Cool Colligative Properties, Really Really Cool

So, as I mentioned before there are a lot of things I love about August. There is one thing I don't, the heat. When the thermometer gets above about 95 degrees I start melting like dropped ice cream in Phoenix in July (okay if I'm being honest its more like 90, okay 85). It's not pretty. But it can be counteracted by none other than ice cream. A delectable treat with almost mystic origins, especially when you make it yourself. How does a little ice and a little salt make it cold enough to freeze milk, sugar, and other really good stuff?

When liquid is cold enough the molecules stop moving around as much and it becomes a solid. It happens at a very exact temperature for each pure substance. The freezing temperature can be manipulated by adding something to it to create a solution. The change is known as a colligative property. When an impurity is added to a pure substance it starts to get in the way. It breaks up the bonds that creates a solid, making it harder and harder for it to freeze. Like when you are at a concert and that annoying guy at the bar is standing between you and the people you came with. This is know as freezing point depression (in a related process, when a liquid is near the boiling point the impurities cause less molecule of the pure substance to be near the surface, meaning less can escape into the gas phase making it harder to boil, this is known as boiling point elevation). When you add salt to ice the salt begins to mix in and lowering the freezing point. The ice melts. But melting requires energy, a lot of energy. When you insulate the ice from the outside it starts to take that energy from the inside container (see Marvelous Mixtures Part 4 for info on heat transfer, temperature, and energy). The inside mixture gets colder and colder and colder until it freezes. The constant stirring keeps everything inside soft, instead of a hard crystal like ice, and makes sure that the freezing occurs at an even rate. After a long time of stealing energy and stirring, tah-dah!, a delectable summer treat of ice cream, sorbet, or, my favorite, gelato!

So, I mentioned in an earlier post that I have a favorite way to eat my sweet summer peaches. The suspense is over, here is the answer. I like my peaches sliced in half with a 1/2 Tablespoon(ish) of butter, a Tablespoon(ish) of brown sugar, and a big marshmallow where the pit use to be. Wrap the whole thing in a greased piece of tin foil and throw it on a hot grill until everything is gooey and melted. Then serve it up with this cinnamon gelato. This particular gelato is very lightly flavored so if you want a little more cinnamon flavor I would add some extra ground cinnamon when you are letting everything seep.

Cinnamon Gelato

Ingredients

2 cups whole milk

1/2 cup heavy cream

4 cinnamon sticks

5 egg yolks

2/3 cup light brown sugar

1/2 teaspoon vanilla extract

1/2 teaspoon salt

Combine the milk, cream, and cinnamon sticks in a large saucepan over medium heat. Cook for several minutes, until the mixture starts to bubble around the edges. Cover, remove from heat and let set for 30 minutes. Discard cinnamon sticks. Beat yolks, brown sugar, vanilla and salt at medium speed until the mixture is thick and pale (the spatula should scoop up long ribbons that do not instantly dissolve back into the mixture). Add milk, return to sauce pan and cook over medium heat until custard thickens slightly (stir constantly). Strain through a fine mesh strainer to remove all the lumps. Chill for 4 hours - 1 day. Use an ice cream machine to freeze.

Exceptional Expanding Gases, aka Awesome Amaretti Cake

Hello fellow internet cooks/scientists, sorry it's been so long since I've posted anything. Things have been crazy here, because... I've moved to new digs in Minneapolis. In fact today is the first time in a few weeks that I have had groceries and used an oven. And how did I start? I made an Italian birthday cake. Appropriate since my birthday is this week.

European cakes, especially Italian cakes, don't use chemical leavening agents (baking soda, baking powder, etc.) to create expanding gases. Instead they use the gases that is already there to raise their cakes. How this works is that egg whites are whipped. As the get stirred up the proteins get further and further apart creating little pockets of trapped atmospheric gases. Eventually the mixture becomes a stiffened protein network with air pockets. Something that we call a meringue. This is then carefully folded (so that the air isn't released) into a cake batter and baked. Most gases follow the ideal gas law (see Preservation Part II). So, because the gases, for the most part can't escape, as the temperature goes up the pressure increases, pushing on the liquidy cake batter harder and harder, until it expands. This creates more volume for the gases to occupy, and the cake to rise.

This cake, like most European cakes is rich and dense, and served without frosting (as a side note my cake was denser than most, I only used 2 eggs, I don't know why so don't ask). The cookies are key because they give it a characteristic sweet almond flavor. I got my tin of cookies online, but you can also find them in Italian groceries and some specialty stores. I highly recommend serving it with whipped cream, vanilla ice cream, or cherries. You can find the original recipe here: http://www.ciaoitalia.com/Recipes.aspx?id=1072

Torta di Amaretti e Cioccolato

Ingredients

2 sticks butter

1 cup sugar

5 eggs

10 pairs amaretti cookies

1/2 cup flour

2 oz semi-sweet chocolate, coarsely chopped

Preheat the oven to 350 degrees. Grease and flour the sides and bottom of a 9 inch spring form pan. Cream the butter and sugar together with an electric mixture. Separate the eggs and beat the yolks one at a time into the batter. Save the whites. Process the cookies and flour in a blender until powdery. Add the pieces of chocolate and process until finely chopped and mix in to the batter. Beat the egg whites until stiff peaks form. Stir 3-4 Tablespoons of the egg whites into the cake batter to loosen the batter up. Then carefully fold in the remaining egg whites. Pour into the prepared pan and smooth until even. Bake until a toothpick inserted in the center comes out dry (~1 hour)*. Remove the cake from the pan and enjoy.

*be sure to put a baking sheet under the pan to keep the drippings from smoking up your kitchen (like I did mine).