April 9, 2010

Free and Bound Water

Free and Bound Water
Water is abundant in all living things and consequently, in almost all foods, unless steps have been take to remove it.

Most natural foods contain water up to 70% of their weight or greater unless they are dehydrated, and fruits and vegetables contain water up to 90% or greater.

Water that can be extracted easily from foods by squeezing or cutting or pressing is known as free water, whereas water that cannot be extracted easily is termed as bound water.

Bound water is usually defined in terms of the ways it is measured; different methods of measurement give different values for bound water in a particular food.

Many food constituents can bind or hold water molecules such that they cannot be easily removed and they do not behave like liquid water. Some characteristics of bound water include:
  • It is not free to act as a solvent for salts and sugars
  • It can be frozen only at very low temperatures (below the freezing point of water).
  • It exhibits essentially no vapor pressure
  • Its density is greater than that of free water
Bound water has more structural bonding than liquid or free water thus it is unable to act as a solvent.

As the vapor pressure is negligible the molecules cannot escape as vapor; and the molecules in bound water are more closely packed than in the liquid state. So the density is greater.

An example of bound water is the water present in cacti or pine tree needles – the water cannot be squeezed or pressed out; extreme dessert heat or a winter freeze does not negatively affect bound water and the vegetation remains alive.

Even upon dehydration food contains bound water.
Free and Bound Water

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