Chemical Composition Food: Water

Front 1

contains intramolecular polar covalent bonds

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Water

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3 Forms of Water

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Free water

Bound water

Water of hydration

Front 3

Is not a true bond. It is an IMFA.

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H Bond

Front 4

Directly correlated to shell life.

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Water Activity

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A part of water Potential

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Water Activity

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Helps two reactants meet

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Mobility Catalyzer

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2 Types of Moisture

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Wet Basis

Dry Basis

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Used as reference standard for ERH

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Salt Solution

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ERH

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Equilibrium Relative Humidity

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Significance of Water Activity

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It tells the direction of water movement.

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When is food equilibrium reached?

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When the food reaches a constant weight

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What is the formula for finding the moisture in dry basis?

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Moisture of water÷Moisture of Solid

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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.

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Hysteresis

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What is the formula in finding the moisture content of wet basis?

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Moisture of water ÷ Moisture sample

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A single layer of water molecules in the surface

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Concept of Monolayer Water

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They are less likely to have pathogenic microorganisms

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High Acid

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Occurs when there is irreversible changes in food

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Hysteresis

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Effects of polar covalent bonds on water

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Boiling point

Freezing point

Vapor pressure

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Represents the amount of water available for chemical and biochemical reactions

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Water Activity

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What is the formula for Water Activity?

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A = P/ P o = ERH

P = Vapor pressure of water in a food

P o = Vapor pressure of pure water

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Must be “available” in foods for the action of both chemical and enzymatic reactions.

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Water

Front 22

Based on a scale of 0.0 to 1.0

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Water Activity

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Water that can be easily removed from a food

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Free Water (Capillary Water/Type III)

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Water that can be utilized for microbial growth

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Free Water (Capillary Water/ Type III)

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Water that is tied up by the presence of soluble solids. Salts, vitamins, carbohydrates, proteins, emulsifiers, etc.

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Bound Water (Adsorbed Water/Type II)

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Water held in hydrated chemicals. Needs ultra high temperatures to be removed.

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Water of Hydration (Structured or Type I)

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How to “Control” water?

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You can either remove water (dehydration or concentration).

Convert the free water to bound water.

You can freeze the food.

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What happens when you remove water from food(dehydration or concentration)?

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Can change the physical nature of the food.

Alters the color, texture, and/or flavor.

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How do you convert free water to bound water?

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Through the addition of sugars, salts, or other water-soluble agents.

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What happens when you remove water from food(dehydration or concentration)?

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Can change the physical nature of the food.

Alters the color, texture, and/or flavor.

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Structure of Water

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Polar in nature, hydrogen bonding

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Can occur in many forms (S,L,V)

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Water

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Acts as a dispersing medium or solvent

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Water

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The “available” water represents the degree to which water in a food is free for:

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Chemical Reactions

Enzymatic Reactions

Microbial Growth

Quality Characteristics

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Quality Characteristics of Water in Food

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Related to a simple loss of moisture

Related to gel breakdown

Food texture (gain or loss)

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All foods have a water activity less than 1.0 because

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Any food substance added to water will lower water activity.

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Shows the relation between water activity and moisture content at a given temperature.

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Moisture Sorption Isotherms

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Shows the relation between water activity and moisture content at a given temperature.

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Moisture Sorption Isotherms

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What happens when you freeze food?

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Immobilizes the water which cannot be used by microorganisms.

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Things to consider when freezing food?

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Not all foods can be or should be frozen.

Frozen foods will eventually thaw, and the problem persists.

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Happens when a “wet” food is placed in a dry environment.

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Desorption

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Generally a slow process, with moisture gradually decreasing until it is in equilibrium with its environment.

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Desorption

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What happens in desorption?

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Food looses moisture

Analogous to dehydration

Aw decreases

Front 44

Are used to keep things moist

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Humectants

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Plays an important role in the dehydration process.

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Water Activity

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Happens when a “dry” food is placed in a wet environment

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Adsorption

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Water Activity with regard to adsorption

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As foods gain moisture, the Aw increases