Chemicals in Food

• How food preservatives work — dehydration, antimicrobial, and antioxidant mechanisms
• Role of antioxidants (BHA/BHT, Vitamins C and E) as free-radical scavengers
• Classification and relative sweetness of artificial sweeteners; phenylketonuria (PKU) constraint
• Distinction between natural and synthetic food colours with examples
• Emulsifiers and the HLB (Hydrophilic-Lipophilic Balance) concept

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Food Preservatives

Preservatives prevent spoilage caused by microorganisms (bacteria, fungi) and chemical oxidation.

Sodium benzoate is permitted up to 0.1% in food. Most effective in acidic foods (pH < 4.5) — used in jams, squashes, pickles.

K₂S₂O₅ + H₂O → 2KHSO₃ → releases SO₂, which acts as both antimicrobial agent and antioxidant.

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Antioxidants

Prevent rancidity (oxidation of fats/oils) which produces off-flavours and toxic peroxides.

Mechanism — Free Radical Scavenging:

Fat + O₂ → R• (free radical) → ROO• (peroxy radical) → rancidity

Antioxidant (AH) + ROO• → ROOH + A•  (antioxidant radical — stable, non-reactive)

BHA and BHT are phenolic compounds — the phenolic –OH group donates H• to quench peroxy radicals.

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Artificial Sweeteners

Used in place of sugar; provide sweetness without significant caloric value.

PKU (Phenylketonuria): Genetic disorder where phenylalanine cannot be metabolised. Aspartame contains phenylalanine → dangerous for PKU patients. Products must carry warning label.

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Food Colours

Synthetic food dyes are regulated by FSSAI (India) and FDA (USA). Maximum permitted levels are strictly defined.

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Emulsifiers

Emulsifiers stabilise mixtures of immiscible liquids (e.g., oil + water) in food products.

How they work: Emulsifiers have both hydrophilic (water-loving) and hydrophobic (fat-loving) ends — they orient at the oil-water interface to reduce interfacial tension.

HLB (Hydrophilic-Lipophilic Balance):

• Scale: 1–20
• HLB 1–6 → water-in-oil emulsions (e.g., margarine)
• HLB 8–18 → oil-in-water emulsions (e.g., milk, salad dressings)
• Higher HLB = more hydrophilic

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Why BHA/BHT are Effective — Phenolic Radical Stability

Phenolic antioxidant: Ar–OH

Reaction: Ar–OH + ROO• → Ar–O• + ROOH

The Ar–O• (phenoxy radical) is RESONANCE-STABILISED by the aromatic ring
→ cannot propagate the oxidation chain
→ chain termination → rancidity prevented

The bulky tert-butyl groups in BHA/BHT prevent the phenoxy radical from reacting further — steric protection combined with resonance stabilisation.

Aspartame Stability — Temperature Dependence

Aspartame is a dipeptide methyl ester (L-aspartyl-L-phenylalanine methyl ester). At high temperatures (>150°C), the ester bond hydrolyses and the aspartyl linkage cyclises → loss of sweetness and formation of diketopiperazine. This is why aspartame-sweetened products (diet sodas) carry "not for cooking/baking" labels.

Sucralose vs Sugar — Structural Difference

Sucralose = 1,6-dichloro-1,6-dideoxy-β-D-fructofuranosyl-4-chloro-4-deoxy-α-D-galactopyranoside (simplified: sucrose with 3 –OH replaced by –Cl). The Cl substitution:

1. Prevents digestion by intestinal enzymes → zero calories
2. Enhances sweetness (electronegativity effect on taste receptor binding)
3. Increases thermal stability (C–Cl bond stronger than C–OH)

Sodium Benzoate Antimicrobial Mechanism

In acidic media: C₆H₅COONa + H⁺ → C₆H₅COOH (benzoic acid, undissociated)

Undissociated benzoic acid crosses bacterial cell membrane → inside neutral bacterial cytoplasm → dissociates → releases H⁺ → lowers intracellular pH → inhibits glycolysis enzymes → microbial death.

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Example 1: Identify the correct sweetener for different scenarios

Problem: Which sweetener should be chosen for each case?
(a) A diabetic patient who needs to bake cookies at 180°C
(b) A child diagnosed with PKU needing a low-calorie soft drink
(c) A product requiring extremely high sweetness (2000× sugar) for cost efficiency

Step 1: Case (a) — baking at 180°C
  → Aspartame: EXCLUDED — unstable above ~150°C, loses sweetness
  → Sucralose: SUITABLE — thermally stable, 600× sweetness, zero calorie
  Answer (a): Sucralose

Step 2: Case (b) — PKU patient
  → Aspartame: EXCLUDED — contains phenylalanine, toxic for PKU
  → Saccharin: SUITABLE — no phenylalanine, excreted unchanged
  → Sucralose: also SUITABLE
  Answer (b): Saccharin or Sucralose (NOT aspartame)

Step 3: Case (c) — maximum sweetness per gram
  → Alitame: ~2000× sweeter than sucrose
  → Smallest quantity needed → most cost-efficient
  Answer (c): Alitame

Summary:
  Baking         → Sucralose (thermally stable)
  PKU patients   → Saccharin or Sucralose (no phenylalanine)
  Max sweetness  → Alitame (2000× sucrose)

Example 2: Mechanism of antioxidant action of BHT

Problem: Explain how BHT prevents rancidity of a fat-containing food product.

Step 1: Initiation of rancidity
  Unsaturated fat + O₂ + heat/light →
  Fat radical (R•) formed
  R• + O₂ → ROO• (peroxy radical) — very reactive

Step 2: Propagation (without antioxidant)
  ROO• + RH → ROOH + R•  (chain reaction continues)
  Accumulation of ROOH → rancid odour and toxic products

Step 3: Chain termination by BHT
  BHT = 2,6-di-tert-butyl-4-methylphenol (phenolic compound)
  
  BHT–OH + ROO• → BHT–O• + ROOH
  
  BHT–O• is resonance-stabilised (delocalised over aromatic ring)
  + steric protection from tert-butyl groups
  → cannot propagate chain
  → CHAIN TERMINATION → rancidity halted

Step 4: Why fat-soluble BHT works in fatty foods
  BHT is fat-soluble → dissolves uniformly in fat phase
  → accessible to fat radicals where oxidation occurs
  
Answer: BHT donates H• to peroxy radical, forming a stable
phenoxy radical that terminates the oxidation chain, thereby
preventing rancidity.

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Q1. Which artificial sweetener is contraindicated for patients with phenylketonuria (PKU)?

A) Saccharin
B) Sucralose
C) Alitame
D) Aspartame

Q2. BHT acts as an antioxidant by:

A) Chelating metal ions that catalyse oxidation
B) Donating hydrogen atoms to peroxy radicals, terminating the chain
C) Absorbing UV light that initiates rancidity
D) Increasing water activity in food

Q3. The HLB value of an emulsifier determines:

A) Its melting point in the food product
B) Whether it forms oil-in-water or water-in-oil emulsions
C) Its relative sweetness compared to sucrose
D) Its antimicrobial efficacy

Q4. Sodium benzoate is most effective as a preservative when:

A) Added to alkaline foods like baked goods
B) Used in neutral pH dairy products
C) Used in acidic foods like fruit juices and jams
D) Combined with sucrose in confectionery

Q5. Which of the following antioxidants is water-soluble?

A) BHA
B) BHT
C) Vitamin E
D) Vitamin C

Answer Key: 1-D | 2-B | 3-B | 4-C | 5-D

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• NCERT Class 12 Chemistry — Chapter 16: Chemistry in Everyday Life
• Pages 486–492 (chemicals in food)
• Key sections: 16.3 Chemicals in food (preservatives, antioxidants, sweeteners)
• Table 16.2: Artificial sweeteners and relative sweetness
• Cross-reference with Class 12 Biology Chapter 4 (molecular basis of inheritance) for PKU context

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🎯 Exam Tips

• Sweetness ratios are directly asked: Saccharin (550×), Sucralose (600×), Alitame (2000×), Aspartame (100×) — memorise these numbers
• PKU + Aspartame is a classic JEE/NEET one-liner — never forget this contraindication
• BHA and BHT are fat-soluble; Vitamin C is water-soluble — this distinction appears in MCQs
• HLB value concept may appear in assertion-reason type questions

📊 Weightage

• Chemistry in Everyday Life: 1 question per JEE Main paper (~4 marks)
• Food chemicals sub-topic shares space with drugs — about 50% of chapter questions
• High recall, low derivation — efficient to master with flashcard revision

🔗 Related Topics

• Biomolecules (fats and oils — rancidity context)
• Surface Chemistry (emulsification and micelle parallels)
• Organic Chemistry — Functional Groups (phenols in BHA/BHT, esters in aspartame)
• Carbohydrates (sucrose structure → sucralose modification)

📝 Revision Checklist

• List 3 natural and 2 synthetic preservatives with mechanism
• Explain free-radical scavenging by BHA/BHT with equation
• State sweetness ratio of all 4 artificial sweeteners
• Identify which sweetener is PKU-contraindicated and why
• Define HLB and state range for oil-in-water vs water-in-oil emulsions
• Name 2 natural and 2 synthetic food colours
• Explain why sodium benzoate needs acidic pH to work