MEASUREMENT AND INSTRUMENTATION
FIGURE 28 Chilled-mirror hygrometer for water activity measure- ments (courtesy Decagon Devices, Inc.). A chilled-mirror hygrometer specifically designed for measurement of water activity. The instrument includes special containers for the substances to be measured. Both dew point and temperature of the vapor immediately above the substance are measured and relative humidity (water activity) is calculated by a microprocessor.
FIGURE 29 %RH instrument for measuring water activity (courtesy Rotronics, Inc.). A relative humidity instrument to meausre %RH in the vapor immediately surrounding the specimen to be analyzed. Special containers are included in the system.
In essence, the aw of a food determines whether it will gain or lose moisture in a given environment. For example, if a cracker and a slice of meat were independently transferred into an atmosphere of 75% RH, we would find that over time the meat will lose water and the cracker will gain water. They will come not to equal water content, but rather to equal aw. The basis of protection by packaging is to slow this process down as much as possible within economic as well as food safety and quality limits. To do this, one must know both the sorption isotherms (moisture content versus aw) of the food product and the transport properties of the packaging film. A typical chilled mirror hygrometer for water activity measurements is shown in Fig. 28 and a %RH instrument in Fig. 29. 2. Paper Industry Machinery for printing paper or cardboard, coating paper with aluminum, and other types of unique applications are quite sensitive to the properties of the product to be transformed and to any variation of some physical phenomena. One parameter that has long been recognized as influencing the properties of paper and cardboard is moisture. Equilibrium relative humidity and the relative humidity of the storage and work areas are responsible for changes occurring in the moisture content of the product. Various studies have demonstrated the importance of %ERH control. FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +9199994621
Mechanical BE (Measurement and Instrumentation)