Lab Cat

13 Oct 2007

Effect of Solutes on Water

It seems that the stars are aligned or something for me to continue my discussion on water and material science. Not only did Dario comment on my Water Activity post asking me to discuss the relationship between glass transitions and water activity, but in a recent Friday Sprog post on Janet’s blog, she discussed phases of matter and a commenter talked about materials that was both solid and liquid. These materials are known as amorphous materials – there seems to confusion as to whether they are solid or liquid because they can have the behavior of both phases. Amorphous materials do not have the regular, ordered pattern of crystalline solids and they typically have more structure than a liquid.

Many foods are amorphous. In fact, very few foods are true crystalline solids or true liquids. Those that are; salt, sucrose, oils; are generally pure compounds. In most foods, water is present with solutes; salts and sugars; and polymers; proteins, complex carbohydrates, and lipids. These interfere with the standard behavior of pure water.

To get you started Janet has a great post about the typical three phases of matter that you meet in high school chemistry class, namely solid, liquid and gas. In that post she represents a great phase diagram:

Phase diagram from AES

 

This is a great representation for materials such as water that have a crystalline solid, ice, and when pure easily convert into a liquid. The point of change between ice and water is the melting point if you increase the temperature or the freezing point if you decrease the temperature.

As you add solutes to water the temperature of freezing/melting changes. Freezing occurs at a lower temperature. This is one of the colligative properties and is the reason why, as long as you don’t live anywhere really cold like Minnesota, a dilute solution of salt can be used as an antifreeze, as it freezes at a lower temperature than pure water. Here is a phase diagram from Spark Notes showing both the pure solution and the solution with dissolved solutes:

From Spark Notes

This is true of whatever solute you use, so sugar would have a similar influence on the freezing temperature. Thus, food freezes at a lower temperature than pure water. If you have ever seen the Alton Brown episode on Thanksgiving the turkey guy talks about the different temperatures and definitions of fresh, hard chilled and frozen turkey. They are also given here. For instance:

Any turkey labeled “fresh” in a food store has never been cooled to a temperature lower than 26°F, which is the point that turkey meat begins to freeze.

And:

[…]in order to be considered “frozen” a turkey must be cooled to a temperature of 0°F or below.

Obviously, hard chilled turkeys are stored between 0 and 26F.

So where does that get us?

Pure water has three phases: ice, water and steam (or solid, liquid and gas for other materials) and the phase change temperatures are influenced by the presence of solutes and polymers present in the liquid. Additionally, as a food freezes, the water typically freezes out, leaving behind a more and more concentrated solute liquid phase. This can result in an change in pH, viscosity amongst other properties. In some instances, the solute-liquid phase might never solidify. If a food is rapidly cooled to below the freezing temperature, a glass may be formed. A glass is an amorphous solid that has a disordered structure while behaving like a highly viscous solid. In some frozen foods, water will freeze to from crystalline ice and the solute-liquid phase will form a glass.

In my next post, I will discuss in detail the effect of sucrose on water and water activity. This is interesting because it is why we can make different candy types.

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7 Comments »

  1. I’m guessing you be aware that octanol water partition coefficients are used to characterise drug molecules. The idea is that the octanol mimics the hydrocarbon core of the lipid bilayer. While octanol works well experimentally, it’s actually quite a complex system since the hydroxyl groups tend to seek each other out. To make things even more complicated, the octanol is also wet. I think about 2M in water. In 3-4 weeks, I’ll be running a short series of Crapshoots on contribution of hydrogen bonding to binding of drugs to their targets. I’ll let you know when that happens so you don’t need to check.

    Comment by Great Molecular Crapshoot — 18 Oct 2007 @ 5:38 pm

  2. I was checking out your posts with the label chemistry…and I saw this one. Couldn’t help but point out that your description of things that are both solid and liquid as amorphous is different from mine, as I come from a similar but different background – I would have immediately called those viscoelastic materials, and then jumped into the science of viscoelasticity.

    Comment by Flicka Mawa — 7 Apr 2008 @ 7:13 am

  3. Flicka Mawa

    I would interested in reading your posts on the same topic. I don’t come from a material science background so I do not immediately think viscoelastic. So I would definitely learn something from you.

    Comment by Cat — 8 Apr 2008 @ 7:08 pm

  4. As of yet I haven’t made any posts that actually discuss the science I work with or have learned about in my training, since I’ve chosen to divulge a lot of other signifying details about who I am on my pseudonymous blog. But while we have different backgrounds, I actually currently do some work relating to food science, so I like reading your posts!

    Also, thanks for your comment on my post about less traditional science professor careers. If you have any blog posts that have already addressed your experiences around that, I’d love for you to direct me to them!

    Comment by Flicka Mawa — 9 Apr 2008 @ 5:20 pm

  5. […] Teaching — Tags: Links, Teaching — Cat @ 10:15 am In a recent comment to one of my posts Flicka Mawa wrote: Also, thanks for your comment on my post about less traditional science […]

    Pingback by Non Traditional Academic Careers « Lab Cat — 10 Apr 2008 @ 10:15 am

  6. their are no photos with well information about salt oscillator

    Comment by Ryan Kharbotli — 20 Feb 2009 @ 12:42 pm

  7. Your own blog post, “Effect of Solutes on Water | Lab Cat” ended up being well worth writing a comment on!
    Simply needed to announce u really did a superb job. Thanks for the post -Horace

    Comment by yahoo.com — 19 Mar 2013 @ 11:41 am


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