Friday, July 28, 2006

Perfecting Puree's

Like myself, you fellow dessert makers have no doubt found yourselves, at one time or another, pureeing berries such as a strawberry, or perhaps a raspberry. A simple task, requiring you to throw the berries in a blender, perhaps with some sugar, spin and strain. Often finding myself pureeing some berry or another, but not settled with random results, or just what ever comes out of the blender cup, I have worked on a few additional steps to ensure certain results. Could I let a puree be simple? Of course not.

The little alterations to the aforementioned process, simple in and of themselves, result in a puree that is thicker, brighter, and more luxurious.

First, because I have flats of berries passing through my hands, I choose only those that are ripe ripe ripe. Often these berries are so ripe that they don't look pretty enough to serve. The ripest berries have the sweetest flavor and are perfect for puree's. Don't feel that you need to let your berries get over-ripe, however. If they are ripe enough to eat, then they are ripe enough to puree.

Second, after washing, I freeze them. Spread in a single layer on a sheet pan, the berries are left to freeze solid, then gathered and stored overnight in a container with a tight, protective seal.

Third, when defrosting them, I transfer them strait from the freezer into a bowl, and toss them with a little sugar. When they are defrosted most of the way, but the temperature is still near freezing, I puree them. The puree is strained through a fine mesh strainer to remove all the seeds.

After the straining, the puree is super thick and luscious, and the color is as bright as can be. The puree stands up tall on a plate, working perfectly as a sauce. The thicker puree folds into recipes better, covers desserts better as it both sticks to them without running off, and is easier to pick up with your spoon making for a very happy mouth.

It is often enough to simply know a process works. I have followed these steps many times, observing successful results, and can enter this endeavor with confidence that I will achieve these results every time. But could I let it be that simple? Of course not.

I have to ask "why?"

When I ask why, I go to two places. First, Harold McGee. Well, not him, per se, but his book.

Then, when I find what I can from Harold, I ask my friend Chris Young. Chris is the research assistant to Heston Blumenthal, and runs his "Atomic Kitchen" or laboratory and the intense study of gastronomy that goes on inside. I often post him questions like, "Why does Elderflower taste so familiar to everyone, yet like nothing they have ever tasted?"

Or "Why does freezing my berries make such a huge impact on the resulting thickness, color, and flavor?" The answer is simpler than I thought. Ice.

The information I found in On Food and Cooking discussed the damages ice crystals cause on vegetable matter when frozen, and how to avoid this. Because I am not avoiding this process, rather using it to my advantage, I went to Chris.

Anyone who has placed liquid in the freezer is aware that it expands. Thus, when we freeze our berries, the water molecules inside the cells expand. The sharp crystals of ice damage the cell walls of the fruit, causing for a better extraction of liquid, carrying both pigment and aroma molecules (Remember that flavor is made of 5 tastes on our tongue, and about a billion aromas in our nasal receptors).

So freezing makes for more release of liquid. Logically, more liquid would seem to make a runnier, thinner puree. But not so. What this process also does is break down the cell walls themselves. When the blade of the blender tears apart the cells, breaking them open to extract the liquid, it also breaks some of the cell wall down into particles small enough to remain in the puree. The damage from the ice allows for more of the cell wall to break down and become part of the puree. Made from carbohydrates, the particles of cell wall will act to thicken the puree.

Finally, the freezing temperatures slow the enzymes that naturally deteriorate the bright hues of berries. Pureeing the fruit while still icy cold slows these enzymes from discoloring your fruit while the pigments are released.


Blogger lee said...

OOOOOOoooo! This is so exciting. I wondered for a long time why raspberry sauce made from frozen berries was better than that made from fresh. Thanks for doing my homework for me! I recently made some apricot puree and didn't make it into something as quickly as I had planned. It discolored a little so I added extra lemon because over the years I've noticed that it helps to add citric acid even AFTER fruit has started to brown. (I'm sure it's more desireable to never let it brown in the first place but sometimes I have more fruit than I can reasonab;y handle.) Anyhoo, my apricot vanilla butter is delicious, and pretty, too.

July 31, 2006 1:16 PM  
Anonymous Silverbrow said...

And what is the answer to the elderflower question?

August 01, 2006 5:19 AM  
Anonymous joy said...

this was totally fascinating. i've been too lazy to read my mcgee, but now i'm re-motivated! i wonder if the same explanation applies to the frozen-then-strained pumpkin puree techique? (i am the girl who got really excited about your dec 7 post regarding fresh pumpkin puree :))
perhaps after freezing, the mixture releases more liquid when strained, resulting in a thicker puree?

(and i also want to know the answer to the elderberry question, please!)

August 02, 2006 7:17 PM  
Blogger musuBman said...

Very cool,
I wonder if the same method could be used for herb oils (basil, chive, parsley, etc.) Gotta try that now, see... a SMART cook, is a BETTER cook. Although it can put me to sleep at time I do defer to "On food and cooking" often, just last week needed to know why our pee smells after we eat asparagus, mercaptin is the culprit.

October 04, 2006 2:19 PM  
Anonymous Anonymous said...

Thank you for explaining the process so well.

I love your pics! They're so well composed & the colours are so inviting!

October 12, 2006 10:11 PM  
Anonymous Anonymous said...

I'll have this with a friend!

- Steven Burda, MBA

October 17, 2006 10:52 PM  
Anonymous Anonymous said...

Thank you sooooo much i can;t wait to try this out, i'm gonna blow my chefs mind!!!!
Thanks kendall

May 09, 2007 9:17 PM  
Blogger Josh said...

This is fascinating, however I hate to have to remind you that cell membranes are not composed of carbohydrates. Sorry.

November 13, 2007 9:27 AM  
Blogger Dana said...

Josh- I hate to have to stick it to you, but the cell wall of plants are indeed made of carbohydrates. This includes cellulose and pectin as the major carbohydrates. Sorry.

November 13, 2007 11:56 PM  

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