Tuesday, June 24, 2014

The Science of Ice Cream

Even though it's been a warm couple of months already, it's officially summer. A delicious, science-filled way to beat the heat? Making homemade ice cream.

Image Credit: St0rmz via Flickr
Over at Physics@Home there's an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?

Does the milk you use make a difference? 


Most ice cream recipes call for heavy cream, whole milk, or half and half. Does it matter if you use skim milk instead of whole milk? Yes.

There are several different kinds of milk that you see at the grocery store. The defining difference is the amount of fat. Of the ‘regular’ types of milk, whole milk has the highest fat content (3.25%) with skim milk containing almost no fat. Half and half is a mixture of cream and milk, and heavy whipping cream has the most fat. Buttermilk is usually a low fat milk with a (completely safe) bacterial culture added. So if you have that around, you could use that as a substitute. The taste is pretty different, but there are plenty of buttermilk ice cream recipes out there, too.

When making ice cream, the higher fat content in the milk allows for a better texture in your finished dessert. Most milk you buy in a grocery store is homogenized, which means that the fat molecules are already distributed evenly with the other liquids. Fats and water don't naturally mix together smoothly, they normally want to separate. Homogenized milk has already taken care of the problem, and the fat molecules are already safely suspended in clusters among the other liquids.
(A) Two types of liquids like oil & water (B) The liquids start to mix together (C) An emulsion forms, allowing for even distribution like oil molecules in water. Image Credit: Modified from Wikipedia
When you mix the ice cream base together, the fat clusters start to break apart. As you mix, you also introduce air, and the recently disturbed fat clusters keep the air pockets in place. When the air pockets are more stable, you get a smoother ice cream. It’s part of why low-fat/fat-free ice cream in the store has such a different feel in your mouth than the expensive, high-fat kind.

Can you skip cooling the ice cream mix before freezing? 


This also has to do with making a better texture ice cream. For a creamy, smooth ice cream you want to have lots of small, evenly dispersed ice crystals. If the base is room temperature before you put it in your ice cream machine, it takes a long time for the ice cream to freeze, and you get large ice crystals. But if you refrigerate the ice cream base so that it’s cold from the get go, it will freeze quickly and you will get small ice crystals. Even though you may want to skip this step and get to ice cream as soon as possible, it’s worth it to chill your ice cream base before you make it.

This is yet another reason why liquid nitrogen ice cream tastes so great. To make it you mix up a regular ice cream base and just add liquid nitrogen. It freezes the ice cream incredibly fast, but it is possible to over-freeze the base and end up with slightly crunchy ice cream.

Making liquid nitrogen ice cream. Image Credit: H. Michael Miley via Flickr

Why do so many recipes call for salt on the ice? 


If you’re making your ice cream with a crank machine or a bag, almost every recipe calls for ice and rock salt. It's for the same reason cold parts of the country spread salt on wet roads when it's close to freezing. Adding salt lowers the freezing temperature of the water and for wintery roads, it means that the water won’t freeze as easily.

For our ice cream, it allows the temperature of the mixture around the ice cream to get colder. Since the ice cream isn’t just water, it needs to be a little below 32°F to freeze. When using a crank ice cream machine, you frequently have to add more salt and ice, so you can watch the process of melting ice more than once. (It's like watching paint dry, only there's ice cream at the end!)

An electric ice cream maker that rotates a container holding the liquid ice cream in an ice/salt mixture.
Image Credit: Wikipedia
A lot of recipes call for rock salt (including our Physics@Home one), but rock salt isn’t the same thing as table salt. Rock salt isn’t processed as much as table salt, and is much chunkier. It also contains a lot of impurities, which is why it isn’t used for eating. Could you use table salt to lower the the melting temperature of ice? Sure. But there's another reason to use rock salt: it's usually cheaper.


So the next time you go to make homemade ice cream, hopefully you have a sense of what you can swap out, and what steps you really should follow.

27 comments:

  1. But why do we get ice cream headaches?!?!?!?

    I really want to know. - Buzz

    ReplyDelete
    Replies
    1. When you eat something cold, it causes the blood vessels in your head to get larger. It may cause the nerve center above the roof of your mouth to over-react and try and 'heat' your brain.

      Delete
    2. I thought the blood in the blood vessels cools while its passing up towards your brain and so we are acutally cooling our blood to our brain. IDK.. Though, its hard to know what is true anymore about anything. lol. it sucks. but at least were thinking.

      Delete
    3. Okay...

      Physician here to explain what a "brain freezes" is (AND, how to get rid of them faster!). I am compelled to give a more scientific explanation as the previous posts are largely inaccurate.

      First know that there are 12 Cranial Nerves (CN's).

      CNIX and CNX (the glossopharyngeal and vagus nerves, respectively) are the primary cranial nerves (cranial nerve = an autonomic, parasympathetic nerve from the brain) in your body that are responsible for a brain freeze sensation.

      CNIX is responsible for taste in the posterior 1/3rd of the tongue. In addition to taste, however, CNIX also carries sensations of pain and noxious temperatures (hot and cold) to the "higher" levels of the brain from the soft palate (roof of the back of mouth) and posterior pharynx (the beginning of the throat). CNX then carries signals of visceral, or organ-related, pain, from the esophagus, stomach and G.I. Organs all the way down to the distal 2/3rds of the transverse colon (a relatively proximal section of the large intestine).

      [Note that, although CNVII is responsible for taste in the anterior 2/3d's of the tongue, it is not involved in a brain freeze (just the good ole taste of ice cream]

      There are two key differences between the way cranial nerves (autonomic, parasympathetic nerves) send pain message signals to your brain and the way a common peripheral sensory nerve root (for example: nerve root L4 to your leg) sends pain message signals to your brain:
      1) LOCATION of the pain
      2) KIND of pain sensation
      These will be explained below.

      1) LOCATION of pain
      The "location" of the pain from a cranial nerve will often be located in an area separate to the actual area sensing pain. A well-known example of this would be the pain of a having a "heart attack" (acute myocardial infarction): pain in jaw, pain radiating down the left arm, etc...which are carried by CNX. A bruised rib would hurt directly over the injured area, be reproducible when pressed and would not likely radiate into the jaw or arm.

      2) KIND of pain sensation:
      From the same example of someone having a heart attack, the pain will feel more like a "pressure," "unwellness," "sweating," etc... A rib bruise, in turn, will feel like a dull, achy, sharp or achy pain that does not make one feel "sick" (assuming there are no other injuries aside from a simple bruised rib)

      The situation of a brain freeze is no different. You're experiencing "visceral" pain from your throat and soft palate. Although the pain from a brain freeze ORIGINATES from your soft palate and throat (swallowing cold ice cream) it's SENSED in the frontal and outer portions of your head ("brain"). Then, the KIND of pain from a brain freeze isn't exactly the same as tasting or touching a delicious cold ice cream. Instead, from your cranial nerves sensing an abrupt cooling of your soft palate and throat, you experience the unpleasant sensation of a sharp gnawing headache associated with a feeling of unwellness. Although transient, one usually feels unwell enough to joke with their friends about the experience of having a brain freeze, as it is very unique and disturbing sensation (as is a heart attack). Thankfully, a brain freeze is not serious.

      To get rid of a brain freeze faster, simply swallow something either room-temperature or slightly warm immediately after sensing the brain freeze. I personally swallow my spit (which is near body temperature) as many times as I can until the brain freeze goes away.

      Hope this is as interesting for you as it is for me :)

      Delete
    4. Okay...

      Physician here to explain what a "brain freezes" is (AND, how to get rid of them faster!). I am compelled to give a more scientific explanation as the previous posts are largely inaccurate.

      First know that there are 12 Cranial Nerves (CN's).

      CNIX and CNX (the glossopharyngeal and vagus nerves, respectively) are the primary cranial nerves (cranial nerve = an autonomic, parasympathetic nerve from the brain) in your body that are responsible for a brain freeze sensation.

      CNIX is responsible for taste in the posterior 1/3rd of the tongue. In addition to taste, however, CNIX also carries sensations of pain and noxious temperatures (hot and cold) to the "higher" levels of the brain from the soft palate (roof of the back of mouth) and posterior pharynx (the beginning of the throat). CNX then carries signals of visceral, or organ-related, pain, from the esophagus, stomach and G.I. Organs all the way down to the distal 2/3rds of the transverse colon (a relatively proximal section of the large intestine).

      [Note that, although CNVII is responsible for taste in the anterior 2/3d's of the tongue, it is not involved in a brain freeze (just the good ole taste of ice cream]

      There are two key differences between the way cranial nerves (autonomic, parasympathetic nerves) send pain message signals to your brain and the way a common peripheral sensory nerve root (for example: nerve root L4 to your leg) sends pain message signals to your brain:
      1) LOCATION of the pain
      2) KIND of pain sensation
      These will be explained below.

      1) LOCATION of pain
      The "location" of the pain from a cranial nerve will often be located in an area separate to the actual area sensing pain. A well-known example of this would be the pain of a having a "heart attack" (acute myocardial infarction): pain in jaw, pain radiating down the left arm, etc...which are carried by CNX. A bruised rib would hurt directly over the injured area, be reproducible when pressed and would not likely radiate into the jaw or arm.

      2) KIND of pain sensation:
      From the same example of someone having a heart attack, the pain will feel more like a "pressure," "unwellness," "sweating," etc... A rib bruise, in turn, will feel like a dull, achy, sharp or achy pain that does not make one feel "sick" (assuming there are no other injuries aside from a simple bruised rib)

      The situation of a brain freeze is no different. You're experiencing "visceral" pain from your throat and soft palate. Although the pain from a brain freeze ORIGINATES from your soft palate and throat (swallowing cold ice cream) it's SENSED in the frontal and outer portions of your head ("brain"). Then, the KIND of pain from a brain freeze isn't exactly the same as tasting or touching a delicious cold ice cream. Instead, from your cranial nerves sensing an abrupt cooling of your soft palate and throat, you experience the unpleasant sensation of a sharp gnawing headache associated with a feeling of unwellness. Although transient, one usually feels unwell enough to joke with their friends about the experience of having a brain freeze, as it is very unique and disturbing sensation (as is a heart attack). Thankfully, a brain freeze is not serious.

      To get rid of a brain freeze faster, simply swallow something either room-temperature or slightly warm immediately after sensing the brain freeze. I personally swallow my spit (which is near body temperature) as many times as I can until the brain freeze goes away.

      Hope this is as interesting for you as it is for me :)

      Delete
  2. One problem I always have with this...it's a hot summer day, the coldest thing in the whole system is fresh water ice at 0 C. Although the brine stays liquid at 0 C, as soon as it drops below 0 C, it is now colder than the fresh water ice and thus can no longer melt it. The molecules in the brine are moving more slowly than those in the ice, so the ice no longer slows them down and thus the temperature should not drop further. I know this is wrong in practice but I am clearly missing something. I asked my college physics professor this and he could not give me a satisfactory reply.

    ReplyDelete
    Replies
    1. ice is in solid form. Therefore, its particles are actually moving more slowly (still locked in lattice position) than those in the liquid brine (sliding past). Any thermal energy absorbed from the cream will first be used to raise the temperature of the brine (which requires more energy to accomplish because of the solute). Once the average kinetic energy of the molecules (aka: temperature) in the brine has increased enough, then they will begin to transfer energy to the ice, and it will again begin to melt. The temperature of the brine will not increase above the melting point until all of the ice has melted (all energy is used to accomplish phase change before increasing temperature)

      Delete
  3. I am doing a science experiment with homemade ice cream and I found this site quite helpful. I will probably use your easy recipe that you had a link for.

    ReplyDelete
  4. I still dont get it. :( Given the fact ice(snow) melts faster with salt. And ice basically melts slower with ice on it when making ice cream is a total non sense to me.
    So basically, if I put salt on an ice cube it melts faster than the other one without salt. Correct? Then on the other side, how come when making ice cream with the same procedure, when I put salt on ice its temperature gets lower so it will melt slower. So wheres the sense? oh please, i getting frustrated i really dont get it.

    ReplyDelete
    Replies
    1. Yes, Water is a very strange thing. so the misconception here is that the salt affects the average temperature of the ice ether making it hotter or colder depending on who you ask. it really doesn't. Liquid water conducts the transfer of heat very well, but in its frozen form it cannot transfer heat nearly as efficiently. in fact it acts more like an insulator than a conductor. in order to make ice cream you need the water to be cold enough to freeze (it must be below 32 degrees) but it can't be frozen or it won't be able to transfer the heat out of your ingredients fast enough to freeze it. the salt allows this to happen simply by allowing water to be in its liquid form at temperatures much lower than 32 degrees. so the ice DOES melt, but it stays the same temperature it was when it was frozen with the added benefit of being able to conduct heat much better.

      Delete
    2. This comment has been removed by the author.

      Delete
  5. Snow doesn't melt faster put an ice cube with salt and one without and see what happens,

    ReplyDelete
  6. I always see the cream/whole milk mixture in ingredients. Can one make ice cream using only heavy cream?

    ReplyDelete
  7. Couldn't you just say that the ice cube with the salt melts because with the salt it can't stay in it's solid frozen form because it now needs to be colder than what it was?

    ReplyDelete
  8. What makes ice cream cold?

    ReplyDelete
  9. What happens if you use milk that is not homogenized?

    ReplyDelete
  10. What will the texture be like if you use milk that is not homogenized?

    ReplyDelete
  11. why is milk used in many other deserts????

    ReplyDelete
  12. One of the comments mentioned there was an ice cream recipe link. I'm not seeing it. Was it removed?

    ReplyDelete
    Replies
    1. It's linked as the text "buttermilk ice cream" at the end of the second paragraph under heading "Does the Milk..."
      Here's the URL, though:
      http://smittenkitchen.com/blog/2009/04/buttermilk-ice-cream/
      Thanks for reading!

      Delete
  13. I bought an icecream maker on the cheap on ebay. Apparently it had been in a closet for a few years. When I opened the freezer bowl, there was 'dust' (white crystal-like granules' that spilled out and 'corrosion' collected on the side and top of the unit. There's a leak at the top of the freezer unit. I can't get all of the coolant off of the unit and some of it is in the inside part where the ice cream would be. How should I dispose of this thing?! Because I clearly can't use it to make ice cream. Or can I?

    ReplyDelete
  14. You scientific morons forget the discussion was about ice cream.

    ReplyDelete
  15. THANKS DUUUUUUUUDE!

    ReplyDelete
  16. This is facebook for nerds lol!

    ReplyDelete
  17. Hi, I have made non churn ice cream in the freezer with sweetened condensed milk and heavy whipping cream and flavoring. I whip it together with a mixer, then freeze. It is very rich. The problem is that some of my family does not like the condensed milk taste. I was wondering if u can use whole milk and heavy whipping cream and sugar and flavoring, instead if the sweetened condensed milk, or will the whole milk and heavy whipping cream separate during freezing, even though i have whipped it during mixing? I do not have an ice cream maker and have to use a non churn, freezer method. I am trying to find a better tasting ice cream. What can I use in place of the sweetened condensed milk? Would whole milk work? Thank you!��

    ReplyDelete