Internal quality standards for eggs

　　With the improvement of people's living standards and the overcapacity of egg production, consumers' demand for egg quality is increasing, so improving egg quality has become urgent.

　　 The intrinsic quality of eggs includes: Egg nutrient content, albumen condition, yolk condition, air cell condition, content odor, blood spots and meat spots rate, as well as chalazae condition and embryo condition, etc.

　　I. Egg Nutrition

　　Eggs are nutritious foods containing proteins, fats, lutein, lecithin, vitamins, and essential minerals such as iron, calcium, and potassium.

　　II. Albumen Condition

　　1. Albumen Quality

　　Albumen quality is an important material basis for egg freshness and the most important indicator of internal egg quality. Egg freshness is measured by three indicators: albumen height, Haugh unit, and albumen pH. The higher the albumen height, the fresher the egg. However, albumen height is related to egg size, so using it to measure albumen quality is not perfect. Therefore, using the Haugh unit to measure egg freshness is more scientific. The higher the Haugh unit, the better the albumen viscosity, the higher the albumen quality, and the fresher the egg.

　　2. Haugh Unit

　　The Haugh unit is an indicator for testing and expressing egg freshness stipulated by the US Department of Agriculture egg product standards. It is also an important indicator for international egg quality testing, obtained from the regression relationship between egg freshness and egg albumen height and egg weight.

　　AA-grade fresh eggs refer to eggs produced within 48 hours by certified breeding bases with a Haugh unit value above 72.

　　The Haugh unit uses hyperspectral (900～1700 nm spectral non-destructive detection) imaging technology combined with chemometrics algorithms for non-destructive detection of the Haugh unit of eggs. This improves upon traditional detection methods which are highly destructive, time-consuming, inefficient, and have limited coverage, easily leading to detection loopholes.

　　The Haugh unit should be tested at 7-15℃.

　　3. Albumen pH Value:

　　The pH of fresh egg white is 7.6～8.5. During storage, the pH of the egg white increases with temperature changes, reaching a maximum of about 9.7.

　　The decline in intrinsic egg quality is due to the loss of water and carbon dioxide in the egg, leading to changes in the egg's pH. This depends on the dissociation equilibrium between carbon dioxide, bicarbonate ions, carbonate ions, and proteins, resulting in the loss of the thick protein structure and watery changes in the egg white.

　　Studies have found that regardless of the storage temperature between 3℃～35℃, after 21 days of storage, the egg white pH is close to 9.4. Studies have also found that when the loss of carbon dioxide in the egg is prevented by oiling the eggshell, the egg white pH is 8.3 after 7 days of storage at 22℃ and does not change. For oiled eggs stored at 7℃, the egg white pH drops from 8.3 to 8.1 within 7 days.

　　For egg white pH measurement, a homogenizer is used to homogenize the egg, stir it evenly, and measure it with a pH meter.

　　Common non-destructive detection methods for internal egg quality include machine vision, near-infrared detection, and hyperspectral detection.

　　III. Yolk Condition

　　Yolk state refers to the regularity of the yolk shape, the clarity of the outline, and its mobility, which can reflect the freshness of the egg.

　　1. Yolk Ratio

　　The yolk ratio is an important indicator for measuring egg nutrition. The larger the yolk ratio, the higher the nutritional level of the egg. Increasing the yolk ratio can significantly increase the economic benefits of the enterprise.

　　The yolk ratio is expressed as yolk weight ÷ whole egg weight × 100%, which is the percentage of the yolk in the total egg weight. The yolk ratio is sometimes also expressed as the yolk-to-albumen ratio. The yolk ratio of eggs is generally around 30%-33%.

　　2. Yolk Color:

　　Yolk color is an important indicator of egg quality. On the market, eggs with lighter yolk color are priced lower than those with darker yolk color.

　　Yolk color is related to genetics, nutrition, disease, and management. Under the same conditions, the deposition ability of brown-shelled laying hens is higher than that of white-shelled laying hens, and the average difference in yolk color index in response to the same dose of lutein is 1.4.

　　 Lutein in feed is the main factor affecting yolk color.

　　Yolk color is detected using a Roche colorimeter, divided into 0-15 levels. The yolk color of fresh eggs for export must reach level 8 or above.

　　3. Yolk pH Value

　　The pH of the yolk is usually close to 6.0 when the egg is freshly laid, but it gradually increases to 6.4～6.9 during storage. The change in yolk pH is relatively small.

　　4. Yolk Index

　　Yolk index = yolk height H (mm) ÷ yolk diameter W (mm). The higher the index, the fresher the egg. As the storage time increases, the yolk height decreases, and the diameter increases.

　　The yolk index of fresh eggs is generally 0.401-0.442, ordinary eggs 0.35-0.4, edible 0.3-0.35. When the yolk index is less than 0.25, the yolk membrane ruptures and the yolk spreads.

　　IV. Air Cell Height

　　Air cell height is an important parameter for judging whether an egg is fresh. The air cell height of a fresh egg is less than 3 mm. As storage time increases, the water inside the egg evaporates and the air cell becomes larger. Eggs with larger air cells are stale eggs.

　　Air cell height = air cell left height + air cell right height ÷ 2, unit mm.

　　Air cell height standard: AA grade ≤3.2mm; A grade ≤4.7mm; B grade >4.7mm.

　　Air cell height is determined using an air cell height measuring ruler.

　　V. Egg Content Odor

　　Fishy odor syndrome in laying hens is caused by gene mutations that prevent the body from metabolizing trimethylamine (TMA) normally. When the hen's ability to metabolize TMA cannot meet the body's needs, TMA gradually accumulates and deposits in the follicle, forming fishy-smelling eggs. This seriously affects the quality of the eggs and causes certain difficulties for egg producers.

　　When the TMA content in eggs accumulates to 1.0～1.5 μg/g (TMA content in the yolk accumulates to 4 μg/g), people can identify the fishy smell of the eggs through their sense of smell.

　　Fishy odor syndrome is relatively common in brown-shelled laying hen breeds, because the main paternal breed of commercial brown-shelled laying hens is Rhode Island Red, and fishy odor syndrome in laying hens was first discovered in Rhode Island Red. When rapeseed oil, rapeseed meal, fish meal, or high doses of choline are added to the diet, it is very easy to induce brown-shelled laying hens to produce fishy-smelling eggs.

　　VI. Blood and meat spots in eggs

　　Blood spots are a common defect inside eggs, mostly appearing as red spots or streaks. Blood spots are mostly found in the yolk and are caused by the rupture of the ovary during ovulation, with blood clots descending with the ovum and being surrounded by the albumen.

　　The main causes of blood spot eggs are ochratoxin A; insufficient choline in the diet, and insufficient or excessive vitamin K.

　　The US Department of Agriculture's grading standards for poultry egg consumption stipulate that AA and A grade eggs are not allowed to have blood spots inside. Eggs with blood spots but with blood clots no larger than 1/8 inch (approximately 3.2 mm) in diameter are graded as B, while those exceeding this size are considered "inedible." Only AA and A grade eggs are allowed to be sold in supermarkets.

　　China's fresh egg hygiene standard GB2748-2003 stipulates that fresh eggs must not contain blood clots or other foreign objects such as chicken tissue.

　　Most meat spots are tissues from the hen's body organs, but some may be partially differentiated blood spots. Meat spots are often brown and mainly appear in the thick albumen, chalazae, or yolk, with a diameter ranging from 0.5 mm to more than 3 mm.

　　Meat spots in eggs are related to breed, vitamins A and K, mold, lighting programs, and fright.

　　The incidence of meat spots in eggs varies with the breed, ranging from 2% to more than 4%, and increases with age. The incidence may be higher in brown-shelled eggs.

　　Blood spots and meat spots affect egg quality and hatchability. The heritability of blood and meat spots is 0.15～0.25. Through selection, the rate of blood and meat spots can be reduced to 0.5%～2.0%, and the diameter of blood and meat spots can be reduced to less than 1 mm, but it is very difficult to eliminate them completely.

　　The proportion of blood and meat spots in brown-shelled laying hens is higher than that in white-shelled laying hens, and the proportion of blood and meat spots in artificially inseminated flocks is higher than that in non-artificially inseminated flocks.

　　Monitoring method: Online spectral monitoring technology for blood spot eggs

　　VII. Others

　　1. Embryo status

　　After fertilized eggs are exposed to a certain temperature, the embryo will begin to develop. This temperature is called the "physiological zero point", also known as the critical temperature. The physiological zero point for chickens is approximately 23.9 degrees Celsius. The quality of fertilized eggs stored at room temperature in hot seasons will definitely be affected.

　　This situation only applies to eggs produced by breeding farms and mixed/free-range chicken flocks. Generally, eggs from large-scale caged chickens will not have this situation.

　　Fresh eggs have no signs of heat or embryo development, and the germinal disc is small and not swollen.

　　2. Chalazae status

　　The function of the chalazae is to keep the yolk in the center of the egg white, so that the yolk stays in the center and does not touch the eggshell.

　　The chalazae are made of thick albumen structure. They are part of the albumen and are a source of high-quality protein. They are elastic, but after long storage, the elasticity of the chalazae weakens, and they detach from the yolk.

　　Eggs with thin chalazae that have detached from or even disappeared from the yolk are inferior eggs.