One day, two years later, Angela's brother-in-law came over with Sue for a visit. Curt is an industrial chemist who is now consulting on environmental remediation. He's a great guy. Over a few beers one night I told them about my search for the perfect Polkagris recipe. (That same night we invented the game "See's Candies Knife Attack", but that's another story.) Curt was a wealth of information. He's one of those rare people who can put their knowledge of scientific principles to practical work. He also was able to teach me. Here's what I think I learned.
Curt told me that the physical characteristics of the final product would all depend on the crystalline structure of the sugar and water mixture once it sets. The ratio of sugar to water in the syrup when it crystallizes will be key. As the mixture boils the sugar does not just dissolve, but it melts. This puts the sugar into an amorphous state.
As water steams off, the ratio of sugar to water increases. A given ratio of sugar to water boils at a particular temperature. By monitoring the temperature of the boiling syrup we're also monitoring the sugar to water ratio. The amount of sugar and water at the start will not matter since we'll boil the syrup until the right ratio is achieved. If there's more water at the beginning we'll just have to boil it longer. In essence we are forming a super-saturated solution of sugar in water which would eventually crystallize if we left it to cool - which is just what we want to happen.
As the mixture cools a crystal pattern will form and quickly spread through the whole of the cooling syrup. It's important to have the proper crystals form. This is why it is very important to wash down the sides of the cooking syrup pot. You must also be careful to keep granulated sugar bits off the marble top. If you have a few sugar crystals on the side of the pot or on the marble top, then they will be the seed crystals that grow as other sugar molecules join them. So, if you have sugar crystals in the mix when you pour the syrup out it will all crystallize in the form of the regular table sugar instead of our new candy form. Since table sugar does not have much water in it, as the candy solidifies it will also shed liquid water.
As the syrup boils it is important to wash any little splashes off the sides of the pot too. If you leave little drops of syrup mixture on the side of the pot it will evaporate at a different rate than the rest of the syrup mix. Essentially we'll have some of the mix at a higher sugar/water ratio than the rest. When we cool the syrup these "hotter" bits will form crystals first and the rest of the candy will follow these aberrant models. We'll end up with "harder" candy than we would expect from measuring the temperature of the bulk of the mix.
Having said all this, we also have to be careful of the utensils we use to stir the mixture. If we stir the mixture with a spoon and remove the spoon from the pot, then when we put the spoon back in it will be coated with a syrup mixture at a different sugar/water ratio than the rest of the pot. Either utensils should remain in the pot, and get too hot to handle, or they must be washed off each time they leave the pot.
When we have the right syrup mixture we'll need to cool it down very quickly so that the crystals form all at once. This also keeps the crystals smaller; one big crystal of candy would not have the smoothness we want. Since different little areas of the syrup might be at different temperatures it would be possible for different crystal structures to form and spread through the cooling candy. Cooling it very quickly below its crystallization temperature will "lock" the crystal in one form. Curt suggested cooling the marble top before pouring out the candy.
Curt thinks the acetic acid is used to keep the sugar from oxidizing and turning brown when it boils. He says to add some but he doesn't think the exact amount will matter very much, we just need enough to make the mix acidic. Also, as the mixture boils the acid will not boil off so it will be fine to add the vinegar at the beginning. In fact, since the vinegar has a lot of water in it, it should be added early.
Cream of Tarter (seriously old name for potassium hydrogen dextro-tartrate) is acidic, so it could also be used to prevent browning.
There is probably some trapping of other ingredients in the crystalline matrix such as oil (peppermint), which will make the candy smooth rather than rock-hard.
The foaming from the soda will introduce air into the mixture. It might cause it to cool by bringing in more air, but more likely the bubbles will just make the crystal sugar parts thinner and make the overall candy more brittle. (Ahh, this is why all the peanut brittle recipes call for baking soda! They also add it at the very end since just heating the sodium bicarbonate will make it decompose to carbonate, water and CO2.) I need to consider if foam is part of the polkagris experience. Right now I think not.
Since the sugar/water ratio is so crucial, maybe a very high humidity day might be different than a very low humidity day, but Curt doesn't think this is going to be such a big deal for me. Also, the very act of boiling off all this water will raise the humidity in the kitchen more than the weather (duh!). If anything, I might need to dehumidify the kitchen. I won't try to control this variable yet.