it covers different methods used for determination of moisture content and total solids analysis of herbal products

1. Mohini Upadhye
Department of Pharmacognosy
PES Modern college of Pharmcy (For
Ladies), Moshi, Pune
Moisture And Total Solid Analysis of
Herbal Products

2. Importance of Moisture Assay
1. Legal and Labeling Requirements
2. Economic
3. Microbial stability
4. Food Quality
5. Food Processing Operations

3. Forms of Water in Foods
 Free water
 Water retains its physical properties
 Capillary water
 Water is held tightly within spaces within a food
that are surrounded by a physical barrier
 E.g. An emulsion droplet
 Bound water
 Water is bound physically (e.g. in protein) or
chemically (e.g. Na2SO4. 10H2O)

4. Determination of Moisture: Methods
1. Drying Methods
 Oven drying methods
2. Distillation method
 Dean and Stark Method
3. Chemical Methods
 Karl Fisher
 Gas production
4. Physical Methods

5. Drying Methods
 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/crude drug
before and after the water is removed by
drying.
 Basic Principle
 Water has a lower boiling point than the other
major components within foods such as lipids,
protein and carbohydrate.

6.  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

7.  Moisture and total solids contents of food can be
calculated using one of the equations below:
 % Moisture (wt/wt) = wt H2O in sample X 100
wt of wet sample
 % 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

8. 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

9. 2. Forced Draft Oven
 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

10. 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

11. 4. Microwave Oven
 Weighed samples are
placed in a microwave
oven for a specified
time and power-level
and their dried mass is
weighed.

12.  In microwave oven,
water evaporation is
due to absorption of
microwave energy,
which causes them to
become thermally
excited.
 Advantage
 simple
 Disadvantage:
 Care must be taken to
standardize the drying
procedure and ensure
that the microwave
energy is applied
evenly across the
sample.

13. 5. Infrared lamp drying
 Principle of drying:
Similar to microwave
oven
 Advantages: rapid and
inexpensive
 This is because the IR
energy penetrates into
the sample

14.  To produce consistent results, one must control
 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.

15. 6. Moisture Analyzer
 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

16. Practical Considerations
 Sample Dimensions
 Surface area of material exposed to the
environment is important
 Clumping and surface crust formation
 Mix the sample with dried sand to prevent clumping
 Decomposition of other food components
 High temp or too long drying time can cause
decomposition of some heat-sensitive components
in the food

17.  Volatilization of other food components
 Food with high volatile components should be
analyzed using distillation or chemical method.
 High moisture samples
 Dried in two stages to prevent spattering
 Sample pans
 Should be dried in the oven and stored in a
dessicator prior analysis
 Avoid contact with bare hands. Use tongs or cotton
gloves

18.  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.

19. Distillation Methods
 Direct measurement of the amount of water
removed from a food/crude drug sample by
evaporation
 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

20.  Advantages:
 Suitable for low moisture foods and foods
containing volatile oils such as herbs and spices
 Cheap, easy to set up and operate
 Disadvantage:
 Destructive, time consuming, involve flammable
solvent, not applicable for some types of foods

21. 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.

22. Distillation Methods

23. Practical Consideration
 There are a number of practical factors that
can lead to erroneous results:
 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.

24. Chemical Methods
 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:
 Karl Fischer Titration
 Gas production Methods

25.  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.

26.  Chemical reaction do not involve application
of heat.
 Therefore, they are suitable for:
 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).

27.  Karl-Fischer Titration
 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

28.  Procedure
 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.

29. 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

30.  Gas Production Methods
 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.

31.  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

32. Physical Methods
1. Electrical methods
 Dielectric Method
 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.

33.  Conductivity method
 The conductivity of an electric current increases with the
percentage of moisture sample
 Must keep the temperature constant

34. 2. Hydrometry
 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.

35. 3. Refractometry
 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.

36. 4. Infrared Analysis
 Principle:
 Measure absorption of radiation by molecules in
foods
 Different functional groups absorb different
frequencies of infrared radiation
 For water, near-infrared (NIR) bands (1400-
1450, 1920-1950 nm) are characteristic of the –
OH stretch of water molecule

37. Methods to determine Water in
Different Molecular Environments
 Vapour Pressure Methods
 Determination of water activity
 Bound water is less volatile than free water
 Thermogravimetric Methods
 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

38.  Calorimetric Methods
 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.

39.  Spectroscopic Methods
 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

40. Comparison of the Methods
 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.

41.  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.

42. Nature of Sample
 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

43. Summary
 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:
 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)