1. Lecture 7 – Water Potential
- HW due Friday 2/13 - Lab report due Wednesday 2/18 - ASABE student branch meeting 2/10
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

2. Lecture 7 – Water Potential
Water Activity:
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.
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

3. 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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

4. 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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

5. Lecture 7 – Water Potential
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.
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

6. 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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

8. 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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

9. Lecture 7 – Deformation and Viscoelasticity
Compression
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

10. 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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

11. Lecture 7 – Deformation and Viscoelasticity
Example:
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

12. 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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

13. 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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

14. 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
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

15. HW#6 Assignment Lecture 7 – Deformation and Viscoelasticity Problem 1:
Explain in YOUR OWN WORDS how an osmometer works and give an example of how one might be used in food engineering. (you will need to do some outside research…web, library…) Limit your answer to one page in MSWord 12pt. Font, double spaced, 1” margins
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7

16. HW#5 Assignment Lecture 7 – Deformation and Viscoelasticity Problem 2:
A piece of carrot is cut in a cylindrical shape. The dimensions of the carrot cylinder are 30.1 mm in diameter and 19.3 mm in height. It is compressed to a height of 17.9 mm with a force of N. Calculate the stress εz , and strain σz on the potato cylinder.
2/01/2008
BAE2023 Physical Properties of Biological Materials Lecture 7