Table 2.
Effects of drying methods on the quality of dried materials.
| Drying Method | Color | Structural Properties | Polyphenols Content | Antioxidant Activity | Volatile Compounds | Essential Oil (EO) Content | Sensory |
|---|---|---|---|---|---|---|---|
| Convective drying (CD) | color changes, generally darkening of the product (blueberries, black mulberries) improved color in case of blackcurrant powder |
product hardening, high shrinkage, dense structure, low porosity, high bulk density; when combined with ultrasounds-higher capacity of dehydration in mushrooms, Brussel sprouts, cauliflowers | reduction of TPC in i.e., chokecherries, chokeberry, chokeberries, moringa leaves, and mango cubes | high reduction of antioxidant activity in many products (chokecherries, blueberries, chokeberries, mango cubes) | generally high loss of volatiles; higher content than for other methods on the studies on shitake mushrooms and chanterelle | higher yield of essential oil than during MD of herbs (rosemary and basil) | generate off flavors, decrease of fresh, floral, herbaceous attributes |
| Microwave drying (MD) | better preservation of color than CD | high porous materials in the studies in potato and carrots decreased porosity in the studies on apple and banana | retention of polyphenols in moringa leaves | retention of antioxidants in moringa leaves | high loss of volatiles, but lower than CD | better yield and preservation in basil and coriander | - |
| Vacuum drying (VD) | - | high porosity in the studies on apple and banana low porosity on the studies of potato and carrot | retention of polyphenols in moringa leaves | retention of antioxidants in moringa leaves | - | - | - |
| Vacuum-microwave drying (VMD) | Improved color in case of blackcurrant powder | low shrinkage in comparison to CD but higher than FD, porous structure, better than CD in the studies on chokeberries, faster reconstitution, lower bulk density than CD | higher than CD in the studies on sour cherries | higher than CD in the studies on sour cherries | higher loss of some compounds than CD | increased EO yield in garlic, higher loss of EO than CD of rosemary | decrease of fresh, floral, herbaceous attributes, increase of sweetness, bitterness and adhesiveness |
| combined convective drying followed by vacuum-microwave drying (CD-VMD) | Slight degradation of color (better than CD and VMD); Improved color in case of blackcurrant powder | lower bulk density than CD | higher than CD and VMD in the studies on sour cherries, chokeberries, | higher than CD and VMD in the studies on sour cherries, Saskatoon berries, chokeberries | higher retention in the studies on chanterelle than other drying methods | increased EO yield in thyme, oregano, and rosemary | - |
| Freeze-drying (FD) | good preservation of natural color in many studies (i.e., black mulberries) | no shrinkage, no collapse, highest porosity, loss of elasticity, viscous material, lower bulk density than CD | preservation of TPC (black mulberry, chokeberries) | preservation of antioxidants | major loss in drying of parsley, low loss of flavor and aroma | preservation of most EOs | - |
| Osmotic dehydration (OD) | good preservation of color, change of color due to the osmotic solution properties (when concentrated juice is used as osmotic solution) | when combined with FD—strengthen the material structure when used as a pre-treatment before CD or CD-VMD—increase porosity lower bulk density than CD | increase when dehydrated in chokeberry, sour cherry solution, degradation in the studies on sour cherries | degradation in the studies on sour cherries increase when carried out in concentrated pomegranate and chokeberry juices |
- | - | - |
| Heat pump drying (HP) | improved color in rosemary and parsley brown areas when applied on nuts | good preservation of the structure in some herbs than other drying methods | good preservation of polyphenols in drying of herbs | good preservation of polyphenols in drying of herbs | volatiles retention on the studies on ginger | - | - |
| Fluidized bed drying (FB) | good color retention | - | no significant reduction in kafir leaves | no significant reduction in kafir leaves | - | - | - |
| Refractance window drying (RW) | decreased browning reaction in pomegranate leather | positively affected | retention of polyphenols high content in pestil pomegranate | retention or improved antioxidant activity in the studies on asparagus, sweet corn and tomatoes high content in pestil pomegranate | - | - | - |
| Intermittent drying | reduce the color degradation | maintain the product microstructure obtaining a porous structure similar to the fresh sample | Retention of polyphenols | retention of ascorbic acid, carotenoids, and so increasing in the antioxidant activity | retains the lower volatile compounds (due to microwave energy penetration which accelerates the disruption of the cell membranes that ultimately releases the volatile compounds faster) | - | protect cells from oxidative injury, providing better sensory quality |