In within the food matrices. Characteristics of bound

In
the paper the author introduces us to the sorption property of foods and the
concept of water activity (aw). Control of moisture content in food is
necessary to avoid microbial and chemical spoilage. The first researcher to
relate relative water vapor pressure to microbial growth leading to food spoilage
was Walter in 1924. The theory of water activity was later developed which
determines the shelf life of food. It shows the intensity with which water
associates with food and hence its availability to participate in various
reactions.

 

Food
reactions and food quality during food processing is highly affected by water
content since natural food has large quantity of water.
The ability of food to hold inherent water, under a specific set of conditions,
is referred as ‘water holding capacity’ or ‘water binding capacity’. Water in
the biological system exist as either free or bound water. Free water is the
most mobile water in food and can be easily removed, whereas, bound water is
restricted in it movement due to stronger hydrogen bonds and physical
entrapment within the food matrices. Characteristics of bound water are that it
does not freeze at low temperatures, has lower vapor pressure and is
unavailable as solvent to dissolve additional solutes. Bound water is
difficulty to remove from food.

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Water activity (aw ) is the
most common concept used in food industries. It is defined as:

aw = p/p0 = relative
humidity%

where,
p is the partial pressure of water in food (atm), and p0 is the vapor
pressure of pure water at the same temperature (atm).

 

The
author further talks about Moisture Sorption Isotherm, which is a
graphically expressed relation between water content of a food versus water
activity at a constant temperature.

Adsorption
isotherms are obtained by adding water to previously dried samples, and,
desorption isotherms are prepared by removing water from samples over a range
of values.

The
water in region A of the isotherm represents the most strongly sorbed and least
mobile. Water in region B are less firmly bound, exerts a significant
plasticizing action on solutes and does of freeze at normal freezing points.
Water in region C has decreased viscosity and has all properties of bulk water.
This region readily supports microbial growth.

Based
on Van der Waals adsorption of gases on various solid substrates, researchers
have classified adsorption isotherms into 5 general types. Water sorption
isotherm can be determined by gravimetric method, manometric method or
hygrometric method.

 

Hysteresis in foods is the phenomenon
by which at constant water activity (aw) and temperature, a food
adsorbs a smaller amount of water during adsorption than during a subsequent
desorption process. During adsorption, micro-cracks and fissures form in the
food to expose additional sites. Exposed sites unable to adsorb moisture on the
way up to higher water activities (aw) because of inappropriate
surface energies adsorb additional moisture on return to lower (aw)  at appropriate (aw)  and surface energies to exhibit a hysteresis
loop. The nature of the pore size distribution and the driving force involved
in changing water activity play a role in hysteresis.

 

The effect of
hysteresis on food is important to increase shelf life. Several theories have
been put forth to explain hysteresis but no theory has given a complete
insight. This could be because food is a complex combination of various
components which can sorb water independently and interact amongst themselves. At
elevated temperatures there is decreased total hysteresis and limited loop span
along isotherms.

There is no single
equation that can mathematically explain moisture sorption isotherms of food
materials though a large number of models are available. This is because water
is associated with food matrix by different mechanisms in different water
activity regions. 6 different equations are described in the literature.

The author
introduces the term Isosteric heat of sorption since the heat of vaporization
of sorbed water may increase to values above the heat of vaporization of pure
water as food is dehydrated to low moisture levels to increase shelf life. This
is important for designing equipment’s for dehydration processes. It can be
determined from moisture sorption data using the equation which is derived from
the Clausius–Clapeyron equation. This equation gives the heat adsorption and
desorption for food materials which is necessary to estimate heat load during
the drying of food materials.

The major conclusion of this review were:

1.      Most
of the food processing operations require regulation of moisture content.

2.      For
the prediction of shelf life of dried foods, moisture sorption isotherms are
useful as it is sensitive to moisture changes.

3.      Hysteresis
loops exists in moisture sorption isotherms showing a non-equilibrium state due
to complex nature of food materials.

4.      The
isosteric heat of sorption is necessary to evaluate the thermodynamic functions
of water sorbed in foods.