Water activity

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Technical Seminar

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Presenter:-Name: Ashish KumarSemester:- 7th Roll no:- Tech 02Faculty of Agricultural Engineering

Bidhan Chandra Krishi Viswavidyalaya Mohanpur, Nadia, West Bengal - 741252

Water activity concept and Its role in Food Preservation

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2 Content INTRODUCTION. MOISTURE CONTENT AND WATER ACTIVITY. WATER ACTIVITY AND EQUILIBRIUM RELATIVE

HUMIDITY. EFFECT OF WATER ACTIVITY ON FOOD QUALITY. EMPERICAL EQUATIONS. CONCLUSION.

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4water32%

Solid68%

solid68%

water32%

MOISTURE CONTENT=

WET BASIS DRY BASISOR

water32%

Solid68%

water32%

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5 Introduction Water activity:-Water activity is a fundamental

property of aqueous solutions, and by definition it is the ratio of the water vapor pressure of food (p) to that of vapor pressure of pure water (p o) at the same temperature.(Chickhester et al. 1963)

Water activity ranges from zero (water absent) to 1.0 (pure water).

Water activity is a measure of how efficiently the water present can take part in a chemical reaction ,biochemical transfer or exchange through a semi permeable membrane.

Unbounded water is responsible for water activity of any food items.(Gustavo V Barbosa-Canovas et al.)

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6 Relationship between water content and water activity

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7 Relationship between Water activity and Equilibrium Relative Humidity(Gustavo V Barbosa-Canovas et al.)

aw=

aw=

The ERH of a food product is defined as the relative humidity of the air surrounding the food at which the product neither gains nor loses its natural moisture and is in equilibrium with the environment.

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8 EFFECT OF WATER ACTIVITY ON FOOD QUALITY(Labuza et al. 1972)

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11 Water activity and Shelf Stability

It can predict which microorganism will be the potential sources of spoilage and infestation.

The water activity of a food is instrumental in maintaining its chemical stability.

Every retail food establishment needs to know what will happen to their products as they sit on the shelf, even under ideal conditions of temperature and humidity.

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12 Prediction of Water activity in practical application

Slight reduction of water activity, control of pH ,mild heat treatment, addition of preservatives and anti browning additives were the factors selected to formulate the preservation procedures.(Gustavo V Barbosa-Canovas et al. 2007).

Water activity can be reduced using:- Water can be removed by a dehydration,

evaporation or concentration process. Adding additional solute. Phosphoric or Citric acids are generally used

to reduce the syrup’s pH so that the final pH of the fruit-syrup system is in equilibrium in the desired range(3.0-4.1).(Alzomora,S.E. et al. 1995).

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13 Emperical Equations

WATER ACTIVITY AT HIGH MOISTURE CONTENTS Raoult’s law is very accurate within the range of 0.95-1.0

water activity.It applies to solutions of small molecular weight compounds for which the calculated aw > 0.95.It is given by

For ideal solutions, the water activity is equal to the mole fraction of water, Xw.

aw = Xw

Xw=

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14 WATER ACTIVITY AT HIGH AND INTERMEDIATE MOISTURE CONTENTS

The Norrish equation is used for solutions, and is valid when the molecular weight and the Norrish k value are known. For a single solute, the Norrish equation reduces to:

aw = Xw × exp(−k i× Xi2)

ln =ki(1-Xw)2

where: aw = water activity; XW = mole fraction of water; Xi = mole fraction of solids in ingredient i; ki = Norrish constant for ingredient i. Norrish equation is reasonably accurate down to about 0.55.

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15 WATER ACTIVITY AT HIGH AND INTERMEDIATE MOISTURE CONTENTS

The water activity of a mixture involving several components can be determined from the product of the water activity of each component separately if all the water present in the mixture is mixed with individual components. Following equation was derived by Ross from Gibbs-Duhem Equation

aw=(aw)o1×(aw)o

2

(aw)0 represent the activity of water in a system with only one solute and water in the mixture.

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16 WATER ACTIVITY AT LOW MOISTURE CONTENTS

In 1938  Stephen Brunauer, Paul Emmett, and Edward Teller developed a model isotherm that takes that possibility into account. Their theory is called BET theory, after the initials in their last names. They modified Langmuir's mechanism and come to the derivation as:

where aw=Water activityX=The moisture content in g H2O/g dry matter.Xm=The mass fraction of water in the material equivalent to a unimolecular layer of

water covering the surface of each particle.C=C is a constant temperature and is related to the heat of adsorption of water on

the particles and is temperature dependent.

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17 WATER ACTIVITY AT LOW MOISTURE CONTENTS

Irving Langmuir was the first to derive a scientifically based adsorption isotherm in 1918. It is based on four assumptions:

I. All of the adsorption sites are equivalent and each site can only accommodate one molecule.

II. The surface is energetically homogeneous and adsorbed molecules do not interact.

III. There are no phase transitions.IV. At the maximum adsorption, only

a monolayer is formed. Adsorption only occurs on localized sites on the surface, not with other adsorbates.

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18 GUGGENHEIM-ANDERSON-DE BOER (GAB) EQUATION

The GAB model was found to be the most suitable for describing the sorption data. The monolayer moisture content was estimated using the Brunauer- Emmett-Teller (BET) equation. The BET model (Brunauer et al. 1938) gives the best fit to the data at aw of up to 0.5 (Bell and Labuza, 2000 and Roos 1995). The GAB equation has a similar form as that of BET, but has an extra constant, K. BET is actually a special case of GAB.

The ‘GAB’ (Guggenheim, Anderson, and de Boer) model is considered one of the most useful ones which in many simple sorption cases can be used for the approximation of the experimental data at water activity of up to 0.9 (Van den Berg 1984). ‘GAB’ equation is usually presented in the form (Biozt (1983)):

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19 GUGGENHEIM-ANDERSON-DE BOER (GAB) EQUATION

The GAB equation is a three-parameter equation with K, C, and Xm as constants. C and Xm have similar significance as in the BET equation. k is a third parameter that corrects for the difference in properties of adsorbed water relative to liquid water and permits the GAB equation to hold over a wider range of moisture content than the BET. Both K and C are temperature dependent.Multiple nonlinear regression techniques must be used to fit and evaluate the parameters for this equation from moisture-water activity data.

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21 Conclusion Water is the most abundant constituent of food and

in terms of food safety the most significant. Its presence, quantity and nature determines many chemical and biochemical processes important for the control of product safety and quality. A better understanding of water activity aw can assist in developing robust and scientifically supported food safety plans.

The binary combination of aw and pH acts as a relevant hurdle in many of these products preventing proliferation of pathogenic microorganism, while the rest (antimicrobials, thermal treatment etc) plays a secondary role mainly against spoilage flora.

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22 References

Sandulachi E.,Dr.,assoc.prof.,Technical University of Moldova ,Water activity concept and its role in food preservation.

05/02/2023By Ashish

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