Tuesday, May 31, 2016

Ice Cream Stabilizers

Shunned and embraced, demonized and defended, shouted and mispronounced, these ingredients are the most widely missunderstood, proving equally befuddling to ice cream lovers on all sides of the never-ending, stupid lively arguments. I’m hoping to shed some light here.

What Are They?

Stabilizers are any ingredients used to thicken the water in ice cream. They make ice cream smoother, by slowing the growth of ice crystals. And they can improve the texture generally, by adjusting the body, the speed of melt, and the finish.

Technically, they are hydrocolloids — suspensions of fine particles in water that form a network, increasing the water's viscosity, and in some cases forming a gel. When you thicken gravy with flour, make pudding with cornstarch, or mix Jell-o, you’re making hydrocolloids. Which is to say: you’ve probably been using stabilizers for a long time. 

Properly stabilized ice cream not only resists developing an icy texture over time, but will actually form smaller ice crystals to begin with.1 Improperly stabilized ice cream (like improperly cooked food) can indeed be awful. We'll be discussing how to use these ingredients well, with the belief that in most cases, it's possible to make a better ice cream with stabilizers than without.

Stabilizer Examples:

Egg Custard. Custard is indeed a stabilizer. It’s among the most effective at creating great textures, but only middling at slowing ice crystal growth. This is likely because custards exhibit synerisis, or weeping: they let water seep out of their gelatinous structure. The smoothest egg custard ice creams use additional stabilizers. Custard's other drawback is that it damps the release of flavors more than most other stabilizers—especially lighter and more aromatic flavors, and water-soluble flavors (fruits, etc.). 

Custard viscosity varies with the concentration of yolks and cooking temperature. For thickening and stabilization, ice cream requires at least 3% egg yolk by weight, which is between 1.5 and 2 egg yolks per liter. 4 to 6 yolks per liter is more common, and some people go higher, making a dessert that's more a frozen custard than an ice cream, with egg flavors and textures dominating.

Cooked egg flavor, like viscosity, increases with concentrationand cooking temperature, and possibly also with cooking time. With yolk concentrations below 4%, eggy hydrogen sulfide compounds will probably be undetectable except after extreme cooking. For more egg-rich recipes, it may be beneficial to keep the cooking temperature between 70°C and 72°C for longer cooking, or below 82°C for shorter cooking.

Refined Starches, like cornstarch and tapioca, have been used in ice creams for a long time. Cornstarch is most famously used in Southern Italian gelatos, which are often made without eggs or even cream. It’s become popular in many home recipes because it’s easy to find and easy to use, and gives reasonably good texture and stability. Starches generally give better flavor release than custard, but not as good gums.

I’ve seen some recipes that use arrowroot starch, which is possibly the best of the refined starches for savory applications, but should be avoided in ice cream. Arrowroot reacts with dairy ingredients to create unpleasant, snot-like textures.

Gelatin is the oldest known non-egg ice cream stabilizer. And it’s a very good one. Its ice crystal suppression and texture make it arguably superior to starches, and it’s equally easy to use. Gelatin has fallen out of favor partly because of cost, and partly because it’s an animal product. Even non-vegetarians are occasionally skeeved by knowing their ice cream contains rendered beef and pork tissue. But I would still encourage experimentation with gelatin, if you want to play with stabilizers but aren’t ready yet to buy a whole arsenal of hydrocolloids from the molecular ingredient sites.


These are the mack-daddy stabilizer ingredients. They work in minute quantities, have superior powers of ice crystal suppression, offer almost infinite textural possibilities, have no detectable flavor of their own, give superior flavor release (they don’t mute the flavors of the ice cream), and can be almost endlessly confusing.

We’re going to look at a small selection of gums individually, although much of the strength of gums comes from their synergies; they work best in combinations. The synergies are often such that the gums reinforce one another, and offer capabilities in combination that they did not offer individually. 1 + 1 = 3, etc.

The challenge in creating a gum blend lies is balancing the qualities of the individual gums with each other, as well as with the rest of the recipe, while taking into account the various synergies between those individual gums.

Gums are all polysaccharides—big molecules made up of lots of small sugar molecules linked together. They are close cousin to starches ... you can think of them as superstarches.

Locust bean pods

Locust Bean Gum, also called carrob bean gum, is made by milling the seeds of the locust tree. It’s been used as a thickener since at least 79 AD, and possibly much longer. LBG has the most powerful ice crystal suppression of all the conventional gums. It favors a smooth, creamy, natural texture that does not draw attention to itself. It manages this by forming a weak gel that is stable while frozen, but that is highly shear-thinning, so once the ice cream melts and starts moving, most of the added viscosity vanishes. These characteristics make it the most important of the gums in ice cream.

LBG needs to be heated to hydrate. Most varieties require heating above 80°C, which is higher than ideal for many ice cream processes. Varieties sold by TIC gums and Willpowder hydrate at much lower temperatures.

Guar beans

Guar Gum is milled from the seeds of the guar plant, which is a legume. Guar gum is chemically similar to locust bean gum, and the two are often used together. Guar is not quite as effective as LBG at ice crystal suppression, but gives greater viscosity than LBG at similar concentrations. In combination, guar and LBG strengthen each other—when using the two together, you can use a lower total quantity of gums.

Guar gum has only been in use since the 1950s, but the guar bean has been cultivated in India as a protein source for hundreds of years.

Guar’s main use, besides strengthening the effect of LBG, is to add body. In high concentrations, it can make ice cream that’s chewy and elastic—either a flaw or a benefit, depending on your point of view. In New England, they like a lot of guar.

Irish Moss / Carrageenan
Carrageenans are extracts from Irish Moss seaweed (chondrus cripus). They’ve been used as food thickeners since the 15th century. Modern versions of carraggeenan are divided into types based on the details of their molecular structure. The most common in the culinary world are Kappa, Iota, and Lambda. These types have different solubility temperatures, gelling characteristics, and interractions.

The most useful type in high quality ice cream is Lambda Carageenan, since the others form gels in the presence of calcium (dairy products). See note on gels, below).

Carrageenan has a moderate effect on ice crystal suppression, and a strong effect on texture, especially of the melted ice cream. Carageenan creates a rich and creamy mouthfeel similar to egg custard, but does so without adding any flavor of its own, and without muting other flavors. 

Carrageenan’s secondary role is to prevent wheying-off, a phenomenon of milk proteins preciptating out of suspension, aggregating, and creating grainy textures. Locust bean gum and carboxymethylcellulose can induce whey-off, so when these stabilizers are used you’ll usually see at least a minute amount of carrageenan.

Giant kelp / brown algae. Where we get the alginate.
Sodium Alginate is another seaweed extract, made from a brown seaweed grown in cold water areas. It’s a popular stabilizer, especially in low-fat and fat-free ice creams, because it forms a gel in the presence of calcium ions in the dairy. Its gelling quality makes it less useful in standard recipes (see the note on gels, below). The gel breaks into a fluid gel when the ice cream is spun, creating a unique body and viscosity. It’s quite effective at ice crystal suppression.

Sodium Carboxymethyl Cellulose: a big-ass molecule

Carboxymethylcellulos, also called cellulose gum, technically called sodium carboxymethylcellulose, is synthesized from plant cellulose. It may have the strongest ice crystal suppression of any known gum. It adds body and chew comparable to guar, and is synergistic with locust bean gum, guar, and carrageenans—it forms a gel in combination with these ingredients, with can be problematic (see note on gels, below).

There are low-viscosity varieties of CMC that suppress ice crystal formation with very little increase in base viscosity, if they're used in a non-gelling blend. These theoretically allow you to control iciness and texture completely independently. Examples include TIC Gums CMC PH-15.

CMC is not popular in higher quality ice creams, because it is a synthetic ingredient. While the word “natural” is rather ambiguous, CMC lies outside most interpretations of natural. This is perhaps more a marketing issue than a real one—there are no health concerns associated with the stuff. It's just a big polysaccharide like the gums that come from ground up seeds. Nevertheless, I’ve only experimented with it once and was dissuaded by its gel-forming with other gums.

You can get xanthan at the supermarket these days.

Xanthan Gum is created by bacterial action, when the organism Xanthomonas campestris chomps on glucose, lactose, or table sugar. It’s a fermentation product, much like cheese and booze. 

Xanthan is often called the “wonder gum,” because it’s easy to dissolve at any temperature, it thickens at any temperature, works at a wide range of acidities, can tolerate alcohol, freezing, thawing, and just about anything else.

While most gum recipes require a scale that reads to 0.01g and careful dispersion and heating, cooks find xanthan pretty friendly in an old-fashioned-ingredient way. Need to stabilize a vinnaigrette? Toss in a pinch of xanthan and whisk until it’s dispersed. Want to add a bit of body to a sauce? Make a slurry with a pinch of xanthan, and whisk in just as you would with cornstarch or arrowroot. 

Xanthan is not, however, my first choice in ice cream stabilizers. It does an acceptable job, but is not the most powerful ice crystal suppressor. And it forms a gel when used with locust bean gum. This makes the mix harder to handle. See note on gels, below. I do recommend xanthan for anyone interested in starting experiments with stabilizers. You can get it anywhere now, and it’s worth having around for a million other uses.

Dondurma vendor offering a cold chew.
Salep, Mastic, Gum Arabic, and Konjac Flour are specialty stabilizers used in Dondurma, a traditional taffy-like ice cream popular in Turkey and Azerbaijan. Also called Maraş, this ice cream is both chewy and resistant to melting. Salep (flour made from the root of the Early Purple Orchid), and Mastic (hardened sap the Mastic Tree) are traditional. Gum arabic (hardened sap of the Acacia Tree) and Japanese konjac flour (starch from the Konjac, aka Elephant Yam) are more readily available substitutes.  A combination of gellan gum (a microbial gum like xanthan, which forms gels) and guar can also substitute

Denatured Whey Proteins: Some of the whey proteins, which make up 20% to 25% of the total protein in milk and cream, can form a gel-like network when heated to the right temperature for the right amount of time. This network functions in the same way as the other hydrocolloids discussed here. Whey proteins may also be denatured chemically or enzymatically, using processes that are probably out of reach in the home and pastry kitchen. We don't know what they do at Haagen Dazs, but they're doing something. See extended discussion in the post on Emulsifiers.

Blends and Variations

Blend 1: Easy-to-find ingredients

Gelatin : Xanthan Gum

3 : 1

1g gelatin 0.33g xanthan For 1 liter of ice cream (0.15% total)

Gelatin hydrates when cooked to 60°C / 140°F. Any standard cooking step will take care of this.

Both the gelatin and the xanthan suppress ice crystals and increase the viscosity of the mix. 

The gelatin forms a weak gel that melts at body temperature and strengthens in the cold, so its effect is most pronounced on the ice cream in the frozen state. The xanthan gum’s activity is almost completely independent of temperature. So its effect is most pronounced on the ice cream in its melted state. So if you want more body, increase the proportion of the gelatin. If you want a creamier melt, increase the proportion of xanthan.

You can experiment freely, but be warned that at much higher concentrations, xanthan’s mouthfeel goes from creamy to slimy. If you’re not getting the results you want from this blend at modest concentrations, you should move on to the other gums.

Blend 2: General Purpose

Locust Bean Gum : Guar Gum : Lambda Carrageenan

4: 2 : 1

0.8g  0.4g  0.2g  for 1L  (0.15% total)

Mix should be cooked at least to the hydration temperature of the locust bean gum. TIC Gums versions hydrate at 74°C / 165°F—most brands hydrate at temperatures higher than what’s ideal for most ice creams. 

All three gums suppress ice crystals and affect texture, but not equally.

The Locust Bean Gum is the most powerful at suppressing ice crystals. It has a subtle effect on increasing the body of the ice cream and the creaminess of the melt.

The Guar amplifies the power of the locust bean gum, and has the strongest effect on the body of the frozen ice cream. Significantly increasing the guar will make the ice cream chewy and elastic.

The Lambda Carrageenan has the strongest effect on the consistency of the melted ice cream. Its mouthfeel is similar to that of custard, although it has a somewhat cleaner finish. If the melt feels too milky or watery, you can subtly enrich it with a bit more LCG.

I use this blend at 0.15% in a 15% milk fat ice cream that uses 2 yolks per liter. For a richer, more custardy mix, you could experiment with using as little as 0.1%. For a lighter ice cream, or one that needs a long shelf life, you could try 0.25%

Blend 3: Eggless Ice Cream

Soy Lecithin: Locust Bean Gum : Guar Gum : Lambda Carrageenan

10: 4: 2: 1

3g 1.2g  0.6g  0.3g  for 1L  (0.7% total)

Mix should be cooked at least to the hydration temperature of the locust bean gum. TIC Gums versions hydrate at 74°C / 165°F—most brands hydrate at temperatures higher than what’s ideal for most ice creams. 
This is the same as the standard formula, but increased 50%, and with soy lecithin added. Egg custard has thickening and stabilizing benefits, so its elimination requires a higher concentration of gums. The eggs also act as emulsfiers (see the next post).

The lecithin content of this blend is equal to two large egg yolks. You could use less—as little as 1/5 this much. I specify this quantity because egg-free bases are most useful when using ingredients that add a lot of fat—like chocolate. In these cases, some extra emulsifier can help ensure a smooth texture.

All the notes for manipulating the standard formula apply here. Be careful if increasing the lecithin. You probably don’t have to. If you go too far, it will actually impede the whipping of the ice cream.

Blend 4: Sorbet

Locust Bean Gum : Guar Gum : Iota Carrageenan : Lambda Carrageenan

4: 2: 2: 1

1.33g  0.66g  0.66   0.33g  for 1L  (0.3% total)

This should be blended into the syrup portion of the sorbet, and brought to a simmer. At this point, flavors can be infused into the syrup (sugar syrup is a powerful solvent for both polar and non-polar flavor molecules). Syrup should be chilled for several hours to allow gums to fully hydrate. Then it can be mixed with chilled fruit puree and frozen into sorbet.

This is similar to the standard formula, but with iota carrageenan added, and total gum quantity doubled. Sorbets have no cream, and are low on solids. They have little to no inherent creaminess, and a lot of water to stabliize. The iota carrageenan forms a weak gel in combination with the locust bean gum, adding viscosity and body. The gel breaks easily into a fluid gel under shear and then re-forms. It's freeze-stable and helps give a creamy texture.

Most sorbets have no fat content and so have no need for emulsifiers. If you wish to make a sorbet with chocolate, nut butters, olive oil or other fatty ingredients, you may get smoother results by adding some lecithin (maybe start with 1.5g / Liter).

We'll look at all this in greater depth in a future post on sorbets.

Notes on Using Gums

Three aspects of gums demand attention: measuring, dispersing, and hydrating.

They can be tricky to measure for small batches because the quantities are minute. You should have, in addition to a higher capacity scale, a small scale that reads to 0.01g. There are many available on Amazon and Old Will Knott Scales for under $40. 

I use a small cup or bowl, and use it to weigh all the ingredients that total less than a few grams. This typically includes the stabilizers and the salt. After the ingredients are measured out, I stir them together, and then thoroughly mix them into to the other dry ingredients (sugars, dry milk, etc.). This helps with dispersion:

Dispersion means mixing the dry ingredients into the wet ingredients evenly, without clumping. Gums make this tricky. So does milk powder. The first defense against clumps is to thoroughly stir all the powdered ingredients together. Locust bean gum will only clump with locust bean gum; dry milk will only clump with dry milk, etc.. So if you mix all the powders together, including the sugar (which makes up a lot of bulk and doesn’t clump at all) you can keep the individual powders separate long enough for them to disperse without clumping.

The second defense is proper use of a blender. Adjustable speeds are handy for this: start on the lowest speed that makes a vortex down the center of the blender jar, and pour in the dry ingredients. Once they’re in, crank the speed all the way and blend for a minute or so. Thorough dispersion is required for thorough hydration:

Hydration, of course, means soaking up water. Gums don’t do anything until they’re hydrated, and many of them need time, or heat, or both. Lambda carrageenan hydrates fairly quickly at room temperature. Guar hydrates at room temperature but can take over an hour to reach full viscosity. Locust bean gum needs to be heated, to 74°C–85°C, depending on brand. It may take many minutes at temperature to hydrate fully. 

Some pastry chefs, including Francisco Migoya, save time by mixing a large batch of stabilizer. Then it can be stored and measured out as a single ingredient, like a commercial blend. This approach demands throrough mixing. It makes the most sense if you use the same stabilizer blend most of the time.

If you experiment, pay special attention to the finish—the flavors and textures left behind in your mouth after swallowing. A successful stabilizer blend won't be detectable. The ice cream flavors should linger and continue to develop, but shouldn't be indelible. The feeling of creaminess should gradually dissipate. It should not devolve into pastiness or stickiness. These kinds of textural flaws point to over-stabilization, or to poor choices in stabilizing ingredients. I like the locust-guar-lambda blend as much for its transparency as for its effectiveness.

Note on Gels

Some common stabilizer ingredients form a gel in ice cream. Examples include Sodium Alginate and Kappa Carrageenan (which gel in the presence of the dairy's calcium). Other ingredients form gels in combination with each other. Examples include xanthan gum with locust bean gum, locust bean gum with kappa or iota carrageenan, and carboxymethylcellose with locust bean gum, guar gum, or any carrageenans.

Gels are solids that exhibit properties of a liquid. Technically they are colloidal dispersions in which the solid forms the continuous phase, while the liquid (we're always talking about water in the ice cream world) forms the dispersed phase. The solids create a network, with either physical or chemical bonds, and typically work in very small quantities—often less than 1% the weight of the water.

Gels can be strong or weak, yielding or elastic, brittle or tough, high or low viscosity. Under shear, some gels exhibit brittleness and crumble (like flan), others stretch and bounce back (like gel-o), others deform (like clay), others form a fluid gel that after sheer reforms into a gel (like iota carrageenan), others form a fluid gel that after sheer stays fluid (like agar).

To gel or no to gel? For most ice creams, I prefer  non-gelling stabilizers. They tend to have a less intrusive texture, and to work more predictably, and to be easier to handle. Often with gelling stabilizers, the mix will be too thick after aging to spin efficiently in the ice cream machine. It will have to be thinned with a blender first, turning it into a fluid gel. If possible, I like to avoid this added step.

If you're making low-fat or fat-free ice creams, or sorbets, gelling stabilizers become useful. They can add body and creaminess that's often lacking in these recipes. 

Closing Thoughts

I hope this post has made a case for the usefulness of stabilizers—of custard, at least, but preferably something with a bit more effectiveness and flexibility. The questions should be about how deeply involved you want to get.

Commercial blends are easy to find and easy to use. They go into many of the world’s best ice creams every day. Pastry chefs who know more than i do—about basically everything—rarely have a clue about the level of hydrocolloid micromanagement we’re discussing here. So why bother rolling your ownI?

Well, you’ve read this far, so you’re probably a geek who likes to tinker under the hood of the world generally. And you may appreciate the level of control afforded by the DIY approach. Blending your own stabilizer is like the next step beyond using curry powder from the supermarket—discovering that there are a dozen spices in there, that they can be varied by proportion and by how they’re added to the dish. 

I have another motive, inspired by my days as a darkroom-obsessed photographer. In any technological medium, your creative process can become dependent on proprietary manufactured products. And when these products are discontinued, or “improved,” you’re screwed, at least until you scramble to figure out an alternative. I learned the hard way the benefits of mixing my own formulas from generic ingredients, after watching some favorite photographic papers and developers vanish. I gradually weened myself from proprietary stuff whenever possible. The kitchen's no different. A company like Cuisine-tech could stop making Cremodan tomorrow—but someone’s always going to be selling plain old locust bean gum.

In the end, stabilizers are just ingredients like any other. If they seem daunting, it’s only because textbooks and cookbooks and culinary websites haven’t addressed the topic adequately. Until they do, I hope the information here proves useful.

Next post: some equally breezy gossip about emulsifiers.


Ingredient sources

Kalustyans (In NYC ... better to visit than to suffer their web store)

For further reading

Ice Cream, 7th Edition, H. Douglas Goff, Richard. W. Hartel. p. 75–82
Stabilizer Blends and their importance in Ice cream Industry 

Ask the experts

TIC Gum Gurus: (800) 899-3953
CP Kelco support: 1 (800) 535-2687

(The tech support people at both companies, when I've called, have been exceptionally friendly women, completely over-qualified—probably chemistry PhDs,—and have seemed happy to talk ice cream. If you're nice they may send you product samples.)

1"Microscopic investigation revealed that stabilized ice cream (locust bean gum and carrageenan) had significantly smaller mean ice crystal diameters both initially and as a result of heat shock and storage (24 weeks) compared to those of ice cream without stabilizer. However, the differences grew larger over time."
—Caldwell, K. B.; Goff, H. D.; and Stanley, D. W. (1992) "A Low-Temperature Scanning Electron Microscopy Study of Ice Cream. II. Influence of Selected Ingredients and Processes," Food Structure: Vol. 11: No. 1, Article 2.
Available at: digitalcommons.usu.edu/foodmicrostructure/vol11/ iss1/2

Part 1 of this series: Introduction
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
Part 11 of this series: Introduction to Flavor
Part 12 of this series: Ice Cream Flavor: Coffee

Monday, May 30, 2016

Sugars in Ice Cream


They’re sweet, and they keep the ice cream soft. If you’ve had homemade ice cream with the consistency of concrete, it’s because the level of solids—especially sugars—was too low. 

Some bloggers and cookbook authors tell you to soften the ice cream by adding alcohol. This works, but you can do better. While alcohol depresses the freezing point, it does so at the expense of smoothness. By increasing the unfrozen portion of water in the ice cream, while doing nothing to help control that water, it will encourage ice crystals to grow larger. You’ll end up with a softer but grainier texture. 

Personal narrative and manifesto: 

The quick fix would be to add more sugar, but most ice creams are too sweet already. A typical home recipe is 17% or more table sugar by weight: kid stuff. You cannot taste any subtlety through cloying sweetness—you can’t taste the dairy, and you can’t taste any of the more delicate, aromatic flavors we’re going to work so hard to put in there. 

It’s not just home recipes. Haagen Dazs is too sweet. Ben and Jerry’s is too sweet. Talenti is too sweet. Cold Stone is too sweet. Every small town “homemade” ice cream shop I’ve ever wandered into: too fricking sweet. 

I once managed an ice cream shop in Colorado, making ice cream that the owners and I were proud of. It was too goddam sweet, of course, but I had no reasonable frame of reference, until after I’d quit and taken a trip to Paris, where I was lucky enough to be invited to dinner at Taillevent—a restaurant which at the time had three Michelin stars and which had once been considered the finest in the city. After uncountable savory courses, we were put in the hands of pastry chef Gilles Bajolle, who would soon become the first chef that I’d shamelessly steal or reverse-engineer recipes from. He was most famous for his marquise au chocolat with pistachio crème anglaise (which indeed I stole and worked on for many years) but the dish that opened up the heavens for me was the single unadorned quenelle of thyme ice cream.

There’s nothing surprising today about an herb flavored ice cream. But back in the 20th Century, for someone who’d been making flavors like “rocky mountain road,” an herb flavor besides mint was a sucker-punch to the imagination. And the flavor itself: let’s just say that I took a bite and sat there, very quietly, for a long time. The sensations kept developing, unfolding, surprising, telling stories. It was obvious that this was the first truly good ice cream I’d ever had. 

Only later did I realize that one of its secrets would be easy to duplicate: make the ice cream less sweet. Let the herbs and the dairy do what they do.

End Rant.

The Problem

Conventional ice cream is too sweet, but reducing the sugar content makes it too hard at serving temperatures.

The Solution

Use different sugars.

Michael Laiskonis, former executive pastry chef at Le Bernardin and current Creative Director at the Institute of Culinary Education, taught me how to think about using sugars. 

Freezing point suppression is dependent on the molecular weight of a dissolved ingredient. The lower the molecular weight, the smaller the molecule, and so the more molecules per gram—and the greater the reduction of the freezing point.  Consider the following table:

The sugars we care most about are sucrose, dextrose, and invert syrup. 

Sucrose is table sugar. Since it's the most familiar, and has the flavor we most expect, we use it as our foundation. It also provides a frame of reference for understanding the other sugars.

Dextrose is about 3/4 as sweet as sucrose, but has nearly double the effect on freezing point suppression. Simply by decreasing sucrose and increasing dextrose, you can lower the sweetness while simultaneously softening the texture. Magic! 

Invert syrup is a liquid sugar that I think of separately. It’s slightly sweeter than sucrose, and offers slightly stronger freezing point suppression, but the differences are too small to matter much. Invert syrup is fantastically useful, however, as a texture modifier. It adds a bit of body, and suppresses the growth of ice crystals—much like milk solids and stabilizers. And it does so while acting as a sweetening ingredient. It does all this without any downsides. 

It’s especially helpful with flavors that require adding non-dairy fats, like cocoa butter (chocolate) and nut oils (nut butters). These fats tend to freeze harder milk fat, and give ice cream a dry, stiff, crumbly texture. Increasing the proportion of invert syrup can bring the softness and smoothness back.

Remember that invert syrup is only about 80% sugar solids by weight when you calculate the solids in your recipe (the rest is water). See notes on invert syrup below in the appendix.

In Practice

My starting point is 13% sugar by weight (not counting the lactose in the milk).

The blend consists of:
60% sucrose 
26% dextrose
13% invert syrup

So, as an example, if the total recipe is 1000g, we start with 130g sugar:

78g sucrose
34g dextrose
17g trimoline

Then the tweaking begins:

Does the ice cream need to be softer? Reduce the sucrose and increase the other sugars.

Does it need to be less sweet? Increase the ratio of dextrose to invert syrup. If that's not enough, Reduce the sucrose and increase dextrose by 0.75 times the change in sucrose.

Are there ingredients that add fats which could harden the ice cream—like chocolates or nut butters? If so add more invert syrup, and subtract an equal amount of sucrose. (My chocolate ice cream’s sugar blend is over 40% invert syrup, with no sucrose beyond what's in the bittersweet chocolate).

Are you adding flavor ingredients that have their own sugars? Like fruit, chocolate, gianduja, or liqueur? Calculate (or estimate) the amount of added sugar and reduce the sucrose by the same amount. 

With fruit, look up the actual composition of the fruit (it usually contains sucrose, fructose, glucose, and other sugars). You can compensate by reducing the glucose as well. We’ll discuss this in detail in a future post on fruit flavors.

Finally, are there flavoring ingredients that will directly effect the freezing point—namely alcohol? If there’s a lot, the ice cream may need all the help it can get to harden enough. Eliminate the glucose. Reduce the sucrose, too, if there’s any room to lower the sweetness. Add a bit of nonfat dry milk to get th solids up, and increase the stabilizers. We’ll discuss this in detail in a future post on booze flavors.


Other Important Structural Sugars

Maltodexrin adds solids and bulk with minimal effect on sweetness or freezing point. It's a bit of an anti-sugar in this sense. It's useful in flavors which by their nature are low on solids, and so need something to combat their innate wateriness—typically sorbets like lemon and watermelon. These flavors are built from fruit juices that are mostly water. We'll address sorbets generally in another post.

As a bulking ingredient, I usually prefer milk solids to maltodextrin, since the latter does all the bad things to your body that sugar does, without the benefit of tasting like anything. But milk solids are generally not an option in sorbets, which everyone expects to be dairy-free.

Fructose is the monosaccharide, which, along with glucose (dextrose) makes up both table sugar and invert syrup. It has the same high freezing point suppression of dextrose, but is much sweeter—about 25% sweeter than table sugar. We could use fructose, like invert syrup, as one of the controls of relative sweetness and freezing point.

Fructose, in combination with dextrose, can replace invert syrup. This has the advantage of adding no water to the recipe (invert syrup is typically around 20% water) and is also a little easier to handle. Invert syrup is a sticky goo that clings to spoons and fingers and has a finite shelf life. I've asked a top-shelf pastry chef why this isn't a popular solution, and he said, simply, that he has invert syrup in the pantry, because he uses it for a million things. He doesn't have other uses for fructose.

If you'd like to try substituting dry sugars for invert syrup, just leave out the invert syrup, and replace with dextrose and fructose. Each of these sugars should be measured to 40% the weight of the invert syrup. So if the recipe called for 30g invert syrup, replace it with 12g each dextrose and fructose. This is in addition to any dextrose that's already in the recipe. Mix in with all the other dry ingredients.

Variety Sugars

Honey is a useful sugar in some ice cream flavors. It behaves mostly like invert syrup (because it IS mostly invert syrup—around 75% by weight), and tastes rather strongly  ... of honey. Because it adds about 20% water to the recipe, and increases body, it's generally not a good idea to substitute honey for all the sucrose. But up to 50% works fine. It can be interesting to experiment with some of the more exotic and intense honey varieties, like buckwheat, heather, and chestnut. You'll probably want to use these honeys in moderation. Mild honeys like clover and alfalfa are most traditional.

There are other varieties of glucose, including atomized glucose powder, corn syrups (typically around 1/3 glucose by weight) and various glucose syrups, identified by their DE number for dextrose equivalence. The DE number technically refers to the percentage of reducing sugars—in this case meaning either glucose or fructose. The higher the DE number of a glucose syrup, the more glucose it likely contains, and the greater the freezing point suppression. Atomized glucose is just spray-dried glucose syrup. It contains more water than anhydrous dextrose. Here's all you need to know: Don't use any of this stuff unless it's all you can get your hands on. Pure Dextrose powder and invert syrup are more useful, and make it a lot easier to know what you're getting.

Caramel is useful as a flavor ingredient. A little goes a long way, which is convenient—because it's hard to know how caramel will effect the ice cream's texture and freezing point. Caramelizing sugar is a gradual process by which some portion of the sucrose molecules break down into smaller molecules, and combine into larger, more complex, more flavorful ones. I like to use a small quantity of caramel, but to cook it to a fairly dark and flavorful degree. This way it will behave less like sugar in the recipe, and will have maximum effect on flavor.

You might also experiment with using caramels browned to different degrees—like a medium caramel, for more traditional toasted flavors, and a dark caramel, for the more complex and bitter burnt sugar flavors.

Molasses is unrefined syrup centrifuged off from sugar cane syrup after it crystalizes. It contains all kinds of stuff, including water, so it's best to use in small quantities just for flavor. The primary sugar component is sucrose.

Maple syrup is also useful as a flavoring. Like molasses, its primary sugar is sucrose (typically 52%), and it contains water (typically 45%) plus around 3% invert syrup.. It's not easy to know precisely how much water is in there, since syrup is boiled down to whatever level the maker desires. Fortunately, a little goes a long way. Grade B is the most flavorful. The grade signifies darkness and not quality; annoyingly, many grocers don't know their trade and stock only the inferior Grade A. It's worth it to find a reliable local source of the good stuff. Maple syrup is so expensive these days, you should get all the flavor you can from every ounce.

Non-Caloric Sweeteners

It’s not easy, but it’s possible, to make decent sugar-free ice creams. The trick is finding ingredients that taste like sugar, adequately suppress the freezing point, and won’t give you a bellyache. 

By these standards, the perfect ingredients do not exist—although some of the sugar alcohols, like erythriol, come pretty close. We’ll discuss these in a later post (although I’m no expert on the topic).

In the next post we'll explore the dark arts of stabilizers.

Appendix 1: Invert Syrup

How to make Invert Syrup

250g sucrose
120g water (approx)
0.25 – 0.5g citric acid or cream of tartar (tartaric acid)

Mix ingredients in a saucepan and bring to a boil.

Once the mixture boils wash away any sugar crystals stuck to the side of the pan with pastry brush dipped in water. Water from the brush won't affect the outcome.

On medium heat without stirring boil the mixture to 235°F / 113°C. Remove from heat and cover the pan. Let cool at room temperature until it’s reasonably safe to handle. transfer to plastic container. Store in a refrigerator. Invert sugar will last at least a few months. 

You can melt and re-cook it if starts to crystalize. Toss it if you see mold.

Most professional kitchens just buy the stuff.

So—What is Invert Syrup?

Sucrose is a disaccharide, meaning a sugar molecule made up of two smaller monosaccharides: glucose and fructose. When we make invert syrup, we split these two monosaccharides apart, with the addition of water—a reaction called hydrolysis. Hydrolysis can occur with just the addition of water and heat, but an acid catalyst improves the efficiency of the reaction.

Typically, we can split (invert) about 85% of the sucrose. Manufacturers may be able to invert more of the sugar, by using other chemical or enzymatic catalysts.

When you cook your own, you control the final water content with the cooking temperature. Cooked to 113°C–114°C the final syrup will contain a bit under 20% water. This is dry enough to work in ice cream without adding too much water, and gives a long life in the fridge. But it's not unreasonably gluey. 

The stuff is great to have around. In addition to magic it works on ice cream, substituting about 10% invert syrup for sucrose in most desserts will improve moistness and add shelf life.

Why is it “Inverted?” 

This may be the most useless piece of knowledge in the entire blog series. But you asked. 

Chemists measure the composition of optically-active solutions with a polarimeter, which passes plane-polarized light through the solution being measured. When the solution contains sucrose, the light rotates to the right. When the solution breaks down to glucose and fructose, the light rotates the other way, hence the inversion: 

C12H22O11 (sucrose, Specific rotation = +66.5°) + H2O (water, no rotation) C6H12O6 (glucose, Specific rotation = +52.7°) + C6H12O6 (fructose, Specific rotation = −92°) net: +66.5° converts to −19.65° (half of the sum of the specific rotation of fructose and glucose). 

This inversion of polarized light has no known application in the kitchen. Not even Nathan Myhrvold has suggested that we run out and buy a polarimeter. Just try to remember that inverting sugar does not mean turning the bag upside-down. 

Appendix 2: Sample Recipe

Quartet of Dark Sugars Ice Cream

I wanted to create a recipe that gets all its flavor from the sweeteners, and that explores the possible depth and range of those flavors. It uses caramel, molasses (from the dark muscovado sugar), maple syrup, and chestnut honey. This ice cream has a lot of layers. Background hints of vanilla and salt take it even farther. It's not kid stuff—it isn't even especially sweet.

Muscovado sugar is a semi-refined brown sugar that's heavy on molasses and has a deep, complex flavor. Chestnut honey is dark, bitter, and challenging. For the maple syrup, look for one marked "Grade B", which is darker and more flavorful than grade A. The grade has nothing to do with quality.

I've written this for cooking in an immersion circulator, but it adapts fine to the stovetop or other heating methods.

Makes 1 to 1.3 liters

55g dark muscovado sugar
50g nonfat dry milk

3g salt 
0.8g locust bean gum (TIC or Willpowder)
0.4g guar gum
0.2g lambda carrageenan

35g (approx) water
35g granulated sugar
30g (approx) water

460g  whole milk
2 egg yolks (36g)
27g chestnut honey 
25g maple syrup

270g  heavy cream
10g (2 tsp) alcohol-based vanilla extract

-thoroughly stir together muscovado sugar, dry milk, gums, and salt.
-set immersion circulator to 75°C

-heat 1st portion of water and granulated sugar in a saucepan. Cook to medium-dark caramel.
-turn heat low. Deglaze with second portion of water. Water will boil off and caramel will clump.
-when water is mostly gone, add milk and stir to disolve caramel. Stir in the honey and maple syrup.

-pour milk mixture into blender.
-set blender speed to create a vortex; add powdered ingredients. cover and blend on high for 30 seconds to disperse the stabilizers.
-add yolks, cream and vanilla extract.
-briefly blend again on high speed.

-pour mixture into 1gal ziplock bag.
-add weight (recommended, to keep bag from floating) and evacuate the air.
-cook in water bath for 45 minutes to set custard, hydrate stabilizers, denature milk proteins.
-gently agitate bag after 5 and 15 minutes. if you see air accumulated in the bag after 15 minutes, release it, and carefully reseal bag.
-mix will be pasteurized (pasteurization time after reaching this temperature is under 2 minutes).

-remove bag from water bath. open and pour hot mix into clean blender container (or a square container if using a homogenizer or stick blender). remove weight (with tongs). use bag to squeegie off any mix. temporarily seal bag and keep handy. 
-blend on highest speed for 30 seconds to homogenize. 
-pour mix back into ziplock bag.

-chill bag in ice water bath (use ice bath to evacuate the air when sealing bag). carefully agitate to cool. Try to cool to refrigerator temperature. 
-refrigerate at least 4 hours, below 38°F / 3°C to age mix / pre-crystalize fat.

-pour into ice cream machine: snip off bottom corner of bag, and squeeze out mix as if using a pastry bag. or squeeze out into an intermediate container that’s easy to pour from.
-spin in the ice cream maker. With a mulitispeed machine, use a slow setting (this recipe works best with a low overrun). Ideal drawing temperature is 23°F / -5°C.

-harden in a blast freezer for several hours, or overnight in a cold standard freezer (should be set to -5°F / -20°C or lower). Ice cream will have to warm up several degrees before serving. 20 to 30 minutes in the fridge works well. Ideal serving temperature is 6 to 10° F / -14 to -12°C.

Shameless Plug:

Keep an eye out for my photography book on the Brooklyn Domino Sugar Refinery. To be published October, 2017.

Part 1 of this series: Introduction
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
Part 11 of this series: Introduction to Flavor
Part 12 of this series: Ice Cream Flavor: Coffee

Friday, May 27, 2016

Ice Cream Technique

So you have a formula. You have great ingredients. You have a digital scale1. You’ve measured everything. What next? Where’s the wand that turns these ungainly liquids and powders into magic?

Every Ice cream process includes, at least, mixing, cooking, aging (in the fridge), and spinning (in an ice cream machine or some substitute). We’re going to expand on these steps a bit in order to get the best results. The process consists of:


1. Mixing


A high-powered blender is ideal, for its ability to disperse powdered ingredients like gums (t can also be useful later, for the homogenizing step). A regular blender or stick blender is fine. If you’re mixing by hand with a whisk, you might have a hard time keeping ingredients like dry milk and gums from forming clumps that won’t dissolve. 

One trick is to thoroughly mix all the dry ingredients before adding to the liquids. An ingredient will only form clumps with itself, so if there are particles of another ingredient running interference—like sugar—the powders tend to stay separate long enough to dissolve.

If you’re using a blender, pick a speed that creates a vortex without any splashing, and pour the well-mixed powders down the center. Then put on the lid and blend on the highest speed for 30 seconds or so. 

It’s best to add add yolks afterwards, and blend just briefly to incorporate. Egg in the mix can make gums harder to disperse, and can also encourage foaming. 

2. Cooking

Old school.

We cook the mix to:
-Hydrate the gums and / or set the egg proteins 
-Partially denature (cook) the whey proteins in the milk, to improve their stabilizing and emulsifying characteriestics
-Pasteurize the mix (especially important if it contains egg)
-In some cases we may also use the cooking step to infuse flavors

Cooking methods and tools

The traditional method is a saucepan on the stovetop, but others are hands-off and give more precise and consistent temperature control.

Anova immersion circulator in a water bath

My preference is an immersion circulator, for its precision and complete prevention of evaporation—and because it's such a useful tool to have around for a thousand other things. It can be less convenient than some other options, especially for larger batches.

Using a circulator means cooking the mix sous-vide, in a Ziploc freezer bag. Pour the mix into the bag straight from the blender, add a weight (you can use a spoon, or make your own weights from stainless steel rod stock) and evacuate the air by partially immersing in some water before zipping. 

To help get all the mix up to temperature, I like to pull the bag out of the water bath to agitate it a couple of times—5 minutes and 15 minutes after starting cooking.

If these ideas are unfamiliar, please see our sous-vide series.

Lab hotplate with magnetic stirrer and thermocouple probe

A laboratory hot plate is a brilliant device and still a novelty in kitchens. It combines a hot plate and a magnetically driven stirring rod (just drop it into your pot and it spins, as if by magic). The better ones (which you’d want) include a probe thermocouple, so the heater can automatically maintain a fixed temperature. You’d want to rig some kind of lid with a hole for the probe, to slow evaporation. 


I have no experience with the Thermomix. They’re not so common in the U.S., but people across the pond rave about them. It combines a blender and a thermostatically controlled hot-pot, and probably a few other things. It should work well for cooking ice cream.

Kitchenaid heated bowl

Kitchenaid makes its own electronically controlled heating bowl. I have no experience with this either. If you already have one of their mixers, it could be worth investigating.

A spatula with a built-in thermometer might help on the stovetop.

If you choose a saucepan on the stove, just be aware that precise temperature control won’t be possible, and that you’ll be losing water to evaporation. Some people make use of this evaporation in order concentrate milk solids, but between temperature control, evaporation control, and constant stirring, this gives you a lot to handle. See the note on evaporation, in the Time and Temperature section.

A bit of good news: if you go the stovetop route, you can dispense with the old-time practice of tempering your egg yolks. It serves no purpose. Just toss the yolks into the pot once you’ve dissolved the solid ingredients and start heating. A flat-bottomed silicone or bamboo spatula works better than a wooden spoon for stirring and scraping the bottom of the pan. You’ll probably want to keep with the traditional practice of straining the mix afterwards; a little bit of overheating and curdled yolk can be hard to avoid. 

Time, Temperature, and Proteins

The traditional French method is all about traditional French custard: throw in a lot of yolks, put the pan on a hot flame, stir until the custard thickens, stop. This method will pasteurize the mix by default, and gets it hot enough to hydrate just about any gums, but pays no attention to the milk proteins. It also, surprisingly, pays little attention to the texture and flavor of the custard.

We used to be taught that custard sets at a precise temperature, and that the only variable was the concentration of yolks. We now know that viscosity rises on a continuum, not just with yolk concentration but with temperature2. Custard forms, at varying thicknesses, between 70°C / 158°F and 88°C / 190°F. Additionally, at higher yolk concentrations and higher temperatures (above 75°C), yolks begin to produce hydrogen sulfide gas, and can develop unpleasant flavors. These flavors may intensify with prolonged cooking—which isn’t done during traditional custard making, but we’re going to explore its benefits.

The Chemistry

Food scientists have known for a long time that milk proteins turn into more powerful emulsifiers and stabilizers, just from being cooked to the right degree3,4. During cooking, whey protein molecules (which make up around 25% of the total milk proteins) begin to unfold (denature), exposing reactive surfaces that had previously been concealed. These are able to form new bonds with casein molecules (which comprise most of the remaining protein). In doing so, they work in similar ways to the emulsifying ingredients we traditionally include in ice cream recipes (like egg yolk). The denatured whey protein can augment, and in some cases replace, traditional emulsifiers. See the post on Emulsifiers.

The partially denatured whey proteins can also form aggregates with other whey proteins, and form a weak hydrocolloid gel—much like the stabilizing ingredients. With careful management, the cooked proteins can serve most of the purposes of an egg custard.

In pure milk, we see effective protein denaturing taking place between 70°C an 85°C. There is, unfortunately, little scientific literature available on this process in the specific context of ice cream. Since the cooking environment—including pH and fat and sugar and dissolved solids levels—throws so many variables into the mix, we can only use the research for broad guidelines. 

Fortunately, some ice cream pioneers have done the empirical research for us. Engineers at Haagen Dazs aren’t sharing, but Jeni Britton-Bauer, of Jeni’s Splendid Ice Creams, devised a process that includes pasteurizing her egg-free mix at 75°C for up to an hour. Ice cream mix pasteurizes to federal standards in about two minutes at this temperature; the rest of that cooking time works the proteins. Between using a higher than normal level of nonfat mix solids, a long, controlled cook, and a small addition of relatively weak starch stabilizer, she creates egg-free ice creams that have a rich mouthfeel normally associated with custard bases.


Britton-Bauer also favors this time / temperature combination because it gives a custardy “cooked milk” flavor to the mix, which she favors. My own experiments don’t bear this out. I made identical, unflavored samples of ice cream mix (15% fat, 10% nonfat milk solids, 2 yolks/liter), cooked sous-vide at temperatures from 72°C to 80°C, for times ranging from 15 to 60 minutes. In a series of double-blind triangle tastings, subjects found the differences almost too close to call. There was a slight preference for 75°C held for 30 minutes, but there was no sense of cooked milk flavor, and it was not more “custardy” than others. It’s likely that with flavoring ingredients added, there would be no differences. I take this as good news: at least if your mix is low on egg yolks you shouldn’t have to worry about flavor when choosing a cooking time and temperature.

You may want to run your own tests. Variables like the number of yolks and the level of nonfat milk solids might influence the results.


If you're cooking the mix for more than a few minutes in an open container (every method mentioned here besides the immersion circulator, Thermomix, or the hot plate with a covered container), enough water will evaporate to effect the results. You'll see a decrease in the total mass of the recipe, which will correspond to an increase in the percentage of solids. This will improve most cookbook recipes. But it can hurt a well designed professional recipe that already has optimal solids.

If you're doing long cooks in open containers, weigh the mix again after cooking, and recalculate your formula based on the final mass. You'll find you can add less dry milk.

Some people use evaporation deliberately to increase nonfat milk solids. This can work, of course, but I'll suggest that you're sacrificing precision by juggling too many variables at once. Evaporation rate will vary with the size and shape of the cooking container, the volume of ice cream mix, and even the ambient temperature and humidity. How are you going to control both the evaporation and the cooking of the milk proteins?

Some people chose evaporation over dry milk powder on the assumption that dry milk powder tastes bad. Some of it does; the good stuff doesn't. Higher quality versions are pure, spray-dried skim milk, with the water evaporated at lower temperatures and under more precise controls than you'll create in the kitchen. You're doing yourself and your ice cream no favors by taking on this role.

Time, Temperature, and Protein Conclusions

If your base has a typical quantity of nonfat milk solids (in the 10% range) I believe that optimizing your cooking can subtly increase smoothness and creaminess, but will not completely substitute for eggs or other emulsifiers and stabilizers. 

My current preference: 75°C / 167°F for 45 minutes. 30 minutes is all that’s needed once at final temperature. I add 15 minutes to give the ingredients time heat up from their starting temperature. I usually use an immersion circulator.


Make sure you’re paying attention to the minimum temperatures needed to hydrate your stabilizing ingredients. Commercial stabilizer blends are often formulated to hydrate at room temperature, but check the instructions just in case. Some gums, like locust bean gum, won’t fully hydrate until they’re hot. Ideally, find a version that works at your preferred cooking temperature. TIC Gums makes Locust Bean Gums that hydrate at 74°C, so I use these. Most other brands require temperatures higher than i prefer.


Required for commercial ice cream, a pretty good idea for any ice cream, and luckily, something that’s going to happen on its own with most standard cooking steps. Pasteurization is defined as a reduction in pathogens (harmful microbes) by a degree that’s legally considered safe. In most cases this refers to a log-6.5 reduction (abbreviated 6.5D) of the most harmful or hardest to kill pathogen. This means that only three bugs in ten-million will survive pasteurization.

Pasteurization requires the right combination of temperature and time. The values vary with the type of food (and therefore the type of resident pathogens).

Here are pasteurization times / temperatures for ice cream and other high sugar / high fat dairy products5. Times are rounded up: 

70°C     15 minutes
71°C     10 minutes
72°C     7 minutes
73°C     5 minutes
74°C     3 minutes
75°C     2 minutes
78°C     30 seconds
80°C     15 seconds
85°C     2 seconds

3. Homogenizing

A rotor-stator homogenizer generates higher shear forces than any blender

Homogenizing means blending with such high shear forces that the big fat globules are broken up into many much smaller ones. This improves smoothness, whipability, and stability of the foam structure.

Even if you start with homogenized milk, it helps to homogenize again after cooking. At cooking temperatures, the fat globules melt and start glomming onto one another. Meaning, the milk will start to de-homogenize.

If you can get your hands on a rotor-stator homogenizer, this is ideal, but a high-powered blender (Vitamix, etc.) does a pretty good job. A stick blender is better than nothing.

The idea is to blast the mix with high shear forces, while it’s still hot. This will let the blades of your blender or homogenizer work on the fats in their liquid state. 

If you’re cooking the mix sous-vide, you’ll have to empty the bag into your blending container. Use this opportunity to remove the weight (use tongs—it will be hot). Seal the bag again to keep contaminants out, so you can re-use it.

If you’re homogenizing in a high-powered blender, beware that these things generate enough blade friction to add heat. You might want to stop and check the mix with a thermometer the first couple of times you try this, to make sure you don’t heat the mix beyond your cooking temperature. In my experience, this has never happened with blending times of a minute or so; the mix loses more than enough heat just by being poured into a new container. 

Blend on the highest speed. Ideally for a full minute.

4. Chilling

Cool the mix as fast as possible before putting in the fridge. The fridge takes much too long, and could allow spores of anaerobic bacteria, which are not killed by pasteurization, to activate. This would be highly unlikely, but there’s no reason to mess around. A couple of quarts of hot ice cream mix can also warm up everything else in the fridge.

Pour the mix into a large Ziploc bag. If you cooked it sous-vide, just reuse the same bag. Then drop it into an ice water bath. Stir it around and gently squeeze / agitate the bag to keep the mix moving. You should be able to chill it to 5°C / 40°F in just a few minutes.

5. Aging

This step—leaving the mix in a refrigerator (below 3°C / 38°F) for 4 to 8 hours—is critical. It allows the fat globules to partially crystalize, so they’ll be whipable. And it gives the ice cream machine a head start in freezing the mix. I've read suggestions that the only important factor is temperature, and that time is unimportant. But research suggests that milk fat crystalizes slowly when in an ice cream mix—much more slowly than when in the form of pure cream. 4 hours at refrigerator temperature is the minimum needed to guarantee good texture.

To age the mix, just put the bagged mix in the fridge. Make sure the place you put it is cold enough. I like to place the bag upright in an open plastic container, to reduce the chance of leaks.

6. Spinning (Freezing)

Carpigiani Maestro and Labotronic machines, along with similar models by Bravo, are about as good as they come.

There are bajillions of ice cream machines available, ranging from inexpensive freezer-bowl models to prosumer compressor models to rock-salt-and-ice-filled soccer balls you churn by kicking around the lawn to high end professional machines with automated cycles and multiple adjustable parameters.

From the perspective of ice cream quality, only three factors matter: the design of the dasher (the spinning blade), the dasher speed, and the cooling power (either the power of the compressor or the heat capacity of the freezer bowl). 

The first two factors determine the overrun (how much air gets whipped into the ice cream). Some machines have some combination of interchangeable dashers, adjustable speeds, and adjustable power cycles. These give you control beyond the ice cream formula itself. 

The second factor determines the speed of freezing ("residence time"). The faster you can freeze the ice cream, the better. The physics are quite simple: the more quickly water freezes, the smaller the crystals. While we can influence crystal size with the dasher speed and design, and with the solids and stabilizer content of the mix, all else being equal, a faster freeze will give smaller ice crystals. 

Fill one side with your mix, the other with salt and ice, and give to some ADHD kid to kick around. 

You’ll find some old-time ice cream machine makers touting the benefits of a long and slow freeze. They’re blowing smoke. If your machine takes longer than 30 minutes to freeze a batch of ice cream, it’s going to favor an icier product. You can still get smooth results by virtue the recipe, but you’ll be doing so in spite of the machine, not because of it. Machines like the top-of-the-line Carpigiani and Bravo models can freeze a batch in well under 5 minutes. The results are almost supernaturally smooth. 

Ice cream attachment for mixer. Works well if your freezer's set to -5°F or lower.

You don’t necessarily have to spend a lot of money. If you don’t need a lot of capacity, some of the freezer bowl models can freeze a quart of ice cream in under 15 minutes, if you keep your freezer turned low enough. And the more powerful home compressor models can do it in 20 minutes or under. I'm a bit skeptical of inexpensive compressor models; many of them are underpowered and specify a freezing time of over 40 minutes. It's difficult to get smooth ice cream with freezing times this long. 

Lello Musso gelato machine. Among the best compressor-driven home models.

Filling the Machine

If your mix has been aging in a Ziploc bag, the easiest way to get it into the machine is to snip off one of the bottom corners and use the bag like a pastry bag: squeeze the mix out. Some mixes can be quite viscous, or even gelatinous, especially if there are lots of added fats from chocolate or nut butters, or if your stabilizer blend forms a gel (which should be avoided).

If your mix has formed a gel that’s too viscous for the machine to churn efficiently, squeeze the mix into a bowl or plastic container, and blast it with a stick blender. This will turn the stiff gel into a fluid gel that’s much more pourable.

Drawing Temperature

It's good practice to aim for a consistent drawing temperature; a temperature when the consistency is correct and it's time to remove the ice cream and head to the next step. It's tempting to just look at texture and ignore the temperature, but this won't let you fine tune your mix. Here's how to do it:

Use an instant read thermometer (a Thermopen or equivalent) to check the temperature of the ice cream when the consistency looks right—dry on the surface, and relatively smooth, with a viscous, whipped, soft-serve texture). At this point the ice cream has achieved the right structure. In the interests of ideal consistency at serving temperature, and in limiting ice crystal growth, we want the mix to reach this consistency as close to -5°C / 23°F as possible.

If the ice cream measures warmer than this when the consistency is right, the recipe should be adjusted to increase freezing point suppression. And of course if it's colder, the recipe should be adjusted the other way.

If your machine has the cooling power to freeze the ice cream colder that -5°C, you may find it advantageous to let it do so—provided that the rate of cooling is still faster than that of the freezer you use for the hardening step. If it take your machine an additional 20 minutes to shave off a few more degrees, then waiting will likely hurt the texture.

Alternative Freezing Techniques

Meet Paco.

The Paco Jet starts with mix that's been frozen solid; it uses high-speed blades to shave and fluff the ice into a foam with nano-sized ice crystals. The ice cream is usually made to order at small cafes and restaurants, so recipes can be looser and freer than ones designed for conventional machines. A smooth texture is guaranteed, provided someone eats the ice cream right away. If you need to make the ice cream ahead and store it in a freezer, then all the usual considerations apply again.

Ahhh, the cryo life.
Liquid Nitrogen came onto the scene in the 1980s, possibly discovered by the founder of Dippin' Dots as a viable ice cream freezing method. LN2 boils at -196°C / -321°F, so as you might imagine, it freezes ice cream fast. This means almost preternaturally smooth ice cream, which can be made in just a couple of minutes in an ordinary stand mixer.

That is, if you have a laboratory dewar full of liquid nitrogen handy. You can buy a dewar for several hundred dollars, and you can probably find a service that will deliver to your kitchen. The expense and inconvenience may still be discouraging. LN2 isn't terribly expensive, but you need a lot—several times the volume of your mix. And delivery services may have minimums. So LN2 ice cream has mostly been popular with people who have other reasons to keep liquid gas around, like chefs who work with contemporary techniques, or biologists who like to party.

To make LN2 ice cream, wear the appropriate safety gear—especially shoes or boots that are closed and covered by your pants cuffs. Make sure there's good ventilation (otherwise, if you spill a bunch, nitrogen will displace the oxygen from the room and you'll never know what hit you). Pour your mix into a stand mixer work bowl, turn the machine on low, using the flat beater, and slowly pour the LN2. Enjoy the disco fog. Stop pouring and stop the machine when you get the consistency of soft serve. Don't try to over-harden it ... your mixer won't be happy.

Dry Ice offers some similar advantages to LN2. With a melting / sublimation point of -79°C / 109°F, it's positively balmy compared to nitrogen, but it's a lot colder than your ice cream machine. Dry Ice isn't as easy to find as it once was—those gel-filled cold packs actually do a better job of keeping beer chilled in the cooler, and won't burn anyone. If you can find some locally, make sure it's food grade. A lot of the stuff is technical grade and can be contaminated with god-knows-what.

A unique quirk of dry ice: it will carbonate your ice cream. The C02 bubbles are quite unstable, so the carbonation won't last long. If you don't want it, hold the ice cream in your freezer for a couple of days. If you do want it, serve the ice cream as soon after hardening as possible. I'll post my recipe for sparkling champaign sorbet in a later post.

To use dry ice, grind it into snow or small chunks in a blender, and then churn into the mix with a stand mixer, as with LN2.

7. Hardening (Static Freezing)

Blast freezer!
Most ice cream machines will only get the ice cream down to soft-serve temperatures. You then need to harden the ice cream, and do so as fast as possible—again, to keep the ice crystals from getting too big. Crystals grow rapidly at the relatively warm drawing temperature—the clock is ticking as soon as stop the ice cream machine. 

Commercial ice cream kitchens and better equipped pastry kitchens use blast freezers, typicallly set to -40°. If you only have a standard freezer, the tricks to a quick freeze are to set the freezer as low as possible (most newer freezers can get down to —20°C / -5°F, and to freeze the ice cream in small batches. Pints are ideal. 16oz polypropylene takeout containers are especially convenient. They get brittle in the freezer, so they crack easily, but you can always stock up by ordering some more takeout.

If the ice cream won't be eaten immediately, you can add some insurance against iciness and off-flavors (from the freezer) by pressing a layer of plastic wrap over the surface, before snapping the lid  on. Be sure to use odor-free plastic wrap (made from polyethylene) rather than the commercial types (made from polyvinyl chloride). All the consumer brands are ok. The commercial brands form a better oxygen barrier, but will make the ice cream taste like a shower curtain.

Lots of these.

If you’re using a blast freezer, you’ll wait until the ice cream is hardened throughout, and then transfer to a standard freezer. It will be as hard as concrete, and will take many hours to resemble dessert. If you’re using a standard freezer for hardening, then after 8 hours or so you’re done. Moving the ice cream to the fridge 15 minutes or so before serving will make it more scoopable. Ideal serving temperature is 6 to 10° F / -14 to -12°C.

Closing Thoughts

In this post we’ve covered just about everything, except, conspicuously enough, flavor. We’re leaving flavor ingredients for later posts, because these fundamentals are probably enough for right now. 

In the next post we’ll take a closer look at sugars.

1You do, right? Two, actually—one that reads to 1 gram and can hold several KG, and one that reads to 0.01g and can hold 100g or so? No? Stop right now and buy something like this and this.

2Myhrvold, Nathan M. Modernist Cuisine, Vol. 4, p. 84]

3Mantovania, Raphaela de Araujo; Ângelo Fazani, Luiz Cavallieria;, Lopes da Cunhaa, Rosiane. O/W emulsions stabilized by whey protein: Influence of heat treatment and high pressure homogenization, presented at 11th International Congress on Engineering and Food (ICEF11)]

4Raikos ,Vassilios. Effect of heat treatment on milk protein functionality at emulsion interfaces. A review, Food Hydrocolloids 24 (2010) 259–265;

5Myhrvold, Vol 1, p. 192

Also see Goff, H. Douglas. "Partial Coalescence and Structure Formation in Dairy Emulsions," in Food Proteins and Lipids, Volume 415 of the series Advances in Experimental Medicine and Biology pp 137-148

Part 1 of this series: Introduction
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
Part 12 of this series: Ice Cream Flavor: Coffee