Research on Chinese consumers' shell egg consumption preferences and the egg quality of functional eggs

Abstract

The purpose of this study is to investigate the characteristics of egg consumption in China and the production of functional eggs, and finally enrich the types of shell egg products. Trial 1 explored the influence of egg quality on Chinese consumers' willingness to purchase eggs through a questionnaire, which investigated 1,317 consumers' preferences for egg appearance, factors influencing egg purchase, and purchase of functional eggs. The results showed that about 65% of respondents ate more than 4 eggs per wk, pink eggs were the most popular in China, about 65% of consumers preferred eggs with an egg weight of 48 to 58 grams. For functional eggs, 75.32% of consumers have never heard of them. Preferences for eggshell color and yolk color varied by geographic region, with darker colors preferred in Northeast China. Based on the survey results of functional eggs consumption in Trial 1, the dwarf layers of China Agricultural University were used in Trial 2 to produce functional eggs. The eggs are small and pink in color, which is in line with the preferences of Chinese consumers. Three hundred dwarf layers were divided into 4 groups, using the linseed oil added, marigold extract added, and yeast selenium added diets to produce normal, n-3 fatty acid-enriched, lutein-enriched and selenium-enriched eggs by feeding for 28 d, determined the eggs’ nutrient content and egg quality. The results showed that the n-3 fatty acid, lutein and selenium contents of the eggs of dwarf layers were significantly increased by changing the diets and did not affect the egg weight, eggshell strength, Haugh units or the proportion of egg parts. The results of this study are helpful to understand the trend of egg consumption preferences in China, and on this basis to produce functional eggs that meet the consumers’ expectations.

INTRODUCTION

Eggs are an important source of animal protein for humans because they are rich in many essential nutrients and are easy to prepare (Chang et al., 2018). With the growth of global population, the demand for table eggs is also constantly increasing (El-Sabrout et al., 2022). Since 1985, China has been the number one producer of eggs, and in 2020, China produced around 40% of the world's total egg production (Chen et al., 2023), with a vast market for egg consumption. Rapid economic growth, improved supply chains and low prices have led to continued growth in egg consumption, with rural egg consumption growing more rapidly than urban egg consumption (Yang et al., 2018). It is estimated that 70% of eggs worldwide are consumed directly as fresh table eggs and that 30% are processed into egg products, whereas over 95% of eggs in China are sold as table eggs (Yang, 2021). Price, sensory attributes and nutritional value influence consumers' purchase intentions. Preferences for egg weight, shell color and yolk color also vary by country and region, consumers in Malaysia, Ghana, and Spain prefer larger eggs over smaller ones (Rondoni et al., 2020). North and Central America, the Middle East, India and the Philippines prefer white shell eggs, Latin America and Europe prefer brown shell eggs (Preisinger, 2018), and in China, the market share of eggs with different colors of shells is 55% for Chinese brown eggs, 43% for pink eggs, 1% for white eggs and 1% for blue eggs (Yang, 2021).

As people's living standards improve and knowledge of dietary nutrition becomes more widespread, consumers are paying more attention to the relationship between dietary composition and health, and therefore, they are demanding higher-quality eggs. By changing the diet of layers, the content of specific nutrients in eggs can be increased to produce functional eggs, and these nutritionally fortified eggs are called “designer eggs” (Zaheer, 2015). Examples include n-3 fatty acid-enriched eggs, which are better for cardiovascular health and brain development; lutein-enriched eggs, which improve yolk color and retinal development; selenium-enriched eggs, which reduce the incidence of cancer and joint inflammation; iodine-enriched eggs; and vitamin-enriched eggs (Singh et al., 2012). Different breeds of chicken eggs differ in their ability to deposit nutrients(Scheideler et al., 1998). There are 114 unique native chicken breeds with diverse egg production in China (Yang, 2021). When producing functional eggs, it is necessary to comprehensively consider consumers' preferences for the appearance of eggs and their ability to deposit functional nutrients.

Based on the above background, this study aims to explore the impact of egg quality on consumers, willingness to purchase, and to evaluate the potential consumption prospects of functional eggs in China. The research systematically analyzes the preferences of Chinese consumers on the appearance of eggs and the factors that affect their purchasing behavior. After that, choosing a breed of laying hens that meets the preferences of most consumers—the dwarf layers of China Agricultural University, and explore its ability to produce functional eggs.

MATERIALS AND METHODS

Survey on Egg Consumption Habits

A questionnaire was used to assess the egg consumption preferences of Chinese consumers. The questionnaire was designed through Questionnaire Star and published online in November 2022. The questionnaire recorded information about the respondents' age, gender and region of residence, and it obtained information about the respondents’ frequency of egg consumption, their preferred egg weight, shell color, and yolk color, the factors influencing egg purchases, and the respondents’ knowledge and purchase of functional eggs. The options for yolk color and shell color were given based on the corresponding colors on the yolk color fan and the shell color fan. This research was anonymous and did not include personally identifiable information such as names or mailing addresses. In order to ensure the representativeness of the results of the study, the respondents of the questionnaire included all the provinces in China, the number of questionnaires in each province was at least 30, and as many as possible covered all age groups and education levels. After screening and sorting, 1,317 real and valid data points were finally obtained and integrated into SPSS for analysis.

Production of Functional Eggs

Animal Ethics

The animal experiments in this study were carried out in accordance with the Guidelines for Experimental Animals as set forth by the Animal Care and Use Committee of China Agricultural University, with the permit number AW10803202-1-2. Additionally, all animal experiments were conducted in compliance with the ARRIVE (Animal Research: Reporting of In Vivo Experiments) guidelines.

Animals and Diets

The research was carried out on 3 hundred 24-wk-old dwarf layers. Layers were randomly assigned to 4 experimental groups, 75 layers per group, 1 layer per cage. Prior to the start of the experiment, all chickens were fed the control diet for 4 wk. The control diet was laying hen feed 324, purchased from the Charoen Pokphand Group (Charoen Pokphand Group, Beijing, China). The control diet composition and nutrition level were prepared according to NRC (1994) and Chinese Chicken Feeding Standard (2004), and the nutritional components of the diet are shown in Supplementary Table 1. Feed and water were offered for ad libitum consumption. The room temperature was maintained between 22 and 26℃, and light exposure was controlled with a light/dark cycle of 16:8 h. Illumination was provided by incandescent lamps with an intensity of 10 lx (at bird-head level). After the initial 4 wk, refer to the dosage added in previous studies, nutritional additives were supplemented to the diet as follows: CON—control diet, LO—supplemented with 25 g of linseed oil/kg feed (Shahid et al., 2020). ME–supplemented 2 g of marigold extract/kg feed (Grčević et al., 2019), and SY–supplemented 0.2 g of selenium yeast/kg feed (Han et al., 2017). The experimental period lasted for 28 d. All procedures, as well as the care, housing, and handling of the animals, were conducted according to accepted commercial management practices. All birds remained healthy during the feeding period. No birds were culled, and none received any medical intervention. The animal experiment was conducted in the Experimental Unit for Poultry Genetic Resource and Breeding of China Agricultural University.

Determination of Fatty Acid Content

On d 28, 10 eggs were randomly selected from each of the CON and LO groups. The yolks were separated, lyophilized and ground into yolk lyophilized powder, and they were stored at -20°C until fatty acid determination. The content of fatty acids in egg yolk was determined by using gas chromatography (HP 6890 series GC system, Agilent Technologies, Palo Alto, CA) with reference to the national standard GB-5009.168-2016. In short, a certain amount of egg yolk lyophilized powder was weighed, toluene and 10% acetylchloromethanol were added, shaken, mixed, and then heated in a water bath at 80°C. A sodium carbonate solution was added, centrifuged, and passed through a 0.22 μm filter membrane for measurement. A capillary column was used with an injector at 270°C and a detector at 280°C with a gradient temperature increase, the carrier gas was nitrogen, the shunt ratio was 100:1, and the injection volume was 1.0 μL.

Determination of Lutein Content

On d 28, 10 eggs were randomly selected from each of the CON and ME groups, and the yolks were separated and stored at -20°C until lutein determination. A high-performance liquid chromatograph (VP series, Shimadzu, Kyoto, Japan) was used to determine the lutein content of egg yolk with reference to the national standard GB-5009.248-2016. In short, a certain amount of egg yolk was weighed, added to dibutylhydroxytoluene, ethanol and sodium hydroxide solution and then shaken and mixed. It was subsequently saponified at room temperature, protected from light for 30 min, extracted, washed, concentrated, and passed through a 0.45 μm filter membrane for measurement. Chromatographic separation was performed on a C30 column (5 μm, 250 mm × 4.6 mm) at 30°C with the mobile phase methanol : water = 88:12, gradient elution, a flow rate of 1 mL/min, an injection volume of 50 μL, and a detection wavelength of 445 nm.

Determination of Selenium Content

On d 28, 10 eggs were selected from each of the CON and SY groups. They were shelled and homogenized with an electric mixer under frozen conditions and stored at -20°C until selenium determination. An atomic fluorescence spectrometer (AFS 830, Titan., Beijing, China) was used to determine the selenium content of eggs with reference to the national standard GB-5009.93-2017. In short, a certain amount of sample was weighed, digested by adding nitric acid-perchloric acid (9:1), adding hydrochloric acid and potassium ferricyanide solution, and fixed with water to be measured. The conditions of the atomic fluorescence spectrometer were as follows: high voltage of 340 V; lamp current of 100 mA; atomization temperature of 800°C; furnace height of 8 mm; carrier gas flow rate of 500 mL/min; shielding gas flow rate of 1000 mL/min; delay time of 1 s; reading time of 15 s; liquid addition time of 8 s; and sample volume of 2 mL.

Determination of Egg Quality

Upon completion of the experimental period, 30 eggs from each group were used in the analysis to determine indicators of egg quality (weight of eggs and the ratio of their main parts, shell strength, yolk color, height albumen, Haugh units [HUs]). The weights of eggs and their main parts were measured by an electronic scale. Shell strength was measured by an automatic device, that is, Eggshell Force Gauge Model II (Robotmation Co, Ltd., Tokyo, Japan), and the values were expressed in kg/cm². The thickness of the eggshell was measured with an electronic micrometer at the equatorial part, the blunt end and the sharp end of the egg and expressed as the average of the 3 points. Albumen height, HUs, and yolk color were determined by using the Egg Multi-Tester EMT-5200 device (Robotmation Co, Ltd., Tokyo, Japan).

Statistical Analysis

Data analysis was performed using SPSS 25.0. Consumer preferences for various egg indicators by gender and age group and the factors influencing consumer purchase intentions were tested using the Kruskal‒Wallis test for independent samples, with 2-way comparisons at P < 0.05. Egg quality was determined using 1-way ANOVA with significance set at P < 0.05 and Duncan test for post hoc tests. Fatty acids, lutein and selenium were determined using Student t test.

RESULTS

Characteristics of Survey Respondents

Out of 1,317 respondents, 38.50% were male, and 61.50% were female. The age of the respondents was divided into 3 categories, among them, those under 25 yr old accounted for 35.16%, those between 25 and 45 yr old accounted for 36.07%, and those over 45 yr old accounted for 28.78%. Based on the administrative geographic regions of China, the respondents were geographically divided into 7 geographic regions, that is, Northeast China (NE), East China (EC), North China (NC), Southwest China (SW), Northwest China (NW), South China (SC), and Central China (CC) (Supplementary Figure 1), with the largest number of respondents in Northeast China (23.31%) and the smallest number of respondents in Central China (6.07%). Detailed characteristics of the respondents are shown in Table 1.

Table 1.

Characteristics of the survey respondents (n = 1317) in China.

Characteristics	Number	Percentage
Gender
Male	507	38.50
Female	810	61.50
Age (yr)
<25	463	35.16
25–45	475	36.07
> 45	379	28.78
Region
Northeast China	307	23.31
East China	266	20.20
North China	206	15.64
Southwest China	168	12.76
Northwest China	150	11.39
South China	140	10.63
Central China	80	6.07

Consumers' Egg Purchasing Preferences

The results show that the largest proportion of people, 36.07%, ate 4 to 6 eggs per wk, and the smallest proportion, 5.85%, ate less than 1 egg per wk. Regarding the weight of eggs, 65.07% of respondents said they preferred eggs with an egg weight of 48 to 58 g. In terms of the choice of egg purchasing platform, the largest number of people bought eggs from large supermarkets (58.77%), and the smallest number of people bought eggs through e-commerce platforms (41.46%). A total of 58.54% of respondents expressed that they did not pay attention to the trademark code and other information when buying eggs (Table 2).

Table 2.

Consumers' egg purchasing preferences.

Characteristics	Number	Percentage
Frequency with which the respondent eats eggs
Less than 1 egg per wk	77	5.85
1–3 eggs per wk	383	29.08
4–6 eggs per wk	475	36.07
More than 6 egg per wk	382	29.01
Preferred weight of eggs
Weighs less than 48 g	107	8.12
48–58 g	857	65.07
Weighs more than 58 g	170	12.91
No preference	183	13.90
Egg buying platform
Supermarkets	774	58.77
Farmers' Market	465	35.31
E-commerce platforms	78	5.92
Whether the respondent pays attention to the trademark and spray code of eggs
No attention	771	58.54
Attention	546	41.46

Consumers' Egg Purchasing Preferences

The top 5 preferred eggshell colors are 3, 6, 4, 5, and 7. The least preferred eggshell colors are 16, 13, 12, 14, and 15 (Figure 1A). Geography had a significant effect on consumers' preferred eggshell color (P = 0.013), with Northeast China consumer group preferring a darker eggshell color than the southwest consumer group (Figure 1B). Gender had a significant effect on the eggshell color preference (P < 0.001), with men preferring darker eggshells than women (Figure 1C). Age had no significant effect on the eggshell color preference (P > 0.05) (Figure 1D).

Figure 1.

Consumer preferences for the eggshell color. (A) Shell color preference. The X-axis represents the number of the shell color fan, and the Y-axis represents the number of people who like the eggshell color. (B) The influence of the region on the eggshell color preference of consumers, using different colors to represent the average value of the eggshell color preference in a region. The map is based on the standard map downloaded from the standard map service website of the Ministry of Natural Resources with the review number GS (2020) 4630, and the base map has not been modified (the same below). (C) The influence of gender on the eggshell color preference. ns represents no significant difference, and *, **, and *** represent significant at the P < 0.05, P < 0.01, and P < 0.001, levels, respectively. (D) The influence of age on the eggshell color preference. ns represents no significant difference, and *, **, *** represent significant at the P < 0.05, P < 0.01, and P < 0.001 levels. Abbreviations: CC, Central China, EC, East China; NC, North China; NE, Northeast China; NW, Northwest China; SC, South China; SW, Southwest China.

The top 5 yolk colors preferred by consumers were 5, 6, 16, 8, and 7. The least preferred eggshell colors were 2, 1, 3, 13, and 14 (Figure 2A). Geography had a significant effect on consumers' preferred yolk color (P < 0.001), with Northeast China consumer group preferring a darker eggshell color than the southwest consumer group (Figure 2B). There was no significant effect of gender on the yolk color preference (P > 0.05) (Figure 2C). Age had a significant effect on the yolk color preference (P < 0.001), with consumers over 45 yr of age preferring the darkest yolk color, followed by consumers between 25 and 45 yr of age, and consumers below 25 yr of age preferring the lightest yolk color (Figure 2D).

Figure 2.

Consumer preferences for the yolk color. (A) Yolk color preference. The X-axis indicates the sequence number of yolk color comparison cards, and the Y-axis indicates the number of people who prefer a yolk color. (B) The X-axis represents the number of the yolk color fan, and the Y-axis represents the number of people who like the yolk color. (C) Effect of gender on the yolk color preference. ns indicates no significant difference, and *, **, and *** indicate significant at the P < 0.05, P < 0.01, and P < 0.001 levels, respectively. (D) Effect of age on yolk color preference. ns indicates no significant difference, and *, **, and *** represent significant at the P < 0.05, P < 0.01, and P < 0.001 levels. Abbreviations: CC, Central China, EC, East China; NC, North China; NE, Northeast China; NW, Northwest China; SC, South China; SW, Southwest China.

The Importance That Consumers Attach to Various Indicators of Eggs

Consumers ranked the importance of these indicators in descending order as follows: quality and safety, nutritional value, appearance, price, and brand (Figure 3A). The trend was the same across age groups (Figure 3B) and genders (Figure 3C). The concern for brand, quality and safety, and nutritional value differed significantly among different age groups (P < 0.05). The concern for brand and nutritional value among different age groups was as follows: over 45 yr old > 24 to 45 yr old > below 25 yr old; respondents above 45 yr old were more concerned about quality and safety than those below 25 yr old (Figure 3B). Women were more concerned about quality and safety, nutritional value and appearance than men (Figure 3C).

Figure 3.

Consumer concern for each indicator of eggs. P-price; A-appearance; B-brand; N-nutritional value; and Q-quality and safety. Figure 3A is a comparison between A, P, B, Q, and N, and Figure 3B and Figure 3C are comparisons within each group. (A) Comparison between different indicators. ^a-e The means between indicators with no common superscript differ significantly (P < 0.05). (B) ^a-c The means among age groups with no common superscript differ significantly (P < 0.05). (C) ^a,b The means between gender groups with no common superscript differ significantly (P < 0.05).

Consumers' Knowledge and Purchase of Functional Eggs

The survey on the knowledge of functional eggs showed that 47.08% of the respondents said that they had never heard of functional eggs. Regarding the price of functional eggs, the highest percentage of people, 41.15%, could accept that each functional egg costs ￥0 to 0.5 more than a regular egg, followed by $ 0.6 to 1.0, 34.17%. For only 10.94% of the respondents did functional eggs account for more than 20% of their egg consumption (Table 3).

Table 3.

Consumers' knowledge and purchase of functional eggs.

Characteristics	Number	Percentage
Ever heard of functional eggs
Heard	697	52.92
Never heard	620	47.08
Can accept a higher price per functional eggs than regular eggs
￥< 0.5	542	41.15
￥0.6-1.0	450	34.17
￥1.1–1.5	179	13.59
￥1.5–2.0	86	6.53
￥2.1–2.5	23	1.75
￥2.6–3.0	33	2.51
￥> 3.0	4	0.30
Percentage of functional eggs in egg consumption
Never purchased	992	75.32
Less than 20%	261	19.82
20–50%	104	7.90
50–80%	27	2.05
More than 80%	13	0.99

Determination of Fatty Acid Content

The results show that the addition of linseed oil to feed significantly increased the n-3 fatty acid content of eggs (P = 0.004), and improved the n-6:n-3 ratio (P = 0.002), but had no significant effect on n-6 fatty acid content (P = 0.705) (Table 4). The detailed composition of egg fatty acids is shown in Supplementary Table 2.

Table 4.

Effect of linseed oil on n-3 and n-6 fatty acids (mg/egg).

	CON	LO	P
n-3	38.1 ± 1.6	240.5 ± 14.3⁎	0.004
n-6	760.4 ± 37.3	764.1 ± 34.1	0.705
n-6:n-3	20.0 ± 0.6	3.2 ± 0.1⁎	0.002

The results are expressed as the mean ± SEM.

^⁎indicate significant at the 0.01 level.

Determination of Lutein Content

The results show that the addition of marigold extract to feed significantly increased the lutein content of eggs (P = 0.004) (Table 5).

Table 5.

Effect of marigold extract on the lutein content of eggs (μg/egg).

	CON	ME	P
Lutein	181.4 ± 8.2	268.8 ± 14.0⁎	0.004

The results are expressed as the mean ± SEM.

^⁎indicate significant at the 0.01 level.

Determination of Selenium Content

The results show that the addition of yeast selenium to feed significantly increased the selenium content of eggs (P = 0.016) (Table 6.).

Table 6.

Effect of selenium yeast on the selenium content of eggs (μg/egg).

	CON	SY	P
Selenium	4.0 ± 0.1	8.9 ± 0.3⁎	0.016

The results are expressed as the mean ± SEM.

^⁎indicate significant at the 0.05 level.

Egg Quality Determination

The results show that the addition of 2.5% linseed oil, 0.2% marigold extract and 0.02% yeast selenium to layer diets did not affect eggshell strength, egg white height, Haugh units, egg weight, the yolk ratio, the shell ratio or the egg white ratio. The addition of 0.2% marigold extracts significantly improved yolk color (P < 0.001) (Table 7).

Table 7.

Effect of different dietary treatments on egg quality.

	CON	LO	ME	SY	P
Shell strength (kg/cm2)	3.22 ± 0.14	3.06 ± 0.14	3.16 ± 0.14	3.18 ± 0.10	0.849
Albumen height (mm)	7.11 ± 0.15	7.5 ± 0.16	7.34 ± 0.18	7.57 ± 0.19	0.255
Yolk color	8.27 ± 0.08bc	8.19 ± 0.12a	9.26 ± 0.12b	8.57 ± 0.12c	< 0.001
Hus	87.97 ± 0.85	89.59 ± 0.82	88.28 ± 1.01	89.18 ± 1.07	0.585
Egg weight (g)	49.40 ± 0.82	49.15 ± 0.76	50.52 ± 0.63	51.71 ± 0.87	0.086
Yolk ratio (%)	26.68 ± 0.34	26.66 ± 0.42	27.19 ± 0.31	27.35 ± 0.30	0.381
Shell ratio (%)	9.86 ± 0.16	9.84 ± 0.19	9.91 ± 0.12	10.12 ± 0.17	0.597
Albumen ratio (%)	63.46 ± 0.39	63.51 ± 0.43	62.91 ± 0.32	62.52 ± 0.38	0.212

The results are expressed as the mean ± SEM.

^a–cMeans within a row with no common superscript differ significantly (P < 0.05).

DISCUSSION

The consumption habits and ability to consume eggs vary from country to country. This study shows that most consumers in China buy eggs in supermarkets (58.77%), and 89.5% of consumers in the United States buy eggs in grocery stores (Kosa et al., 2015). Previous studies have shown that people in most countries consume 2 to 4 eggs per wk, with people in the US consuming 248 eggs per person per yr, higher than 4 per wk, whereas people in Brazil, Chile, the Netherlands and the UK all consume less than 4 per wk (Lemos et al., 2018). China is a major producer and consumer of eggs (Yang et al., 2018). With a population of approximately 21% of the world's population, China produces and consumes approximately 40% of the world's eggs, and the country's per capita egg consumption is approximately twice the world average (Li and Li, 2018). After the African swine fever epidemic, China's hog industry was hit, whereas the egg industry has seen a period of opportunity for development, with per capita egg possession reaching 20.09 kg in 2019 and a large increase in demand in the egg consumption market (Zhu and Qin, 2020). The survey of this study showed that 65.08% of respondents eating more than 4 eggs per wk, confirming the enormous egg consumption market in China. At present, China's egg industry chain is intelligent, digitalization is accelerating, new retail and new consumer subjects are rising (Sun et al., 2023), and the competitiveness of consumption is at a high level. However, the competitiveness of the egg industry is lower than that of countries such as Spain, Japan, and the United States and is overall at a moderate to low level (Ma and Wang, 2021).

There are regional differences in consumer preferences for egg weight, which can influence the choice of egg strain. In this study, 65.07% of the respondents preferred eggs with an egg weight of 48 to 58 g, whereas Arthur and O'Sullivan (2005) showed that the average weight of eggs worldwide is 60 to 61 g. The market preference in South Africa is approximately 58.5 g. Consumers in Malaysia, Ghana, and Spain prefer larger eggs over smaller ones(Rondoni et al., 2020).Compared to other countries around the world, Chinese consumers prefer smaller eggs, which may be related to China's egg consumption tradition. On the one hand, consumers usually prefer eggs produced on family farms to those produced on industrial poultry farms (Berkhoff et al., 2020). Family farms usually raise indigenous double-purpose chicken breeds with egg weights less than 60 g. This may have led to a consumer preference for smaller eggs. On the other hand, smaller eggs are thought to have a higher yolk percentage and better flavor (Yang, 2021). Understanding regional differences in consumer preferences for egg size and the factors that influence these preferences can help inform the choice of egg strain and ultimately improve egg production efficiency.

Eggshell color is an important characteristic that affects consumer preferences (Lu et al., 2021). Although it does not affect the nutritional content or health benefits of eggs, people's preference is influenced by culture and tradition. Some consumers believe that brown eggs are more natural and healthier (Scott and Silversides, 2000). Johnston et al. (2011) reported that producers of brown eggs are better at using packaging and promotional tools and therefore sell at higher prices. According to this study, the top 3 eggshell colors preferred by Chinese consumers are eggs with numbers 3 to 6 on the shell color fan, which are pink eggs. These results are in line with the findings of Preisinger (2018), who showed that North and Central America, the Middle East, India and the Philippines prefer white eggs, Latin America and Europe prefer brown eggs, and Japan and China prefer pink eggs.

Yolk color is an important influencing factor for consumers to purchase eggs, and it is related to the pigmentation in the yolk. In this study, more than 7.5% of consumers preferred yolk color fan numbers 5, 6, 7, 8, and 16. Consumers in Ghana prefer the yolk color to be dark yellow than light yellow, which they believe implies better nutrition and health status (Ayim-Akonor and Akonor, 2014).

Color affects a person's psychological perception and, in turn, his or her mood. Warm colors, such as red evoke positive emotions such as liveliness and warmth. However, cooler colors such as blue evoke negative emotions such as anxiety and coldness (Elliot and Maier, 2014). The yellow and orange colors of the egg yolk belong to warm colors, and the color of the egg shell usually belongs to warm colors as well. This study showed that geography had a significant effect on both the shell and yolk color preferences (P < 0.05), with darker shells and yolks being more popular in Northeast China, whereas lighter shell and egg white colors were more popular in Southwest China. This result may be related to temperature differences due to differences in latitude, Compared to Southwest China, Northeast China has higher latitude and lower temperature, so it needs more positive emotions brought by warm colors. Among warm colors, orange has a stronger effect than yellow, which may be the reason why the egg yolk color favored by Northeast China people is darker.

The results of this study show that the factors influencing Chinese consumers' egg purchases are, in descending order, quality and safety, nutritional value, appearance, price, and brand. However, the study by Berkhoff et al. (2020) shows that the most important factor consumers are concerned about is price. Rondoni et al. (2020) indicated that sensory attributes and nutritional value are the most critical factors in determining consumer egg purchases. The most important factors influencing the purchase of animal products in France are the date of production, taste, and origin (Walley et al., 2015). A study in Ghana showed that the factors influencing the purchase of eggs were, in descending order, size, price, neatness, and color. Brazilian and Chilean consumers were more concerned about health, animal welfare, and the farm environment (Lemos et al., 2018). The factors influencing consumer purchases are related to the country and the level of economic development.

In this study, 47.08% of the respondents had never heard of functional eggs, 75.32% had never bought functional eggs, and only 3.04% of the respondents was their egg consumption more than 50% functional eggs. These results indicate that there is still much room for the promotion and consumption potential of functional eggs in China. Compared to traditional eggs, functional eggs have more nutritional value and are more expensive. However, most consumers are not willing to pay such high prices (Chang et al., 2010), which is an important factor limiting the promotion of functional eggs. Therefore, the development of cheaper functional additives to reduce the production cost of functional eggs is one of the directions for the egg industry.

The survey in this study showed that eggs with an egg weight of 48 to 58 g were most popular with Chinese consumers and that most consumers preferred eggs with a shell color score of 3 to 6, which are pink eggs. Based on such research results, The dwarf layers of China Agricultural University were selected for this study, with egg weights of 49.2 to 51.7 g and laying pink eggs, which are in line with the preference of Chinese consumers. Compared with normal layers, dwarf layers are smaller in size and consume less metabolism to maintain themselves. Thus, they have the advantage of less feed consumption and a higher feed conversion ratio (Yang, 2021), and their egg weight is also smaller than that of normal layers. The results showed that the production of n-3 fatty acid-enriched, lutein-enriched, and selenium-enriched eggs using the dwarf layers of China Agricultural University did not affect egg weight, shell strength, Haugh units, or the proportion of egg parts. These results suggests that the dwarf layers of China Agricultural University can be utilized to produce functional eggs that meet the preferences of Chinese consumers.

The composition and content of dietary fatty acids affect the fatty acid composition of the yolk. Therefore, the diet of layers can be modified to obtain functional eggs rich in n-3 fatty acids (Singh et al., 2012). At present, the commonly used dietary supplements for n-3 fatty acids are fish oil, linseed, or microalgae (Fraeye et al., 2012). However, there have been some negative reports that the use of fish oil for egg production can lead to the development of a fishy taste (Gonzalez-Esquerra and Leeson, 2000), and the high fiber content of linseed and its antinutritional components, such as cyanogenic glycosides, phytic acid and mucilage, can also negatively affect nutrient absorption (Bean and Leeson, 2003). The use of linseed oil instead of linseed is effective in reducing the fiber content and facilitating the absorption of eggs (Shahid et al., 2020). The results of this study showed that the addition of 2.5% linseed oil was effective in enriching eggs with n-3 fatty acids without affecting egg quality. The addition of 10% linseed to feed has been reported to cause a reduction in egg Haugh units, but the addition of linseed oil does not have this effect (Shahid et al., 2020). Lee et al. (2021) also showed that the use of linseed oil in egg production did not affect egg quality.

Marigold extract is rich in lutein and has been marketed as a natural source of pigment for poultry (Grčević et al., 2019). The lutein-enriched eggs produced in this way are one of the main functional eggs in China. The results of this study showed that feed supplementation with 0.2% marigold extract significantly increased the lutein content of the eggs of the dwarf layers (P = 0.004) and increased the yolk color by 1 yolk color fan, without affecting other egg quality indicators. The trial by Skřivan et al. (2015) showed that the addition of lutein supplementation did not affect eggshell strength or thickness. Grčević et al. (2019) showed that the addition of lutein supplementation did not change the shell strength, egg white height or Haugh units of fresh eggs. Lokaewmanee et al. (2011) reported no significant effect of lutein supplementation on yolk, egg white, shell ratio or HUs. For egg weight, egg weight was lower in the lutein supplemented group in the trial of Skřivan et al. (2015) and higher in the lutein-supplemented group in the trial of Grčević et al. (2019), whereas the effect of lutein supplementation on egg weight was not significant in the present study, which is the same as the results of Jang et al. (2014).

The addition of selenium to feed can improve the health of poultry and produce selenium-enriched eggs. The selenium additives used include both inorganic selenium (sodium selenite) and organic selenium (yeast selenium and selenium substitute protein). The results of this study showed that the addition of 0.2% yeast selenium to feed effectively increased the selenium content of eggs (P = 0.016) but did not affect other egg quality indicators, which is consistent with previous studies (Han et al., 2017).

CONCLUSIONS

China has a huge egg consumption market, with 65.08% of respondents eating more than 4 eggs per wk. Most consumers prefer eggs with an egg weight of 48 to 58 g. Pink eggs are the most popular in China. The factors that influence consumers' purchase are, in descending order, quality and safety, nutritional value, appearance, price, and brand. A total of 75.32% of consumers have never purchased functional eggs, indicating that there is still much room for the promotion of these eggs in China. The dwarf layers of China Agricultural University meet the preferences of Chinese consumers, and the production of n-3 fatty acid-enriched, lutein-enriched and selenium-enriched eggs with them increases the nutritional content without negative impact on egg quality, so they are a good breed for the production of functional eggs.