NFS386 Lecture 2: Water
As a constituent of food (part of other foods)
As a beverage
Water as a constituent of food
Water and Ice
Differences in structures implications for food quality (freezing affects quality
Availability of water in a food system (for chemical reactions) implications for
food quality (e.g. raising keeps for long time; water not available for reactions)
Amount of water available for chemical reactions (e.g. microbial growth)
Ice has an ordered hexagonal structure: crystal lattice structure.
The structure of liquid water is described by the mixture model: Water retains some
hexagonal structure but is more disordered. “Ice like” clusters.
When water freezes, it increases in volume.
Density of water 1g/mL
Density of ice 0.92g/mL
Density of water > Density of ice
Ice formation has 2 steps
a. Initiation of ice crystal formation
2. Crystal growth
a. Increase in the size of the ice crystal
The size of ice crystals depends on the rate of cooling.
As the rate of cooling increases
o Number of nuclei increases
o Decreased size of crystals because of more nuclei
FAST cooling = more small nuclei
Freeze concentration: as water freezes, solutes are concentrated in unfrozen zones.
Small crystals dilute solute
Large crystals solute moved to regions outside crystal, more concentrated in
Freeze concentrations slow chemical reactions off-flavors.
Freezing will damage the cellular structure of plant/animal foods
Fruits/vegetables/raw meat have high water/moisture content.
During freezing, extracellular water freezes first, drawing water by osmosis out of
intracellular space. Large crystals/slow shrinkage of cell
Small crystals/fast less shrinkage of cell
Water moves from intracellular extracellular
Thawing drip loss
Properly freeze: cut into thin slices (reduces time cold penetrates), blanch before freezing.
Freezing promotes two opposing forces
o Slow chemical reaction rates
o Slow growth of microbes, but not completely kill
Freeze concentration increases reaction rates
Inadvisable to refreeze food
Increases level of microbes
Freezer burn: ice at surface undergoes sublimation
o ice water vapor
o solid gas
o food dries at surface
o prevented by proper packaging
measure ability of water in food to participate in chemical reactions and support
the growth of micro-organisms
predictor of shelf life
depends on water interaction with other food constituents, not just amount of
increased water activity = increased reactions = increased microbial growth
depends on amount of water in food * interaction with other food
How water associates with other food constituents
o H-bond to polar components of food
o Does not behave like solvent
o Unavailable for freezing, chemical reactions
o Can not support growth of micro-organisms
o Physically entrapped in food structure
o Behave like solvent
o Available for freezing, chemical reactions, supports growth of micro-
Continuum: types of bound water Constitutional water
o Bound to interior of molecule (e.g. Inside fold of protein)
o Bound to ou