1. ERT 456: POST HARVEST ENGINEERING
MOISTURE CONTENT & EQUILIBRIUM MOISTURE CONTENT (EMC)
MDM AIMI ATHIRAH AZNAN
Biosystems Engineering Program
School of Bioprocess Engineering, UniMAP

2. The amount of water in food & agric. products affects:
quality
storability - perishability
However, perishability is not directly related to MC.
It varies among products even with the same MC !!!!. Eq: bean vs fruit

3. A much better indicator of perishability is:
the availability of water in the product to support degradation activities such as microbial action.
The term water activity (aw) is widely used as an indicator of water available in a product.
Water activity (aw) is defined as (Labuza, 1984)

4. Quantity of water/unit weight
MOISTURE CONTENT (MC)
The quantity of water contained in a material, such as grain, leaves or fruit.
Grain MC (% wet basis: the total weight of the grain including the water):
Ratio of weight of water that can be removed without changing the grain chemical structure to the initial weight of the grain
Quantity of water/unit weight

5. IMPORTANCE OF MEASURING MOISTURE CONTENT
Accurate moisture content testing is important in managing and marketing the grain.
Inaccurate moisture content measurements lead to:
Extra drying cost and harvesting loss if paddy is harvested wetter than necessary
Spoilage if the grain is too wet in storage
Extra drying cost and loss of quality if paddy is dried too far
Lower head rice when milled at wrong MC
Weight loss (loss in profit) if grain is sold too dry

6. MOISTURE CONTENT OF A MATERIAL
MCwb = Mass of water x ….(1)
Mass of Wet Product (Undried)
MCdb = Mass of water x …..(2)
Mass of Dry Product
Mass of Dry Product= Mass of Wet Product (Undried) - Mass of Water
Mass of water/Mass of wet product (Undried)
Mass of water/Mass of dry product

7. MOISTURE CONTENT CONVERSION
MCwb = MCdb____ ….(3)
100 + MCdb
MCdb = MCwb___ ….(4)
100 - MCwb

8. EXAMPLE PROBLEMS

9. MOISTURE CONTENT MEASUREMENT
MC Determination Methods
Direct Measurement (Primary – by removing of moisture)
Chemical Reaction: Karl Fischer Method
Heating (Oven)
Distillation
Microwave Radiation
Infra-red Radiation
Indirect Measurement (Secondary – based on electrical property of the grain)
Resistance
Capacitance
Relative Humidity

10. Chemical Reaction Method (Direct Method)
The most accurate method to determine MC of grains.
Karl Fischer tritation method
Iodine + water (in the presence of sulfur dioxide & pyridine) → hydriodic acid & sulfuric acid (Hart and Neustadt, 1957)
Considered as standard for measuring grain MC
Seldom used for grain because of time consuming and complicated features
Advantages
Disadvantages
Chemical reaction based on the presence of water should theoretically produce high accuracy
All water may not take part in the reaction

11. Oven Method (Direct Method)
Sample is heated under specified conditions
Loss of weight is used to calculate the moisture content of the sample.
The amount of moisture determined is highly dependent on the type of oven used, conditions within the oven, and the time and temperature of drying.
Advantages
Disadvantages
Simple and direct
Heat may cause decomposition of dry matter creating water not originally present
Other volatile materials may be driven off

13. Distillation Method (Direct Method)
Require toluene or oil
The chemical boil under atm pressure at temperature >100°C
Direct measurement of the amount of water removed from a food sample by evaporation
When ground grain is boiled, moisture escapes in form of vapour.
Condense water, measure volume of water to determine MC
Advantages
Disadvantages
Suitable for low moisture foods and foods containing volatile oils such as herbs and spices
Destructive, time consuming, involve flammable solvent, not applicable for some types of foods
Cheap, easy to set up and operate

14. Infrared & Microwave Method (Direct Method)
Infrared drying involves penetration of heat into the sample being dried, as compared to heat conductivity and convection with conventional ovens.
IR Advantages: rapid and inexpensive
Microwave moisture analysis (microwave drying) was the first precise and rapid technique that allowed some segments of the food industry to make in-process adjustment of the moisture content in food products before final packaging.
For example, processed cheese could be analysed and the composition adjusted before the blend was dumped from the cooker.
The ability to adjust the composition of a product in-process helps food manufacturers reduce production costs, meet regulatory requirements, and ensure product consistency.
Weighed samples are placed in a microwave oven for a specified time and power-level and their dried mass is weighed.

15. Capacitance & Resistance Type (Indirect Method) Does not measure MC directly
The electrical properties of water are used in the dielectric method to determine the moisture content of certain foods.
Measuring the change in capacitance or resistance to an electric current passed through a sample.
These instruments require calibration against samples of known moisture content as determined by standard methods.
Measure capacitance/resistance → relate to MC
Capacitance type is the most accurate amongst the 3 indirect methods.
These techniques can be very useful for process control measurement applications, where continuous measurement is required.
These methods are limited to food systems that contain no more than 30–35% moisture.

16. Capacitance & Resistance Type (Indirect Method)

17. Relative Humidity Type (Indirect Method)
Measure RH in the interstices of a grain sample
RH of air surrounding the grain sample directly depends on grain MC and temperature
Accuracy affected by the uniformity of moisture distribution in the kernels.
Requires 1-2hrs after collection of grain sample to obtain reliable reading.
RH type is the least expensive, least accurate & lease used.
Unreliable at grain MC above 20-22% w.b when equilibrium RH >85%.
Interstices: an intervening space, especially a very small one.

18. EQUILIBRIUM MOISTURE CONTENT (EMC)

19. EQUILIBRIUM MOISTURE CONTENT

20. Moisture is exchanged between a material and its surroundings until the material reaches some EMC at which there is no net gain or loss of moisture.
The EMC is known to be a function of:
Temperature RH
Physical properties of the material
Previous moisture history of the material

21. no net gain or loss of moisture

22. The Importance of EMC Determination
To investigate the stability of food product in storage.
Solving drying problem because knowing the temperature and RH specified for the process, the final MC and water activity of the food can be evaluated.

23. Equilibrium Moisture Content Isotherms
Equilibrium Moisture Content Isotherm = A plot of EMC vs. RH at a constant temperature

24. Sorption isotherm: Indicates that the equilibrium was reached in a wetting environment. (Water gain)
Desorption isotherm: Indicates that the equilibrium was reached environment. (Water loss)
EMC isotherms for most materials are sigmoidal in shape.
The desorption isotherm being above the sorption isotherms.

25. Hysteresis
Phenomenon of exhibition of different equilibrium moisture characteristics depending upon whether the eq. is reached by adsorption/sorption or desorption of the moisture.

26. EMC MODELS
Equations suggested to describe the equilibrium moisture content curves of various products

27. EMC MODELS
No single equation is suitable for all products; however, most products can be represented by one of several equations available.
EMC Model
Description
Halsey
Less complicated equation
No temperature parameter, thus diff constant must be used for each products and temperature of interest
Henderson
Modified Henderson
Slightly more complicated models
Used for grains and related products
Modified Halsey

30. Computed using Modified Henderson and the constants from Table 10
Computed using Modified Henderson and the constants from Table for shelled corn.
Note that curves begin at 10% relative humidity and are terminated before reaching 100% relative humidity.
Prediction curves are generally written to describe conditions in the middle ranges of relative humidity. They do not predict well for extreme conditions.
In addition, reliable experimental data are difficult to obtain in those regions.

31. At 30°C, RH 70%, EMC is reached at …………………%
When it is ventilated by air at 50% RH, the corn …………………. moisture and reach EMC at …………%
When it is ventilated by air at 80% RH, the corn …………………. moisture and reach EMC at …………%

32. Methods to Determine EMC
Gravimetric
Manometric
Hygrometric

33. Gravimetric Method Record weight changes
Determined continuously & discontinuously in static and dynamic systems
Continuous: Use electro or quartz spring balance
Discontinuous: Sulphuric acid or salt solution is placed in a vacuum with the food/products to give a measure of the ERH using desiccator.

34. EMC using Desiccator
Desiccator: A glass container or other apparatus holding a drying agent for removing moisture from specimens and protecting them from water vapor in the air.
The lower compartment of the desiccator contains lumps of:
silica gel,
freshly calcined quicklime,
Drierite or
anhydrous calcium chloride to absorb water vapor.

35. Manometric Method Use sensitive manometers
Oil is used as the manometer fluid is stead of mercury to improve accuracy (relative displacement of oil is greater than mercury).
Measure vapor pressure of water in equilibrium with a food material at a given MC.

36. Hygrometric Method
Measure ERH of air in contact with food material at given MC.
Dew-point hygrometer: detect the condensation of cooling water vapor.
Electric hygrometer: measure changes in conductance/capacitance of hygro-sensor
Hair hygrometer: measurement based on the stretching of human hair.

37. THE END