Physical methods for food analyses

Mehods

1) Densimetry( 密度法）2) Refractometry （折光法）3) Polarimetry （旋光法）4) Colourimetric method （色度）5) Viscosimetry （粘度）

Densimetry

The density of many substances is a characteristic

physical property and serves for identification

purpose.

1) Density, ρ, is defined as ρ = mass/volume = m/v

2) Absolute density （绝对密度） , ρ t , at t is defined as℃ ρ t =m(g)/Vcm3 (cm3) V, total volume not including air gap

3) Relative density （相对密度） , dt1 t2 , at t is defined as℃

dt1 t2 =m(g)/Vml (ml) By definition, 1ml = 0.001 part of the volume of 1kg of pure

water at 3.98 , it is dimensionless; t℃ 2, the temperature of sample, t1, the temperature of water

4 ） Apparent density （表观密度） , ρt1, at t is defined as℃ ρ t1=m(g)/Vcm3 (cm3) V, total volume including air gap in a state of nature

Definition of density

Influencing factor for density analyses

1)Temperature

The density decreases by about 0.03% per ℃rise for the water at room temperature.

Measurement of the density of liquids

Pycnometric Method

1. Types of pycnometer

Principle:

Measuring the weight of a known volume of liquid in a vessel, the volume of which was calibrated in terms of the weight of pure water that the vessel holds.

Matters need attention

1. Before use, the pycnometer must be thoroughly cleaned with a mixture of potassium dichromate and concentrated sulfuric acid, rinsed thoroughly with water, and dried.

2. To avoid changes in volume , the pycnometer must not be subjected to excessive temperature or pressure changes.

3. Bubbles are the most common sources of relatively large errors.

Hydrometry (Archimedes principle)Based on the principle that the same body displaces equal

weights of all liquids in which it floats. If V1 and V2 are the volumes of two liquids displaced by the same floating body, and D1 and D2 their respective densities, then

V1D1 =V2D2 D1/D2 = V2/V1 Thus, the volumes of different liquids displaced by the

same floating body are inversely proportional to the densities of the liquids. If the floating body is an upright cylinder of uniform diameter, the volumes displaced are proportional to the depths (H1,H2) to which the body sinks.

D1/D2 =H2/H1

Types of hydrometers

Ordinary Densitometer （普通密度计）Alcoholometers （酒精计）Salometers （盐度计）Oleometers （油度计）Baumē hydrometers （波美计）Saccharometer （糖度计）Lactometers （如稠计）

AlcoholometersAlcoholometers are used for determining the percentage of alcohol. SalometersSalometers are used to determine the percentage of saturation of salt brines.OlemetersOlemeters are used for vegetable and sperm oils, the

y have a scale of 50-0o that corresponding to densities of 0.870-0.897.

Baumé hydrometers

Baumé hydrometers are of two kinds: heavy

Bé for liquids heavier than water, and light Bé for liquids lighter than water. In the first, 0o corresponds to a density of 1.000 and 66 o to a density of 1.842. In the lighter than water, Bé corresponds to a density of 0.745. For Bé degree on a scale of densities above 1.000.

Desnisty = 145/(145- Bé reading)

Saccharometers

The use of saccharometers is to determine the composition of various sugar blends (sucrose, corn syrups, invert sugar, high-fructose syrups) in solution

LactometersLactometers are used to determine dry milk solids(and ad

ulteration by water addition) of milk from density determinations. The Soxhlet lactometer has a scale of 25-35 (density 1.025-1.035) subdivided into suitable divisions. The Quevenne scale has a range of 14-42o Quevenne in 1o divisions, and corresponding scales to indicate percentage of water in whole or skim milk.

Hand-held Densitometer

Measurement of the density of solids

Instrumentation for determining

apparent density of particles

Hand-held Densitometer

Refractometry （折光法）The amount of refraction is a characteristic of every

substance. It is commonly expressed by the refractive index (RI) n （折射率） . RI measurements have long been used for the qualitative （定性的） identification of unknown compounds by comparing the RI of the unknown with literature values of various known substances.

Definition 1. Reflection phenomena and law （反射现象和定律

）

2. Refraction phenomena and law （折射现象和定律）

When a ray of electromagnetic radiation strikes a flat surface at an angle, the ray may bent upward (reflected) or bend downward (refracted) as illustrated in Fig.. Notice that the ray does not go straight through the material. The Refractive index (RI) n, defined as

n = sinα1/sin α2 = υ1/ υ2

Because,n =C / υ （ velocity in empty space)So, sinα1/sin α2 = n2 /n1

Total reflection ( 全反射）and critical angle1.Optically thinner medium( 光疏介质）2.Optically thicker medium （光密介质）3.Critical angle ( 临界角）

sinα1/sin α2 = n2 /n1

n1 = n2 sinα2

n2, refractive index of prism;

α2,critical angle;

n1, refractive index of liquid

Three rays of monochromatic radiation strike a medium of different density. Three of these rays are refracted and would produce light on the other side of the medium. However, the fourth ray and all other rays having an angle of incidence equal to or greater than it are not refracted. Thus no light gets through the medium at this point and a dark field is produced.

When α2 is the critical angle, α1 = 90o

Refractometers （折射计）

Abbe Refractometer( 阿贝折光仪）The Abbe refraction is probably the most common t

ype employed today. A schematic diagram of it is shown. the refractometer can be used for direct reading and requires only one or two drops of sample.

Polarimetry ( 旋光法）

There are millions of rays coming from yhour lamp and the direction of the electric and magnetic components are puely random and may look like (a), this radiation is said to be unpolarized. If , However, by some means we can get all of the rays to have their electric and magnetic components all in the same direction, then the radiation is plane polarized, as shown in (b).

Optical rotation (optical activity)Compounds that can rotate polarized light （偏振光） are said

to be optical active( 旋光性） , In the old system of nomenclature, if you were looking toward the light source and the rotation was clockwise, then the compound was said to be dextro(+)( 右） ; if rotation was counterclockwise, then the compound was levo(-) （左） .

The new system of nomenclature, the Cahn-Ingold-Prelog system, is not based on rotation at all but on the configuration of atoms around a particular center. As an example of how an optically active compound is designated in this system, consider the molecule of 1-chloro-1-ethanol.

Cl

H OH

CH3

C

1. Optical rotation （旋光度）

Angular dimension( 角度大小） of polarization plane （偏振面） of polarized light that is rotated by optical sample , is called optical rotation.

2. Specific rotatory power （比旋光度） [α]λ

t ＝ α/Lcα, Optical rotation L, Length c, Concentration of sample

Polarimeter( 旋光计）

Polarimeter

The instrument is used for measuring optical rotation. A polarizer ( 偏光镜） polarizes the radiation, the sample rotates the radiation, and an analyzer measures how much the rotation is.

糖度计

Colorimeter( 比色计）

viscosity

viscosity is the resistance a material has to change in form.  This property can be thought of as an internal friction

Kinematic viscosity( 运动粘度）Absolute viscosity ( 绝对粘度）

Viscometer

质构仪