From Haute Cuisine to Soft Matter Science Lab 4 – Heating, Cooling, & Tempering
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SPU 27 – Science & Cooking Lab 4 Science of the Physical Universe 27 Science and Cooking: From Haute Cuisine to Soft Matter Science Lab 4 – Heating, Cooling, & Tempering Week of Thurs Sept 30 – Tues Oct 5, 2010 ~ Molten Chocolate Cake ~ (with Ice Cream) At the end of this lab, you should be able to: (i) Explain the time-temperature curves for various locations within a food. (ii) Explain the parameters that affect the length of time you heat or cool food. (iii) Understand how temperature and time can engender molecular transitions that determine the texture of foods. Introduction: Transitions between phases occur at defined temperatures that depend on the strength of interactions between the molecules that comprise a material. Knowledge of the temperature at which these transitions occur in food, and the temperature distribution within a food is central to cooking. Chefs are experts at manipulating the temperature distribution within a food: they can induce a temperature change via a method of heating or cooling (e.g. baking, frying, sous vide) for a defined length of time to achieve the desired final texture of a food. In lecture, you learned that the temperature of a food changes with time according to: (−t /τ ) T(t) = (Tinitial − Texternal )e + Texternal . 1 of 6 SPU 27 – Science & Cooking Lab 4 Here, Texternal is the final temperature outside the food (e.g. in the oven, in the ice bath), Tinitial is the initial temperature of the food, and t is the timescale for heating or cooling, given by the equation of the week t = R2/D, where R is the distance of the location where the temperature is measured to the heat source, and D is the diffusion constant for the food. In this lab, you will apply these equations to measure the diffusion constant for chocolate molten cake. Using thermometer probes positioned inside the cake, you will measure the temperature distribution within the molten chocolate cake as it bakes. You will then use this data to determine the D for molten chocolate cake. You will also apply this equation to gain insight into how we can manipulate the texture of ice cream. PART I – Molten Chocolate Cakes Cooking time is critical to achieve the liquid center of these chocolate cakes. Here you will investigate how the texture of the cake’s interior changes with baking time, and determine the ideal baking time to maintain a molten center. Makes 6 individual cakes. Ingredients: 120 g dark chocolate chips 120 g butter 120 g sugar 200 g eggs (~4 eggs) 60 g flour 0.5 g (pinch) salt Instructions: • Your TF has wrapped two thermometer probe wires around the top wire rack of the oven, so that the tips of the wires are about 1 cm above the bottom rack of the oven. Verify that the wires are positioned correctly by placing an empty ramekin in position beneath the wires such that the tips of the wires can be positioned at the edge and center of a ramekin. • Preheat the oven to 375 °F/190 °C and coat six ramekins with non-stick baking spray. • Over double boiler melt chocolate and butter. Stir and make sure it doesn’t burn. • In a medium bowl, beat together the eggs and sugar until very well combined. • In a separate bowl, weigh out your flour and add your pinch of salt to the flour, and combine well. • Little by little, add the chocolate mixture into the egg mixture, to avoid overheating the eggs. • Delicately incorporate the flour mixture into your wet ingredients until just combined. 2 of 6 SPU 27 – Science & Cooking Lab 4 • Portion out the batter into greased cups or ramekins with approximately 80g per cup. • Place all six ramekins on the bottom rack in the oven. Position one of the cakes directly underneath the thermometer probes, so that the tip of one is near the edge of the ramekin and one is in the center. – close the door carefully so that the thermometer wires do not pull out the probes out of the cake. • Every 30 seconds, record the temperature. Bake until a thin, solid skin appears on the cakes. At this point, remove the cakes from the oven at one-minute intervals, leaving the one with the thermocouples in the oven until the end. After removing each cake, wait 60 seconds to allow it to cool slightly, then carefully cut in half and record the minimum and maximum thicknesses of the solid layer. Use the chart below to record your observations. Allow cakes to cool slightly before eating. Enjoy with ice cream (Part II). Baking Time Minimum Maximum (min) Lcooked (mm) Lcooked (mm) PART II – Ice Cream Heat transfer is also essential when cooling foods. For example, the texture of ice cream is highly dependent on how quickly the milk and cream is cooled. In this demonstration, your TFs will prepare ice cream both using an ice cream o maker filled with ice and salt (Texternal ~ -20 C) and will also use liquid nitrogen o (Texternal ~ -200 C). Varying Texternal in the equation, (−t /τ ) T(t) = (Tinitial − Texternal )e + Texternal , you will observe how the rate of cooling differs in these two methods, and also taste the resultant effect on ice cream texture. Why does the heat transfer rate matter? The size of ice crystals that form depend on how quickly the liquid is cooled. Once ice crystals form, they begin to grow – recall the ‘supercooled’ dessert by Joan Roca. When you cool the mixture very quickly, you create many small crystals all at once; this can impart a 3 of 6 SPU 27 – Science & Cooking Lab 4 smoother texture to your ice cream. (Note: similar physics can be used to understand why uniform crystal formation of cocoa butter in chocolate is critical for its texture). Making ice cream with liquid nitrogen also affects the aeration of the ice cream. By incorporating the liquid nitrogen into the cream mixture, you can exploit the liquid-gas phase transition: as the nitrogen boils, tiny bubbles of air are created that also contribute to the ice cream texture. Ingredients: 460 mL Heavy cream or crème fraiche 600 mL Milk 250 g Sugar 0.5 g (pinch of salt) Ice Coarse salt Instructions: • Your TF has prepared the setup for making ice cream in advance. • Observe the process of cooking of ice cream using liquid nitrogen. Compare the time it takes to freeze the ice cream using liquid nitrogen compared to the ice cream maker. Also compare the textures of the two ice creams – can you taste a difference between the liquid nitrogen and ice cream maker methods? • At home, you can easily make this recipe by mixing the ingredients, and then placing the mixture in an ice cream maker and following standard procedure for making ice cream. • If you don’t have an ice cream maker do not fear – here are a few alternatives: o You can use the ‘rock salt’ method by filling a large bowl with ice and rock salt, and placing a smaller bowl containing your ice cream ingredients on the ice bowl. Whisk constantly until frozen. o Alternatively you can use Ziploc bags: Fill a large Ziploc bag with your ice and rock salt mixture. Place a smaller Ziploc bag containing your ice cream mixture inside – be sure it is well sealed. Shake until frozen. o Last but not least you can place the ice cream mixture in a shallow baking pan and put it in the freezer. Take it out every now and then to break up the chunks of ice that form. This method doesn’t produce the smoothest ice cream, but it works. 4 of 6 SPU 27 – Science & Cooking Lab 4 APPENDIX For this lab we will need (type of space and safety requirements): · Convection ovens · Liquid nitrogen Equipment list: · Dual-input thermometer · Two thermometer probes with heat-stable wire · Heat protection gloves (or dry bar towels) · Cold protection gloves · Safety glasses Food Equipment list: · Ramekins (4) · Small saucepan · Bowls (3) · Whisk · Digital kitchen scale · Ice cream maker · Kitchen aid mixer Shopping list (refrigeration needed): · Dark chocolate chips · Butter · Flour · Salt · Sugar Notes for each TF: In this lab you will demo ice cream making. You will make ice cream with both liquid nitrogen and using an ice cream maker, and have the students observe the time required for ice cream formation. You will use the same base recipe for both; the quantities shown below are for the TOTAL amount you will use for both methods. Make this mixture and keep in fridge until ready to use: Ingredients (2.5x the recipe given for the students): 1150 mL heavy cream (~2.5 pints) 1500 mL whole milk 625 g sugar Syrup or flavorings of choice (added later if you want two different flavors, and/or swirls, etc. Opened syrups are in fridge.) 5 of 6 SPU 27 – Science & Cooking Lab 4 Additional ingredients for the machine: Ice Salt (1 lb, ~1/3 of a box) Additional ingredients for the LN2: Full 4L dewar of LN2 from Room 158 (first floor, go up stairs on the north side of the building right outside of our lab). Contact person is Zach Zinsli <[email protected]> 1. For the ice cream machine method: Get started on this right when you get to the lab because it takes time to churn. Weigh out recipe above directly in the ice cream maker’s aluminum canister. Pour about 1/3 of mixture into the Kitchenaid bowl. Put this 1/3 in fridge to reserve for the LN2 ice cream.