Assessment of near infrared (NIR) diffuse reflectance analysis for measuring moisture and water activity in raisins

Near Infrared (NIR) Analysis was applied to assess its potential for measur ing water activity (a(w)) and moisture in California Seedless Raisins in mo isture equilibrium and nonequilibrium conditions. Samples covered a wine ra nge of anticipated moisture contents. An NIR calibration for a(w) was made for whole raisins that were fully equilibrated For ground raisins, NIR cali brations were made both a(w) and vacuum oven moisture, covering a large ran ge of moisture contents and equilibrium conditions. Direct measurement of a (w) by a dewpoint device gave highly repeatable results, especially for gro und raisins. Vacuum oven analysis of moisture was less precise than a(w) me asurements. Full spectrum NIR calibrations indicated that NIR could predict a(w) of raisins in ground form, irrespective of their state of hydration b eyond three hours following processing. A much larger sample population wil l be needed to establish precise calibrations for vacuum oven moisture.