Field Lab 7: Reverse Spherification
A molecular gastronomy example of cross-linking polymers

Seaweed as Natural Resource:   Seaweed is collected around the world and one product extracted from the walls of the plant is alginic acid, which is similar to cellulose.  Both are made from chains of sugar molecules.   Alginic acid is neutralized with sodium hydroxide (NaOH), which converts alginic acid to water soluble sodium alginate.  

The top and bottom of the image are parts of the sodium alginate polymer.  When a solution of sodium alginate is mixed with a solution containing calcium ions, the calcium ions cross-links the alginate chains, which causes them to no longer be soluble.

Alginate uses:     The ability to go from a liquid to gel to solid by cross-linking is useful because one can control the viscosity of the final solution.  It can be thick or thin.   To make dental impressions, a lot of calcium ions are used to create many cross-links which will make the impression stiff.   In other items like ice cream, cosmetics, and wound dressings, there are less calcium ions, so they turn into more of a gel.

Reverse Spherification is different from spherification in that the additives are reversed.  In other words, calcium chloride (or some other salt of calcium) is added to the liquid you which is encapsulate.   That liquid is then dropped into a bowl of sodium alginate.  In regular spherification, the sodium alginate is added to the liquid you want to encapsulate, which then gets dropped into a bowl containing calcium ions.  Either way, the calcium ions will cross-link the sodium alginate chains causing them to solidify.

In this image, calcium chloride was added to grape juice.  Using a syring, it was then placed in a bowl of 0.5% w/v solution of sodium alginate.  

When the drops of grape juice enter the bowl, a skin (membrane) forms around the grape juice creating spheres of grape juice. These encapsulated spheres of grape juice are then added to other food to give a burst of grape juice flavor when eaten.

Both Spherication and Reverse Spherification create spheres of edible liquids (juices, sauces, alcoholic beverages, etc.)  Reverse spherification involves adding a calcium salt such as calcium chloride, calcium lactate, or calcium lactate gluconate to the liquid one wants to be encapsulated within a sphere.   That liquid is then dropped into a bowl that contains sodium alginate.   When the two liquids contact each other, calcium ions in the juice cross-links the alginate in the sodium alginate solution.   That creates an alginate membrane around the drop of juice.   So you end up with edible liquids encased in spheres, which are used to provide bursts of flavor.

Your take-home kit consists of 2 bottles of 0.5% w/v sodium alginate.  Each bottle contains 125 mL.  So that's a total of 250 mL.  The kit also has 2 grams of calcium lactate gluconate which will be added to whatever juice, beverage, or sauce you may use.   There is also a new syringe and a new empty bottle.

1)  Decide what kind of edible liquid you want to encase in spheres.  I had already tried a juice, so here I am trying ketchup.  The formula is to add 2 grams of calcium lactate gluconate to 100 grams (or 100 mL) of the lquid.   You can use the empty plastic bottle to place the juice or other beverage, or a sauce in. 

A1)  What liquid did you choose to get encapsulated as spheres?

2)  Add the packet of calcium lactate gluconate to the bottle (unless you are using a thick sauce).  I made the mistake of adding the calcium first and then adding the ketchup.  Then it becomes very difficult to mix the ketchup with the calcium inside of that bottle.   If your liquid is like juice or a thinner sauce like soy sauce, you can add the calcium first then the liquid.  You will be able to shake the bottle to mix it. 

With a thick sauce, you can add it to the bottle just to get the correct volume, then pour it out into a small bowl.  Then add the calcium powder and mix it. 

2)  Squeeze out the sodium alginate from both of the bottles into a shallow bowl, tray, or food container. 

Here I used the syringe to draw in some ketchup from the small bowl.  Again, if using a thinner liquid, you can draw up the liquid straight from the bottle where you mixed it.

4) Use the syringe to create spheres of the liquid in the sodium alginate bath. You can allow drops to fall into the sodium alginate bath, or you can place the tip of the syringe under the surface and squeeze out spheres of the liquid.

I noticed a problem in using a thick sauce like ketchup.  Ketchup is so thick that when squeezed out of the syringe, it wants to form tubes rather than spheres.    I was able to make spheres if I placed the syringe under the sodium alginate solution at the bottom of the shallow container and then squeezed out the ketchup slowly while keeping the syringe still.

Since the thick ketchup wanted to form strings rather than spheres, I decided to be creative and make more strings of ketchup rather than more spheres of ketchup. 

If I had used tomato sauce instead of ketchup, I realize that I could create noodles (strings) of tomato sauce that could be added to spaghetti noodles.    Then you'd have a bowl of spaghetti that would be all skinny noodles with half made from flour (spaghetti) and half made from tomato sauce (from reverse spherification).

A2) If you can, try to take a picture of your bowl (tray) that has your reverse spherification spheres (or whatever shape you made).  Attach picture to an email or place it in a Word document; or you can send it to my phone from your phone.  If sending by phone, be sure to include your name.

If you can't take a photo, write down a good description of what shapes and sizes where your spheres.

After about 3 minutes, you should remove your encased spheres (or in my case spheres and noodles) from the sodium alginate bath.

Two options for rinsing off the sodium alginate solution from your spheres.

5) Option 1:  Use a spoon to pick up the encased liquid sphere and transfer them to a clean bowl with tap water.  Once in the clean bowl stir to rinse the sodium alginate off of your spheres.  Then pour excess water out of the bowl.

6) Option 2:  Pour the contents of your shallow bowl through a strainer (or colander) to trap the encased spheres and letting the sodium alginate solution pass through.  If done with making more spheres, you can just let the sodium alginate run down the sink.

7) Gently run water over the spheres (spheres and noodles in my case) to rinse off the sodium alginate. 

Since I figured the noodles (strings) of the ketchup would be fragile, I allowed them to sit in the alginate for about half an hour. However, I noticed that instead of stiffening the strings, it just made a thicker wall of the cross-linked alginate around the strings.   I believe if I wanted the strings to become stiff (like gummy bears), I would have do the opposite with the sodium alginate and calcium lactate gluconate.  In other words, I would have had to add sodium alginate to the ketchup and drop that into a bath made for calcium lactate gluconate.   That process is called Spherification.  What we do in this lab is called Reverse Spherification, which apparently keeps the encased liquid remaining as a liquid.

I have to admit that my results don't look very appetizing because these strings look like worms, but those could be a fun food on Halloween.

I ended up not being able to eat my spheres and strings because after I made them, I check the expiration date on the bottle of ketchup.  It expired last summer.  

A3) If you or someone does eat your spheres, did they like them? (Remember they can be added to other food.)

Either send your results to chm107@chemistryland.com or print (or write them down) and hand them to Mr. Costello in the lab class.