Measurement of

Moisture Content

Specific Gravity

Humidity

• In developing countries more than 30 percent of fresh food lost after harvesting
• Post-harvest losses are primarily due to moisture-related microbial growth, which can make food unfit for human consumption, leading to illness or EVEN death
• Microbial growth rate depends on a variety of factors, such as pH, temperature, and water activity (a_w).
• Water activity is a measurement of the availability of water for biological reactions
• Moisture or water content is a measurement of the total water contained in a food product, usually expressed as a percentage by weight on a wet basis
• To avoid microbial growth, the moisture content and water activity must be kept below approximately 10% and 0.60–0.65, respectively depending on the type of food
• Fresh foods, which are high in moisture content often have a water activity close to 0.99 and are particularly prone to microbial growth

• Drying the food reduces the amount of moisture available to support microbial growth, thereby increasing the product shelf life, which is an ideal solution when appropriate storage is not available

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• Often, the inspection of these products is done by sight and feel, with no formal method of grading the produce.

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• Given the concerns of postharvest losses for farmers and food safety for consumers, it is essential for the moisture measurement technology to be low cost, easy to transport, employing a relatively accurate method, and broadly applicable for different food materials.

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• Most substances have an optimum moisture content for obtaining the best possible processing results and therefore attaining maximum quality.
• Furthermore, moisture content impacts on price and there are statutory rules for some products governing the maximum permissible moisture content (e.g. as defined by national food regulations).

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Measurement of Moisture

Direct methods measure moisture content without an intermediate variable, including measuring the moisture content by weighing or titration before and after drying

Indirect measurement methods determine the moisture content by measuring the variation of a physical property (e.g., temperature, refractive index, pressure) relative to a baseline or over time. This property, transformed into a signal (e.g., wavelength shift, electric current), is then correlated to moisture content

• Both direct and indirect moisture measurement methods can be used in a variety of applications including laboratories, processing lines, and in the field.
• Only indirect methods can be used for automation and continuous measurement

Gravimetric method

• Standard laboratory technique
• Sample is weighed, then dried (usually for a prescribed period of time under specific conditions of temperature and often under vacuum) and weighed again.
• Moisture content is calculated based on the initial and final weights of the sample, which assumes that all weight loss is due to the removal of water and ignores the loss of other volatiles
• The sample preparation and drying conditions (e.g. time, temperature, type of oven, humidity, and pressure) influence the efficiency of moisture removal

Karl Fischer titration method

• Used to measure moisture content in liquids and solids.
• Moisture content is determined with a calibration curve correlated to the volume of reagent used to titrate the water of a sample.
• The amount of water in the original sample solution is determined based on the amount of titrant solution added before re-coloration occurs

Colorimetric titration method

• Incorporation of cobalt chloride (CoCl2) into desiccant materials as an indicator for their degree of water pick-up.
• In its un-hydrated form, cobalt chloride (CoCl2) is light blue in color.
• As a dihydrate, cobalt chloride (CoCl2·2H2O) takes on a purple coloration which turns to pink when the compound is hydrated even further to give a hexahydrate (CoCl2·6H2O).

Indirect methods

Indirect methods include optical, dielectric, nuclear, and hygrometric approaches, which are considered to be quantitative methods.

Most indirect methods, with the exception of hygrometric methods,

use electromagnetic radiation to determine moisture content by exploiting the strong influence of water in a material on the electromagnetic field

If a known electromagnetic field is applied to a sample before and after drying, the moisture content can be

determined by comparing the changes in physical properties, such as refractive index (e.g., optical methods), caused by the alternating electromagnetic field in the presence of water

The manner in which the waves propagate and interact with an applied electromagnetic field is influenced by the electrical, magnetic, physicochemical (e.g., homogeneity, texture, size, shape) properties of a material, and the distance at which the sensor is placed

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Material characteristic calibration curves are needed to convert the radiation signal into moisture content.

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The choice of spectrum range depends on the absorption, reflectance, and transmittance of the targeted material, the expected moisture content in the sample, the type of water bonds, and on the contrast in optical properties between the targeted material and water

Optical methods

• Optical measurement methods are most useful for measuring samples with a homogenous surface, whereas rough surfaces or variations in color can result in errors

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• Another limitation is the penetration depth of the electromagnetic radiation, which is influenced by material absorption and reflection characteristics

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• Both the material and moisture content influence the radiation path length, making calibration of these methods, material-specific

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• Optical methods have been used in the food industry for color sorting and detection of surface defects primarily in large processing plants

Infrared spectroscopy

• Infrared (IR) spectroscopy is one of the most common methods for remote sensing of moisture content measurement in a variety of biological samples, including food.

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• The measurement principle is based on the capacity of molecules and atoms to be excited by light absorption at different wavelengths

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• IR electromagnetic radiation can be used to quantify moisture content based on the reflection and absorption of this radiation by water molecules especially prominent at around 3300 cm−1

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• One widely used variation of IR spectroscopy is Fourier Transform Infrared (FTIR) spectroscopy, which uses a mathematical procedure (Fourier transform – FT) to transform raw data into the actual spectrum.

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Infrared (IR) thermography

• The measurement principle is based on detecting the difference in IR emitted from a dry sample and a wet sample.

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• An IR camera detects thermal energy (primarily in the 3–20 μm range) emitted by the surface of an object and transforms this into a visible “temperature map” of the surface.

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• The amount of radiation emitted depends on the surface characteristics and temperature of the object.

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Hyperspectral imaging (HSI).

• Useful method for non-destructive measurement and visualisation of moisture content in food materials
• Combines both conventional imaging and spectroscopy
• Reflectance is the most common mode for hyperspectral imaging and is usually carried out in the Vis – NIR (400–1000 nm) or NIR (1000–1700 nm) range.
• Hyperspectral imaging has been used to detect defects, contaminants and quality attributes of fruits, vegetables and meat products
• key advantage of HSI is its ability to carry out non-destructive measurements of irregular shaped objects during the “on-line” drying process.

Dielectric methods

• Water molecules have a high dielectric constant, or relative permittivity, due to their permanent dipole.
• The dielectric constant is “the ability of a material to store electromagnetic energy”.
• Water molecules rotate and align their electric dipole moments in response to an applied electromagnetic field and this reorientation produces an electrical polarization noise that can be used to measure the dielectric properties (dielectric constant and dielectric loss) of a material and thus determine its moisture content
• Dielectric properties of a material depend on its composition, moisture content, ionic conductivity, temperature, scattering due to nature of heterogenous materials (food samples), density variations, and properties of the frequency used as well as the geometry of the particle and the orientation in relation to the applied electrical field

Electrical conductivity

• Moisture content is determined based on the variation in capacitance or resistivity of the dielectric properties of a material before and after drying

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• Small electrical current is passed using a contact method with the sample. The amount of resistance correlates to the amount of moisture in the material

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• Low-cost hand-held moisture meters that are based on measuring electrical conductivity (or resistance) for evaluating moisture content.

Microwave method.

• Similar to IR, waves, moisture content can be determined based on transmitted, absorbed, or reflected microwaves, or by using resonance techniques

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• The moisture content can be correlated with the attenuation the wave suffers in the presence of a wet material.

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Radio frequency (RF) method

• This method determines the moisture content by measuring the propagation delays of the electromagnetic waves in a material using a single frequency or various frequencies

Hygrometric methods

Hygrometric methods are based on equilibrium relative humidity

(ERH) instead of moisture content.

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ERH is numerically equal to water activity but expressed as a percentage of the actual amount of water in the air versus the amount of water the air could hold at saturation

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RH or water activity sensors are relatively easy to use in the context

of foods. The measurement approach is relatively simple but regular

calibration using desiccant salts (e.g., sulphuric acid, NaCl) is required

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Sensors based on capacitance and resistivity may be used, but their measurement may be affected by high temperatures and humidity.

• short measurement time
• no sample preparation
• non-destructive testing
• Sensitivity
• does not require a reagent
• Real time data acquisition
• Monitoring and control of processes

Advantages

Specific Gravity Determination

Specific gravity is the ratio of the density of a substance to the density of a reference substance;

The reference substance is nearly always water at its densest (4°C) for liquids; for gases it is air at room temperature (21°C)

• Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa). Temperatures for both sample and reference vary from industry to industry

Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials, or of quality control: to evaluate physical changes, or determine the degree of uniformity between samples or lots.

https://www.youtube.com/watch?v=5w3IKnovlng

https://www.youtube.com/watch?v=t9XAiRbL7t4

A hydrometer is an instrument used for measuring the relative density of liquids based on the concept of buoyancy. They are typically calibrated and graduated with one or more scales such as specific gravity.

Specialized hydrometers

Alcoholometer

Lactometer

Saccharometer

Salinometer

Acidometer

Digital density meters (Typical ASTM Test method D4052)

Vibrating Element Transducers:

• This type of instrument requires a vibrating element to be placed in contact with the fluid of interest.
• The resonant frequency of the element is measured and is related to the density of the fluid by a characterization that is dependent upon the design of the element.
• In modern laboratories precise measurements of specific gravity are made using oscillating U-tube meters.
• Widely used in the brewing, distilling, pharmaceutical, petroleum and other industries.
• Continuous measurement - ONLINE

Digital density meters based on the oscillating U-tube technique: The sample to be measured is filled into a U-shaped tube which is induced to vibrate. The eigen frequency of the oscillation of the U-tube is influenced by the mass and therefore by the density of the sample.

https://www.gpsil.co.uk/our-products/density-meters/measuring-principle/