bio-process engineering group, dept. agricultural & bioresource engineering, u of s 2008 csbe...

BIO-PROCESS ENGINEERING GROUP, Dept. Agricultural & Bioresource Engineering, U of S

2008 CSBE International Meeting

Microwave drying characteristics of two varieties of red lentils

Opoku, A., L.G. Tabil and V. Meda

Department of Agricultural and Bioresource EngineeringUniversity of Saskatchewan


Introduction

Lentils yearly export earnings $240 million (Canadian)

Lentils nutritious and healthy food, low in fat

Swath 1/3 of pod turns yellow or straight-cut fully mature

Threshed at 16 to 20% wb to reduce shattering losses

Needs drying to 12 -14% safe storage


Introduction

Conventional convective drying longer time and energy intensive

Microwave drying is energy efficient and faster heating in processing foods compared to convective drying

Microwave drying reduce retrogradation and has potential to increase the commercial use of lentil starch.

To better understand design, control and efficient operation of microwave drying systems for lentils, drying kinetics should be investigated.


Objectives

The objective of this study was to investigate the drying and color characteristics of red lentil varieties (Impact and Robin) using microwave drying system and to compare with convective drying.


Material

Robin lentils obtained from Pure T. Organics, Regina, SK

Impact lentils supplied by Reisner Seed Farm, Limerick, SK

Preconditioned moisture content Robin (9.85%) and Impact (6.50%)

Added water, rotated 5 h and stored at 5oC for a week

Conditioned moisture content Robin (20.82%) and Impact (21.86%)


Microwave drying

A combined microwave-convective dryer, Model NN-C980W (Panasonic Canada Ltd, Mississauga, ON)

Sample size of about 700 gMicrowave power levels

P10 (713 W), P7 (606 W) and P4 (330 W)

Sample was removed and weighed at regular intervals

Cooled for 10 min and stored


Convective drying

A convective oven dryer ((Model 28, Precision Scientific Group, Chicago, IL) ) was used

A sample size of about 700 g was placed on container

Two samples dried at the same time

Samples were removed and weighed at regular interval

The samples were dried at 70oC.


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


Data analysis - Drying models

Model name Drying model

Diffusion MR = aexp(-kt) + (1 - a)exp(-bkt)

Page MR = exp(-ktn)

Wang and Singh MR = 1 + at + bt2


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, and b

• Equilibrium moisture content (EMC) was assumed to be zero for the microwave drying data and 3.80% dry basis (db) selected from Menkov (2000)

• Drying rate determined as the amount of water removed per time (kg of water per kg of dry matter per hour)


Results - Microwave drying

Effect of microwave power levels on lentils drying

0

5

10

15

20

25

30

0 50 100 150 200 250 300

Time (min)

Mo

istu

re c

on

ten

t (%

db

)

70-IM

70-R

P4-IM

P7-IM

P10-IM

P4-R

P7-R

P10-R


Results – Microwave and convective drying rates

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 50 100 150 200 250 300

Time (min)

Dry

ing

rate

(db

/h)

70-IM

70-R

P4-IM

P7-IM

P10-IM

P4-R

P7-R

P10-R


Results

Model name Drying model

R2 range SE

Diffusion MR = aexp(-kt) + (1 - a)exp(-bkt)

0.9995 – 1.0000

0.0061 – 0.0013

Page MR = exp(-ktn)

0.9989 – 1.0000

0.0077 – 0.0012

Wang and Singh

MR = 1 + at + bt2

0.9947 – 0.9995

0.0168 – 0.0042


Results – L, a, and b

Treatment Color before drying Color after drying

L a b L a b

P10-R 37.40 6.24 10.19 38.28 6.23 11.32

P7-R 38.54 6.04 10.36 38.60 6.83 11.76

P4-R 38.99 6.19 10.51 37.37 6.44 10.94

P10-IM 38.11 7.40 9.69 37.69 6.65 9.96

P7-IM 37.64 7.00 8.98 37.38 7.27 9.94

P4-IM 40.54 6.84 9.43 38.28 6.38 9.52

70oC-R 44.94 7.83 16.92 44.70 8.29 17.68

70oC-IM 45.53 7.95 14.37 41.45 9.56 15.15


Results – Color change

Treatment Color change

ΔL Δa Δb ΔE

P10-R -0.88a 0.02bcd -1.13ab 1.44bc

P7-R -0.05ab -0.79ab -1.40a 1.62cd

P4-R 1.62c -0.25bc -0.43cde 1.72cd

P10-IM 0.43b 0.74d -0.27de 2.05de

P7-IM 0.26b -0.26bc -0.97ab 1.11ab

P4-IM 2.26c 0.46cd -0.08e 2.41e

70oC-R 0.24b -0.46bc -0.76bcd 0.92a

70oC-IM 4.08d -1.61a -0.78bc 4.45f


Conclusions

Times required to dry lentil samples were shorter compared to convective drying and decreased with increasing power levels.

Microwave drying resulted in higher drying rates compared to convective drying.

There was no difference in the drying rate between the two lentil varieties

Initial moisture content might have affected the drying time between the two varieties


Conclusions

Diffusion model provided a better fit for almost all the drying conditions, with higher R2 and lower SE compared to the other models.

Microwave and convective drying had more effect on the Impact variety than the Robin variety.

Convective drying of the Robin variety produced the lowest total color change.

The highest total color change was produced by convective drying of the Impact variety.


AcknowledgmentSaskatchewan Pulse

GrowersNSERC

Thank You

bio-process engineering group, dept. agricultural & bioresource engineering, u of s 2008 csbe...

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