Hello again and welcome back to the Soft Matter Kitchen! Life has been quite busy for me lately but somehow I’ve managed to stay healthy and have in fact been successful at losing a significant amount of weight. With that being said, I believe now is the perfect time for me to get more into deep-fried foods, and one of my recent favorites is a dish new to me called deep-fried milk.
What is deep-fried milk?
Different versions of “fried milk” dishes are found in many countries and cultures including Italy, Spain, and India; the version that I chose to make is a very simple snack that originated in Cantonese cuisine. For this dish, one first forms a gelled solid out of the milk; after cooling, the gelled milk is then robust enough to withstand the breading and frying processes. Key to the gelling process is starch, which is commonly used as a thickener in sauces, but here is present in much higher concentrations.
Plant starches form as solid granules that are composed of chains of glucose sugar molecules that are bonded to each other. On heating to about 60 °C, these solid granules absorb a significant amount of water and become similar to the microgels that I’ve mentioned in previous entries. Eventually, portions of the starch granules will dissolve in the water and enter the surrounding liquid. As the solution is cooled, the starch molecules form stable bonds with each other, creating a stiff gel network at sufficiently high concentrations. Over time the starch molecules become more organized and bond more tightly with each other.
Making the deep-fried milk
After cooling down, the milk forms a stiff gel that can be cut into blocks
The recipe I followed can be found here. Adding sugar and cornstarch to whole milk, the original mixture is quite low in viscosity, but after cooking ends up forming a sticky yield-stress fluid that you can see in the video. From playing around with the mixture while it was still warm, I estimate that the yield-stress was around 100 Pa, however the rheology of this material is highly temperature dependent. After cooling in the fridge and being brought to room temperature the material was much stiffer and could be easily cut into blocks; at these temperatures I would classify it as a brittle solid rather than a yield-stress fluid since it did not flow and re-heal on yielding.
Because the gel is so stiff after cooling, it can survive the standard breading process
After cutting the milk gel into blocks, the frying procedure was fairly standard: I dipped the blocks into flour, egg yolk, then breadcrumbs; once the oil was hot enough, I submerged the blocks until they turned a golden brown. I found that an important aspect for my enjoyment of this dish is the temperature-dependent behavior of the starch milk gel. If you have ever reheated rice, you may be familiar with the fact that terribly hard day-old cooked rice stored in a fridge can largely recover its soft sticky texture simply by microwaving it with high heat. This behavior is due to the weakening of the starch gel structure at elevated temperatures, which also occurs with the milk gel during frying. Shortly after frying, the stiff gelled block transformed back into a creamy yield-stress fluid which formed a very nice contrast with the crispy breadcrumb exterior.
Shortly after frying, the creamy interior texture formed a nice contrast with the crispy exterior
Once the food cooled down to room temperature or lower—while the sweet taste was still pleasant—I found the texture underwhelming. This got me thinking about if it would be possible to formulate a different milk gel that retained the creamy yield-stress fluid texture closer to room temperature. The difficulty would likely be in ensuring that the material could survive the breading and frying process. One possibility to address this could be to use a material that in some ways behaves oppositely to starch; in my past research I studied methylcellulose, an additive which is typically a liquid in solution at room temperature, but forms a gel at elevated temperatures. Incorporating methylcellulose into the milk could be part of a strategy for producing a cold version of this dish which currently is often served as a dessert—perhaps similar to fried ice cream.