Week 2 - Water, Physical and Chemical Nature of Food

FST1800

What are the major chemical changes in food?

1. Lipid oxidation and hydrolysis

2. Enzymatic and non-enzymatic browning

3. Denaturation and hydrolysis of protein

What affect do chemical changes have on food?

• colour

• flavour

• texture

• taste

• nutritional content

Food browning can be divided into___?

1.Non-enzymatic browning

• Maillard browning

• Caramelization

• Ascorbic acid browning

2. Enzymatic browning —> undesirable

Maillard reaction occurs between___ +_____?

carbohydrates (reducing sugar mainly D-glucose) and amino group of protein

• When heated or stored together for some time

Roles of water in food

• texture

• Chemical reactions - hydrolysis of carbs and proteins

• Growth of microorganisms

• Conduct heat

• Mobilization of large macromolecules

Between what phase transitions does amorphous glass exist in?

Solid and liquid

Describe amorphous materials

• non crystalline solids

• lack long range order - random arrangement

• no sharp melting point

• no latent heat involved

• Undergoes second order phase transition

What occurs at the second order phase transition?

• solid (glassy state) to rubbery state at glass transition temperature, Tg

Explain the phrase isothermal phase transition

• Liquid and gas coexists

• Iso(constant) Thermal (temperature)

First order phase transition

• changes occur isothermally

• Isothermal phase transition line between different states

Addition of solutes effect on freezing point and boiling point

• depresses the freezing point

• Increases the boiling point (Boiling Point elevation)

Factors that influence the boiling point

Temperature at which vapour pressure has just exceeded atmospheric pressure.

• addition of solute

• Altitude

How and what does the addition of a solute e.g NaCl effect the boiling point of a substance

Vapour pressure is lowered, increase in boiling point.

• Partially negative oxygen atoms are attracted to the sodium atoms

• Partially positive hydrogen atoms are attracted to the chloride atoms

• ion surrounded by water molecules and held by ion-dipole interactions \

• less water molecules can escape into the air

Explain how altitude effects the boiling point of substances

Higher altitude —> Atmospheric pressure decreases due to gravity

• easier for vapour pressure to exceed atmospheric pressure (boil)

Types of water

1. Free water —> Squeezed out of food, promotes growth of microorganisms

2. Bound water

• Attracted to the surface of molecules like polysaccharides and proteins

What type of water causes the spoilage of foods

Free water

What is moisture content?

Bound + free water

• (weight before x weight after) / weight before x 100

• Not a reliable predictor of microbial response and chemical reactions

• same moisture content may still differ in perishibility

What is water activity (aw) ?

Indicator of how much free water *ONLY FREE WATER!!!

• energy status or escaping tendency of water into a sample

• if easy for water to escape (aw HIGH) —> shelf life short

aw = VP of H2O in sample/VP of pure H2O

• aw scale 0 (no water) to 1 (pure water)

what aw can microorganisms not produce at

aw<0.5

Moisture migration

• only refer to WATER ACTIVITY!

• Move from high to low water activity

Altering water activity

• drying

• dehydration

• Dry infusion (coating)

• Direct formulation (add humectant - component that absorbs moisture)

Explain enzymatic browning

• oxidation of polyphenolic compounds present in food

1. Oxidation of polyphenolic compounds to form quinones(highly reactive)

2. Polymerisation of quinones —> form insoluble brown pigments - melanin

3 factors required for enzymatic browning

• Polyphenolic compounds

• Active enzyme oxidase —> PPO, polyphenol oxidase (in food)

• Oxygen from air

What affects the rate of enzymatic browning

• concentration of PPO, polyphenolic compounds, pH, temperature and O2

Explain caramelization

—> heating of sugar above its boiling point without nitrogen-containing compounds (facilitated by small amounts of acid and salts)

1. Inversion: Dissaccharide breaks to form monosaccharide

2. Dehydration

3. Polymerization

Diacetyl (buttery), Furan (nutty), Acetadehyde (rum)

Explain ascorbic acid browning

—> brown discolouration in citrus fruit juices and concentrate during storage or heating

• degredation of ascorbic acid

• reaction with degraded product with amino acid release co2

Lipid oxidation and hydrolysis

• oxygen reacts with double bond of unsaturated fatty acids

• forms hydroperoxide (unstable) which is broken down into small, volatile molecules

• aldehyde, ketones, alcohol, acid —> rancid flavours

Autoxidation

• promoted by heat, light, metal, enzymes (lipoxygenases)

Lipid hydrolysis

Interaction of lipid with water

• FFA free from glycerol and forms a rancid smell

• Catalysed by heat and enzyme lipase

ways of heat transfer (7)

1. Conduction

2. Convection

3. Mechanical energy

4. Mass transfer

5. Radiation

6. microwave

7. Ohmic heating

Conduction

Energy is transferred from one molecule to adjacent molecule

Convection

Circulation of currents of hot air and cold air

Mechanical energy

Energy transferred through physical movement

Mass transfer

• movement of food component into or out of heated food

Radiation

direct transfer of heat energy from heat source to food

—> heat transferred as EM waves

Irradiation —> exposing food to ionizing radiation

• Gamma

• X-ray (bombardment of e)

• Microwave - heating thru vibration of water, fats or sugar

-microwave heat dipoles —> molecules vibrate —> heat

Ohmic heating

• alternating current flow between 2 electrons

• rapid and uniform heating

• useful for proteinaceous food

• prevent degredation of flavour