Abstract: The present study concentrated on the volume expansion mechanism of raspberry fruit chip during microwave heating. To investigate the puffing characteristics of raspberry fruit chips, four-filed coupling models based on the theories of electromagnetic field, heat transfer, structural mechanics and the Transport of Diluted Species physics were established to describe temperature distribution, moisture content and expansion deformation as a function of microwave intensity and initial moisture content, respectively, and they were evaluated by comparison with the experimental results. The results obtained indicated that high pressure inside fruit chips was generated due to water evaporation from the interior during microwave heating, leading to expansion of the chips. On the other hand, water evaporation from the surface resulted in shrinkage of fruit chips. The volume change of raspberry fruit chips was dependent on both expansion and shrinkage. The temperature distribution was mainly dependent on the penetration depth of microwave into the material. The moisture content was decreased gradually with the increase of microwave heating time. The increased elastic modulus of raspberry fruit chips was due to the increase of temperature and the decrease of moisture content, which influenced the volume expansion of fruit chips. Microwave intensity of 20–40 W/g and initial moisture content of 26% gave a better product with a higher expansion percentage of 3.91.

Key words: raspberry chips, initial moisture content, microwave intensity, microwave puffing, puffing mechanism