L5 FOOD DECOMPOSITION Flashcards
What are physical, chemical, microbiological and other reasons for food decomposition?
Physical temperature humidity light physical damage
Chemical
enzymatic reactions
rancidity
oxidation reactions
Microbiological
bacteria
yeasts
molds
Others Insects Rodents Animals Birds
How do temperature humidity and light affect food?
Higher is the temp. and humidity faster will be food decomposition. Exposure of light also accelerates the food deterioration reaction.
How do chemical factors affect food?
Chemical factors such as presence of certain enzymes (enzymatic browning), hydrolytic rancidity (water in the lipids releases free fatty acids) or oxidative rancidity produces off flavours.
How do microbial factors affect food?
Higher microbiological load or favourable conditions to their growth also pose food safety issues.
Give examples of physical changes in food systems.
fat structure in chocolate changes and chocolates lose its snap and shine overtime period.
Slow freezing leads to big ice crystals in ice cream, loosing the soft texture of ice creams.
Bread stalls as starch molecules crystallise, bread needs water molecules if not network changes and becomes rigid.
Give some examples of chemical changes due to environmental or microbial factors.
Oxidative rancidity of unsaturated oils
Hydrolytic rancidity in oils
Presence of water breaks down triglycerides into free fatty acids and these free fatty acids produce off-flavours
Fish odour
What is randicity and what types of fats are the most prone?
Rancidity is a term used to describe the off flavours in foods caused due to the action of oxygen, water or light on fats and oils present in that particular food. Unsaturated fatty acids are more prone to oxidative rancidity that is the major cause of off flavours of fats and oils at the room temperature.
Explain the hydrolysis of fat, the factors that affect it and why free fatty acids spoil food.
Hydrolytic rancidity occurs in foods when the lipid (fat) is hydrolyzed by the water contained in food to fatty acids. Some of the liberated fatty acids are volatile and some have very unpleasant odours and flavours. Some of the factors that impact the fat hydrolysis are the presence of heat and enzyme lipase. Higher is the temperature of oils, faster is the hydrolysis reaction.
Draw the structure of a triglyceride being hydrolysed.
draw it :
Define oxidative randicity and autooxidation.
When rancidity occurs due to air, it is termed as Oxidative rancidity. The oxidation of acylglycerols which occurs in air, without the presence of enzymes, is called autoxidation.
Explain chemically the initiation step in oxidative randicity and draw the process.
An unsaturated lipid loses hydrogen and an electron to and oxygen molecule. The unsaturated lipid now becomes a lipid radical and the hydrogen + oxygen forms a hydroperoxy radical which later stabilises itself to form water.
RH or RH +O2 = R’
Draw
Explain chemically the propagation step and draw the process.
In the propagation step, the lipid radical can react with an O2 molecule adding it and forming a lipid peroxyl radical. This radical can then go on to react with another fatty acid taking one electron and hydrogen to form a hydroperoxide. The lipid which had its H removed now becomes a lipid radical and can continue to affect other lipid molecules.
R’+O2 = ROO’ (Lipid Peroxyl Radical)
ROO’ + RH = ROOH (Hydroperoxide) + R’
Explain chemically the termination step and draw the process.
Termination is whenever 2 radicals combine to form non-radical products.
F’+G’=F-G
Explain Lipid Oxidation step by step.
Initiation: Initiation of oxidation by singlet oxygen is an important aspect of oil oxidation. Activation of oxygen can be induced by electronic excitation
such as photosensitization, metals and natural pigments.
Propagation: The peroxy free radical reacts with an unattacked unsaturated fatty acid to form a fat hydroperoxide and an alkyl free radical (propagation). This product is tasteless and odourless. Hydroperoxide can break down into other molecules like aldehydes or ketones and acid which give oxidised fats and oils their rancid flavours.
The reaction continues until there is a depletion of oxygen or when a fatty radical reacts with a stable
antioxidant radical or when two unstable radicals react
What are the products of oxidation of fats?
Fats and Oils to Primary Oxidation products (hydroperoxide) to Secondary Oxidation products (aldehydes ketones) to Tertiary Oxidation Products (dimers, polymers).
