1 FOOD 2400
Unit 1: Water
Unit 2: Lipids
Unit 3: Carbohydrates
Unit 1: Water
List some unique chemical and physical properties of water and ice
high specific heat, large difference in density b/w water and ice
high melting and boiling temperature, surface tension, capillary action, high heat of
freezes more rapidly than it thaws
Describe some interaction of water with other molecules
Water binding/hydration depends on substances hydrophilic nature, considering salt,
composition, pH and temp.
water holding capacity - eg gels of pectin and starch - syneresis.
Define what is meant by water activity'
Water activity is the relationship between water content and perishability. Not the same
as water content - simply an indicator of how much water is available to reactions and
Aw 1.0 = roughly 50% water.
Sorption Isotherm is a plot of Aw vs water content.
Explain the 3 regions represented in a sorption isotherm
2 FOOD 2400
A) Adsorption of a monomolecular (Langmuir) layer - most strongly absorbed, not
freezable- almost part of the solid.
B) adsorption of additional layers of water - Multilayer and bonded to itself.
C) condensation of water in capillaries and pores of the material (Less strongly bound,
most mobile. this is the bulk phase of water. Freezable)
Understand the importance of sorption isotherms to food stability
Section C allows for microbial growth and chemical reactions at greater rates.
Basically, an indication of how free water is to undergo reaction and crystallization.
Define what is meant by hysteresis'
Hysteresis depends on the nature of the food. It is the effect that the adsorption and
desorption curves of a given food are not often superimposable. The best explanation is
the ink-bottle theory. During adsorption, the body of the capillary fills first, causing a
more steep curve. During desorption, the neck of the capillary empties first, causing a
more gradual curve.
Explain the differences between bound water and free water in food systems
Bound water is the water that remains unfrozen at some prescribed temperature below
0C, usually -20C. Bound water is also the amount of water in a system that is
unavailable as a solvent. Free water, on the other hand, is freezable at any temperature
below 0C and is a solvent.
Explain the process of ice crystallization
Nucleation occurs rapidly at temperatures much lower than the freezing temperature -
Crystallization occurs rapidly at temperatures close to the freezing point.
Explain the significance of nucleation and seeding' during ice crystal formation
Seeding can be used to increase the number of initial nuclei present and to encourage a
finer crystalline structure. It involves the addition of nuclei to liquids prior to freezing.
Explain the effect of cooling rate on ice crystal formation
Fast cooling - high rate of nucleation, slow crystal growth, fine crystal structure.
Slow cooling - low rate of nucleation, fast crystal growth rate, fewer/larger ice crystals.
Describe the changes which occur in foods during frozen storage; specifically freeze
concentration and ice crystal damage.
Freeze concentration - concentration of non-aqueous constituents in the unfrozen
phase. Concentration of these causes changes in pH, and nonenzymatic reactions - an
increase in certain reaction rates.
Decompartmentalization - Cellular disruption occurs because of increase in volume
from water to ice. This can break cells, releasing contents (enzymes) that can cause
chemical and structural changes.3 FOOD 2400
Recrystallization - Ice crystals enlarge because tiniest crystals melt first.Therefore,
temperature fluctuations are especially damaging to frozen foods - form larger crystals
which can damage the food.
Explain the concept of glass formation in foods and how this impacts on food stability
Glass formation is not the same as freezing. A glass is a very viscous fluid. Glass
transition temperature is much lower than the crystallization temperature. At high
moisture contents, when the temperature is lowered, the system can go through a
detrimental rubbery state. A glass can form in a partially frozen food. The glass
immobilizes dissolved solids, and reduces problems with freeze concentration. This is
known as cryoprotection.
Explain the significance of water activity in relation to microbial growth/inhibition, chemical
reactions, enzyme activity, and packaging requirements.
Water activity is closely related to food spoilage. IMF foods have moisture content b/w
20-40%, and have water activities above 0.5. They usually have their Aw decreased by