Lecture 7 Water Potential

Transcription

1 Lecture 7 Water Potential - HW due Friday 2/13 - Lab report due Wednesday 2/18 - ASABE student branch meeting 2/10 1

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

3 Lecture 7 Water Potential Water Activity: ratio of vapor pressure above solution to vapor pressure of pure water p w aw = p 0 w 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 3

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 4

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. 5

6 Deformation due to applied forces varies widely among different biomaterials. Depends on many factors Rate of applied force Previous loading Moisture content Biomaterial composition 6

7 Force deformation studies Texture of raw and processed, cooked and uncooked New variety selection Study damage during harvesting and handling Failure studies, cracking/splitting 7

8 Deformation of solids and liquids/semisolids Chapter 4: solids Chapter 6 & 7: liquids/semi-solids Chapter 4: Solids Damage to fruits, vegetables, grains, seeds during harvesting and handling 8

9 Compression 9

10 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 10

11 Example: 11

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 12

13 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 13

14 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 14

15 Problem 1: Lecture 7 Deformation and HW#6 Assignment 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 15

16 Problem 2: Lecture 7 Deformation and HW#5 Assignment 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 ε z, and strain σ z on the potato cylinder. 16