Oils and Fats Flashcards

Flashcards covering the chemical mechanisms of autoxidation, hydrolytic rancidity, oil extraction, refining, and physical property manipulation (hydrogenation and interesterification) based on the lecture transcript.

What is the specific chemical mechanism of non-enzymic oxidative rancidity in vegetable oils?

Non-enzymic oxidative rancidity is a free-radical chain reaction called autoxidation, which occurs at the carbon-carbon double bonds ($-HC=CH-$) of unsaturated fatty acids.

How is singlet oxygen ($¹O_2$) generated during the initiation stage of autoxidation?

It is generated from triplet oxygen ($³O_2$) by heat, light, or metal catalysis.

How much faster does singlet oxygen ($¹O_2$) react compared to triplet oxygen ($³O_2$)?

$1,500$ times faster.

What occurs during the Propagation stage of autoxidation?

A carbon radical reacts with atmospheric oxygen to form a peroxy radical ($ROO•$), which then abstracts a hydrogen atom from another fatty acid to form a hydroperoxide ($ROOH$) and a new radical ($R•$).

Which compounds are responsible for the characteristic rancid off-flavours and odours produced during the decomposition of hydroperoxides?

Short-chain carbonyl compounds such as aldehydes, ketones, and secondary alcohols.

What carcinogenic substances are produced in animals when deep-fat frying accelerates the decomposition of hydroperoxides?

Polycyclic aromatic hydrocarbons.

How do phenolic antioxidants ($AOx$) inhibit the process of oxidative rancidity?

They interrupt the propagation step by donating a hydrogen atom to the peroxy radical ($ROO•$), forming an antioxidant radical ($A•$) that is stabilised by resonance delocalisation.

List three common synthetic phenolic antioxidants used commercially and their typical usage limit.

Butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and propyl gallate (PG), used at up to $200\,ppm$.

How do carotenoids like $\beta$-carotene and lycopene act as natural antioxidants?

They quench singlet oxygen, converting $¹O_2 \rightarrow ³O_2$.

What is the role of synergists like citric acid in protecting vegetable oils?

They chelate pro-oxidant metals such as $Fe$ and $Cu$ to prevent metal-catalysed initiation and enhance antioxidant action.

What chemical reaction defines hydrolytic rancidity in food lipids?

Lipase enzymes de-esterify triacylglycerols ($TG$) into free fatty acids ($FFA$) and glycerol.

Why is butter more susceptible to noticeable hydrolytic rancidity than beef fat?

Butter contains high concentrations of short-chain fatty acids ($SCFA, C4–C12$) like butyric acid ($C4:0$), which have low odour thresholds; beef fat contains long-chain fatty acids ($C16, C18$) that are less volatile.

In which specific foods is controlled hydrolytic rancidity considered a desirable effect for flavour development?

Blue-veined cheeses (Roquefort and Stilton), Cheddar cheese, and some fermented sausages.

At what temperature can lipoxygenases remain active in plant foods like soybeans and wheat?

They remain active even at $-18\,^{\circ}C$.

Why is blanching essential before freezing vegetables?

To deactivate lipoxygenases which cause off-flavours (carbonyl compounds) at the methylenic groups of polyunsaturated fatty acids.

What are the four primary steps in the extraction of oils by pressing?

1. Conditioning (moisture adjustment), 2. Cleaning, 3. Crushing, and 4. Cooking ($\sim 70\,^{\circ}C$).

What are the specific temperature and product requirements for cold pressing olive oil?

The temperature must be below $27\,^{\circ}C$ (< 27\,^{\circ}C) for extra virgin olive oil.

Which solvent is typically used in the solvent extraction of low-oil seeds like soybeans?

Hexane or a light petroleum fraction.

What is the purpose of the Degumming step in oil refining?

To remove resins, gums, proteins, and phosphatides by adding phosphoric acid ($H_3PO_4$) or brine.

How are free fatty acids ($FFA$) and trace heavy metals like $Fe$ and $Cu$ removed during the Neutralising stage of oil refining?

By spraying in dilute $NaOH$ or $KOH$, which converts $FFA$ into soaps and neutralises residual acids.

What materials are used in the Bleaching stage to adsorb pigments and oxidative products?

Fuller's earth or activated carbon.

What is the mechanism of the Deodorisation stage in oil refining?

Vacuum steam distillation at $170\text{--}230\,^{\circ}C$ to remove volatile off-flavour compounds.

What are the standard industrial conditions for the hydrogenation of vegetable oils?

$0.01\text{--}0.2\%$ Ni catalyst, $160\text{--}220\,^{\circ}C$, and $2\text{--}10$ atmospheres of $H_2$ pressure.

What is the primary chemical change during hydrogenation?

The addition of $H_2$ across double bonds, converting $-CH=CH-$ to $-CH_2-CH_2-$, which increases the melting point.

What is a significant nutritionally undesirable consequence of partial hydrogenation?

The formation of trans fatty acids (isomerisation) due to high temperatures.

State the melting point change when oleic acid is converted to elaidic acid during hydrogenation.

Oleic acid ($C18:1$ cis) has a m.p. of $10.5\,^{\circ}C$, while elaidic acid ($C18:1$ trans) has a m.p. of $43.7\,^{\circ}C$.

What is the purpose of winterising vegetable oils?

To prevent cloudiness in oils (like cottonseed oil) used for salad dressings by holding them at $\sim 5\,^{\circ}C$ and filtering out crystallised saturated fatty acid fractions.

How does interesterification differ from hydrogenation in terms of saturation?

Interesterification rearranges fatty acids within and between triacylglycerol molecules without altering the degree of unsaturation or adding $H_2$.

What are the conditions required for random chemical interesterification?

A sodium methoxide catalyst and temperatures between $50\text{--}200\,^{\circ}C$.

What is directed or controlled interesterification?

An enzymatic process using specific lipases that target particular positions on the triacylglycerol, such as the $sn-2$ position.