CHAPTER 4: Food Chemistry
Functional Properties Physical and chemical properties of food molecules that affect
behaviour in foods during formulation, processing and storage. Include sensory and mechanical
- The dipole nature of water affects its physical characteristics like boiling point. Results in
Solubility Food molecules can form hydrogen bonds with water so they can dissolve or
Hydrophilic Compounds Compound’s that hydrogen bond easily to water to form solutions
or colloidal dispersions. Generally charged or polar.
Hydration Water molecules surround and interact with solutes by acting as a solvent
Amphiphilic molecules contain both hydrophilic and hydrophobic regions in their structures. In
water form micelles Clusters of molecules in which the hydrophobic groups are directed away
from the water, while the polar (charged) groups are exposed on external surfaced. The non-polar
hydrophobic groups form a stable inner core due to hydrophobic interactions. Micelles are
stabilized structures of amphiphilic molecules.
Known as noncovalent interaction and include hydrogen bonding, ionic interactions and
Presence of water in foods, is known as moisture content or as the water activity of food.
Moisture refers to absolute amount of water present in a food, while water activity has to do
with the form the water exists in.
Moisture Amount of water present in a food, as a component, relative to all the solid
Most water in foods is called free water. Free water is lightly entrapped, thus easily pressed
from food matter. Acts as a dispersing agents and solvent and can be removed by drying foods.
Absorbed Water Associates in layers vi intermolecular hydrogen bonds around hydrophilic
Bound Water (Water of hydration) is a third form of water in food. Exists in a tight
chemically bound situation, such as within crystalline structure, via water-ion and water-dipole
interactions. Does not show typical water properties, failing to freeze at 0 and fail to act as
solvent. Water Activity (A )wMeasure of the availability of water molecules to enter into microbial,
enzymatic or chemical reactions. Determines shelf life of food. Bound water is inversely related
to water activity, as % of bound water increases, water activity decreases.
A w P/P 0 P is the vapour pressure P 0s vapour pressure of pure water at same temp
Relative humidity: RH%= 100 x A w
Water sorption isotherms (MSI, moisture sorption isotherms) are graphs of data that interrelate
the water (moisture) of a food with its water activity at a constant temp. A sorption isotherm
indicates water activity at which a food is stable and allows predictions of the effect of changes
in moisture content on A awd storage stability.
Water activity will increase an increase in temperature. WSI plots are used to determine rate of
drying, frozen storage temp and moisture barrier properties required in food packaging materials.
Water as Component of Emulsion:
- Emulsion is a type of colloidal dispersion, system containing two liquids or phases that
normally do not mix: a dispersed phase and a continuous phase.
- Water as aq component can function in either a dispersed or continuous phase. Water
phase is hydrophilic while fat phase is lipophilic.
Water and Heat Transfer:
- Water molecules always possess kinetic energy as long as temp is above 0 Kelvin. With
the addition of heat energy, kinetic energy of water molecules increase. Water acts as a
conductor of thermal energy to food molecules, process called heat transfer.
Water as a Plasticizer:
- Water acts as a plasticizer, especially in low moisture and frozen foods. A plasticizer
when added to polymer food system, lowers the glass transition temperature (T ). Thg
glass transition temp refers to temp at which a change in the physciochemical state and
mobility of water and polymer of molecule constituent of a food occurs. A decrease in Tg
with an increase in water activity being linear.
- A plasticizer acts as a food system softener, increasing food polymer molecular volume
Food Acid Structure:
- A typical food acid is a carboxylic/organic acid, containing the carboxylic acid group
(COOH). Acids lacking carboxylic group are inorganic.
- Some foods that remain free-flowing have acids added to them because acids exhibit low
hygroscopicity, which means a low attraction for moisture. Without benefit of acid,
clumping of ingredients due to moisture would occur. Acid Strength:
- Food acids donate protons. Food acids are termed weak acids, which are mainly in form
of COO- + H . Strong acids have large amounts of dissociated ions.
- Weak acids have a small dissociation or ionization constant, K .aK ia given for ionization
reaction HA H +A- , where HA is unionized acid and H+ and A- are ionized species
derived from HA acid.
- pKa of an acid is also measure of strength. Lower pKa the stronger the acid.
Fumaric Acid and Dough Softening:
• When added to products like flour doughs, certain acids, like fumaric acid, can soften
o The carboxylic acid groups in fumaric acid establish what are called
“reducing conditions” in the dough, by donating H+
o Under such conditions, the hudrogen adds to the sulphur atoms present in
the disulfide bonds, converting them back to SH
o Therefore, fumaric acid acts as a reducing agent, by adding hydrogen to