Raspberries are one of Serbia’s best-known and most widely exported fruits. Due to market fluctuation, producers are looking for ways to preserve this fresh product. Drying is a widely accepted method for preserving berries, as is the case with freeze-drying. Hence, the aim was to evaluate convective drying as an alternative to freeze-drying due to better accessibility, simplicity, and cost-effectiveness of Polana raspberries and compare it to a freeze-drying. Three factors were in experimental design: air temperature (60, 70, and 80 °C), air velocity (0,5 and 1,5 m · s−1), and state of a product (fresh and frozen). Success of drying was evaluated with several quality criteria: shrinkage (change of volume), color change, shape, content of L-ascorbic acid, total phenolic content, flavonoid content, anthocyanin content, and antioxidant activity. A considerable influence of convective drying on color changes was not observed, as ΔE was low for all samples. It was obvious that fresh raspberries had less physical changes than frozen ones. On average, convective drying reduced L–ascorbic acid content by 80.00–99.99%, but less than 60% for other biologically active compounds as compared to fresh raspberries. Convective dried Polana raspberry may be considered as a viable replacement for freeze-dried raspberries.
Drying is one of the oldest methods for food preservation that removes the water from fruit and makes it available for consumption throughout the year. Dried fruits can be produced by small- and large-scale processors, which makes them a very popular food among consumers and food manufacturers. The most frequent uses of drying technology include osmotic dehydration, vacuum drying, freeze-drying and different combinations of other drying technologies. However, drying may provoke undesirable changes with respect to physiochemical, sensory, nutritional and microbiological quality. Drying process energy efficiency and the quality of dried fruits are crucial factors in fruit drying. Recently, innovative technologies such as ultrasound, pulsed electric field and high pressure may be used as a pretreatment or in combination with traditional drying technologies for process intensification. This could result in quality improvements of dried fruits and enhanced efficiency and capacity of the production process, with a positive impact on environmental and economic benefits.
Freeze-drying has proven to be one of the best drying processes thus far.
The purpose of this paper is to examine the effects of storage and treatment with sulfur dioxide of different concentration on the overall change in the color of dried apricots. Upon sulfurization of the prepared apricot samples, they were dried using the combined osmotic-convective drying technology, and subsequently kept either in a storage unit at t ≈ 25 o C or in a cooled environment at t ≈ 3 o C over a period of six months. The sample color was measured on the inside and the outside of the dried apricot fruits. Statistical analysis was performed using the Dependent Samples T-test so as to determine changes in the color of dried apricots after 3 and 6 months of storage. After six months of storage, the smallest color change of ∆E = 6.56 was measured in the apricot samples stored in a cooled environment. After six months, the greatest color change of ∆E = 49.38 was measured in the apricot samples stored at an air temperature of 25 o C.
The objective of this research was to investigate the influence of osmotic dehydration as a pre-treatment to the air drying of strawberries. Fresh, untreated strawberries were sliced and dried in a sucrose solution at a temperature of 50 o C and concentrations of 50 and 65 o Bx. After osmotic dehydration, the slices were dried in a thin layer at air temperatures of 60, 70 and 80 o C, and an air velocity of 1 m/s. After osmotic dehydration, the moister content and solid gain at a sucrose solution concentration of 55 o Bx were 3.44 g w /g dm and 0.062 g dm /g, respectively. However, the moister content and solid gain in at a sucrose solution concentration of 65 o Bx were 4.08 g w /g dm and 0.0944 g dm /g (65 o Bx), respectively. The effective moisture diffusivity of air drying varied from 1.57 x 10-9 to 4.43 x 10-10 m/s 2 , increasing with an increase in air temperatures and decreasing with an increase in pretreatment source concentrations. Lower air temperatures exert a positive influence on the rehydration time. A shorter air drying process positively affects the total changes in colour. The impact of pretreatment on colour changes in strawberries was not recorded after drying.
This study aimed to determine the effects of osmotic dehydration on the kinetics of hot air drying of apricot halves under conditions that were similar to the industrial ones. The osmotic process was performed in a sucrose solution at 40 and 60 °C and concentrations of 50% and 65%. As expected increased temperatures and concentrations of the solution resulted in increased water loss, solid gain and shrinkage. The kinetics of osmotic dehydration were well described by the Peleg model. The effective diffusivity of water 5.50–7.387 × 10−9 m2/s and solute 8.315 × 10−10–1.113 × 10−9 m2/s was calculated for osmotic dehydration. Hot air drying was carried out at 40, 50, and 60 °C with air flow velocities of 1.0 m/s and 1.5 m/s. The drying time shortened with higher temperature and air velocity. The calculated effective diffusion of water was from 3.002 × 10−10 m2/s to 1.970 × 10−9 m2/s. The activation energy was sensitive to selected air temperatures, so greater air velocity resulted in greater activation energy: 46.379–51.514 kJ/mol, and with the osmotic pretreatment, it decreased to 35.216–46.469 kJ/mol. Osmotic dehydration reduced the effective diffusivity of water during the hot air drying process. It also resulted in smaller shrinkage of apricot halves in the hot air drying process.
Serbia is one of the leading producers and exporters of raspberry in the world, and considering the short shelf life of raspberry, the processing, storage, and transport are some of the main issues to be addressed. A comparative experiment was conducted in order to find the suitable process parameters for convective drying that may be considered as the alternatives to freeze‐drying, which is a widely used preservation method for raspberry even though it is a costly and energy‐consuming method. Twelve convective drying regimens were applied with a combination of three influencing factors: air temperature (60°C, 70°C, and 80°C), air rate (0.5 and 1.5 m·s−1), and stage of raspberry (fresh and frozen). The final product, a dried raspberry, was assessed for chemical, physical, and mechanical properties and rehydration capacity. Deep ranking analysis by power eigenvectors (DRAPE) and sum of ranking differences (SRD) were used to uncover the differences and similarities between the applied drying methods. SRD showed that convective drying of fresh raspberries proved to be more similar to freeze‐dried raspberries than convective drying of frozen ones. Fresh samples dried at 60 °C air temperature and 1.5 m·s−1 air flow proved to be the most similar to the reference freeze‐drying method. This convective regimen gives samples with the lowest color change, shrinkage, and shape deformation. With the mechanical and chemical properties of these samples being observed, statistical Duncan's test show that there is no significant difference (P < .05) in terms of hardness, shear force resistance, total phenolic, and total flavonoid preservation, compared with freeze‐dried samples. DRAPE gave similar results, but it added the variable importance in ranking as well, and total phenol reduction was defined as the most important variable. These results can help practitioners to develop cheaper and simpler drying methods that would replace the freeze‐drying but keep the same quality of the dried products.
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