Here is one of our latest kitchen chemistry adventures: Liquid Nitrogen Pumpkin Ice Cream. First the list of ingredients:
- 3/4 c. brown sugar
- 1 c. canned solid-pack unsweetened pumpkin
- 1 tsp. cinnamon
- 3 egg yolks
- 1/4 c. granulated sugar
- 1/4 tsp. grated nutmeg
- 1/8 tsp. ground cloves (this was a little too much for me)
- 1/2 tsp. ground ginger
- 2 c. heavy cream
- 2 c. milk
- 1/8 tsp. salt
- 1 tbsp. vanilla extract
Combine cream, milk, brown sugar and granulated sugar in a medium-sized saucepan over medium heat. Cook, stirring, until the mixture is hot but not boiling, about six minutes.
Whisk eggs in a medium bowl. Gradually whisk 1 c. of the warm cream mixture into the bowl with the eggs.
Pour the egg mixture back into the saucepan, reduce the heat to medium-low, and cook, stirring, until the mixture thickens enough to coat the back of a spoon, 5 to 10 minutes. Do not boil.
Strain the custard into another bowl and cover it partially with plastic wrap. Cool at least one hour at room temperature.
Combine pumpkin, vanilla, cinnamon, ginger, nutmeg, cloves and salt in a medium bowl and blend well. Add to the cooled custard and mix so that all the ingredients are evenly distributed. Refrigerate, covered completely, in the coldest part of your refrigerator until the mixture is very cold, about six hours.
Stir the cold pumpkin mixture, then pour into the mixing bowl of an ice cream maker. Freeze according to ice cream maker manufacturer's directions.
Put pumpkin ice cream in a container with a tight cover. Freeze at least three hours before serving.
Some helpful hints:
When thickening the custard so that it coats the back of a spoon (step 3), the temperature of the mixture should register at least 160 degrees F on a candy thermometer.
The refrigerated custard that has yet to be frozen in the ice cream maker (step 5) can stay covered in the refrigerator as long as three days.
The finished pumpkin ice cream tastes best if eaten within four days of making it.
When heating the custard in the thickening phase (step 3), do not boil it. The egg yolks will curdle.
Now for the cool part. Instead of making the ice cream in your ordinary every day ice cream maker we used N2(l). Nitrogen makes up 78.1% of the air we breath and is completely harmless. When it is cooled to 77K (-195.79 °C, -320.42 °F) it becomes a liquid. We bought our N2 from the chemistry department for $1 per liter. If you don't have a chemistry department handy you can purchase it from AirGas (my local store sells it for $2/L but you need a DOT approved container) Next we dumped the N2 into the chilled custard and watched it harden into ice cream.
There are several videos of people doing this on YouTube so you can see how it's done. The Guinness Book of World Records says that the current record for making one L of ice cream is 18.78 seconds held by polymer physicist Peter Barham from the University of Bristol, UK.
Maybe I'll go for the record.
The first step is the protonation of the bicarbonate (HCO3- ) to form carbonic acid (H2CO3). Carbonic acid is in equilibrium with carbon dioxide and water as seen by the following reaction.
The equilibrium constant (k1/k-1)in this reaction explains why the CO2 evolves rapidly. The rate constant for the forward reaction, k1, is 23 s-1. The rate constant for the reverse reaction, k-1, is 0.039 s-1. So the equilibrium constant (k1/k-1) in this reaction is ≈590 which lies heavily in favor of the products.
