1. Lecture 7Announcements
Lab #1 due Today, 1/31
HW#5 due MONDAY, 2/5
HW#6 due Wednesday, 2/7
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

2. Moisture Relationships Chpt 3, Pages 92-107
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.
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

3. Moisture Relationships Chpt 3, Pages 92-107
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
Yeats 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
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

4. Moisture Relationships Chpt 3, Pages 92-107
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
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

5. Moisture Relationships Chpt 3, Pages 92-107
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.
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

6. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
Deformation due to applied forces varies widely among different biomaterials.
Depends on many factors
Rate of applied force
Previous loading
Moisture content
Biomaterial composition
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

7. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
Force deformation studies
Texture of raw and processed, cooked and uncooked
New variety selection
Study damage during harvesting and handling
Failure studies, cracking/splitting
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

8. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
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
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

9. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
Compression
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

10. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
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
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

11. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
Example:
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

12. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
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
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

13. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
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
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

14. Deformation and Viscoelasticity Chapter 4, pg. 111 - 169
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
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

15. BAE2022/BAE4400 Physical Properties of Biological Materials
HW#6 Assignment
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
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials

16. BAE2022/BAE4400 Physical Properties of Biological Materials
HW#6 Assignment
Problem 2:
A piece of carrot is cut in a cylindrical shape. The dimensions of the carrot cylinder are 25.8 mm in diameter and 20.3 mm in height. It is compressed to a height of 18.2 mm with a force of N. Calculate the stress εz , and strain σz on the potato cylinder.
1/26/07
BAE2022/BAE4400 Physical Properties of Biological Materials