No abstract

Saskatoon berries (Amelanchier alnifolia Nutt., cv. Thiessen and Smoky) were dehydrated using freezedrying (FD), vacuum microwave drying (VMD), air-drying (AD), and a combination of air-drying and vacuum microwave drying (CD) methods. All processed berries had significantly reduced (P < 0.05) total phenolics and anthocyanin contents in addition to reduced antioxidant activities, as compared with fresh frozen (FF) berries. FD berries rendered the highest amount (P < 0.05) of anthocyanin and antioxidant activities, followed by VMD berries. The CD dehydration resulted in intermediate results between VMD and AD. The substantial decrease in both total phenolics and anthocyanin content in berries was corresponded to significant (P < 0.05) losses in antioxidant capacities measure in different model systems.

The effects of calcium pretreatment, vacuum level, and apple variety on the texture of apple chips, processed using a combination of air drying and vacuum microwave dehydration (VMD), were determined. Pretreatment of apple slices by immersion in 1-5% aqueous CaCl 2 significantly increased crispness of chips as determined by instrumental and sensory analysis; however above 1%, chips were perceived as bitter. Higher vacuum applied during VMD significantly lowered density and increased crispness of chips. This effect was mediated by the vaporization of water in the interior of the chip, which caused expansion of the tissue. Chips made from Fuji apples had higher calcium contents, and were crisper than Red and Golden Delicious apple chips. Microstructure of the chips evaluated by scanning electron microscopy indicated that chips with thicker cell walls and large internal voids were crisper.

Vacuum to lower boiling temperature and microwaves for energy transfer can provide very rapid dehydration at low temperatures. Tomato sections were dehydrated in a batch convection air dryer (AD), a 16 kW vacuum microwave (VM) dryer, or by 1 of 3 combination processes. Drying rate of the 100% VM process was 18 times that of the 100% AD process. Only a slight falling rate effect was noted in VM drying AD and VM in sequence allowed the operator to choose any process time between 0.8 and 14.75 h. In this instance the least energy consumption occurred in the 100% VM process. Lowest energy cost was found for the 70% AD / 30% VM process. These results are expected to be strongly dependent upon the scale and design of dryers. Tomatoes finish-dried by VM exhibited a puffed structure associated with faster rehydration.

Basil (Ocimum basilicum L.) was dried using conventional hot air or the recently developed vacuum-microwave dryers. The effect of the drying method on the relative abundance of major flavor volatiles, rehydration rate, color, and structural integrity of the plant was evaluated. Dynamic headspace analysis of volatiles present in fresh or dried basil revealed that linalool and methylchavicol (estragole) were the two major headspace volatile compounds of the plant sample. Vacuum-microwave dehydrated basil yielded approximately 2.5 times the linalool and 1.5 times the methylchavicol of the air-dried samples. Furthermore, the vacuum-microwave-treated samples yielded more volatiles than fresh basil, due to chemical reactions during drying. Air-dried samples of basil had darker and fewer green hues than those prepared by vacuum microwave. Vacuum-microwave-dried samples had a higher rehydration rate, whereas the potential of the plant material to rehydrate was hindered in air-dried samples. This is likely attributed to the dramatic and pronounced structural collapse of the air-dried cells as revealed by the scanning electron microscope.