Chemistry of Cooking

Content Understanding

High school students are in the position to make more food choices since they are practicing healthy decision making skills during adolescence. The issues with making appropriate food choices are twofold, regarding student finances and health. For example, I often see students walking into class late with McDonald’s bags or Starbucks Frappuccino in their hand. I am concerned with both the financial impact on students who purchase these foods many times each week, and the nutritional value, or lack thereof, from the foods students are choosing to consume. More students need to take advantage of the food provided daily on campus. For many of them, our campus provides the main or sole source of entire meals. The food provided to schools by state funding often arrives frozen, is then heated in the morning, and finally stored in insulated drawers. While our Nutritional Service Team makes every attempt to prepare fresher food to supplement the main part of the meal, a good portion of the food is processed to ensure quick heating time. Whether students receive their food from home, fast food restaurants, or at school, they need to be aware of how to preserve perfectly edible leftovers to avoid foodborne illnesses. This practice of preserving leftovers is also economical since food is not being thrown away due to spoilage.

Spoiled food is inedible because substances that cause illnesses in the consumer have contaminated it. Food spoils when biological, chemical, or physical interactions occur with external sources. The necessary level of understanding of microbiology and biochemistry for this unit is just past high school level; however, the details provided in this curriculum unit will be more than enough for anyone teaching this unit. An overview on food preservation methods and storage container materials will provide critical background information. The majority of lesson activities will lead students to understand the chemical interactions and reactions involved in food spoilage. Additionally, comparing different storage methods and options will provide students with many opportunities to practice design techniques and engineering practices.

Spoilage of Food

The process of food spoilage is both biological and chemical. The biology of spoilage involves bacteria, yeast, and mold. The chemistry of spoilage includes enzymatic reactions that break down carbohydrates, proteins, and fats into different molecules. Changes in molecules alter the appearance, taste, smell, and texture of food items, making the food less desirable.

Bacterial species vary greatly, but many of them have similar basic needs as humans, including the need for water, oxygen, food, and a warm place to thrive. Other specific environmental needs includes pH above 4.5 to 7 and temperature ranging from 4-60 degrees Celsius (^°C).¹ Many of our natural food sources are perfect homes for these single-celled organisms to consume and multiply exponentially. Specific species of bacteria can be found in one food type, but not found in others. Table 1 shows some of the more common bacteria that can spoil food and drinks.

Table 1. Common bacteria and enzymes associated with different common foods and drinks. Compiled from Huis in’t Veld, 1996 and references therein.²

The commonality amongst all of these bacteria is that they render our food inedible due to the growth of their colonies. The bacteria with its byproducts are harmful for human consumption. Additionally, the nutrients humans need from the food have been partially or completely consumed by the bacteria.

Enzymes also break down the chemical composition of the nutrients in food that the human body would normally perform during metabolism. Enzymes are proteins that are the “Wreck-It Ralphs” of the biochemical world. An enzyme that helps us in preparing our food is bromelain that is found in pineapple and used in meat tenderizers.³ Bromelain breaks down meat protein, hence making the meat very tender. This also occurs when humans eat too much pineapple and end up with a raw feeling in their mouths. Enzymes that reduce the quality of food also target specific molecules to break, and the result is organic properties that warn us not to consume the food. A common example is when lipase catalyzes the hydrolysis of triglycerides in milk and milk products to result in rancidity. 

Food Preservation Methods

The intention of food preservation is to limit biological, chemical, and physical interactions within food. Only preservation techniques that align to the strategies and activities for this unit will be discussed in detail.  The following methods encompass the scope of this unit: dehydration, freeze-drying, pasteurization, and identifying ideal storage temperatures. These methods are mostly commercially done to provide ready-to-eat meals for consumers who would continue the preservation process at home.

Dehydration

This is an umbrella for preservation techniques that remove varying degrees of water from the food source. The atmospheric conditions necessary to achieve dehydration include: temperature, humidity, pressure, portion size, and length of storage intended for the food.⁴ Salting, smoking, and hanging in dry air are some common forms of dehydration. Salting dehydrates cells of organisms via the process of osmosis. Cells contain water that is drawn out when the salt concentrations in the environment are greater than the concentration inside the cell.⁵

Freeze-drying

Also known as lyophilization, freeze-drying is under the broader category of dehydration methods. Freeze-drying is an extremely sophisticated way of removing water from food and drinks that not only limits microorganism activities, but also reduces the overall volume of the foodstuff. Temperature and pressure are manipulated to reduce the water content of frozen food resulting in a dry, stable product. Simply stated, three steps accomplish this:

1) Freezing the food/drink;

2) Sublimation (solid water directly changed into vapor);

3) Desorption (resulting in a dry, stable product)⁶

All of this is achieved in a vacuum to avoid transitions into the liquid phase. The sophistication of this process exists in the modification of temperature and pressure for each ingredient in the food source. Since the phase change occurs differently in each ingredient, the engineering of vacuum-sealed chambers to achieve sublimation must be taken into consideration for all of these variations. The result leads to very expensive freeze-dry products with shelf lives averaging a decade. Freeze-dried products like meals-ready-to-eat (MRE), from brands like Backpack Country, serve as great emergency rations for natural disasters because of their long shelf lives, or lightening the backpack load on a hiking trip.

Pasteurization

Foods typically pasteurized include milk, liquid eggs, fruit juices, and beer.⁷ This mild heat treatment targets specific bacteria species in a two-step process. First, the food item is heated. Secondly, the food is sealed in a hermetic package that includes two processes for packaging. Hot-fill pasteurization involves having the food item at a hot temperature for a short period of time. The inside of the container will be pasteurized during this short span of contact with extremely hot food product. Cold-fill pasteurization typically occurs in frozen foods. The food and container are joined in a very sterile environment during the cold-fill process. This method of pasteurization requires taking extra precautions to avoid microorganism interaction.⁸

The engineering of pasteurizers is dependent on targeting a specific type of bacterial species, while maintaining the foods’ flavor and consistency. Milk and creams are common liquid products that are pasteurized to eliminate common bacteria like Escherichia coli (abbreviated as E. coli), Salmonella, and Listeria.⁹ Even with the intention to create an environment too hot for some bacteria, refrigeration is still necessary to delay the growth of bacterial not killed by the pasteurization process.

Storage Temperature

The intention for storing food and drinks in cold temperatures is to delay bacterial growth. Perishable food like beef, fish, dairy products, and agricultural foods are dependent on cold storage for longer shelf life.¹⁰ Many bacteria will react to hot and cold temperatures just like humans do. If the weather is freezing cold, humans are likely found huddling with many layers of jackets and blankets to keep warm. If the weather mimics the warm shores of Hawaii, humans would be running around soaking up the warmth of the sun. Bacteria act similarly by how they slow down growth and stay huddled in cold temperatures and speed up growth and spread with warm temperatures.

Types of Food Containers

The purpose of food packaging is to protect food against biological and chemical spoilage, while also providing the consumer with a container to store their food according to their preferences. Packaging manufacturers take many factors into consideration when designing and producing these different forms of food containers. Packing considerations include: cost, compatibility with the food, shelf life, modifiable sizing, production speed, impermeability, tamper resistance, and consumer convenience.¹¹ The materials used for creating food containers vary from metal to edible films. Each type of food container material addresses the form of the containers available, how the materials work in preventing spoilage, and the potential chemical interactions that it may have with food.

Metals

Metal food containers commonly take the shape of cans, trays, bottles, or wraps. It prevents spoilage by creating a barrier from oxygen and light. Two commonly used metal containers are either made of steel or aluminum. Steel is molded into cans and bottles for processed food like fruits, vegetables, and various juices. Aluminum is molded into trays and wraps that are lighter, yet less sturdy compared to steel. Since foods come into direct contact with the metal, chemical reactions may occur. One issue with these metals is its reaction with acids. Over an extent of time, the acid begins to corrode the metal. Packing companies typically coat steel with noncorrosive materials like tin, chromium, or aluminum.¹² These coating materials must be inert to acids, withstand sterilization temperatures, and resist breakage to prevent contamination.¹³

Glass

Glass bottles and jars serve multiple food containment purposes. This material prevents vapor and oxygen loss in food when appropriately sealed. In addition, the thickness of glass can vary to allow for minimal breakage. Since glass is a derivative of metal oxides, like silicon dioxide (commonly known as sand), the modification of thickness is not difficult to achieve; however, the production cost will increase since more material is used. Coatings of silicon or waxes are used to assist with minimizing nicks and scratches on the container. This allows for continued reuse of the container even after the original food item is consumed. Glass and glass coatings are inert and do not react with acids, oxygen, nor bacteria, making this an ideal material for containers aiming to prevent bacterial spoilage by blocking their interaction with material inside the container. However, there are necessary precautions to store glass containers to avoid contamination. Leaving the lid off or loose lids provides an open invitation for bacteria and other contaminants to enter the sacred food space. Extended exposure to air on food products will also create an oxygen-rich environment for bacteria to grow.

Paper

Depending on the thickness and type, paper containers include bags, wraps, or carton form. Paper containers are often coated with laminates, plastics, resins, and waxes to prevent leakage, bursts, rips, vapor loss, and to block contaminants. Paper of minimal thickness is good for bagging or wrapping baked goods. The next type of paper container is thicker than the previous and is appropriate for holding drinks, like milk and juices. Paper with limited thickness and coating allows for permeability of gases, which is useful when paper containers are used in cooking. For instance, wrapping a slab of sea bass in parchment paper with some shitake mushroom, ginger, and soy sauce, then broiling it in an oven produces a wonderfully steamed meal. Some of the gases permeate through the paper while letting heat through to cook the food products. These containers are sophisticated with the addition of susceptors to allow for more versatility beyond storage and extend into the cooking process. Susceptors are found in microwavable popcorn bags and Hot Pocket wrappers.

Plastics

Plastics provide the most diverse forms of containers since they are flexible, stretchable, lightweight, resistant to breakage, heat sealable, and form almost all shapes. Different forms of plastic serve purposes for specific types of storage. For example, polyethylene with ethyl vinyl acetate is in frozen food containers, while polystyrene is Styrofoam for to-go containers.¹⁴ Other forms of plastics include bottles, jars, closures, films, pouches, bags, tubes, and trays. The plastic material is good for creating a barrier to oxygen, light, and moisture. Like glass, plastic is inert to bacterial and chemical spoilage hence making it an appropriate container for many different food sources. The limitations with plastic are its inability to withstand high temperature and its permeability to certain liquids and gases.

Edible Film

This innovative form of storing food is edible! Edible films are made of food products in combination with additional sugar, starches, casein, or gelatin. Prime examples of edible films are the candy coating on M&Ms and casing on sausages. The film provides a minimal barrier to the food from spoilage, analogous to the tortilla of a burrito. Since the barrier is also a food product, the risk of spoilage is the highest in this form of container material. Edible films are mainly for the purposes of consumer’s convenience and preferences.

Refrigerators

Temperature plays an important role in regulating spoilage of food and drinks. In general, hot temperatures speed up chemical reactions and cold temperatures slow down reactions. Since the process of spoilage is biochemical, placing food items in a regulated cold environment delays or hinders bacteria from reacting with the food. Examples of cold storage used by individuals domestically include refrigerators, water coolers, chillers, and deep freezers. Most households around the world have some variations of sophistications in refrigeration techniques. A refrigerant removes the heat from the food, resulting in a lower temperature.¹⁵ Refrigerators and other cold storage equipment provide space for cold temperature to extract energy from food. This is the process of latent heat of vaporization. If a system is placed on a very cold surface, eventually the system and surface will reach equilibrium between the two original temperatures. Refrigerators have mechanisms to ensure that energy is constantly removed from the system to prevent the temperature from reaching equilibrium with the outside of the refrigerator.

Settings

Studies about the knowledge and understanding of refrigerator settings show that very few people know the appropriate temperature to prevent food spoilage and cross-contamination. There are still discrepancies with optimal temperatures for cold and freezer compartments. The recommended temperatures for refrigerators housing perishable food is less than 4.4^°C (40.0^°F).  A survey of 200 refrigerator owners showed that less than 18% have a visible thermometer to read the exact temperature in fridges.¹⁶ Even with highly intelligent refrigerators that display everything from temperature to listing dates of when items were placed inside, we still try to hide a slice of cake behind the low-fat Greek yogurt. Different sources recommend getting a visible refrigerator thermometer to accurately monitor temperature throughout the fridge,^17,18 as there are suggested temperatures for the three different compartments of the refrigerator.

Organization

Most fridges have three compartments, each with separate optimal temperature ranges to allow for differentiated food and drink storage. The fresh food storage compartment is for unfrozen food at a temperature of 0 to 8^°C, the chill compartment is for very perishable food at a temperature of -2 to 3^°C, and the cellular compartment is for all food and drinks at temperature warmer than the previous two compartments.¹⁹ When Americans were surveyed, there were few who knew how to separate different food items onto the many shelving options available depending on fridge design. Refrigeration storage has become more reflective of personal style of the users rather than the functionality of the fridge. Although instructional manuals recommend food locations within the compartments, there are no real consequences to misuse. People tend to place food and drinks where it is convenient, based on categories of food items, or because it looks more organized one way than another. Food placement variations result in a high chance of cross-contamination.

Food Safety

When food items are stored improperly, the likelihood of cross-contamination is increased and provides opportunities for bacterial and chemical spoilage. Foodborne illnesses are defined as:

“…[I]nfections or irritations of the gastrointestinal (GI) tract caused by food or beverages that contain harmful bacteria, parasites, viruses, or chemicals.”²⁰ The lack of knowledge about appropriate food storage contributes to the rise in illness due to spoilage of food and drinks.

Statistics Of Foodborne Illness

Though not all foodborne illnesses are reported to the public health department, an estimated 1 in 6 Americans experience foodborne illnesses.²¹ The Center of Disease Control (CDC) places foodborne illnesses into one of two categories: known pathogens and unspecified agents. There are numerous hurdles in obtaining data on foodborne illnesses since the mechanism of infection differs in each individual. We eat and metabolize food slightly different from one person to the next. Unfortunately, medical records from doctor and hospital visits, as well as autopsy reports can only provide a rough number of occurrences related to foodborne illnesses. The CDC estimates that 48 million sicknesses, 128,000 hospitalizations, and 3,000 deaths occur each year from foodborne illnesses.²² To aid in the reduction of these numbers, local and national public health departments created informational flyers and pamphlets to better understand the  causes and recognize the symptoms of these illnesses.