Intro: Delicious Dry-ice Ice Cream and Underlying Science!
Hey everyone, I wanted to share with you my dry-ice ice cream recipe as it's a relatively simple and easy recipe for a home-cook to execute without investing in expensive scientific tools and ingredients. The use of dry-ice also offers an innovative twist to a classic dessert that provides a real contribution to texture, taste, and cooking through the understanding of underlying material science. This is not a recipe that pursues novelty for its own sake, but uses our understanding of underlying ice-cream crystal growth and applies the tools available(this being dry-ice) in a way to make ice-cream more delicious and stimulating. With that being said, lets get right into it!
Step 1: Step 1: Understanding the Science
So lets talk science!
What is ice cream exactly? At face value the ice cream base is just: milk, sugar, and your flavoring of choice...right? Well yes, but lets break it down even more. Milk is made up of mostly fat, water and proteins. The fat inside milk is in tiny microscopic droplets that are thoroughly dispersed throughout the water phase. Proteins in the milk help to stop the fat from coagulating and keep the ice cream as an emulsion. (Think Mayonnaise)
But wait, when you churn the ice-cream base while freezing, you are also incorporating air into your milk. This means ice cream also has a gaseous phase that has become dispersed within a liquid phase or in other words a foam. (Another example are whipped up egg-whites forming stiff peaks). So, ice cream is both an emulsion and a foam forming a complex dynamic multiphase system of fat, water, proteins and air all in balance with each other to form a creamy, smooth delicious dessert. (see the picture above) Ice-cream makers for years have been attempting to master this process with the right amount of air, fat, water all the while trying to keep it stable through the additions of the right emulsifiers.
So this takes us to the ice cream base
Whatever ice cream base you use, adding egg yolks to your mixture will allow for a more stable, creamy final product. How does this work? Well egg yolks contain mostly fat and protein. Fat, which freezes differently and less hard than water, makes ice cream soft and creamy. More valuable,the egg yolks contain protein which forms a thick gel when heated to 70C, the point at which the yolks proteins begin to set. A good analogy from seriouseats are that the proteins coagulate into a thick gel which, if you could zoom in on them at a microscopic level, would resemble a mesh of fibers. Water gets trapped in this mesh like dryer lint in a lint tray, and when it's trapped and surrounded by protein, it can't form chunky ice crystals. No chunky ice crystals = smooth, creamy ice cream.
The other important aspect of egg yolks are the fact that they contain a little protein called lecithen that is amphiphilic. Amphi meaning "both" and philic meaning "loving" or in other words lecithen has both water-loving and fat-loving properties that allow it to act as an emulsifier between the fat and water within milk. This is important because fat droplets typically have a hard time coalescing(merging) due to proteins on the surface. Emulsifiers actually help fat to partially coalesce during the churning process which significantly improves an ice creams ability to hold air. Essentially, partially coalescedfat droplets are very good at holding air, and emulsifiers like lecithen are what allow the fat droplets to stay stable in this state, ultimately resulting in a fluffy ice cream. This significantly improves texture by leveraging more of the fat within the milk while also helping with stability, thus improving shelf-life.
Colloidal aspects of ice cream structure | Food Science
Step 2: Step 2: Making the Ice Cream Base
Thanks for sticking with me while I prattle about all the sciency stuff! If you got anything from reading the last step, then make sure you use EGGS in your ice cream base. Here is my personal recipe utilizing black-tea but any ice cream base recipe will really do:
- 2 cups of cream
- 2 grams of black-tea leaves or 1-2 bags of english breakfast tea
- 1/4 cup of sugar
- 3-5 egg yolks
- In a small pot, simmer milk, sugar and tea until sugar completely dissolves, about 5 minutes.
- Remove pot from heat. In a separate bowl, whisk yolks. Whisking constantly, slowly whisk about a third of the hot cream into the yolks to temper them, then whisk the yolk mixture back into the pot with the cream.
- Return pot to medium-low heat and gently cook until mixture is thick enough to coat the back of a spoon (about 160 degrees on an instant-read thermometer). Strain through a fine-mesh sieve into a bowl.
- Cool mixture to room temperature. Cover and chill at least 4 hours or overnight. This will help during the freezing stage
Note: Its really important you try to hit 160 F or 70 C while heating the pot. When this temperature is reached you will know that the proteins in the egg yolk have set and formed a good gel. A good indicator is that your ice cream base can coat the back of the spoon.
Step 3: Step 3: Ice Cream Crystalization
What happens during Ice Cream Crystallization!
Our goal is to have a creamy, smooth ice cream texture. Studies show that smaller ice crystals produce a better mouth feel and provide a better texture. Now if you want to get smaller ice crystals, we have to understand the science behind crystal nucleation and growth.
Nucleation is the birth of a crystal. In a typical homemade ice cream freezer, this only occurs at the wall of the freezer where the temperature difference is high enough to form new crystals. The rate of nucleation is important because it affects the number of crystals in a product, which in turn affects how large each crystal can become. See image above. Several factors affect nucleation. The most important factor is the driving force—the temperature difference between the solution and its freezing point. A greater driving force increases nucleation. However, when the temperature is too low, decreased molecular mobility or diffusion within the ice cream base slows nucleation. An example of this is if I take a big chunk of dry ice and plop it into my ice cream base. Only the ice cream around the chunk of dry ice will freeze and the rest of the ice cream base will not. 
Crystal growth happens when the thermodynamics of your mixture prefers to grow the current crystals that are already there rather than nucleating new ones. This typically happens during slow freezing when the latent heat from freezing can not be removed fast enough.
The last thing I want to talk about is recrystallization. Recrystallization is the growth and change of crystal shape that occurs during storage or directly after freezing. This is essentially the same process as when a blacksmith will temper a blade at a higher temperature to make it less brittle after quenching(or freezing depending on how you look at it). After the freezing process, your ice cream base that was formed at -40 C (using dry ice) will go into the freezer stored at close to 0 C. It will begin to recrystallize and increase the size of its crystals to come to equilibrium with the new 0 C, similar to a tempering process. This is why ice cream in your fridge typically tastes worse and worse over time due to increased crystal growth and re crystallization from the freeze-thaw cycle.
 Ice Cream Freezing Curves | Food Science https://www.uoguelph.ca/foodscience/node/1997/
 Importance crystallization https://www.uoguelph.ca/foodscience/book-page/importance-crystallization-rate
Step 4: Step 4: Ice Cream Churning!
Now that we understand why freezing something faster creates smaller ice crystals, its time to use our dry ice to produce our delicious ice cream. This next step is a little bit more involved, but here are the things you will need:
What you will need:
- Dry Ice - I found this at my local grocery store, Acme. It might be a bit hard to find, but luckily its super cheap to buy.
- Small hammer
- Food processor
- Stand Mixer
- Winter or CryoGloves!!!! PLEASE BE CAUTIOUS AND SAFE WHEN HANDLING DRY ICE
- Optional: Sieve
- Prior to starting this, make sure your stand mixing bowl is in the freezer at least 4 hours beforehand and your ice cream base is sufficiently chilled
- Take your dry ice and begin by smashing it with a small hammer until it is in small chunks
- Use a blender or food processor to turn the small chunks of dry ice into a powder. NOTE: If you don’t have a blender or food processor, just keep smashing until you have a fine powder. Large chunks of dry ice can be dangerous to eat! A good way to ensure you have no large chunks is to sieve your powder.
- Place your ice cream base into the stand mixer with wire whisk attachment and begin to churn at medium speed.
- Begin to add dry ice powder spoonful at a time into the stand mixer until ice cream is thick enough to scoop. Should take around 5 minutes.
- Your mixture is now beautiful and almost ready to eat. Right now your ice cream should be incredibly soft and creamy. Transfer to freezer to temper a little bit before serving. You want to make sure the ice cream is not so soft as its like whipped cream.
Step 5: Step 5: ENJOY!
I hope you enjoyed my tutorial about the science of ice cream. This is just a small facet of the intricacies and complex interactions of a very common and delicious dessert.
This was also my first instructable and hopefully I'll do some more soon!