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BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
2008 CSBE International Meeting
Microwave-vacuum drying and quality characteristics of
sour cherries
Opoku, A., V. Meda and L.G. Tabil
Department of Agricultural and Bioresource EngineeringUniversity of Saskatchewan
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Introduction
Sour cherry provides a good source of fiber, vitamins A and C, high amounts of anthocyanins and low in fat
Healthy food choice for prevention of heart disease, diabetes, and obesity
Seasonal crop Saskatchewan frozen storage or dry to maintain quality, nutritional value and longer shelf-life
Several hours needed to dry using convective hot-air dryer
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Introduction
Microwave-vacuum drying (MVD)
– Rapid and efficient energy usage
– Yields dried products with excellent quality
– Drawback is non-uniform heating
Drying behavior is necessary for equipment design and optimal drying
Microwave-vacuum drying kinetics and quality characteristics of sour cherry need to be studied
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Objectives
The objective of this study was to investigate The objective of this study was to investigate the drying and quality characteristics of cherries using a microwave-vacuum dryer
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Material
Frozen, pitted sour cherries obtained from Riverbend Plantation Gourmet Foods (Saskatoon, SK, Canada)
Thawed at 5oC for more than 24 h
Juice drained and thawed cherry stored at 5oC
The initial moisture content of the cherries was about 83.44% wb
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Microwave and Microwave-vacuum drying
A combined microwave-vacuum dryer, Model VMD 1.8 (ENWAVE Corp., Vancouver, BC)
Sample size of about 150 g Microwave power levels P10
(404 W), P8 (344 W) and P6 (300 W)
Two vacuum pressure levels V20 (67.0 kPa) and V10 (33.5 kPa)
Sample was removed and weighed at regular intervals
Dried to 40 to 45%, cooled and stored
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Color measurement
•Hunterlab Color Analyzer (Hunter Associates Laboratory Inc., Reston, VA, U.S.A.)
•Measured L, a, and b values before and after drying
•Determined change in color, ΔL, Δa, and Δb
•Total color difference ΔE
222 baLΔE
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Methods – Water activity
Measured using Pawkit water activity meter (Decagon Devices, Inc., Pullman, WA, U.S.A.)
Accurate to +0.02 aw
Two replicates were made for each sample
3 readings for each replicate
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Data analysis - Drying models
Model name Drying model
Page MR = exp(-ktn)
Wang and Singh MR = 1 + at + bt2
Modified Page MR = c + exp(-ktn)
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Data analysis
• TableCurve 2D (Jandel Scientific, San Rafael, CA) was used to determine the parameters of the models
• Coefficient of determination (R2) and the standard error (SE) were determined for the models
• Regression models were fitted to describe drying rate constant (k in min-1) and empirical constants: n, a, b, and c
• Equilibrium moisture content (EMC) was assumed to be zero for the microwave-vacuum drying data
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Results – Microwave-vacuum drying
Effect of microwave power and vacuum levels on sour cherry drying
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70 80 90
Time (min)
Mo
istu
re c
on
ten
t (%
wb
)
P4-V20
P7-V20
P10-V20
P4-V10
P7-V10
P10-V10
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Results
Model name Drying model
R2 range SE
Page MR = exp(-ktn)
0.9954 – 0.9996
0.0174 – 0.0054
Wang and Singh
MR = 1 + at + bt2
0.9946 – 0.9995
0.0225 – 0.0062
Modified Page
MR = c + exp(-ktn)
0.9954 – 0.9997
0.0174 – 0.0016
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Results – L, a, and b color values
Treatment Color before drying Color after drying
L a b L a b
P4-V10 19.09 17.71 10.23 18.19 13.64 6.70
P7-V10 21.19 20.59 10.03 19.59 11.32 5.76
P10-V10 22.23 20.60 14.47 20.84 11.87 5.82
P4-V20 20.81 14.65 8.19 19.88 7.29 3.53
P7-V20 20.37 23.20 14.11 18.66 9.69 4.07
P10-V20 20.37 23.20 14.11 18.74 9.14 4.61
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Results – Color change
Treatment Color change
ΔL Δa Δb ΔE
P4-V10 0.89a 4.07a 3.53a 5.46a
P7-V10 1.60c 9.27d 4.26b 10.32c
P10-V10 1.39b 8.74c 8.64d 12.37d
P4-V20 0.94a 7.36b 4.66c 8.76b
P7-V20 1.71c 13.52e 10.04f 16.92e
P10-V20 1.63c 14.07f 9.51e 17.05e
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Results – Water activity
Treatment Water activity
(aw)
Temperature
(oC)
Moisture
content (% wb)
P4-V10 0.81 28.4 45.50
P7-V10 0.80 29.0 39.31
P10-V10 0.80 29.0 44.41
P4-V20 0.82 28.9 45.45
P7-V20 0.79 28.1 41.46
P10-V20 0.78 28.6 39.46
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Conclusions
The vacuum pressure levels did not greatly influence the drying time of the sour cherries compared to the microwave power levels
Increasing the microwave power level produced faster drying times for the samples.
The polynomial and exponential models were fitted to the drying data
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Conclusions
Vacuum pressure and the microwave power levels significantly influenced the final quality of the dried cherries.
Total color difference was dependent on microwave power and vacuum pressure levels.
The lowest total color change was produced at power level P4 and vacuum pressure of 33.5 kPa. The total color difference was highest at power level P10 and vacuum pressure of 67.0 kPa
Water activity of the samples after drying ranged from 0.78 to 0.82 for moisture content range of 39.31 to 45.50% wb.
BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S
Thank You
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