BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S 2008 CSBE...

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


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


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


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


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


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


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


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


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)


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


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


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


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


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


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


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


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.


Thank You

BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S 2008 CSBE...

Documents

Transcript of BIOPROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S 2008 CSBE...