How to use the moisture meter for liquid product testing?
At first glance, determining the amount of water in a liquid seems contradictory. After all, liquid is water, right? But consider the difference in taste between a good vegetable juice brand and the watered-down flavour of a cheaper brand. That's why processors of good liquid products like juices and syrups use moisture analysers to determine whether their products meet their own and industry standards.
Drying ovens to remove excess moisture is part of the processing steps in preparing products such as vegetable juices. However, spot checks using moisture analysers should be performed prior to packaging. These devices, also known as moisture balances, consist of halogen heaters mounted on precision analytical balances. They are programmed to determine moisture content by thermogravimetry and to display the loss of weight on drying (LOD). "Drying" seems to be the wrong term here, as the liquid is never completely "dry". What we are looking for is a target moisture content for suspended solids in a liquid.
Setting up a moisture meter
The method of sample preparation for measuring the moisture content of liquids such as carrot and/or orange juice is slightly different than that used for "dry" products (e.g. sugar, coffee or dried vegetables). The sample is not "opened" by the sample tray, but covered by a glass fibre filter. This ensures an even distribution of the liquid on the sample disc due to capillary effects, prevents droplet formation and supports rapid evaporation due to exposure to the larger surface of the halogen heat.
The target moisture content will vary depending on the product being tested and is determined by industry or government standards. These parameters are programmed into a moisture analyser such as the Torbal ATS 60 available from Tovatech.For liquids, such as vegetable juices, the moisture analyser should be set to fast drying rather than standard drying. During the initial phase of the rapid drying cycle, the heater output is 40 per cent higher than the set temperature. This compensates for the heat absorption of the vapour and accelerates the drying process.
Prior to testing, the sample tray and glass fibre filter were placed in the sample tray holder of the moisture analyser and tared. Remove the glass fibre filter and spread a 4 g sample of vegetable juice onto the sample tray using a pipette and cover with the filter. The process is initiated by closing the lid of the moisture balance. For vegetable juices, a typical time/temperature cycle at 140°C may be 14-16 minutes.
The Moisture Balance will switch off after (1) a fixed period of time or (2) when the weight loss is no longer stable as no more water is being removed. The moisture content is shown on the LCD display. For record keeping, the information can be logged to a printer or PC.
