COMPARISON OF THE MORPHOLOGICAL STRUCTURE AND CHEMICAL COMPOSITION OF EGGS IN RATITES

Danuta Szczerbińska, Monika Wiercińska
Department of Poultry and Ornamental Brids Breeding, West Pomeranian University of Technology in Szczecin, Poland

ABSTRACT

The study aimed at comparative analysis of the morphological structure and chemical composition of eggs in ostrich, emu and rhea. Examinations were carried out on 30 eggs, 10 eggs per each species. Eggs were from the peri-peak egg laying period. Based on the carried out analyses, both significant species differences and common features were found referring to the morphological and chemical composition of eggs. It was demonstrated that the largest yolk quantity occurred in emu eggs, while the least in ostrich ones (38 and 22%, respectively). Relative albumen weight in ostrich and rhea eggs was similar (59%), whereas significantly smaller in emu eggs (by about 12%). The largest participation of egg shell (19%) was found in ostrich eggs, whereas the least in rhea ones (12%). Chemical analysis of the egg content showed a larger content of dry matter and total protein in ostrich eggs, both in albumen and yolk. It was found that the quantity of fat in ostrich and emu eggs was similar (35–36%), whereas significantly smaller in the yolk of rhea eggs. It was demonstrated that the albumen of emu eggs was characterised by the smallest quantity of mineral elements. Only the ash content in yolk was similar and very high (1.4–1.5%), irrespective of the evaluated bird species. The content of most chemical elements building the egg shell, except calcium, phosphorus and sodium, was similar in respective bird species under examination.

Key words: ostrich, emu, rhea, egg quality.

INTRODUCTION

Numerous ovological studies referring to egg structure and its morphological and chemical composition, egg shell thickness and strength, and even its ultrastructure were carried out for many poultry species [1,10,11,15]. However, such a scope of knowledge has bot been achieved with respect to evaluation of eggs in birds of the superorder Palaeognathae. It includes ostriches from Africa, emus and cassowaries from Australia, rheas and tinamous from South America and kiwis from New Zealand [4]. In Poland, the farming of three of the above-mentioned species has drawn attention in the mid 90s, in particular of ostriches [7] and emus and rheas to a smaller extent [19], primarily in order to acquire meat, skins and feathers [2,13,14]. Based on sparse osteological studies and egg morphology and egg shell structure examinations, similarities between the afore-said birds can be suspected, resulting from their relationship. No complex evaluation of the quality of eggs in these birds has been carried out so far in order to verify this hypothesis.

The study aimed at comparative analysis of the morphological structure and chemical composition of eggs in utility ratites, i.e ostriches, emus and rheas.

MATERIAL AND METHODS

Egg quality evaluation was carried out in the breeding flocks of African ostriches (Struthio camelus), emus (Dromaius novaehollandiae) and rheas (Rhea americana). Birds were kept on three farms: ostriches and rheas on private farms situated in the Western Pomeranian and the Lubusz Provinces, whereas emus on an experimental farm of the Department of Poultry and Ornamental Birds Breeding of the Agricultural University in Szczecin. Ostrich flock comprised 9 specimens – 6 females and 3 males, whereas the emu one 22 specimens, with equal participation of both sexes; in case of nandu, the flock was composed of 20 birds, including 16 females and 4 males. Emus and rheas were 5 years old when the study started and they were in the 3rd year of the laying period, whereas ostrichs were 4 years old and were laying eggs in the second laying season. During the whole observation period, the birds were kept under open-range maintenance system and fed complete feeds and green forage. In winter season, ostrich and nandu fodder was supplemeneted with lucerne and clover hay whereas the emu one with carrots, cabbage and leek. Quality analysis was done on 30 eggs (10 eggs per each species) from the peri-peak laying period. Eggs with similar weight and shape and properly coloured and normally formed and undamaged egg shell were chosen for examination. Eggs were weighed and measured with a slide calliper, determining their shape index based on long axis length to short axis lenght ratio. After breaking the egg, yolk and thick albumen indices were determined, as well as participation of yolk, albumen, egg shell membranes and egg shell in the egg weight. Egg shell thickness was measured with egg shell membranes and without them in three locations: on the blunt end of the egg, in its equatorial part and on the sharp end of egg, using an electronic micrometer screw gauge. Analysis of the chemical composition of egg albumen and yolk (dry matter, total protein, raw fat and ash) was made by means of classical methods.

The content of chemical elements in egg shells was determined with inductively coupled argon plasma-optical emission spectroscopy (ICP-OES). Analyses were made by means of Perkin Elmer Optima 2000 DV apparatus. Samples for spectrometric analyses were mineralised in 65% HNO₃ in a high-pressure microwave mineralisation system. As a reference material, certified multielemental standard was used. Standard solutions were supplmented with an addition of nitric acid in such a concentration which was occurred in mineralised samples. All measurements of the intensity of emitted radiation were made selecting a longer, axial optical path in spectrometer.

After data completion and entering them into a Statistica 7.1 PL database, mean values and standard deviation were calculated. The results were subject to one-factor analysis of variance using the Duncan's test.

RESULTS AND DISCUSSION

Despite the resemblance and similar biology of ostriches, emus and rheas emphasised in literature [4,6,19], significant differences were shown during own study in the morphological structure and chemical composition of eggs. One can think that differences in the traits expressed as absolute values result first of all from different weight of the eggs of birds under evaluation, whereas differences in chemical and physical traits given as relative values point to certain species distinctiveness in this respect. (Table 1). In the present study, ostrich egg weigh was over 1500 g, being 2.5-times higher when compared to that in other species under evaluation. Egg shape index was typical for evaluated bird species, as the egg weight, not being different from the results of earlier studies [3,5,12,17]. It was shown that emu eggs had the largest quantity of yolk (38%) out of the examined representatives of Palaeognathae (Table 1). On the other hand, relative albumen weight in ostrich and rhea eggs was similar (about 59%) but significantly smaller in emu ones (by 12%). The percentage of egg shell in eggs of respective species differed significantly. The largest quantity of egg shell was found in ostrich eggs (19.4%), whereas the least in rhea ones (12.4%). The egg shell thickness, depending on the evaluated bird species, differed significantly and ranged from 2.03 (ostrich eggs) to 0.96 mm (rhea eggs). Specific morphological structure of the eggs of bird species under evaluation results undoubtedly from environmental conditions they live in wild and their evolutionary adaptation. In case of ostrich natural environment, thick egg shell – constituting a large percentage in the egg weight, is to protect an embryo from the effect of extreme thermal and humidity conditions and withstand the weight of a brooding adult bird. In turn, such a large quantity of yolk in emu eggs is udoubtedly connected with the length of hatching period in this species and the way of survival during the first days of life under difficult environmental conditions.

Table 1. Egg quality analysis ( ± SD)

Analysis of the chemical composition of egg content (Table 2) showed significantly larger quantity of dry matter in ostrich eggs (11.5%). Furthermore, the largest content of protein in both egg parts was found in this species. The quantity of fat in emu eggs and ostrich ones was similar (35.7% and 34.9%, respectively). On the other hand, significantly smaller quantity of this component was found in the yolk of rhea eggs (33.6%) but only when compared to emu eggs. Also the smallest, significantly different quantity of mineral elements in albumen was demonstrated in this species (0.57%). As it results from the above analysis, the chemical composition of Palaeognathae eggs significantly differed in most cases, only the ash content in yolk was similar and very high (about 1.4-1.5%), irrespective of the evaluated bird species. This may result from the fact that it is essential for chicks leading a nomadic life and requiring a strong skeleton to build strongly mineralised bone tissue. Large quantity of mineral elements in the yolk of ostrich eggs (from 1.44 to 2.6%) called also the attention of Superchi et al. [16] as well as Szczerbińska et al. [18] in emu eggs (1.96%).

Table 2. Chemical composition [%] of the egg content ( ± SD)

Chemical analysis showed that a main chemical element comprised in the egg shell is calcium, as in other bird species (Table 3). Its content ranged from 357 to 376 mg/g of the egg shell in rhea and emu eggs, respectively. Other chemical elements building the egg shell and being found in it in considerable quantities were phosphorus, sodium, magnesium and potassium. The content of the majority of twelve chemical elements assayed in egg shells was similar in respective bird species, with significant differences being found only for calcium, phosphorus and sodium. The greatest concentration of these microelements was found each time in emu eggs, with significant differences being reported in case of calcium in relation to rhea egg shells only. There are not much data in the literature about the chemical composition of egg shell in the birds under evaluation. One of a few studies on the chemical composition of emu egg shell is a paper by Klös et al. [8]. These authors reported that main elements of the egg shell of those birds are calcium, magnesium and phosphorus. Similar results referring to Ca and Mg contents in emu egg shell were obtained by Szczerbińska [17], only phosphorus content was twice smaller than that obtained by Klös et al. [8]. Majewska et al. [9] reported, when analysing the chemical composition of emu egg shell, that calcium, magnesium and phosphorus were the chemical elements being found in the greatest quantity (396.7 mg/g, 1.22 mg/g and 0.71 mg/g, respectively). These results differ from those obtained in the present study in relation to magnesium only.

Table 3. Chemical composition of the egg shell ( ± SD)

CONCLUSIONS

1. The chemical analysis of egg content showed significantly higher content of dry matter and total protein, both in the albumen and yolk of ostrich eggs. It was found that fat content in ostrich and emu eggs was similar (35–36%), while being significantly smaller in the yolk of rhea eggs.

2. It was showed that the albumen of emu eggs was characterised by the smallest quantity of mineral elements. Only the ash content in yolk was similar and very high (1.4–1.5%), irrespective of the evaluated bird species.

3. The content of most chemical elements building the egg shell, except calcium, phosphorus and sodium, was similar in respective bird species under examination.

REFERENCES

1. Burton F. G., Tullett S. G., 1983. A comparison of the effects of eggshell porosity on the respiration and growth of domestic fowl, duck and turkey embryos, Comp. Biochem. and Physiol., 75A, 2, 167–174.

2. Cooper, R.G., Horbańczuk J.O., 2002. The anatomical and physiological characteristics of ostrich (Struthio camelus var. domesticus) meat determine its nutritional importance for man, Anim. Sci. J., 73, 3, 167–173.

3. Deeming D. C., 1995. The ratite egg [In: Ratite Encyclopedia]. Ed. C. Drenowatz. Ratite Records Inc., San Antonio, 93–102.

4. del Hoyo J., Elliott A., Stargatal J., 1992. Handbook of the Birds of the World. Lynx Edicions.

5. Gonzales A., Satterlee D. G., Moharer F., Cadd G. G., 1999. Factors affecting ostrich egg hatchability, Poult. Sci., 78, 1257–1262

6. Horbańczuk J.O., 2002. The ostrich. European Ostrich Group, Denmark.

7. Horbańczuk J.O., Tomasik C., Cooper R.G., 2008. Ostrich farming in Poland – its history and current situation after accession to the European Union, Avian Biol. Res., 1, 2, 65–71.

8. Klős H. G., Langner H. J., Boenigk G., Wandelburg K., Pohl H., Grund S., Eichberg J., Steglich W., 1976. Chemische und physikalische Untersuchungen an Eierschalen von vier Laufvogelarten (Struthioniformes), Zentralblatt für Veterinarmedizin Reihe A, 23, 413–428.

9. Majewska D., Szczerbińska D., Tarasewicz Z., Ligocki M., Dańczak A., Safader Nędzusiak L., Sammel A., 2008. Age-related changes in the quality of emu (Dromaius novaehollandiae) eggs, Arch. Geflűgelkd., 72, 4, 168–173.

10. Mróz E., Michalak K., Faruga A., Horbańczuk J.O., Ormowska O., 2008. Shell microstructure and hatchability of turkey eggs, Anim. Sci. Papers and Reports, 26, 2, 129–140.

11. Peebles E. D., Brake J., 1987. Eggshell quality and hatchability in broiler breeder eggs, Poult. Sci., 66, 596–604

12. Sales J., Poggenppoel D. G., Cilliers S. C., 1996. Comparative physical and nutritive characteristics of ostrich eggs, World's Poult. Sci. J., 52, 45–52

13. Sales J., Horbańczuk J.O., 1998. Ratite meat, World's Poult. Sci. J., 54, 1, 59–67.

14. Sales J., Horbańczuk J.O., Dingle J., Coleman R., Sensik S., 1999. Carcase characteristics of emus (Dromaius novaehollandiae), Br. Poult. Sci., 40, 145–147.

15. Solomon S. E., 1997. Egg and Eggshell Quality. Iowa State University Press, Iowa, USA.

16. Szczerbińska D., 2002. Charakterystyka użytkowości reprodukcyjnej emu (Dromaius novaehollandiae) ze szczególnym uwzględnieniem ultrastruktury skorupy i jej związku ze wskaźnikami wylęgowości jaj [The characteristic of reproductive performance in emu (Dromaius novaehollandiae) with particular reference to eggshell ultrastructure and its association with egg hatchability indices]. Rozprawy 210 AR Szczecin [in Polish].

17. Szczerbińska D., Dańczak A., Tarasewicz Z., 1999. A relationship between emu (Dromaius novaehollandiae) egg quality and hatching rate, Arch. Geflűgelkd., 63, 185–187.

18. Szczerbińska D., Majewska D., Dańczak A., Tarasewicz Z., 2007. Emu. Monografia [The emu – a monograph]. Wydawnictwo Naukowe Akademii Rolniczej w Szczecinie [in Polish].

Accepted for print: 24.11.2010

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