PY107Lab 5. Heat transfer and emulsions. Physics in Food Lab 5

Physics in Food Lab 5

PY 107 Lab 5 
  Lab 5:  Heat transfer and emulsions -- Molten chocolate cake and lactonnaise  
 Objective I: Understand what an emulsion is, and how to stabilize an emulsion. Describe why an emulsion of two liquids can behave as a solid.  
 Objective II: To understand role of diffusion in cooking and heat transport. To relate temperature and cooking time. 
 Introduction: Emulsions and Foams While oil and water naturally want to separate from each other, oil can be mixed with water by shaking to form drops, as anyone who has mixed oil and vinegar to make salad dressing knows.  However, these drops are not stable: they coalesce and the oil quickly separates from the vinegar.  To make the emulsion stable, additional ingredients can be added: for example, mustard helps to stabilize a vinaigrette, whereas in mayonnaise it is the egg. Such ingredients contain molecules that accumulate at the boundary between the oil and water because they are amphiphilic: these molecules have both hydrophilic parts, which have an affinity for water, and hydrophobic parts, which have an affinity for oil.  Amphiphilic molecules can act as surfactants or emulsifiers and are added to improve the emulsion’s stability and prevent phase separation on timescales of cooking and eating.   Fortunately, the cells that comprise living matter contain many excellent emulsifiers such as the lipids and amphiphilic proteins that comprise cell membranes, which provide a boundary between the cell and its exterior.  Thus, mashing up the cells in a garlic clove releases emulsifying molecules that can help to stabilize the droplets of olive oil in water, yielding the creamy, garlicky aioli sauce.  Similarly mustard seed and egg yolk contain large proportions of lecithin, which is a mixture of lipids found in egg yolk and can also be readily extracted from soy beans, making it a common food additive.  Emulsions are ubiquitous in the chef’s palette, and in addition to aioli and mayonnaise, include a variety of sauces, butter, and milk. Interestingly while liquids are materials that flow, emulsions are mixtures of two fluids that in some cases behave as a solid.  A common example is mayonnaise, a mixture of water, oil and egg, where tiny oil droplets are dispersed in water. The solid-like behavior of such emulsions is the result of the large number of interfaces between the drops that make up the emulsion; these interfaces are squeezed together, making the two liquids behave as a solid. Foams are similar to emulsions, but consists of gas entrapped in a solid or liquid matrix; foams are central to the texture of myriad foods including bread, mousse, beer, cappuccino, and whipped cream; culinary foams comprised of beet or licorice root were introduced into the gastronomy world by Adriàn Ferran.  Typical foams, such as whipped cream, consist largely of gas in an aqueous phase. The origin of the solid-like behavior of a gas in a foam is the same as the origin of the solid-like behavior of a liquid in an emulsion: the gas in a foam is formed into bubbles, which are squeezed together deforming their interfaces and making them solid-like.  Similar to emulsions, a major challenge in using foams in cooking is that they are not stable: their structure changes over time as density differences between the gas bubbles and the liquid in which they are suspended leads to drainage of liquid to the bottom of the foam.  Moreover, the pressure within bubbles varies with their size: it is preferable to form larger bubbles, as smaller bubbles have higher pressures and therefore the smaller bubbles gradually shrink while the larger ones grow, leading to overall growth in the bubble size.  There are various approaches to stabilizing culinary foams.  To hinder drainage, the liquid’s viscosity can be increased: adding molecules to the fluid to increase its viscosity (such as hydrocolloids or thickeners) can help to impart foam stability.  Foam stability can also be achieved by solidifying the continuous phase: bread and meringues are good examples of how temperature can be used to create solid foams; mousse is a foam whose solid characteristics are imparted by gelation. In this lab you will make an emulsion, and investigate the effects of the stabilizing additive, a surfactant.  You will take pictures of your emulsions under the microscope, calculate the volume fraction, and use the equation of the week to estimate the elasticity.   
PY 107 Lab 5   Lab 5:  Heat transfer and emulsions -- Molten chocolate cake and lactonnaise   Objective I: Understand what an emulsion is, and how to stabilize an emulsion. Describe why an emulsion of two liquids can behave as a solid.   Objective II: To understand role of diffusion in cooking and heat transport. To relate temperature and cooking time.  Introduction: Emulsions and Foams While oil and water naturally want to separate from each other, oil can be mixed with water by shaking to form drops, as anyone who has mixed oil and vinegar to make salad dressing knows.  However, these drops are not stable: they coalesce and the oil quickly separates from the vinegar.  To make the emulsion stable, additional ingredients can be added: for example, mustard helps to stabilize a vinaigrette, whereas in mayonnaise it is the egg. Such ingredients contain molecules that accumulate at the boundary between the oil and water because they are amphiphilic: these molecules have both hydrophilic parts, which have an affinity for water, and hydrophobic parts, which have an affinity for oil.  Amphiphilic molecules can act as surfactants or emulsifiers and are added to improve the emulsion’s stability and prevent phase separation on timescales of cooking and eating.   Fortunately, the cells that comprise living matter contain many excellent emulsifiers such as the lipids and amphiphilic proteins that comprise cell membranes, which provide a boundary between the cell and its exterior.  Thus, mashing up the cells in a garlic clove releases emulsifying molecules that can help to stabilize the droplets of olive oil in water, yielding the creamy, garlicky aioli sauce.  Similarly mustard seed and egg yolk contain large proportions of lecithin, which is a mixture of lipids found in egg yolk and can also be readily extracted from soy beans, making it a common food additive.  Emulsions are ubiquitous in the chef’s palette, and in addition to aioli and mayonnaise, include a variety of sauces, butter, and milk. Interestingly while liquids are materials that flow, emulsions are mixtures of two fluids that in some cases behave as a solid.  A common example is mayonnaise, a mixture of water, oil and egg, where tiny oil droplets are dispersed in water. The solid-like behavior of such emulsions is the result of the large number of interfaces between the drops that make up the emulsion; these interfaces are squeezed together, making the two liquids behave as a solid. Foams are similar to emulsions, but consists of gas entrapped in a solid or liquid matrix; foams are central to the texture of myriad foods including bread, mousse, beer, cappuccino, and whipped cream; culinary foams comprised of beet or licorice root were introduced into the gastronomy world by Adriàn Ferran.  Typical foams, such as whipped cream, consist largely of gas in an aqueous phase. The origin of the solid-like behavior of a gas in a foam is the same as the origin of the solid-like behavior of a liquid in an emulsion: the gas in a foam is formed into bubbles, which are squeezed together deforming their interfaces and making them solid-like.  Similar to emulsions, a major challenge in using foams in cooking is that they are not stable: their structure changes over time as density differences between the gas bubbles and the liquid in which they are suspended leads to drainage of liquid to the bottom of the foam.  Moreover, the pressure within bubbles varies with their size: it is preferable to form larger bubbles, as smaller bubbles have higher pressures and therefore the smaller bubbles gradually shrink while the larger ones grow, leading to overall growth in the bubble size.  There are various approaches to stabilizing culinary foams.  To hinder drainage, the liquid’s viscosity can be increased: adding molecules to the fluid to increase its viscosity (such as hydrocolloids or thickeners) can help to impart foam stability.  Foam stability can also be achieved by solidifying the continuous phase: bread and meringues are good examples of how temperature can be used to create solid foams; mousse is a foam whose solid characteristics are imparted by gelation. In this lab you will make an emulsion, and investigate the effects of the stabilizing additive, a surfactant.  You will take pictures of your emulsions under the microscope, calculate the volume fraction, and use the equation of the week to estimate the elasticity.   
Introduction Part II. Heat Transfer:  Many transformations of food during cooking require precise temperature control, through either heating or cooling. Heating protocols in recipes can be quantitatively understood with a few simple principles from the physics of heat transfer.  (i) Different foods (Meat, fish, eggs, chocolate, etc.) require different target temperatures for the cooking to be successful, depending on the molecular transitions that must occur.   (ii) Heat is supplied by radiation from the heating element in an oven, whereas it is supplied by conduction from the heater via the bottom of the pan when grilling or cooking on a stove-top. Heat is also transported by convection if cooking in a liquid medium, such as in boiling or frying. Radiation can occur in vacuum. In conduction energy is transported from molecule to molecule by diffusion of heat. In convection molecules sink from higher density and rise from regions of lower density setting up convection currents. Convection occurs in liquids, while conduction in solids. Heat diffuses  in conduction while molecules diffuse and move in convection. Convection requires the liquid to flow. Different heating methods (baking, frying, microwaving, etc.) require different heating  protocols for optimal cooking, since they cause different temperature distributions within the food during cooking. (iii)  In all cases heat is eventually transferred through the food by diffusion. Heat is transferred between molecules through random collisions, resulting in the diffusion of heat via random walks similar to those discussed previously. The diffusion equation can be used to estimate cooking time. In this lab you will see that cooking time control is critical to achieve the liquid center of the 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.