At first it might seem that determining moisture content in liquids is a contradiction in terms. After all, liquids are moisture, right? But think of the difference in taste between a quality brand of vegetable juice and the watered-down taste of a bargain brand. That’s why processors of quality liquid products such as juices and syrups use a moisture analyzer to determine if their product meets their own and industry standards.
Drying ovens to remove excess moisture are part of the processing steps in preparing products such as vegetable juices. But spot-checks using a moisture analyzer should be made prior to packaging. Also called moisture balances, these devices consist of a halogen heater mounted on a precision analytical balance. They are programmed to determine moisture content by the thermogravimetric process and display loss of weight on drying (LOD). “Drying” might seem a misnomer here because the liquid is never fully “dried.” What we are seeking is the target moisture content against the suspended solids in the liquid.
Setting up the Moisture Analyzer
Slightly different sample preparation is employed for measuring the moisture content of liquids such as carrot juice and or orange juice than apply to “dry” products such as sugar, coffee or dried vegetables. Instead of being “open” in the sample pan the sample is covered with a fiberglass filter. This assures even distribution of the liquid on the sample pan due to capillary effect, it prevents the formation of drops, and it supports quick evaporation due to the larger surface exposed to the halogen heat.
Target moisture content will differ depending on the product being tested and is determined by industry or government standards. These parameters are programmed into a moisture analyzer such as the Torbal ATS 60 available from Tovatech. In the case of liquids such as vegetable juices the moisture analyzer should be set for fast drying rather than standard drying. During the initial stages of the rapid drying cycle the heater output is 40% higher than the set temperature. This compensates for the endothermic heat of vaporization and accelerates the drying process.
Before running a test the sample pan and the fiberglass filter are put in the moisture analyzer sample pan holder and tared. Remove the fiberglass filter and use a pipette to spread a 4-gram sample of vegetable juice on the sample pan then cover it with the filter. Closing the cover of the moisture balance initiates the process. For vegetable juice a typical time/temperature cycle could be 14-16 minutes at 140˚C.
The moisture analyzer will turn off after (1) the fixed time period or (2) when weight loss levels off because no more moisture is being driven off. The moisture content is shown on the LCD. For record keeping the information can be recorded to a printer or PC.
What are the moisture contents of liquids processed in your facility? How often do you check moisture content as part of your quality control procedures? Contact the scientists at Tovatech for help on selecting and operating a precision moisture balance.