Why is bread not glue if they’re both water and starch?

After a pinata party, Maria e-mailed me the following question:

Hey, why does water with flour become glue for pinatas? Is there a scientific explanation for the stickiness? How come it doesn’t work like that when it makes bread? Is bread sticky too? I’m really confused!

Off the bat, I knew that flour has starch and the starch makes the glue sticky, but then I started wondering too. Why is starch sticky? And why isn’t bread dough more like glue?

 

Starch

Starch is the major component of flour. Starch exists in tiny granules, which swell and break when boiled in water. This releases the starch molecules, which then all stick together to make a goopy, gluey mess. [1]

 

Starch glue

You can make starch glue from flour and water, by mixing one cup of flour in some cold water, and adding this mixture to 3 cups of boiling water. You can even use less flour, and it will still work. [1]

Starch glue is good at sticking paper to paper, to make pinatas for example, because paper itself contains cellulose, which is quite similar in structure to starch. They’re both polysaccharides. You can’t use starch glue to glue plastic or metals – only other polysaccharides. [2] (In making pinatas, the first layer of paper sticks to the balloon. I think this works because while the glue is still wet the wet paper will stick to the balloon. Once the glue is dry, it actually doesn’t stick to the balloon anymore, but at that point the paper has been permanently shaped around it.)

 

Bread

Like starch glue, bread dough also contains water and flour. Still, it’s not as sticky – you can’t use bread dough to glue paper together. Let’s look at the ingredients for a super simple recipe to find out what’s different.

Ingredients for bread
1 c. milk (Hot)
1/4 c. sugar
4 T. margarine
1 tsp. salt
1 pkg. yeast
1 egg
4 c. all-purpose flour
1/4 c. warm water
[3].

Instead of 1 cup (or less) of flour and 3 cups of water, we now have 4 cups of flour and 1.25 cups of warm water and milk. (The rest of the ingredients don’t add a lot of water or starch, so I ignored them, although they will make a difference to the final dough: the yeast, for example, makes the dough rise.)

The ratio between the flour and water is completely different between glue and dough. Would this be crucial in whether or not it’s glue?
According to this website on the science of bread ingredients, the amount of water is indeed the answer:

In bread making not as much water is added as when making a sauce or gravy, and gelatinisation isn’t completed – the starch granules swell, and many don’t burst to form a gel. This forms a network of bloated starch granules all touching at the edges. [4]

So there you go: water and flour glue is sticky only for paper and other cellulose-based materials. It’s the large amount of water that makes it sticky, because it releases the starch from starch granules. Bread dough doesn’t have that much water, and therefore isn’t as sticky.

Save

Eva

Eva Amsen is a writer, science communicator and blogger, interested in the overlap between science and music, art, pop culture, and daily life. Portfolio | Twitter | Contact

You may also like...

13 Responses

  1. Maria says:

    Wow, thanks. It is so interesting and I knew you would find the answer! Does this mean we can actually eat the glue and not get sick?

  2. eliane says:

    Now, as a housewife, I could have told you this right away. Making bread dough, you start out with dry flour. And then you add a little bit of water and then you start kneading. At first you have some sticky stuff and lots of dry dough, it looks like a mess. But then if you continue, you will end up with a beautiful ball and clean hands!

  3. eliane says:

    lots of dry four I meant to say. Not lots of dry dough, sorry.

  4. eliane says:

    flour, not four. I am a mess myself, today.

  5. Lab Cat says:

    Also the water temperature is important. The hotter the temperature the more the starch granules absorb and swell.

    Bread dough also is different because kneading also causes the protein, gluten to form. This gives the dough it elasticity.

  6. Shelley says:

    Adding salt to the bread also matters because it stabalises the gluten, making it stretchy instead of sticky. The yeast and sugar are for raising, together they make pockets of air trapped by the strands of gluten. So they are important for nice bread but not for your stickiness discussion. Everything else is just flavouring or for changing the texture of the bread.

  7. fornetti says:

    I do not believe this

  8. uffda says:

    OK OK, now i know not to be so careless with adding the water….to avoid another sticky mess the likes of which i made last night! had to cut the bums in half and pop them back into the oven to dry out the middle. couldn’t find the molasses, so added more brown sugar and added water. THAT’s probably where all that provoking sticky came from. molasses is on the shoping list. thanks for being on line.

  9. uffda says:

    “cut the bums in half”….i’m zonked! LOL cut the BUNS in half….

  10. cal mccune says:

    If I mix flour with cold water and do not boil and then use as a glue, is the process safe from bacterial development if I only use mix for about two hours?

  11. ADISA GEORGE says:

    Tank u very much for your explantation,but there is one problem that i have now,how to disolve starch glue to make it to be smonth befor apply with cloth,if you can tell me i will appreciate.

    Thank u God bless.

  12. zeeky says:

    is this the same glue used in offices and schools

  13. Eva says:

    Zeeky, it’s the kind of paste sometimes used in kindergartens (that you put on with a paintbrush). Not like glue-sticks or the transparent and alcohol-smelling bottles of glue, but white and runny goop.
    Hope that made sense!