Analysis of Influencing Factors in Production of Selenium-enriched Eggs

Hui HU, Xu YANG, Yangchao CHEN, Jiangong XIANG

1. Huaihua Polytechnic College, Huaihua 418000, China; 2. Huaihua Second Middle School, Huaihua 418000, China

Abstract Selenium is an essential trace element for human health. Eating selenium-enriched eggs is conducive to easily and effectively solving the problem of selenium deficiency. In the production process of selenium-enriched eggs, different selenium sources, additive doses, feeding time, and the addition of other vitamins, trace elements, and methionine in the diet will influence the deposition of selenium in eggs. Through this review, it is intended to provide a reference for the practical, safe and economical production of selenium-enriched eggs.

Key words Laying hens, Selenium-enriched eggs, Influencing factors, Egg selenium conversion rate

1 Introduction

In the 1970s, related studies showed that selenium is an indispensable trace element for humans and animals. Selenium has magical and unique physiological functions, and is recognized by the world medical community as "the element of longevity", "the seed of life", "the king of tumor suppression",etc.Selenium deficiency may lead to a low immune system, insufficient fertility, premature aging, hypothyroidism, and increased susceptibility to viral infection[1]. Severe deficiency of selenium can lead to cardiomyopathy and myocardial failure, such as: Keshan disease, Kashin-Beck disease, low sperm motility, decreased immunity,etc.In addition, selenium deficiency may also lead to more than 40 kinds of chronic diseases.

China is a country with severe selenium deficiency: 72% of the country are deficient in selenium, and 28% of counties and cities are severely deficient in selenium[2]. Selenium deficiency in soil will cause selenium deficiency in animals and plants, and it is difficult for people living in selenium-deficient areas to obtain enough selenium through local common food. Common ways of selenium supplementation include medicine, selenium nutritional supplements and dietary selenium supplementation. Among them, dietary selenium supplementation is recognized to be the safest way to supplement selenium. At present, selenium-enriched food has become a research hotspot and focus. With advantages of being rich in high-quality protein, vitamins, and minerals, and inexpensive and easy to cook, eggs are one of the best sources of nutrition for humans and the most common animal food on the table.

The selenium in eggs is an organic selenium that can be easily absorbed and utilized by the humans, so eggs have become an ideal functional food carrier to provide organic selenium. Eating eggs to supplement trace elements is practical, inexpensive, effective and convenient. Therefore, developing selenium-enriched eggs is the most convenient and effective way to solve the problem of dietary selenium deficiency in China. In view of this, we analyzed the factors affecting the selenium content of eggs, to provide a reference for the safe, effective and economical production of selenium-enriched eggs.

2 Appropriate selenium content in eggs

China has issued many national or industry standards for agricultural products, such as selenium-enriched tea (NY/T600-2002), selenium-enriched rice (GB/T22499-2008), selenium-enriched potatoes (NY/T3116-2017), selenium-enriched garlic (NY/T3115-2017),etc.However, there is currently no national or industry standard for selenium-enriched poultry meat and eggs, and only a few provinces have formulated relevant local standards. For example, according to the selenium content standard of selenium-enriched agricultural products in Hunan Province, the selenium content of egg products should be greater than or equal to 0.15 mg/kg. The formulation of this standard will greatly promote the healthy and orderly development of the selenium-enriched industry and agricultural products in Hunan Province, produce selenium-enriched agricultural products that meet the standards, regulate the market of selenium-enriched agricultural products, safeguard the rights and interests of consumers, and ensure the standardization of the selenium-enriched industry in Hunan Province. It is of great significance to sustainable development of the selenium-enriched industry.

In accordance with China’sNationalFoodSafetyStandardProceduresforToxicologicalAssessmentofFood, it can be considered that selenium-enriched eggs with a selenium content below 1.2 mg/kg are safe. The upper limit of selenium has been abolished inNationalFoodSafetyStandardMaximumLevelsofPollutantsinFoods(GB2762-2012), and there is no upper limit for selenium content in food in the current standard. The adult consumption of selenium recommended by the Chinese Nutrition Society (CNS) is 60-400 μg daily[3]. If eating two eggs daily, the selenium content should be 0.5 mg/kg. Therefore, 0.15-1.20 mg/kg selenium-enriched eggs are safe, and 0.5 mg/kg selenium-enriched eggs are easier to supplement daily.

3 Composition principle of selenium-enriched eggs

Selenium is an essential micronutrient for the human health. Selenium of selenium-enriched eggs is mainly found in egg yolks. In laying hens, the selenium content in the blood exceeds the human body’s requirement for selenium. Micronutrients are added to diet, drinking water or intramuscularly to transfer selenium into human tissues and organs by active transport. This stabilizes selenium levels in tissues at homeostatic levels. When the glutathione content in cells is normal, selenium will cross the ovarian barrier, leading to an increase in selenium content in the reproductive system and eventually selenium is deposited in eggs and increasing the selenium content in eggs[4].

4 Factors influencing the selenium content in eggs

4.1 Effects of different selenium sources on egg selenium depositionIn livestock and poultry breeding, the commonly used selenium sources are divided into inorganic selenium and organic selenium. Inorganic selenium is often selenate and selenite. Organic selenium can be divided into biotransformed organic selenium (selenium-enriched maize, selenium-enriched alfalfa, selenium-enriched yeast,etc.) and synthetic organic selenium (selenocystine, selenomethionine, Kappa-selenocarrageenan,etc.). The conversion efficiency of different selenium sources in laying hens was significantly different. Extensive studies have demonstrated that the conversion rate of organic selenium to egg selenium is higher than that of inorganic selenium under the same amount of selenium, because inorganic selenium must be combined with organic ligands in the intestine before it can be absorbed by the body. However, there are various factors that cause the gut to compete with selenium for organic ligands, and they are easily combined with vitamins. After selenium is absorbed by the body, it mainly exists in the blood. When the content of inorganic selenium exceeds the nutritional or production needs of the human body, the excess inorganic selenium is mainly excreted through urine[5]. Therefore, the stability, absorption rate and bioavailability of inorganic selenium are low. Organic selenium is formed by combining selenium with amino acids through biotransformation. It usually exists in the form of selenomethionine. After entering the body, it will follow the metabolic pathway of methionine and participate in protein synthesis. Organic selenium can be stored under good nutritional conditions. The results showed that inorganic selenium had certain negative effects on broilers, the proportion of liver, stomach and carcass was low, and the length and diameter of the small intestine were small. Adding selenite to fermented rice bran could reduce the harm of inorganic selenium. Organic selenium and inorganic selenium absorb the same amount of total selenium in the body, but organic selenium is more likely to be deposited in eggs, liver and muscle tissue than inorganic selenium. Therefore, organic selenium can be used as the preferred selenium source for selenium-enriched eggs, but the cost of using organic selenium is higher than that of inorganic selenium.

4.2 Effects of different additive doses on the egg selenium conversion rateThe varieties used for commercial laying hens in China include Hy-line Variety Gray, Hy-line Variety Brown, and Roman laying hens. The selenium-enriching ability of animals during the growth process will vary to some extent. It is very important to select good varieties of laying hens for the production of selenium-enriched eggs, but there are few research reports on this aspect after consulting data. The demand of poultry for selenium is low, and only 0.06 mg/kg of selenium can be added to the feed of laying hens to meet their daily needs[6]. According to theSpecificationsfortheSafeUseofFeedAdditives(AnnouncementNo.2625oftheMinistryofAgricultureofthePeople’sRepublicofChinain2017), the upper limit of selenium in compound feed or total mixed diet is 0.5 mg/kg. Therefore, the amount of selenium in the feed of laying hens in selenium-deficient areas should be 0.06-0.50 mg/kg. Through consulting literature, it is found that except Su Xiaofei[7]who fed laying hens with 0.1 mg/kg selenomethionine and sodium selenite for 56 d, their egg selenium concentrations were 0.125 7 and 1 466 mg/kg, respectively, which did not meet the standard of selenium-enriched eggs, the addition of selenomethionine, sodium selenite and yeast selenium in the grains with a concentration of 0.1-0.5 mg/kg can make the egg selenium concentration above 0.15 mg/kg, which can reach the standard of selenium-enriched eggs. Due to excessive selenium addition, it is easy to cause poisoning, and then lead to disorder of estrogen and progesterone secretion, abnormal ovulation, and even sudden stop of egg production, which affects the egg-laying performance of laying hens[8].

4.3 Effects of different feeding time on selenium content in eggsAfter selenium additives were added to the diet, the selenium content of eggs increased differently with different selenium sources and different additive doses. Usually, selenium-enriched eggs could be produced after 7-14 d of feeding, but the feeding time for stable selenium content in eggs was different in each experiment. Relevant experiments have shown that when the selenium content in the diet remained unchanged, the selenium content of eggs was basically stable. At two weeks of stopping selenium supplementation, the selenium content in eggs started to decline. At four weeks after stopping selenium supplementation, egg selenium levels decreased to the basal diet-fed poultry level. From the perspective of cost, the continuous production of selenium-enriched eggs should stop the supply of selenium-enriched diets for no more than three weeks, so that it is possible to ensure the level of selenium-enriched eggs. Xiao Yingetal.[9]selected local native chickens and young hens, and added an appropriate amount of selenium solution to the daily feed. After continuous feeding, it was found that the selenium content in eggs increased significantly after 60 d, reaching 0.467 mg/kg. With the extension of feeding time, the content of selenium in laying hens also increased, which fully met the industry standard of 0.3-0.5 mg/kg for the production of selenium-enriched eggs, and improved the flavor and taste of chicken and eggs.

4.4 Effects of vitamin supplementation on egg selenium contentSelenium and vitamin E have protective effects on cells, and together they form the body’s defense system against free radical damage. When selenium and vitamin E exert physiological functions, if selenium is sufficient and vitamin E is insufficient, the body is also prone to selenium deficiency symptoms; if selenium is insufficient and vitamin E is sufficient, there will also be excess peroxides and unsaturated lipids to chemically react to damage cells[10]. These indicate that when adding selenium to the feed, a sufficient amount of vitamin E should be ensured, which can increase the selenium content of eggs. According to findings of Tan Fangetal.[11], adding 1.0-2.0 mg of selenium and 83.32-166.64 IU of vitamin E to the diet of 1 kg of laying hens can not only improve the production performance of laying hens, but also greatly increase the deposition of selenium and vitamin E in eggs. Vitamin C and vitamin E have a synergistic effect with selenium, which can enhance the resistance of broilers to acute heat stress. Vitamin B2constitutes glutathione peroxidase (GSH-Px) and participates in many redox reactions in metabolism in the form of coenzyme. Studies have shown that the combined use of selenium and vitamin B2has no synergistic effect on the laying performance of laying hens and the changes in the content of selenium and vitamin B2in eggs[12].

4.5 Effect of trace element addition on egg selenium content

Related reports indicated that adding various trace elements to the diet also influences egg selenium deposition. Yang Lieetal.[13]added different contents of iron, zinc, iodine and selenium to the diet to feed laying hens for 30 d. They found that compared with the control group, the egg yolk selenium content of the two high dose groups increased by 352.54% and 560.10%, respectively. Through experiment using different zinc sources and different amounts of zinc and selenium additions in laying hens, Liu Qinghuaetal.[14]found that the effects of zinc and selenium content in the diet on the deposition of selenium in eggs reached a very significant level, and the enrichment of selenium in eggs showed a downward trend, and the longer the feeding time, the more obvious the rules. Zinc sources also significantly affected selenium deposition in eggs. The content of selenium in eggs in the zinc oxide group was significantly higher than that in the compound group of zinc oxide and zinc sulfate. In addition, selenium can antagonize and reduce the toxicity of mercury, chromium, thallium, arsenic and other elements in the body. Whether the antagonistic or synergistic relationship between trace elements can be used to produce selenium-enriched eggs is worthy of further study. Using the diet containing 0.5 mg/kg iodine and 0.2 mg/kg selenium as the control, and the diet containing 50 mg/kg iodine and 1.2 mg/kg selenium as the experimental group I, and the diet containing 100 mg/kg iodine and 2.2 mg/kg selenium as the experimental group II, Cao Shengfengetal.[15]found that compared with the control group, egg yolk and protein selenium in the experimental groups were significantly increased.

4.6 Effects of different selenium sources on selenium content in eggsZhou Jianjunetal.[16]selected several different selenium sources to compare the production performance, egg production quality and egg selenium content of laying hens, and they found that in terms of egg selenium content, the Kappa-Selenocarrageenan group, the active selenium-enriched yeast group, the mechanically broken selenium-enriched yeast group, and the enzymatically hydrolyzed selenium-enriched yeast selenium group all increased significantly (P<0.05). Among them, the selenium content in the eggs of the enzymatically hydrolyzed selenium-enriched yeast group was the highest, and the selenium content in the eggs of the sodium selenide group and the Kappa-Selenocarrageenan group increased by 145.5% and 47.1%, respectively, and the selenium content increased with the feeding period.

5 Discussion

In China, many areas are selenium deficient. As a functional agricultural product, selenium-enriched eggs are easily accepted by people, thus have great market prospects and obvious economic benefits in R&D and production. According to the survey by the Chinese Academy of Sciences, there are as many as 22 selenium-deficient provinces in China, and 28% of the counties and cities are severely deficient in selenium. Now most of the population in China lives in a severe selenium-deficient environment. World Health Organization (WHO) of the UN pointed out that dietary intake of selenium is the most ideal way of selenium supplementation. At present, studies on the addition of selenium to laying hens’ diets are mainly concentrated the use of different selenium sources, different dosages of selenium and its effects on the physiological functions of laying hens. The method of adding high levels of selenium to obtain selenium-enriched eggs has a greater impact on the production performance of laying hens than high-selenium diets. However, under normal circumstances, long-term addition of high levels of selenium in the diet may lead to selenium poisoning in laying hens, thereby influencing the egg production performance. What’s worse, long-term high selenium supplementation in the diet is also potentially harmful to the environment. Different laying hens, the optimum amount of selenium source addition and feeding time at different ages will influence the conversion rate of selenium in eggs, and the quality of selenium-enriched eggs has little to do with the selected chicken species. It is necessary to balance the relationship between feeding mode, feed quality, chicken growing environment, selenium enrichment method, and between different vitamins, trace elements, and selenium, so as to obtain the best egg selenium conversion rate, to achieve economical, safe and efficient production of selenium-enriched eggs. The continuous deepening of research on selenium-enriched eggs and the provision of high-quality selenium-enriched products are very important to the healthy development of human beings.