Haugh Unit and Yolk Index — What They Measure

Correct Answer: Height of albumin & Spherical nature of yolk

• Haugh unit measures the height of the thick albumin after breaking an egg on a flat surface. A taller albumin mound = higher Haugh unit = better quality. Standard value = 72 (range 65–85).
• Yolk index measures the spherical nature of the yolk. Formula: Yolk Index = Yolk Height ÷ Yolk Width. Standard value = 0.50 (range 0.40–0.50). A fresh yolk is round and stands high (yolk index close to 0.50); a stale yolk flattens out (yolk index falls).

Why These Values Decrease with Age

As eggs age, the enzyme lysozyme breaks down ovomucin in the albumin, causing the thick white to liquefy (albumin height drops → Haugh unit falls). Similarly, the vitelline membrane weakens over time, causing the yolk to flatten (yolk index decreases). Both are therefore direct indicators of egg freshness.

Instruments Used for Egg Quality Measurement

• Haugh unit: Tripod micrometer / spherometer
• Yolk index: Ruler / micrometer after egg break-out
• Egg Shape Index: Vernier caliper
• Shell thickness/strength: Screw gauge
• Specific gravity: Brine flotation technique