Development of a Microwave–Vacuum-Based Dehydration Technique for Fresh and Microwave–Osmotic (MWODS) Pretreated Whole Cranberries (Vaccinium macrocarpon)

Abstract

A bench scale microwave-vacuum (MWV) dryer was developed using a modified consumer-grade microwave oven. MWV dehydration was first tested as a standalone method on whole frozenthawed berries. Subsequently, a new combination drying technique was developed employing microwave osmotic dehydration under continuous-flow medium-spray (MWODS) conditions together with MWV as a secondary drying operation. Fresh (frozen-thawed) and MWODS pretreated berries were dried under a range of MWV treatments employing continuous and decreasing microwave power settings (duty cycles). Initial microwave power density for all treatments was approximately 10.2 W/g and magnetron power-on and power-off times varied from 3 to 15 s and 27 to 15 s, respectively. Drying times to 20% (db) were recorded and energy consumption was calculated according to the total magnetron power-on time where overall it was found that drying times and energy consumption decreased with increasing MWV process intensity, where drying times for all MWV treatments were significantly shorter than those of conventional air drying. Drying kinetics were fit using two models (exponential and Page's empirical model), where Page's model better fit the experimental data. The quality of the berries was monitored visually through evidence of scorching in order to screen treatments and establish upper limits of treatment intensity for further studies.

Microwave‐Osmotic/Microwave‐Vacuum Drying of Whole Cranberries: Comparison with Other Methods

“…As one of the most energy intensive processes in the food industry, energy consumption is another important consideration when comparing different drying methods is their energy consumption. Wray and Ramaswamy (a), demonstrated that the MWV process provides energy consumption benefits as compared to air drying and freeze drying. However, there has been no work on the energy needs of the MWODS or MWODS‐MWV treatments.…”

“…The bench‐top scale microwave vacuum dryer used for these experiments was the same as previously described in Wray and Ramaswamy (a) and consisted of a consumer grade microwave in which a custom made cylindrical glass vacuum chamber was suspended from an analytical balance, a setup which allowed the monitoring of the sample mass without interrupting the process. The vacuum chamber was connected to a cold water condenser/water trap and a vacuum pump on 1 side and a standard analog vacuum gauge on the other.…”

“…The chamber was able to maintain ∼93% vacuum (≈6 kPa abs). The initial power density for a 20 g sample was calorimetrically determined to be 10.2 W/g, and the duty cycle used were 20% and 10% (Wray and Ramaswamy a).…”

“…The vacuum pump was then engaged and allowed to bring the setup to the operating pressure, after which the microwave was then engaged and the sample mass monitored using the integrated balance. Wray and Ramaswamy (a) determined that the most advantageous MWV treatments employed the 20% power setting for the first 10 min of the process, after which the setting was reduced to the 10% until completion. In this study, the process was adapted slightly so that the 20% setting was employed until the product reached a moisture content of 50% (wb), which was determined visually to be the point beyond which product charring would begin to occur.…”

“…In the earlier studies, MWODS pretreated samples were hot air dried (Azarpazhooh and Ramaswamy a, b). Additionally, initial work focused on developing a microwave‐vacuum (MWV)‐based drying technique for both fresh and MWODS pretreated berries using a single moderate MWODS process to screen for suitable MWV treatments by varying power levels (Wray and Ramaswamy a). The objective of this work is to therefore expand the scope of previous studies by determined the effects of MWODS input variables on product quality as well as to compare the finished product to cranberries dried via traditional techniques such as convective, freeze, or vacuum drying.…”