Salt-Cured Egg Yolk

May 31, 2021

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What are cured eggs?

Curing is the general process of preserving or flavoring a food using salt. Salted egg yolk originates in Chinese cuisine, mostly with duck eggs. In this tradition, as well as generally in Singapore, whole duck eggs are submersed in a salt solution (brined). Adding salt can extract water from food as well as from any bad bacteria that are present, slowing their growth and preserving the food. Good bacteria are also found in egg yolks, like Lactobacillus which is one of the types that I mentioned is used in yogurt fermentation. This good bacteria can be encouraged to grow by the inclusion of sugar in the curing powder, and can acidify their surroundings. I’m aware of acids being capable of solidifying egg white, but I was unable to find a reference to acidity affecting the texture of the yolk itself. Doing a quick experiment, I added vinegar to a separated egg yolk and did not observe any thickening or solidification. It is possible that a stronger acid is necessary to have a visible effect, but I believe that the textural changes that occur in cured eggs are mainly due to the extraction of water.

Making the Cured Egg Yolks

The recipe I followed can be found here, and differs from the traditional preparation. Rather than brining a whole egg (I used chicken eggs), I separated the yolks from the white, and cured them in a powder bed of salt and sugar. Leaving the egg yolks for 4 days, I dusted them off and rinsed them. I was surprised at how hard the texture was; I would compare the stiffness to that of a pencil eraser which has an elastic modulus of around 1 MPa. At this stage the yolks were a bit malleable, and I could cut off a small slice to play around with. They likely had a yield stress in the range of thousands of Pascals. After rinsing, I placed the gem-looking yolks in the oven at approximately 60C to dry for a few hours at which point they became quite hard. The yolks that I made were significantly different from my understanding of the traditional brined variety. Brining uses a salt solution on whole eggs, and thus less water is extracted from the yolk and the texture is apparently more like that of a soft cream. It may be possible to achieve this same texture by using the powder bed method for a shorter amount of time, but I expect one would have to be much more careful about extracting the soft yolk from the powder.

The flavor of the yolks I made was quite salty as you might imagine, but with an interesting eggy tanginess. I ended up grating the yolks and enjoying them over some ravioli. Certainly, the taste of the yolks was heavily affected by the curing process, but I’m curious about if the particular mechanical properties and texture that resulted could be achieved in a different way. Since I believe that the textural changes of the yolk occurred mainly due to the extraction of water, I’d be interested sometime in the future to find out if I could achieve similar mechanical properties by skipping the salt entirely and only slowly drying the yolks in the oven. Overall, I recommend this recipe if you’re curious, just know that it uses A LOT of salt and sugar.

As I mentioned at the top though, for this entry I was actually less concerned with the rheology of the yolks and more interested in that of powder. Rheological methods for powders are in some ways less developed than more traditional rheometry, and powder flow is extremely complex. An example of this complexity that was documented in the NY Times last year is that when making an hourglass filled with sand grains of known sizes and shapes, there is no reliable way to predict how long it will take to flow through; you just have to try it. In my current research, my team is concerned with producing pharmaceutical drug particles that we think will flow more easily than conventional powders and thus be easier to handle in precise quantities. It is unrealistic to produce huge quantities of powders to test them in an actual industrial setting. Instead, we engineer small powder quantities in the lab and characterize their “flowability” in a powder rheometer. We then use these experiments to make decisions about what formulations and processing methods are likely to produce flowable powders when scaled up to an industrial setting.

One of the simplest tests we can perform to get a general idea of powder flowability can actually be done at home. By flowing powder through a funnel to form a heap, we can look at its “angle of repose”. I turned a circular-bottomed drinking glass upside down to have a known size of the base of the heap. I then gradually flowed powder through a kitchen funnel and allowed it to build up. With this test we are generally looking at how the cohesive and frictional forces between powder particles compare to the gravitational force. Higher cohesive forces make it easier for a taller heap to build up, increasing the angle of repose. Based on the angle, the powder can be classified into different standardized flowability categories. In my experiment I compared the flow behavior of the salt-sugar powder before and after the curing process. Before curing, the powder forms a very short heap, with an angle of 34 degrees that classifies it as “free-flowing”. After curing, a significant amount of water has been extracted into the powder, greatly increasing the cohesive forces between the powder particles. As a result, the formed heap is quite tall, with an angle of 46 degrees that classifies it as a “cohesive” powder.