Moisture content of food products is essential to be scientifically determined mainly for the following reasons:
Food quality – the extent of moisture is one of the major contributory factors to foods’ desirability and acceptability. In terms of deterioration, the shelf life is highly dependent on the amount of moisture, hence the necessity of ensuring adequate percentage of moisture in various foods.
Microbial durability – presence of sufficient water in food is one of the main growth factors for harmful micro-organisms to survive. Many types of foods may be dried to be protected against microbial development and preserved for longer safely. The level of water is initially determined which can be subsequently used for dehydrating foods to specific point (less than the critical moisture content).
Food legislation – there are some certain regulations in relation to the permitted amount of water in specific types of foods which must be complied with by manufacturers.
Food processing – moisture evaluation of foods is usually required to obtain scientific information about the food characteristics and its reactions during various processing e.g. freezing, modified atmosphere packaging, heat treatment, filling, etc.
Chemical evaluation of water in food products involves numerous analytical methods among which distillation and oven drying procedures are basically discussed in this report:
Distillation (using Dean-Stark apparatus)
This technique involves a burette-like distillation tube (with volumetric graduation) attached to a condenser from the top. The burette is connected to a fractionating column via an inclined side-arm tube towards the distillation / reaction flask used for experimenting food sample.
The flask containing a known weight of solvent is initially prepared; usually xylene or toluene may be used which is required to have the following chemical properties:
- Immiscible in water
- high boiling point
- Lower density than water
Accurate weight of the experimental sample is then added into the flask and allowed to distil for a certain period of time. Subsequently the vaporized substance of solvent and moisture are discharged from the flask through the condenser and gently distilled into the graduated tube (drops of liquid collected in the burette). Two separate phases (immiscible liquids) are now created in the tube; the bottom phase is water and the top is the immiscible solvent which has less density than water. The solvent then travels back through the side-arm tube to the fractionating column and finally to the original flask.
When the collection is complete, the quantity of bottom phase is then directly observed and recorded – the bottom level of concave meniscus is the accurate figure. The recorded amount of collected water (v/w) is calculated to obtain the percentage of moisture i.e. 100 x value of the collected water, and then divided by the original weight of sample.
Distillation procedure is usually suitable for low moisture content foods and those containing significant proportion of volatile oils found in various parts of plants e.g. seeds, herbs, spices, etc. generally for essential oil evaluation. However it has been reported that the obtained results determined by distillation technique may be sometimes unreliable as the evaluated values of moisture content are possibly less than the actual amount present in the experimental food. In addition, extra safety cautions are required to be taken when handling the flammable solvents as they are extremely hazardous to human life e.g. wearing gloves and goggles and all flames are necessary to be extinguished when handling the solvents in the same atmosphere.
Oven drying methods
This is primarily based on evaporation process by which the reduced weight of the sample can be calculated to work out the percentage of water of the experimental sample. This type of analysis may be conducted using one of the following principles:
- Drying oven at 100°C — is mostly suitable for food samples with higher stability at this temperature
- Infrared drying lamps
- Vacuum ovens at 70°C – usually for food samples with less tolerance and likely to be decomposed at 100°C e.g. sugars
- Vacuum desiccators – suitable for foods liable to be decomposed at above ambient temperature e.g. baking powders
Evaporation-based technique is constantly used for water estimation of some food products basically due to its straight forward procedure with fairly reliable outcome (when conducted properly). In addition it is a generally inexpensive experiment.
This method however includes some disadvantages including its unsuitable application for some food ranges especially those enriched with a high proportion of volatile oils; water is not easily expelled during the evaporation. The accuracy of the obtained results is directly dependent on the extent of association of water with other existing components in the sample during evaporation process. Free water which has no chemical or physical linkage with macro-molecules is easily vaporized during the drying process. By contrast, evaluation of food samples containing bound water will require some special conditions and meticulous implementation in order to achieve accurate results and avoid the possibility of other components interference to the analysis in consequence of their degradation during the evaporation.