Background
To prepare myself for writing this unit, I want to create a context that is relevant to the student population. Thus, I will have to read about George Washington Carver’s experiments and bulletins and his true history as an agricultural chemist. Students ought to be given an opportunity to truly understand the magnitude of his contribution to creating an economy. Secondly, I had to conduct research on the chemical separation techniques used in extracting fats. A prime example would be the cocoa bean. Finally, I conducted research on the different methods used to extract juices from fruits. The extraction of juices alone provides the setting for a variety of separation techniques commonly used throughout an introductory chemistry course. Overall, I want the students to experience the power of creating their own food products and for students to connect how chemical separation techniques can actually change how a food is marketed to an economy.
Phases of Matter
By the time this unit is taught, my students will already know the basics of chemistry such as the states of matter. In the past, I have never gone into the depth of solids and liquids at the beginning of the year even though I spend an entire unit just on gas laws during the spring. Therefore, concerning the liquid state, I will introduce to them terms such as solute, solvent, suspension, immiscible, emulsions. On the avenue of solids, I will also introduce the terms crystalline, amorphous, and polycrystalline. All of these words are necessary when talking about mixtures, especially in processing foods.
Solids can be classified into three organizational structures or phases. Solids are considered crystalline when its atoms or molecules are organized according to one general pattern. A primary example of a crystalline solid is table salt. If you had to picture this pattern on a macroscale, imagine a mound of oranges on display at the grocery store appearing stacked and organized. Solids are classified as being polycrystalline when there are multiple patterns found within the molecular structure. Examples include minerals and gems found in the earth’s crust. An example of a polycrystalline food is Himalayan salt and a good representation of a polycrystalline material is a cookies ‘n creme chocolate bar. Himalayan salts come in various shades of pink and also appearing colorless like table salt, which is a result of organized crystals, but with variations in the chemical composition in different locations of the salt crystals. In the cookies ‘n creme chocolate bar, the white chocolate and the cookies represent two differently organized regions, yet they are mixed together.
Liquid mixtures can also have phases. A substance that is dissolved (solute) in another (solvent) typically makes a solution. Only one phase is present, which is a homogenous liquid. This is easily achieved with salt water and sugar water. However, when oil and water are mixed, they create a mixture known as a suspension. They separate because one is polar, the other is nonpolar, and they cannot achieve favorable interactions with each other, forming two distinct layers, in which oil would float on top. This separation is described as the two liquids being immiscible.
However, the food industry has created solutions where phases of matter can be trapped in spheres called colloids. These colloids are stable in the presence of an emulsifier, which arrange themselves at the junction where the two liquids meet because the emulsifier contains atoms that allow it to favorably react with both substances. This is common in mayonnaise, toothpaste, and cosmetic creams.
Basic Separation Techniques
This unit will look at classic separation techniques as well as a few that have been used in food chemistry. Provided that students have sufficient science exposure prior to high school, most students will have encountered six basic separation techniques that are taught throughout the physical sciences: flotation, sifting, magnetic, filtration, distillation/evaporation, and panning. Based on my short experience as a teacher, most students are unfamiliar with sifting because it differs from filtration and panning by sifting for objects of specific size. Students often recognize evaporation, but not distillation, since distillation is a process that typically involves an additional apparatus.
Microwave-Assisted Extraction
The first additional separation technique commonly used in the food industry is microwave-assisted extraction (MAE). MAE has been used to extract aromatic compounds from foods such as garlic, lavender flowers, and orange peel.2 The advantages of using MAE are low solvent consumption, short extraction times, low energy consumption, and reproducibility.
Microwave ovens at home typically rely on water molecules present in food as a heat absorber. Solvents have a chemical property called a dielectric constant. The dielectric constant is a measure of a substance’s ability to absorb energy while present in an electromagnetic field. Water has a high dielectric constant that causes it to absorb microwave radiation. The absorbed microwave energy is transformed into heat, which is why food becomes hot.
The type of solvent used in MAE is important. A molecule with permanent or induced dipoles will be rotated by the microwave’s oscillating electromagnetic field. The rotation of the molecule as it moves in alignment with the electromagnetic field releases the energy in the form of heat.
Chemistry of Chocolate
There are mainly 3 types of commercialized chocolate depending on the fat content and the amount of cocoa solid. Dark chocolate contains the most cocoa solids. Milk chocolate contains between 20-30% cocoa solids. To improve the taste, vanillin and butyric acid are added. Finally, white chocolate contains no cocoa solids, but contains additions such as sugar and milk.
There are three types of molecules primarily found in foods: carbohydrates, fats, and proteins. Cocoa butter is classified as a triglyceride fat meaning that it is composed of a glycerol molecule and 3 fatty acids: palmitic (P), stearic (S), and oleic (O) acids. The most common symmetrical mono-unsaturated forms are POP, SOS, and POS.3 Occasionally, cocoa butter will contain small quantities of triolein OOO, tripalmitin PPP, and tristearin SSS. Occasionally, there are small amounts of linoleic acid present.
Scientists have recently used MAE to isolate the fat contents of cocoa powder using a hexane/isopropanol solvent and cocoa nibs using petroleum ether solvent.4 Results revealed the accuracy of fat separation using MAE is just as efficient as other processes. Hexane is a nonpolar molecule with a dielectric constant of 1.9, while polar molecules like isopropanol, petroleum ether, and water have dielectric constants of 18.3, 4.3, and 80.4, respectively.5 A solvent with a low dielectric constant is considered relatively unaffected by microwaves. Since hexane is a nonpolar molecule and cocoa butter is also nonpolar, the cocoa butter will be extracted from the cocoa powder. The MAE speeds up the extraction process by heating the polar water and isopropanol molecules in the mixture.
Since this unit is designed for African American students, the extraction of fat from cocoa is essential in producing cocoa butter, a common cosmetic product used by that community. In the event that this unit is not administered to African American students, the other common product of cocoa butter is white chocolate. Thus, teaching this advanced separation technique has relevance in simply making other candies and desserts.
Separation Techniques used in Processing Fruit Juices
There are four basic methods for producing fruit juices.6 Juice extraction by pressing is the most common separation technique and is mostly associated with juices that still contain pulp, such as orange juice. This is accomplished mechanically by applying a force to the outside of the fruit to create enough tension to drain out the liquid, called the juice yield. The juice yield can then be used as is, or further treated with other separation techniques.
The type of fruit typically determines whether or nor is it readily consumable or needs additional processing. The categories are citrus, pomaceous fruits, stone fruits, grapes, and berries. Fruits such as apples and oranges are consumable without additives. However, berries such as black and blueberries are acidic berries that requires a sugar syrup or a juice concentrate in order to be enjoyable. The most common concentrate additives are apple and grape. Fruit pulps can also be used to create nectars by adding a type of sugar syrup.
Juice extraction via water solvent relies on molecular diffusion of the fruits components. The rate of diffusion, or diffusion coefficient, and the permeability of the cell walls are controlled between temperatures of 50-70 °C.6 Also, increasing the available surface area of the fruit by chopping the fruit in the smaller pieces improves the transfer process. The most common but subtle example of extraction via water solvent is in restaurants. Oftentimes, customers will order a glass of water with lemon. Some will mechanically squeeze the lemon, while others will simply let the lemon juice naturally diffuse into their beverage. Hotels often have a mini-concession table in their lobbies with water containing fruits such as lemons, limes, oranges, strawberries, and cucumbers, to provide a refreshing welcome to innstayers. Boiling lemon in water has also become common due to the belief that drinking lemon water in the morning helps to remove toxins from the body, balancing alkalinity, and ultimately improving the immune and digestive systems.
Juice clarification involves an additional step after extraction via pressing out the water solvent. Usually, there are still plant particles present in the juice. Oftentimes, parts or all of the plant particles are removed from the mixture to improve taste, color, and odor. Scientists will continue to improve the quality of the juice with the addition of enzymes to break down larger particles or clarifying agents that will further precipitate unwanted particles. Clarification can also occur mechanically with centrifugation or filtration.
Juice concentrate is produced using evaporation, freezing, and microfiltration techniques. Evaporation risks losing some of the benefits while freezing risks retaining small plant matter. Microfiltration has a distinct advantage in which microbes can be filtered out of the juice concentrate.
A Healthy Balance: Chocolate, Fat, and Juice
Now that we have looked at chocolate and juices, we can begin to explore the relationships between the two. Since chocolate is high in fat and sugar, food scientists have sought to develop a healthier chocolate. By using a Pickering stabilizer, scientists, such as Stefan A. Bon, began with substituting water in place of fat through emulsification.7,8 However, the dispersion of water in chocolate risks the structural stability of the chocolate, meaning that it will not maintain its ‘snap’. Bon has also taken it a step further by infusing juices into chocolate using chitosan, a popular polysaccharide used in the biomedical field to reduce bleeding and to deliver drugs through the skin. With this discovery in mind, the idea of making infused chocolate confectionaries lends itself directly to the study of mixtures, their phases, and the chemical separation techniques used to manipulate them into desired products.
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