MOISTURE AND TOTAL SOLID

ANALYSIS

SFA3023Food Analysis

Importance of Moisture Assay

1. Legal and Labeling Requirements2. Economic3. Microbial stability4. Food Quality5. Food Processing Operations

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)

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

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.

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

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

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 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 Weighed samples

are placed in a microwave oven for a specified time and power-level and their dried mass is weighed.

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.

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

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.

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

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

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

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 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

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

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

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.

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

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.

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).

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

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.

Major Difficulties and Source of Error1. Incomplete water extraction

Fineness of grind is important2. 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

dried4. Interferences from certain food constituents

Oxidation of ascorbic acid to dehydroascorbic acid

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.

The amount of gas produced can be measured by

1. The volume of the gas produced2. The decrease in the mass of the sample

after the gas is released3. The increase in pressure of a closed

vessel containing the reactants

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.

Conductivity method The conductivity of an electric current

increases with the percentage of moisture sample

Must keep the temperature constant

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.

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.

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

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

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.

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

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.

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 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

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 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)