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Experiment No. 6

Moisture Sorption Isotherm

Queenie Gene Gadong

Date Performed: February 22, 2012

Date Submitted: March 17, 2012

I. IntroductionWater in a food substrate is composed of two classifications, the free water and the bound water. Free

water is that moisture that can be easily removed through evaporation from the food surface. There is also

the bound water for which forced removal of water would be hard and could lower the quality of the

product since the water is chemically bound to the food matrix. Many preservation processes attempt to

delay spoilage of the food material by reducing the availability of water to microorganisms and

undesirable chemical changes. (John M. deMan, 1999)

Removal of all the free water after a period of time would result in a constant amount of water left in

the food. This definite moisture content of the food product is known as the Equilibrium Moisture content(Xe). The equilibrium word in itself indicates that the moisture removed is the maximum amount of water

that can be absorbed by the drying medium, in this case the air at a given temperature and humidity, at

which point there is no more exchange of moisture from the material to the air. (Geankoplis, 1995) It can

also be argued that the equilibrium moisture content of the food material is the maximum amount of freewater which can be removed from the food material. (Earle, 1983)

Water activity, Aw on the other hand, is the representation of the amount of water present in a

substrate, which may be able to support microbial growth. The term water activity (aw) was developedto account for the intensity with which water associates with various nonaqueous constituents.(Fennema, 1996) This relationship between water activity and association with solid constituents is based

on the fact that already bound water is less likely to engage in degradative activities. Biologically active

organisms, which spoil organic substrates, are unable to live and proliferate in a water free environment.(Reynolds, 1998)

Water activity can determine the shelf-life of a food commodity since spoilage and even pathogenic

microorganisms are unable to proliferate in water activity just below 0.91. Fungi can only survive until

0.80, below this fungal growth is stunted and further removal of moisture to 0.6 would halt other

microbial activity. Also the enzymes are unable to promote undesirable changes in the food, which

renders a longer shelf-life. Aside from this, water activity also influences the vitamin activity and it could

even affect the sensory attributes of the food such as the color, taste and aroma. (Andrade, 2011)

Moisture Sorption Isotherm (MSI) is graphical representation of the relationship between the

equilibrium moisture content of the food and its corresponding water activity. Water activity can be

considered as the ratio of water vapor pressure, (the waters tendency to leave the solution) in the foodsystem and the pure water vapor pressure at a constant value of pressure and temperature. (Fennema,

1996)

(1)

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Another possible definition for moisture sorption isotherm is the plot of moisture content against the

relative humidity of the air of which the food is at equilibrium in terms of temperature and pressure. A

relationship between the water activity and the relative humidity is represented by equation 2 as

(2)

where, Aw is water activity, P is the water vapor pressure and Po is the saturated vapor pressure. Thus,the water activity of food is equal to the relative humidity of the air around it divided by 100, which

means that equilibrium has been reached, constituting a form of measurement of the water amount

available in food. (Andrade, 2011)

Moisture Sorption Isotherm is used in food technology in determining the tendency of a food material

for microbial growth and enzymatic action. It can also predict the affinity of the food material for

deteriorative mechanisms such as browning and lipid oxidation. Thus it could also predict the shelf-life,

storage stability, and food quality. Moisture Sorption Isotherm can also be used in food engineering since

it could help in determining drying equipment to ensure process utilization. (Fennema, 1996)

Moisture sorption isotherms are sigmoidal in shape for most foods, a relationship which shows that an

increase in moisture content is accompanied by an increase in water activity in a non-linear way. Thereare some types of food however, foods that contain large amounts of sugar or small soluble molecules,

that have a J-type isotherm curve shape. The moisture sorption isotherm of a food is obtained from the

equilibrium moisture contents determined at several water activity levels at constant temperature. (John

M. deMan, 1999)

There are three types of isotherm curves; adsorption (starting from the dry state), desorption (starting

from the wet state), or working (native state). An isotherm prepared by adsorption will not necessarily be

the same as an isotherm prepared by desorption. This phenomenon of different moisture contents for the

same aw is called moisture sorption hysteresis, and is exhibited by many foods. Some reasons for the

hysteresis are: differences in the filling and emptying of pores and capillaries, swelling of polymeric

materials, transition between glassy and rubbery state and super saturation of some solutes during

desorption Moisture Sorption Isotherm curves are thought of as the fingerprint of a food system andcan only be determined experimentally. (Decagon Devices, 2004).

Figure 1: Adsorption and Desorption Isotherms

Source: (Fennema, 1996)

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The objective of this laboratory exercise is to construct a Moisture Sorption Isotherm from the

obtained raw data for the raisin sample. This experiment would teach the students to identify the water

activity range for which moisture desorption and absorption occur and be able to differentiate between the

two.

II. MethodologyPreparation

The sample used for this experiment is raisins. Prior to the start of the laboratory period the group

prepared 8 wire screen baskets, where in the samples during the course of the experiment will be placed.

The initial weight of the samples were determined same with its initial moisture content using the IR

moisture analyzer.

Eight (8) saturated salt solutions were prepared by dissolving the crystals in separate petri dishes with

2mm liquid layer. The solutions were deemed saturated when the solution was unable to dissolve

anymore the salt crystal added to it. These salt solutions were Potassium Nitrate, Ammonium Sulfate,

Sodium Chloride, Sodium Nitrate, Potassium Carbonate, Magnesium Chloride, Potassium Acetate and

Lithium Chloride.

A sample was then placed in a separate dessicator hanging on top of a saturated solution and stored

for 1 week or until equilibrium was reached. The weighing of samples after 7 days for a single reading

was then performed at a recorded room temperature. The temperature in each separate dessicators on the

other hand was done daily.

III. ResultsTable 1.a: Summary of calculated results of Moisture Sorption Isotherm for raisins, replicate one.

Sample Salt X, dry basis(kg/kg) Aw values at 30C

1 potassium nitrate 0.4341 0.907

2 Ammonium sulfate 0.2319 0.8

3 Sodium chloride 0.1749 0.756

4 Sodium Nitrite 0.0983 0.633

5 Potassium carbonate 0.0226 0.435

6 potassium acetate -0.0239 0.22

7 magnesium chloride 0.014 0.328

8 lithium chloride -0.0256 0.118

Table 1. b: Summary of calculated results for Moisture Sorption Isotherm for raisins, replicate two.

sample Salt X, dry basis(kg/kg) Aw values at 30C

1 potassium nitrate 0.8094 0.907

2 Ammonium sulfate 0.4795 0.8

3 Sodium chloride 0.8684 0.756

4 Sodium Nitrite 0.5399 0.633

5 Potassium carbonate -0.1576 0.435

6 potassium acetate -0.4893 0.22

7 magnesium chloride -1.1777 0.328

8 lithium chloride 0.2384 0.118

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Figure 1.a: Moisture Sorption Isotherm Plot for raisins, replicate one.

Figure 1.b: Moisture Sorption Isotherm Plot for raisins, replicate two.

To be able to obtain a sigmoidal shaped curve data points that greatly deviate were discarded,

particularly the negative moisture content.

IV. DiscussionIgnoring the negative data points, the plot for the moisture sorption isotherm of the sample raisins

shows an increasing trend in the graph. The moisture content of the samples increased significantly when

the dried samples were exposed from different saturated salt solutions. We can also see from the results

that salt solutions containing higher water activity greatly increases the moisture content of the samples

compared to those saturated salts having lower water activity. From the graph, it is interpreted that there

is adsorption of moisture of the dried food sample from the saturated salt solutions. The isotherm can be

described as an adsorption isotherm curve, in which the sample starts from a dried state then to an

increasing weight because of adsorption of moisture.

-0.1

0

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0 0.2 0.4 0.6 0.8 1

X,

dry

basis(kg/

kg)

Aw

MSI 1

0

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0.2

0.3

0.4

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0 0.2 0.4 0.6 0.8 1

X,

dry

basis(kg/kg)

Aw

MSI 2

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V. ConclusionAt the end of the experiment, the group concluded that moisture sorption isotherm is a good way of

showing the relationship between water content and equilibrium humidity of the material. With the use of

the MSI, a person could see the response of the sample under different humidity conditions. . Moisture

sorption isotherm shows that Aw is directly proportional to the moisture content, but in a nonlinear

trend.For the group sample which is raisins, the isotherm shows an increasing trend in the process. Thismeans that the product absorbs moisture from its surrounding, therefore increasing its initial weight, and

term is referred to as adsorption.

VI. References Decagon devices.2004. moisture sorption isotherm method. Available from:

http://www.decagon.com/pdfs/app_notes/moisturesorptionisothermmethod.pdf.

accessed date: 03/14/12

Andrade, r. L. (2011). Models of Sorption Isotherms for Food: Uses and Limitations. In Vitae,Revista de la Facultad de Qumica Farmacutica (pp. 325-334). Medelln, Colombia: Universidad

de Antioquia, .

Fennema, O. R. (1996). Water and Ice. In o. R. Fennema, Food Chemistry 3rd ed. (p. 1051). 270Madison Avenue, New York, New York 10016: Marcel Dekker, Inc.

Geankoplis, c. J. (1995). Transport Processes and Unit Operations 3rd ed. University ofMinnesota: Prentice-hall International, Inc.

John m. DeMan, p. (1999). Principles of Food Chemistry 3rd ed. Gaithersburg, Maryland: AspenPublishers, Inc.

VII. AnnexTable 2: Initial Moisture Content of sample using IR analyzer.

TrialMoisture

Content

1 5.99

2 3.59

3 2.26

ave. 3.946666667

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Table 3.a:Summary of raw and calculated data for MSI replicate one.

Sample Salt wt. basket (g)wt. before

storage

wt. after

storage (g)W1 W2

X, dry basis

(kg/kg)

1 KNO3 2.3802 3.9047 4.4802 1.5245 2.1 0.4341

2 (NH4)2SO4 1.9789 3.5521 3.8405 1.5732 1.8616 0.231941

3 NaCl 3.1116 4.7567 4.9678 1.6451 1.8562 0.174681

4 NaNO3 2.3695 3.8457 3.9269 1.4762 1.5574 0.098354

5 K2CO3 2.7935 4.2265 4.201 1.433 1.4075 0.022562

6 KC2H3O2 1.9435 3.3515 3.2636 1.408 1.3201 -0.02391

7 MgCl2 2.1168 3.6068 3.5681 1.49 1.4513 0.014048

8 LiCl 1.8509 3.4154 3.3151 1.5645 1.4642 -0.02566

Table 3.b:Summary of raw and calculated data for MSI replicate two.

Sample Salt wt. basket (g) wt. beforestorage

wt. afterstorage (g)

W1 W2 X, drybasis

(kg/kg)

1 KNO3 0.9282 2.4304 3.0261 1.5022 2.0979 0.42284535

2 (NH4)2SO4 2.4253 3.9362 3.8553 1.5109 1.43 -0.0457443

3 NaCl 1.0087 2.5948 2.7599 1.5861 1.7512 0.11836875

4 NaNO3 0.7379 2.2499 2.3404 1.512 1.6025 0.07231382

5 K2CO3 0.9086 2.6782 2.9744 1.7696 2.0658 0.18425992

6 KC2H3O2 1.8952 3.6578 3.539 1.7626 1.6438 -0.0601698

7 MgCl2 3.324 5.0689 5.0289 1.7449 1.7049 -0.01386598 LiCl 2.8906 4.6882 4.5724 1.7976 1.6818 -0.0570661

Table 4: %RH of salts at 30C

Salt %RH at 30C Aw values at 30C

KNO3 90.7 0.907

(NH4)2SO4 80 0.8

NaCl 75.6 0.756

NaNO3 63.3 0.633

K2CO3 43.5 0.435

KC2H3O2 22 0.22

MgCl2 32.8 0.328

LiCl 11.8 0.118

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Sample Calculations:

Determination of Moisture Content:

X, dry basis (kg water/kg solid)

( ) (

)

( )

( ) (

)

( ( )

)

Determination of Aw, water activity value:

Aw = 0.907