Science Buddies: "Ask an Expert"

Hi, please can someone help make sense of something for me?
I've found a paragraph in a book that says "starch is useful because its molecules are both compact and insoluble, but are readily hydrolyzed to form sugar when required".

I don't understand how something can be insoluble yet easily hydrolyzed at the same time. I thought hydrolyzed meant that something dissolved when mixed with water?

Hello and welcome to Science Buddies!

That's an excellent question regarding a seemingly confusing name.

Let's first think about what solubility means, because that's the easier concept of the two. When something - let's say, table salt (NaCl) - "dissolves" in water, this is due to polarity. Water is a polar molecule, with a positive end towards the hydrogens and a negative end towards the oxygen. Salt is also composed of polar components: a sodium ion (Na+) and a chloride ion (Cl-). Outside of water, these ions are attracted strongly to each other, making a single solid compound. But water breaks them down into their ions by attracting and surrounding them, pulling them apart - pointing their negative oxygens towards the positive Na+ and their positive hydrogens towards the negative Cl-.

This is the basic principle of solubility in water. If it is polar or made of polar components, water will "break it down" and dissolve it. Note that dissolving something does not actually cause a chemical reaction; in other words, neither the water nor the salt are chemically changed into something new.

Now for the other one. "Hydrolyzed" refers to a chemical's ability to undergo hydrolysis, a special type of chemical reaction that involves water as a reactant. Here, water turns from H2O into a hydrogen ion (H+) and a hydroxide ion (OH-). Then, the hydrolyzed substance - like starch - will break a bond to take the H+ ion. Note that here, both the water and the starch have been involved in a reaction that changes both of their structures. Water has been "ionized" into two different things that aren't water, and the starch has had a chemical bond actually broken.

In starch specifically, the hydrolysis reaction will break it down. Starch is a polysaccharide, which just means it's a bunch of molecules called "saccharides" strung together on a series of chemical bonds. Hydrolysis breaks these bonds down, cutting starch into smaller and smaller strings of saccharides. Think of it like a string of Christmas lights: each light is a saccharide, and the water is taking a pair of scissors to the wire between them. (Note: do not try this at home. )

Hope that helps! Feel free to ask more questions!

Hi,

You are right that hydrolysis involves water and results in an aqueous solution, but there's more to it (naturally!)

The word 'hydro lysis' means taking apart (lysis) by water (hydro), but what does the taking apart is the key. Sometimes water alone is sufficient as in the hydrolysis of acids and bases. https://en.wikipedia.org/wiki/Hydrolysis

In the case of corn starch, however, its relative insolubility prevents hydrolysis https://en.wikipedia.org/wiki/Glucose_syrup

If you have made a corn starch based sauce in a recipe you know that heating the corn starch in liquid transforms it into a nice creamy sauce. The heating in water breaks down some of the bonds to give it a jelly-like consistency https://www.scientificamerican.com/arti ... ooey-gels/

This is a kind of hydrolysis by heat, but there are two others that are commonly used--acid and enzymes. Corn starch can be partly broken down (hydrolyzed) by an enzyme called amylase that is a special type of protein that is able to break certain chemical bonds in the starch to release its sugars. This is a type of enzymatic hydrolysis.

Starch can also be chemically broken down by heating in acid and the resulting compounds will be water soluble.

Check out the websites I listed and if you have more questions let me know.

Sybee

thank you so much guys - those were both really helpful!! I've just started a course and am not very science minded, so struggle with the basics!!

So basically, hydrolyzing is breaking it down into smaller pieces rather than dissolving it?

Hydrolysis is a chemical reaction because some of the bonds in the starch are broken. When a chemical like sodium chloride dissolves in water it is usually considered a physical change because the ions separate but no bonds are formed or broken.

After the starch is hydrolyzed, the sugars are dissolved in water so you could say that this is an example of a chemical or enzymatic reaction that leads to a solution.

If you don't understand a lot of this stuff, go to YouTube and watch some videos. There are plenty of good chemistry videos that will give you a quick education in a very useful subject.

Sybee