1. MOISTURE AND TOTAL SOLID ANALYSIS
SFA3023
Food Analysis

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

3. Forms of Water in Foods Free water Capillary water Bound 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
Drying Methods
Oven drying methods
Distillation method
Dean and Stark Method
Chemical Methods
Karl Fisher
Gas production
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 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
% Total solids (wt/wt) = wt of dry sample X 100

8. Types of Oven 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. 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. Temperature and power level variations
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 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: Disadvantage:
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
Incomplete water extraction
Fineness of grind is important
Atmospheric moisture
External air must not be allowed to infiltrate the reaction chamber
Moisture adhering to walls of unit
All glassware and utensils must be carefully dried
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
The volume of the gas produced
The decrease in the mass of the sample after the gas is released
The increase in pressure of a closed vessel containing the reactants

32. Physical Methods 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 ( , 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. Intended Purposes
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

44. 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)