Scanning electron micrograph of ice crystals |
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.
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.
*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 Wuh?? 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.
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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...
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I see that you're a photographer, but you don't post pictures of the ice cream you've made! I'd be very interested in seeing your results.
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You mentioned a post about sorbets!
You also mentioned somewhere about a chocolate sorbet you are making.
Any plans on posting the post and/or the recipe?
I really love the work you 've done here.
Thanks.
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Do you have any pointers for dairy-free icecream? I'd like a starting point if you have any experience in this area.
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I've been working through your posts and have to say, this is AWESOME! Thanks for putting this together. But, truth be told, I was never great at chemistry, and am having a hard time applying the information you are giving us into my recipes.
I've been working on a recipe for a frozen nonfat Greek yogurt desert that I add citrus flavored fish oil to. Sorry if you just threw up a little, but hear me out. I'm trying to develop a "healthy" desert. I love ice cream, but I don't like what it does to me, so I said, "Hey. Why can't I make it healthier?" So, here's the recipe I've come up with with weights in grams. Oh yeah, I also keep it at a 1:1:1 ratio of calories from carbs, protein, and fats. That's the portion control mechanism I use:
Nonfat Greek Yogurt 684.9285171
MPC 85 684.9285171
Maltodextrin 114.1547529
Honey 296.8023574
Orafti P95 114.1547529
Orange Juice Concentrate 228.3095057
Citrus Flavored Cod Liver Oil 308.2178327
Skim Milk 570.7737643
Guar Gum 10
Locust Bean Gum 10.00
I'll add flavoring to make it more like a sherbet once I get the texture down.
I heat it to 165, cool it in the fridge, and then turn it into sherbet in a condenser style ice cream maker.
It still gets real hard after freezing it in 16oz deli containers to the point that I have to let it sit out for 45 minutes.
The main health benefit from this from the fish oil. Its a tasty way to incorporate it into my diet.
What can I do to enhance the texture while keeping the 1:1:1?
Thanks,
Wally
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Thank you for an insight on ice-cream.
I have been trying to make fruit paletas with 1000ml water, 400 GM sugar n 4 GM's of stabilizer (407,410,412,415,466,471)
My end product has a glue like feel on the outside.
Can you tell me as to where am I going wrong.
Have been going crazy with experimenting. But no success yet.
Looking forward to a solution
Tia
Love from india
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And I have some experience with sorbets, which I'll write about eventually.
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300g milk
100g cream
85g non fat milk powder
100g buttermilk
200g raspberry puree
10g freeze dried raspberry
75g sugar
40g dextrose
20g trimoline
2g lecithin
0.8g locust
0.6g guar
0.4g lambda
0.7g salt
1g citric acid
So basically I lowered the amount of cream and milk and add raspberry puree and a little bit of buttermilk/freeze dried raspberry/citric acid to brighten up the flavors. I also add a little bit of sugars because my freezer temperature is around -21c, so the ice cream would not be so hard..
But anyway, do you have any experience about berry/fruit ice creams without egg yolk? Should I add more trimoline, milk powder or stabilizer to compense the added amount of raspberry puree? (raspberry puree = lot of water, not a good thing!) Thanks!
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