Food Biochemistry and Food Processing (2 edition)

BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come

102 Part 1: Principles/Food Analysis

vapor, liquid, and solid phases or in solutions and food react and

interchange in any equilibrium system. The tendency to react

and interchange with each other is called thechemical potential,

μ. At equilibrium, the potential of water in all phases and forms

must be equal at a given temperature,T. The potential,μ,ofthe

gas phase may be expressed as:

μ=μw+RTln(p/pw),

where R is the gas constant (8.3145 J mol−^1 K−^1 ),pis the

partial water vapor pressure, andpwis the vapor pressure of

pure water atT. The ratiop/pwis called thewater activity aw

(=p/pw), although this term is also calledrelative vapor pres-

sure(Fennema and Tannenbaum 1996). The difference is small,

and for simplicity,awas defined is widely used for correlating

the stability of foods. For ideal solutions and for most moist

foods,awis less than unity (aw<1.0; Troller 1978).

The Clausius-Clapeyron equation, mentioned earlier, corre-

lates vapor pressure,P, heat of phase transition,
H, and tem-

perature,T. This same relationship can be applied to water ac-

tivity. Thus, the plot of ln(aw)versus 1/Tgives a straight line, at

least within a reasonable temperature range. Depending on the

initial value forawor moisture content, the slope differs slightly,

indicating the difference in the heat of phase transition due to

different water content.

Both water activity and relative humidity are fractions of the

pure-water vapor pressure. Water activity can be measured in

the same way as humidity. Water contents have a sigmoidal re-

lationship (Fig. 5.16). As water content increases,awincreases:

aw=1.0 for infinitely dilute solutions,aw>0.7 for dilute so-

lutions and moist foods, andaw<0.6 for dry foods. Of course,

the precise relationship depends on the food. In general, if the

water vapor in the atmosphere surrounding the food is greater

than the water activity of the food, water is adsorbed; otherwise,

desorption takes place. Water activity reflects the combined ef-

Figure 5.16.Nonequilibrium or hysteresis in desorption and

adsorption of water by foodstuff. Arbitrary scales are used to

illustrate the concept for a generic pattern.

fects of water-solute, water-surface, capillary, hydrophilic, and

hydrophobic interactions.

Water activity is a vital parameter for food monitoring. A

plot ofawversus water content is called anisotherm. However,

desorption and adsorption isotherms are different (Fig. 5.16)

because this is a nonequilibrium system. Note that isotherms in

most other literature plot water content againstaw, the reverse

of the axes of Figure 5.16, which is intended to show thatawis

a function of water content.

Water in food may be divided into tightly bound, loosely

bound, and nonbound waters. Dry foods contain tightly bound

(monolayer) water, andawrises slowly as water content in-

creases; but as loosely bound water increases,awincreases

rapidly and approaches 1.0 when nonbound water is present.

Crisp crackers get soggy after adsorbing water from moist air,

and soggy ones can be dried by heating or exposure to dry air.

Water activity affects the growth and multiplication of mi-

croorganisms. Whenaw<0.9, growth of most molds is inhib-

ited. Growth of yeasts and bacteria also depends onaw. Microor-

ganisms cease growing ifaw<0.6.In a system, all components

must be in equilibrium with one another, including all the mi-

croorganisms. Every type of organism is a component and a

phase of the system, due to its cells or membranes. If the water

activity of an organism is lower than that of the bulk food, water

will be absorbed, and thus the species will multiply and grow.

However, if the water activity of the organism is higher, the or-

ganism will dehydrate and become dormant or die.Thus, it is not

surprising that water activity affects the growth of various molds

and bacteria. By this token, humidity will have the same effect

on microorganisms in residences and buildings. Little packages

of drying agent are placed in sealed dry food to reduce vapor

pressure and prevent growth of bacteria that cause spoilage.

Aquatic Organisms and Drinking Water

Life originated in the water or oceans eons ago, and vast popula-

tions of the earliest unicellular living organisms still live in water

today. Photosynthesis by algae in oceans consumes more CO 2

than the photosynthesis by all plants on land. Diversity of phyla

(divisions) in the kingdoms of Fungi, Plantae, and Animalia live

in water, ranging from single-cell algae to mammals.

All life requires food or energy. Some living organisms re-

ceive their energy from the sun, whereas others get their energy

from chemical reactions. For example, the bacteriaThiobacillus

ferrooxidansderive energy by catalyzing the oxidation of iron

sulfide, FeS 2 , using water as the oxidant (Barret et al. 1939).

Chemical reactions provide energy for bacteria to sustain their

lives and to reproduce. Many organisms feed on other organisms,

forming a food chain. Factors affecting life in water include min-

erals, solubility of the mineral, acidity (pH), sunlight, dissolved

oxygen level, presence of ions, chemical equilibria, availability

of food, and electrochemical potentials of the material, among

others.

Water used directly in food processing or as food isdrink-

ing water, and aquatic organisms invisible to the naked eye

can be beneficial or harmful. TheHandbook of Drinking Wa-

ter Quality(De Zuane 1997) sets guidelines for water used in