Organic Farming and Mineral Content of Chicken Eggs

Chapter 10

Organic Farming and Mineral Content of Chicken Eggs Kamil Küçükyılmaz* and Mehmet Bozkurt** *Department of Animal Science, Faculty of Agriculture, Eskis¸ehir Osmangazi University, Eskis¸ehir, Turkey; **Erbeyli Poultry Research Institute, Aydın, Turkey

INTRODUCTION Eggs are among the most important, nutritious foods in the human diet. Eggs serve as a source of protein, essential vitamins and minerals, and other bioactive compounds; therefore, they contribute substantially to a healthy diet. Moreover, eggs are included in several food products to fulfill different functions such as a thickener, emulsifier, or a foaming agent. Indeed, a whole egg contains all the nutrients required to turn a single cell into a baby chicken.

MINERAL CONTENT OF HEN’S EGGS AND FACTORS AFFECTING ITS NUTRITIVE VALUE Eggs contain most of the minerals that the human body requires for health. Minerals, such as phosphorus, iron, zinc, iodine, and other trace elements, are present in the egg. In particular, eggs are an excellent source of iodine, which is required for the synthesis of thyroid hormone, and phosphorus, required for bone and teeth health, normal nervous system function, and energy metabolism. Eggs are also a rich source of biologically available zinc needed for proper functioning of the immune system. Selenium from eggs serves as an antioxidant. Calcium, a critical mineral needed for bone growth and maintenance, muscular contraction, and nervous system functioning, is largely concentrated in the eggshell with only minor concentrations deposited in the egg yolk. Although eggs contain significant amounts of iron, most of the iron in the yolk is bound to a phosphoprotein called phosvitin. Therefore the low bioavailability of the iron present in egg yolk is due to its tight binding to phosvitin and the formation of an insoluble phosvitin-iron complex (Ishikawa et al., 2007; Li-Chan and Kim, 2008). The mineral content of egg yolk is approximately 1%. Phosphorus is the most abundant mineral component, 61% of which is contained in phospholipids (Sugino et al., 1997). Yolk also includes calcium, chloride, potassium, sodium, sulfur, magnesium, and manganese as well as other minerals present at trace levels. The major components of egg whites are sulfur, sodium, and chloride followed by potassium, calcium, and magnesium (Li-Chan and Kim, 2008). Table 10.1 shows the concentrations of major minerals in fresh eggs. Just 1 egg (50 g edible portion) provides > 25% of the recommended daily allowances (RDA) of selenium, > 10% RDA of zinc and phosphorous for children up to 9 years of age, and 48% of RDA of iodine for children aged 4–8 and 36% for children aged 9–13 (Yalçın and Yalçın, 2013). Eggs are not important in meeting the RDA for calcium and magnesium. There is a large variation in reported mineral content of eggs. These differences could be due to the age of the analytical data, the use of different analytical methods, or could represent the normal variation for biological material. Variation in mineral content can also be affected by the age of the hens because the albumen to yolk ratio of eggs changes as the hen ages (Ahn et al., 1997). Differences in feeding regimens can also affect the content of some minerals in eggs. Whereas trace element content can vary depending on the laying hen diet, the macroelement content remains relatively stable. There is a strong relationship between the trace mineral content of the hen’s diet and the concentration of minerals in the egg, particularly in the yolk where most trace minerals are deposited. Trace mineral enrichment of eggs can be achieved by manipulating the diet of laying hens. Several studies have confirmed that it is possible to produce novel eggs with enhanced levels of certain important trace minerals, in particular, selenium, iodine, ferrum, and zinc. The addition of zinc, copper, manganese, iodine, and Egg Innovations and Strategies for Improvements. http://dx.doi.org/10.1016/B978-0-12-800879-9.00010-X Copyright © 2017 Elsevier Inc. All rights reserved.

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TABLE 10.1 The Content of Major Minerals in Fresh Egg Mineral

Whole Egg (mg/100 g)

Egg White (mg/100 g)

Egg Yolk (mg/100 g)

Sodium

154

185

52

Potassium

145

149

124

Calcium

46

6

149

Magnesium

13

12

12

Phosphorus

179

12

600

Iron

1.72

0.01

6.24

Copper

0.05

0.02

0.16

Zinc

1.12

0.01

4.03

Chloride

180

159

163

Manganese

0.030

<0.010

0.110

Iodine

0.050

0.004

0.130

Selenium

0.023

0.008

0.059

Source: Data taken from Roe, M., Pinchen H., Church S., Finglas P., 2013. Nutrient Analysis of Eggs. Analytical Report. Department of Health, London, United Kingdom. Available from: https://www.gov.uk/government/publications/nutrient-analysis-of-eggs.

selenium to the hen’s diet increased their corresponding concentrations in the egg yolk. Some enrichment levels can be very high. Iodine and selenium contents can reach up to 60 and 10 times higher than the initial content, respectively (Stadelman and Pratt, 1989). Such enriched or fortified eggs could be used to improve both human nutrition and chick embryo viability (Giannenas et al., 2009). Inorganic forms of iodine in feed are absorbed very efficiently from the intestines of hens (Underwood and Suttle, 2001). The iodine concentrations in egg yolk, egg albumen, and the whole egg increased with increased iodine supplementation (Yalçın et al., 2004). Iodine supplementation to the hen's diet with levels up to 5 mg/kg is recommended to obtain iodineenriched eggs (Yalçın et al., 2004). Hens that are fed high levels of zinc produced eggs that contained as much as 25% more zinc than the eggs produced by the hens fed the control diets (Schiavone and Barroeta, 2011). The concentration of zinc in the yolk was reduced in the eggs of hens fed a copper supplemented diet and vice versa, probably due to a zinc–copper antagonism. It seems evident that higher levels of dietary zinc or copper could overcome the negative effects of the antagonizing element (Skrivan et al., 2005). Also, selenium may negatively affect zinc absorption; research has shown that dietary selenium supplementation reduces the amount of zinc in the egg (Bargellini et al., 2008; Rizzi et al., 2009). Egg yolk is rich in iron. However, the poor bioavailability of iron in egg yolk, which ranges from 1% to 10% of total content, is due to the fact that iron is present in ferric form and interacts with the phosvitin found in egg yolk (Hallberg et al., 1997). The iron content of eggs could be increased by 5% to 18% through dietary supplementation of iron in both organic and inorganic forms (Park et al., 2004). Also, iron supplementation in the hen’s diet, in combination with zinc and copper, increased the iron content in the egg by 34.9–36.7% (Skrivan et al., 2005). Experimental studies that compare inorganic versus organic trace mineral sources have shown that both trace mineral mixes increase the mineral content of the egg, but the bioavailability and efficacy of organic sources of zinc, manganese, copper, iron, and selenium are superior to the more traditionally used inorganic forms (Swanson, 1987; Davis et al., 1996; Cantor et al., 2000; Payne et al., 2005; Dobrzanski et al., 2008; Paik et al., 2009; Sun et al., 2012). Organic sources of zinc, manganese, copper, and selenium in the diets of broiler breeders increased the retention of these corresponding minerals in the whole egg when compared to inorganic forms (Sun et al., 2012). Dietary supplementation with organic cooper (Saccharomyces cerevisiae yeast enriched with copper) to the hen’s diet increased egg copper concentration (Dobrzanski et al., 2008). The greater bioavailability of organic minerals is probably related to different absorption mechanisms and to better protection from binding to dietary constituents such as phytates (Schiavone and Barroeta, 2011). Inorganic selenium is passively absorbed, whereas selenium bound to an amino acid is absorbed by active transport, which is consistent with the absorption process of amino acids (Underwood and Suttle, 2001; Surai, 2002). Organic selenium (selenmethionine and selenized yeast) is mainly deposited in the egg white, whereas inorganic selenium and nonselenomethionine compounds are primarily deposited in the yolk (Davis et al., 1996).

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Outside of dietary factors, the mineral content in egg yolk is only slightly affected by embryogenesis. Although there were some minor differences, especially with magnesium, the mineral content in yolk fractions were similar between table eggs and fertile eggs incubated for 5 days (Bäckermann and Ternes, 2008).

EFFECT OF ORGANIC REARING SYSTEM ON EGG MINERAL CONTENT Current commercial hen egg production can be divided into four different husbandry systems: cage eggs, barn eggs, freerange eggs, and organic eggs. Consumer demand as well as the European Union Directive (1999) that ordered a ban on conventional cages in the European Union beginning in January 2012 resulted in many research institutions evaluating alternative production systems for the housing of laying hens. This research continues to contribute to the improvement in the design of enriched cages, aviaries, and floor systems to better enhance laying hen welfare. Raising egg laying strains of chickens under organic conditions is an additional production system of interest to the consuming public (Kijlstra and Eijck 2006; Küçükyılmaz et al., 2012a). Free-range and organic eggs have increased in popularity among consumers, resulting in a greater proportion of the market share. The production of organic eggs is based on specific standards (European Union Directive, 2007). There is no worldwide standard for organic egg production, but most nationally developed standards evolved from those developed in Europe by the International Federation of Organic Agriculture Movements. To be defined as organic, the feed provided to hens is void of antibiotics and any conventionally grown feedstuffs that use artificial fertilizers. Such feedstuffs as genetically modified organisms or crops, synthetic additives, and animal by-products are not allowed in the organic rations. Only organically grown oil seeds, cereals, and roughage are used as feed ingredient sources. Besides feed, an additional standard is that hens must be cage-free and have outdoor access which generally lowers stocking intensity. There is a general perception among consumers that organic eggs are safer, tastier, and more nutritious than eggs from hens housed in other systems, such as conventional cages, because hens are consuming organic feed with access to vegetation on pasture. Unfortunately, there is no consistent or firm scientific evidence to support this claim (Kouba, 2003). Mainly because of the cost of organic feed ingredient sources, consumers who purchase organic eggs pay a premium price as compared to eggs produced in conventional cages. The higher price paid for organic eggs establishes high expectations among consumers who assume that these eggs will be of better quality as compared to conventional ones. Based on the increasing public interest in organic eggs, studies have focused on organic production systems and have shown that the egg nutritional composition can be manipulated through use of feed ingredients in the organic diet of hen (Cherian et al., 2002; Samman et al., 2009; Mugnai et al., 2009; Küçükyılmaz et al., 2012a). Most of the research shows that the organic rearing system influences the fatty acid composition of eggs, including saturated (SFA) and polyunsaturated (PUFA) fatty acid content, the total amounts of omega-3 and omega-6 fatty acids, as well as the ratios of PUFA/ SFA and omega-6/omega-3 fatty acids through feed ingredient composition. The effect of the hen rearing system on egg protein content is less pronounced than its effect on fat content. The PUFA and omega-3 content of eggs is enhanced by organic feeding and is achieved by the use of appropriate dietary ingredients and feeding green grass in the outdoor area (Küçükyılmaz et al., 2012a). Also, pasture feeding of hens has been shown to increase the vitamin E content of their eggs (Karsten et al., 2010). Although there is growing experimental evidence that examines the effect of organic production systems on the performance of hens (Küçükyılmaz et al., 2012a), little data is available in the literature concerning the effects of organic rearing procedures on egg mineral content. Unfavorable attributes to organic rearing systems in terms of egg phosphorus content have indicated that organic eggs have considerably lower phosphorus content compared with conventional eggs (Matt et al., 2009; Küçükyılmaz et al., 2012b; Bologa et al., 2013, Table 10.2). The reasons for this trend of lower phosphorus in the edible portion of organic eggs are not clear. In the study of Küçükyılmaz et al. (2012b), the chickens rapidly depleted the pasture during the pullet rearing period, so vegetation on pasture during the egg-laying phase was lacking. Therefore, pasture vegetation during egg laying did not serve as a source of variation for the phosphorus content of organic eggs compared to those produced in conventional cages. Any phosphorus reserves retained by pullets from the consumption of vegetation prior to egg laying would contribute to more phosphorus rather than less in organic eggs. The composition and intake of the organic versus conventional feeds by the hens in the respective housing system also did not provide explanation for the lower values of phosphorus in the edible portion of organic eggs. Although available levels of phosphorus were not calculated, the organic (0.65%) and conventional feeds (0.66%) had similar total phosphorus levels (Küçükyılmaz et al., 2012b). Both feeds were not supplemented with phytase to make the phosphorus from soybean meal more available. Sunflower meal is an excellent source of phosphorus, but this feed ingredient was double the amount in the organic as compared to the conventional feed. Hens in organic systems generally have higher feed intake than conventionally caged hens (Küçükyılmaz et al., 2012a) which would make more phosphorus available for organic hens to deposit

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TABLE 10.2 The Mineral Content of the Edible Portion and Shell of Eggs From Hens Reared in an Organic as Compared to a Conventional Housing System Edible Egg Part

Eggshell

Mineral

Organic (ppm)

Conventional (ppm)

Organic (ppm)

Conventional (ppm)

Calcium

2,460

2,633

469,000

481,000

B

A

Phosphorus

5,778

1,638

1,507

Magnesium

578.2

532.8

4,596A

4,174B

Iron

81.2

95.4

1.57

1.6

Zinc

41.6B

60.0A

2.3B

2.8A

Copper

11.6

12.8

4.5

6.2

9,800

A,B

Within a row and housing system, values with differ superscripts denotes significant difference (P < 0.05). Values are on a dry weight basis. Source: Adapted with kind permission from Elsevier: Küçükyılmaz, K., Bozkurt, M., Yamaner, Ç., Çınar, M., Çatlı, A.U., Konak, R., 2012. Effect of an organic and conventional rearing system on the mineral content of hen eggs. Food Chem. 132, 989–992.

into the edible portion of the egg which of course did not occur. The same logic can be applied to the consumption of soil (∼10 g per hen per day) by hens with access to the outdoors which would provide more phosphorus for deposition into the egg rather than less. Because there is no indication that feed contributed to the lower phosphorus content in organic eggs, the physiological utilization of this mineral may differ between hens of the two housing systems. Organic hens on littered floor system with outdoor access may have used more of their body mineral reserves for increased activities, such as greater frequency of walking, running, wing flapping, and scratching the soil, which are inherent behaviors of hens in organic systems (Küçükyılmaz et al., 2012b). Although one of the major roles of phosphorus is as a hydroxyapatite component of bone, it is also an essential component of adenosine triphosphate involved in almost every aspect of metabolism. Phosphorus is involved in skeletal growth and maintenance, muscle coordination, nervous tissue function, the metabolism of amino acids, carbohydrates, and fat, and the transportation of fatty acids and lipids (Leeson and Summers, 2001). Therefore, phosphorus needs for energy metabolism and stronger bones of organic hens may have resulted in less phosphorus available for internal egg deposition. Negative but inconsistent results were reported for egg calcium content. Matt et al. (2009) reported that organic eggs had considerably lower calcium content (2.8×) than conventional eggs, but Küçükyılmaz et al. (2012b) reported that the egg calcium contents of both the edible portion of the egg as well as the eggshell of conventional and organic eggs were not different (Table 10.2). However, the magnesium content in the eggshell was 10% higher in organic eggs when compared to conventional eggs (Table 10.2) perhaps due to the ingestion of soil that may have contained abundant magnesium. It is estimated that hens with outdoor access can consume 10 g of soil daily (De Vries et al., 2006) which could represent 7% of their total intake if feed consumption is at 128 g per day (Küçükyılmaz et al., 2012a). The rations fed to the hens of the two production systems were similar relative to analyzed values of feed magnesium (2442 versus 2408 ppm of magnesium in conventional and organic diets, respectively). However, hens of organic systems generally eat more feed due to increase exercise than conventionally caged hens resulting in more magnesium intake and therefore, availability for shell deposition. In contrast to shell content, consuming more magnesium by organic hens did not result in a concomitant increase in magnesium in the edible portion of the egg with concentrations being similar between the organic and conventional housing systems. The magnesium content of the edible portion of the egg is about 8 times lower than the amount in eggshell (Table 10.2, Küçükyılmaz et al., 2012b). Conventional eggs contained more zinc than organic eggs (Giannenas et al., 2009; Küçükyılmaz et al., 2012b; Bologa et al., 2013, Table 10.2). The organic ration (114 ppm) was similar to conventional feed (117 ppm) relative to zinc content (Küçükyılmaz et al., 2012b) with presumed greater zinc intake for organic as compared to conventionally caged hens due to greater feed and soil intake as described for phosphorus (Küçükyılmaz et al., 2012a). Because zinc ingestion may have actually been greater for organic hens whose eggs were lower in zinc content, intake is once again ruled out as a contributing factor for differences in mineral composition between organic and conventional eggs. As with phosphorus, the hens in the organic system may have a higher requirement for zinc than hens in conventional cages resulting in less zinc available for egg deposition. Hens in organic systems are generally exposed to more variable environmental conditions compared to hens in indoor housing containing conventional cages, hence they might have utilized more zinc to support their immune systems against external stimulation, thus reducing zinc’s deposition in the egg (Küçükyılmaz et al., 2012b). To support this hypothesis that zinc utilization is sequestered more for immune function rather than deposition into the egg of organic

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TABLE 10.3 Trace Element Concentration in Yolk and Albumen of Eggs From Conventional and Organic Husbandry Systems Egg Yolk

Egg Albumen

Trace Element

Organic (ng/g)

Conventional (ng/g)

Organic (ng/g)

Conventional (ng/g)

Selenium

410A

313B

54.5

62.0

B

A

Zinc

18,225

20,676

1,029

1,003

Manganese

797

836

35

33

Cobalt

4.6

4.6

1.14

1.36

Copper

1,233

1,357

189

212

Molybdenum

246

260

19.5

26.0

Vanadium

13.2

12.5

13.6

13.2

Chromium

82.9

66.2

48.2

48.2

Nickel

58.4

63.3

56.3

64.2

Thallium

1.5

1.4

0.51

0.72

Arsenic

12.5

13.9

4.4

5.4

Cadmium

1.6

1.4

0.8

0.6

A,B

Within a row and housing system for egg yolk, values with differ superscripts denotes significant difference (P < 0.05). Hens for both husbandry systems were housed on littered floor systems. The organic housing system had outdoor access. Source: Adapted with kind permission from Elsevier: Giannenas, I., Nisianakis, P., Gavriil, A., Kontopidis, G., Kyriazakis, I., 2009. Trace mineral content of conventional, organic and courtyard eggs analysed by inductively coupled plasma mass spectrometry (ICP-MS). Food Chem. 114, 706–711.

hens, a more robust antibody titer to Newcastle Disease vaccination was noted with organic as compared to conventionally caged hens (Küçükyılmaz et al., 2012a). Zinc serves as a cofactor for a number of metalloenzymes involved in the hen’s immune response. Mononuclear phagocytes (heterophils, monocytes, and macrophages) produce a superoxide radical (O −2 ) used to destroy pathogens. The enzyme, superoxide dismutase, uses cofactors, such as zinc, to either remove or add electrons to superoxide radicals transforming them into less damaging molecules of hydrogen peroxide or normal molecular oxygen. Thus, zinc contributes to enzyme activity that helps maintain the integrity of mononuclear leukocytes. The role of zinc in cellular biology is diverse, offering host protection against bacteria, fungi, parasites, and viruses. Its role in poultry immunology and disease resistance is of key importance (Kidd, 2005). In contrast to the pattern observed for phosphorus and zinc, Giannenas et al. (2009) reported that organic egg yolks, but not albumen, contained more selenium and numerically more chromium than yolks from eggs of hens housed on littered floor with conventional feed and no outdoor access (Table 10.3). The selenium content of the organic and conventional diets of the hens were similar, but the chromium was 1.7 times higher in the organic diet. According to the authors, the increase in the selenium and chromium content of organic eggs was mainly due to the hen’s soil and grass intake when the hens were in the outdoor area.

HAZARDOUS HEAVY METAL RESIDUES IN ORGANIC REARING SYSTEM Heavy metals are persistent contaminants in the environment that can cause serious environmental and health hazards. They are released into the environment from natural as well as man-made activities. Some heavy metals, like copper and iron, are essential to maintain proper metabolic activity in living organisms, whereas others, such as lead and cadmium, are nonessential, have no biological role, and are toxic in high concentrations (Li et al., 2005). Deficiency of essential elements (zinc and copper) results in impaired biological function, but when their intakes exceed the recommended level, the consequences can be more severe. Heavy metals present a food safety concern because these chemicals accumulate throughout the food chain and are not readily cleared from the body of humans and other animals. Therefore, chronic exposure to heavy metals can lead to adverse health effects emphasizing the need to minimize exposure (Holt et al., 2011). Consumers and government regulators are demanding healthy products, free of any chemical contamination that could endanger human and animal health. Data on the heavy metal content of eggs is relatively scarce. Heavy metal residues in eggs may result from accidental inclusion in the feed or water or both. The laying hen may also come in contact with residual contaminants in the environment. Independent of the housing systems, the exposure to this class of chemicals from water or commercial feeds is

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expected to be uniform across all large production settings. However, exposure to several of these chemicals could potentially be greater in free-range and organic systems than in other systems, because organic hens come in direct contact with the outdoor environment and ingest soil or organisms in the soil. Organic rearing systems that provide outside access to animals are more susceptible to contaminants in the environment for which exposure is difficult to control. The most widely reported chemical contaminate of eggs associated with free-range and organic flocks are dioxin-like compounds (Chang et al., 1989; Harnly et al., 2000; De Vries et al., 2006). Very little research has compared the effects of alternative housing systems on heavy metal content in eggs. The amount of available data on the level of heavy metals in feeds exceeds the amount of information examining levels of heavy metals in eggs which is not surprising as quality control checks are most often performed on feeds. Furthermore, there is a very low transfer of metals from the feeds to the animal products. Küçükyılmaz et al. (2010) reported that there were no differences in the concentration of heavy metals (lead and cadmium) between conventional or organic labeled feed and eggs. In free-range eggs from backyard flocks in Belgium, the heavy metals of lead, mercury, cobalt, and thallium had median concentrations 2–6 times higher than those of commercial eggs presumably due to soil contamination (Van Overmeire et al., 2006). Radu-Rusu et al. (2013) showed that lead levels were below the detection limit and slightly higher levels of cadmium were found in the yolks produced in free range system. However, none of the heavy metal levels exceeded the toxicity limits for humans which is due to the very low transfer of dietary cadmium to the egg (Leach et al., 1979). Several authors have shown that cadmium primarily accumulates in the liver and kidneys (Salisbury et al., 1991; Ysart et al., 2000) rather than eggs. Heavy metal content in eggs varies widely depending on their origin with some geographical regions have more problems than others. Heavy metal residues are sometimes found on farms located in heavily-populated areas close to factories, industrial complexes, and roadways. As an example, the farm’s distance from roadways had no effect on egg copper and manganese content; however, an effect on egg cadmium and lead content was observed (Şekerog˘lu et al., 2013). It is postulated that environmental pollution is at the origin of the higher contamination of eggs. Time spent in the outdoors appears to influence the level of environmental contaminants ingested by hens in free-range systems (Kijlstra et al., 2007). De Vries et al. (2006) estimated that a hen with outdoor access consumes 10 g of dry soil, 7 g of dry vegetation, and 20 g of insects and earthworms a day. Nevertheless, these amounts of extraneous feed sources outside of the complete ration are subject to wide variations among range systems. Limiting the risk of transfer of contaminants from the environment into eggs requires chemical analysis of soil that the hens will have access to and evaluating the rearing practices that are likely to favor soil, plant, worm, and insect ingestion (Jondreville et al., 2011). Organic or outdoor poultry production is advisable only for those areas with low environmental pollution. In these areas, it is likely that both organic and conventional eggs contain undetectable levels of heavy metal residues.

STRATEGIES FOR IMPROVEMENT It is noteworthy that one of the most significant uncertainties and shortcomings in the experimental procedures of organic rearing studies in poultry is the unclear definition of plant cover on the outdoor area. This shortcoming makes it very difficult to precisely estimate or measure the additional mineral intake provided by plants. Furthermore, differences in mineral content of organic and conventional eggs may not be due simply to the consumption of vegetation on pasture, but could also possibly be affected by the mineral composition and content of soil, pebbles, insects, and worms consumed by hens with outdoor access. Additional evaluation is needed to verify unequivocally if sources of nutrients coming from the pasture contribute to the mineral content of eggs. Chemical analysis of soil for heavy metals and other contaminants, such as dioxin, should be conducted prior to allowing the hens access to the outdoors or preferably before initiation of construction of new range facilities.

ANALYTICAL METHODS For sensitive, precise, and accurate determination of mineral concentrations, inductively coupled plasma optical emission spectrometry or inductively coupled plasma mass spectrometry are used (Roe et al., 2013). Trace elements are determined using flame and graphite atomic absorption spectrometry (Kılıç et al., 2002). Specific wavelengths for each element are used. Egg shell and edible egg part samples are dried at 70°C (158°F). One g of the dried sample is ashed in a furnace at 550°C (1022°F). Ultra pure nitric acid (5 mL) is added to each ash sample until completely dissolved followed by 20 mL of deionized water. Samples are digested in acid under oxidizing conditions using sealed bombs in automated microwave digestors. Each sample is filtered using Whatman 42 filter paper followed by dilution with deionized water to the final volume of 100 mL (Küçükyılmaz et al., 2012b).

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CONCLUSIONS l

Although there is a substantial amount of information on the mineral composition of the chicken egg, there is currently a dearth of literature which investigates the mineral profile of eggs laid by hens reared under alternative systems. l It has been hypothesized, but not proven, that outdoor access may impart benefits in additive mineral intake, thus potentially enhancing the mineral profile of eggs produced under an organic system. l There are a few studies indicating that the mineral content of eggs is affected by an organic rearing system. Contrary to the hypothesis, the edible portion of organic eggs as compared to conventional eggs contained lower levels of phosphorus and zinc. Yolk selenium, however, was higher in organic eggs as compared to conventional eggs. l Organic egg production systems which require outdoor access may affect egg safety through heavy metal contamination, such as cadmium and lead; however, areas without environmental pollution most likely will result in organic and conventional eggs both having undetectable levels of heavy metal residues. l A general conclusion cannot be drawn as to the effect of pastured feeding on egg mineral contents owing to the scarcity of studies on this subject.

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