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. 2024 Sep 19;103(12):104332. doi: 10.1016/j.psj.2024.104332

Towards the perfect soft-boiled chicken eggs: defining cooking conditions, quality criteria, and safety assessments

Yuanhang Shi *,1, Sirui Chen †,1, Yu Liu *, Jiayi Liu *, Lin Xuan *, Guangqi Li *, Junying Li *, Jiangxia Zheng †,2
PMCID: PMC11525136  PMID: 39413700

Abstract

In recent years, ready-to-eat soft-boiled chicken eggs, with coagulated whites and semi-solid yolks have become popular among Chinese consumers due to their convenience, tender texture, and nutritional benefits. However, the standards for these products are currently inconsistent, and quality evaluation parameters and food safety issues remain unclear. Softness ratio, representing the semi-liquid yolk proportion in a cross-section, was defined through a survey of different brands of ready-to-eat soft-boiled chicken eggs. By accounting for egg weight and cooking time, optimal softness under specific conditions was determined. A quality evaluation method was established based on the softness ratio and shelling score. Finally, the safety of soft-boiled chicken eggs was assessed by measuring yolk center temperature. The optimal softness ratio was 0.46 to 0.64. Optimal cooking times in boiling water (100°C) with a 5:1 water-to-egg ratio were 300 s for 43 to 48 g eggs, 330 seconds for 48 to 53g, 350 s for 53 to 58 g, and 370 s for 58 to 63g. After cooking, eggs were cooled for 6 min in an ice water mixture with a 3:1 water-to-egg ratio. Shelling scores (0–5) depended on the egg white surface exposed post-shelling, peaking at an air cell diameter of 21.55 ± 2.26 mm. Egg weight and shelling score had a correlation of −0.41, while egg white springiness and shelling score had a correlation of 0.86. Ensuring core yolk pasteurization standards was difficult, thus stricter management or whole egg irradiation was suggested for safety.

Key words: soft-boiled egg, quality evaluation, softness ratio, shelling score, food safety

INTRODUCTION

Eggs are a high-quality source of animal protein with a balanced amino acids composition, high digestibility (Réhault-Godbert et al., 2019; Goto et al., 2021; Nolasco et al., 2021), and a variety of processing methods (Zheng et al., 2023; Jiang et al., 2024). In the past 30 y, global egg production has increased by 1.5 times, reaching 86,999,551 tons of fresh shell eggs worldwide. China has become the largest egg producer, accounting for 34% of the world's total production (FAO, 2022). In order to meet the diverse needs of consumers and solve the problem of short shelf life of fresh eggs (Barbut et al., 1987), the types of egg products have been steadily increasing. Products such as dried eggs, liquid eggs, frozen eggs, and salted eggs have gained wide market acceptance (Kelly et al., 1962; Kraft et al., 1967; Lechevalier et al., 2013; Ma et al., 2023). The most common egg products on the market are shell-free, which reduces transportation cost and complexity. In addition, egg products have undergone various processing and seasoning to enhance the taste and convenience of consumption (Zheng et al., 2023), but significantly weaken the original flavor and texture of eggs.

In China, soft-boiled eggs are a traditional snack, often served as part of breakfast or as a side dish to noodles (Ding et al., 2021). The production process is short-time high-temperature cooking and low temperature rapid cooling. Onsen tamago (Fong et al., 2022), a Japanese hot spring egg, is similar in appearance to soft-boiled eggs and is a popular dish in Japanese cuisine. However, its cooking method is different as it is slowly heated in water at approximately 70°C. This process takes longer than soft-boiled eggs and requires control of water temperature, which makes the technique more complex. Vacuum-packed ready-to-eat soft-boiled chicken eggs appeared on the Chinese market in 2019, and since then, hundreds of poultry egg product manufacturers and snack producers have been selling this product. At present, the ready-to-eat soft-boiled chicken eggs of brands on the market can be divided into shell-free and shell-on according to the production process. Shell-free soft-boiled chicken eggs are generally shelled marinated egg with flavors such as salty baked and abalone sauce. Shell-on soft-boiled chicken eggs retain the original flavor after processing from. The yolk of ready-to-eat soft-boiled chicken eggs is orange and tender, which solves the shortcomings of harder texture and dry yolk of hard-boiled eggs (Liu et al., 2023). The taste and flavor are very similar to traditional homemade soft-boiled chicken eggs, and are very popular among consumers.

According to data from the online shopping platform Taobao (China) (http://www.taobao.com), there are currently 13 brands with more than 1,000 transactions per month for ready-to-eat soft-boiled chicken eggs. The prices of different brands range from 1.5 yuan to 6 yuan per egg, with an average price of 3 yuan. Some brands have accumulated online sales of up to 500,000 eggs per month. In 2022, the “White Paper on the Nutrition and Safety of Soft-Boiled Eggs (2022)” was officially released, which outlined and popularized knowledge about egg nutrition, the nutritional value of soft-boiled chicken eggs, and the healthy and safe consumption of soft-boiled chicken eggs. However, research on the quality evaluation and safety assessment of soft-boiled chicken eggs is still insufficient. The softness ratio of soft-boiled chicken eggs varies greatly, the liquid-to-solid ratio of the yolk is unstable, and egg white adherence is prone to occur during shelling. There is a lack of scientific and unified quality evaluation methods. In addition, the safety of soft-boiled chicken eggs is often questioned by consumers, which is also a major factor restricting the further growth of its sales. Consumers are concerned about whether soft-boiled chicken eggs meet pasteurization standards and whether there is a potential risk of infection with Salmonella enteritidis. In response to the above issues, this study focuses on commercially available ready-to-eat soft-boiled chicken eggs and those made from White Leghorn eggs. It aims to explored the parameters and influencing factors of soft-boiled chicken egg quality evaluation, established a quality evaluation method, and evaluated the safety of soft-boiled chicken eggs.

MATERIALS AND METHODS

Ethics Issues

According to the ethical issues for the protection of animals, this project does not require the consent of the competent ethics committee for animal experiments.

Determination of Soft-Boiled Chicken Egg Quality Evaluation Parameters and Their Influencing Factors

Softness Ratio and Optimal Softness Ratio

Branded ready-to-eat soft-boiled chicken eggs (shell-on) were purchased from a Chinese online shopping platform (http://www.taobao.com), 30 eggs each of brands A, B, and C. The total market share of transaction volume for the 3 brands is 19.67%. The market share and origin of each brand are shown in Figure 1.

Figure 1.

Figure 1

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Regional distribution map of soft-boiled chicken egg brands. Pie charts A, B, and C represent the online sales market shares of brands A, B, and C, respectively, based on the monthly transaction volume from the Taobao online platform. Market share=Monthly transaction volume of the brand / Total transaction volume of the thirteen brands. The production origins of brands A, B, and C are Guangdong Province, Sichuan Province, and Jiangsu Province, respectively. SO is shell-on branded soft-boiled eggs. SF is shell-free branded soft-boiled eggs.

The branded ready-to-eat soft-boiled chicken eggs were shelled and then cut vertically through the center of the yolk. Position the cut surfaces upwards and took photographs. Using the free selection tool in ImageJ (ImageJ Fiji software version 1.53t, NIH) select the soft center part and the entire yolk part in the photo. Click measure to obtain the areas, and calculate the ratio of these areas to determine the softness ratio.

After measuring the softness ratio of all 90 ready-to-eat soft-boiled chicken eggs, the mean ± SEM represents the optimal range for the softness ratio.

Best Cooking Conditions for Soft-Boiled Chicken Eggs of Different Weights

A total of 150 forty-seven-wk-old slow-feathering White Leghorn eggs from the Genetic Resources and Breeding Experimental Base of China Agricultural University were stored at 20 ± 3°C for no more than 24 h from collection until use.

Thirty eggs were used in the preliminary experiment. According to the National Trade Industry Standard SB/T 10638-2011 of the People's Republic of China, 120 eggs were divided into 4 groups of 43 to 48 g, 48 to 53 g, 53 to 58 g, and 58 to 63 g by weight, with 30 eggs in each group. The dietary ingredients for laying hens references Shi et al (Shi et al., 2023).

The ratio of water to egg weight during cooking is 5:1, and the ratio of water to egg weight for cooling is 3:1. Position the eggs vertically in a plastic egg rack and fully immerse them in boiling water at 100°C (Ding et al., 2021). After a brief heating period, immediately transfer the eggs to an ice-water mixture for a 6-min cooling duration.

The temperature of the yolk and egg white interface when preparing soft-boiled eggs was determined to be 70 to 73°C, in the preliminary experiment of 30 eggs. Substitute this temperature as Tyolk and Tegg of 20 ± 3°C into Formula (1) to calculate the cooking time for eggs of different weights (Table 1). Subsequently, verification experiments were conducted using eggs of various weights to determine the optimal cooking time for preparing soft-boiled eggs. The final determined cooking time required for eggs of different weights to achieve the desire softness is shown in Table 3.

t=0.415M2/3ln[0.76×Tegg−TwaterTyolk−Twater] (1)

(Barham, 2001)

Table 1.

Cooking time for eggs of different weights calculated by the formula.

Egg weight / g
43 48 53 58 63 68
Max cooking time / s 259 279 294 316 334 352
Min cooking time / s 204 220 235 249 263 277
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Table 3.

Optimal cooking time for soft-boiled eggs of different weights.

Egg weight / g
43-48 48-53 53-58 58-63
Cooking time / s 300 330 350 370
Softness ratio 0.55 ± 0.020 0.53 ± 0.016 0.53 ± 0.001 0.55 ± 0.013
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Data are expressed as mean ± SEM.

Tegg = Initial temperature of the egg (°C); Twater = Water temperature (°C); Tyolk = Expected temperature at the interface between egg white and yolk (°C); t = Cooking time for the egg (min); M = Egg weight (g).

Shelling Score

Forty-seven-wk-old slow-feathering White Leghorn eggs were obtained from the Beijing Huadu Yukou Poultry Industry Co., Ltd, Beijing, with a total of 900 eggs. Eggs were stored at 20 ± 3°C and divided into 8 groups according to storage time: 1, 3, 5, 7, 11, 14, 18, and 21 d.

Soft-boiled chicken eggs were prepared according to the optimal cooking conditions to achieve the desired softness. Manual shelling was performed in a standardized manner, in which the eggshell was cracked at the blunt end and then gently tapped along its length from the blunt end to the pointed end to produce uniform cracks. Shelling was started from the air cell end to minimize the influence of shelling technique on the shelling results. The shelling score standard was established by referring the evaluation method for eggshell surface contamination in USDA egg quality standards (USDA, 2024).

The eggshell thickness was measured using an ultrasonic thickness gauge from Dakota Ultrasonics, USA. After zero, apply a coupling agent to the probe and the eggshell surface, and set the sound velocity to 3456 m/s. Press the probe against the surface of eggshell. Measured the eggshell thickness at 3 points around the equator, and took the average of the 3 points as the eggshell thickness for the egg.

The diameter of the air cell was measured with a digital caliper from Mitutoyo, Japan. Using an egg candler to illuminated the air cell and used a pencil to mark the outline of the air cell. Used a digital caliper to measure the length of the major and minor axes and took the average of these two values as the air cell diameter. Eggs were weighted with electronic scale from Yoke Instrument Co., Ltd., China.

The texture profile analysis (TPA) of the egg whites was evaluated using the TA-XTplus texture analyzer from Stable Micro Systems, UK. For the shear force, a cylinder egg white sample with a diameter of 3 cm was taken from the equatorial region of the egg and placed in moist petri dishes to maintain humidity. The cylinders were sheared using the wire blade. The test speed was 0.50 mm/s and deformation were 90%. For the springiness and hardness, a cylinder egg white sample with a diameter of 1cm was taken from the equatorial region of the egg and placed in moist petri dishes to maintain humidity. Measured using a cylindrical probe with a diameter of 50 mm. The test speed was 0.50 mm/s, the compression distance was 2.00 mm, and holding time after penetration was 30.00 s.

The yolk centering ratio was measured using the method proposed by Grunden et al. (Grunden et al., 1975), which was improved by taking cross-sections images of soft-boiled eggs. Using ImageJ (ImageJ Fiji software version 1.53t, NIH) the scale was set along the linear axis of the horizontal diameter of the yolk. The distance from the outer edge of the yolk to the outer edge of the egg white was measured, and the specific measurement positions denoted as R1, R2, R3, and R4 in Figure 2. The yolk centering rate from left to right is defined as the side/side ratio, while the yolk centering rate from top to bottom is defined as the top/bottom ratio.

Side/sideratio=sideofshorterdistance/sideoflongerdistance
Bottom/topratio=bottom(bluntend)/top(pointedend)
Figure 2.

Figure 2

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Diagram of yolk centering ratio. For this egg, side/side ratio = R3 / R4, top/bottom ratio = R2 / R1.

Safety Evaluation

A total of 240 forty-seven-wk-old slow-feathering White Leghorn eggs were obtained from the Genetic Resources and Breeding Experimental Base of China Agricultural University. One hundred and twenty eggs were randomly divided into groups of 1, 2, 3 and 4, and the remaining 120 eggs were categorized into 5, 6, 7, 8groups of 48 to 53 g, 53 to 58 g, 58 to 63 g, and 63 to 68 g by weight, with 30 eggs in each group.

The electronic food thermometer from PEOPLE, China, was used to measure the yolk center temperature. To determine the yolk center temperature during early stages of cooking, eggs in groups 1, 2, 3, and 4 were cooked for 120, 210, 240, and 270s, respectively. The eggs in remaining 4 groups were cooked according to the optimal cooking times, and the center temperature of the yolk was measured. The soft-boiled eggs were removed from the cooking process, cutting in half vertically at the equator, and the center temperature of the yolk was immediately measured. To meet the pasteurization standards for whole eggs, the center temperature is maintained at 57°C for 57.5 minutes in the United States and at 63.3°C for 2.5 min in China (Keener, 2017).

Statistical Analysis

Data analysis was conducted using SPSS 26.0. Research data on soft-boiled chicken eggs were processed using one-way analysis of variance (ANOVA), which can be represented in the following model:

Yij=μ+τi+ϵij

Yij is the observation from the j-th subject in the i-th treatment group. μ is the overall mean. τi represents the effect of the i-th treatment. ϵij is the error term.

Factors influencing shelling scores were analyzed using non-parametric tests (Kruskal-Wallis) and post-hoc comparisons. OriginPro 2022 was employed for graphical representation, fitting logarithmic models, and computing parameters. Pearson correlation analysis was performed on shelling scores, egg weight, days of storage, etc., and correlation plots were generated.

RESULTS AND DISCUSSION

Determination of Soft-Boiled Chicken Egg Quality Evaluation Parameters and Their Influencing Factors

Softness Ratio and Optimal Cooking Condition

Through market research, the top 3 brands of shell-on soft-boiled chicken eggs, based on cumulative sales rankings on Taobao (http://www.taobao.com), were selected. The brand ranked third had sales exceeding 50,000 soft-boiled chicken eggs per month, while the top 2 brands had sales exceeding 450,000 per month. The weight of eggs from the 3 brands was significant different (p < 0.001) (Table 2). The range of the most common softness ratio is represented by the mean ± SD of the 3 brands of soft-boiled chicken eggs, so the most common softness of brand soft-boiled chicken eggs was between 0.46 and 0.64. The illustrative diagrams of soft-boiled chicken eggs with different softness ratios were shown in Figure 3.

Table 2.

Determination of optimal softness ratio and peeling score based on brand soft-boiled eggs.

Egg weight / g Softness ratio Shelling score
Brand A 62.87 ± 0.40a 0.53 ± 0.013b 4.24 ± 0.21a
Brand B 57.18 ± 0.28c 0.62 ± 0.011a 3.10 ± 0.25b
Brand C 60.80 ± 0.21b 0.49 ± 0.014b 3.93 ± 0.19a
p-value 1.24 × 10−18 2.20 × 10−9 0.005
Total 60.28 ± 0.29 0.55 ± 0.010 3.76 ± 0.14
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Data are expressed as mean ± SEM.

Figure 3.

Figure 3

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Schematic diagram of soft-boiled egg cross-section and softness ratio. The upper portion represents the cross-section of a soft-boiled egg, while the lower portion corresponds to the respective softness ratio.

The preparation of soft-boiled chicken eggs was carried out according to the cooking times corresponding to the egg weights in Table 1, ensuring that the softness ratio was within the most common range for soft-boiled chicken eggs (0.46-0.64). After experiments, the optimal cooking time and softness ratio for different egg weights were determined, as shown in Table 3.

The difference between soft-boiled chicken eggs and hard-boiled eggs and raw eggs is the semi-solid yolk. Inconsistent softness ratio of ready-to-eat eggs will affect product quality. If the softness ratio is too low, the texture of the soft-boiled egg is similar to that of the hard-boiled egg, thus losing its uniqueness. On the contrary, if the softness ratio is too high, the proportion of liquid yolk is too high, may leading to food safety issues (Whiley et al., 2017). Therefore, the stability of the soft-boiled egg's semi-liquid yolk consistency is beneficial for its quality evaluation. When preparing soft-boiled chicken eggs, we should cook them according to the optimal cooking times for eggs of different weights to control the softness within a certain range. After cooking, the eggs should be immediately immersed in ice water to cool down to prevent the yolk from continuing to absorb heat and causing the softness to decrease (Sahachairungrueng et al., 2023). During the cooling process, add ice to the ice-water mixture as needed to keep the water temperature below 4°C.

Shelling Score

The shelling score is defined based on the proportion of egg white adhering to the shell (USDA, 2024), with 5 to 0 indicating increasingly severe adhesion. The scoring criteria are shown in Table 4.

Table 4.

Shelling score criteria.

Egg white adherence ratio
 Schematic diagram of egg white surface adherence ratio
Score Single point(SP) Cumulative(C)
0 SP >14 C >12 Image, table 4
1 18 < SP <14 14 < C < 12
2 116 < SP < 18< 18 < C < 14
3 132 < SP < 116 116 < C < 18
4 SP <132 C < 116
5 ≈0 ≈0
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When the scores were 0, 1, 2, or 3, the egg white surface exhibited significant shell adhesion. At a score of 4, there was slight shell adhesion, and at a score of 5, the egg white surface was smooth.

The method of judging the freshness of eggs was studied by measuring the diameter of the air cell. At the same storage temperature, the correlation between days of storage and air cell diameter was 0.93 (p = 1.27 × 10−11), and no significant correlation between air cell diameter and egg weight, as shown in Figure 4. The air cell diameter increased significantly with the extension of storage time. The scatter plot of air cell diameter and storage days (Figure 4) showed that there was a functional relationship between air cell diameter and days of storage.

Figure 4.

Figure 4

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The relationship between air cell diameter and storage time. Logarithmic curve fitting yielded the equation y = 12.983 +3.492lnx (p = 2 × 10−6, R² = 0.98). In this model, 12.983 represents the minimum air cell diameter, and 3.492 represents the rate at which the air cell diameter increases over time.

The formation of the air cell in eggs occurs within a few minutes after laying (Reizis et al., 2005). It is because the external environment is cooler than the internal temperature of the hen, causing thermal expansion and contraction, which cause the 2 shell membranes to separate and eventually form a cavity call an air cell. As the storage time of eggs increases, carbon dioxide and moisture gradually escape through the pores of the eggshell, resulting in the diameter and initial height of the air cell to increase (Samli et al., 2005; Mao et al., 2007). Therefore, the measurement of air cell diameter and height is often used as an indicator of egg freshness (Dong et al., 2020; Adriaensen et al., 2022). The method for measuring air cell diameter is lossless simple, fast, and easy to measure (Phillips et al., 1992; Mao et al., 2007). In addition, some non-destructive methods for measuring the air cell diameter has been developed (Aboonajmi and Abbasian Najafabadi, 2014; Nakatomi et al., 2023), which further improves convenience and speed. The freshness of eggs has a significant impact on egg quality. During storage, egg weight loss, albumen pH, yolk pH, yolk ratio, and lipid peroxidation increase, while Haugh units, albumen proportion, decrease (Huang et al., 2012; Quan et al., 2021), resulting in a decrease in egg quality. The R2 value of the logarithmic model in this study is high, indicating that the simulation accuracy is high, which is consistent with the results of previous researches. the air cell diameter can be used as an indicator egg freshness.

Subsequently, the factors affecting the shelling score were analyzed. The survey of the top 3 best-selling brands of soft-boiled chicken eggs on Taobao online shopping platform showed that there was a significant difference in the shelling scores between Brand B and Brands A and C. The shelling scores of the 3 brands were 3.76 ± 1.29 (Table 3). According to the definition of shelling score, the egg white of each brand of egg is slightly adhered to the eggshell, with a cumulative adhesion rate of 0 to 1/8.

Figure 5 shows the correlation between the shelling score of soft-boiled chicken eggs and storage time, eggshell thickness, egg weight, air cell diameter, and TPA. The shelling score is negatively correlated with egg weight (p = 0.03). The smaller the egg weight, the higher shelling scores. The shelling score is positively correlated with storage time, air cell diameter, and egg white springiness (p < 0.05). The lower the freshness and the larger the air cell diameter, the higher shelling scores, indicating that soft-boiled eggs made from eggs with lower freshness had lower egg white adhesion after shelling. This finding is consistent with previous research on shellability of hard-boiled eggs. Although soft-boiled eggs with larger air cell diameters had higher shelling scores, the increase in storage time led to a decrease in egg quality (Jones and Musgrove, 2005; Cozzolino et al., 2024). Therefore, there should be certain requirement for the freshness of the eggs used for soft-boiled eggs, and eggs with poor freshness should not be used simply to improve the shelling scores. Springiness is related to the rapidity and degree of recovery from a deforming force (Rahman and Al-Farsi, 2005; Nakatomi et al., 2023), which affects consumers’ sensory experience of egg white texture (Meullenet et al., 1998; Di Monaco et al., 2008). The springiness of egg white is influenced by heating temperature, heating time, salt concentration and pH (Kato and Nakai, 1980; Woodward and Cotterill, 1986). We may analyze the factors influencing egg white springiness by examining the laying hens' diet or genetic factors.

Figure 5.

Figure 5

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Correlation between peeling score and quality characteristics and texture properties. The upper right section of the matrix represents the correlation coefficients, while the lower left section displays the corresponding p-values.

When the shelling score was 5, the average air cell diameter was 21.55 ± 2.26 mm (Figure 6B). And there was a significant difference in air cell diameter between the shelling scores of 5 and the shelling scores of 2, 3, 4 (p < 0.01). There was a significant difference in egg weight between the shelling scores of 2 and the shelling scores of 4 (p < 0.05), as shown in the Figure 6A. As shown in Figure 6C, the springiness of egg white exhibits significant differences when comparing a shelling score of 2 with shelling scores of 3, 4, and 5, with p < 0.05, p < 0.01, and p < 0.001, respectively. According to the results of this study, when processing soft-boiled chicken eggs, it is inferred that the shelling score is best when raw eggs with an air cell diameter of 21.55 ± 2.26 mm are selected. The smaller the egg weight, the higher the shelling scores. The average weight of branded soft-boiled chicken eggs (with shells) is 60.28 ± 2.76g, which exceeds the egg weight range of 48-58g most favored by Chinese consumers (Chen et al., 2023). Its average shelling score is less than 4, as shown in Table 2, which may be due the low shelling scores caused by the excessive egg weight.

Figure 6.

Figure 6

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Differences in egg weight, air cell diameter and springiness for different shelling scores. * in superscripts indicate significant differences with p < 0.05. ** in superscripts indicate significant differences with p < 0.01. *** in superscripts indicate significant differences with p < 0.001. A, B, and C represent the differences in egg weight, air cell diameter, and springiness, respectively, at various shell shelling scores.

The quality assessment of soft-boiled chicken eggs in vacuum packaging is influenced by the adhesion and ease of shell removal of egg whites during shelling. Severe adhesion of egg whites to the shell reduces the quality of semi-cooked eggs and leads to wastage. Studies on shellability have found that the ease of shelling is primarily associated with storage environment pH, storage duration, and coating emulsion (Reinke et al., 1973; Imai, 1981; Britton and Fletcher, 1987). Longer storage times, higher pH levels, and a coating emulsion consisting of fermented starch and vegetable oil can facilitate easier shelling of fully cooked eggs.

The yolk centering ratio reflects the position of the yolk in the egg. The closer the value is to 1, the more central the yolk is. This study found that the yolk centering ratio of soft-boiled chicken eggs gradually decreased with the extension of storage time, indicating that the yolk deviated from the center (Table 5). This observation is consistent with the results of Grunden et al. (Grunden et al., 1975) on hard-boiled eggs. As storage time increases, the gel-like structure of the chalazae depolymerizes, reducing its ability to stabilize the yolk (Pu et al., 2024), resulting in yolk displacement, which reduces the distance between the yolk membrane and the eggshell. In addition, long-term storage changes the structure of the yolk membrane, potentially giving microorganisms the opportunity to enter the yolk (Heath, 1976). Furthermore, the thinning of the albumen during storage may cause the egg to stick to the eggshell when shelling, which may directly expose the yolk, which is not conductive to consumption. Grunden et al. (1975) found that the position of the yolk is related to the direction (horizontal or vertical) when the egg is cooked. In some strains of White Leghorn chickens, the yolk is more centered when the egg is cooked vertically (Grunden et al., 1975). Therefore, vertical cooking was chosen in this experiment. In order to alleviate the adverse effects of yolk displacement on shelling, Stadelman and Rhorer (1984) found that rotating eggs during boiling results in more centered yolks in hard-boiled eggs. This method has been implemented in production and can also be used to improve the yolks centering in soft-boiled chicken eggs.

Table 5.

Yolk centering ratio at different storage times.

Days of storage / d
1 3 7 21
Yolk side / side ratio 0.45 ± 0.032 0.40 ± 0.035 0.39 ± 0.035 0.31 ± 0.024
Yolk top / bottom ratio 0.51 ± 0.037 0.37 ± 0.027 0.23 ± 0.027 0.19 ± 0.018
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Data are expressed as mean ± SEM.

Safety Assessment of Soft-Boiled Chicken Eggs

The yolk center temperature at different cooking times is shown in Figure 7. When the cooking time is 210s to 370s, the yolk center temperature exceeded 57°C. The center temperature of the whole cooking process was less than 63°C. Additionally, immersing the eggs in an ice-water mixture immediately after cooking will cause the temperature of the egg contents to drop. According to the pasteurization regulations for whole eggs, the center temperature is maintained at 57°C for 57.5 min in the United States and at 63.3°C for 2.5 minutes in China (Keener, 2017). So it is difficult for the soft-boiled chicken eggs to reach the pasteurization standards.

Figure 7.

Figure 7

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Yolk center temperature at different cooking times.

During laying, transportation and storage, eggshells are susceptible to microbial contamination. These microorganisms can penetrate the eggshell and invade the interior of the egg, with Salmonella (Humphrey et al., 1991), Listeria (Farber et al., 1992), Staphylococcus, Escherichia coli (Hsu et al., 2023), potentially present in both the egg white and yolk. The aforementioned foodborne pathogens exist in various strains, and consumption can lead to gastroenteritis, manifesting symptoms such as diarrhea, nausea, vomiting, abdominal pain, dehydration, and shock (Zhao et al., 2014; Teklemariam et al., 2023). Studies have indicated that with prolonged storage, there is a decline in yolk membrane weight, protein content, and glucosamine content, which may result in increased membrane permeability (Fromm, 1967). This alteration in the yolk membrane could facilitate the invasion of Salmonella into the yolk (Humphrey and Whitehead, 1993). When raw eggs contaminated with foodborne pathogens are used to produce soft-boiled chicken eggs, there is a potential risk of microbial residue in the semi-cooked portion, thereby compromising food safety.

In addition to the traditional heat pasteurization method, non-thermal sterilization methods, such as irradiation, can also be used. This method uses irradiation (gamma rays, electron beams, and X-rays) to cause cross-linking of microbial DNA, thereby preventing its reproduction. In 2000, the United States Food and Drug Administration approved this method for use in eggs (Harder and Arthur, 2017). The process of making soft-boiled chicken eggs is difficult to maintain the required pasteurization time. Therefore, in the production of soft-boiled chicken eggs, it is recommended to strictly control the safety quality of raw eggs, or to add whole-egg irradiation or other sterilization steps are suggested in the soft-boiled egg process.

CONCLUSIONS

The quality evaluation parameters for soft-boiled chicken eggs include yolk softness and shelling score. Optimal boiling conditions were established to maintain yolk softness within the ideal range (0.46-0.64) as follows: for eggs weighing 43 to 48 g, 48 to 53 g, 53 to 58 g, and 58 to 63 g, the boiling times are 300 s, 330 s, 350 s, and 370 s, respectively. Factors such as egg freshness, egg white springiness, and egg weight were identified as significant influencing (p < 0.5) the shelling score. The temperature and duration used for processing soft-boiled chicken eggs did not comply with the standards for whole egg pasteurization, leading to potential food safety issues. This study primarily explores the evaluation parameters and influencing factors of soft-boiled eggs. In addition, consumer sensory evaluation, eating interest, and the effects of genetics and nutrition on the quality of soft-boiled eggs remains to be further investigated.

DISCLOSURES

The authors declare no conflicts of interest.

ACKNOWLEDGMENTS

This work was financially supported by the National Key Research and Development Program of China (2022YFD1300100), the China Agriculture Research Systems (CARS-40), the National Key Research and Development Program of China (2021YFD1200803) and the Natural Science Foundation of Sichuan Province (Grant No. 2024NSFSC0389).

REFERENCES

  1. Aboonajmi M., Abbasian Najafabadi T. Prediction of poultry egg freshness using Vis-Nir spectroscopy with maximum likelihood method. Int. J. Food Prop. 2014;17:2166–2176. [Google Scholar]
  2. Adriaensen H., Parasote V., Castilla I., Bernardet N., Halgrain M., Lecompte F., Réhault-Godbert S. How egg storage duration prior to incubation impairs egg quality and chicken embryonic development: contribution of imaging technologies. Front. Physiol. 2022;13 doi: 10.3389/fphys.2022.902154. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Barbut S., Arrington L.C., Maurer A.J. Hard-cooked egg shelf life. Poult. Sci. 1987;66:1941–1948. [Google Scholar]
  4. Barham, P. 2001. The science of cooking. (P Barham, Ed.).
  5. Britton W.M., Fletcher D.L. Influence of storage environment on ease of shell removal from hard-cooked eggs. Poult. Sci. 1987;66:453–457. [Google Scholar]
  6. Chen R., Jiang C., Li X., Shi X., Zhuang L., Zhou W., Zhou C., Xuan L., Xu G., Zheng J. Research on Chinese consumers’ shell egg consumption preferences and the egg quality of functional eggs. Poult. Sci. 2023;102 doi: 10.1016/j.psj.2023.103007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Cozzolino D., Sanal P., Soumeh E.A., Dekkers M.H., Anderson M., Boisen S., Hoffman L.C. Implications of the storage conditions and production system on the physical properties of eggs: a multivariate data analysis study. Int. J. Food Sci. Technol. 2024;59:3235–3244. [Google Scholar]
  8. Di Monaco R., Cavella S., Masi P. Predicting sensory cohesiveness, hardness and springiness of solid foods from instrumental measurements. J. Texture Stud. 2008;39:129–149. [Google Scholar]
  9. Ding B., Chen Y.L., Dou Y.B., Li B., Liu H.N. Optimization of processing technology of soft−boiled egg. Sci. Technol. Food Industry. 2021;42 :135-141+148. [Google Scholar]
  10. Dong X.G., Zhang B., Dong J., Lu B., Hu C., Tang X.Y. Egg freshness prediction using a comprehensive analysis based on visible near infrared spectroscopy. Spectrosc. Lett. 2020;53:512–522. [Google Scholar]
  11. FAO. FAOSTAT: Livestock Primary. Accessed Oct. 2024. https://www.fao.org/faostat/en/#data/QCL.
  12. Farber J.M., Daley E., Coates F. Presence of Listeria spp. in whole eggs and wash water samples from Ontario and Quebec. Food Res. Int. 1992;25:143–145. [Google Scholar]
  13. Fong F.L.Y., Poon P.M.Y., Chung K.C.W., Leung M.L., Sze P.K., Leung K.Y., Sze E.T.P. Suppression of salmonella enteritidis in preparation of Japanese onsen tamago. J. Food Process. Preserv. 2022;46 [Google Scholar]
  14. Fromm D. Some physical and chemical changes in the vitelline membrane of the hen's egg during storage. J. Food Sci. 1967;32:52–56. [Google Scholar]
  15. Goto T., Shimamoto S., Ohtsuka A., Ijiri D. Analyses of free amino acid and taste sensor traits in egg albumen and yolk revealed potential of value-added eggs in chickens. Anim. Sci. J. 2021;92:e13510. doi: 10.1111/asj.13510. [DOI] [PubMed] [Google Scholar]
  16. Grunden L.P., Mulnix E.J., Darfler J.M., Baker R.C. Yolk position in hard cooked eggs as related to heredity, age and cooking position. Poult. Sci. 1975;54:546–552. [Google Scholar]
  17. Harder M.N.C., Arthur V. Pages 177–185 in Egg Innovations and Strategies for Improvements. Elsevier; San Diego, USA: 2017. Effects of gamma radiation for microbiological control in eggs. [Google Scholar]
  18. Heath J.L. Factors affecting the vitelline membrane of the hen's egg. Poult. Sci. 1976;55:936–942. [Google Scholar]
  19. Hsu S.-C., Chen H.-L., Chou C.-F., Liu W.-C., Wu C.-T. Characterization of microbial contamination of retail washed and unwashed shell eggs in Taiwan. Food Control. 2023;149 [Google Scholar]
  20. Huang Q., Qiu N., Ma M.H., Jin Y.G., Yang H., Geng F., Sun S.H. Estimation of egg freshness using S-ovalbumin as an indicator. Poult. Sci. 2012;91:739–743. doi: 10.3382/ps.2011-01639. [DOI] [PubMed] [Google Scholar]
  21. Humphrey T.J., Whitehead A. Egg age and the growth of Salmonella enteritidis PT4 in egg contents. Epidemiol. Infect. 1993;111:209–220. doi: 10.1017/s0950268800056910. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Humphrey T.J., Whitehead A., Gawler A.H.L., Henley A., Rowe B. Numbers of Salmonella enteritidis in the contents of naturally contaminated hens’ eggs. Epidemiol. Infect. 1991;106:489–496. doi: 10.1017/s0950268800067546. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Imai C. Effect of coating eggs on storage stability. Poult. Sci. 1981;60:2053–2061. [Google Scholar]
  24. Jiang S., Huang Y.T., Tang X.L., Wang T., Li Q., Wang H.P., Meng X.G. Traditional cooking methods decreased the allergenicity of egg proteins. J. Food Sci. 2024;89:3847–3857. doi: 10.1111/1750-3841.17118. [DOI] [PubMed] [Google Scholar]
  25. Jones D.R., Musgrove M.T. Effects of extended storage on egg quality factors. Poult. Sci. 2005;84:1774–1777. doi: 10.1093/ps/84.11.1774. [DOI] [PubMed] [Google Scholar]
  26. Kato A., Nakai S. Hydrophobicity determined by a fluorescence probe method and its correlation with surface properties of proteins. Biochim. Biophys. Acta BBA - Protein Struct. 1980;624:13–20. doi: 10.1016/0005-2795(80)90220-2. [DOI] [PubMed] [Google Scholar]
  27. Keener K.M. Pages 165–175 in Egg Innovations and Strategies for Improvements. Elsevier; San Diego, USA: 2017. Shell egg pasteurization. [Google Scholar]
  28. Kelly J.M., Newcomer J.L., Borsenik F. Freeze-dried whole egg solids, other processed eggs, and fresh eggs . Comparison of flavor preferences. J. Am. Diet. Assoc. 1962;40:31–34. [PubMed] [Google Scholar]
  29. Kraft A.A., Torrey G.S., Ayres J.C., Forsythe R.H. Factors influencing bacterial contamination of commercially produced liquid egg. Poult. Sci. 1967;46:1204–1210. doi: 10.3382/ps.0461204. [DOI] [PubMed] [Google Scholar]
  30. Lechevalier V., Nau F., Jeantet R. Pages 484–512 in Handbook of Food Powders. Elsevier; San Diego, USA: 2013. Powdered egg. [Google Scholar]
  31. Liu Y., Wang K., Ma J., Wang Z., Zhu Q., Jin Y. Effect of yolk spheres as a key histological structure on the morphology, character, and oral sensation of boiled egg yolk gel. Food Chem. 2023;424 doi: 10.1016/j.foodchem.2023.136380. [DOI] [PubMed] [Google Scholar]
  32. Ma Z., Chi Y., Chi Y. Cryoprotective role of saccharides in frozen egg yolks: water/ice tailoring effect and improved freeze–thaw stability. Food Hydrocoll. 2023;145 [Google Scholar]
  33. Mao K., Murakami A., Iwasawa A., Yoshizaki N. The asymmetry of avian egg-shape: an adaptation for reproduction on dry land. J. Anat. 2007;210:741–748. doi: 10.1111/j.1469-7580.2007.00737.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Meullenet J., Lyon B.G., Carpenter J.A., Lyon C.E. Relationship between sensory and instrumental texture profile attributes. J. Sens. Stud. 1998;13:77–93. [Google Scholar]
  35. Nakatomi C., Hsu C.-C., Ono K. Correlations of sensations of hardness and springiness of agar and gelatin gels with mechanical properties in human participants. J. Oral Biosci. 2023;65:316–323. doi: 10.1016/j.job.2023.09.001. [DOI] [PubMed] [Google Scholar]
  36. Nolasco E., Yang J., Ciftci O., Vu D.C., Alvarez S., Purdum S., Majumder K. Evaluating the effect of cooking and gastrointestinal digestion in modulating the bio-accessibility of different bioactive compounds of eggs. Food Chem. 2021;344 doi: 10.1016/j.foodchem.2020.128623. [DOI] [PubMed] [Google Scholar]
  37. Phillips L., Brake J., Ellner S., Oukama R. A mathematical model for estimation of broiler egg weight loss from physical dimensions and air cell size during incubation. Poult. Sci. 1992;71:625–630. doi: 10.3382/ps.0710625. [DOI] [PubMed] [Google Scholar]
  38. Pu J., Hu J., Xiao J., Li S., Wang B., Wang J., Geng F. Integrated landscape of chicken egg chalaza proteomics. Poult. Sci. 2024;103 doi: 10.1016/j.psj.2024.103629. [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Quan C.L., Xi Q., Shi X.P., Han R.W., Du Q., Forghani F., Xue C.Y., Zhang J.C., Wang J. Development of predictive models for egg freshness and shelf-life under different storage temperatures. Food Qual. Saf. 2021;5:fyab021. [Google Scholar]
  40. Rahman M.S., Al-Farsi S.A. Instrumental texture profile analysis (TPA) of date flesh as a function of moisture content. J. Food Eng. 2005;66:505–511. [Google Scholar]
  41. Réhault-Godbert S., Guyot N., Nys Y. The golden egg: nutritional value, bioactivities, and emerging benefits for human health. Nutrients. 2019;11:684. doi: 10.3390/nu11030684. [DOI] [PMC free article] [PubMed] [Google Scholar]
  42. Reinke W.C., Spencer J.V., Tryhnew L.J. The effect of storage upon the chemical, physical and functional properties of chicken eggs. Poult. Sci. 1973;52:692–702. [Google Scholar]
  43. Reizis A., Hammel I., Ar A. Regional and developmental variations of blood vessel morphometry in the chick embryo chorioallantoic membrane. J. Exp. Biol. 2005;208:2483–2488. doi: 10.1242/jeb.01662. [DOI] [PubMed] [Google Scholar]
  44. Sahachairungrueng W., Tonpho P., Veeradechakul M., Rosnim T., Thompson A.K., Terdwongworakul A., Teerachaichayut S. Transient heat modeling for non-destructive assessment of boiled eggs. Heat Mass Transf. 2023;59:1895–1901. [Google Scholar]
  45. Samli H.E., Agma A., Senkoylu N. Effects of storage time and temperature on egg quality in old laying hens. J. Appl. Poult. Res. 2005;14:548–553. [Google Scholar]
  46. Shi X.F., Liang Q.N., Wang E.L., Jiang C.Y., Zeng L.S., Chen R.C., Li J.Y., Xu G.Y., Zheng J.X. A method to reduce the occurrence of egg translucency and its effect on bacterial invasion. Foods. 2023;12:2538. doi: 10.3390/foods12132538. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Stadelman W.J., Rhorer A.R. Quality improvement of hard cooked eggs. Poult. Sci. 1984;63:949–953. [Google Scholar]
  48. Teklemariam A.D., Al-Hindi R.R., Albiheyri R.S., Alharbi M.G., Alghamdi M.A., Filimban A.A.R., Al Mutiri A.S., Al-Alyani A.M., Alseghayer M.S., Almaneea A.M., Albar A.H., Khormi M.A., Bhunia A.K. Human salmonellosis: a continuous global threat in the farm-to-fork food safety continuum. Foods. 2023;12:1756. doi: 10.3390/foods12091756. [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. USDA. 2024. Accessed Oct. 2024. https://www.ams.usda.gov/grades-standards/shell-egg-grades-and-standards.
  50. Whiley H., Clarke B., Ross K. Knowledge and attitudes towards handling eggs in the home: an unexplored food safety issue? Int. J. Environ. Res. Public. Health. 2017;14:48. doi: 10.3390/ijerph14010048. [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Woodward S.A., Cotterill O.J. Texture and microstructure of heat-formed egg white gels. J. Food Sci. 1986;51:333–339. [Google Scholar]
  52. Zhao X., Lin C.-W., Wang J., Oh D.H. Advances in rapid detection methods for foodborne pathogens. J. Microbiol. Biotechnol. 2014;24:297–312. doi: 10.4014/jmb.1310.10013. [DOI] [PubMed] [Google Scholar]
  53. Zheng M.T., Chen S.P., Yao Y., Wu N., Xu M.S., Zhao Y., Tu Y.G. A review on the development of pickled eggs: rapid pickling and quality optimization. Poult. Sci. 2023;102 doi: 10.1016/j.psj.2022.102468. [DOI] [PMC free article] [PubMed] [Google Scholar]

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