The consumer is asking for a table egg of high quality. However, preferences vary a lot. Large or small eggs, ones with a pale or striking golden yellow and/or orange yolk colour – the perfect egg to meet everyone’s preference. But, how high is the dry matter content of an egg? Who is even interested in knowing that? A question which has hardly or never even crossed the mind of many consumers

The development and release of an egg from the ovary requires an enormous metabolic performance of the hen. The egg formation starts with the ovulation in an ovary of 60 g. Due to ruptures in the follicle wall, the yolk granule is released into the 60 cm fallopian tube. In the first stage, the egg will be fertilised, i.e. provided that semen is available in the sperm glands. Various egg white proteins that contain bactericidal effects and influence the coagulation properties will then be accumulated. The albumen will then be covered with the egg membrane before the calcification of the eggshell begins. The fine-pored eggshell is 0.2 to 0.3 mm thick and guarantees the necessary exchange of air and moisture for the embryo. The cuticle of the eggshell protects the inside of the egg in terms of microbial invasion and dehydration

The modern layer lays an egg almost every day. The formation of an egg takes about 24 hours. Considering that the egg is a small “miracle of nature” as it ensures the life of offspring and it’s multipurpose application in human nutrition, this is indeed a very short time frame.

Is the dry matter content of an egg of importance to anyone?

For consumers it only plays a minor role. Their attention is focused mainly on aesthetical traits as the eggshell and yolk colour that have no influence whatsoever on the nutritional value of the egg. On the other hand, the stability of the eggshell is a very functional trait particularly since the egg has to withstand all forces from the point of oviposition right up to the delivery to the consumer without getting any ruptures. The freshness of an egg that is measured by the air chamber on the blunt end of an egg, as well as blood and meat spots or inclusions of insects and manure, are important quality characteristics for table eggs. The dry matter content of an egg, on the other hand, is of importance to the embryo and has an impact on the hatchability. The egg yolk has a dry matter content of 50 % whereas the egg white contains up to 88 % of water. The egg yolk is the main supplier of nutrients and is the source of food for the embryo. The yolk sac serves as a means of nutrition for the day-old chicks. Therefore, an insufficient yolk proportion can influence the chick’s body condition and its liveability. The egg processing industry demands a dry matter content of 24 % for the whole egg in order to produce a maximum quantity of saleable egg products. The production of various egg products indirectly affects the consumer. Egg yolk, egg white as well as the whole egg can be acquired pasteurised, refrigerated or dried. Furthermore, the egg is processed in convenience food, e.g. mayonnaise, noodles or baking premixes

The impact of breeding on the dry matter content

During the last decades, the proportion of the egg yolk has reduced slightly due to intensive breeding for laying performance. Today, layers have the potential to lay 300 eggs per year, i.e. the hens lay an egg on subsequent days on end. These eggs contain a high proportion of egg white to maintain the desired high egg weight. The high amount of aqueous egg white lowers the dry matter content of the whole egg and with that, the content of valuable nutrients. The dry matter content of an egg can be genetically influenced by the egg yolk percentage or the dry matter content itself. To calculate the yolk proportion, the breeder captures data on the egg and egg yolk weight. More efforts are needed to record the dry matter content with the so-called “sea-sand” method. In order to do this, the egg yolk and egg white have to be homogenised. 2g of this mixture of egg fluid is filled into a 35 g of sea-sand and the sample will then be placed in a drying oven at 103 °C for four hours. Afterwards, the difference of the weight before and after drying is measured to calculate the dry matter content.

An alternative to the sea-sand method

Since the sea-sand method is very costly in terms of time and materials, there is a necessity to find a simpler method. In view of this, two different refractometers were tested for a comparative study. A refractometer works with the refraction of light to determine the density of a substance and finally, the dry matter content. In a study conducted by Lohmann Tierzucht, 150 eggs each of LSL and LB layers were tested. The layers were 41 weeks of age and the eggs were tested for the dry matter content. The dry matter content was measured with the sea-sand method, once per egg, and three times per egg with each of the two refractometers. One refractometer was provided by the company A. Krüss Optronic, and the second by Kyoto Electronics Manufacturing Co. Ltd. The measurements were statistically and individually analysed for each flock. The average egg and yolk weight as well as the average dry matter content from each of the three measurements were calculated for the LSL and LB eggs. Additionally, phenotypic correlations between all traits as well as between the repeated measurements were estimated. The phenotypic correlation reflects the relationship between different traits and varies between rp= -1 to rp= +1. The closer the values respond to – 1 or + 1, the stronger the negative or positive relationship.

Results

The egg weight is related to the age of the hen and its origin. However, there is a difference between white and brown eggs. The average egg weight of the LSL hens is around 62 g, whereas the eggs of the brown layers show an average egg weight of 65.9 g. The yolk proportion is negatively related to the egg weight. Therefore, the yolk proportion of the white eggs is with 28.1 % higher than that of the brown eggs with an average of 26.8 %. Due to the higher yolk proportion of the white eggs, the dry matter content of these eggs is also higher as compared to the brown eggs. Measuring the dry matter content with the sea-sand method, the average value is 23 % for the white eggs and 21.7 % for the brown eggs. The dry matter contents measured with the refractometer tend to be similar. As for the LSL eggs, the average values are around 24.1 % (KEM) and 23.9 % (Krüss). The lower dry matter contents for the brown eggs are calculated with 23.2 % (KEM) and 22.5 % (Krüss). The phenotypic correlations between the egg weight and yolk proportion, or respectively, for each of the dry matter measurements, are negative. The estimated correlations range between rp= – 0.12 to rp= – 0.26 for the white eggs and between rp= – 0.24 and rp= – 0.40 for the brown eggs. The consistent positive correlation between the yolk proportion and dry matter content with all of the methods tested clearly shows that the yolk proportion determines the dry matter content of the whole egg. Regardless of the origin of the flock, the estimated correlation is rp= + 0.60. Furthermore, the highly positive correlations of rp= +0.70 between the three repeated measurements with the refractometers indicate a high accuracy of the refractometer devices.

Comparable results were obtained with all three methods, i.e. the sea-sand method and the two refractometers of Kyoto Electronics Manufacturing Ltd. and A. Krüss Optronic. Moreover, there are some advantages for the refractometer to measure the dry matter content of the whole egg. Due to a less extensive sample preparation and no drying time, the recording of the dry matter content is much faster with a refractometer as compared to the traditional sea-sand method. When testing a large amount of tested eggs, the application of the Krüss refractometer has its advantages. Integrated digital data storage is very useful to handle the data measured. Additionally, both refractometers can be used directly on the farm. No special laboratory is necessary to test the eggs for their dry matter content.

In summary:

» The dry matter content of the egg white is around 12 % and for the yolk, around 50 %

» A high yolk proportion has a positive influence on the embryonic development, the liveability of young chicks as well as for further egg processing

» From the breeders point of view, an increase in the dry matter content of the whole egg can also be reached by an increase

of the yolk proportion or rather, a shift in the proportion of egg yolk to egg white » As an alternative to the traditional seasand method, the dry matter content can be measured with a refractometer

» In general, white eggs have a higher dry matter content as compared to brown eggs.

» The refractometers of A. Krüss Optronic und Kyoto Electronics Manufactoring Co. Ltd. have been tried and tested in the tests performed by Lohmann Tierzucht

Dr. Wiebke Icken