3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 1 Water Activity: (remainder of Chpt 3) –Used when referring to foods –EMC used when referring to grains and seeds –Chemical potential of a “system”…solution and the air above the surface of the solution –Determines how states will change or interact. Lecture 8 – Water Potential

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 2 Water Activity: ratio of vapor pressure above solution to vapor pressure of pure water –Strongly influences microbial activity –Molds don’t generally grow at less than 0.7 –Yeasts don’t grow at less than 0.8 –Bacteria don’t grow at less than 0.9 –Oxidation is at a minimum at 0.4 –Browning reaches a maximum at 0.8 Lecture 8 – Water Potential

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 3 Water potential: describes the transfer of water into and out of cells and movement of water through cells. Characterizes the water status of cells in fruits and vegetables Osmotic pressure: measured with an osmometer Lecture 8 – Water Potential

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 4 Osmotic Pressure depends on turgor potential and total water pressure Directly related to the solutions water activity When turgor potential = 0, plant tissue is flacid, not stretched or extended. When cells are placed in water that has a water potential different from the water potential in the cell, water will move across the cell membrane. Lecture 8 – Water Potential (Chapter 4)

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 5 Deformation due to applied forces varies widely among different biomaterials. Depends on many factors –Rate of applied force –Previous loading –Moisture content –Biomaterial composition Lecture 8 – Deformation and Viscoelasticity

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 6 Force deformation studies –Texture of raw and processed, cooked and uncooked –New variety selection –Study damage during harvesting and handling –Failure studies, cracking/splitting Lecture 8 – Deformation and Viscoelasticity

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 7 Deformation of solids and liquids/semi- solids –Chapter 4: solids –Chapter 6 & 7: liquids/semi-solids Chapter 4: Solids –Damage to fruits, vegetables, grains, seeds during harvesting and handling Lecture 8 – Deformation and Viscoelasticity

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 8 Compression Lecture 8 – Deformation and Viscoelasticity

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 9 Definitions –Normal stress: force per unit area applied perpendicular to the plane –Normal strain: change in length per unit of length in the direction of the applied normal stress Lecture 8 – Deformation and Viscoelasticity

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 10 Example: Lecture 8 – Deformation and Viscoelasticity

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 11 Stress strain relationship –Strain not recovered = plastic strain –Recovered strain = elastic strain –Ratio of plastic strain to total strain = degree of plasticity –Ratio of elastic strain to total strain = degree of elasticity Lecture 8 – Deformation and Viscoelasticity

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 12 Stress strain relationship –Strain not recovered = plastic strain –Recovered strain = elastic strain –Ratio of plastic strain to total strain = degree of plasticity –Ratio of elastic strain to total strain = degree of elasticity Lecture 8 – Deformation and Viscoelasticity

3/7/2016BAE2023 Physical Properties of Biological Materials Lecture 8 13 Modulus of elasticity –Linear region of stress strain curve –E = σ/ε –For biomaterials: apparent E = σ/ε at any given point (secant method) –Tangent method: slope of stress/strain curve at any point Lecture 8 – Deformation and Viscoelasticity