Food Analysis Lecture 13 (3/2/2010) Infrared (3) Qingrong Huang Department of Food Science

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 ( nm) is more widely used for quantitative analysis of foods than are mid-IR.

Near-IR Spectroscopy

Reflectance (R) is defined as: R=I/I 0 I: the intensity of radiation reflected from the sample at a given wavelength I 0 : 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, meters 2 ), 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 stress When 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

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 =0] Newtonian Model: [n=1; k=µ;σ 0 =0] Power law Model: [=0] Power law Model: [σ 0 =0] Bingham Plastic Model [n=1;K=µ pl ] Bingham Plastic Model [n=1;K=µ pl ]