The production of shell eggs is an important component of American agriculture and the American economy. The nation-wide production of shell eggs in 2000 was 71 billion. Of this total production, 56% was sold to the public at retail outlets, 31% was purchased by the processed foods industry primarily for the manufacture of cake and pudding mixes, pasta, ice cream, mayonnaise, candies and bakery products. About 1% of the total shell egg production was exported. The per-capita consumption of eggs in the United State in 2000 was 260 and has been rising yearly throughout the late 20th and early 21st centuries [1]. This trend toward increasing egg consumption is due, at least in part, to the fact that reputable research, such as the Harvard University study that followed 117,000 health care workers [2], indicated that moderate egg consumption does not adversely affect healthy people. In their recent dietary guidelines, the American Heart Association indicated that the coronary health of people who are not otherwise ill is not adversely affected by moderate egg consumption [3].

In addition to the importance of shell eggs as economic and dietary components, the shells themselves have become popular in applications that seek to introduce metabolic calcium into living systems. Eggshells have been shown to be an effective source of bio-active calcium in humans, mammals and plants [4 – 11]. Suguro et al. [4] report that powdered eggshell has a higher absorptivity and availability in mature mammalian cells with increased oral phosphate binding activity than other mineral calcium sources. They attribute this increased availability of the calcium in eggshell to the porous structure of the eggshell. In animal studies, Omi and Ezawa [5] found that eggshell calcium was efficient in increasing bone strength and mineralization in females after removal of the ovaries. Kikuchi et al. fed eggshell to lactating rats that were maintained on an otherwise low-calcium diet [6]. They found that eggshell calcium was more effective for bone mineralization than other calcium sources. Scheideler [7] evaluated eggshell quality as a function of eggshell fed to laying hens. She measured the mineral calcium digestibility and rate of lay and eggshell quality . She concluded that, while eggshell calcium is highly available for egg production, large-particle calcium mineral, for example from crushed oyster shell, should be included with ground eggshell for optimum eggshell quality. Eggshell products are used in cosmetics [11] and pulverized eggshell has been shown to increase the shelf life of orchard fruits when used as a fertilizer component [10].

Of the approximately 70 billion eggs produced annually in the United States, 6 - 8 percent of them, or around 5 billion eggs, are cracked or broken during handling and transport [12]. This represents a tremendous loss in edible protein and nutrition, and is about seven times the amount of eggs that are exported. Egg shell breakage is a combination of factors including genetics of eggshell strength, handling and nutritional variables [13].

Most studies of eggshell strength in the literature have focused on the strengths of white and brown eggshells [13-20] with some studies focusing on white and brown eggshells from commercial lines [17,18]. Washburn has given a comprehensive review of the work on eggshell strength that was done from about 1900 to 1990 [13]. Washburn [13] indicates that the strength of eggshells may not be solely dependent on eggshell thickness or color [14]. Washburn and co-workers [14] found in general the shells of white layers to have a greater strength despite the fact that they are not always the most dense. The shell strengths of commercial layer lines were evaluated by Hunton [18] and Hamilton et al. [17]. Their results confirmed the work of Washburn in the finding that, on average, white eggshells have superior strength than do brown. There are also differences in eggshell properties between different breed lines and from breed to breed [13-20].

Washburn in his 1990 review [13] discusses the variables that are thought to account for the intra-species variation in shell strength (thickness, shape, size, ability to accommodate deformational forces). Potts and Washburn [19] and Engsrtom et al. [20] performed statistical studies of the heritability of shell strength and incidence of shell cracking. This work indicates that the incidence of eggshell cracking is a trait that is passed on through family lines.

Eggshells are formed around the ovum as it passes through the latter part of the oviduct. In the early 1970s Katzukura and Tamate [21] measured the calcium mobilization at locations along the oviduct in laying hens, which is divided into five regions called vagina, uterus, isthmus, magnum and infundibulum. The infundibulum accepts the yolk into the oviduct as it breaks away from the ovary. The yolk travels the length of the oviduct where layers of albumin are deposited. The eggshell is put on just before the lay event. Katzukura and Tamate [21] found that calcium was secreted onto the shell membrane from the glandular regions of the isthmus posterior.

Krampitz et al. [22] studied the calcium chemistry in the shell gland fluid of hens. The fluidized calcium found there was chelated to ligands having molecular weights of around 15,000 daltons. These chelating ligands contained segments of carbohydrate, peptides and sulfate esters. The calcium binding ligands from other body tissues were found to be dissimilar. They were able to get calcium carbonate crystals to form from this fluid by treatment with the bicarbonate anion. Nys and Mongin examined the hen gut permeability to water and cations of calcium, potassium and sodium [23]. Finding no permeability differences between immature pullets and hens before and during eggshell calcification, they proposed that the increase in calcium absorption from the gut during shell calcification was due to enhanced solubility of CaCO₃ in the upper digestive tract.

Micronutrients and environmental pollutants are known to affect eggshell strength in different ways [24-30]. Agricultural chemical pesticides, such as DDT and DDE [24-28] are known to be causative agents in the decrease of eggshell thickness, weight and strength in wild avian species. Lundholm [24] suggested that the DDE pesticide reduces eggshell thickness by inhibiting the calcium binding to the membranes of the cells of the eggshell gland. Kolaja et al. [26] measured the amount of calcium per gram of mallard duck eggshell with and without DDT in the poultry diet. They found that the pesticide does not alter the calcium per gram ratio despite a significant eggshell thinning effect. The proposal was put forward that calcium-ATPase activity may be inhibited by the pesticide.

Vitamin D is associated with enhanced calcium uptake in avian species [29-32] and contributes to eggshell thickness, weight and strength. Large effects due to vitamin D have been observed in developing chick embryos [31,32]. Eggshell porphyrins have been linked with plasma chemistry and uterus glyoxalase II activity [29]. Eggshell porphyrins have also been shown to promote bone calcification in poultry when incorporated in the feedstuffs [30].

Organic substances are part of eggshell and they may appear either on the exterior as bloom, the interior of the shell as the membrane and within the eggshell mineral matrix. The bloom is thought to be essentially a sealant for the eggshell pores [13], but it probably also functions as an important lubricating medium during the lay event. The blue/green pigment of Araucana eggshells is due to liver compounds like biliverdin that are present throughout the shell. Porphyrins within the mineral matrix and on the eggshell exterior could have internal biochemical as well as external dietary sources. Although to our knowledge it has not been verified scientifically, many people believe that eggshells of free-range hens are stronger than those from confinement hens. The heme of red blood contains a porphyrin that binds the iron and the part of the chlorophyll that binds the magnesium is a porphyrin ring system.

All of the calcium that gets to the shell gland comes through the blood stream and is therefore in a soluble form. Many proteins and nitrogenous substances like porphyrin are efficient chelating agents for common soil and plant cations like Ca²⁺, Mg²⁺, Zn²⁺, K⁺, Cu²⁺ . It is very probable that some concentration of these chelating agents remains in the eggshell matrix. This could help explain the enhanced bioavailability of eggshell calcium. Organics remaining in the eggshell may also, depending on the concentrations, influence properties of the eggshell such as strength, weight, density, granularity of the crystallites of calcium carbonate. The concentrations of these organic compounds in the eggshell may be a heritable trait. Certainly the darkness of the brown eggshell, the chromophore of which is a heme compound, is a heritable trait.

Since the biological mechanism of eggshell formation uses the skeletal system as a warehouse for calcium with as much as 25% of the calcium in an eggshell coming from the hen's skeleton, one might consider that the skeletal system is part of the eggshell formation system. There are some recent results with respect to bone strength in the literature that are counterintuitive. Williams et al. [33] measured the bone strength of chickens at two weeks of age that were being raised on a variety of diets with different calcium / phophorous ratios. They found no significant mineral content effect on bone mineral or bone breaking strength on the diets tested. They concluded that the dietary Ca:P ratio in the diet has little effect on the mechanical compentence of bone. In a recent analysis of bone strength data existing in the literature, Taylor [34] concluded that the underlying structure of bone changes with animal size in order that the tendency to develop stress fractures is essentially the same in all animals. These results suggest that factors other than mineral content are most probably influencing the strength of the bone material.

References
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