Normally, fresh chunks of apple frozen in ice cream are as hard as a rock. But if you infuse them with a sugar solution, they remain soft and tasty. This sugar solution acts like a kind of anti-freeze because it has a lower freezing point than the apple (see graph below for the explanation).

This I'ble will show you how to achieve this. You will end with a subtle ice cream: at first you just taste the cinnamon, but once you bite trough a piece of apple, it's like suddenly a second flavor is released (apple).

The fastest way of infusing this sugar solution into apple is by using a vacuum chamber (look here for my previous Instructable).
However, in the case of apple, there's a secret: you don't need vacuum, just a little bit more time!

Note
It took me quite some time & effort to get this recipe right. I froze many batches of infused apple in which I varied: vacuum pressure, infusion temperature, time, sugar concentration and chunk size.
By coincidence I discovered that a reference sample that was merely soaked for a long period (without vacuum) showed nice results. It should be noted that this is true for apple. For more details on the process, see step 2 of this I'ble.

Step 1: Why Can It Work Without Vacuum?

If you're only interested in the recipe just skip this section.

Infusing (or impregnation) is forced by two phenomena: capillary force and osmosis.
Roughly one could say that the capillary force infuses the liquid through the pore space of the apple, osmosis runs through the matrix of the apple.

Vacuum will speed up the capillary impregnation by removing the air in the apple and allowing the sugar solution to infuse into the pore space. As a result the contact area increases and osmotic impregnation will speed up as well.
Given enough time, any fruit can be saturated with a sugar solution without the use of vacuum. However, fruit with a softer texture (like strawberry) tends to become really weak when it is soaked in a sugar solution for a long period of time. The end result is no-where near the original fruit and tends to be jelly-like (similar to strawberries in jam). By using a vacuum chamber, this exposure time can be reduced.

Apple has a firm texture that, even after a couple of hours of soaking, remains fairly firm and close to the original. Therefore, apple is a good fruit to infuse without a vacuum chamber (provided you have time because the proces takes longer).

Step 2: Ingredients

This will make approximately 1 liter of ice cream (part of this is air!).

Apple & sugar solution
* 120 g of Apple cubes.
* 180 g of sugar
* 180 g of water
(180g of sugar and 180g of water gives a sugar solution of 50%).

You can choose whatever amount of apple you want. However, keep in mind that the weight of the sugar solution has to be much larger than the apple. I have used a factor 3.
So for 120 g of apple, you require 360 g of sugar solution. Aiming at a weight percentage of 50%, follows 180 g of sugar and 180 g of water.

Ice cream mix
* 0,3 l milk (3% fat)
* 2 sticks of cinnamon
* 4 eggs (yolk)
* 150 g brown sugar. I use brown sugar here for the color contrast with the apple.
* 0,3 l cream (30% fat)

Note
I have seen quite some books on ice cream and have noticed that East and West of the Atlantic Ocean, opinions differ on how ice cream should be made. This one is custard-based.
If you have your own recipe, go ahead and use it, just as long it provides a nice contrast with the apple. This means, it may not be too sweet nor may the cinnamon dominate.

Step 3: Infusing the Apple

Cut the apple in small chunks (smaller chunks will be infused faster, however small chunks have a less dramatic effect in the end result). I have chosen for chunks of approximately 4x4x4 mm. If you want larger chunks, your required saturation time rises (exponentially!)

Dissolve the sugar into the water. If necessary, slightly heat up the water so that the sugar will dissolve more easily (you're close to the saturation point at room temperature). Watch out not to heat it too much (above say 35 C, because apple will start browning).

Place the apple and sugar solution in a bowl and cover it with a foil to seal it from the air. Place a second bowl on top so that the apple remains in the liquid all the time. In this way you don't have to fear browning of the apple.
Leave the bowl at room temperature for 4 hours (placing it in the fridge will slow down the process!).

If you choose to age your ice cream mix over night, be careful how you time this step. Alternatively, you can freeze the chunks on a tray in your freezer (pass them over a strainer first and make sure they don't stick together).

PS I made several attempts to "recycle" the spent sugar solution into the ice cream mix as it contains some of the Apple taste. In theory you can do this by correcting your recipe for water, fat, sugar and solids. However, in practice this has led to poor results (so far).

Step 4: Ice Cream Mix

As stated earlier, if you have your own recipe, go ahead and use it just as long it provides a nice contrast with the apple.
Here's my suggestion for a rich and creamy ice cream (custard-based). It is a short version, assuming that you have basic knowledge of ice cream making. If it doesn't make sense to you, just let me know and I will clarify any questions.

Picture 1
Pour the milk into a pan and add the sticks of cinnamon. Gently heat it up (nearly boiling). Turn off the heat completely and let it soak for 15 minutes.

Picture 2
Meanwhile whisk 4 egg yolks and brown sugar in a separate bowl until it is fluffy.

Picture 3
Take out the sticks and place the pan back on a low fire. Transfer some of the milk into the bowl with egg mixture and mix it gently. Repeat this step until everything is mixed.
Transfer everything back into the pan and slowly increase the temperature to approximately 80 C while gently stirring. You will notice that the mixture is becoming thick and creamy.
Watch out not to overshoot this temperature!

Picture 4
If you don't have any means of measuring the temperature, here's how you can tell: if the mixture's is thick enough to cover the back side of a spoon while you pull it out of the mixture, it is ready.

Remove the pan from the fire and cool it as rapidly as possible (placing it in a bowl of cold water).
Once the mixture is cooled, add the cream and mix it genltly.

Let the mixture "age" in the refrigerator, preferably over night.

Step 5: Churning

Start your ice cream machine and in the meanwhile pass the apple through a strainer (unless you have pre-frozen the chunks).
At the moment that your ice cream is almost ready add the chunks of apple (just at the same time you would add nuts).

Place it in a container in the freezer and let it harden further.