Study a new atmospheric freeze drying system incorporating a vortex tube and multi mode heat input 5

Figure 5.3 shows the temperature variation at different locations inside the drying chamber with time at a fixed operating pressure of 6 bar also.. Drying condition-Two stage process: Pr

CHAPTER 5 EXPERIMENTAL RESULTS AND DISCUSSION

This chapter starts with an investigation of the subzero air temperature distribution

produced by the vortex tube The AF drying kinetics along with quality parameters of

the model products tested (color, rehydration properties, scanning electron

micrographs (SEM) etc) under fixed bed and multimode heat inputs are presented

Subsequently, effect of osmotical pretreatment on AFD was examined Also,

investigation of a vibro-fluidized bed dryer in AFD with addition of an adsorbent to

the model product is presented and discussed Finally a comparison between vacuum

freeze drying (VFD), atmospheric freeze drying (AFD) and heat pump drying (HPD) is

made in terms of both drying kinetics and quality aspects

5.1 Fixed Bed AFD with Multimode Heat Input 5.1.1 Temperature distribution in the AFD chamber

The measured temperature distribution generated by the vortex tube as well as inside

the drying chamber at inlet air pressure of 4 and 6 bar pressure (absolute) is shown in

Figure 5.1 The air temperature inside the drying chamber drop from the ambient

temperature to about -10°C and –3°C within 15 minutes of start-up of the experiment,

at operating compressed air pressures of 6 bar and 4 bar absolute, respectively The

rate of decrease of the chamber temperature becomes slower with time and stabilizes at

about -19°C and –6°C, respectively. The air temperature, immediately after the vortex tube, decreases rapidly and remains constant at -26°C and -17°C, respectively, after

only about 4 minutes Tangential injection of compressed air at room temperature

into the vortex tube at high velocity produces a vortex, which spins annularly along the

Chapter-5 Experimental Results and Discussion

tube inner wall as it moves axially down the tube A part of this air is adiabatically

expanded inward to the centre, according to the explanation of the flow in a vortex

tube (Crocker 2003)

Figure 5.1 Temperature distributions inside drying chamber at constant pressure at the

inlet of the vortex tube

The decrease in pressure during expansion causes a drop in temperature, which

provides a cooler central column of air directed out of one end of the tube Following

the vortex tube, the air passes through a muffler to reduce the noise level and expands

suddenly into the well-insulated drying chamber As a result, the temperature of the air

rises somewhat inside the drying chamber Figure 5.2 shows the effect of inlet air

pressure on the carrier gas temperature inside the drying chamber as well as the vortex

Air temperature after vortex tube at variable pressure:

6 bar (0 to 14 min) and 4 bar (after 14 min) Air temperature inside drying chamber at variablepressure: 6 bar(0 to 14 min) and 4 bar (after 14 min)

T2 T4

T1

T3 T5

T5

Figure 5.2 Temperature distributions inside drying chamber at variable pressure at the

inlet of the vortex tube

Figure 5.3 Temperature distributions inside drying chamber with time at 6 bar absolute

pressure

Chapter-5 Experimental Results and Discussion

tube exit Initially the pressure was set at 6 bars absolute and the corresponding

temperature after the vortex tube and also inside the drying chamber was found to

reach about -30oC and -17oC, respectively Inlet pressure of 4 bars also was set at

elapsed time of 14min, which results in an instantaneous change in the air temperature

of the cold stream to about –16oC, while the chamber air temperature reached about

-11°C Figure 5.3 shows the temperature variation at different locations inside the

drying chamber with time at a fixed operating pressure of 6 bar also It can be seen

from their figure that all points inside the chamber show similar pattern of temperature

distribution After 40 minutes, temperature at all locations approaches asymptotic

values between -16°C to -18°C This indicates a relatively uniform temperature

distribution at different locations within the drying chamber in the absence of heat

input

Figure 5.4 Variation of product and drying air temperature with time

Drying condition-Two stage process:

Product - Potato (Disc type)

Variation of local temperature distribution of potato sample as well as the carrier gas in

the drying chamber with time under single and two stage heat input schemes is shown

in Figures 5.4 and 5.5, respectively Figure 5.4 shows that for single stage drying at

-6oC a sharp increase of frozen product temperature from -20oC to -9oC within about 5

minutes; placed inside the refrigerator to make the product in freezing state prior to

start the experiments It is likely that the air temperature (-6oC) is comparatively higher

than that of the frozen product (-25oC) Also the high value the of convective heat

transfer co-efficient due to the high velocity of compressed air contributes to heat

transfer from the air to the product On the other hand, air temperature drops down

from ambient to operating temperature (-6oC) within the same time interval

Figure 5.5 Variation of product (potato) and drying air temperature with time for the

two stage drying process

Chapter-5 Experimental Results and Discussion

Product - Carrot (Disc type)

After about 20 minutes both temperatures approach a stable condition and maintain

nearly constant temperature difference of about -3oC between the carrier gas and the

product This is due to sublimation of ice layer from evaporation of the product

During sublimation only latent heat transfer takes place and keeps the temperatu

nearly constant through out the course drying Figure 5.5 shows results for the local

temperature distribution for the two stage drying process of the product (-14oC) and the

carrier gas temperature (-11oC) Unlike the first stage of drying upto 4 hours of drying

time at an operating pressure of 4.4 bar absolute without supplying any additional h

input In the second stage, an increased and stable air temperature was obtained (-6°C

at the same operating pressure by adding radiation and conduction heat sources The

same temperature difference of about 3oC was also observed between the product and

air temperature, which are both well below the melting temperature of the samples

Figure 5.6 Variation of product (carrot) and drying air temperature with time for the

a

re

eat

)

Po tato A FD-Two s tag e: -11C &

-6 C (Rad iatio n & Co n d u ctio n )

In itial M as s o f all

s amp le = 0.3g

This ensures frozen integrity of the potato samples in both stages of drying - an

essential requirement for sublimation and hence to maintain product quality

Figure 5.7 Variation of dimensionless moisture content for potato sample with time

Figure 5.6 shows the variation of the measured local temperature of carrot and the

carrier gas in the two-stage process Almost similar nature of the temperature

distribution with somewhat higher temperature differences (4oC) was found for carrot

samples which again imply the frozen integrity of the product as well as the

consistency of the experimental results

5.1.2 Drying kinetics

Plots of the dimensionless moisture content with drying time for potato samples for the

four-heat input schemes are shown in Figure 5.7 It is observed that after 4 hours of

drying time, the drying rate gradually dropped under the constant heat input scheme at

-11oC It is likely that supply of energy for the sublimation of evaporation front is not

Chapter-5 Experimental Results and Discussion

Two stage: -11C & -6 C (Rad & Cond)

Initial mass of all carrot

sample = 0.3g

enough due to the lower intensity drying condition at this stage Moreover, as drying

progresses the evaporation front recedes deeper into the product and the highly porous

structure of the dry layer decreases the thermal conductivity of the product As a result

the evaporation front does not receive enough heat to sustain a higher drying rate

Figure 5.8 Variation of dimensionless mo

The change in gradient of moisture content after 4 hours suggests that the drying air

temperature should be elevated at this time to enhance the moisture removal gradient

This phenomenon is used in the two-stage process using multimode heat input; it was

shown to yield a significant improvement in the atmospheric freeze drying kinetics

Final dimensionless moisture contents for Case2, Case3 and Case4 were 0.0775,

0.0491, and 0.03, respectively Case4 showed better drying performance than case2

and case3 This can be explained by the fact that a higher process temperature provides

isture content for carrot sample with time

greater energy to the product surface due to increased heat transfer inside the drying

chamber It is worthwhile to note that radiation heats the product superficially without

heating the surrounding and penetrates gradually with time into the product (Ratti and

Mujumdar, 1995) Conduction heat from the bottom of the product also provides

energy for the sublimation front as drying progresses Note that the increased heat

transfer must be achieved without melting of the frozen product Figure 5.8 shows

variation of the dimensionless moisture content with drying time for carrot samples for

the various input schemes tested Similar phenomena were observed for carrot samples

as well Unlike, potato, the two-stage process coupled with conduction and radiation

heat input showed the higher drying rates for the carrot samples

Variation of mass flux for potato with dimensionless moisture content is shown for all

four cases in Figure 5.9 It can be seen from this figure that the initial drying rate for

case-1, case-2, case-3, and case-4 were 0.228 kg/m 2h, 0.149 kg/m 2h, 0.158 kg/m 2h,

0.164 kg/m 2h and 0.176 kg/m 2h, respectively Results show that, except for the drying

condition -6oC; for all other cases the drying rate was nearly same in the first four

hours of drying because of identicle operating conditions during this period The

application of higher constant heat input at -6oC increases the drying rate However, a

quality analysis performed in parallel showed that melting occurs due to the higher

initial temperature causing some product degradation in terms of the internal structure,

which effects rehydration rate when compared with other heat input schemes studied in

this work

Figure 5.9 also shows linear behaviour of the mass flux in the falling rate period for all

four cases for carrot No constant drying rate period was observed A rapid increase in

drying rate was observed in all tests of the two-stage process when the dimensionless

Chapter-5 Experimental Results and Discussion

Two stage: -11C & -6C (Rad & Cond)

Two stage: -11C & -6C (Rad & Cond) Figure 5.9 Variation of drying rate for potato with dimensionless moisture content

Figure 5.10 Variation of drying rate for carrot with dimensionless moisture content

moisture content reached 0.5 and the corresponding drying time was about 4 hours

This can be attributed to the fact that the higher heat transfer rates attained in the two

stage process accelerate the rate of sublimation at the ice vapor interface It causes a

higher pressure gradient to develop between the interface and the drying medium This

high pressure gradient acts as driving force to increase migration rate of moisture from

the vapour interface o the surface of the product After sudden increase of drying rate,

a falling rate was observed during the remaining course of drying when the moisture

content reached at 0.4 As drying progresses the sublimation layer moves inside the

product which in turns increases the diffusion path of the moisture to travel to the

surface of the products and hence reduce the drying rate Finally, it was observed that

Case 4 heat input scheme shows higher mass flux than that for all other operating

conditions

Figure 5.10 shows variation of the drying rate with dimensionless moisture content for

carrot The initial drying rate for case-1, case-2, case-3, and case-4 were about 0.271

kg/m2h, 0.162 kg/m2h, 0.139 kg/m2h, 0.173 kg/m2h, and 0.195 kg/m2h, respectively A

similar pattern of mass flux curves was observed for carrot samples in all cases Only a

minor discrepancy was observed in the drying rate of carrot during the beginning of

second stage in all two-stage processes A minor improvement in drying rate was

observed instead of a sudden increase in drying rate at the beginning of the second

stage process after the 4 hours of drying time attributable to the increases of air

temperature

Figure 5.11 shows a comparison of the freezing point temperature of potato with solid

fraction obtained from Schwartzberg’s empirical correlation and the measurements for

Chapter-5 Experimental Results and Discussion

Figure 5.11 Variation of freezing temperature with weight fraction of solid

Figure 5.12 Comparison of dimensionless moisture content for potato and carrot

samples with time

case2 and case4 These two cases represent two extreme cases of low and highly

intense drying conditions The correlation is consistent with our experimental data,

which implies that the air and product temperatures attained in this study satisfy the

basic requirement for AFD

A comparison of the variation of dimensionless moisture content for potato and carrot

sample with time is shown in Figure 5.12 Case-1 and case-4 represent the low and

high intensity drying conditions, respectively It can be seen from this figure that in

both cases the carrot sample shows higher drying rate than the potato sample The final

dimensionless moisture content for potato and carrot sample was about 0.4 and 0.29

for case-1 and 0.04 and 0.02 for case-2, respectively This can be explained noting that

carrot has a higher diffusivity (Wollf and Gibert, 1990b; George et al 2002) It is

important to note that for case-4, differences between dimensionless moisture content

of potato and carrot sample gets closer at the end of drying It is likely that at the end

of drying, moisture content of the product approaches the equilibrium value of

moisture content Therefore, moisture removal rate from inside to the surface of the

product and finally, to the carrier gas, is independent of the external drying conditions;

rather it depends on the material properties

5.1.3 Quality analysis

Figure 5.13 shows the variation of relative mass index of the dried sample (potato)

with time It shows that the relative mass index for case-4 was approximately equal to

5.5, while for the other two stage processes; case-2 and case-3 were almost same at

about 4.8 after 4 minutes This can be explained in terms of the fact that for case-4 the

final dimensionless moisture content was about 0.03, as shown in Figure 5.7, which

implies an almost completely dried product of porous structure because of the

Chapter-5 Experimental Results and Discussion

Carrot-Two stage: -11C & -6C (Rad & Cond)

sublim

Figure 5.13 Variation of rehydration ratio for dried potato with time

ation process during the course of the experiment Therefore, during rehydration

the dried product under case-4 absorbed more water into the porous matrix inside the

product and hence shows a higher rehydration ratio Poor rehydration quality (3.4) was

observed for the single stage process at –6oC; this implies that melting may

occurred during drying due to the high initial drying temperature On the contrary,

rehydration quality at -11oC was even worse (2.18) although it was a mild drying

operation This is due to the incomplete drying of the product as can be seen in Figure

5.7; a large amount of moisture exists inside the product and hence it deteriorates the

rehydration quality

Variation of the relative mass index with time for carrot for all cases is shown in

Figure 5.14 The final rehydration ratios for case-1, case-2, case-3 and case-4 were

2.18, 7.27, 6.73, 6.74 and 8.71, respectively Unlike for potato a higher relative mass

flux was found for case-4 and poor rehydration quality was observed for case-1 at

Chapter-5 Experimental Results and Discussion

Figure 5.15 shows the comparison of the absorbed water by dried potato and carrot

samples during rehydration for case-4 Better rehydration quality was found for potato

than for carrot Absorbed water for the first 1 and 2 minutes during rehydration for

potato was about 0.188 gm and 0.013, while for carrot it was 0.155 and 0.019,

Figure 5.16 (a) SEM photographs of cross section (x 80 200μm) of dried potato

and carrot

Figure 5.16 (b) SEM photographs picture of horizontal surface (x 80 200μm) of dried

potato and carrot (Drying condition-Two stage: -11°C (convection) and -6°C (convection, radiation and

Porosity

Cell

Cell

respectively Figures 5.16(a) and 5.16(b) shows the microscopic picture of cross

sectional area and horizontal surface area for potato and carrot sample, respectively for

case-4 The SEMS results show a clear porous structure of the dried potato for both

areas This result is consistent with the observed better rehydration quality as shown in

Figure 5.13 It is also evidence of sublimation which occurs during AFD drying

However, for carrot the product does not have a highly porous structure This is due to

low freezing point of carrot, which causes melting of ice during experiment and hence

damages the structure

Measured changes in colour due to drying are shown in Table 5.1 Overall colour

differences (ΔE) for case1 (-11oC), case2, case3 and case4 were found to be 11.30,

26.14, 28.02 and 11.14, respectively, for potato and 10.63, 11.34, 19.24, and 7.17,

respectively, for carrot It is apparent from the results that case4 drying scheme yielded

Table 5.1 Changes in colour before and after drying for potato sample-effect of

various drying conditions

Chapter-5 Experimental Results and Discussion

better colour in the dried samples, which was close to the original colour of the

product From analysis of data for all trials, the two stage process operating at

–11oC and –6oC, with radiation-convection and conduction heat input (case 4) showed

faster drying kinetics together with better quality of dried product in terms of internal

structure, rehydration and colour Therefore, in subsequent analysis case-4 is used as

Potato AFD- Two stage: -11C &

-6 C (Rad & Cond)

benchmark for the AFD system

5.1.4 Comparison between AFD, VFD and HPD

A comparison was made between the AFD process under case4 conditions with

vacuum freeze drying (VFD) and heat pump drying (HPD) to evaluate the viability of

the proposed novel AFD system as shown in Figure 5.17 This figure compares the

drying kinetics for AFD, VFD and HPD for potato samples

Figure 5.17 Comparison of dimensionless moisture content with time for different

method of drying for potato

Figure 5.18 Comparison of dimensionless moisture content with time for different

method of drying for carrot

Final dimensionless moisture content of 0.05 was obtained after 1.2, 3, and 7.5 hours

of drying time for HPD, VFD and AFD, respectively Thus AFD required 4.5 hours

longer time to achieve the same final moisture content, as compared to the VFD

process In practice, this difference can be cost effective due to the lower cost of AFD

AFD is about 30% less expensive than VFD according to Wolff and Gibert (1990a)

Comparison also carried out using carrot is shown in Figure 5.18 Similar results are

noted for carrot, which gives support for viability of the proposed AFD system

Although a full technoeconomic evaluations was not made

Figure 5.19 and Figure 5.20 show the comparison of drying rate data for potato and

carrot between AFD, VFD and HPD At the beginning of the drying mass flux for

potato was about 0.649 kg/m2h, 0.416 kg/m2h and 0.176 kg/m2h for HPD, VFD

t AFD-Two stage: -11C & -6C (Rad &

)

Chapter-5 Experimental Results and Discussion

Figure 5.19 Comparison of drying rate with dimensionless moisture content for

different drying processes for potato

Figure 5.20 Comparison of drying rate with dimensionless moisture content for

different drying processes for carrot

and AFD, respectively, while for carrot it was 0.631 kg/m2h, 0.366 kg/m2h and 0.195

kg/m2h, respectively Results show a comparable drying rate between VFD and AFD

for both samples, while a higher drying rate was observed for HPD relative to VFD

and AFD However, quality tests showed that HPD caused significant product

degradation in terms of the internal structure, rehydration rate and colour

Figure 5.21 Comparison of relative mass index with time for different drying processes

for potato

Figures 5.21 and 5.22 show the relative mass indices for potato and carrot slices dried

using VFD, AFD and HPD The relative mass index was found to be about 6.24, 5.62

and 4.82, respectively, for potato, while 2.55, 7.27 and 6.55, respectively, for carrot

after 4 minutes Comparable rehydration quality was observed between VFD and AFD

However, poor rehydration quality was observed for HPD This can be well explained

by examination of the SEM images (Fig.5.23)

Chapter-5 Experimental Results and Discussion

Atmospheric freeze dried potato

Heat pump dried potato

Figure 5.23 Comparison of microscopic picture for different drying processes of potato

sample (Drying condition-Two stage: -11°C (convection) and -6°C (convection, radiation and

Cell

Chapter-5 Experimental Results and Discussion

Figure 5.23 show that the AFD product yields very similar structure to the VFD

sample while porosity is hardly noticeable in the HPD product This is again a clear

indication of the sublimation process AFD

Table 5.2 shows the change of brightness in colour of the dried potato and carrot

product obtained using the three different drying processes Results show that

brightness of the dried product of 74.87, 53.49 and 26.65 for potato, and 47.8, 40.32

and 33.94 for carrot, which is denoted by L for VFD, AFD and HPD, respectively,

while the original brightness for potato and carrot, is 42.58 and 49.72, respectively

Products dried by VFD and AFD show brighter colour in the final dried samples

Table 5.2 Colour measurement of potato and carrot samples dried using different

Table 5.3 Comparison of colour changes of dried potato and carrot samples

due to different drying techniques

However, degradation in colour was observed in the HPD product due to exposure of

the product to hot air causing enzymatic browning reactions (Krokida et al, 2001)

Color change of dried products for VFD, HPD and AFD are 32.34, 16.17 and 11.16,

respectively, for potato and 3.69, 12.30, and 23.14 for carrot is shown in Table-5.3

The overall color difference is represented by ΔE As the difference of initial and final

measurement of color parameters is minimum, it can be concluded that color of the

product at the end of drying is as close to that of the original product VFD dried

potato sample was brighter in color than the original color probably due to change of

its microstructure AFD-dried potato had color comparable with VFD dried product

Figure 5.24 and 5.25 shows photography of dried potato and carrot samples using

VFD, AFD and HPD A similar size and shape of finally dried product was observed

for VFD and AFD Figure 5.24, which shows the puffed and undeformed shape of

Chapter-5 Experimental Results and Discussion

Vacuum freeze dried potato

Atmospheric freeze dried potato

Heat pump dried potato

Figure 5.24 Photographs of disc and rectangle-shaped dried potato sample using VFD,

Vacuum freeze dried carrot

Atmospheric freeze dried carrot

Heat pump dried carrot

Figure 5.25 Photographs of disc and rectangle-shaped dried carrot samples using VFD,

AFD and HPD

Chapter-5 Experimental Results and Discussion

AFD-dried potato sample, which implies that only sublimation, occurred during the

course of drying This helps preserve the internal structure of the product This result is

a proof of good comparison of AFD with VFD in terms of dried product quality A

slightly deformed shape in the dried carrot sample was observed due possibly to minor

condensation at the ice front during drying This agrees with other quality analyses i.e

rehydration and SEM results for carrot sample For HPD a curl and deformed

structured was observed In HPD, drying takes place due to migration of moisture in

liquid form Therefore cell structures of the HPD dried product are collapse due to

surface tension and capillary forces during travel of moisture towards the surface of the

product Hence, dried products for HPD tended to curl due to uneven drying and

drying-induced stresses

5.1.5 Osmotic treatment

Figure 5.26 presents typical experimental AFD drying data obtained for osmotically

treated banana slices as well as the untreated ones It was observed that the osmotically

pretreated samples have, as expected, a lower initial moisture content compared to the

untreated sample It is also noted that osmosis in sugar solution lowers the moisture

content to a greater extent than does salt solution at the same temperature e.g from 2.3

kg/kg db to 0.95 kg/kg db and 1.03 kg/kg db, respectively As sugar is more soluble in

water than salt it creates higher pressure gradient that act as a driving force to increase

the migration rate of water from the product to the solution and hence lower the initial

moisture content This result agrees with the previous results (Rahman and Mujumdar,

2007) The effect of osmotic pretreatment on drying rate of banana samples is also

shown in Figure 5.27, in the form of dimensionless moisture content for better

comparison This figure shows higher drying rate when the sample is pretreated in

Atmospheric freeze drying

Two stage process:

Disc type : D-16 cm and Thickness-1 mm

Figure 5.26 Effect of osmosis using concentrated salt and sugar solution on variation

of moisture content with time for banana

Figure 5.27 Effect of osmosis using concentrated salt and sugar solution on variation

of dimensionless moisture content with time for banana

Chapter-5 Experimental Results and Discussion

Two stage process:

Disc type : D-26 cm and Thickness-1 mm

Figure 5.28 Effect of osmosis using concentrated salt and sugar solution on variation

sugar solution The final dimensionless moisture content of the products for untreated

samples and ones treated with sugar and salt solution were 0.12, 0.17 and 0.22,

respectively, from the same initial dimensionless moisture content of 1 after a drying

time of 8 hours Presence and precipitation of sugar (Karathanos et al., 1995) and salt

crystals inside the product inhibits the diffusion of water molecules and thus decreases

the drying rate to some extend relative to that of an untreated sample

of moisture content with time for potato

In contrast, although it is apparent from the Fig 5.26 that moisture content reduces

significantly for the treated sample, however after 6 hours of drying time moisture

content for the product treated with salt and sugar solution reaches nearly equilibrium

moisturecontent of 0.3 kg/kg db and 0.26 kg/kg db, respectively, while for untreated

Cod fish-Osmotic in salt solutionAtmospheric freeze drying

Two stage process:

Disc type : D-26 cm and Thickness-2 mm

samples it is about 0.5 kg/kg db This scenario is quite contrary to observations with

other drying methods (Rahman and Mujumdar, 2007) It is likely that a porous

structure is formed in the product during AFD Therefore, the moisture has room to

migrate from inside to the surface of the product inspite of the precipitation of salt and

sugar, which otherwise would clog the passage, during the course of drying These

results are consistent with the findings for the potato samples as well (Fig.5.28)

Figure 5.29 Effect of osmosis using concentrated salt and sugar solution on variation

of moisture content with time for biological origin product Plots of the variation of moisture content with time for osmotically treated cod fish and

beef using a concentrated salt solution as well as for untreated sample are shown in

Figure 5.29 The initial moisture content of cod fish and beef was reduced from 2.4 to

1.2 kg/kg db and 2.6 to 1.4 kg/kg db, respectively, after osmotic treatment in salt

Chapter-5 Experimental Results and Discussion

Beef-Osmotic in salt soluitonAFD: Two stage process:

Disc type : D-26 cm and Thickness-2 mm

Figure 5.30 Effect of osmosis using concentrated salt and sugar solution on variation

solution The final moisture content for cod was found to be about 0.65 kg/kg db and

0.45 kg/kg db after 20 hours of drying time for untreated and treated samples,

respectively, while for beef it was about 0.13 kg/kg db It is important to note that both

biological products dried faster throughout the course of drying compared to the

treated samples as shown in Figure5.30 This phenomenon is quite different from other

osmotic drying processes The dissolved salts seems to precipitate near the surface of

the sample from the early stage of drying in the case of such biological products,

of dimensionless moisture content with time for biological origin product

causing a “case-hardening” effect; this reduces substantially the effective moisture

diffusivity (Uzman and Sahbaz, 2000) and hence reduces the drying kinetics