growth Rate and Grade Standards After formation, the air cell grows as moisture evaporates through the pores. The rate of growth depends primarily on storage temperature and humidity. Refrigerated eggs (4 degrees Celsius, 70 to 75 percent relative humidity) lose moisture slowly, and the air cell grows from under 3.2 millimeters to perhaps 5 to 6 millimeters over 4 to 6 weeks. Room-temperature eggs in low humidity can reach the same size in a week. USDA egg grading uses air cell depth as one of three primary criteria (the others are interior quality of the white and yolk, assessed by candling, and shell condition): Grade AA: Air cell no deeper than 3.2 millimeters (1/8 inch). White is thick and clear. Yolk is centered and firm. Grade A: Air cell no deeper than 4.8 millimeters (3/16 inch). White is reasonably clear. Yolk is well-defined. Grade B: Air cell deeper than 4.8 millimeters. White may be watery. Yolk may be flattened or off-center. Most eggs sold in retail stores are Grade A. Grade AA is more common in premium markets. Grade B typically goes to commercial food processing where appearance matters less. ## The Float Test Revisited The air cell growth directly drives the float test. A Grade AA egg with a 3.2-millimeter air cell has average density well above water and sinks flat. As the air cell grows, the blunt end becomes progressively more buoyant. A 1 to 2 week old egg tilts blunt-end-up at the bottom of a glass. At 3 to 4 weeks under refrigeration, the egg may stand nearly vertical. Eggs with very large air cells float horizontally. Each of these positions corresponds to a different air cell depth and a different rough age. For incubation: the air cell must grow to a specific size for the developing chick to pip internally and take its first air breath. Hatcheries monitor air cell size by candling and by weighing eggs to track moisture loss, targeting a 15 to 18 percent weight reduction over the first 18 days of incubation.