Water Sorption Isotherms of Water Sorption Isotherms of Vegetables as Influenced by Seed Vegetables as Influenced by Seed Species and Storage TemperatureSpecies and Storage Temperature

Water Sorption Isotherms of Water Sorption Isotherms of Vegetables as Influenced by Seed Vegetables as Influenced by Seed Species and Storage TemperatureSpecies and Storage Temperature

Department of Plant Production, College of Agriculture, King Saud University,

P.O. Box 2460, Riyadh 11451, Saudi Arabia

Department of Plant Production, College of Agriculture, King Saud University,

P.O. Box 2460, Riyadh 11451, Saudi Arabia

Abdullah A. Alsadon

IntroductionIntroductionIntroductionIntroduction

• Vegetables are widely grown in Saudi Arabia. Vegetables are widely grown in Saudi Arabia. Among the common grown crops are beet, cabbage, Among the common grown crops are beet, cabbage, lettuce and okra. lettuce and okra.

• Vegetables are widely grown in Saudi Arabia. Vegetables are widely grown in Saudi Arabia. Among the common grown crops are beet, cabbage, Among the common grown crops are beet, cabbage, lettuce and okra. lettuce and okra.

• Vegetable seeds are mostly imported by agricultural Vegetable seeds are mostly imported by agricultural companies and sold to farmers. companies and sold to farmers.

• Vegetable seeds are mostly imported by agricultural Vegetable seeds are mostly imported by agricultural companies and sold to farmers. companies and sold to farmers.

• Some farmers may use part of the seeds to grow their crops and keep the remaining seeds for the following season.

• Some farmers may use part of the seeds to grow their crops and keep the remaining seeds for the following season.

• Seeds may be stored in farm storage houses in an environment that might not be suitable to preserve seed viability.

• Seeds may be stored in farm storage houses in an environment that might not be suitable to preserve seed viability.

• The dry environments in the central region and humid environment in the coastal areas along with higher temperature all play major influence on seed longevity.

• The dry environments in the central region and humid environment in the coastal areas along with higher temperature all play major influence on seed longevity.

• Controlling water content of seeds and reducing storage temperature can insure greater seed germinability for several years (Walters et al., 1998).

• Controlling water content of seeds and reducing storage temperature can insure greater seed germinability for several years (Walters et al., 1998).

• The behavior of moisture sorption isotherms can be illustrated by the relationship between equilibrium moisture content (EMC) and relative humidity (RH).

• The behavior of moisture sorption isotherms can be illustrated by the relationship between equilibrium moisture content (EMC) and relative humidity (RH).

• A small change in seed moisture content greatly affect the storage life of seeds (Hanson, 1985). The temperature and moisture content of stored seeds are key factors in seed longevity.

• A small change in seed moisture content greatly affect the storage life of seeds (Hanson, 1985). The temperature and moisture content of stored seeds are key factors in seed longevity.

• The isotherms can be used for determining the approximate RH required for seed storage or drying (Walters and Hill, 1998).

• The isotherms can be used for determining the approximate RH required for seed storage or drying (Walters and Hill, 1998).

• Various models have been proposed to describe the relationship between RH and EMC or relationship between RH, EMC and temperature using best fit models to generate three-dimensional response services (Fang et al., 1998).

• Various models have been proposed to describe the relationship between RH and EMC or relationship between RH, EMC and temperature using best fit models to generate three-dimensional response services (Fang et al., 1998).

• The GAB equation is one of the most widely accepted models for sorption isotherms (Ayaranci et al., 1990; Tsami et al., 1990).

• The GAB equation is one of the most widely accepted models for sorption isotherms (Ayaranci et al., 1990; Tsami et al., 1990).

• The Henderson equation (Henderson, 1952, in Roberts, 1972) predicts greater changes in EMC for a given temperature change at higher RH than at lower RH.

• The Henderson equation (Henderson, 1952, in Roberts, 1972) predicts greater changes in EMC for a given temperature change at higher RH than at lower RH.

The objectives of this study were The objectives of this study were (a): to determine the water sorption isotherms of four (a): to determine the water sorption isotherms of four

vegetable species at four storage temperatures (5, vegetable species at four storage temperatures (5, 15, 25, and 35 ºC), and 15, 25, and 35 ºC), and

(b): to correlate experimental sorption data to sorption (b): to correlate experimental sorption data to sorption isotherm equations.isotherm equations.

The objectives of this study were The objectives of this study were (a): to determine the water sorption isotherms of four (a): to determine the water sorption isotherms of four

vegetable species at four storage temperatures (5, vegetable species at four storage temperatures (5, 15, 25, and 35 ºC), and 15, 25, and 35 ºC), and

(b): to correlate experimental sorption data to sorption (b): to correlate experimental sorption data to sorption isotherm equations.isotherm equations.

ObjectivesObjectivesObjectivesObjectives

Materials and MethodsMaterials and MethodsMaterials and MethodsMaterials and Methods

• Seeds of four vegetable species were selected for this Seeds of four vegetable species were selected for this study: carrot (study: carrot (Daucus carota Daucus carota L.), cucumber L.), cucumber ((Cucumes sativusCucumes sativus L.), onion ( L.), onion (Allium cepaAllium cepa L) and L) and tomato ( tomato ( Lycopersicon esculentumLycopersicon esculentum Mill.). Mill.).

• Seeds of four vegetable species were selected for this Seeds of four vegetable species were selected for this study: carrot (study: carrot (Daucus carota Daucus carota L.), cucumber L.), cucumber ((Cucumes sativusCucumes sativus L.), onion ( L.), onion (Allium cepaAllium cepa L) and L) and tomato ( tomato ( Lycopersicon esculentumLycopersicon esculentum Mill.). Mill.).

Table 1. Seeds sources, initial aw, and initial MC

for the four vegetable species.

Species Cultivar

Sourceinitial

aw

initial MC (db)Company

Test date

Initial germ. (%)

Carrot Nantes 2-Tito

Nickerson – Zwaan, Barendrecht, Holland

6/99 80 0.378 4.36

Cucumber Special California – Ventura, CA, USA.

2/99 95 0.383 4.28

Onion Red Creole SunSeeds, Porma, ID, USA.

12/98 84 0.274 3.10

Tomato Tanshet Star

Genetics Int. Inc., Modesto, CA, USA.

2/99 85 0.298 2.88

100

RHaw = water activity = initial MC (db): (g H2O/g dry weight).

• Eight saturated salt solutions (Table 2) were prepared corresponding to a range of water activities from 0.113 to 0.985.

• Eight saturated salt solutions (Table 2) were prepared corresponding to a range of water activities from 0.113 to 0.985.

• Glass desiccators containing the salt solutions were kept in temperature controlled chambers at 5, 15, 25, and 35ºC).

• Glass desiccators containing the salt solutions were kept in temperature controlled chambers at 5, 15, 25, and 35ºC).

• The desiccators were tightly sealed from the outside atmosphere using high vacuum silicone grease.

• The desiccators were tightly sealed from the outside atmosphere using high vacuum silicone grease.

Table 2. Water activity of saturated salt solutions at 5, 15, 25, and 35ºC.

Saturated salt solution

Water activity (aw)Z

5°C 15°C 25°C 35°C

Lithium chloride 0.113 0.113 0.113 0.113

Potassium acetate 0.291 0.245 0.225 0.215

Magnesium chloride 0.336 0.335 0.328 0.325

Potassium carbonate 0.431 0.432 0.432 0.433

Sodium bromide 0.644 0.62 0.584 0.545

Sodium chloride 0.757 0.755 0.753 0.752

Potassium chloride 0.877 0.865 0.843 0.831

Potassium sulfate 0.985 0.985 0.973 0.962

Z = Water activity data were checked for all saturated salt solutions at 25oC using Aqua Lab.

(Model CX-21, readability 1 mg, Decagon Devices Inc., Washington).

The data for other temperatures were then adapted from

(Winston and Bates, 1960 and Rizvi, 1995).

• The moisture isotherms of seeds at 5, 15, 25 and The moisture isotherms of seeds at 5, 15, 25 and 3535ººC exhibited a reverse sigmoidal shape.C exhibited a reverse sigmoidal shape.

• At the first half of the curve (the region with At the first half of the curve (the region with low relative humidity) seeds sorbed relatively low relative humidity) seeds sorbed relatively lower amounts of moisture (Fig. 1).lower amounts of moisture (Fig. 1).

• The moisture isotherms of seeds at 5, 15, 25 and The moisture isotherms of seeds at 5, 15, 25 and 3535ººC exhibited a reverse sigmoidal shape.C exhibited a reverse sigmoidal shape.

• At the first half of the curve (the region with At the first half of the curve (the region with low relative humidity) seeds sorbed relatively low relative humidity) seeds sorbed relatively lower amounts of moisture (Fig. 1).lower amounts of moisture (Fig. 1).

Results and DiscussionResults and DiscussionResults and DiscussionResults and Discussion

Evaluation of vegetable species sorption isotherm response

Evaluation of vegetable species sorption isotherm response

• In general, the sorption isotherms curves of the four vegetable species were similar at lower RH (Fig. 2).

• However, difference between species isotherm curves became obvious when seeds were kept at RH higher than 60%.

• In general, cucumber seeds had the least sorption response followed by tomato, onion, and then carrot seeds.

• Thus, it is expected that the longevity of cucumber seeds would be higher than that of the other species in this study.

• In general, the sorption isotherms curves of the four vegetable species were similar at lower RH (Fig. 2).

• However, difference between species isotherm curves became obvious when seeds were kept at RH higher than 60%.

• In general, cucumber seeds had the least sorption response followed by tomato, onion, and then carrot seeds.

• Thus, it is expected that the longevity of cucumber seeds would be higher than that of the other species in this study.

5oC

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50 60 70 80 90 100

RH (%)

E.M

.C.

(g H

2O

/g d

w)

tomato

cucumber

onion

carrot

Fig 1 (a) Water sorption isotherms of the four vegetable species at 5ºC.

15oC

0

0.1

0.2

0.3

0.4

0 10 20 30 40 50 60 70 80 90 100

RH (%)

E.M

.C.

(g H

2O

/g d

w)

tomato

cucumber

onion

carrot

Fig 1 (b) Water sorption isotherms of the four vegetable species at 15ºC.

25oC

0

0.1

0.2

0.3

0.4

0 10 20 30 40 50 60 70 80 90 100

RH (%)

E.M

.C. (

g H

2O

/g

dw)

tomato

cucumber

onion

carrot

Fig 1 (c) Water sorption isotherms of the four vegetable species at 25ºC.

35oC

0

0.1

0.2

0.3

0.4

0 10 20 30 40 50 60 70 80 90 100RH (%)

E.M

.C. (

g H

2O

/g

dw)

tomato

cucumber

onion

carrot

Fig 1 (d) Water sorption isotherms of the four vegetable species at 35ºC.

The effect of temperature on sorption isotherms

The effect of temperature on sorption isotherms

• For most seed species, it was found that the increase of temperature increased water activity or RH. At 35ºC, the water sorption was lower at any given RH (Table 3).

• For most seed species, it was found that the increase of temperature increased water activity or RH. At 35ºC, the water sorption was lower at any given RH (Table 3).

• Seed deterioration manifested by fungus growth was obvious as relative humidity increased. At 97% RH, all seeds deteriorated before reaching equilibrium at 25 and 35ºC (Table 3).

• Seed deterioration manifested by fungus growth was obvious as relative humidity increased. At 97% RH, all seeds deteriorated before reaching equilibrium at 25 and 35ºC (Table 3).

• Under the conditions of this study, it was found that storing seeds at 5 or 15ºC reduced the possibilities of seed deterioration.

• Under the conditions of this study, it was found that storing seeds at 5 or 15ºC reduced the possibilities of seed deterioration.

Table 3 (a) . Experimental EMC (dry weight basis) for the four vegetable species corresponding to various

water activities at 5oC (aw = [RH/100]).

Table 3 (a) . Experimental EMC (dry weight basis) for the four vegetable species corresponding to various

water activities at 5oC (aw = [RH/100]).

__* : Not available since seeds deteriorated due to higher relative humidities at 25o and 35o C.

Water activity (aw)

Moisture Contents (g H2O / g dry weight)

carrot cucumber onion tomato

5oC

0.113 0.032577 0.029901 0.027166 0.025024

0.291 0.042494 0.039405 0.042375 0.039996

0.336 0.044051 0.043276 0.039238 0.035025

0.431 0.049536 0.048297 0.049626 0.045294

0.644 0.072376 0.062324 0.082424 0.075772

0.757 0.119078 0.098395 0.124737 0.109728

0.877 0.168632 0.132832 0.173026 0.153718

0.985 0.317402 0.208052 0.308704 0.257759

Table 3 (b). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various

water activities at 15oC (aw = [RH/100]).

Table 3 (b). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various

water activities at 15oC (aw = [RH/100]).

__* : Not available since seeds deteriorated due to higher relative humidities at 25o and 35o C.

Water activity (aw)

Moisture Contents (g H2O / g dry weight)

carrot cucumber onion tomato

15oC

0.113 0.042961 0.035710 0.042370 0.057458

0.245 0.054153 0.047976 0.050423 0.068554

0.335 0.058650 0.054286 0.058522 0.075092

0.432 0.064125 0.059442 0.063833 0.078853

0.620 0.072356 0.079203 0.095065 0.112016

0.755 0.110135 0.095388 0.128637 0.133573

0.865 0.283784 0.125131 0.168391 0.168234

0.985 0.367635 0.21914 0.334225 0.295785

Table 3 (c). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various water activities at 25oC (aw = [RH/100]).

Table 3 (c). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various water activities at 25oC (aw = [RH/100]).

__* : Not available since seeds deteriorated due to higher relative humidities at 25o and 35o C.

Water activity (aw)

Moisture Contents (g H2O / g dry weight)

carrot cucumber onion tomato

25oC

0.113 0.035413 0.029414 0.037006 0.047709

0.225 0.046731 0.041751 0.045498 0.061095

0.328 0.051232 0.050291 0.054798 0.070348

0.432 0.054313 0.054007 0.059084 0.068712

0.584 0.117191 0.068740 0.087404 0.093718

0.753 0.155089 0.125194 0.148617 0.153130

0.843 0.226300 0.250919 0.366381 0.227461

0.973 __*

Table 3 (d). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various water activities at 35oC (aw = [RH/100]).

Table 3 (d). Experimental EMC (dry weight basis) for the four vegetable species corresponding to various water activities at 35oC (aw = [RH/100]).

__* : Not available since seeds deteriorated due to higher relative humidities at 25o and 35o C.

Water activity (aw)

Moisture Contents (g H2O / g dry weight)

carrot cucumber onion tomato

35oC

0.113 0.028031 0.011204 0.014305 0.019406

0.215 0.024139 0.021888 0.024522 0.020256

0.325 0.035939 0.035539 0.035421 0.030610

0.433 0.041590 0.039783 0.043663 0.038326

0.545 0.053423 0.049783 0.055250 0.044768

0.752 0.097561 0.084243 0.104767 0.086506

0.831 0.108772 0.106595 0.143286 0.113824

0.962 __*

Fitting sorption data to isotherm models

Fitting sorption data to isotherm models

A) GAB equationA) GAB equation

)1)(1( www

wm

Ckakaka

CkaMM

The GAB (Guggenheim, 1966, Anderson, 1946; de Boer, 1953) equation is one of the most widely accepted

models for sorption isotherms (Ayaranci et al., 1990; Tsami et al., 1990) and can be written as follows:

The GAB (Guggenheim, 1966, Anderson, 1946; de Boer, 1953) equation is one of the most widely accepted

models for sorption isotherms (Ayaranci et al., 1990; Tsami et al., 1990) and can be written as follows:

Where,Where,

EMC = equilibrium moisture content, g water/g dry matter.

Mm = monolayer moisture content, g water/g dry matter.

C = constant related to heat of sorption of monolayer.

K = constant related to total heat of sorption.

aw = water activity

EMC = equilibrium moisture content, g water/g dry matter.

Mm = monolayer moisture content, g water/g dry matter.

C = constant related to heat of sorption of monolayer.

K = constant related to total heat of sorption.

aw = water activity

B- The Henderson EquationB- The Henderson EquationThe Henderson equation (Henderson, 1952, in Roberts, 1972 and Toledo, 1991) predicts greater changes in EMC for a given temperature change at higher RH than at lower RH. On the other hand,

The Henderson equation (Henderson, 1952, in Roberts, 1972 and Toledo, 1991) predicts greater changes in EMC for a given temperature change at higher RH than at lower RH. On the other hand,

The Henderson equation can be written as follows:

Where,

aw is the water activityEMC is equilibrium moisture content (g water/g dry matter), and a and b are constants.

The Henderson equation can be written as follows:

Where,

aw is the water activityEMC is equilibrium moisture content (g water/g dry matter), and a and b are constants.

bw EMCaa )(exp1

Table 4. Estimated parameters for GAB and Henderson equations for the four vegetable species.

Table 4. Estimated parameters for GAB and Henderson equations for the four vegetable species.

ModelVegetable species

carrot cucumber onion tomato

GAB

Mm0.039 0.029 0.038 0.002

c 34.517 304.100 14.622 24.913

k 0.923 0.932 0.919 -0.037

R2 0.998 0.999 0.997 0.969

Hendersona 22.280 49.465 19.798 36.424

b 1.391 1.568 1.302 1.522

R2 0.949 0.962 0.953 0.923

R2 = Correlation Coefficient.Mm = Moisture content (g H2O/g dry weight)

c = Constant , a = Constant , b = Constant

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

5152535

Fig 2 (a) Diagram of tomato data fitting with GAB (________________) and Henderson (--------------) models.

RH (%)

E,M

.C. (

g H

2O/g

(d

w)

oC

oC

oC

oC

0 10 20 30 40 50 60 70 80 90 100

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

5

15

25

35

E,M

.C. (

g H

2O/g

(d

w)

oC

oC

oC

oC

0 10 20 30 40 50 60 70 80 90 100

Fig 2 (b) Diagram of cucumber data fitting with GAB (________________) and Henderson (--------------) models.

RH (%)

0 10 20 30 40 50 60 70 80 90 100

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

5

15

25

35

RH (%)

E,M

.C. (

g H

2O/g

(d

w)

oC

oC

oCoC

Fig 2 (c) Diagram of onion data fitting with GAB (________________) and Henderson (--------------) models.

0 10 20 30 40 50 60 70 80 90 1000 10 20 30 40 50 60 70 80 90 100

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

5

15

25

35

RH (%)

E,M

.C. (

g H

2O/g

(d

w)

oC

oC

oC

oC

Fig 2 (d) Diagram of carrot data fitting with GAB (________________) and Henderson (--------------) models.

0 10 20 30 40 50 60 70 80 90 1000 10 20 30 40 50 60 70 80 90 100

ConclusionConclusionConclusionConclusion

• Water sorption isotherms of carrot, cucumber, onion and tomato seeds were highly dependent on temperature.

• Water sorption isotherms of carrot, cucumber, onion and tomato seeds were highly dependent on temperature.

• In general, species were not significantly influential on the curves of water sorption isotherms except at higher RH.

• In general, species were not significantly influential on the curves of water sorption isotherms except at higher RH.

• The experimental data fitted well the two-sorption models of GAB and Henderson equations.

• The experimental data fitted well the two-sorption models of GAB and Henderson equations.

• Comparing the two models, it is evident that GAB model fits experimental data better than the Henderson model as indicated by the high value of R2.

• Comparing the two models, it is evident that GAB model fits experimental data better than the Henderson model as indicated by the high value of R2.