Radha Maholtra(1), Michelle Miao(1), Manshi Patel(1), Julia Shi(1), and Caroline McNeil(2)
1 - Rice University Wiess School of Natural Sciences, contributed equally to this work
2 - Rice University Department of Chemistry
ABSTRACT
Using current methods, the process of making century eggs takes roughly 30 days and causes eggs to completely gel. UB Preserv, a Houston restaurant, wants to develop a faster way to prepare century eggs. Two separate experiments were carried out in order to study the impact of the two variables on speed of century egg preparation. First, the concentration of Ca(OH)2 in the egg pickling solution was increased to study its impact on egg gelation speed, and second, egg shells were dissolved to study the impact of increase in the diffusion rate of the pickling solution into the egg. The Ca(OH)2 experiment was carried out using chicken eggs, and the shell experiment used duck eggs. Both experiments recorded a negative result. For the first experiment, the control eggs had a higher internal pH than the experimental. Additionally, none of the chicken eggs completely gelled. All of the eggs became noticeably more viscous over time. For the second experiment, the duck eggs with shells had higher internal pH’s than the eggs without shells. The eggs without shells were significantly stained by the pickling solution and the yolks solidified, preventing yolk pH from being taken. Both duck eggs with shell and duck eggs without shell became more viscous over time, and one control duck egg achieved complete gelation.
INTRODUCTION
Century eggs, also known as pi dan, are a traditional Chinese delicacy. It is made by soaking eggs (traditionally duck) in a mixture of alkaline salt and then fermenting the eggs for a lengthy period while wrapped in solid material which prevents oxygen flow. Through the process, the yolk becomes green while the white becomes dark brown[1]. The transforming agent in an alkaline salt, which raises the pH of the egg to around 10 by the end of the process[2].
Century eggs are part of a specialty dish served at UB Preserv, a Houston-based restaurant. Due to issues with the consistency of commercially sourced century eggs, UB Preserv is seeking a way to make century eggs in house. Unfortunately, the process is much too long for UB Preserv to realistically make this delicacy. UB Preserv reached out to us to see if we could find a way to reduce the length of the egg preparation process.
Previous literature has defined the century egg making process, determined the process raises the final internal pH of eggs[2], and suggested that various cations may impact the speed of egg gelation [3]. Given a study by Ganasen et. al which suggested that Ca2+ ions could decrease egg gelation time, we hypothesized that adding Ca(OH)2 to supplement the alkaline base could speed up the century egg preparation process. In a separate experiment, we dissolved egg shells to increase diffusion rate of the solution across the egg membrane. We hypothesize that this will decrease the time needed for egg gelation by promoting movement of base from the solution into the egg. This movement is what leads to the increase in internal egg pH[1]. The aim in both experimental setups will be to achieve both an increase in internal pH of the eggs and gelation.
MATERIALS AND METHODS
Duck Eggs
Four liters of pickling solution (.62M NaCl, .53M NaOH, 10g/L jasmine tea leaves, milli-Q H2O) were prepared. Shell-less duck eggs—which had been soaked in household vinegar for four days—and shelled duck eggs were rinsed using milli-Q H2O, dried, then placed in the prepared solutions at room temperature.
After soaking for one day, two eggs were removed from each solution, dried, then wrapped in saran wrap to ferment for two weeks at room temperature. This process was repeated every 2-4 days until all the eggs had been removed. After fermentation, the eggs were weighed before separating the yolks and whites. The pH of each component was measured.
Chicken Eggs
Four 1000mL solutions of varying amounts of Ca(OH)2 were prepared (.62M NaCl, .53M NaOH, 10g/1L jasmine tea leaves, milli-Q H2O). Forty-eight chicken eggs were massed and twelve placed in each solution at room temperature.
After 3 days, two eggs were removed from each solution, dried, wrapped in Saran Wrap, and left to ferment for 2 weeks at room temperature. This was repeated every 2-4 days. After fermentation, the eggs were weighed before separating the yolks and whites. The pH of each component was measured.
RESULTS AND DISCUSSION
Chicken Eggs
Our first experiment focused on chicken eggs in solutions with different concentrations of Ca(OH)2. We soaked eggs in solutions of varying Ca(OH)2 content to test the impact of Ca2+ on egg pH. Based on previous literature, we decided to prepare solutions consisting of NaOH, NaCl, jasmine tea leaves, and Ca(OH)2 [1]. Control eggs were placed in solution with no Ca(OH)2.
The general trend for the average pH of chicken egg yolks over time for all solutions was an initial decrease followed by a period of increase and then decrease. The control solution recorded the highest pH of the four solutions compared. The largest difference between the control and the remaining three solutions was at 15 days: the control yolks had an average pH of 9.85, 56% times more than the average yolk pH of solution 2 and 52% times more than the average yolk pH of solution 1 and 3. (Figure 1.)
Figure 1.
Average pH of chicken egg yolks over time. Two eggs were removed from each solution every 2 days and the pH of the both yolks was measured and averaged. The solutions had Ca(OH)2 concentrations of .000 mM, 67.5mM, 135mM, and 202mM.
For the pH of egg whites, the trend lines generally remained constant before reaching a peak and dropping off. Once again, the control solution recorded the highest pH of the four solutions compared. There was a notable decrease in pH between Days 11-13 for all solutions except the 135mM solution, followed by a peak point at Day 15 in control and the 202mM solution. Between the solutions themselves, the differences in pH between them are not noteworthy; the graph lines often intersect. (Figure 2.)
Figure 2.
Average pH of chicken egg whites over time. Two eggs were removed from each solution every 2 days and the pH of the both whites was measured and averaged. The solutions had Ca(OH)2 concentrations of .000 mM, 67.5mM, 135mM, and 202mM.
Qualitatively, the chicken eggs became more viscous over time, but the whites and yolks did not change color. Additionally, the egg yolks did not have any noticeable changes in consistency. An outlier in the control solution appeared on Day 13 which displayed both the dark brown egg white and green yolk which are characteristic of century eggs.
We expected that by increasing amounts of Ca(OH)2 in each solution, the diffusion rate across the egg shell would increase, making the eggs more basic. By increasing the egg pH faster, we hoped to produce century eggs in less time as a previous study by Ganasen et. al found century eggs to have an average pH of roughly 10 by the end of the process[2]. However, our results disagreed with our expectations: higher Ca(OH)2 concentrations led to lower pH of the egg yolks and whites.
This result may be because we were unable to get the Ca(OH)2 to fully dissolve. We believe this to be because the solutions were fully saturated. A potential consequence of this is that the solution surrounding the eggs did not diffuse into the eggs. The saturation of solution could have led to a hypertonic solution when compared with the egg, leading any diffusion to occur from the egg into solution, rather than the other way around. Given these results, we suggest that Ca(OH)2 actually makes the process of achieving egg gelation slower because our experimental eggs recorded lower pHs than our control eggs and because the Ca(OH)2 may have caused the solution to be hypertonic to the egg which would interfere with the diffusion of the solution into the egg.
Duck Eggs
Our second experiment sought to determine whether the absence of the egg shell would increase the diffusion rate of the pickling solution into the eggs. Eighteen experimental duck eggs were soaked in household vinegar to dissolve the duck egg shells without destroying the egg membranes. Eighteen control duck eggs were not placed in vinegar so that their shells were maintained.
The pHs of the duck egg yolks remained mostly constant throughout the experiment, with the exception of a slight decrease in pH at Day 4 in the control eggs and slight increase in pH on the same day in the experimental eggs. Our control eggs consistently had higher yolk pH than our experimental eggs (Figure 3.).
Figure 3.
Average pH of duck egg yolks determined every 2 days. After Day 6, pH testing of the shell-less eggs ceased due to solidified yolks.
For the average pH of whites, the experimental eggs remained constant throughout the experiment until the end, where there was a sharp increase in pH. The control eggs had a slight increase in pH before dropping at Day 11 and then increasing again. Once again, our control eggs had a higher pH than our experimental eggs. The largest difference could be seen after 8 days in solution, where the pH of control egg whites was pH of 10.65; this was 69% more than the pH of our experimental egg whites, which was 6.3 (Figure 4.).
Figure 4.
Average pH of duck egg whites determined every 2 days. The pH of egg whites without shell was not taken for eggs after Day 13 as there was significant fungal growth on the eggs, which would have introduced a confounding factor into the data.
Qualitatively, the experimental egg yolks became solid at Day 6 while the whites became more viscous over time. The control egg whites and yolks remained largely unchanged. When massed, experimental duck eggs were significantly larger than control duck eggs. Shell-less eggs were also notably more stained outside than were shelled eggs, and the experimental egg yolks became a muddy yellow color while control egg yolks remained bright yellow. An outlier control egg that was in solution for 13 days achieved complete gelation of the egg white.
We expected the removal of the egg shell to lead the eggs to reach higher pH faster. By removing the shell, we remove a barrier between the eggs and solution, increasing diffusion. The movement of base into the egg is what leads to the increase in egg pH, so we hoped that removing the shell would increase the speed of diffusion and thus help the eggs increase pH[1]. However, our results disagreed with our expectations; the eggs without shells had a lower pH than our control eggs. Additionally, the pH of the control egg whites increased sooner than the pH of experimental egg whites, which had mostly constant pH. This may have been due to the vinegar used to dissolve the egg shells. Vinegar contains acetic acid, which has a low pH. This could have impacted the pH of the duck eggs’ yolks and albumen significantly.
Noticeable fungal growth was observed on our experimental eggs on Day 13. The growth was due to humid and warm laboratory conditions which were outside of our control. These conditions promote fungal growth[4]. The pH of the duck eggs without shells could have been influenced by the growth of fungi, which would introduce a confounding variable into our study. Thus, we did not include these final few eggs in our results.
CONCLUSION
We carried out two separate experiments in order to study the impact of Ca(OH)2 and removal of the egg shell on egg pH and speed of gelation. Both experiments recorded negative results; the control egg groups in both experiments had higher internal pH than the experimental egg groups. Additionally, none of the chicken eggs completely gelled, though the eggs became noticeably more viscous over time. These results may be due to undissolved Ca(OH)2 in the pickling solutions in the first experiment and the impact of the egg shell removal process on egg pH in the second experiment.
In future experiments, the process of dissolving the egg shells could be modified so that the shell is instead partially dissolved. This would allow the diffusion rate to increase by removing some of the shell while preventing the duck eggs’ pH from being influenced to the same extent by the vinegar as in our experiment. Furthermore, the chicken eggs could be pickled with metal cations (e.g. Zn2+), which have been suggested to help eggs retain solution[5]. Fermentation time could also be reduced to prevent excess denaturation, which can lead to liquidation of the egg yolk after solidification and fungal growth [4]. Fungal growth specifically would make the eggs inedible, which defeats the purpose of decreasing the time needed to prepare century eggs so that they can be served in a restaurant. Environmental factors such as humidity and temperature should be better controlled in future studies to ensure that the results are consistent and not impacted by external variables such as fungal growth.
ACKNOWLEDGEMENTS
This work was supported by Rice University Wiess College of Natural Sciences. Additional thanks to Dr. Jamie Catanese, Sydney Parks, and Kevin Sun for their guidance and Dr. Lesa Tran Lu for providing the duck eggs utilized in our experiments.
REFERENCES
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