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Water, with a density of 1000 kg m^-3, is denser than the oil components of foods;

oils and fats typically have densities in the range 850–950 kg m^-3. Glycerols and sugar solutions are denser than water.

Unlike the solid phase of most other liquids, ice is less dense than liquid water;

ice has lower thermal conductivity than water. These properties have an effect on the freezing of foods that are predominantly water based; the formation of an ice layer on the surface of liquids and the outside of solids has the effect of slowing down the freezing rate.

Because a molecule of water vapor is lighter (molecular weight = 18) than that of dry air (molecular weight = about 29), moist air is lighter than dry air at the same temperature. This is somewhat unexpected, because the popular conception is that humid air (which contains more water) is heavier than dry air.

At room temperature, water has the highest specific heat of any inorganic or organic compound, with the sole exception of ammonia. It is interesting to spec-ulate why the most commonly occurring substance on this planet should have one of the highest specific heats. One of the consequences of this peculiarity in the food industry is that heating and cooling operations for essentially water-based foods are more energy demanding. To heat a kilogram of water from 20 to 50°C requires about 125 kJ of energy, whereas heating the same mass of vegetable oil requires only 44 kJ.

A sponge holds most of its water as liquid held in the intestacies of the sponge structure. Most of the water can be wrung out of the sponge, leaving a matrix of air and damp fibers. Within the sponge fibers the residual water is more tenuously held

— absorbed within the fiber of the sponge.

If the sponge is left to dry in the sun, this adsorbed water will evaporate, leaving only a small proportion of water bound chemically to the salts and to the cellulose of the sponge fibers. Like water in sponge, water is held in food by various physical and chemical mechanisms (Table 3.1). It is a convenient oversimplification to dis-tinguish between “free” and “bound” water. The definition of bound water in such a classification poses problems. Fennema (1985) reports seven different definitions of bound water. Some of these definitions are based on the freezability of the “bound”

component, and others rely on its availability as a solvent. He prefers a definition in which bound water is “that which exists in the vicinity of solutes and other non-aqueous constituents, exhibits reduced molecular activity and other significantly altered properties as compared with ‘bulk water’ in the same system, and does not freeze at –40°C.”

The moisture content can be measured simply by weighing a sample and then oven drying it, usually at 105°C overnight; the difference in mass is the moisture content in the original sample. However, much confusion is caused by reporting the moisture content simply as a percentage without specifying the basis of the calcu-lation. It should be made clear whether the moisture content is calculated on a wet basis (moisture content divided by original mass) or on a dry basis (moisture content

divided by the “bone dry” or “oven dry” mass). Even the term bone dry mass can cause confusion among non-English speakers; it was once misinterpreted as the

“mass of the dry bones.” For example, in foods containing significant quantities of fat or salt, the moisture content may be calculated as the mass of water in a sample divided by the dry solids that are not salt or fat; in this case, the moisture content should be reported as calculated on a salt-free, fat-free, dry basis.

3.3.2 SORPTION ISOTHERMS AND WATER ACTIVITY

3.3.2.1 Principle

Since 1929 it has been recognized that the chemical and microbial stability, and thus the shelf life, of foods is not directly related to its moisture content, but to a property called water activity (Tomkins, 1929). Essentially, water activity is a measure of the degree to which water is bound within the food, and thus unavailable for further chemical or microbial activity.

Water activity is defined as the ratio of the partial pressure of water vapor in or around food to that of pure water at the same temperature. Relative humidity of moist air is defined in the same way, except that by convention, relative humidity is reported as a percentage, whereas water activity is expressed as a fraction. Thus if a sample of meat sausage is sealed within an airtight container, the humidity of the air in the headspace will rise and eventually equilibrate to a relative humidity of say 83%, which means that the water activity (a_w) of the meat sausage is 0.83.

The relationship between water activity and moisture content for most foods at a particular temperature is a sigmoidal-shaped curve called the sorption isotherm (Figure 3.10). The term equilibrium moisture content curve is also used. Sorption

TABLE 3.1

Classification of Water States in Foods

Class of

Water Description

Porportion of Typical 90% (Wet Basis) Moisture Content Food Constitutional An integral part of nonaqueous constituent <0.03%

Vicinal Bound water that strongly acts with specific hydrophilic sites of nonaqueous constituents to form a monolayer coverage; water–ion and water–dipole bonds

0.1–0.9%

Multilayer Bound water that forms several additional layers around hydrophilic groups;

water–water and water–solute hydrogen bonds

1–5%

Free Flow is unimpeded; properties close to those of dilute salt solutions; water–water bonds predominate

5% to about 96%

Entrapped Free water held within matrix or gel that impedes flow

5% to about 96%

isotherms at different temperatures can be calculated using the Clausius–Clapeyron equation from classical thermodynamics:

(3.5)

where a_w is the water activity, T is the absolute temperature, ∆H is the heat of sorption, and R is the gas constant.

A complication arises from one of the methods of measuring sorption isotherms for food. Food that has previously been dried and then is rehydrated will have a different sorption isotherm (adsorption isotherm) from that which is in the process of drying (desorption isotherm). This difference is due to a change in water-binding capacity in foods that have been previously dried.

Many mathematical descriptors for sorption isotherms have been proposed. One of the more famous is that of Brunauer et al. (1938), the B.E.T. isotherm, which is based on the concept of a measurable amount of monomolecular layer (vicinal) water for a particular food. Wolf et al. (1985) compiled 2201 references on sorption isotherm data for foods. An example of the type, detail, and accuracy of sorption isotherm data available in the literature is presented in Table 3.2.

Iglesias et al. (1975) proposed the following three-parameter equation to fit sorption isotherm data for a range of foods:

a_w = exp(–a' θ^r)

where a' and r are the parameters listed in Table 3.2, and θ = X/Xm. FIGURE 3.10 A typical sorption isotherm for food.

Desorption

Adsorption

Water activity

Moisture content (dry basis)

0.2 0.4 0.6 0.8 1.0 4.0

3.0

2.0

1.0

d(lna_w) d 1 T( ⁄ ) --- ∆H

---R

=

X is the equilibrium moisture content and X_m, in units of g/100 g dry basis, is the B.E.T.

monomolecular moisture content for the food listed in Table 3.2. However, there are nearly as many equations to sorption isotherms as there are researchers in this field.

3.3.2.2 Measurement of Water Activity

Many methods of measuring water activity have been developed by researchers.

These include direct vapor pressure measurement, equilibration with a stable hygro-scopic substance that has a known sorption behavior, and various types of hygrom-eters (Doe, 1998).

Water activity is most conveniently measured by the measurement of relative humidity in the headspace over a food sample in a sealed container. Commercially available instruments for water activity determination use various methods for measuring the relative humidity: hair hygrometer (Lluft), electrical hygrometer (Nova Sina), and dew point temperature (Aqualab). The hygrometer-based instru-ments are prone to drift and must be calibrated regularly against saturated solutions of various inorganic salts. Hygrometer-based instruments are also prone to hys-teresis at high humidities.

The Aqualab CX2 water activity meter (Decagon Devices Inc., Pullman, WA) detects water condensation on a chilled mirror (dew point temperature). The instru-ment is sensitive to water activity units of <0.001. Readings take 5 min or less and are accurate to ±0.003 water activity units.

Care must be taken with any measurement of water activity to ensure that the sample is representative of the food under test. Dried fish, for example, will have moisture and salt contents, and thus water activity, varying widely from thin, exposed flesh to the relatively moist interior. If the worst-case scenario for the growth of potentially toxic or spoilage organisms is of interest, the sample of flesh for water activity determination should be excised from the thickest, most moist region of the fish.

TABLE 3.2

Sorption Isotherm Data for Cod and Corn

Product

Temperature

(°C) X_m r a'

Cod^a 30 7.68 1.2398 1.3490

Corn^b 4.5 8.30 2.2345 1.9748

15.5 7.68 2.4862 2.0949

30 7.30 2.5663 1.7950

38 6.35 2.3711 1.8618

50 6.89 2.1203 1.5936

60 5.11 2.2185 1.7430

a Adsorption, after Jason (1958).

bDesorption, after Chen and Clayton (1971).

Source: From Iglesias, H.A. et al., J. Food Technol., 10, 289, 1975.

3.3.3 W^ATER A^{CTIVITY AND} S^HELF L^{IFE OF} F^OODS

Many of the chemical and biological processes that cause deterioration of foods — and ultimately spoilage — are water dependent. Microbial growth is directly linked to water activity. No microbes can multiply at a water activity below 0.6. Dehydration is arguably the oldest form of food preservation; the sun drying of meat and fish has been traced to the beginning of recorded history. Drying relies on removing water, thus making it unavailable for microbial growth.

Salting or curing has the same effect. A saturated solution of common salt has a water activity of close to 0.75. Thus, by adding sufficient salt to foods, the water activity can be lowered to a level where most pathogenic bacteria are inactivated, but the moisture content remains high.

Intermediate moisture content foods (IMFs) such as pet food and continental sausages rely on fats and water-binding humectants such as glycerol to lower water activity. Fat, which is essentially hydrophobic, does not bind water, but acts as a filler for IMFs to increase the volume of the product.

The effect of several humectants is for each to sequester an amount of water independently of the other humectants that may be present in the food. Each thus lowers the water activity of the system according to the equation of Ross (1975):

a_wn = a_w0 · a _w1 · a _w2 · a _w3 · etc.

where a_wn is the water activity of the complex food system, and a _w0 etc. are the water activities associated with each component of the system.

For example, the water activity of a food with a moisture content of 77% (wet basis) and a salt content of 3% (wet basis) can be calculated as follows: 100 g of the food comprises 77 g of water, 20 g of bone dry matter, and 3 g of salt. The contribution to the water activity due to the salt can be calculated (according to Raoult’s law of dilute solutions), using the molecular weights of water (18) and salt (58.5), as:

a _w1 = (77 × 18)/(77 × 18 + 3 × 58.5) = 0.89

The water activity for the salt-free solid matter of the food is found from its sorption isotherm at that moisture content, e.g., a _w0 = 0.90. Thus the water activity of the salted food is:

a _wn = a _w0 · a _w1 = 0.9 × 0.89 = 0.8

None of the dangerous pathogenic bacteria associated with food, such as Clostridium or Vibrio spp., which cause botulism and cholera, can multiply at water activity values below about 0.9. Thus drying or providing sufficient water-binding humectants is an effective method of preventing the growth of food-poisoning bacteria. Only osmophilic yeasts and some molds can grow at water activities in the range 0.6–0.65. Thus, by reducing the water activity below this value, foods are microbially stable — unless the packaging is such that the food becomes locally

wet again, in which case local spoilage can occur, e.g., when condensation occurs within a hermetically sealed package subjected to rapid cooling.

There are various chemical reactions that proceed and may be accelerated at low values of water activity. Maillard reactions leading to lysine loss and brown color develop peaks at a_w values around 0.5–0.8. Nonenzymatic lipid oxidation increases rapidly below a_w = 0.4. Enzymic hydrolysis decreases with water activity to a_w = 0.3, after which, it is negligible.

3.4 WATER SUPPLY, QUALITY, AND DISPOSAL