Polymers, Biodegradability, and Food Preservation Chemistry

Introduction to Alkenes and Functional Groups

• Simplest Alkene: Ethylene ($C_2H_4$) is identified as the simplest alkene.
• Functional Groups: Ethylene serves as a fundamental functional group in organic chemistry. Though not discussed extensively in previous sessions, it is central to the formation of polymers.
• Molecular Scale: When billions of ethylene molecules are combined through chemical reactions, they form a polymer.

The Fundamentals of Polymerization

• Polyethylene: This is the polymer version of ethylene.
• Representation in Chemistry:
  • Polymers are specifically described using brackets in chemical notation.
  • Instead of drawing an infinite chain, chemists draw a single repeating unit within brackets.
  • The subscript $n$ (e.g., $[CH_2-CH_2]_n$) represents the repetition of these units, indicating that there are "n" number of those molecules joined together.
• Example: Polystyrene:
  • Styrofoam is the common commercial name for polystyrene.
  • Polystyrene is the polymer formed from the monomer styrene.

Reaction Mechanisms: Radical Reactions

• Mechanism Type: Polymerization is often achieved through radical reactions.
• Connection to Previous Lessons: This connects to discussions on antioxidants, which are used to manage radicals.
• The Propagation Step:
  • A radical is "kick-started" (initiation).
  • The process leads to a propagation step, which continues the reaction nearly indefinitely.
  • This continuous addition results in the creation of exceptionally long carbon chains, consisting of billions upon billions of units.

Structural Properties: Low-Density vs. High-Density Polyethylene

• Property Determinants: The specific nature of the carbon chain and the interactions between polymer molecules dictate the physical properties of the plastic.
• Low-Density Polyethylene (LDPE):
  • Commercial Example: Grocery bags.
  • Physical Characteristics: Easy to break, flexible, and stretchy.
  • Molecular Structure: The polyethylene branches are more separate from each other. There is significant "branching," which creates weaker interactions between the individual polymer strands.
• High-Density Polyethylene (HDPE):
  • Commercial Example: Milk jugs.
  • Physical Characteristics: Hard, rigid, and much more difficult to pull apart or break.
  • Molecular Structure: The polyethylene molecules are packed much closer together. There is minimal branching, allowing for stronger intermolecular interactions.

Environmental Impact and Sustainability in Packaging

• Solid Waste Statistics: A research paper from approximately 15 years ago stated that $31\%$ of solid waste in the United States originated solely from packaging waste. This number is estimated to be significantly higher today.
• Recycling Challenges:
  • Cities often refuse to recycle materials contaminated with food.
  • Plastic grocery bags generally cannot be placed in standard recycling bins; they require specific bins often located at store entrances.
  • Cardboard boxes (like pizza boxes) are often excluded from recycling if they contain food components.
• Chemical Stability: Traditional plastic carbon chains are highly unreactive and stable. While this makes them durable, it makes them extremely difficult to break down in the environment.

Biodegradable Polymer Alternatives

• Polylactic Acid (PLA):
  • Production: PLA is created through fermentation by microorganisms.
  • Fermentation Process: Microorganisms (like yeast or bacteria) consume nutrients to produce lactic acid. This is the same biological process seen in sourdough bread production.
  • Degradability: PLA contains ester bonds that can be hydrolyzed. Hydrolysis breaks the ester into a carboxylic acid and an alcohol, allowing the polymer to decompose more easily than pure carbon chains.
• Polyhydroxyalkanoates (PHA):
  • Production: PHAs are produced directly by bacteria.
  • Mechanism: Scientists isolate specific cell lines of bacteria and engineer them to produce these polymers as they grow.
  • Sustainability: Like PLA, these can be broken down through hydrolysis.

Bio-Engineering and the Central Dogma

• The Industrial Process: Growing polymers via bacteria is analogous to how proteins are manufactured in a lab.
• The Central Dogma of Biology: Information flows from DNA to RNA to functional proteins.
• Engineering Microorganisms:
  • Scientists use prokaryotes (like $E. coli$).
  • A plasmid (a double-stranded DNA molecule) is engineered with specific genetic information.
  • The bacteria act as "factories," reproducing and translating that genetic information into the desired protein or polymer.

Natural Polymers: Polysaccharides

• Starch and Cellulose: These are well-known polysaccharides found in plants that function as natural polymers.
• Sodium Alginate:
  • Source: A natural product extracted from seaweed.
  • Structure: Composed of multiple sugar monomers linked together.
  • Applications: Used to create thin films (similar to plastic wrap) and edible membranes.
  • Benefits: It is biodegradable and edible.
  • Limitations: Films made from sodium alginate are currently very fragile and break easily compared to synthetic plastics.
• Chitin:
  • Source: Found in the exoskeletons of insects and crustaceans (e.g., crabs).
  • Conversion: Chitin can be converted into Chitosan.
  • Applications: Used to create gels and biodegradable films.

Limitations of Plastic Packaging

• Permeability: While plastics appear solid, they are permeable to very small molecules such as water ($H_2O$), oxygen ($O_2$), and carbon dioxide ($CO_2$).
• Flavor/Odor Issues: Because small molecules can pass through the plastic, food smells can escape, and external odors can enter. Unlike glass, plastic containers often retain the smell of the food they previously held.
• Staining: Plastics are prone to staining. A common example is the permanent orange stain left on plastic containers by spaghetti sauce.

Recap of Food Preservation Principles

• Three Main Preservation Materials:
  1. Glass.
  2. Metal cans (Aluminum and Tin).
  3. Plastics.
• Primary Goals of Preservation:
  • Prevent Oxidation: Exposure to oxygen causes lipids/oils to oxidize, creating a "rancid" smell.
  • Prevent Microorganism Growth: Inhibiting bacteria and mold.
  • Inhibit Enzyme Activity: Preventing natural enzymes in food from breaking it down or oxidizing it prematurely.

Questions & Discussion

• Question: How would you describe a grocery bag?
• Answer: It is easy to break or stretch. This is due to the low-density interaction and branching of the polyethylene molecules.
• Question: What is the Central Dogma?
• Answer: It is the process of information transfer in biological systems: DNA to RNA to functional proteins.
• Question: Are insects used in biology for this?
• Answer: Students noted they usually learn about plants and bacteria in biology rather than insects, but the instructor clarified that chitin from insect exoskeletons is a significant polysaccharide used in polymer science.

Upcoming Laboratory and Assignments

• Spherification Lab (Tomorrow):
  • StartTime: 08:10 AM.
  • Procedure: Combining Sodium Alginate and Calcium Chloride ($CaCl_2$) to create edible spheres/membranes.
• The "Martini" Recipe Assignment:
  • Due tomorrow.
  • Requirements: Must be creative. A simple two-step recipe is insufficient; the instructor is looking for complex processes that allow for the discussion of multiple chemical steps.
• Monday Schedule: The class will experiment with "Miracle Berries" at 02:15 PM.
• Additional Resources: Scientific articles regarding biosensors (to detect food oxidation) and containers with integrated antioxidants will be posted for summer reading.