The moisture content of foodstuff items varies widely, and so it is necessary to select the most appropriate sample handling procedures and measurement method for each individual product
The food category encompasses a wide range of substances with varying properties.
It includes agricultural, livestock and marine products and processed products, as well as food additives.
The moisture content of these items also varies widely.
It is necessary, therefore, to select the most appropriate sample handling procedures and measurement method for each individual product.
Test methods for foodstuffs are mostly stipulated in the JAS standards.
In most cases moisture content is measured using heating and drying methods, but Karl Fischer titration is specified for some items.
In the case of chemically defined foodstuffs, such as sugars and additives, Karl Fischer titration can be applied using the same methods as for normal chemicals.
Food items with complex structures, such as cereals and meats, contain not only adhesion moisture but also moisture incorporated within cells.
Because many food items do not dissolve in organic solvents, the moisture vaporisation method is used extensively.
However, foodstuffs commonly become unstable when heated, and water may be formed as substances break down.
Care must be taken, therefore, when setting analytical conditions.
Food additives and flavourings: key points.
Food additives are substances used in the manufacture, processing and preservation of foodstuffs.
There are many types with varying attributes.
Most are solid, but those that dissolve in dehydrated solvents or electrolytes can be titrated directly using Karl Fischer titration.
First, it is necessary to determine whether or not interference substances are present.
Direct titration should be avoided in the case of insoluble substances; the moisture vaporisation method is recommended.
As the moisture content of these substances is generally high, volumetric titration is used.
Coulometric titration can also be used if the sample quantity is kept small.
When using moisture vaporisation, it is necessary to set the measurement conditions after determination of the heating temperature.
Official specifications for food additives commonly stipulate the loss-on-drying method, so conformance with that method should be checked.
Cereals and dried vegetables: key points.
Volumetric titration is suitable for cereals and starches which have high moisture content (approximately 10%).
If the sample is in powder form it can be dispersed in dehydrated solvent, allowing the moisture content to be extracted for direct Karl Fischer titration.
When using a dehydrated solvent with sugars, solubility is sometimes enhanced if the temperature is raised to 40-45C before titration.
The moisture vaporisation method is suitable for the many items that do not dissolve in solvents.
Care needs to be taken with the heating temperature for moisture vaporisation.
Temperatures in the 130-180C range are commonly used.
High temperatures may cause various phenomena, including discolouration, abnormal results or failure to reach an endpoint.
In such cases the measurement must be repeated at a lower temperature.
Moisture vaporisation should be used with coulometric titration.
Cereals cannot be titrated directly as they do not dissolve in electrolytes.
Cereals are sometimes sampled after preliminary processing (eg, pulverisation).
Care is needed to avoid moisture vaporisation or absorption during these processes.
Volumetric titration is the principal method used for dried vegetables.
The sample is added to the dehydrated solvent and stirred well for a period of 5-10 minutes before direct titration.
Moisture vaporisation is used when the moisture content cannot be fully extracted from samples in this way.
Karl Fischer titration can be carried out using either the volumetric or coulometric methods.
As moisture content tends to be high, volumetric titration is usually preferable.
The precautions stipulated for moisture vaporisation in connection with cereals also apply to dried vegetables.
Sugars and condiments: key points.
Sugars are dissolved in a sugar-type dehydrated solvent and titrated.
Volumetric titration is used.
If a substance is slow or difficult to dissolve, the solvent can be heated (up to 50C) and titrated.
(If the temperature exceeds 50C the formamide in the sugar-type dehydrated solvent will gradually break down.
The Karl Fischer reagent will be consumed in small amounts leading to uncertainty about the end point.) Condiments are normally dispersed in general-use dehydrated solvent.
This is followed by extraction and titration.
If the ingredients of a particular item give high sugar content, a sugar-type dehydrated solvent is used.
An oil-type dehydrated solvent is used for products with a high fat content.
Moisture vaporisation should generally be avoided for sugars as these tend to break down when heated.
Moisture vaporisation is unnecessary for condiments, since in almost all cases moisture content can be measured using direct titration.
When using the moisture vaporisation method the optimal heating temperature must be determined first.
Changes in the sample could lead to errors.
Confectionery: key points.
Because sugar is the main ingredient of confectionery, these items are dissolved in a sugar-type dehydrated solvent for titration.
Volumetric titration is used.
Coulometric titration is normally avoided as these products will not dissolve in the anolytes.
The moisture vaporisation method is also unsuitable as heating causes sugars to break down and produce water.
Caramels, candies and similar items are titrated after they have been dissolved in a sugar-type dehydrated solvent that has been heated (up to 50C).
(If the temperature exceeds 50C the formamide in the sugar-type dehydrated solvent will gradually break down; the Karl Fischer reagent will be consumed in small amounts and this will lead to uncertainty about the end point.) The moisture vaporisation method can be used for snack-type confectionery.
