Food AnalysisLecture 13 (3/2/2010)

Infrared (3)

Qingrong Huang

Department of Food Science

http://foodsci.rutgers.edu/huang/Food_Analysis/FA2010.htm

Reading Materials

Reading Chapter 24 & chapter 30

Questions: Page 515, Study Questions: 1-3

ATR-FTIR

ATR-FTIR: Attenuated Total Reflectance FTIR- A surface sensitive technique;- The evanescent wave penetrates only a small distance through the crystal (typically about a micro meter), depending on the refractive index of the IR-transparent crystal, the internal reflectance element (IRE).

Near-IR Spectroscopy

NIR (800-2500 nm) is more widely used for quantitative analysisof foods than are mid-IR.

Near-IR Spectroscopy

Near-IR Spectroscopy

Reflectance (R) is defined as:

R=I/I0

I: the intensity of radiation reflected from the sample at a given wavelength

I0: the intensity of radiation reflected from the reference at the same wavelength

Reflectance data are expressed most commonly as log (1/R),analogous to absorbance in transmission spectroscopy.

NIR Applications

Rheological Principles for Food Analysis

Qingrong Huang

Department of Food Science

Reading Chapter 30

Questions: Page 515, Study Questions: 4-6

Flow Properties of Foods

Fundamentals of Rheology

Rheology: the study of the deformation and flow of all materils.

Viscosity: defined as the internal resistance to flow.

Stress (): the measurement of force, defined as the force (N) divided by area (A, meters2), Pascals (Pa)

Two types of stress: • Normal stress: the force directly perpendicular to a surface

- tension or compression, eg. Chewing gum• Shear stress: the force parallel to the sample surface

- e.g. the spreading of butter over a slice of toast

Normal vs. Shear Stress

Normal stress Shear stressWhen a stress is applied to a food, the food deforms or flow.

Strain (): is a dimensionless quality representing the relative deformation of a material.

Shear Rate

Shear flow between parallel plates

Shear rate =d(L/h)/dt or

h

UL

dt

d

h

)(

1

Fluid Viscosity

Newtonian Fluids:

where is the stress, is the shear rate, and is the Newtonian

the viscosity. Apparent viscosity () is defined as

If is a constant, then it is a Newtonian fluid; otherwise it is non-Newtonian fluid.

Newtonian Fluids

Shear Thinning vs. Shear Thickening

• Most of the fluid foods are not Newtonian fluids;

•

Rheological Fluid Models

• Herschel-Bulkley Model

• Newtonian Model: [n=1; k=µ;σ0=0]=0]

• Power law Model: [Power law Model: [σ0=0]=0]

• Bingham Plastic Model [n=1;K=µBingham Plastic Model [n=1;K=µplpl]]