A physicist's guide to ice cream | Hacker News Reader

A physicist's guide to ice cream

physicsworld.com

122 pointssohkamyung60 comments

I started cooking ice cream this summer. I bought an ice cream machine that can yield 1L of ice cream in 30 minutes, and prepared about 30L, sometimes sourcing material from the garden (strawberries, raspberries, cherries, figs and blackberries). If you enjoy the taste of fresh figs (which I don't normally because of the texture of the fruit) it is an incredible way to preserve it.

The best ice cream I made was blueberry + coco cream + lime. Highly recommend it. Coco cream as a substitute for water in sorbet is an amazing trick.

Shared this with a friend yesterday:

    1. **Creaminess (smooth, creamy texture)**  
    Creaminess ↑ ⇔ Cream ↑ + Eggs ↑ + Sugar ↑  
    Creaminess ↓ ⇔ Water ↑ + Fresh fruits (high in water) ↑  
    
    2. **Lightness (airy texture)**  
    Lightness ↑ ⇔ Milk ↓ + Cream ↓ + Eggs ↓ + Sugar ↓  
    Lightness ↑ ⇔ Air incorporation (whipping) ↑  
    
    3. **Stability (avoiding crystallization)**  
    Stability ↑ ⇔ Sugar ↑ + Eggs (yolks) ↑ + Alcohol ↑  
    Stability ↓ ⇔ Water ↑ + Uneven freezing temperature ↑  
    
    4. **Density (weight in the mouth)**  
    Density ↑ ⇔ Fats (cream, butter) ↑ + Eggs ↑ + Sugar ↑  
    Density ↓ ⇔ Incorporated air ↑ + Water ↑  
    
    5. **Melting Temperature (softer or harder out of the freezer)**  
    Melting Temperature ↑ (softer) ⇔ Sugar ↑ + Alcohol ↑ + Air ↑  
    Melting Temperature ↓ (harder) ⇔ Water ↑ + Freezing temperature ↓  
    
    6. **Sweetness (perceived sugar)**  
    Sweetness ↑ ⇔ Sugar ↑ + Sweet fruit puree ↑  
    Sweetness ↓ ⇔ Acidity (lemon, vinegar) ↑ + Acidic fruits ↑  
    
    7. **Crystallization (grainy or smooth texture)**  
    Crystallization ↑ ⇔ Water ↑ + Poorly mixed ingredients ↑  
    Crystallization ↓ ⇔ Fats ↑ + Sugar ↑ + Proper agitation ↑

Have you tried incorporating herbs (like basil with strawberries)? Or you do not like that kind of things?

Yes, I have, and this is a bad idea (strawberry + mint). The problem is that the herbs become harder than the ice cream, creating an unpleasant sensation in the mouth. I had a similar issue with a toffee ice cream where I tried incorporating small cubes, but they became too hard when frozen.

One possible solution could be to candy the herbs (though it’s a lot of work) or to soften the toffee by turning it into a kind of cream, freezing it separately, and then folding it into the ice cream at the last moment. This challenge is addressed in the article,

>We need to add sugar to the fruit to make sure it is softer than the ice cream itself – you don’t want to bite into ice cream and find a hard, frozen berry.

and considering you almost never see artisanal ice creams with chunks (unlike industrial ones like Ben & Jerry’s, for example), this is where culinary engineering becomes essential I guess.

Perhaps if you grind the herbs very finely they'll disperse better in the ice cream

Indeed this is what I do with lemon zests. I put them in a blender with the juice for 10 minutes at maximum power (as a result, the mixture must become hot). Add two egg whites beaten into stiff peaks for 3/4L, and you'll get the softest, tastiest lemon ice cream you've ever eaten.

If one is too lazy to infuse, it’s nice to add homemade caramel or compote. Snacks like Lotus biscuits are excellent too.

Or infuse them in the milk/cream and then strain out?

If you don't care about having the bits for texture and only care about the taste you could infuse the cream. Heat the cream (stop before boiling of course), add the bits, let it rest for a while and then sieve it. I will add some time but it should work.

I'm also not a fan of ice cream with solid stuff. If you can't lick it pleasurably, it's not really ice cream in my book.

I would blend the herbs in rum, then strain and use the rum with simple syrup.

>The problem is that the herbs become harder than the ice cream, creating an unpleasant sensation in the mouth.

The obvious solution to this is to make an syrup infusion and use that instead of sugar or as a topper.

I make herb and floral syrup all the time. I do 1 cup water, 1 cup sugar, handful of herbs. Heat and simmer for around 10 minutes. Strain.

I see what you mean about the herbs and the texture issue...

Thank you very much for this! I remember eating a very "cheap" icecream as a kid, that was more "ice" or "frozen" than the creamy (expensive) goop I find everywhere these days. I think it's down to a combination of less creaminess, and more crystallization (to follow your list). But this kind of concise guide or explanation really let's me try to recreate that texture, as I don't think it's something that anyone would "sell" traditionally.

The ice-cream was "cheap" I think because it was made in a "failing" country that still had a relatively functioning dairy and manufacturing chain, so they were doing their best to still make icecream whilst dealing with the loss of key ingredients and dwindling margins stemming from the economic situation.

that would be a sorbet?

Might be? But I've only ever had fruity or flavored sorbets. So maybe I just need to find one that's leaning more on the plain/traditional ice cream flavors like vanilla and chocolate.

The worst offender is when the ice-cream feels "airy". Icy is nice for fruity bases, while not great for creams.

Airy ice cream is also lighter. Beware pints that weigh less than expected. At a minimum, it indicates a ton of air whipped into the mix. But it's also a decent proxy for low-quality ingredients or garbage fillers.

In the grocery store / mass market context, it's hard to do much better than Häagen-Dazs. Just be aware that their "pints" are now only 88% of a pint (this baffles me -- are the ingredients really a meaningful contributor to total COGS?).

Well, for Haagen-Dazs, perhaps yes. It’s decent stuff because it’s mostly cream and eggs. Also, smaller packs are not just about COGS: you’ll probably buy more or buy again sooner.

I know this is a simplified version but sugar is not detailed enough (different types of sugar, like dextrose), also I see no mention of carob seeds, also very important to get a creamy texture.

Indeed, I tried glucose to get a sligthly smoother texture, but it's a bit hard to source and pricey. I take note of carob seeds.

The "ice cream parameter space" above was generated by ChatGPT and I've been interested in building a recipe website where that kind of representation would be generated and later improved by users. I think this could prove to be quite unique.

I think dextrose or also called "grape sugar" is very commonly used in Italy to get decrease the freezing point without feeling o overly sweet.

The sweetness intensity is different across different types of sugar.

kind of weird question is it cook? does one cook ice cream? always assumed cook implied fire or some sort of heat source

The cheap answer is "yes" -- see custard ice creams as an example.

Typically though, "cooking" requires heat, and most non-custard ice creams wouldn't be cooked. Even in the same phrase when talking about cooking other things, you'd say something like "I'm cooking steak and also making ice cream."

I'm sure some regions use the word differently, and adding a bit of ambiguous context might make on-the-fence regions more likely to use "cook" for cold-prepared dishes. E.g., if you talk about cooking dinner or cooking a meal and then list the things being prepared (one of which is ice cream), the verb "cook" is, in some sense, being applied to the ice cream. That ambiguity can color the verbs you subsequently choose when referring to just the ice cream.

oh cool to know always learning something new about English in my country we don't have ice cream but we have words that loosely translate to make or cook food but none of them feel ergnomic with ice cream

> does one cook ice cream?

Yes. Many recipes require heating of the ingredients.

Here is one example picked randomly: https://www.bbcgoodfood.com/recipes/ultimate-vanilla-ice-cre...

"Step 4: Return the pan to a low heat and cook, stirring all the time with a wooden spoon, for 8-10 minutes, until the custard is thick enough to coat the back of the spoon."

Yes, you typically cook the custard before freezing it.

You can also make ice cream what's called "Philadelphia-Style" which doesn't have eggs and requires no cooking before churning/freezing.

But the custard method is more typical.

Short of constantly stirring a pot full of the ice cream custard, do you have any solutions for getting the mixture up to temperature without scalding the milk/cream?

ChatGPT suggest an immersion heater. Maybe add a pump or a magnetic stirrer.

I received two downvotes for this comment, presumably because of the ChatGPT mention and the "weird" recommendation that triggered this knee-jerk anti-hype reaction.

https://gourmantissimes.com/creme-anglaise-sous-vide-inratab...

https://www.milord-sous-vide.com/categorie/cuisson-sous-vide...

GP could have asked how to guarantee an even sugar density when the sugar content of the fruits is unknown/subject to variation, and I would have replied: "ChatGPT recommends using a refractometer or an areometer". Pretty sure I'd have received the same reaction, even though this is the very first subject Escoffier talks about in his well-known Culinary Guide:

https://gallica.bnf.fr/ark:/12148/bpt6k65768837/f753.item

https://www.deco-relief.fr/fr/accueil/1518-refractometre-suc...

Was one of these supposed to be Lightness ↑ and the other ↓?

The best ice cream recipe ever I got from a physicist:

Cream + sugar + vanilla, stir until dissolved. Slowly add liquid nitrogen while stirring fervently until sufficiently frozen.

The stirring in of the liquid nitrogen not only freezes the cream, it prevents any cristallisation and fluffs up the ice cream by evaporating. Truly delicious!

I can confirm that this works and is delicious.

if you have LN2, you can add high spirits as well - bitter/sweet liquors are a classic in that combo.

You can add too much liquor and it will still freeze nicely. Spiced rum was pretty good.

How much of each?

Just look up any ice cream recipe. Doesn't really matter.

I've used 1l of plain cream, 100g of sugar and the marrow of 1 1/2 vanilla pods.

Add sugar to taste, while liquid. IMO you can never add too much vanilla.

The fat content seems too high. Don't you get a film of fat covering the top of you mouth?

Depends on the cream you use. But basically it doesn't really matter, you can also use milk, even skimmed milk. Any liquid icecream-mix works, only chunky stuff is problematic.

Where do you get liquid nitrogen as a home cook? I’ve tried to get my hands on both that and dry ice without any luck

I know in upstate New York the only option is to hope there's a gas supplier‡ relatively close or try ordering it online.

‡: like this: https://noblegassolutions.com/

One time in grad school we used liquid argon that was going to boil off. Since argon is heavier than Alex the ice cream had a very interesting bubbly consistency.

> argon is heavier than Alex ..

Al(ex)chemy is so counter intuitive and suprising!

Hah. (Supposed to be air, if not obvious. DYAC).

Call me slow but I still do not get the "Alex". Alchemy?

Pretty much, it was weak at best, cozzyd seemed amused which was the intent, we all make mistakes and typo'ing Alex rather than air was worth a mild poke.

I started making my grandmother's recipe again recently and it's amazing and amazingly simple. A pint of cream, a tin of condensed milk and a teaspoon of vanilla extract. Whip the cream until it lightly peaks, stir in the condensed milk and the vanilla and freeze for at least 4 hours. Done. You can add choc chips or whatever flavor you want at the condensed milk stage.

Last summer I experimented making ice cream without ice cream machine. The principle I used was to freeze it in plastic bags, take them out once or twice to "massage them" and then to squeeze them out into a frozen bowl where they rest in the freezer until eating time.

The recipes are important too of course. The sugars need to be a mixture of fructose and dextrose. Also used a stabilising agent. But eventually I could create really good ice cream with a minimum of equipment.

The basic Ben& Jerry's recipe in an ice cream machine is pretty much unbeatable if you're looking for traditional ice cream. On the other hand, I've tried, without much luck, lower calorie ice creams using substitute sugars like allulose. Unfortunately, they haven't turned out very well. It really is the combination of real sugar, fat, protein, and all that that comes together in the cold to make the magic happen.

In my experience after a decade of trial and error, inulin fiber sweetened with stevia and monk fruit[1] is pretty much a perfect sugar substitute. If anything, the texture might be a little better than sugar.

Inulin is slightly tricky to work with because it clumps up easily when it comes into contact with liquid (likely for the same reason, when baking with it I prefer ghee to butter), but it mixes into the heated custard base on the stove without too much trouble as long as you stir well while pouring it in.

Another tip that's worked well for me: instead of milk or half-and-half, I use a 50:50 ratio of heavy cream to egg white + coconut water. In other words, I use whole eggs and then add an amount of coconut water that matches the difference between volumes of heavy cream and egg white. I don't recall offhand how the math on that works out (it's somewhere in an old ChatGPT log that I've been meaning to properly document), but it's a lot easier than separating yolks and it gets a great result with less sugar than milk or half-and-half.

1: https://www.lowcarbfoods.com/low-carb-white-sugar-sweetener-...

I wonder if that's at all related to different sugars configuration at different temperatures as described by AppliedScience [0].

Maybe there are other substitute sugars out there which work well at low temperatures?

0: https://youtu.be/kiuXasyB3L0?si=b_-6iNw1TD35alG4

My local supermarket only sells the basic flavors vanilla, strawberry and chocolate. But then they sell dozens of variants of Ben and Jerry's and similar brands. I wish this was the other way around. Just give me boxes of single flavors like in the Italian gelaterias.

I guess supermarkets get higher margins on B&J's :(

I completely agree. It's difficult to get quality vanilla ice cream except at a gelataria or fancy market shop. Reading this thread however makes me brave enough to try making my own once summer appears.

I'll probably go for something inconvenient so I need to pre-freeze the bowl and thus avoid an unhealthy constant flow of ice cream, but tech these days seems pretty good to make homemade gelato:

https://www.seriouseats.com/the-best-ice-cream-makers

Costco’s “super premium vanilla” is quite nice—-better than any other mass market vanilla.

my view of ice cream has changed over time. I grew up close to the biggest factory for ice cream in the UK where it was adjacent to the biggest bacon factory in the UK.

The myth at the time was that the fat from the bacon went next door to the ice cream factory. At the time the label just said 'animal fats' rather than 'dairy' so there was nothing to dispel the myth.

Since then the formulation has changed, now you get the usual palm oil. As far as ultra-processed foods go, is there anything worse than commercial ice cream? You have got animal fats or palm oil to block your arteries, refined sugar to spike your glucose and lots of emulsifiers and other 'e numbers' to disrupt your digestive tract?

Most ice cream is sold in tourist venues where the vendor knows he will never see the customer ever again. There is zero motivation to make it a healthy product or to use quality ingredients when you are not expecting repeat customers.

In other developments, there is a move to have more efficient freezers for ice cream in stores, which means changing the temperature so it does not need to be stored at such a low temperature. The likes of Unilever can spend a fortune on the food science to get the temperature up, to roll out new freezers.

> There is zero motivation to make it a healthy product

Heh. I mean, uh, oh no!

While clearing out an old tub of ice cream I noticed that industrial ice cream doesn't melt in the same way as 'real' ice cream. It holds its shape well beyond what you'd expect. Left at room temperature real ice cream puddles, industrial ice cream slumps. There's emulsifiers and stabilizers that keep the consistency for a larger range of temperatures, so that's it's soft to scoop at -18C whereas real ice cream is as hard as concrete. The ingredients list includes things such as whey powder rather than milk. Like so many ultra processed foods it's just cheap powdered ultra processed ingredients held together and flavoured by additives.

I have seen scoops of cheap ice cream, accidentally dropped onto pavement on a hot sunny day in the Southern US, maintain their scoop-like shape for hours.

Cheap ice cream is not ice cream. Do not eat.

>Cheap ice cream is not ice cream

Most of the UK's "ice cream" would be called Frozen Dairy Dessert in the US.

In the United States, cheap ice cream is quite literally not ice cream. It's "frozen dairy dessert".

> Since then the formulation has changed, now you get the usual palm oil. As far as ultra-processed foods go, is there anything worse than commercial ice cream?

The ice cream that Walmart sold from its own personal line was messed up. It did not melt as fast as other ice cream and was oily.

I'd rather eat no ice cream than eat that.

History of Ice Cream is starting to sound realer by the day

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