• i. Ten grams (10g) of corn kernels were

dried in a convection oven before using

them for volume and specific gravity

measurements. The final weight and the

moisture content of the sample were 8.1 g

and 8% db, respectively. Calculate the

initial moisture content of corn kernels on

wet and dry basis.

10g Final

WB

Final 8.1 g 8% dry basis 0.074074 7.41%

Initial MC

Moisture in

Dry mass final 0.60021 7.49979

db moisture% 0.333371 33.30%

wb 0.250021 25.00%

Free water versus bound water

• Water activity is sometimes defined as

"free", "unbound", or "available water" in

a system. Water activity instruments

measure the amount of free (unbound or

active) water present in the sample.

• A portion of the total water content

present in a product is strongly bound to

specific sites on the chemicals of the

product.

Read More: http://www.fst.ohio-

state.edu/olympiad/Laboratories/Olymp/Lab%201_W

aterActivity.htm

• These sites may include the hydroxyl

groups of polysaccharides, the carbonyl

and amino groups of proteins, and other

polar sites.

Hydrogen bonds, ion-dipole bonds, and

other strong chemical bonds tightly bound

water.

Some water is bound less tightly, but is still

not available (as a solvent for water-

soluble food components).

Many preservation processes attempt to

eliminate spoilage by lowering the

availability of water to microorganisms.

Reducing the amount of free--or unbound--

water also minimizes other undesirable

chemical changes that occur during

storage.

• Eg. concentration, dehydration, freeze

drying and Freezing. Water in frozen foods

is unavailable to microorganisms and for

reactions with food components

Basic concepts

Moisture content = mass of water / unit mass of dry solids

Equil. moisture content = f ( temperature, humidity )

Sorption isotherms : curves relating the m.c. of the material and the humidity of the atmosphere with which it is at equilibrium at different temperatures.

Sorption isotherms of foods are also expressed as moisture content vs. water activity.

Water activity aw = P/Po

(vapor pressure of food moisture/ saturation vapor pressure of pure water)T

Relative humidity of atmosphere = pw / ps

(partial pressure of water vapor/ saturation partial pressure of pure water)T

At equilibrium: pw = P , ps = Po

Sorption behaviour of foods is important in:1. Studying mechanisms of drying and designing dehydration

processes,2. Predicting storage stability of dried foods.

PP0

TºC

RH

%

Water activity (aw)

Eq

uilib

riu

m m

ois

ture

co

nte

nt

(%d

b)

0

10

20

30

40

50

60

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Equilibrium moisture curvesIsotherm

Curve shows the relationship between water

activity (aw) and equilibrium moisture

content (%db).

Typical shape of a sorption isotherm

Relationship between EMC & ERH

ERH

EMC

Water Activity

0 ----------------------------50%--------------------------- 100%

0 --------------------------- 0.5----------------------------- 1

• Different Materials gives different curves

Moisture Sorption Isotherm

aw

Mo

istu

re c

onte

nt

(d.w

.b.)

Zone 3

Zone 2

Zo

ne 1

Zones in Isotherms

• Zone 3: Bulk water

• Zone 2: Loosely bound water

• Zone 1: Tightly bound water.

• EMC depends on

Relative Humidity (RH)

Temperature

Time

MC%

T1 , RH1%

T1 , RH2%

EMC1

EMC2

•Therefore EMC should be given with the environmental conditions

• RH EMC

T EMC

RH1 > RH2

Temperature effect on isotherms

0

5

10

15

20

25

30

35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Water activity

Eq

uilib

riu

m m

ois

ture

co

nte

nt(

%d

b)

Series1

Series2

Series3

45o C

55o C

35o C

Sorption isothermsfor some selecteddried fruits

Equilibrium moisture curves of different

materials at room temperature (25ºC)

Back to previous

Hysteresis effect

True for

many

foods

Uses of EMC curves

• Can find moisture content in a product in a

given temperature and relative humidity.

• Use in selecting the Storage conditions of

the products.

• Use in selecting the Drying conditions of

the products.

• Use in ingredient selection of foods.

Critical

Source: T.Labuza (1992).

User’s manual,

Water analyzer programs

Preservation aw Limits

• bacteria ~ 0.91 yeast~0.88

• molds ~ 0.7 all microbes ~0.6

Rate of Oxidation of Potato

Chips

0.1

1

10

100

0 0.2 0.4 0.6 0.8

aw

Rela

tive r

ate

consta

nt

Monolayer moisture

0

0.05

0.1

0.15

0 0.5 1

aw

Mois

ture

conte

nt

(dw

b)

The GAB Model

Mm Cka

ka kaw Cka

w

w w

0

1 1( )( )

mo -monolayer value

k- multilayer parameter

C -temperature dependency parameter

db

Texture ChangesM

ois

ture

conte

nt

(d.w

.b.)

Soft

Crispy/crunchy

0.2-0.5

Powder ChangesM

ois

ture

conte

nt

(d.w

.b.)

Agglomerated

Free flowing

~0.4

Conditions provided

• Temperature - Using incubators

• Relative humidity -Using salts

• Duration - 3 weeks

Method of Developing EMC curves

Two methods:

1. Static method (Wink’s weight equilibrium method) -

slow (3-4 weeks) –probs. of fungi attacks

2. Dynamic method – fast – conditioned air is

agitated or passed over the product

Creation of constant humidity environments

Name of the

chemical

Relative Humidity at

35o C 45o C 55o C

KF

MgCl2.6H2O

K2CO3

NaBr

NaNO2

NaCl

KCl

K2SO4

24.59

32.05

43.64

54.55

62.84

74.87

82.95

96.71

21.46

31.10

42.99

51.95

60.11

74.52

81.74

96.12

20.60

29.93

42.40

50.15

57.64

74.41

80.70

95.53

Table 01- Greenspan values for saturated salt solutions by FDA.

Usually used for low RH trials but safe use is important

Model fitting

• Guggenheim, Anderson,deVreis (GAB) equation -

m = (mo*Kb*C*aw) / (1- Kb*aw) (1- Kb*aw+ C*Kb*aw)

• Modified Handerson equation - Good for grains

m = (ln(1- aw) / (-A*(T+C)))^(1/B)

• Chung and Pfost - Accurate between 20-90%

m = (-1/B) ln ( - (T+C) / A ln (aw))

Method using ‘statistica’ software

• Determination of EMC isotherms and appropriate mathematical

models for canola

• Ali Zomorodian, a, , Zahra Kavoosia and Leila Momenzadeha

• a Agricultural Engineering Department, Shiraz University, Shiraz,

Iran

• Received 29 April 2010;

• revised 10 October 2010;

• accepted 18 October 2010.

• Available online 4 November 2010

http://www.sciencedirect.co

• Equilibrium Moisture Content of Thai Red

Chillies

•

• Sukruedee Nathakaranakule*,

Wattanapong Rakwichian*, Rak

Dandamrongrak**, Sirichai Thepa

• - Posted to MIS

Evaluation of goodness of fit of a model

• Coefficient of determination (R2),

• Mean Relative Error (MRE),

• the Standard Error of Estimate (SEM)

• and residual plots (Mohamed et al., 2004).

N

i i

preii

M

MM

NMRE

1 exp,

,exp,100

f

N

i

preii

d

MM

SEM

1

2,exp,

Where Mi, exp is the ith experimental moisture content, Mi,

pre is the ith predicted moisture content, N is number of

observations and df is the number of degrees of freedom

of the regression model.

Evaluation of goodness of fit –Contd.

• Mean Relative Deviation Modulus (P)

(P < 5%)

P = (100/n)* (Mi – Mpi) / Mi

Mi - Moisture content observation i

Mpi- Predicted moisture content

n - Number of observations