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Cookie Dough Fudge Mint Chip, 2.0

April 24, 2010

So here we are again, making CDFMC ice cream. This time, it a chocolaty-fudgy ice cream, with bits of cookie dough and chocolate covered mint patties. I should be using mint flavored chips, but those are not available where I live.
Ignoring that minor peeve, the science of ice cream today is about the cream itself, milk, ice, and emulsions.
Lets start with a basic quaestion:

Water and oil don’t mix. Why?
Usually, different substances don’t mix because the bonds between similar molecules are stronger than the bonds between different molecules.
That’s Enthalpy. However, with water and oil, the reason is Entropy, the desire of a system to have as much disorder as possible. You would think that mixing would create more disorder, but that requires a lot of space, so that the molecules could avoid each other, like in the gas phase. In a liquid it’s different- the molecules are too close together and they can affect each other.
A water molecule has 6 different ways for hydrogen bonding with two other water molecules. Replacing one of those molecules with one that cannot form hydrogen bonds reduces the potential conformations by half, limiting the movement of the molecules and forcing a higher degree of order on the system. A higher degree of order requires a greater amount of energy to maintain, and therefore is not a favorable situation.
If oil molecules stay together, apart from water molecules, they reduce their exposure to the water and reduce the entropy loss.
The hydrophobic effect is entropy driven, and the attraction of the water molecules to each other is only a secondary force. It has nothing to do with the way water dissolves polar molecules, such as table salt.
If you can reduce the order of the system, or input a lot of energy, you could force the oil and water to mix. For example, you could boil a lot of water, with a tiny bit of oil, and eventually they’ll mix.
You could also use other molecules as barriers, which will reduce the contact area between water and oil, but at the same time allow them to mix together. That is what emulsifiers do.

What are emulsifiers?
Molecules with a dual nature, just like soaps and detergents.
Their dual nature allows them to remain in the interface between water and oil, and to help form and stabilize bubbles of one liquid within another.
The emulsifiers in milk and other emulsions are found in very small quantities, and they cover the bubbles only partially, so their stabilizing effect is not complete.
Emulsifiers can be soaps and detergents, but they can also be large molecules such as proteins and complex sugars, which is the case with milk and beer (in beer they stabilize gas bubbles, remember?).
Because emulsifiers play only a small (but important) part in stabilizing the emulsions, the emulsion’s own attributes become more prominent.

Milk is an emulsion of oil in water
The oil in milk is broken up in to very small bubbles, on the scale of 100nm, which is also the scale of wavelength of visible light. Bubbles that small are somewhat stable and it takes them a while to combine together and form a single layer of fat. The other thing they do, is break the path of the light that goes through them, separate it to its components (red light, blue…) and make a mess of it, so instead of two transparent liquids, we see one white liquid.
Milk straight from the cow isn’t that stable, and it will separate in a few days, because the bubbles are of un-even size and larger than the store-bought milk. The milk we buy at the store has gone through a lot of to become stable for more than a couple of days.
It is heated and pasteurized to prevent it from fermenting into yogurt or cheese.
It is separated into a water layer and a fat layer, along with anything that is soluble in those layers. Then the fat is returned in to the water layer at a specific and predetermined percentage, so that each carton of milk will have the same amount of fat and calories in it. And then the milk goes through a homogenizing machine, which forces the oil bubbles to break into much smaller bubbles, all the same size. The bubbles remain that way for a certain period of time due to the natural emulsifiers found in milk.

Size, and the degree of variation, matters
A long time ago, there lived a man named George Gabriel Stoke. His contribution to today’s post is Stoke’s Law, which is a mathematical representation of the forces that affect a solid round object, as it sinks to the bottom of a vessel, through a liquid. This physical law can be converted and applied to liquid systems, to represent how round blobs of oil behave in a water environment (or vice versa).
According to this law, there are 4 things we need to consider:
The difference of densities between the oil and the water.
The difference in the viscosity of the two liquids.
The size of the bubbles.
The degree of variation in the size of the bubbles.
These four parameters affect the Brownian motion (random motion), the gravitational pull on the bubbles and the contact-induced motion of the bubbles. The gravitational motion is the most prominent of these.
The mathematical representation of the law implies that the bubbles should be as small as possible and as uniform as possible, in order to prevent them from concentrating at the bottom. Also, the emulsion shouldn’t be kept in large and tall containers, but rather in small, short ones.
The purpose of this is to reduce the gravitational pull on the bubbles, and their chances of bumping and connecting.

What are we trying to prevent?
When bubbles meet, there are four processes which might take place:
Creaming or sedimentation- depending on the difference of densities between the water and the oil, the bubbles either float up (creaming) or sink down (sedimentation) in the fluid. The problem is the same- all the bubbles of a certain substance become concentrated in a single place. This process is reversible- all you have to do is shake or stir the container to redistribute the bubbles.
Flocculation- bubbles touch and stick to each other, to form clusters. The clusters can be forced to disperse by using a great deal of force and energy and homogenizing the emulsion once more.
The clustering is caused by the emulsifier molecules, which are very long. They can partially detach themselves from one bubble and attach themselves to another bubble, thus linking the bubbles together.
A different option is that the emulsifier tails on one bubble entangle themselves with the tails of other emulsifiers on the other bubble.
Coalition- if the bubbles are close enough together, and the partial coverage of the emulsifiers allows for uncovered patches on the surface of bubbles, the bubbles may combine to one larger bubble.
To prevent coalition, you can increase the amount of emulsifier, but that costs money, and might result in flocculation. Or you can use an ionic emulsifier, so that the charge on the bubbles will repel them from each other. The problem with using an ionic emulsifier (detergent) is that it may not be enough to stabilize the formation of bubbles to begin with. It’s very difficult to find the right combination of emulsifier characteristics needed for a certain emulsion.
Phase separation- the final result of the previous processes is phase separation, when all the bubbles have gathered in one location, clustered together and combined to form one larger bubble, until just a single bubble is formed- the phase that is separate from the surrounding solvent.
The larger the bubbles are, and the greater their size distribution is, they will cream and flocculate faster. That’s why it’s very important to use the right emulsifier, at the right amount, and also to force the bubbles into small, evenly sized bubbles (homogenize the emulsion).
Milk is not the only food emulsion around. Margarine is an emulsion of water in oil, which uses the fat itself as an emulsifier (tiny globules of fat cover the face of the water bubble and protect it. The coverage and emulsification is physical, not chemical). Mayonnaise is an emulsion of water in oil, where proteins and other molecules in the egg yolk act as emulsifiers between the oil and lemon juice. When making salad dressing from oil and lemon juice, the citric acid in lemon juice acts as an emulsifier.
Making all these emulsions requires the energy of stirring, because the emulsifiers alone are not enough. Using greater amount of emulsifier, or using more efficient emulsifiers, will reduce the amount of stirring needed, up to a point where the presence of the emulsifier would be enough to create an emulsion. That is a special case, known as micellization and micro-emulsions, which lead to liquid crystals- a vast and interesting field, which I might post about some other time.

Ostwald ripening and icicles in your ice cream
A more in depth explanation of flocculation and coalition is Ostwald ripening. This is a process in which the growth of a large cluster is more thermodynamically favored than the separation of that cluster to its smaller components. On the other hand, a small cluster has better chances of disintegrating than growing.
Statistically speaking, a cluster with two monomers has a great chance of disintegrating- just one monomer needs to leave in order to destroy the cluster. A large cluster, of one thousand monomers, has a much smaller chance of disintegrating, because 999 monomers need to leave. It has a much greater chance of growing.
Energetically speaking, the molecules on the surface of a cluster are less stable than the ones hidden in the interior. The greater the ratio of surface to volume, the less stable is the cluster. Larger clusters have a smaller ration and therefore are more stable.
If you let your ice cream out of the freezer to defrost just a little, small icicles will start to form. Even after you return it to the freezer, those clusters will continue to grow, and your ice cream will become gritty and unpleasant to eat.
Home made ice cream is more susceptible to this process, because the mass produced ice cream has the benefit of powerful homogenizing machines, and special emulsifiers, to give it a good start.
The home made ice cream is mostly based on the emulsifying qualities of eggs, which is why a custard-based ice cream is much creamier than a non-custard ice cream.
Your ice cream maker is your homogenizing machine, and the more efficient it is, the creamier your ice cream will be, even without eggs.
The problems start when you’re trying to make a sugar syrup-based sorbet, without an ice cream maker. No emulsifiers and no homogenization. Bad idea.

My new recipe, with pictures, in the next post.

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