Saturday, March 11, 2017

Ice Cream: Solids, Water, Ice

This post is addendum to the post on How To Build a Recipe, and the post on Sugars. I want to clarify the importance of solids—which is really a reflection of the importance of water. These are key ideas—if you master them, you will be well on your way to texture Ninjahood.

Scanning electron micrograph of ice crystals

We’ve discussed how ice cream is made up of three physical systems: an emulsion (fats suspended in water); a foam (air dispersed in a solid fat network); and a sol (solid water dispersed in liquid water).

Here we’re going to look at that sol—the interactions of water and ice. And we’re going to look at the effects of all the nonfat solids that are dissolved or suspended in the water.

It’s important—and instructive—that the ice is mostly pure water. This is the case because freezing acts as a purifying process; when ice crystals form, they expel dissolved and suspended solids. The result, besides much lower concentrations of stuff in the ice, is an increased concentration of stuff in the liquid portion of the water. 

This phenomenon is called Fractional Freezing. It creates an interesting system in which there’s no single freezing temperature for ice cream, or for any water-based solution. Instead there’s a temperature range between the extreme points where none of the water is frozen and where all of it is frozen. Between these points, you get a mix. The colder the temperature, the greater the proportion of frozen water, and the higher the concentration of solutes in the liquid water.

Fractional freezing works because of the colligative properties of water regarding freezing point depression.* The stronger a solution, the lower the freezing point. So when a bit of a solution freezes, strengthening the concentration of the remaining liquid portion, that liquid’s freezing point drops. And so on. This process is continuous and self-regulating. And pretty cool. 

So What? 

We need ice to make ice cream, but we don’t want too much. With too much ice, you get a popsicle. In order to have a lower proportion of ice, we need a higher proportion of other stuff. 

Some of that other stuff is fat. But we likewise don’t want too much, or the ice cream will be too rich and the flavors too muted. Some of that other stuff is air, but we really don’t want too much of that, or the ice cream will be too fluffy and insubstantial. 

The remainder is liquid water, and dissolved or dispersed nonfat solids. These two are intimately related, because 

1) the more nonfat solids, the lower the percentage of total water, and
2) the more nonfat solids, the greater the portion of that water that stays liquid

So, in general, solids are good. Solids with low molecular weights** depress the freezing point the most, while all solids displace some of the water. 

We especially like milk solids—specifically the nonfat portion—because they effectively concentrates the milk. We get more of the functional qualities of milk, like emulsification, freezing point depression, and improved body, but without added water. And milk tastes good, the way ice cream should.

Sometimes we get solids from the flavor ingredients: chocolate and cocoa, fruit pulp, nut butters, matcha powder, coffee solids, etc.

Remember from the How to Build a Recipe post, we typically aim for the following levels of solids in a well-balanced recipe:

Nonfat Milk Solids: 10-12%. Or higher for low-fat ice cream.
(everything in milk besides the water and fat)

Total Nonfat Solids: 22–25% 
(everything in the ice cream besides water, alcohol, and fats)

Total Solids: 37–42%
(total nonfat solids plus total fat. everything besides water and alcohol)

If solids levels are too low, ice cream can lack body, freeze too hard, and may have textural problems like iciness. If solids levels are too high, ice cream can be excessively chewy or cakey. If the milk solids specifically are too high, you can get grainy textures. 

Just remember that by managing the solids, you’re managing the water.

Periodic Table of the Elements. The number int the top left of each box is that element's atomic mass (when a molecule is made of many atoms, we add the individual atomic masses together to find the Molecular Mass). Lower number = smaller molecule = more molecules at a given weight = greater freezing point depression.

*Colligative properties are based on concentration of dissolved solutes in the water, and on the size of their molecules, but not on any special chemical qualities of those solutes. Two equally concentrated solutions of two chemicals that have the same molecular weight will depress the freezing point of water equally. This is handy to know; it gives us just two values to consider when adjusting ice cream’s hardness.

**Molecular Whu?? The molecular weight (also called molecular mass) of substance is a number that tells us the size and mass of its individual molecules. Molecules are made of atoms, and atoms are made of protons, neutrons, and electrons. The protons and neutrons have equal mass, while the electrons are so tiny and insubstantial that we can ignore them. So the molecular weight is the total number of protons and neutrons. 

A high molecular weight means the individual molecules are relatively massive. This means that a gram of something with a high molecular weight contains fewer molecules than something with a low molecular weight. And this, in turn, is why substances with a low molecular weight are more powerful at depressing the freezing point: for a given mass, there are more individual molecules in dispersion in the water, exerting their colligative influence. 

Next post: infusing flavor (yes, we’re eventually getting to the good stuff)

Part 1 of this seriesIntroduction
Part 2 of this series: Components
Part 3 of this series: How to Build a Recipe
Part 4 of this series: Basic Recipe Examples
Part 5 of this series: Techniques
Part 6 of this series: Sugars
Part 7 of this series: Stabilizers
Part 8 of this series: Emulsifiers
Part 9 of this series: Booze
Part 10 of this series: Solids, Water, Ice


  1. Thanks for this Paul, and for all of the other blogposts. This blog, together with the comments people post, has been the single most useful resource in terms of home ice cream making for me.
    I recently had the opportunity to buy a well-reviewed compressor-style ice cream maker at a crazily low price (what would be $85!) and it's on it's way to me. I have a freezer bowl style at the moment, but frustrating to an experimenter like me because you have to wait at least a day in between attempts rather than beast it in an afternoon!

    Hence I've progressed slowly with my knowledge and attempts - adding some xanthan, LBG and a tablespoon of vodka to an egg base was as far as I'd got (although these were all big improvements). But as I already have some lecithin I'm ready to quit the eggs altogether - never enjoyed the laborious custard-making process, and if I swap out LBG for guar I can totally avoid having to heat (other than a little to help dissolution).

    I realise now after reading your blog that what I'd assumed was iceyness was actually chalkiness from too much fat. The recipes I've been using use way too much cream - I'm going to go for a more gelato style and use mostly milk I think. And switch in some invert/dextrose too.

    Cannot wait for this machine to arrive - it better work!

    Have you ever used tapioca starch or arrowroot as stabilisers/thickeners? I like the idea of the glossiness that the arrowroot might provide (saw it somewhere - could have even been this blog but I think it was somewhere else) and I have a big tub of an "instantised" tapioca starch as a versatile sauce thickener that might be worth a go...

    1. Hi Tom, thanks so much for writing. Nice score on the ice cream machine.

      I haven't used Tapioca starch, but it should work fine. Jeni Britton Bauer of Jeni's homemade uses it, and her ice creams are great. However, I believe she chooses it because it's "label friendly" ... it sounds like something your grandma might have used. It doesn't offer any functional advantages over the gums, and will probably not work as well, or offer as much versatility.

      I have tried arrowroot, and give it the big thumbs-down. It gave the ice cream an inedible, slimy texture. I'd read a warning about bad textures with arrowroot in dairy, but had to try for myself, since it's one of my favorite thickeners on the savory side of the kitchen.

      My experience was bad enough that I never felt inclined to try again. This isn't the final word—since then I've seen a recipe that included it, from someone I had no reason to believe was crazy. But at this point, I'm not interested in starches at all.

      Good luck!

  2. Hey Paul! THIS IS AN AMAZING BLOG! I'm actually starting up my own ice cream truck in Arkansas and am using your techniques on stabilization to make all of my flavors egg-free. What are the major differences between these "home" recipes and places like Oddfellows, Jeni's, etc.? It seems to me that even they could benefit from stabilizers. I'm lucky enough to use VAT pasteurized, grass-fed dairy with a great milk fat content. Yet, I still find using an average of 0.1%-0.15% LBG:Guar:Lambda mixture greatly increases my mouthfeel and scoop for the better! I can't wait for your next blog post!

    1. Hi Unknown, thanks so much for writing. I don't know anything about Oddfellows. I know a bit about Jeni's (Jeni herself was kind enough to answer some questions, and a few years ago her company site shared some details about their process).

      Jeni's does stabilize their ice cream. They use tapioca starch, almost certainly because it's label-friendly (those words sound more like food than like "chemicals"). But if the quality of your ice cream is the number one priority, you can do better than starches, by using gums like the one's you're already using.

      Where Jeni's gets interesting is emulsification. Their ice creams are completely egg-free, and they don't add any emulsifiers to make up for this. They get all the emulsifier action from the milk proteins.

      They do it by using a mix that's very high in milk solids. I don't know their process now, because they have their dairy do all the heavy lifting. But back when they made the ice cream in-house, they started with raw milk, and centrifuged it to separate it into cream and skim milk. Then they'd use reverse osmosis to concentrate the milk without adding any heat (you could also use vacuum evaporator). Then they would blend this concentrated, high-solids milk with the cream to get whatever fat level they wanted. They'd maximize the emulsifying power of the milk proteins by cooking at 75°C for an hour, in batches.

      Now I believe they use a continuous process, with higher temps and shorter times. Jeni didn't go into details. She did suggest that it would be very hard to get the results they get if you didn't start with raw milk—presumably because you'd have no control over the time/temperature of pasteurization, which is essential for getting the proteins to work as emulsifiers. But raw milk is challenging to handle safely—Jeni's actually had to shut down some years ago because they discovered some contamination.

      Anyway. If you don't have all the industrial-level toys, the best way to add milk solids it to use nonfat milk powder. If you get good stuff (100% skim milk, low-temperature spray-dried) there are no disadvantages.

      Then the trick is to replace the emulsifying power of the eggs. Cooking the proteins as I describe here should help. Adding an emulsifier should help. If you're making ice creams with low fat or high overrun, this can get tricky, but for more deluxe ice creams that have a fair amount of milk fat and not a lot of air, I think lecithin should work well.

      Let me know how it goes. If the lecithin doesn't work for you, I'd suggest calling tech support at TIC Gums or Kelco. They know a lot. If they help out, please share what they say.

      Good luck!