Moisture mobility mechanism of beef jerky during combined mid-infrared and hot air drying

Abstract: One of the most important processes in food drying is moisture mobility, and different drying methods have different moisture mobility mechanisms. Combined mid-infrared and hot air (CMIHA) drying is a new drying method used in beef jerky that has more thermal efficiency than the hot air (HA) drying often used in Chinese factories. To investigate the reason why CMIHA drying could reduce more time consumption than HA drying, the moisture mobility mechanism of CMIHA drying was studied in this paper. Based on early research, mid-infrared wavelength 2.8-3.1 ?m, drying temperature 70℃, radiation distance 8 cm, radiation intensity 0.48 W/cm2 and wind velocity 1m/s were selected to study the moisture mobility mechanism of CMIHA drying by comparing to those of HA drying (temperature 70℃, radiation distance 8 cm, wind velocity 1 m/s). The Shrinkage rate, moisture diffusivities, and heat transfer characteristic were determined to research the moisture mobility of CMIHA drying from a macro-perspective. Based on the low field nuclear magnetic resonance (LF-NMR) that included magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI), the transverse relaxation time of bound water (T21)，immobile water (T22) and free water (T23), Peak integral area of different moisture ( A21, A22, A23), signal amplitude of immobile water (M22) and free water (M23), and Proton density image of beef jerky were determined to research the moisture mobility of CMIHA drying from the microcosmic aspect. The results showed that, compared to HA drying, CMIHA drying could significantly improve (p<0.05) the inside and outside temperature, reduce (p<0.05) Shrinkage rate, and improve (p<0.05) moisture diffusivities of beef jerky during the drying process, so as to accelerate the moisture mobility. Besides, compared to HA drying, CMIHA drying could significantly improve the transverse relaxation time of immobile water (T22) and free water (T23) and accelerate the process in which immobile water shifts to free water. The peak integral area and signal amplitude of immobile water (A22, M22) were significantly decreased (p<0.05) and the values of A23, M23 were significantly increased (p<0.05); the inside immobile water could change into free water and migrated to the surface by free diffusion. However, all drying methods had no significant (p>0.05) difference in bound water. In conclusion, based on the macro-indexes and microcosmic indexes, CMIHA drying can accelerate the process that immobile water changes into free water. This may be caused by high temperature (55℃) that can make the head of myoglobulin and the tail of myosinogen degenerate and the vibration of moisture molecules by absorbing infrared energy, both of which can decrease the binding force of immobile water. The higher moisture content of inside temperature rather than outside temperature makes free water migrate to the surface through free diffusion. At the same time, HA can accelerate the loss of outside water through the evaporation process and decrease the outside temperature. So, a higher inside moisture content and temperature make moisture have the same direction of heat and mass transfer, which of course can improve drying efficiency and decrease the Shrinkage rate of beef jerky during drying to improve its quality. This study could help provide evidence for the use of CMIHA drying on beef jerky.