Water Activity

Definition:

Wateractivity is a measure of how efficiently the water present can take part in achemical (physical) reaction. If half the water is so tightly bound to aprotein molecule that it could not take part in a hydrolysis reaction theoverall water activity would be reduced. Water activity (a_w) isdefined as

a_w=p/p_o

where p and p_o arethe partial pressures of water above the food and a pure solution underidentical conditions respectively. The tightly bound water has no tendency toescape from a food as a vapor and therefore exerts no partial pressure and hasan effective water activity of zero. Water activity is clearly a function ofcomposition but is also a function of temperature. The approximate wateractivities of some common foods are given below:

0.95	Fresh fruit, meat, milk
0.95-9	Cheese
0.9-0.85	Margarine,
0.85-0.8	Salted meats
0.8-0.75	Jam
0.75-0.65	Nuts
0.65-0.60	Honey
0.5	Pasta
0.3	Cookies
0.2	Dried veg., crackers

Free waterversus bound water

Wateractivity is sometimes defined as "free", "unbound", or"available water" in a system. Water activity instruments measure theamount of free (sometimes referred to as unbound or active) water present inthe sample. A portion of the total water content present in a product isstrongly bound to specific sites on the chemicals that comprise the product.These sites may include the hydroxyl groups of polysaccharides, the carbonyland amino groups of proteins, and other polar sites. Hydrogen bonds, ion-dipolebonds, and other strong chemical bonds tightly bound water. Some water is boundless tightly, but is still not available (as a solvent for water-soluble foodcomponents). Many preservation processes attempt to eliminate spoilage bylowering the availability of water to microorganisms. Reducing the amount offree--or unbound--water also minimizes other undesirable chemical changes thatoccur during storage. The processes used to reduce the amount of free water inconsumer products include techniques like concentration, dehydration and freezedrying. Freezing is another common approach to controlling spoilage. Water infrozen foods is in the form of ice crystals and therefore unavailable tomicroorganisms and for reactions with food components.

Zones of moistureisotherm

Zone 3: Bulk water,effectively a dilute solution, easily removed with minimal impact on foodstability

Zone 2: Loosely bound water,possibly additional layers bound to the Zone 1 water

Zone 1: tightly bound water,exceptionally hard to remove (i.e., needs very intense drying conditions).

Water Activityand Quality

Water activity is a critical factor thatdetermines shelf life (Fig. 8-10). Most bacteria, for example, do not grow atwater activities below 0.91, including pathogens such as Clostridiumbotulinum. Below 0.80 most moldscannot be grown and below 0.60 no microbiological growth is possible. However,there remain a number of food spoilage microbes that can grow within the range0.8 - 0.6. The risk of food poisoning must be considered in low acid foods (pH> 4.5) with a water activity greater than a_w 0.86. Staphylococcusaureus, a common food poisoningorganism, can grow down to this relatively low water activity level. Foods thatmay support the growth of this bacterium include cheese and fermented sausagesstored above correct refrigeration temperatures.

By measuring wateractivity, it is possible to predict which microorganisms will and will not bepotential sources of spoilage. In addition to influencing microbial spoilage,water activity can play a significant role in determining the activity ofenzymes and vitamins in foods and can have a major impact their color, taste,and aroma. It can also significantly impact the potency and consistency ofpharmaceuticals.

With the exception of lipidoxidation, all of the rates decrease at least 100 fold as the zone 2 water isremoved and effectively stop at the monolayer value. This is because whateverthe reagents responsible for a reaction, they always need a solvent to movearound in order to encounter each other and react. As the solvent is removedthe rate decreases and, as monolayer water is not adequately liquid-like to actas a solvent, the reaction stops. Several rates may slightly decrease at highwater activities due to dilution of the reagents.

SorptionIsotherms

Therelationship between water content and water activity is complex. An increasein aw is almost always accompanied by an increase in the water content, but ina nonlinear trend. This relationship between water activity and moisturecontent at a given temperature is called the moisture sorption isotherm. Thesecurves are determined experimentally. Moisture sorption isotherms are sigmoidalin shape for most foods, and a moisture sorption isotherm prepared byadsorption (starting from the dry state) will not necessarily be the same as anisotherm prepared by desorption (starting from the wet state). This phenomenonof different a_w vsmoisture values by the two methods is called moisture sorption hysteresis andis exhibited by many foods. Hysteresis represents the difference in a_wbetween the absorption and desorption isotherms (Figure 8-6).

Ahydrated food can be dehydrated to remove moisture until the desired a_wis reached (desorption) or completely dehydrated and then re-hydrated to thedesired a_w (absorption). A food is more stable against microbialspoilage when its a_w is adjusted by absorption rather than bydesorption

.

Absorption and desorptionisotherms for a food system at a given temperature are plotted using a_walong the horizontal axis and the % water along the vertical. When plotted, thephenomenon of hysteresis can be observed (Fig. 8-6).

Overview ofthe Laboratory Exercise

Theobjective of this lab exercise is to collect the raw data necessary toconstruct moisture sorption and desorption isotherms for a food product and toestimate the initial water activity of a food product if a water activity meteror hydrometer is not available.

To collect the necessary datayou will also have to perform oven drying techniques on the food sample toassess the initial moisture content of the product. In addition, we will use awater activity meter to directly measure the water activity of the product. Youwill compare and contrast the data for each product and each method used todetermine a_w.

Saturatedsalt solutions will be used to create a specific relative humidity within aclosed environment. Food samples will be placed in these environments andallowed to equilibrate.

Atotally dehydrated food (crackers equilibrated over drierite) and anintermediate moisture food (crackers with increased moisture equilibrated overK₂SO₄) will be studied in five different relativehumidity environments using five different saturated salt solutions. Since equilibrium relative humidity(ERH) and a_w are related, saturated salt solutions will be used todetermine the ERH of the five "environments" we will study. The foods with two different wateractivities will be placed in these environments and changes in their moisturecontent will be followed.

Outline of SorptionIsotherm Exercise

Theinitial moisture content of crackers will be measured using an oven dryingtechnique. Also, the water activity of the food product will be measured usinga water activity meter. Prepare tables in your lab notebooks to record ALL datayou will need to complete the lab report both from your own lab group and otherlab groups.

Reagents	Equipment
dried crackers (decreased moisture content → equilibrium with drierite)*	10 mL graduate cylinder
ÒmoistÓ crackers (increased moisture content → equilibrium with K2SO4)**	desicators
crackers (original moisture content)	microwave oven
potassium acetate	Vacuum oven
Mg(NO3) 2¥6H2O	small plastic cups
NaCl	aluminum weigh cups
KCl	weighing pads for use in microwave
KNO3	water activity meter
	water activity sample containers

* %RH <1; ** %RH = 97.5

PROCEDURE

ERH Chambers

Calculate the amount of eachsalt needed to prepare a saturated solution of 5.0 ml volume given thesolubility constants below. Usingthese values prepare 5 ml of a saturated salt solution in water to makerelative humidity chambers with the following %RH:

solute	solubility g/100ml H2O	g/5 mL H2O	%RH
K-acetate	320		22.5
Mg(NO3)2¥6H2O	426		52.0
NaCl	37		75.5
KCl	40		84.5
KNO3	25		93.0

1. Prepare 5 ERH chambers as follows

· * label chambers with thesolute, sample name, and your group number

· * put calculated amount ofsalt and 5 mL of water into the corresponding chamber

· * cut 3triangles/incisions in the bottom of a plastic cup

· * invert the plastic cupin the chamber to use as a sample stand

· * cover the chamber andseal with the lid

Over time, the corresponding ERH will be reachedwithin the enclosed environment

2. Dehydrated food sample(dried crackers)

· weigh an aluminum panand record the weight of the empty pan

· calculate the weight ofthe pan + 2.5 g

· weigh 2.5 g of driedcrackers into the pan and record the actual weight of the pan + sample

· place the sample in theappropriate ERH

· repeat for 4 othersamples, recording in your labbook the weight of each pan + sample going intoeach ERH chamber

· in next week lab periodyou will reweigh your samples to get the final weight of pan + sample

3. Intermediate Moisture Food (crackers with increased moisture content)

· weigh an aluminum panand record the weight of the empty pan

· calculate the weight ofthe pan + 4.0 g

· weigh 4 g of ÓincreasedmoistureÓ crackers into pan and record the actual weight of the pan + sample

· mash the sample with aglass rod to increase surface area after weighing

· place the sample in theappropriate ERH

· repeat for 4 othersamples, recording in your labbook the weight of each pan + sample going intoeach ERH chamber

· in next week lab periodyou will reweigh your samples to get the final weight of pan + sample

Important: Be sureto note in your labbook the balance that you used and use the same balance forall portions of the experiment.

Initial MoistureContent

Initial moisture content ofthe sample will be determined using either the vacuum oven or the microwaveoven drying techniques.

· weigh 1.0 g of "normal"cracker sample and record the value

· press the crackers toincrease surface area

· place the sample in theoven and dry it

· record dried weight of crackers+ weighing pads

Water Activity, a_w

· weigh 2.0 g of sample

· place sample in smallplastic container

· open the drawer of thewater activity meter,

· insert your sample,close and turn the knob to the "read" position

· allow sample toequilibrate

· read a_w andrecord in your lab book

Definition of Terms

a_w=p/p₀= %RH/100

p =vapor pressure of water in food

p₀= partial pressure of water at same temperature

w_ti = initial sample weight

w_tf = sample weight after adjustment to the selectedwater activity

w_td = drysample weight = (1-X_water)w_ti

x_water = decimal fraction of H2O in sample

%moisture final, dry weight = w_{ti -} w_{td /} w_td

%moisture change, dry weight basis = w_{tf -} w_ti/ w_td

Data to be recorded in lab-book:

Raw Data pan weight

initialpan + sample weight

finalpan + sample weight

% water initial in "normal" crackers

a_wof dried crackers

a_wof "increased moisture" crackers

Calculated Data

Crackers: initialweight and % moisture (dry basis)

Dried crackers: initial,and final sample weights (w_ti, w_tf, w_td)

%moisture final on a dry weight basis

ÒIncreased moistureÓ crackers: initial,dry and final sample weights (w_ti, w_tf, w_td)

%moisture final on a dry weight basis

%moisture change on a dry weight basis

Graphs

% moisture change (dry weight) vs. a_w (sorptionand desorption curves)

QUESTIONS

1. Determine the initial a_wof the crackers from the graph of % moisture change vs a_w. Discuss the stability of this productagainst microbial growth.

2. Define saturated saltsolution.

3. Why do absorption anddesorption curves of a moisture isotherm differ?

Sample Data