External quality of eggs

　　Eggs are widely recognized as the most important and inexpensive nutritious food for humans, closely related to consumer health. Egg safety has become a widespread and far-reaching social issue today. High-quality eggshells and good internal egg quality are important components of food safety. Improving food safety production quality and management systems is the social responsibility of every practitioner and is a necessary path for the survival and sustainable development of large-scale chicken farms.

　　To date, the focus of some chicken farmers is on egg prices and market conditions; the health and egg production of the flock, with little attention paid to egg quality and safety. In order to reduce the risk of flock diseases, they frequently and excessively use antibiotics (even banned drugs); in order to maximize the benefits of raising chickens, they use cheap and inferior feed for a long time; in order to pursue the beauty of eggshell and yolk color, they use banned chemical additives and hormones; in order to avoid fluctuations in egg prices, they store eggs at room temperature for a long time (some even in the chicken coop). These factors not only cause nutritional imbalance and quality degradation of eggs, resulting in exceeding drug residues, toxic and harmful substances or bacterial contamination, but also extremely detrimental to human health and the development of the egg industry.

　　 I. Egg Quality Standards

　　The quality indicators of eggs mainly include external quality and internal quality. Eggshell condition, egg weight, eggshell strength, eggshell thickness, egg shape index, eggshell color, egg specific gravity, and surface cleanliness are conventional indicators reflecting poultry egg quality.

　　Abroad, eggs are classified into: fresh eggs (with domestic and export fresh egg standards, further divided into first, second, and third grade standards), refrigerated eggs (divided into first, second, and third grade), and preserved eggs, etc.

　　According to the standards of the United States Department of Agriculture (USDA, 2000), egg quality is judged based on eggshell texture, color, shape, integrity, and cleanliness.

　　 Proportion of common problems (%) in different types of egg products

　　 1. Egg Quality Grades

　　Egg quality detection and grading standards mainly use three major standards: EU standards, US standards, and national standards. Grading is based on indicators such as egg shape and size, weight, eggshell surface defects (cracks, stain area), yolk color, blood spots, and meat spots.

　　In the US poultry egg quality grading standard, qualified poultry eggs are divided into three grades: AA, A, and B, while unqualified poultry eggs are classified as grade C.

　　US Egg Standards

　　China's hygiene standards for fresh eggs mainly stipulate the sensory indicators, physicochemical indicators, and food additives of fresh eggs. The industry standard SB/T10638-2011 for fresh egg grading issued by the Ministry of Commerce stipulates the eggshell, egg white, yolk, foreign matter, Haugh unit, and weight.

　　Quality Grading Requirements for Fresh Chicken Eggs and Fresh Duck Eggs in China

　　 2. Egg Weight Grades

　　Egg weight is an important indicator for judging egg grade, freshness, and structure. It is the most intuitive and commonly used indicator for shell egg grading. Many countries use egg weight as a standard for distinguishing grades, dividing them into five grades. Generally speaking, large eggs are more likely to break than small eggs. 。

　　Egg weight is closely related to breed, nutrition, laying age, laying weight, egg age, ambient temperature, and storage conditions.

　　Automatic egg grading systems are now widely used, with units in grams.

Foreign Fresh Egg Weight Grading Standards

China Fresh Egg Weight Grading Standards

Automatic Grading System

　　 3. Egg Packaging and Grading Requirements

Packaging Product Grading and Judgment Requirements

　　 II. External Quality of Eggs

　　 1. Eggshell Quality

　　Good eggshell quality can reduce food safety risks and reduce economic losses due to eggshell breakage. The quality of the eggshell is determined by its structure. According to different measurement scales, the structure of the eggshell can be divided into four levels: macroscopic structure (naked eye view), microscopic structure (optical microscope view), ultra-microscopic structure (electron microscope view), and crystal structure (X-ray crystal diffraction).

Macroscopic Structure of Eggshells

Schematic diagram of the ultra-microscopic structure of eggshells

　　 2. Eggshell Strength

　　Eggshell strength refers to the maximum stress that the eggshell can withstand. Internationally, it is required that when the egg is placed vertically, it should not break under a pressure of 2.94 × 10?pa (30 atmospheres), and the breakage rate should not exceed 1% under a pressure of 2.65-3.5 × 10?pa. Eggshell strength is related to breed, nutrition, and environment. White-shelled eggs have a relatively higher breakage rate.

　　The longitudinal compressive strength of eggs is higher than the transverse strength, so it is better to store and transport them vertically.

　　Eggshell strength is determined using an eggshell tester, with units of kg/cm² or pa.

Eggshell Strength Tester

　　 3. Eggshell Thickness

　　Eggshell thickness is a major factor contributing to the compressive strength of the eggshell and is an important quality and economic indicator of egg products.

　　The eggshell thickness should generally be maintained above 0.35 mm for good compressive strength, allowing for long-distance transportation and easy storage.

　　Eggshell thickness has a great impact on the breakage rate. For shells with a thickness of 0.38 mm~0.40 mm, the breakage rate is 2%~3%; for shells with a thickness of 0.30 mm~0.27 mm, the breakage rate may be as high as 10%; and for shells with a thickness of less than 0.25 mm, the breakage rate will be as high as 85%.

　　Eggshell thickness is determined using an eggshell thickness gauge or micrometer, with units in mm.

　　Take eggshell fragments from the blunt end, middle, and sharp end of the egg, rinse them with clean water, dry them with filter paper, remove the eggshell membrane, and take the average thickness value, accurate to 0.01 mm.

Eggshell Thickness Tester

　　Generally, the thicker the egg, the greater its eggshell strength. Eggshell strength and eggshell thickness are positively correlated, but not significantly, and are also related to eggshell density.

Relationship between eggshell strength and thickness

　　 4. Egg Shape Index

　　The egg shape index is the ratio of the longitudinal axis to the transverse axis of an egg; it can also be expressed as the ratio of the transverse diameter to the longitudinal diameter, represented as a percentage. Normal eggs are oval, with an egg shape index of 1.3-1.35, or between 72%-76%. Products deviating from this value are considered substandard.

　　Egg shape does not affect its edibility, but it is related to its breeding value and hatchability. It is very important for packaging and transportation. An appropriate egg shape index results in fewer broken and cracked eggs. If the egg shape index is higher than 80% or lower than 70%, the rate of eggshell breakage increases significantly.

　　The two angles of the larger end of a standard egg shape are 60°, and the two angles of the smaller end are 51°.

Standard-shaped eggs

　　Machine vision technology online detection systems are a relatively new method for determining the egg shape index. However, vernier calipers are more commonly used for measurement, with units in mm and an accuracy of 0.5mm.

　　Online detection software system for egg shape parameters

　　 5. Eggshell Color

　　The color of an egg is the combined result of the pigments contained in the eggshell and the shell membrane. There is a certain relationship between eggshell color and strength. Generally speaking, the more pigment, the thicker the eggshell, and the higher the compressive strength. Eggs with darker eggshells generally have higher economic value.

　　Factors affecting eggshell color include: breed, nutrition, age, environment, disease, medication, and stress.

　　Currently, eggshell color is judged by color comparison.

Eggshell Color Grade Classification

　　 6. Specific Gravity of Eggs

　　The specific gravity of an egg is an important standard for distinguishing the freshness of eggs and is also an indirect method for determining eggshell thickness.

　　The specific gravity of an egg is determined using the salt water flotation method, expressed by the buoyancy of sodium chloride solution on the egg. It is divided into nine levels, and the suitable temperature is 34.5℃.

　　Eggs with a specific gravity greater than 1.080 are fresh eggs; those greater than 1.060 are fairly fresh; those greater than 1.050 are somewhat stale; and those less than 1.050 are rotten and spoiled.

Egg Specific Gravity Grades

　　 7. Cleanliness of Eggs

　　The judgment of cleanliness depends on whether there are stains on the eggshell and the size of the stained area. Stains on the eggshell seriously affect the internal and external quality of the eggs and greatly reduce consumer desire to purchase.

　　Regulations such as the "US Shell Egg Quality and Weight Grading Standards," the "Canadian Agricultural Products Act," and the Japanese Industrial Standard (JIS) clearly stipulate that shell eggs must be cleaned, disinfected, graded, and packaged to meet hygiene standards before entering the market. China also requires that packaged eggs have clean shells without visible contamination.

　　The standard of Grade B eggs with surface foreign matter not exceeding 1cm² is unacceptable to modern consumers.

　　 8. Broken Eggs

　　Strictly speaking, the proportion of broken and cracked eggs generally exceeds 6%. The eggshell, as a barrier protecting the contents of the egg from damage, is of extraordinary significance for egg preservation and packaging. After the eggshell is broken, microorganisms gradually invade the inside of the egg, causing the egg to spoil faster.

　　Detection methods for broken eggs include infrared spectroscopy, machine vision detection technology, and mechanical tapping and vibration technology.

Cracked Egg Detection Device

Schematic diagram of egg tapping point

　　After cracked substandard eggs are broken, their color and shape are poor.

　　 9. Other Substandard Eggs

　　Rain-soaked eggs, sweating eggs, frozen eggs, rotten eggs, moldy eggs, and incubated eggs all affect the quality and yield of eggs.

　　These are "rubber eggs" stored outdoors at -10℃, frozen at -20℃, and refrigerated at 0℃.