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Water activity and food stability

Bound water

Solutes (salts)

Hydrate salts

Sugars Hydrophylic bonds

Gels

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Water activity derives fromprinciples of thermodynamicsand physical chemistry.

m = mo + RT ln (f/fo)

- m (J mol-1) is the chemicalpotential i.e. energy per mole ofsubstance;

- mo is the chemicalpotential of the purematerial at thetemperature T (°K);

- R is the gas constant(8.314 J mol-1 K-1) ;

- f is the fugacity or theescaping tendency of asubstance; fo = pure subst.

-The activity of a species isdefined as a = f/fo

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In the case of water, a subscript is designated for the substance,

aw = f/fo

aw is activity of water, or the escaping tendency of water insystem divided by the escaping tendency of pure water. Forpractical purposes, the fugacity is closely approximated by thevapor pressure (f ~ p) so;

aw = f/fo ~ p/po


Also : aw = p/po = ERH (%) / 100

Water activity is a measure of the energy status of the waterin a system. Colligative effects of dissolved species (e.g. saltor sugar) interact with water through dipole-dipole, ionic, and

hydrogen bonds. Capillary effects, hydrogen bondingbetween water molecules, surface interactions with chemicalgroups on undissolved ingredients (e.g. starches and proteins)through dipole-dipole forces, ionic bonds (H3O+ or OH-) and

van der Waals forces (hydrophobic bonds). It is acombination of these factors in a food product that reduces

the energy of the water and thus reduces the relative humidityas compared to pure water.

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Why is water activity important?Water activity (aw) is one of the most critical factors indetermining quality and safety of foods.

Water activity affects the shelf life, safety, texture,flavor, and smell of foods.

While temperature, pH and several other factors caninfluence if and how fast organisms will grow in aproduct, water activity may be the most importantfactor in controlling spoilage.

Most bacteria, for example, do not grow at wateractivities below 0.91, and most molds cease to grow atwater activities below 0.80.

By measuring water activity, it is possible to predictwhich microorganisms will and will not be potentialsources of spoilage.

Water activity--not water content--determines thelower limit of available water for microbial growth. Inaddition to influencing microbial spoilage, wateractivity can play a significant role in determining theactivity of enzymes and vitamins in foods and can havea major impact their color, taste, and aroma.


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Minimum aw for some microorganisms

BacteriaStaphylococcus aureus 0.86halophilic bacteria(Halobacterium spp.) 0.75

MoldsAspergillus flavus 0.78Chrysosporium fastidium 0.69Xeromyces bisporus 0.61

YeastsDebaryomyces hansenii 0.83Torulopsis spp. 0.70Zygosaccharomyces bailii 0.80Zygosaccharomyces rouxii 0.62


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Examples of aw value of several foods

fresh, raw fruits, vegetables, meat, fish > 0.98cooked meat, bread 0.91 - .095cured meats, cheeses 0.91 - .095fermented sausages (some) 0.83 - 0.87jams 0.75 - 0.80honey 0.75dry cereals (some) 0.65 - 0.75pastry fillings 0.65 - 0.71candies 0.60 - 0.65sugars, syrups 0.60 - 0.75cake and pastries 0.60 - 0.90dried fruits 0.60 - 0.75powdered milk, dried pasta, spices 0.20 - 0.60


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Controlling aw in foods•equilibration with atmosphere of knownrelative humidity•water removal (e.g., dehydration)•addition of solutes (humectants)

•sugars•NaCl•polyhydric alcohols (glycerol,sorbitol)•propylene glycol

loss or gain of moisture in packaged foods


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•Measurement•electric hygrometers = consist ofpotentiometer, sample holder, andsensor with immobilized electrolye(e.g., lithium chloride); changes inERH are reflected in changes inconductance of current throughsensor (Beckman, Rotronic); typicallyslow and requires routine calibrationwith standards•dew point instruments = use acooled mirror as condensing surface;mirror is cooled --> condensationoccurs --> temperature = dew point;ERH is derived from psychrometricchart (automatically)

•very fast and accurate•calibration is not needed

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Hurdle Technology Several years ago, hurdletechnology was developed as anew concept for the realization ofsafe, stable, nutritious, tasty, andeconomical foods. It employs theintelligent combination of differentpreservation factors or techniquesto achieve multi-target, mild butreliable preservation effects.

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Preservation factors are hurdlesto inhibit microorganisms.

Hurdle technology deliberatelycombines existing and newpreservation techniques to establisha series of preservative factors(hurdles) that the microorganismsin question are unable to overcome(jump over). These hurdles may betemperature, water activity, acidity,redox potential, preservatives, andothers.


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An increasing list of hurdles

Apart from the most important andcommonly used hurdles, there aremany others of potential value.

Other hurdles include:

ultrahigh pressure, thermo-sonication,photodynamic inactivation, modifiedatmosphere packaging of both non-respiring and respiring products,edible coatings, ethanol, maillardreaction products and bacteriocins.Examples of foods preserved bycombined processes are fruit juicesand heat-processed, cured meatproducts.


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