Food Engineering/Physics How Different Methods of Heat Transfer Change Starch Behavior

Food Engineering/Physics How different methods of heat transfer change starch behavior

Background: Starches are used as thickeners in many foods. Depending on the desired characteristics, this may be a raw starch (e.g. cornstarch) or a more complex thickener such as roux (a mixture of starch, protein and fat). Some recipes, especially when using roux, call for cooking or browning the thickener before adding to liquid. But browning a starch not only affects the flavor and color, it decreases its thickening power. This experiment is designed to familiarize high and junior high school students with two major methods of heat transfer, and to provide insight into how heat transfer can affect the nature of foods. This lesson gives students the opportunity to conduct hands-on physics experiments; make visual observations based on how different methods of heat transfer (conduction and convection) change the appearance of raw starch; make visual observations on the differences between starch gels/sols; and take numerical measurements of the flow (rheology) of starch gels/sols. Students should be divided into groups of 2-4. Each group will receive raw cornstarch and a line spread template. Students will brown 1 Tbsp of cornstarch in a dry pan and allow it to cool. Students will add 1 cup of water to 1) the browned cornstarch and 2) 1 Tbsp raw cornstarch then boil to put the starch through gelatinization. After cooling, students will place a measured volume of each gel/sol onto a line spread template to determine the flow properties. For a longer experiment, each group can prepare a third sample using a different amount of raw cornstarch (e.g. 1 ½ tsp, 2 Tbsp). To incorporate chemistry in this experiment, each group can be given an additional type of starch (rice, potato, tapioca) to prepare (1 Tbsp starch with 1 cup water) and measure spread. Different starches can be treated with an iodine solution to examine the chemical make-up of the starches (amylose turns dark purple, amylopectin turns burgundy).

Vocabulary: Amylopectin – (am-uh-low-PEC-tin) A very large branched-chain molecule made up of individual glucose (sugar) units that is contained in starch granules. All types of starch contain some amylopectin, but “waxy” type starches such as rice and waxy maize have a higher amylopectin content. Amylopectin usually forms clear, very soft gels. Because of the shorter branches, amylopectin shows a burgundy or wine color when dyed with iodine solutions. Note – the standard version of this experiment does not examine the differences between amylose and amylopectin gels. To include this aspect, additional starch samples (rice, tapioca, or waxy corn) can be given to students to boil, cool and measure spread. Amylose – (am-uh-LOHS) A large straight-chain molecule made up of individual glucose (sugar) units that is contained in starch granules. Most types of starch contain 20-30% amylose, though some specialty breeds of corn contain more (high amylose maize ~50%) or less (waxy maize <1%). Amylose can be used to form very strong, firm gels (depending on the amount of starch used). The long, straight chains of amylose show a dark purple to indigo color when dyed with iodine solutions.

Gel – A type of colloidal dispersion where a liquid is dispersed in a solid. Examples include gelatin, jams and cheese. Starch gels are made when starch molecules interact and trap water. When there is not enough starch to form a gel, the dispersion forms a sol. Gelatinization – (gel-at-in-uh-ZAY-shun) A process unique to starches, that involves heating raw starch granules in the presence of moisture. Raw starch granules are made up of layers of rigidly aligned amylose and amylopectin molecules, similar in appearance to the layers of an onion. When heated with moisture, amylose leaves the granule. This process is gelatinization. To form gels (gelation), starches must always be gelatinized first – but a gelatinized starch does not always form a gel. Examples of gelatinization include cooked rice (softens and pulls in water) and Heat Transfer – The way in which heat (energy) is moved from one place to another. The two forms of heat transfer examined in this experiment are conduction and convection. Heat (energy) can also be transferred via radiation (e.g. microwaves). Conduction is the direct transfer of heat from one particle to another. Cooking techniques that use conduction include sautéing, pan-frying, and griddling. Convection is the transfer of heat through waves found in heated gasses and liquids. All convection cooking includes some amount of heat transfer by conduction. Cooking techniques that use convection include boiling (liquid water), steaming (water vapor), and roasting (air). Pyrolysis – (pie-RAWL-uh-sis) The application of heat to break down organic molecules (such as the starch in this experiment). Turning wood into charcoal is an example of pyrolysis. Intense heat is required, much higher than can be reached in the presence of water, so it is not as commonly seen in foods. Creating grill marks on meats and vegetables is one food example of pyrolysis. Rheology – (ree-ALL-uh-gee) The study of how liquids, soft solids, and semi-solids flow. In foods, rheology can be used to study the texture of gels and sols. When we describe the “consistency” of a food, we are most often talking about characteristics that are measured using rheology. Sol – A type of colloidal dispersion where a solid is dispersed in a liquid. This is very different from a solution, where a small molecule (such as sugar or salt) is dissolved in a liquid. In sols, the molecules are too large to dissolve, but are small enough that they won’t settle out of the liquid as it sits. Examples of sols include gravy, cheese sauce, and pudding.

This lesson incorporates the following State of Utah Course Standards: Foods and Nutrition I Strand 3, Standard 3: apply food selection and preparation guidelines related to quick breads, rice, grains and pasta. Foods and Nutrition II Strand 1, Standard 2: Discuss and apply basic food preparation principles. Strand 7, Standard 1: Identify terminology used in making soups and sauces. Performance Objective 7: Actively participate in the preparation of a mother sauce, thick soup, or stock soup. Physics Standard 4, Objective 3(b): Investigate the transfer of heat energy by conduction, convection, and radiation.

Materials Cornstarch Line spread template Cookie or portion scoop (exact size doesn’t matter, as long as the same scoop is used for each of the different types of starch gel/sol)

Preparation: 1. Make enough copies of the line spread template so each student group has one per sample. Templates can be laminated then wiped clean and re-used multiple times. 2. Brown 1 Tbsp of cornstarch ahead of time to show students for an example. The raw cornstarch will not appear entirely brown, but a light tan color will be evident. The dark color will become much more apparent when added to water. If desired, brown enough cornstarch ahead of time so there is 1 Tbsp for each group. They can either be told to skip the browning step, or this can be held as a back-up in case groups have problems with this step. 3. Make enough copies of the data sheets to give one to each student. Make enough copies of the instruction sheets to give one to each group.

Instructions (with teacher notes): 1. Measure 1 tablespoon (1 Tbsp) cornstarch. Put the cornstarch in a dry frying pan and heat it over medium-high heat. Make sure to stir the cornstarch the entire time it is on the stove. Cook the starch until it starts to turn a light tan color. Remove it from the stove and put it in a saucepan to cool.  Provide an example of browned cornstarch for students to reference. This step can take 10-15 minutes plus time for the starch to cool down, so you may want to prepare the browned cornstarch ahead of time. 2. Combine 1 Tbsp cornstarch with 2/3 cup cold water in a saucepan. Heat the cornstarch and water mixture over medium-high, stirring constantly. Bring the mixture to a boil and turn heat to low. Cook, stirring constantly, for 2 minutes. Pour the mixture into a bowl and chill in a shallow ice bath or cold water. How thick does the warm mixture appear? Stir it and record your observations. Let the mixture chill 10-15 minutes.  Ice baths can be made in cake pans or metal mixing bowls. If using a mixing bowl, warn the students not to add so much water that it comes up the sides of the bowl with the cornstarch mixture. 3. Repeat step 2 using the browned cornstarch and 2/3 cup water. Are there any obvious color, aroma, or texture differences between the browned starch mixture and the starch mixture from step 2? Stir it and record your observations. Let the mixture chill 10-15 minutes.  Let students know that they can do steps 2 and 3 simultaneously if there is enough kitchen equipment. 4. Using the cookie or portion scoop provided by your instructor, take a sample of the cornstarch mixture from step 2. Make sure to completely fill the scoop. Carefully empty the scoop over the center of a line spread template (marked with an “X”). Let

the mixture sit on the template for 1 minute. Record the number for the line closest to the mixture in each of the 4 directions. Calculate the average of all four measurements – this is the spread for the cornstarch mixture. Did the mixture spread evenly in each direction? How would describe its texture – a more like gravy, pudding, or a scoop of ice cream or dough? Would you call it a sol or a gel? Record your observations.  Demonstrate for students how to read the concentric rings of the template. Point out that the numbers are only given in 5’s and the lines in between each count as 1. A dry erase marker can be used to draw a shape on a laminated template, then students can practice taking measurements for this first.  Smaller cookie scoops (e.g. #70s) may not be large enough to accurately record spread for the thicker cornstarch mixtures if the template is printed at 100% size. You can reduce the size of the template if larger scoops are not available. 5. Repeat step 4 with the browned cornstarch mixture from step 3. What differences do you see between the two mixtures? Would you call this mixture a sol or a gel?  The browned cornstarch sol can be very runny. To contain messes, put the spread template in a cake pan or on a rimmed sheet tray.  If this sol spreads beyond the last ring on 1 or 2 sides of the template, tell students to use “28” as the number for that side to calculate an average. If it spreads beyond the last ring on 3 or 4 sides, they can report the average as >28.

Discussion points/questions:

1. Which types of heat transfer were used for the regular cornstarch mixture (step 2)? For the browned cornstarch mixture (steps 1 and 3)? Can you think of food examples where starches or starchy foods are cooked using the same type of heat transfer?  The regular cornstarch mixture from step 2 uses convection – the heat is transferred through currents in the water as it is boiling. This causes the starch to go through gelatinization, the first step in being able to form a gel or sol.  The browned cornstarch mixture uses two types of heat transfer. Conduction (direct transfer of heat) is used in step 1, then convection is used in step 3. The intense direct heat in step 1 causes the starch molecules to break down and turn brown.  Examples of starches cooked by convection – boiled (water) or steamed (water vapor) rice, baked potatoes (air), boiled pasta (water), oatmeal (water)  Examples of starches cooked by conduction – sautéed potatoes or hash browns

2. Why doesn’t the browned cornstarch thicken as well? What benefit would there be for browning a starch before adding it to a food? Can you think of food examples where we purposely brown a starch then use it as a thickener?  The intense direct heat in step 1 causes the starch molecules to break down and turn brown. This is called pyrolysis. The effect is similar to taking a long paper chain and cutting random links. There are still pieces of paper chain left, but they are different lengths and can’t reach as far. When the starch molecule amylose is broken down by heat, it leaves several smaller pieces. These smaller pieces can’t reach each other as well, so they aren’t able to form a gel.

 Browning starches creates deep flavors and colors. Examples include the crust on bread and the lightly browned outside of French fried potatoes. We don’t usually brown cornstarch, though, because it doesn’t thicken as well. We do purposely brown flour, though. Roux is cooked longer to develop more flavor and color (white vs blond vs brown roux) depending on how it is going to be used. Sauces with deeper flavors use brown roux, even though it doesn’t thicken as well. Some recipes use brown roux for flavor and white or blond roux for thickening.

Food Engineering/Physics How different methods of heat transfer change starch behavior

Starches are used as thickeners in many foods. The way a starch behaves depends on several things, including how it was heated and what other ingredients are added. When there is a high amount of starch, it might form a gel. Gels are networks of solid molecules trapping water in between. Examples of food gels are gelatin (protein gel) and fruit jelly (pectin gel). If there is not as much starch added, it might form a sol. Sols are made when solid molecules are suspended in water, making it thicker but not as solid. Examples of food sols are gravy (starch sol) and fruit jam (pectin sol). The two main types of heat transfer used in cooking are conduction and convection. Conduction is the direct transfer of heat energy from one molecule to another. Convection is the transfer of heat energy by currents in gas or liquid. Examples of gasses used in cooking are air and steam. Liquids include water and oil (such as in deep frying). Today we will examine how well cornstarch thickens water after it has been heated in different ways.

Instructions: 1. Measure 1 tablespoon (1 Tbsp) cornstarch. Put the cornstarch in a dry frying pan and heat it over medium-high heat. Make sure to stir the cornstarch the entire time it is on the stove. Cook the starch until it starts to turn a light tan color. Remove it from the stove and put it in a saucepan to cool. 2. Combine 1 Tbsp cornstarch with 2/3 cup cold water in a saucepan. Heat the cornstarch and water mixture over medium-high, stirring constantly. Bring the mixture to a boil and turn heat to low. Cook, stirring constantly, for 2 minutes. Pour the mixture into a bowl and chill in a shallow ice bath or cold water. How thick does the warm mixture appear? Stir it and record your observations. Let the mixture chill 10-15 minutes. 3. Repeat step 2 using the browned cornstarch and 2/3 cup water. Are there any obvious color, aroma, or texture differences between the browned starch mixture and the starch mixture from step 2? Stir it and record your observations. Let the mixture chill 10-15 minutes. 4. Using the cookie or portion scoop provided by your instructor, take a sample of the cornstarch mixture from step 2. Make sure to completely fill the scoop. Carefully empty the scoop over the center of a line spread template (marked with an “X”). Let the mixture sit on the template for 1 minute. Record the number for the line closest to the mixture in each of the 4 directions. Calculate the average of all four measurements – this is the spread for the cornstarch mixture. Did the mixture spread evenly in each direction? How would describe its texture – a more like gravy, pudding, or a scoop of ice cream or dough? Record your observations. 5. Repeat step 4 with the browned cornstarch mixture from step 3. What differences do you see between the two mixtures?

Discussion points/questions:

Spread Observations (4 measurements) 1______2______1 Tbsp 3______

cornstarch 4______

Ave______

1______2______1 Tbsp 3______browned cornstarch 4______

Ave______