Ice Cream Junk and Stuff #2

I’ve got some more short topics to talk about...

Atkins Candy Bars

Making regular sugar-based ice cream is easy. Want to make M&M ice cream? Add some M&M’s to vanilla ice cream and you’re done. Want to improve it? Freeze more M&M’s, grind them up into fine particles while frozen, and add that to the ice cream while churning. There is no shortage of sugary stuff that can be added whole or crushed up to ice cream.

Atkins Nutritionals makes a bunch of candy look-alikes. Why don’t I use them? It’s because they use maltitol, inulin, and polydextrose in their products. They claim they don’t significantly raise your blood sugar, which may or may not be true, but they sure a heck upset my stomach in large quantities. It would be SO much easier to walk into a store, buy a box of Atkins candy, chop it up, and be done with it. Unfortunately, I can’t, so I don’t.

Milk Powder

You may have seen that I’ve been introducing low carb milk powder in some recipes. I’ve been experimenting with it and it works really well to turn almond milk into a better whole milk substitute. The non-fat milk solids allow the finished ice cream to turn out less icy.

If you’re trying to cut down the calories of an ice cream recipe, you can substitute some of the heavy cream for almond milk plus low carb milk powder.

The formula is: 1 tablespoon LC milk powder per ¼ cup of almond milk. For ice cream, this translates to:

½ cup heavy cream = ½ cup almond milk + 2 tablespoons LC milk powder

¾ cup heavy cream = ¾ cup almond milk + 3 tablespoons LC milk powder

I wouldn’t replace more than ¾ cup of heavy cream since the ideal ratio of most home ice cream recipes is 2:1 heavy cream to whole milk. I’ve only experimented with the no-cook low carb ice cream base. I haven’t done any cooked custard ice cream bases using almond milk and milk powder other than chocolate.

Now, the ice cream is good, but it’s definitely more prone to having an icy texture when substituting heavy cream for almond milk even with the milk powder. However, I have a solution for that and it’s the next topic of this post.

Mini Freezer

The most important appliance in making good ice cream isn’t the ice cream maker. It’s the freezer. The faster the ice cream gets frozen, the smaller the ice crystals. A professional blast freezer/chiller has an operating temperature of -40 °C (-40 °F) and includes a fan (or fans) to circulate cold air to speed cooling. The cost of a small counter-top blast freezer is well over $5,000. Instead, I found an inexpensive medical freezer online for under $200.

The specs on the website claim the temperature is -20 °C (-4 °F), but I’ve tested it on its coldest thermostat setting and have achieved temperatures below -30 °C (-22 °F)!

There’s not a whole lot of room inside, but it can fit plenty of ice cream quart containers.

According the the Energy Star rating, it should only cost about $26 a year to operate, but since I’m running it colder, it will probably use more electricity. It’s a manual defrost, but for that price, it’s expected and actually desired. I wouldn’t want the freezer to begin a defrost cycle in the middle of a freeze.

The Science of Ice Cream Hardening

An ice cream maker is the first step in the freezing process. It’s job is to cool and incorporate air into the mix. The resulting ice cream is usually the consistency of soft-serve, so additional cooling is required to harden it sufficiently.

In the traditional ice cream process, when ice cream is drawn from a [continuous or batch] freezer and placed in containers, it is of a semisolid consistency and is not stiff enough to hold its shape. Therefore, the freezing process is continued in containers without agitation until the temperature reaches −18 °C (0 °F) or lower, preferably −25 to −30 °C.¹

A typical home freezer (in a full sized refrigerator) usually operates at approximately -18 °C, which is well below the freezing point of water (and ice cream). However, it takes time for the ice cream placed in a freezer to reach equilibrium with its environment.

Convection and Conduction

Cooling thorough convection (the cold air in the freezer) is the first step. Blast freezers help speed up this process by forcing the air to circulate across the food much quicker. Cooling also occurs from the outside of the ice cream container penetrating into the center through conduction. Getting ice cream completely frozen to its core isn’t a linear process. As more water in the ice cream is frozen, the change in state requires additional energy. This has an insulating effect on the core, which slows the freezing process down.

Hartel (1998) shows temperature profiles at different depths into an individual half-liter package of ice cream during hardening in an air-blast freezer at −30 °C. The surface cooled to −18 °C in about 13 min, whereas the center took nearly 17.5 min. More importantly, 6 or 7 min elapsed before the center started to cool. This extended time at −6 °C led to a substantial increase in ice crystal size at the center due to ice crystal ripening. The surface had approximately the same ice crystal size as the initial product (median size of 27–28 mm), whereas the median ice crystal size at the center of the ice cream was nearly 10 mm larger. Clearly, rapid hardening is desired to ensure the smallest ice crystal sizes.²

Ice cream chefs use chemistry to keep the ice cream from getting an icy texture. The most common methods involve using ingredients that bind to water molecules. Glucose, casein, gums, gelatin, emulsifiers, etc. are used by many home chefs to mitigate ice crystal growth. However, we can also attack the problem by increasing the rate at which the ice cream is frozen.

Quantifying freezing speed

I’m not interested in predicting with great accuracy the overall freezing time. I’m more interested in knowing the major factors that effect it and work to shorten the freezing time as best I can.

The governing equation for the rate of heat transfer (Q) is given as:

Q = U × A × ΔT

where:

U is the overall heat transfer coefficient.
A is the exposed surface area.
ΔT is the temperature differential between the freezer and the ice cream core.

The heat transfer coefficient, U, accounts for (1) conductive heat transfer out of the ice cream and through the package and (2) the convective heat transfer coefficient between the blowing air and the package. To maximize Q, each of the three factors should be as large as possible.³

Maximizing A could be as simple as using smaller containers. Of course, the packaging material may act as an insulator, so perhaps the benefit is negligible or non-existent. The variable that is the most straightforward to increase is ΔT and the simplest way to accomplish that is by using a freezer that’s colder.

The difference in temperature between the product and the cooling medium defines ΔT. The greater this difference, the faster heat will be removed; consequently, the higher the temperature at drawing for a given hardening tunnel, the longer the time of hardening. This implies the need for very cold hardening temperatures, such as −35 to −40°C, even though it is not necessary to get core ice cream temperatures this low. As freezing proceeds, the ΔT decreases and thus, the rate of temperature decline toward the end of the process is much slower than the rate of temperature decline at the beginning of the hardening process. Very cold hardening temperatures help to maintain a ΔT to speed up the process.⁴

Did the Mini Freezer Work?

Yes! The texture of the ice cream with almond milk plus milk powder is much smoother and not crunchy. It was a relatively inexpensive investment. I’d like to do the Low Carb Coconut Ice Cream with Saffron again to see how it will come out under the lower freezing temperatures.

References

1. Goff, H. Douglas, and Richard W. Hartel. Ice cream. 7th ed. New York: Springer, 2013. p 299.
2. p. 300.
3. p. 301.
4. p. 301.