Week 6 -8 CHM2962

What is chelation?

The binding of a metal ion (M) by a ligand (L) to form a metal complex (ML): M + L ⇌ ML

What ring size is ideal for a stable metal-chelate complex?

Stable 5- or 6-membered rings

Give two common food chelating agents

EDTA and citrate

Why are metal chelators used in food preservation?

They bind free metals (Ca, Mg, Cu, Fe) and prevent metal-catalysed oxidation reactions

Which metal chelator is used in margarine, mayonnaise, soups and cheese?

Citric acid (E330)

What are antioxidants (in food chemistry)?

Substances added to food to delay onset or slow the rate of oxidation of oxidisable food substrates

Which macronutrient class is most susceptible to oxidative deterioration and why?

Lipids - lipid hydroperoxides can be oxidised by metal ions to form radicals, which break down into volatile aldehydes, ketones and short-chain carboxylic acids that cause rancid odours/flavours

What are the three phases of oxidative rancidity (autoxidation)?

1. Initiation (radicals formed), 2. Propagation (radicals react and generate more radicals), 3. Termination (radicals combine and the chain reaction stops)

Write the initiation step of lipid autoxidation

RH + •OH → R• + H2O

Write the propagation steps of lipid autoxidation

R• + O2 → ROO•; ROO• + RH → ROOH + R•

Write the termination step of lipid autoxidation

ROO• + ROOH → stable end products

How do metal ions (e.g. Fe2+/Fe3+) accelerate lipid oxidation?

They oxidise/reduce stable lipid hydroperoxides (ROOH) to generate new radicals (ROO• or RO•), e.g. ROOH + Fe3+ → ROO• + H+ + Fe2+

What do BHA and BHT stand for?

BHA = butylated hydroxyanisole (E320); BHT = butylated hydroxytoluene (E321)

How do BHA and BHT act as antioxidants?

They act as free radical scavengers/H-donors, reacting with peroxyl radicals (ROO•) rather than R•, interrupting the radical chain reaction

Write the general antioxidant mechanism with AH

ROO• + AH → ROOH + A•

Why are BHA/BHT effective antioxidants?

Their aromatic (phenolic) rings stabilise the resulting radical by resonance delocalisation, making the radicals less likely to propagate new reactions; weaker A-H bonds allow easier H donation

High-yield relationship for antioxidant effectiveness

A weaker A-H bond means easier H donation and a more effective antioxidant

What is sodium erythorbate (E316) used for?

A water-soluble antioxidant used in meat curing; accelerates nitrite curing, improves flavour stability, reduces oxidation/darkening, and helps prevent nitrosamine formation

What are nitrates/nitrites used for in food?

Antimicrobial curing agents in bacon and ham; they prevent growth of Clostridium botulinum

What is the major safety concern with nitrite curing?

Nitrite can react with amines to form nitrosamines (R2NH + NaNO2 → R2N-N=O), some of which are carcinogenic and can react with DNA/enzymes

What is SO2 (E220) used for in food?

A preservative, antiseptic, and antioxidant, mainly in winemaking; controls microbes including wild yeast and preserves dried fruit

Why is sodium benzoate used instead of benzoic acid?

Benzoic acid has low water solubility (~0.27% at 18°C) while sodium benzoate is much more soluble (~66% at 20°C)

What structural property of organochlorine pesticides (e.g. DDT) makes them prone to bioaccumulation?

Their high lipophilicity (fat solubility) and chemical stability/resistance to breakdown, allowing them to accumulate in fatty tissue up the food chain

What are fungicides?

Pesticides that kill or prevent the growth of fungi and their spores (e.g. to control rusts, mildews, blights); examples include Captan and Mancozeb; copper and sulphur have long been used as fungicides

What is fumigation?

A pest control method where a pesticide gas/vapor (e.g. methyl bromide, phosphine) is released into air or soil to kill pests such as termites and bedbugs

Which form of mercury bioaccumulates in the food web and why?

Methylmercury (CH3Hg+) - it is produced by bacteria from inorganic mercury and readily taken up and retained by fish/organisms, biomagnifying up the food chain

Give three examples of naturally occurring pesticides

Pyrethrins (from chrysanthemums), nicotine, rotenone (Derris powder)

What is tetrodotoxin and its LD50?

A naturally occurring neurotoxin (e.g. in pufferfish), LD50 300 µg/kg

What is saxitoxin and its LD50?

A potent neurotoxin produced by certain algae (associated with shellfish poisoning), LD50 5 µg/kg

What is solanine and what two classes of biomolecules does it comprise?

A toxic phytochemical (glycoalkaloid) found in potatoes, comprised of a steroidal alkaloid backbone linked to a carbohydrate (sugar) chain

Why doesn't cooking destroy solanine?

Solanine is thermally stable up to around 260°C, so boiling, cooking or microwaving does not break it down

Why do organisms produce natural toxins?

As a chemical defence mechanism to deter predators/herbivores or competitors (protective/evolutionary role)

What factors affect the sensitivity/LOD of instrumental analysis for food contaminants?

Whether qualitative screening or quantitative measurement is needed, the instrument's inherent Limit of Detection, use of pre-concentration methods (e.g. HS-SPME, solvent extraction), volatility and hydrophilicity/hydrophobicity of the analyte, and availability/cost of standards

Low-MW sugars vs high-MW polysaccharides: how do they lower water activity differently?

Low-MW sugars (sucrose, glucose) hydrogen-bond with and immobilise free water, lowering aw (e.g. jams); polysaccharides (starch, gums) work via hydrodynamic volume, entanglement and mechanical trapping of water in gel networks

Why is cellulose insoluble in water?

Its extended, flat-ribbon beta(1-4)-linked glucose chains form extensive rigid intra/intermolecular hydrogen bonds, creating tightly packed crystalline microfibrils that exclude water

Why do arabinoxylans show variable solubility?

Unsubstituted backbones allow chain-chain alignment and are insoluble; heavily substituted backbones (with arabinose/glucuronic acid branches) sterically hinder inter-chain association, increasing solubility

Describe amylose structure and solution behaviour

Linear alpha(1-4)-linked glucan; the alpha configuration creates a kink causing single helical turns (~6 glucose residues/turn); linear nature allows strong polymer-polymer association over time (retrogradation)

Describe beta-glucan structure and solution behaviour

Unbranched mixed-linkage homopolysaccharide with beta(1-3) and beta(1-4) links forming a worm-like random coil; allows high water binding, solubility and viscosity

Describe amylopectin structure and its effect on water-holding

Highly branched via alpha(1-6) linkages; branch points interrupt regular chain ordering, preventing crystallisation and enhancing water-holding capacity and viscosity

What are the three primary mechanisms of polysaccharide gelation?

1. Electrostatic/ionic gelation (egg-box model), 2. Hydrophobic gelation, 3. Aggregation/physical entanglement

What is the defining, exam-relevant property of polysaccharide gels vs protein gels?

Polysaccharide gels are thermoreversible (melt on heating); protein gels are typically heat-set/irreversible

What are the three primary types of water-protein interactions?

Water-peptide/neutral group H-bonding, water-charged group ion-dipole hydration shells, and hydrophobic clathrates (structured water cages around non-polar side chains)

What intrinsic protein factors control solution viscosity?

Size & shape (larger/elongated proteins increase drag) and net charge (higher charge expands the protein and traps more water)

What protein-solvent and protein-protein factors control viscosity?

Higher solubility traps more water (raising viscosity); protein-protein aggregation forms larger effective particles, making the solution thicker

Describe the mechanism of heat-induced protein gelation

Partial denaturation exposes interior hydrophobic residues and reactive -SH groups; unfolded proteins associate via hydrophobic interactions, hydrogen bonding, electrostatic attraction and disulfide bonds, forming a continuous 3D viscoelastic network

How do proteins stabilise foam in beer?

They act as surfactants, migrating to the bubble boundary, unfolding to position hydrophobic parts into the gas and hydrophilic parts into the liquid, then cross-linking to form an elastic film preventing bubble popping/merging

Why is tempering required in chocolate manufacturing?

Cocoa butter is polymorphous; tempering selectively promotes stable beta crystals, giving chocolate a glossy finish, clean snap, high melting point and resistance to fat bloom

What happens if chocolate is left untempered?

It cools into unstable polymorphs (Forms I-IV) that transform into beta-prime crystals over time, causing dull/soft texture, poor snap, gritty mouthfeel and fat bloom (white hazy surface)

What is the exam-key mechanistic chain for denaturation leading to interfacial stabilisation?

Denaturation → loss of native structure → exposed hydrophobic patches → migration to air-water or oil-water interface → stabilisation of foam/emulsion

What food example combines hydrocolloid (water-holding) and surfactant (foam) protein functions?

Milkshake

How does fatty acid saturation/geometry affect crystallisation and melting point?

Saturated fatty acids with no kinks pack tightly, forming large crystals with high melting points; unsaturated (especially cis double bond) fatty acids have kinks that disrupt packing, giving lower melting points

What is polymorphism in the context of fats?

Fat crystals exist in different polymorphic forms with different packing/stacking and stability, and can transition between forms

Give a worked example of fat-soluble vitamin partitioning into oil

Palm fruit is rich in Vitamins A and E (hydrophobic); these partition into the oil fraction during processing, giving red palm oil its colour

Why does protein concentration control the "printability" of 3D-printed food gel ink?

Protein concentration is the main parameter controlling the gel's textural/rheological properties, determining whether the extruded material can hold its shape (entrap water and form a suitable gel)

What is food rheology?

The study of deformation and flow of matter in food

Give the texture spectrum example used for 3D-printed food gels

Gummy bear (chewy) vs fried egg white (brittle) - two extremes of food gel texture

What is a food additive vs a food contaminant?

An additive is intentionally added to a food for a functional purpose; a contaminant is introduced unintentionally (e.g. via production, processing, or environment) or occurs naturally and is not intended to be present

What are the two main sources of food colour?

Natural (e.g. carotenoids-orange, chlorophyllin-green, anthocyanins-purple, betanin-red) and synthetic (produced through specific chemical reactions, e.g. Quinoline Yellow WS, Patent Blue V, Green S)

What wavelength range can humans see, and what lies outside it?

Approximately 360-780 nm; below 360 nm is ultraviolet, above 780 nm is infrared

What three things are required for colour perception?

A source of illumination, an object that interacts with light, and the human eye/brain to interpret the light

List five molecular origins of colour

Atomic excitation, vibrational/rotational transitions, ligand-field effects (d-orbitals in transition metals), molecular-orbital transitions (e.g. π→π* in organics), band theory (metals/semiconductors)

What determines an object's perceived colour?

The combined effects of light absorption and scattering; an object appears as the complementary colour of the wavelength(s) it absorbs

What is the difference between rods and cones in the eye?

Rods work in low light, aid night vision, and don't detect colour (greyscale); cones need brighter light, detect colour, and humans have three types each responding most strongly to a different wavelength range

What food colour E-number range covers colours?

E100-E199 (includes both natural and synthetic colours)

What gives carotene (E160a) its colour and solubility properties?

A highly conjugated hydrocarbon structure allows visible light absorption; it lacks polar functional groups, making it highly lipophilic/fat-soluble

What is the structure of chlorophyll (E140)?

A metalloporphyrin structure with magnesium at its centre

What happens to chlorophyll's colour during cooking under acidic conditions?

Magnesium is displaced from the porphyrin ring, changing the colour from bright green to dull grey-green or yellow

What determines a dye's "hue"?

The wavelengths of light absorbed; wavelengths not absorbed are transmitted/reflected, producing the perceived complementary colour

What determines a dye's "strength", and what law measures it?

A dye's ability to absorb light, measured via the Beer-Lambert law: A = εcl (A=absorbance, ε=molar extinction coefficient, c=concentration, l=path length)

What determines a dye's "brightness"?

The shape of the UV-visible absorption band: a narrow band gives a bright/vivid colour, a broad band gives a dull colour (e.g. brown, olive green)

Why do food dyes with conjugated structures absorb visible light?

Conjugated (alternating double bond) systems allow π→π* electronic transitions that absorb light in the visible range; the wavelengths absorbed vs transmitted determine the observed colour

What is fluorescence?

A molecule absorbs energy, enters an excited state, and quickly emits light returning to the ground state; some energy is lost non-radiatively, so emitted light has lower energy/longer wavelength than absorbed light

What is phosphorescence and how does it differ from fluorescence?

Light emission following an intersystem crossing to a triplet excited state; it occurs more slowly than fluorescence because the transition back to the singlet ground state is spin-forbidden

What causes the ester ethyl butanoate to form, and what flavour is it associated with?

Reaction of butanoic acid with ethanol (esterification, releasing water); associated with pineapple flavour

What is sourness mainly associated with in food?

Food acids (acidulants), which contain one or more carboxyl groups (-COOH)

List the main functions of acidulants in food

Enhance flavour, assist protein gel formation (e.g. yoghurt), control pH and inhibit microbial growth, chelate trace metal ions, support antioxidant activity, reduce non-enzymatic Maillard browning

Is sourness determined by pH alone?

No - organic acids (e.g. citric acid) can taste more sour than HCl at the same pH; sourness intensity is influenced by hydrogen ions, acid anions, and undissociated acid molecules

What three main compounds produce umami taste?

Glutamate, 5'-inosinate, and 5'-guanylate

What type of sensation is pungency, and what does it produce?

A chemesthetic sensation (not a basic taste); produces hot, sharp or stinging sensations

Name five pungent compounds and their common sources

Capsaicin (chilli), piperine (black pepper), gingerols/shogaols (ginger), eugenol (cloves)

How is pungency measured?

Using the Scoville scale, expressed in Scoville Heat Units (SHU)

Why does water worsen chilli heat, and why does milk help?

Capsaicin is poorly soluble in water, so water spreads it around; milk's fat and casein proteins can dissolve/remove capsaicin from taste receptors

With reference to its structure, why does capsaicin's taste linger in the mouth?

Its long hydrophobic tail allows it to embed in lipid membranes/receptor sites and its poor water solubility means it is not easily washed away by saliva, so it persists on receptors

What does a balanced vs imbalanced charge distribution produce in sweet-tasting molecules?

Balanced charge distribution produces sweetness; imbalance produces both sweet and bitter sensation

Differentiate nutritive and non-nutritive sweeteners

Nutritive sweeteners (sucrose, glucose, fructose) are absorbed and provide energy; non-nutritive sweeteners (saccharin, aspartame) provide little/no energy and are intensely sweet

What is the "triangle of sweetness" / glycophore concept?

Sweet-tasting substances contain a glycophore - appropriately arranged proton-donor and proton-acceptor groups that hydrogen-bond with sweet-taste receptors; their arrangement, size and shape determine sweetness intensity, and the compound must be water-soluble

What is the reference standard for relative sweetness?

Sucrose, with a relative sweetness value of 100%

Do naturally occurring flavours in fresh or cooked food require E numbers?

No - e.g. orange flavour or caramel occurring naturally does not require an E number