Transcript MOISTURE AND TOTAL SOLID ANALYSIS

MOISTURE AND TOTAL
SOLID ANALYSIS
SFA3023
Food Analysis
Importance of Moisture Assay
1.
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3.
4.
5.
Legal and Labeling Requirements
Economic
Microbial stability
Food Quality
Food Processing Operations
Forms of Water in Foods
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Free water
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Capillary water
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Water retains its physical properties
Water is held tightly within spaces within a food that
are surrounded by a physical barrier
E.g. An emulsion droplet
Bound water
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Water is bound physically (e.g. in protein) or
chemically (e.g. Na2SO4. 10H2O)
Determination of Moisture:
Methods
1.
Drying Methods
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2.
Distillation method
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3.
Dean and Stark Method
Chemical Methods
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4.
Oven drying methods
Karl Fisher
Gas production
Physical Methods
Drying Methods
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These methods rely on measuring the mass of
water in a known mass of sample.
The moisture content is determined by
measuring the mass of a food before and after
the water is removed by drying.
Basic Principle
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Water has a lower boiling point than the other
major components within foods such as lipids,
protein and carbohydrate.
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Sometimes, moisture content also reported as
“total solid”
Total solid is a measure of the amount of
material remaining after all the water has been
evaporated
Advantages
 Cheap, easy to use, many samples can be
analyzed simultaneously
Disadvantages
 Destructive, time consuming
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Moisture and total solids contents of food can be
calculated using one of the equations below:
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% Moisture (wt/wt) = wt H2O in sample
wt of wet sample
X 100
% Moisture (wt/wt) = wt of wet sample - wt of dry sample X 100
wt of wet sample
% Total solids (wt/wt) = wt of dry sample X 100
wt of wet sample
Types of Oven
1. Convection Oven

Greatest temperature
variations - because
hot air slowly
circulated with out the
aid of fan, air
movement is
obstructed further by
pans placed in the
oven
2. Forced Draft Oven
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The least temperature
differential across the
interior (< 1oC). Air is
circulated by a fan that
forces air movement
throughout the oven
cavity
Drying period 0.75 – 24
hr, depending on food
sample and its
pretreatment
3. Vacuum oven
 Drying under reduced
pressure (25 –
100mm Hg).
 Able to obtain a
more complete
removal of water and
volatiles without
decomposition within
a 3 – 6 hr
4. Microwave Oven
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Weighed samples are
placed in a microwave
oven for a specified
time and power-level
and their dried mass is
weighed.
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In microwave oven,
water evaporation is due
to absorption of
microwave energy, which
causes them to become
thermally excited.
Advantage
 simple
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Disadvantage:
 Care must be taken to
standardize the drying
procedure and ensure
that the microwave
energy is applied
evenly across the
sample.
5. Infrared lamp drying
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Principle of drying:
Similar to microwave
oven
Advantages: rapid and
inexpensive
This is because the IR
energy penetrates into
the sample
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To produce consistent results, one must control
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The distance between the sample and IR lamp
The dimensions of the samples
IR drying methods are not officially recognized
for moisture content determinations because it is
difficult to standardize the procedure.
6. Moisture Analyzer
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Using a digital balance,
the test sample is placed
on an aluminum pan
and the constant
temperature is applied
to the test sample.
Instrument
automatically weighs
and calculates the % of
moisture or solids
Practical Considerations
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Sample Dimensions
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Clumping and surface crust formation
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Surface area of material exposed to the environment
is important
Mix the sample with dried sand to prevent clumping
Decomposition of other food components
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High temp or too long drying time can cause
decomposition of some heat-sensitive components in
the food
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Volatilization of other food components
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High moisture samples
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Food with high volatile components should be
analyzed using distillation or chemical method.
Dried in two stages to prevent spattering
Sample pans
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Should be dried in the oven and stored in a
dessicator prior analysis
Avoid contact with bare hands. Use tongs or cotton
gloves
 Type of water
 free vs. bound water
 Example: % water in milk vs. non-fat dried milk.
 Temperature and power level variations
 Significant variations in temperature and power
levels within an drying instruments.
Distillation Methods
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Direct measurement of the amount of water
removed from a food sample by evaporation
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Involve co-distilling the moisture in a food
sample with a high boiling point solvent that is
immiscible in water, collecting the mixture that
distills off and then measuring the volume of
water
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Advantages:
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Suitable for low moisture foods and foods containing
volatile oils such as herbs and spices
Cheap, easy to set up and operate
Disadvantage:
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Destructive, time consuming, involve flammable
solvent, not applicable for some types of foods
Dean and Stark Method
 A known weight of sample is placed in a flask with
immiscible organic solvent such as xylene or toluene.
 The flask containing the sample and organic solvent is
attached to a condenser by a side arm and the
mixture is heated
 The water in the sample evaporates and moves up to
the condenser
 Liquid water will be collected in graduated tube
 When no more water collected, distillation is stopped.
Distillation Methods
Practical Consideration
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There are a number of practical factors that can
lead to erroneous results:
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Emulsions can sometimes form between the water
and the solvent which are difficult to separate
Water droplets can adhere to the inside of the
glassware
Decomposition of thermally labile samples can occur
at the elevated temperatures used.
Chemical Methods
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Moisture is determined by the reactions between
water and certain chemical reagents
A chemical reagent is added to the food that reacts
specifically with water to produce a measurable
change in the properties of the system, e.g., mass,
volume, pressure, pH, color, conductivity.
Type of chemical method commonly used:
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Karl Fischer Titration
Gas production Methods
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Measurable changes in the system are
correlated to the moisture content using
calibration curves.
For accurate measurement, chemical reagent
must reacts with all of the water molecules
present, but not with any of the other
components in the food matrix.
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Chemical reaction do not involve application of
heat.
Therefore, they are suitable for:
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Foods that contain thermally labile substances that
would change the mass of the food matrix on heating
(e.g., food containing high sugar concentrations) or
Foods that contain volatile components that might be
lost by heating (e.g. spices and herbs).
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Karl-Fischer Titration
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Determine the low moisture foods (e.g. dried
fruits and vegetables, confectionary, coffee, oils
and fats) or low moisture food high in sugar or
protein.
It is based on the following reaction:
2H2O + SO2 + I2 → H2SO4 + 2HI
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Procedure
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The food to be analyzed is placed in a beaker
containing solvent and is then titrated with Karl
Fisher reagent (a solution that contains iodine).
Iodine will reacts with remaining water in the
samples.
The endpoint color is dark red-brown.
The volume of iodine solution required to titrate the
water is measured and can be related to the
moisture content using a pre-prepared calibration
curve.
Major Difficulties and Source of Error
1.
Incomplete water extraction
 Fineness of grind is important
2. Atmospheric moisture
 External air must not be allowed to infiltrate the
reaction chamber
3. Moisture adhering to walls of unit
 All glassware and utensils must be carefully dried
4. Interferences from certain food constituents
 Oxidation of ascorbic acid to dehydroascorbic
acid
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Gas Production Methods
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Commercial instruments are also available that utilize
specific reactions between chemical reagents and
water that lead to the production of a gas
Example; when a food sample is mixed with
powdered calcium carbide, the amount of acetylene
gas produced is related to the moisture content.
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The amount of gas produced can be measured
by
1. The volume of the gas produced
2. The decrease in the mass of the sample after
the gas is released
3. The increase in pressure of a closed vessel
containing the reactants
Physical Methods
1. Electrical methods
 Dielectric Method
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Moisture is determined by measuring the
change in capacitance or resistance to an
electric current passed through the sample
Limited to food contains not more than 30-35%
moisture.
 Conductivity method
 The conductivity of an electric current increases
with the percentage of moisture sample
 Must keep the temperature constant
2. Hydrometry
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Measuring specific gravity or density
Best applied to the analysis of solutions consisting
of only one component in a medium of water
Commonly used in beverages, salt brines and sugar
solutions
Example: Pycnometer, hydrometer, Westphal
Balance.
3. Refractometry
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Determine the soluble solids in fruits and fruit
products
Rapid and accurate methods
Principle: when a beam of light is passed from one
medium to another and the density differs, then
the beam is bent or refracted.
4. Infrared Analysis
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Principle:
 Measure absorption of radiation by molecules in
foods
 Different functional groups absorb different
frequencies of infrared radiation
 For water, near-infrared (NIR) bands (14001450, 1920-1950 nm) are characteristic of the –
OH stretch of water molecule
Methods to determine Water in
Different Molecular Environments
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Vapour Pressure Methods
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Determination of water activity
Bound water is less volatile than free water
Thermogravimetric Methods
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Measure the mass of a sample as it is heated at a
controlled rate
Free water normally evaporates at a lower
temperature than bound water
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Calorimetric Methods
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Using Differential Scanning Calorimetry (DSC) and
Differential Thermal Analysis (DTA)
Measure changes in the heat absorbed or released by
a material as its temperature is varied at a controlled
rate.
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Spectroscopic Methods
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Nuclear magnetic resonance (NMR) – distinguish
molecules within materials based on their molecular
mobility
Molecular mobility for free water is higher than that
of bound water
Comparison of the Methods
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Oven drying methods: involve the removal of
moisture from the sample and then a weight
determination of the solids remaining.
Distillation methods: Involve a separation of the
moisture from the solids. The moisture content is
calculated directly by volume.
Chemical Methods: reflected as the amount of
titrant used.
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Dielectric and conductivity methods: electrical
properties of water
Hydrometric methods: based on the relationship
between specific gravity and moisture content
Refractive Index: how water in a sample affects
the refraction of light
NIR Methods: absorption at w/length
characteristic of the molecular vibration in water.
Nature of Sample
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Oven Drying: problem with volatile compounds
and chemical degradation
Distillation: minimize volatilization and
decomposition
Karl Fischer: Food with very low moisture or high
in fats and sugars
Pycnometer, hydrometer and refractometer
requires liquid samples with limited constituents
Intended Purposes
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Rapid analysis but less accurate: oven drying
methods, microwave drying, infrared drying and
moisture analyzer
Rapid but require correlation to less empirical
methods: electrical, hydrometric, refractive
index and infrared analysis methods
Summary
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Types of water present: free, adsorbed and hydration of
water.
Major difficulty in many methods is attempting to remove or
otherwise quantitate all water present.
Factors to be considered in selecting moisture analysis:
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expected moisture content,
nature of the other food constituents (i.e. highly volatile),
equipment availability,
speed necessary,
accuracy and precision required and
intended purpose (e.g. regulatory or in-plant control)