Lecture 7 – Water Potential 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. 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Water Potential 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 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Water Potential 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 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Water Potential (Chapter 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. 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity Deformation due to applied forces varies widely among different biomaterials. Depends on many factors Rate of applied force Previous loading Moisture content Biomaterial composition 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity Force deformation studies Texture of raw and processed, cooked and uncooked New variety selection Study damage during harvesting and handling Failure studies, cracking/splitting 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity 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 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity Compression 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity 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 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity Example: 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity 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 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity 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 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7

Lecture 7 – Deformation and Viscoelasticity 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 11/13/2018 BAE2023 Physical Properties of Biological Materials Lecture 7