Commercial strains of egg production layers have been selected primarily for high production of eggs with acceptable market weights whereas little, if any, emphasis has been directed towards egg composition. Additionally, improvements in the management, disease control, nutrition, and genetics of layers as well as advancements in processing technology over the last 40 years have undoubtedly changed egg quality and composition, yet few studies have documented this progress. There have been some studies (Cook and Briggs, 1977; Marion et. al., 1964; Jackson et al., 1986) which looked at breed, strain and age and their impact on egg composition. The general conclusion was the proportion of yolk tended to be greater and the proportion of albumen smaller in small eggs than in larger eggs.
Other studies (Rodda et al., 1977; May and Stadelman, 1960; Rose et al., 1966; Akbar et al., 1983; Hill et al., 1966) have reported that selected commercial strains weighed more and contained higher percentages of albumen, albumen solids and albumen protein. However, this variation among strains has often been related to variation in egg size. In the older literature, more variability was seen in the component parts of eggs from younger hens than eggs from older hens. In general, there was a smaller percentage of yolk (Asmundson, 1933; Olsson, 1936) and a larger percentage of albumen (Olsson, 1936) in eggs from younger birds than in eggs from older birds (see Table 1).
Romanoff and Romanoff (1949) reported that during the first few months of lay, the relative amount of yolk in the egg gradually increases, whereas that of the shell decreased rapidly and the percentage of albumen remained the same.
Bird age has also been reported to have an effect on egg size and composition traits. Most of these trends can be reversed by forced molting, but resumes afterwards. Eggs increase in weight over a production period. This increase is associated with development of both yolk and albumen constituents, although such changes occur disproportionately, with the percentage of albumen increases progressively while the percentage of yolk decreases.
Izat (1983) and Cunningham et al., (1960) found that percentage of albumen solids, albumen protein, and Haugh units decreased with age of bird. Both studies generally reported that as hens age the yolk quality traits generally improve, whereas the albumen traits worsen. However, Tharrington et al. (1999) reported no significant differences in between strains for albumen protein, solids, pH or yolk solids in eggs with significantly different weights. In fact, they reported genetic selection had produced larger eggs containing lower percentage of yolk while overall egg quality had been maintained or improved.
The yolk is enclosed in a thin, pliable envelop, known as the vitelline membrane. The vitelline membrane breaks down as the egg ages. In addition, it is the vitelline membrane strength which allows the yolk to remain intact during the breaking operation. Therefore, the stronger the vitelline membrane the less likely it is to rupture during the breakout process.
One of the most widely used measurements for albumen quality is the Haugh unit. The Haugh unit is an expression relating egg weight and height of thick albumen--the higher the Haugh unit the better the egg quality.
Foaming properties of egg albumen are affected by many factors, including pH, additives, physical treatments, viscosity, and temperature (Yang and Baldwin, 1995). However, the more likely factor for poor foaming properties of fresh, frozen or commercially processed eggs (St. John and Floc, 1931) has long been attributed to the presence of lipid, usually yolk lipid, introduced during the breaking step of the egg albumen production process.
In the modern commercial environment, breeding layer strains for production of large egg sizes to enhance yield is ongoing. However, effects of such breeding efforts on the functional properties of eggs, particularly foaming properties of the albumen from those eggs, has not been studied. Eggs from layer strains bred for optimal albumen production may produce characteristics that detrimentally affect foaming property of the albumen. For example, eggs may be more susceptible to breakage of yolk membrane and contamination of albumen by yolk lipid. Because of poor foaming power of the resulting product, economic advantage of such breeding programs may be negated.
A method to study these problems is to correlate strains of layer hens bred for the egg breaking industry to the quality, functional and biochemical characteristics of resulting egg components of shell, albumen and yolk. This research project allows an evaluation of layer hen breeding characteristics production for albumen production to the resulting functional properties of those eggs.
The information here reported is an overview from several research studies done in collaboration with North Carolina State University. However, all the research studies utilized birds from the North Carolina Layer Management and Performance Test by Anderson (2004). This paper is focused on results from Hy-Line W-36, Hy-Line W-98, Hy-Line CV-20, ISA White, and Bovans White strains. These strains were selected based upon their egg weight, availability, and market share. The breaker market is looking for strains that will produce larger eggs. This would increase the throughput for any given breaking machine. The eggs from the selected strains have these characteristics. In addition, some of these strains hold a significant market share of the current production flocks.
Egg Composition
Egg processors who break eggs to create a variety of egg products, want large eggs that consistently produce high quality egg products. The consistency is the major problem with eggs. It is well known that as the hen ages a number of changes occur in the egg. As stated earlier, bird age has been reported to have an impact on egg size and composition. Egg weights generally increase as the bird ages. This is consistent with the results shown in Table 2. However, once the hens in this study reached the sixth period of production egg size became more consistent. Once egg weights plateaued to a consistent weight, do the contents remain the same? If so, the processor could blend eggs from younger and older flock for the desired component part. Unfortunately, it does not appear to be that simple. Egg weights were also influenced by strain. The strains with the highest (P<0.05) egg weights were the W-98, and ISA White at 61.2 and 61.0 g, respectively. The lowest egg weights were associated with the W-36 hen’s eggs weighing 57.1 g during the first cycle. There were no differences in the percent of large eggs produced. The differences were related to a shift, predominantly related to small and medium sizes and the extra large percentage. Those strains with the heaviest eggs had the greatest percentage of extra large eggs.
The general conclusion from previous research reported was that the proportion of yolk tended to be greater and the proportion of albumen smaller in smaller eggs than in larger eggs. As can be seen in Figure 1, albumen percentage decreases about ten percent over time and yolk increased. In 1977, Stadelman reported a standard range for percent albumen of 56 to 61 percent. This study found the range for albumen to be considerably higher 62 to 70 percent. Egg weight may account for a small part of the difference, but strain may play a bigger role than egg weight relative to the breakdown of component parts of the egg.
The age of the laying hens resulted in the percentage of albumen, yolk and whole egg solids significantly (P<0.05) changing during the production cycle. Albumen solids decrease and yolk solids increased. Whole egg solids continuously increased through 266 days of lay and then plateaued at approximately 24.9% solids. Each of the solids measurements was significantly (P<0.05) impacted by the strain of the hen. See Table 3 for specific strain impacts.
Egg Quality
The Haugh unit is an expression relating egg weight and the height of the thick albumen. It is generally accepted that the higher the Haugh unit value, the better the quality of the egg. It is also important that all eggs being evaluated at the same internal temperature. Age of the hen and season of the year can also impact Haugh unit values. This hen age impact can be seen in Figure 2 where Haugh unit values decreased from 89.6 to 68.8 over the twelve production periods.
The vitelline membrane plays an important role in egg quality. As the integrity of the membrane begins to decline moisture from the albumen begins to leak in to the yolk and the potential for yolk contents to leak in the albumen. Any leakage of fat components into the albumen is very detrimental to foaming functionality. The vitelline membrane strength of the eggs from this study decreased over time (2.27 g to 1.91 g). There are numerous factors (hen age, egg age, season, etc.) that can impact the membrane strength. Figure 3 shows how the vitelline membrane strength changed over the first year of production. This variation in strength could be contributing to the inconsistency seen in functionality of egg products.
Egg Functionality
Albumen foaming properties were evaluated using angel food cake volumes. Angel food cake volumes were not significantly different (P>0.05) in periods 1, 2, 3, 5, 7, and 8 (Figure 4). However, period 4 was significantly lower (P<0.05). During period 4, birds were diagnosed with Osteomalacia, which is related to Osteoporosis, Rickets, etc. Birds were treated initially with vitamins A, D, E, and K, supplement administered in the water, and 5 g of oyster shell/hen was top dressed in the feeder. The hens are still being supplied with 50% of their Ca in the form of oyster shell to provide for a slow release Ca during shell formation. The feed was also analyzed for vitamin content since Ca and P were normal.
However, production and mortality rates returned to normal by sampling period 5. As the bird continued to age the angel food cake volumes differed (P<0.05) from the highest volume at 334.56 mL in period 8 to the lowest at 270.60 mL in period 12, indicating that bird age negatively affected albumen foaming properties. Findings by Izat (1983) and Cunningham et al. (1960) tend to support this conclusion. They reported that the percentage of albumen solids, albumen protein, and Haugh units decreased with the age of the bird. Both studies also stated that as hens aged the albumen quality traits worsened and yolk traits generally improved. As mentioned earlier, this could be due to that decreased integrity of the vitelline membrane during the latter portion of the production cycle which could possibly be allowing fat from the yolk to seep into the albumen.
The greatest variability in functional properties of eggs was found with whole eggs in sponge cakes. Sponge cake volumes changed erratically (P<0.05) with volumes starting out low at 318.40 mL in period 1 to a high of 356.90 mL in period 7 and then dropping down to 307.93 mL in period 11 (Figure 5).
When looking at the variation among strains (Figure 6) a significant difference (P<0.05) was found between Hy-Line W-36 and Hy-Line W-98. These two strains are the most commonly used strains in the egg breaking industry, which could explain the inconsistency in whipping/foaming behaviour that they are reporting.
Emulsion strength of mayonnaise was impacted by bird age (P<0.05) during the first 5 months of production (386.05 to 512.03 g), but that impact lessened as emulsion strength increased and stabilized during the last 7 months (438.54 to 508.91g) (Figure 7). But we have to remember that during the 4th period there was an Osteomalacia problem with the birds. Looking at Figure 7 it makes you wonder if it was the Osteomalacia that impacted the emulsification properties during periods three through five.
Summary
In summary, while vitelline membrane strength (2.33 to 1.92 gms), percentage of shell as a component part (10.33 to 7.83%), and Haugh units (89.63 to 68.76 HU) decreased slightly over time, bird age did have a significant effect on all three. The percentage of albumen and yolk was significantly impacted by bird age. These component parts also followed previously reported data that showed an inverse relationship between the two, where as the percentage of albumen (70.01 to 62.57 %) decreased as the bird aged the percentage of yolk (28.43 to 19.67 %) increased. By comparing and characterizing all egg components this should give the breaking industry desperately needed information that is not available on modern layer strains.
All of the differences in albumen and yolk functionalities are definitely contributing to the variability in whipping properties that continue to plague the egg breaking industry. It is too early to say what definitive impacts these ages are having until the second year of this research project can be evaluated.
Reference
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From Proceedings of the “Midwest Poultry Federation Convention”, St. Paul, Minnesota, U.S.A.
Patricia A. CURTIS
Professor & Director
Poultry Products Safety & Quality Peaks of Excellence Program
Auburn University
Auburn, AL
U.S.A.
