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COMBINED OSMOTIC AND MICROWAVE DRYING OFSTRAWBERRIESK. VENKATACHALAPATHY a & G.S.V. RAGHAVAN aa Department of Agricultural and Biosystems Engineering , Macdonald Campus of McGillUniversity , Ste-Anne de Bellevue, Quebec, Canada, H9X 3V9Published online: 27 Apr 2007.

To cite this article: K. VENKATACHALAPATHY & G.S.V. RAGHAVAN (1999) COMBINED OSMOTIC AND MICROWAVE DRYING OFSTRAWBERRIES, Drying Technology: An International Journal, 17:4-5, 837-853, DOI: 10.1080/07373939908917573

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DRYING TECHNOLOGY, 17(4&5). 837-853 (1999)

COMBINED OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES

K. Venkatachalapathy and G.S.V. Raghavan

Department of Agricultural and Biosystems Engineering Macdonald Campus of McGill University

Ste-Anne de Bellevue. Quebec, Canada, H9X 3V9

Key Words: dehydration, microwave, osmotic, pretreatment, quality, strawberry.

ABSTRACT

Strawberries pretreated with 2% ethyl oleate and 0.5% NaOH were osmotically dehydrated and their osmotic dehydration rate is compared with untreated berries. It was found that treated berries dehydrated better compared to untreated berries. Osmotically dehydrated berries were convective and microwave dried at different power levels and results were compared with respect to drying time and rate. The rehydration ratio, texture, color and sensory values are compared with freeze dried strawberries with the same pretreatment. It was found that microwave drying was short in time and the quality parameters of the microwave dried berries were comparable to those of freeze dried berries.

Copyright 0 1999 by Marcel Drkker. Inc.

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VENKATACHALAPATHYANDRAGHAVAN

INTRODUCTION

The strawberry (Fragaria ananassa) is a highly perishable fruit whose production

has steadily increased due to its popularity and relatively high return on investment.

Because the harvest season is very short and production vastly outweighs thedemand

forthe fresh fruit, there is a large market for the processed forms, includingpreserves,

frozen and dried strawberries. Dried strawberries may be stored for a long time and

are used in the preparation ofmany products, such asjamsandjellies, bakery products

and cereals.

Strawberries are generally freeze-dried. However. this is an energy-intensive

operation and hence expensive. Conventional hot air drying is not recommended due

to low thermal efficiency, lengthy drying timeand volatilization losses of flavour and

aroma upon exposure to heat (Van Arsdel er a/ . 1973; Yongsawatdigul and Gunasekaran, 1996). Combination methods involving osmosis and convection

(Rahman and Lamb, 1991; Sankat et a/ . 1996; Lenart, 1996), or microwaves and

vacuum (Yongsawatdigul and Gunasekaran, 1996) have been investigated inan effort to improve quality or reduce drying time and temperature requirements. Although

volumetric heating by microwaves has the greatest potential for reducing drying time

and energy requirementsofhighmoisture products (Tulasidas et a/ . 1995) prelimimy

experiments have shown that microwave dlying alone cannot be used for whole

strawberries (Venkatachalapathy and Raghavan, 1997) since the skin resistance to

moisture diffusion is too high, such that the vapou pressure increase within the beny

causes bursting. Although a dipping treatment in 2% ethyl oleate and 0.5% sodium

hydroxide prevents bursting and allows the beny to dy completely in the microwave environment without bursting, the long exposure to heat results in significant loss of

flavour and aroma.

Since partial dehydration in an osmotic solution has been reported to improve

flavour retention in convection-dried fruits (Jackson and Mohammed, 1971; Ponting,

1973; Dixon et al. 1976; Flink, 1979; Voilley and Simatos, 1979), it was decided to

investigate the possibility this technique could beusedas apretreatment for microwave

drying to improve the quality of the finished product. The objectives of this study were to find the osmotic dehydration rate of strawberries pretreated with ethyl oleate

and sodium hydroxide, to compare the drying rates of osmotically dehydrated and

microwave dried berries with that ofpretreatedosmotically dehydrated and convection dried and also freeze-dried berries as well as to compare andassess the product quality

by rehydration, texture, colour and sensory evaluation.

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OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES

MATERIALS AND METHODS

Osmotic Dehydration

Strawberries of unknown origin and cultivar were procured from the market and stored in the cold room at I "C for a maximum period of4 to 5 days. The berries were

taken out of the cold room 2 h before the experiment to allow them to reach mom temperature. Damaged berries were removed and the rest were treated with 2% ethyl

oleate and 0.5% sodium hydroxide at 40•‹C for 1 minute (Rahman and Perera, 1996). The treated berries were washed in running water to clear the solution from the h i t surface. They were then cleaned with water absorbent paper and gentle air was blown to remove surface moisture. Their initial moisture content was then determined.

Four 100 g samples of treated berries were taken and mixed with pmular sucmse in ratios of 3: 1 or 4: 1 fruit to sucrose (F:S) (WeighWeight) at room temperature and mixed periodically. The samples were taken out at 12 h, 24 h, 36h and 48 h respectively and rinsed in water at the same room temperature, surface water was removed as before and loss in massdue to osmosis wasdetemined. The strawberries that were osmotically dehydrated for 24 h were used for microwave (MW) drying

experiments, convective b i n g and freeze-drying trials.

Microwave Drying

The experimental convective and microwave drying unit (Figure I ) consists of a microwave genemtor(750 W and2450MHz). waveguideassembly, cavityand forced

air system. The unit permits control of the MW power output in continuous mode rather than as a duty cycle. The dimensions ofthe microwave cavity are 40x35~25 cm.

Power meten are located on the wave guide assembly to monitor the incident and reflected power. The circulator connected between the MW generator and the cavity

absorbs the reflected MW power, and the 3 tuning screws situated on the wave guide

assembly are used to tune and reduce the reflected power to a minimum. A strain

gauge is fixed at the top centre of the cavity and is c o ~ e c t e d to the sample holder to continuously monitor the change in mass of the sample being dried. At the bottom of the cavity, a blower forces the preheated air through a pipe into the cavity. Three electrical heaters placed inside the pipe heat the air to the required temperature. Air

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VEMKATACHALAPATHY AND RAGHAVAN

1 MW Generator 2 Circulator 6 3 :Power Meters 4 'Tuning Screws 5 Strain Gauge 6 :MW Cavity 7 Sample Holder El 8 .Blower

FIGUF! 1. Schlematic diagram of the microwave drying setup.

velocity and temperature controls are provided near the blower. Sensors are

connected to the data acquisition system to monitor ambient, inlet and outlet air

temperatures, relative humidity, incident and reflected power.

In these experiments, the inlet air velolcity was kept constant at 2 d s , whereas the

inlet air temperature was 35 "C or 45 "C. The microwave power used was 0.1 Wlg or

0.2 Wlg based on the initial mass of the samples before osmotic dehydration. For

convective drying experiments, hot air at 35 "C) or 45 "C was used with MW power

turned off. The samples were dried until they reached 0.2 kgkg moisture content (Dry

basis). The dried samples were cooled to ambient temperature, packed in polyethylene

covers and stored in the cold room at 1 "C for rehydration, texture, colour and sensory

evaluation studies. There were three replicates of the experimental conditions

summarized in 'Table 1 as well as three replicates of strawberries pretreated,

osmotically dehydrated for 24 h and then freeze-dried for 24 h in a tunnel freeze dryer

(LYO-TECH, Canada).

The relative drying rate was evaluated for different osmotic and microwave drying

treatments, taking convective drying time at 35 "C as the control and expressing the

relative drying rate as

where, ( is the re:lative d~ying rate (Weitz et al. 1989), A, is the drying time of the

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OSMOTIC AND MICROWAVE DRYING OF STRAWBEPJUES 84 1

TABLE 1. Mean:; separations by Duncan's test for moisture removal % (wet basis) at the two h i t to sugar ratiols, treatment with EO/NaOH

and different times of dehydration.

Dipping Treatment

- Ratio -- 2% EO 0.5% NaOH 29.95 A

None 14.26 B 12 16.2 B

25.1 C

--- 4 8 38.7 E Means with the same letter are significantiy diffeIrent a! the 0.05 level.

control sample (rnin), and A, is thle drying time ofthe tre:atecl sample (min). The control

for this calculation was the drying rate for convection drying at 35 OC.

Quality Evaluation

The colour of dried 'berries was measured using a c:hrornameter (Minolta Chroma

meter CR-300, Minolta Camera Co. Ltd. Azuchi-Machi, Osaka 541, Japan). The

c'hromacity of osmotically dehydrated and microwave dried and osmotically

dehydrated and freeze dried berries was determined by measuring their L, a and b

c'oordinates. The meter was calibrated against a standard white surface plate. The

measurements were repeated thrice for each sample. :L is the lightness indicator and

a and b are the chromacity coordiinates. Colour difference values AL, Aa and Ab are

calculated according to the following

AL= L-L,, Aa= a-q , Ab- b-b, (2)

where L, a and b are the measured values of the specimen, and L, , a, , b, are values of the target colour. The target colours in this experiment are L, a, and b values of the

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842 VENKATACHALAPATHYANDRAGHAVAN

fresh strawberry h i t . The total colour difference AE, is measured using the L, a, b colour coordinates and as defined by the following equation (Minolta, 1991):

AE, = [(AL)' + (Aa)' + (Ab)']" (3)

Rehydration tests were conducted on osmotically treated and microwave dried berries

at different power levels and osmotically dehydrated and freeze dried berries as

recommended by USDA (Anon, 1944). The rehydration ratio was calculated as the

ratioofthe mass oftherehydrated sample to that ofdehydrated sample (driedsample). Texture is the property ofthe h i t (food) which is associated with the senseof feel

or touch experienced by fingers or mouth. Objective methods involving different types

of instruments have been made use of to provide eficient and precise quantitative predictions (Ranganna, 1986). In practice, texture measurements are expressed in

terns of force. In this investigation, texture measurements in terms of toughness were made with a puncture test using a 6 mm probe in an lnstron Corporation Series IX Automated Material Testing System 1.16.

Sensoryevaluation of dried strawberries was based on a Hedonic test with a panel of ten untrained judges. The scale of evaluation ranged from "like extremely" which carries 9 points and "dislike extremely" which carries 1 point. Treatments which obtained a mean score of 5 and above are acceptable.

RESULTS AND DISCUSSlON

The osmotic dehydration rates of untreated strawberries and pretreated with 2% ethyl oleate (EO) and 0.5% sodium hydroxide (NaOH) and osmotically dehydrated in

a fruit t o sugar ratio of 3:1, are shown in Figure 2. The time advantage of pretreatment is quite clear, treated strawberries losing far more moisture than untreated strawberries. This is due to the fact that ethyl oleate dissolves all the waxes from the h i t surface and sodium hydroxide helps by creating micropores on the skin through which moisture may diffuse. However, there was some discoloration of the strawberries during osmotic dehydration. The difference in moisture loss between

those dehydrated with a 3:l F:S ratio and a 4:l ratio was significantly different, however, the increase due to a greater amount of sugar is small and doesn't justify its

use. The advantage due to the pretreatment is clear, dipped'strawberries losing far

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OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES

Time in hours OUntreatedHTreated

FIGURE 2. Osmotic dehydration of treated and untreated strawberries.

more moisture than undipped strawberries. Furthemore, off-odours developed by 36 hours, which is the reason why only berries dehydrated for 24 h were used in drying

and quality assessment in the various regimes tested.

Drying Times

The drying times and relative drying rates of the 24 h osmotically dehydrated berries which were then dried under various regimes are shown in Table 2. A

preliminary analysis (not shown) indicated that the inlet airtemperature, and the power

level (includingconvection, or 0 W g"), significantly inlluenced the drying times, with both reducing the dryingtime. There were also significant interactions between h i t -

to-sugar ratio, power level and temperature, as well as between temperature and power level. However, inspection of Table 2 shows that the microwaved samples are inconsistent with respect to the effect of inlet air temperature.

Only power level and fmit-to-sugar ratio were significant main effects whenonly

the microwaved samples were considered. There were also significant interactions between inlet air temperature i d power level, as well as between fruit to sugar ratio

and power. Nevertheless, inlet air temperature does increase the relative drying rate in the convection situation by about 50%.

Referring to Table 3, inlet air temperature does not have a clear influence on relative drying for microwave trials but does increase the overall relative drying rate.

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VENKATACHALAPATHYANDRAGHAVAN

TABLE 2. Microwave drying time and re(ative drying rate of osmotically dehydrated strawberries.

Treatment Drying Time Relative Drying Air F:S PL, W/g Min Rate

35OC 3:l 0 810 1

0.1 100 8.1

0.2 75 10.8

3S•‹C 4:l 0 810 1

0.1 85 9.53

0.2 60 13.5

45•‹C 3:l 0 510 1.59

0.1 120 6.75

0.2 60 13.5

45-C 4:l 0 570 1.42

0.1 90 9

0.2 60 13.5

TABLE 3: Duncan groupings for mean drying times (min) at two air temperatures, fruit to sugar ratios and power levels.

Air Mean Temp "C

45 83.333 A

35 80.000 A Means with the same letter are not significantly different at the 0.05 level.

F:S Ratio Mean

3:l 88.750 A

4:l 74.583 B

Power Level Mean W g"

0.1 98.750 A

0.2 64.583 B

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OSMOTIC AND MICROWAVE DRYING OF STRAWBERRlES 845

The 3: 1 hit-to-sugar ratio led to significantly longer drying times than the 4:l ratio, which one would not necessarily expect, since after 24 h of osmotic dehydration, the moisture content of the strawberries infused at a 3:l ratio was lower 53% compared

to 58% of the berries dehydrated with 4:l fruit-to-sugar ratio and should therefore have reached the final moisture content more rapidly. One possibility fortheopposite result is that more sugar entered the berries osmotically dehydrated with a 3: 1 h i t - to- sugar ratio, resulting in lower diffusion rates which inhibited the outward flow of

moisture, as suggested by Rahman and Lamb (1991) and Sankat el al. (1996) in air drying studies of osmotically dehydrated fruit. The microwave power levels have significant effect on the drying rates, as expected higher power levels (0.2 Wlg) have shorter drying times. The drying kinetics shown in Figure 3 clearly indicates the rate

advantage due to microwaves. Figure 3 shows the drying rates versus moisture content for pretreated and

osmotically dehydrated at 4: 1 berries at the two MW power levels and for convective

drying. It is clear from this figwe that as the microwave intensity increases dryingtime decreases and at higher power levels the moisture removal rate is high and proves that

there is a great advantage in application of microwaves in faster removal of moisture.

Quality Assessmeot

These drying experiments clearly indicated that there is a significant advantage to

pretreating the berries for the osmotic dehydration stage and that microwaves dramatically enhance the drying rate compared to convection. However, the quality ofthe final pmduct must be considered. In this study, we used the freeze-dried product as the standard of quality.

The rehydration ratios and toughness measurements are presented in Table 4, the means separation ofcolourdifferences in Table 5 and the sensory evaluations in Table

6. The analysis of variance on rehydration ratio showed that there were significant differences due to air inlet temperature or hit-to-sugar ratio among the microwaved strawberries. The range of rehydration ratios (Table 4), for the experimental berries

(1.64 to 2.12) includes that of the freeze-dried pmduct (1.88 to 1.90). The highest rehydration ratios were obtained in the 0.2 W g-' regime (Table 4). and both microwave regimes led to better rehydration than the osmotically dehydrated convection-dried berries. The rehydration ratio of freeze-dried berries was not

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VENKATACHALAPATHYANDRAGHAVAN

.............................

................

......................

0 100 200 300

Time, min

FIGURE 3. Microwave drying of osmotically dehydrated strawberries at different power levels.

TABLE 4. Rehydration ratio and texture measurements of osmotically dehydrated and microwave dried strawberries.

Treatment Rehydration Texture Air F:S PL, W/g Ratio (Toughness)

35•‹C 3:l 0 1.64 0.56

0. I 1.82 0.45

0.2 2.06 0.26

3SaC 4:l 0 1.62 0.65

0.1 1.84 0.55

0.2 2.12 0.46

4 5 T 3:l 0 1.70 0.36

0.1 1.68 0.35

0.2 2.05 0.48

45•‹C 4:l 0 1.69 0.41

0.1 1.93 0.38

0.2 1.79 0.42

Freeze 3:l I .90 0.78 Dried 4:l 1.88 0.78

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OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES 847

TABLE 4a. Means separation of rehydration ratios at the experimental temperatures, fruit to sugar ratios and power levels.

Air Temp Mean I F:S Ratio Mean ( Power Level Mean

0.0 1.66 C Means with the same lener are not significantly different at the 0.05 level.

TABLE 4b. Duncan's groupings for mean toughness (MPa) at two temperatures, fruit to sugar ratios and drying regimes.

I FD 0.80500 A Means with the same letter are not significantly different at the 0.05 level.

Temp Mean "C

35 0.50000 A

45 0.40667 B

FD 0.80500 A

significantly different from those dried in the 0.1 W g" regime. The low rehydration

potential of the convection-dried berries could be due to case-hardening.

These results indicate that microwave-dried berries are equal to or better than

freeze dried berries in rehydration. Thus, it should be possible to determine the precise

microwave drying conditions that yield the same value if that is an essential product

parameter.

Table 4b shows that the toughness expressed in mega pascals (MPa) was

influenced by inlet air temperahure and drying regime, but not by fruit-to-sugar ratio. Application of microwave energy seemed to soften the berries, probably due to the

greater internal heating compared to convectiondrying. The freezedried berries were

significantly tougher than the thermally dried ones which is unlikely to be what the consumer prefers.

F:S, Mean Ratio

4:l 0.48500 B

3:l 0.42167 B

FD 0.80500 A

Power Mean Wlg

0 0.49833 B

0.1 0.45083 B

0.2 0.41083 B

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848 VENKATACHALAPATHY AND RAGHAVAN

TABLE 5. Me:ans sepau-ations of co101.x differences due to drying regime, fruit to sugar ratio and inlet air temperature.

Convection 17.768 13 I I

Drying Regime Me; 1 F:S Mem ---

Freeze-dried 10.23 1 C I --- --- I Means with the same letter Ere not significantly different at the 0.05 level.

Temp Mean

TABLE 6. Sensory evaluation of osrnotica1l.y dehydrated and microwave and freeze dried strawberries.

Treatment Sensory Duncan Air F:S PL, Wlg Score (Mean) Groupings -- --- 45°C 3:l 0 6.50 A

0.1 6.12 A

0 .:2 5.75 A

45°C 4: 1 0 5.75 A

0 .:2 5 .oo A

Freeze Dried 3:l 5.75 A

Freeze Dried 4: 1 --- 5.62 A Means with the same letter Ere not significantly different at the 0.05 level.

The analysis of colour difference showed that air inlet temperature was not

influential, but that both fruit to sugar ratio and power level significantly affected this

parameter. Table 5 shows that the greatest: colow difference with the fresh berries

was obtained at the highest power level. For the fruit-to-sugar ratio, there was a

difference in colour betweein the two 1eve:ls of dehydration. The greater heating

should lead to faster darkening, with some possibility of occurrence of imperceptible

burnt spots. It is difficult to explain why the benries infiised with less sugar should

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OSMOTIC AND MICROWAVIE DRYING OF STRAWBERRIES 849

exhibit a significantly larger colow difference, particularly since this group dried more

quickly than those dehydrated at a 3: 1 ratio. It is interesting to note that the freeze-

dried berries were the lightest, and closest to fresh berry colour.

The results of the sensory evaluation (Table 6) indicate that the panel of judges

had difficulty in distinguishing the drying conditions used, since there were no

significant differences in rating over the various treatment combinations. Interestingly,

the convectively dried product scored highest. The fact that some of the n~icrowave-dried berries scored as high as the freeze-

dried product is encouraging for possible adoption of combined osmotic and

microwave dehydration for strawberries in place of freeze drying.

Empirical Model of Finish Drying with Miicrowmvea

The following approach was taken to generate an empirical model to describe

microwave and convective finish drying of s'trawberries osmotically dehydrated in a

4: 1 h i t to sugar ratio. For each replicate, of convection or microwave finish drying,

an exponential curve of the form of M = IVl, e-"' was fit to each set of dry basis

moisture contents observed in the trials (M is the dry basis moisture content; M, is the

initial moisture, k is a rate constant and t is time). The rate constants thus obtained

(R2 2 0.99 in all cases), were then expressed in terms of the power level using a best-fit

(using CurvexpertTM software version 1.3) second-order linear equation of the form:

where, PL is the microwave power level (0,O. 1 or 0.2 W g-') and the bi are regression

parameter estimates.

This procedure led to the: following equation to predict moisture content as a

function of time and power level:

MC(t, PL) = 5.98 exp ((- 0.0025 - 0.041 3 x PL t 0.0785 x P L ~ ) x t )

( 5 )

The above equation was then used to generate predicted moisture contents for each

power level, as shown in Figures .4 to 6. This approach gave a very good fit to the

convection data and to the data. obtained at a microwave power level of 0.1 W g-' over

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VENKATACHALAPATHYANDRAGHAVAN

0 120 240 3 6 0 480 600 720 I ' . . ' Time, min' '

- , . . . . 8 . ,

, F .

FIGURE 4. Predictid moisture content of strawberries by the exponential model compared with the experimental values (PL = 0 Wig).

............. ~~~~~~

. . . . . . . . . . . . . . . . . .

0 20 40 60 80 100 120, 140

Time, min . , .

FIGURE 5. Comparison of the moisture content of strawberries predicted by the exponential moddl and the experimental values (PL = 0.1 Wig.

0 20 40 60 60 100

Time, min

F I G U P 6: Predicted moisture content of strawberries by the exponential model compared with the experimental values (PL= 0.2 Wlg).

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OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES 85 1

most of the drying period. In the case of 0.2 W g-' moisture content was grossly

underestimated forthe first 45 minutes ofdrying, but tended towardsthe experimental

data near the end of the drying period. As the power level goes up the estimation of

the moisture by the model slightly falls, because of the high rateof moisture removal

by the microwaves.

CONCLUSIONS

The results of this study indicate that dipping in an alkaline ethyl oleate solution

promotes rapid dehydration by osmosis. The osmotic dehydration step has been

shown to be necessary in obtaining a microwave-dried strawbeny of quality close to

that of the freeze-dried product in terms of rehydration characteristics and overall sensory evaluation. The microwave-dried product is nevertheless darker and softer

than the freeze-dried product. A simple exponential decay model with the rate constant expressed in terms of

time and power levels (including 0 W g-'), fit the data reasonably well, particularly for

low moisture levels near the end of the d~ying period. This model could be verified

for intermediate power levels.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the financial support fromFCAR Quebec

and the Natural Science and Engineering Council of Canada (NSERC).

REFERENCES

1. Anon. 1944. Vegetable and Fruit dehydration. USDA Misc. Pub. ~ 5 4 0 .

2. Dixon, G.M., len, J.J., and Paynter, V.A. 1976. Tasty apple slices results from combined osmotic dehydration and vacuum drying process. Food Pro.Dev. 10: 60.

3. Flink, J.M. 1979. Dehydrated carrot slice; Influence of osmotic concentration on air drying behaviour and product quality. Food Proc. Eng. 1.412-418.

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852 VENKATACHALAPATHYANDRAGHAVAN

4. Jackson, T.H., and B.B. Mohammed. 1971. The Shambat process. New development arising fromtheosmoticdehydrationof fruitsandvegetables. Sudan J. food. Sci. and Technology. 3:18-22.

5. Lenart, A. 1996.Osmo-convective Drying of Fruits and Vegetables: Technology and Applications. Drying Technology (ed. Mujumdar, AS.) Marcel Dekker, Inc. New York 14(2): 391-413.

6. Minolta. 1991. Chroma Meter CR-300lCR-3 IOICR-321lCR-33 VCR-331C Instructional Manual. Minolta Camera Co Ltd. 101 Williams Drive, Ramsey, New Jersey 07446, U.S.A.

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