Water activity converted

Water activity

1. Water Activity (aw)
P. Karthika
Assistant Professor
Department of Food Science and Nutrition
PeriyarUniversity, Salem -11

2. Introduction
•Allthefoodproductscontainvaryingamountof
waterintheirtissues.Variousqualitiesand
processingattributeslikedeterioration,dehydration,
spoilageetc.,inrawaswellasprocessedfoods,are
influencedbythewatercontent.Waterexistsin
threeforms:
•Assolid(ice)below0ºC
•Inliquidformaswaterbetween0ºCand100ºCand
•Asgasabove100ºCattheatmosphericpressure.

3. •The water present in foods interacts
with major hydrophilic nutrients such
as carbohydrates, proteins, water-
soluble vitamins, and minerals etc.
(polar hydrogen-bonding
interactions).
•The alkyl chains of fats and proteins
are hydrophobic in nature, and
therefore do not tend to interact with
water.

4. Definition
Water activity is defined as the ratio of the vapourpressure of
water in a food item (p) to the vapourpressure of pure water
(po) at the same temperature.

5. Role of Water Activity in food spoilage
Onthebasisofmoisturecontent,thefoodproductscanbebroadlyputinto
threecategoriesas:
•Lowmoisturefoodse.g.,driedorfreezedriedfoodshavingmoisturecontentof
5-15percent.
•Intermediatemoisturefoodse.g.,cakesanddateswithamoisturecontentof
20-40percent.
•Highmoisturefoodse.g.freshfruitsandvegetablesofgreaterthan40percent
ofmoisture.
Thoughmoisturecontentisimportant,itisthewateractivity(aw)thatis
criticalfactorfortheShelflifeofafooditem.Itmaybeusedtopredictstability
withrespecttophysicalpropertiesliketextureandcaking,ratesof
deteriorativereactionsandmicrobialgrowth.

6. Water Activity and Quality
•Mostbacteria,forexample,donotgrowatwateractivitiesbelow
0.91,includingpathogenssuchasClostridiumbotulinum.
•Below0.80mostmoldscannotbegrownandbelow0.60no
microbiologicalgrowthispossible.Theriskoffoodpoisoning
mustbeconsideredinlowacidfoods(pH>4.5)withawater
activitygreaterthanaw0.86.
•Staphylococcusaureus,acommonfoodpoisoningorganism,can
growdowntothisrelativelylowwateractivitylevel.
•Foodsthatmaysupportthegrowthofthisbacteriuminclude
cheeseandfermentedsausagesstoredabovecorrectrefrigeration
temperatures.

7. •measuringwateractivity-topredictwhich
microorganismswillandwillnotbepotentialsources
ofspoilage.
•wateractivity-determiningtheactivityof
enzymesandvitaminsinfoodsandcanhaveamajor
impacttheircolor,taste,andaroma.
•Stabilityandfoodsecuritydependsonwateractivity
andpHinthefoodenvironment.
•wateractivityishigherwiththeproductsare
perishable.

8. Aw Value Effect on Food
0.2–0.3A monolayer of moisture is formed. It represents the optimal moisture content for the
maximum shelf life of the dehydrated foods.
0.35–0.45The water is available outside the monolayer and the water phase requiring chemical
reactions begin to occur. This may cause changes in physical state, e.g., loss of crispness,
stickiness, and re-crystallization of amorphous state sugars.
0.4–0.5The soft materials like raisins etc., become hard due to drying out.
0.6 Considered a critical point where there is high potential for growth of microbes if the
moisture content increases.
0.6–0.8The rates of chemical reactions that require an aqueous phase increase and cause
deterioration of foods, reaching a maximum. The rates however fall at higher moisture
content.
0.85 Another critical point, above which bacterial spoilage occurs and pathogens begin to grow.
If we want to prevent the food spoilage or the action of microbes and maintain food quality,
the water activity of the food product needs to be lowered. Conventionally, the water
activity in foods has been controlled by drying, addition of sugar or salt, and by freezing.
The added salt or sugar dissolves in the free water and makes it bound or unavailable. In
other words, it decreases the water activity and makes the food less prone to spoilage.
Lowering the temperature checks the activity of the enzymes in the food and also makes
conditions unfavorable for the growth of microorganisms. Drying of the product acts as a
means of preserving the food by lowering the water activity.