Originally posted April 01, 2011

Caveat emptor!!

April 01, 2011

… And, I’m back! Sorry for my absence from blogging for the past couple of weeks. I know all of you have been terribly disappointed that I’ve been gone for so long. It’s been a crazy couple of weeks with classes and grants and working on a gift for my post-doc advisor’s 75th birthday party. (On the suggestion of my good friend Paul, I plan on talking about the gift that I put together in a future blog post. It’ll have a lot of inorganic chemistry, history, and lots of colors.) But, today, I’m back to talking about food. And, to be honest, I’ve missed writing my posts. There was something odd about not writing for this site for the past few weeks. I’m excited … so let’s get to it!

I was paging through my copy of On Food and Cooking recently when I came across this little informational gem: “Canners frequently add calcium salts to firm the cell walls and keep the pieces intact, and this can interfere with their disintegration during cooking. If you want to make a fine-textured dish from canned tomatoes, check the labels and buy a brand that doesn’t list calcium amount its ingredients.”

Curious little tidbit indeed. As usual when I’m reading this book, I find Harold McGee’s factoids so interesting that my curiosity pushes me to dig up some more info! Why would calcium ions (Ca²⁺) make such a big difference in the the way canned tomatoes are cooked?

Pectin

The answer, it turns out, has a lot to do with pectin. (But of course you guessed that already, didn’t you? You’re so clever!) Pectin, like cellulose or amylose (starch), is a long string of sugars that are all connected together, end to end. Also like cellulose and amylose, pectin gives structure and strength to the cell walls of plants.

(top) Individual sugars (D-Galacturonic acid) joined end to end to form the pectin polymer. (bottom) A schematic of a plant cell wall highlighting the structural polymers: cellulose, hemicellulose, and pectin. (Image source for bottom image.)

Pectin is different from cellulose and amylose in that pectin is acidic. That means when pectin is surrounded by water, it gives up H⁺ ions and becomes negatively charged.

The sugars in pectin give up H⁺ ions to become negatively charged.

Now, this is where the calcium ions come into play. Ca²⁺ is positively charged and, therefore, attracted to the negatively charged sugar (specifically to the carrier of that negative charge, the acidic oxygen). Ca²⁺ (a hard metal) is really good at binding negatively charged oxygen atoms (hard ligands). So, if you add Ca²⁺ to canned tomatoes, the Ca²⁺ can bind to sugars in different pectin polymers, holding a network of pectin together. These interactions with Ca²⁺ are what cause pectin to gel when you are making jam (or similar food where pectin is added as a thickener) using low methoxy pectin (aka low sugar pectin).

(left) Ca²⁺ binds to two negatively charged oxygen atoms. (right) A schematic showing how Ca²⁺ ions can hold multiple pectin molecules together. (Image source for image on right)

Back to the canned tomatoes … The canners who add calcium salts to their tomatoes before they can them are really just trying to firm up the structure of the tomato. The calcium stabilizes the pectin that helps to hold the tomato together. The canners want the tomato to look like a tomato (and not a squishy mess) when you open up their product. I understand the how this can be appealing to a consumer. We like to think that we’re getting a fresh product, even if it comes out of a tin. But, for the case of making a tomato sauce, adding calcium is a really bad idea.

A good tomato sauce has a velvet-y texture and “mouthfeel”. The smooth texture that you get in a quality tomato sauce comes from a partially gelled network of pectin that has leaked out of the tomatoes and developed in your sauce. (This pectin gelling works the same way in the process of making jam with the gel in the jam setting to a greater extent.) During cooking, the pectin will seep out of the tomatoes’ cell walls and into the sauce. Initially, the pectin is surrounded by a lot of water and won’t gel. But, as you keep cooking, that water evaporates, the polymers of pectin can find each other, start to gel, and give your pasta sauce a much more satisfying texture. When canners add Ca²⁺ to their tomatoes, the pectin is stabilized in the cell walls, it never has a chance to seep out from the tomatoes, and using these tomatoes will result in a watery, more chunky tomato sauce.

So, in summary, calcium is good for your bones and bad for your pasta sauce.

The next time you purchase a can of tomatoes at the grocery store, make sure that the manufacturers label does not say, “contains calcium salts”. Your pasta sauce will be much better off without them.

Cheers

-mrh