Thursday, June 9, 2016

Ice Cream Emulsifiers

This has so far been the most challenging post to research and write; there’s a lot of arcane and incomplete knowledge on the topic, much of it of questionable relevance to the mission of artisanal ice cream making. If you favor recipes with one or more egg yolks per liter of base, and don’t want extra homework, you can skip this one.

If you’re interested in making egg-free ice creams with the best possible texture, or are just a glutton for minutia, read on.



Electron micrograph of a milk fat globule. Here's where the action is.

Basics


An emulsifier is a two-faced molecule that binds to water on one side, oil on the other. You’ll see one side labelled hydrophilic, the other labelled either hydrophobic or oleophilic—different jargon, same idea.

The adage that oil and water don’t mix still holds true after all these years. This would be a problem for much of the world, especially the plant and animal parts, if it weren’t for emulsifiers helping to hold things together.

Like, for instance, milk. Milk is a colloid made from a polar chemical (water) and non-polar chemical (milk fat), which cannot mix. Technically speaking, they are not miscible. But they can form an emulsion—a more complex arrangement in which immiscible ingredients coexist, with some degree of stability.

Because it’s an emulsion, Milk is creamy, opaque, and homogenous. The fat molecules are divided into many microscopically small globules, which are surrounded by emulsifying chemicals that keep them from glomming together and rising to the top. These emulsifiers include fatty chemicals called phospholipids, and proteins called wheys and caseins.

We’re mostly interested in the caseins, which make up 20% to 25% of the milk proteins. They gather together in clusters called micelles, which expose their hydrophilic and hydrophobic surfaces on different sides, allowing them to bind both water and fat—thereby keeping the milk from separating.1 If you look at milk under an electron microscope, you see big round fat globules, covered in a thin membrane of casein micelles. This casein, with its hydrophilic end facing outwards, keeps the fat globules connected to the water and discourages them from joining each other.


Emulsifiers in Ice Cream


Milk Fat and Casein. Click to enlarge.

Left: fat globule with membrane made of casein micelles

Center: crystalized fat. Crystals protrude, but casein prevents them from bumping into other globules.

Right: crystalized fat partially destabilized by emulsifiers. Most of the protective casein is gone,
so the fat globules are better able to bump into each other and form a larger structure.


Thanks mostly to the casein, milk and cream are already pretty stable emulsions—so why are emulsifiers so important in an ice cream recipe?

The reason is that the milk and cream emulsions are a bit too stable. A big part of making ice cream involves the same process as making whipped cream, which requires destabilizing the emulsion. As long as the emulsion is stable, those fat globules can't join to create a foam network. With traditional methods, you can only whip cream containing over 30-something percent fat. With lower fat percentages, there’s enough water between the fat globules to keep the emulsion stable. Ice cream usually has far too little fat.

So to whip ice cream, you need extra help to partially destabilize the milk fat emulsion. Then the fat globules can partially coalesce—science talk for the fat glomming together just enough to form a stable foam.2

This is what added emulsifiers are for. They weaken the existing emulsion, allowing the ice cream to whip into a foam more easily, and with better texture and stability.



Ice Cream Emulsifying Ingredients



Egg Yolk


Egg yolks may be the most familiar emulsifier in the kitchen; they hold together mayonnaise, hollandaise and béarnaise family sauces, Caesar’s salad dressing, and chocolate marquises.

Yolks are usually cooked to a custard consistency, which also thickens the water portion of the ice cream (as a stabilizer—see the post on Ice Cream Stabilizers). Yolks are occasionally used without taking advantage of their thickening ability, but they must at least be cooked to the point of pasteurization.

Yolks contain a number of proteins and lipids that work as emulsifiers. By far the most important among these is lecithin. For adequate effect on ice cream emulsification, the mix requires 0.5% to 1% egg yolk. This equals 1/3 to 2/3 yolk per liter of mix, so a single yolk is more than adequate. 2 yolks (3 to 4%) are necessary to get significant thickening / stabilization. For the richest French custard-style ice creams, 4 to 6 yolks per liter is more common, with 8 to 10 being close to the outer fringe.

I would consider anything with 10 yolks per liter a frozen custard rather than an ice cream. Custard texture and flavor will dominate, or at least will compete heavily with everything else. 



Non-Egg Emulsifiers


The following are useful primarily if you’re making ice cream without eggs. Egg-free ice creams can be lighter, can have more vibrant flavors and a cleaner finish, and of course will be completely without egg flavors (although ice creams made with as few as two yolks per liter have essentially no egg flavor). Eliminating eggs can be especially useful in ice creams that have a lot of extra fat from the flavor ingredients: chocolate, nut butters, avocado, etc.  With these flavors, eliminating eggs is a way to to avoid the flavor damping and greasiness that can come with too much total fat content.

Lecithin
Lecithins are complex mixtures of fatty components derived either from eggs or soybean oil. They allow egg-free ice creams to be made with the same, all-natural emulsifier used in custard ice creams. Soy lecithin is as effective as egg lecithin and costs less. A single “large” egg yolk contains roughly 1.5g lecithin. An egg-free recipe can be made with 1.5 to 4.5g lecithin per liter (0.15% to 0.45%) Higher quantities may be helpful in recipes with lots of added fat.

It’s possible that in very delicately flavored ice cream, lecithin will have a detectible flavor of its own. I haven’t found this to be the case (I’ve tried straight soy lecithin out of the jar and found its flavor much milder than milk powder). Flavor may vary with the quality grade of the ingredient. Seek a high quality, very refined lecithin that has a mild flavor, and always disperse it thoroughly in a powerful blender.


Buttermilk Powder 
is similar to skim milk powder, but has a greater number of phospholipids that have emulsifying power. There’s some research into using this ingredient in place of traditional emulsifiers. I haven’t seen research on ice cream specifically, so I can’t recommend quantities or techniques. In larger quantities, it will of course taste like buttermilk. Buttermilk flavored ice cream might be good for first experiments.


Polysorbate-80
Is an emulsifier derived from oleic acid and from a form of sorbitan. It works in minute quantities, so it's both inexpensive and completely flavorless. It belongs to a family of emulsifiers called sorbitan esters, which are believed to be the most active at the fat interface of the ice cream, making them the most efficient at destabilizing the fat emulsion.3 

While the health scares you’ll read the internet are imaginary, I can’t find much compelling reason to use polysorbate in artisanal ice cream. Its advantages over lecithin and egg yolks are in cost and and improved whipability. Cost is a minor issue in small volume production, and most high quality ice creams are intentionally dense (low overrun) and so don’t need improved whipping. If you'd like to experiment, standard concentration is between 0.02% and 0.04%.


Mono- and Diglycerides
These are emulsifiers which occur naturally at very small concentrations in seed oils. Commercial products are manufactured, using a glycerolysis reaction between triglycerides and glycerol—components of fats from either plants or animals. This ambiguity concerning their source gets them flagged by some vegans, although there is no chemical difference in the end product. Unlike sorbitan esters, glycerides are believed to be most active at the fat-air interface, helping the ice cream form a finer-textured foam. I have not seen this confirmed by studies.

Because they naturally form together, the mono- and di- versions are usually used together and packaged together. The monoglycerides, however, are thought to be the functional component. These typically make up about 40% of the blend. It's also possibly to use pure, distilled monoglycerides, which contain nearly pure glycerol monostearate or glycerol monooleate. These work in smaller quantities but are more expensive than the usual mon- / di- mixture.

Versions for chefs are often packaged as “glycerin flakes,” although this is a bit of marketing nonsense, since there’s no glycerin in there. The vendors are fighting the bad reputation of ingredients that are manufactured or hard-to-pronounce. There is plenty of internet hysteria about emulsifiers—most of it dreamed up by charlatans like the Food Babe who deserve your active contempt. 

If you'd like to experiment, standard concentration is comparable to lecithin: between 0.1% and 0.2%.
3-D diagram of a ß-Lactoglobulin whey molecule.
When it denatures (cooks), it unwinds.



Denatured Whey Proteins
Whey proteins make up 20%–25% of milk’s protein content, vs. Casein’s 75%–80%. Whey is much less stable than casein when exposed to heat. At temperatures that are typical for pasteurizing ice cream and thickening custard, whey proteins begin to denature—they unfold from their original shape, exposing previously concealed surfaces to the molecules around them.

Another culinary term for using heat to denature proteins: cooking.

When the whey proteins denature, they expose rows of active, hydrophobic (water-hating / oil-loving) molecules to other proteins and fats they encounter. There are several different types of whey proteins in milk, all of which react differently to different temperatures and environmental conditions when they unfold.

In other words, this is complicated. Not only is my own understanding of the science imperfect, but the science itself, with regards to ice cream, is immature.

Here’s what we know: different types of whey protein denature at different temperatures. They partially denature over time above 60°C / 140°F. They don’t fully denature until they reach 90°C / 194°F or higher. When they fully denature, some may form insoluble aggregates that are harmful to the ice cream's texture. 

When they partially denature, they develop different kinds of molecular attractions. Some of them are attracted to the casein micelles. When they bind to the casein micelles, they form stronger bonds than the casein’s original bonds to the fat globules. When enough of this happens, the emulsion partially destabilizes—which is exactly what we use all those other emulsifying ingredients for.

Other denatured whey proteins are attracted to each other. At some temperatures they begin to form aggregates that make a gelatin-like network in the water phase of the ice cream. Which is to say, they behave like a hydrocolloid stabilizer (see the post on Stabilizers). 

What this tells us: we can, in theory, use partially denatured whey proteins as both an emulsifier and a stabilizer. Some industrial manufacturers, like Haagen Dazs, appear to get all of their stabilization from the whey proteins (although they use eggs as emulsifiers). Others, like Jeni’s Splendid, get all of their emulsification from the whey proteins (although they use starch as a stabilizer). In both cases, partially denatured whey proteins probably play a role in the ice cream’s body and texutre.

How much whey do we need? That's unclear, but a recipe that's high on nonfat milk solids is essential. You're not going to get enough from straight milk and cream. I'd suggest adding nonfat dry milk powder to raise the milk solids into the 10% range (which will improve the ice cream for many unrelated reasons as well).

What’s the optimum time and temperature to get these effects? It’s somewhere between 60°C and 90°C, half a minute and 90 minutes. There is no published definitive research on this, and what exists probably expresses ranges with varying characteristics and tradeoffs.

Back when Jeni’s made their egg-free bases in-house, in a big batch processor (as opposed to a continuous processor, which is more common in dairies and manufacturing plants), they cooked it at 75°C for an hour. That's a relatively low temperature and a very long time.

I cook my ice cream at 75°C for 30 minutes, plus 15 minutes for it come to temperature—for a total of 45 minutes in a heated water bath. This works well for my methods—but I’m not counting on the milk proteins for everything. I use gums to stabilize, and a couple of yolks (or some lecithin) to emulsify. I’m only counting on the proteins for some added body and smoothness. 

If you want to rely on the whey completely, you have some experimenting to do. You’ll probably need:

• a formula with a lot of milk solids
• fresh milk and cream that have been low-temperature pasteurized
• or raw milk, and the knowledge required to handle it without killing anyone
• nonfat dry milk that’s been spray dried at low temperatures

And you may need to cook different portions of the whey at different times and temperatures, to optimize them as both stabilizers and emulsifiers.


Ice Cream With No Emulsifiers
It’s possible to make ice cream with just cream, milk, and sugar, skipping the eggs and everything else. The result is called “Philadelphia Style,” although it doesn’t appear to have any roots or any particular following in Philly. This name may be the coinage of a 19th Century Journalist who needed a catch-all for East Coast ice creams that aren’t French. 


This is good news for the citizens of Philadelphia, since the name is rather slanderous. Philly-style ice cream can have vibrant, clean flavors, but it pays for them with a litany of texture defects: it’s icy, it rapidly gets icier, it whips poorly, the foam structure lacks smoothness (it can be grainy) and lacks stability (it deflates). Some people champion this style, and they’re lucky, because it’s easy to make. But I can’t think of other reasons to recommend it.


Closing Thoughts

Egg yolks work really well. If you don't want the flavor of eggs, or the flavor-dulling of heavy custard, you can use just a couple of yolks, or as little as half a yolk. If you really want to banish egg entirely, lecithin works nicely. I make eggless ice cream bases for flavors like chocolate and nut butters, where the flavor ingredients add tons of their own fat. In these cases a bit of lecithin (equal to what's in two or three eggs) and a bit of extra stabilizer takes care of the emulsion perfectly.

If you want to delve into the more efficient manufactured ingredients, there's a world of glycerides and polysorbates to play with. See the ingredient sources listed in the Stabilizers post. If you find any advantages to these ingredients in Artisanal ice cream, I'd like to hear from you.

You can also take advantage of the natural emulsifying power of cooked whey proteins. The whey's already there, and you're already cooking the mix, so why not take advantage? Generally, the emulsifying and stabilizing power of these proteins will be enough to supplement the more conventional ingredients, rather than replace them.

It may be possible to replace the conventional ingredients entirely, although there's only one reason to do so: “label friendliness.” For retail ice cream, the ingredient list has become a marketing document, and therefore a place to play to the whims of an often-uneducated public.

As long as people believe locust bean gum is somehow less natural or less desirable than tapioca starch—even though it’s a superior ingredient—manufacturers will feel pressure to use it. Or to use nothing at all. Denaturing the whey proteins provides a way—even if somewhate ironic and cynical—to use science in the service of pretending your ice cream is as science-free as Grandpa’s. 

Never mind that Grandpa’s ice cream was never all that good. The idea of Grandpa’s ice cream, in all its nostalgic splendor and purity, has created a standard everyone with a retail label aspires to. 

If you’re making ice cream at home or in a pastry kitchen, you’re lucky. You can use the ingredients that make the best product, without concern for labels and mythology. For you, getting some emulsion and texture control from the whey proteins isn’t mandatory—it’s just the cherry on top.


Next Post: Booze!



1Eventually, milk will separate. Fresh from the cow it will separate over time just from gravity (fat is lighter than water) or faster if put in a centrifuge (the way dairies separate cream from skim milk today). Emulsifiers make an emulsion relatively stable, but not perfectly so. We can add stability by thickening the water portion of the emulsion (we call anything that does this a stabilizer) or by breaking the fat globules up into a greater number of smaller ones. This physical process is called homogenization. Most of the milk we buy has been run through a high-pressure homogenizer, which blasts the fat globules into a bajillion nano-sized ones—they’re so minute and plentiful that the emulsion stays stable almost indefinitely.

2Full coalescence would mean whipping the cream into butter, which we’re not trying to do.

3Ice Cream By Robert T. Marshall, H. Douglas Goff, Richard W Hartel, p.85


For further reading:

Oil-in-Water Emulsions Stabilized by Whey Protein—Effects of Heat Treatment and High Pressure Homogenization
Instability and Partial Coalescence in Whippable Dairy Emulsions (abstract)
Studies on heat-induced interactions and gelation of whey proteins
The Effects of Polysorbate 80 on the Fat Emulsion in Ice Cream
Hydrocolloids as Emulsifiers and Emulsion Stabilizers
Ice Cream. Robert T. Marshall, H. Douglas Goff, Richard W Hartel
Emulsifiers in Food Technology Robert J. Whitehurst.
The Science of Ice Cream. Chris Clark




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