Raisins, Part II

So what happens to a grape deferred?  Does it dry up, like a raisin in the sun?

Usually.  Why usually?  Because if you just let a grape sit there, it will more likely mold before anything else.  Mold spores and natural yeasts (hence wine) love grapes, but only when there is sufficient moisture for growth to happen.  If the grape dries out quickly enough, it won't rot because there is not enough moisture to support the food baddies.

Incidentally, this is why things like the 12 year old hamburger exist.  They remain preserved because they are too dry to "go bad," not because McDonald's is an evil mutant hamburger machine.  In fact, the instructions for creating your own self preserving burger specifically say that "The moisture needs to escape the food naturally."  Obviously.  This is after they claim that the burger needs to be obtained from "any place that serves hybridized, chemicalized, genetically altered, hormone/ pesticide-laden food."  This simply reveals their bias against cheap fast food; they have no conclusive evidence that the supposed chemicals inherent in the burger keep them from degrading.

Here's a thought: go get some organic burger buns and organic ground beef.  Prepare a burger with near identical properties to the McD's hamburger, and let it sit out.  I'd put good money on the homemade burger "self preserving" in the same way as the "pesticide-laden" burger.

/Rant.

I apologize for that, but I think you'll find this a bit more interesting: homemade raisins without the use of a food dehydrator or warm sun (which currently doesn't exist in Portland).

Experiment One:

An experiment was carried out to determine the possibility of making grapes by means other than air drying.  Two hygroscopic substrates were used, salt and sugar, to draw the water out the grapes and transform them into raisins.  Twelve Thompson seedless grapes were used, half of them peeled and half of them left whole.  Of these, three peeled grapes were placed into a bowl with 150g sugar, three into 150g salt, three unpeeled into 150g sugar, three unpeeled into 150g salt.

The goal is to get these grapes to turn to raisins through hygroscopically induced osmosis.  The hypothesis is that the peeled grapes in salt will reach this stage first, while the unpeeled grapes in sugar will take the longest.

Let’s define some things.  First of all, what’s this hygroscopic thing all about?  Hygroscopic simply refers to a compound’s tendency to absorb water.  If you’ve ever dumped sugar onto a bowl of strawberries, you’ll know that, after 30 minutes or so, you’ll have limp strawberries floating in delicious syrup.  This is because the sugar has drawn juices out of the strawberries and then dissolved to form a syrup.

Salt does the same thing, but to a greater degree, which is why salt was also used in the experiment.  (And remember our friend lye?  Good ol’ sodium hydroxide is so hygroscopic that it takes moisture straight out of the air around it!)  There is probably some sort of scale to measure hygroscopic tendencies, but I haven’t been able to find it.  Typically, one would just say that compound X can hold up to Y % of its weight in moisture.

Also, osmosis.  Who can tell me about osmosis?  You may need to reach back to ninth grade biology on this one.  Yes, you in the back!

“Wasn’t that a lame cartoon starring Will Smith?”

No.  You are wrong.  (It was Chris Rock, anyway.)

Osmosis has to do with semi-permeable membranes.  When you’ve got two solutions on each side of the membrane, and the solvent (usually water) passes through the membrane from the side of lower concentration to the side of higher concentration.  This happens to equalize the concentration on both sides of the membrane.  This is why you get prune fingers in the bath, and why brining a turkey for Thanksgiving is awesome.

Going back to the grapes, grape skins don’t seem very permeable, which is why some grapes were peeled.  The idea is that, inside a grape, you’ve got water and some sugars.  This forms a solution.  If you place the grape in something hygroscopic, it begins to attract water, from the solution, through the membrane.  A super saturated solution is formed outside the grape, and so more water begins to flow through the membrane to balance the solutions.  The grape’s internal solution begins to concentrate, making it taste sweeter.  The structure begins to collapse, making it small and wrinkled.  A raisin is formed!

This type of raisin is not exactly sun dried, like the famous California raisin brand, nor is it air dried, like many other brands.  It would be more accurate to call this a “cured” raisin!  Pretty neat, eh?

Anyway, observations from the experiment:

Beginning grape weights ranged from 3g to 7g, with the average being 4.75g.

Grapes were placed in 150g sugar or salt.

After 24 hours, the salt and sugar in the peeled grape bowls had solidified.  The assumption here is that sufficient water had been drawn out (and equalized throughout the entire volume of sugar / salt) to cause a very slight amount of dissolution and bonding of the individual granules.  Neat!

Here is a picture of the bowl of sugar.  You can see the sugar collapsing over the grapes due to this dissolution.

After chipping away at the sugar and salt, I removed the grapes for visual observations.  You can see that the unpeeled grapes show no visible change, nor did they show any change in weight.  However, the peeled grapes are visibly smaller, and now all average 2g each.

 A responsible scientist would take down observations like these at systematic intervals, but this experiment was conducted by a slightly irresponsible layman who chose not to check the grapes again until seven days after the initial burying.

But after one week, the peeled grapes had turned to raisins!  They weighed in at 2g total for both batches, so no more than .66g each, indicating a roughly 4g water loss!

The unpeeled grapes were left in for two weeks, after which they weighed in at an average of 1.33g each.  They were much more plump, but, unfortunately, also moldy.  They did not dry quickly enough to prevent mold from forming.  Sad :(

So there you have it: homemade raisins!  I’d recommend the sugar cured method, as salty grapes are weird.  You could rinse them a few times, like with salt cured capers, but still.  Blech.

Any questions?  Stay tuned, I’ve got more raisin rambling up my sleeve!