Sensory Analysis: Taste, Chemesthesis & Industry Applications

Week 9 – Administrative & Housekeeping

• Second-to-last week of the course
  • Session recorded on Tuesday 5 Aug (lecturer repeatedly corrected the date).
  • Next week: VIP guest speaker + in-class quiz → lecturer urged full attendance and asked students to spread the word to fill the lecture hall.
• 52-slide lecture; 10-min break in the middle; extra quiz segment run by Apeksha at the end.

Why the Food Industry Cares About Human Senses

• Three core consumer purchase drivers:
  1. Price / Cost / Affordability
  2. Convenience
  3. Taste / Pleasure (focus of this week)
• Long-standing industrial goal: create a third shelf of foods that “taste good and are good for you.”
• Sequencing of the human genome (~2001) allowed the field to move from empirical formulation to molecular design of taste & aroma → several Nobel Prizes (Linda Buck, Richard Axel, David Julius, Ardem Patapoutian).

Basic Vocabulary & Concepts

• Taste (Gustation) – detection of dissolved chemicals by receptors on the tongue.
• Smell (Olfaction) – detection of volatile chemicals via the nasal cavity (orthonasal & retronasal).
• Flavor – holistic perception produced by the combination of taste + smell (and chemesthesis, texture, sight, sound, context…).
• Chemesthesis / Chemosensory Irritation – tingling, burning, cooling, etc., mediated primarily by trigeminal nerve endings (e.g., capsaicin, menthol).
• Stimulation vs. Perception
  • Stimulation: physical/chemical activation of peripheral receptors.
  • Perception: subjective brain representation (depends on context, mood, physiology).
• Five classical senses: sight, smell, taste, hearing, touch (but course continually emphasises cross-modal interaction).

Context Dependence: The “Gustatory” Illusion Analogy

• Checkerboard example (Adelson illusion): physical luminance of squares A & B identical, yet perceived as black vs white because of contextual processing → similar contextual effects occur with tastes (sweetness, saltiness, temperature, plating, mood, etc.).

Sensory Science Methodology

Human Testing Infrastructure

• Purpose-built sensory booths with cloches for blinded sample delivery (example: six-station lab in Science-Engineering Building).
• Panel types:
  • Trained (can detect $\,\approx\,$3 % salt difference, recognise off-flavours, etc.)
  • Untrained (consumer liking, JAR: Just About Right scales).

Core Test Designs

• 2-AFC / 3-AFC (Alternate Forced Choice)
  • “Find the odd one” among 3 samples → used to determine detection thresholds & difference thresholds.
• GLMS (General Labelled Magnitude Scale)
  • Anchors: barely detectable → strongest imaginable; accommodates individual differences.
• Multidimensional profiling
  • Spider/Radar diagrams – axes for sweetness, creaminess, bitterness, etc.
  • PCA (Principal Component Analysis) – reduces large attribute matrices to PC1/PC2 plots for product mapping.

Animal Behaviour Assays

• Two-bottle preference test (mice): measures consumption of test vs control solution to infer hedonic valence; can incorporate genetic knock-outs to map receptor specificity.

Neuro-Anatomical Pathways

• Taste nerves
  • Chorda tympani (CN VII branch) – anterior tongue
  • Glossopharyngeal (CN IX) – posterior tongue
  • Vagus (CN X) – epiglottis/larynx
  • 1st synapse: Nucleus of the Solitary Tract (NST) → thalamus → Gustatory cortex (insula).
• Trigeminal nerve (CN V) – three roots innervating face, oral & nasal mucosa; responsible for chemesthetic sensations.

Tongue Microanatomy

• What you see: Papillae (fungiform, foliate, circumvallate).
• What actually senses taste: Taste buds embedded in papilla walls.
  • ≈100 polar neuro-epithelial cells per bud.
  • Labelled-line coding: distinct cell subtypes dedicated to single modalities (not cross-fibre).

Molecular Biology of Each Taste Modality

1. Sweet

• Biological role – Signals caloric carbohydrates; innate preference (energy, “happiness”).
• Industrial relevance – Consumer pull; sweeteners, sugar reduction, satiety manipulators.
• Receptor – Heterodimer T1R2 + T1R3 (Class C GPCR).
  • Discoveries published simultaneously by six labs in Feb 2001; Charles Zuker’s group secured key patents.
  • Knock-out mice lacking both subunits show abolished chorda tympani responses to sucrose, saccharin, etc.
• Ligand diversity – Sugars, polyols, high-potency sweeteners (saccharin, aspartame, cyclamate), sweet proteins (thaumatin, monellin), some D-amino acids.

2. Umami

• Biological role – Indicates protein (L-glutamate) & nucleotide richness → “meaty/savoury.”
• Industrial relevance – MSG, nucleotides (IMP, GMP) as flavour enhancers (Ajinomoto, stock cubes).
  • Synergy: \text{EC}{50}\,(\text{MSG}) \approx \text{mM};\ \ +\ 10^{-6}\,\text{M}\,\text{IMP}\ \Rightarrow\ \text{EC}{50}\,(\text{MSG})\rightarrow\mu\text{M}.
• Receptor – Heterodimer T1R1 + T1R3 (shared T1R3 subunit).
  • Double knock-out eliminates responses to MSG ± IMP.

3. Bitter

• Biological role – Avoidance of toxins (alkaloids, metals, spoilage indicators).
• Industrial interest – Masking coffee/beer bitterness; producing “healthy bitter” vegetables.
• Receptors – ~30 T2R GPCRs → broad chemical coverage but convergent “bitter” percept.
  • Species differences (human, mouse, rat, bonobo) documented.
• Cross-activation caveat – High conc. saccharin activates some T2Rs → explains off-notes of artificial sweeteners.

4. Salty

• Biological role – Detects \text{Na}^+, essential for nerve & muscle function; drives salt appetite after depletion.
• Primary detector – ENaC (epithelial sodium channel).
  • Mouse taste: \alpha\beta\gamma subunits; amiloride-sensitive.
  • Human taste: \alpha\beta\gamma\delta tetramer (δ-subunit explains earlier pharmacological anomalies).
• Mechanism – Ion permeation depolarises taste cell directly (channel, not GPCR).

5. Sour

• Biological role – Warns of unripe or spoiled (acidic) foods; helps maintain acid–base balance.
• Candidate sensors – PKD2L1/Pkd1l3 channels (“PiCaDilly” cells), OTOP1 proton channel, etc.
• Industry angle – Managing acidity in yogurt, beverages; acid masking.

6. Fat / “Oleogustus” (emerging modality)

• Nutritional role – Highly caloric, carrier of fat-soluble vitamins, essential FAs.
• Consumer psychology – “The fatter, the better” (ice-cream, potato chips; peak pleasure when finger-licking).
• Candidate detectors – CD36, GPR120, GPR40, K+ channels (texture), yet multi-modal (taste + mouthfeel).
• Case study – Olestra (non-digestible fat mimic) tasted fatty but caused gastrointestinal leakage → market failure.

7. Chemesthesis / Spiciness

• Key TRP channels
  • TRPV1 – heat & capsaicin (red chili) → Nobel 2021 (Patapoutian & Julius).
  • TRPM8 – cold & menthol → gives “cooling” of mints.
  • TRPA1, TRPV3, TRPV4 – mustard oil, gingerol, wasabi, etc.
• Cultural/Health Aspects
  • Spices historically used for antimicrobial, anti-inflammatory, thermogenic, or metabolic effects.
  • Synergistic perception: sub-threshold chili elevates overall taste intensity of foods.

Beyond the Tongue: Extra-Oral Taste Receptors

• PCR screens revealed sweet, umami, bitter receptors in gut entero-endocrine cells.
  • Influence GLP-1, CCK, ghrelin → link to satiety & glycaemic control (basis for modern weight-loss drugs).
• Demonstrates nature’s “re-use” of sensory GPCRs for internal nutrient sensing and metabolic regulation.

Practical Demonstrations Suggested by Lecturer

• Clip nose + blindfold → grated onion vs apple become indistinguishable (proves dominance of smell in flavour).
• Drink coffee while occluding nose, then release → experience olfactory contribution.
• Rub capsaicin vs sugar in eye (NOT recommended) → only chemesthetic stimuli activate trigeminal endings in cornea.

Key Take-Away Matrix (Zuker 2009)

Entrepreneurship Sidebar – Patents vs Papers

• Feb 2001: Six labs simultaneously cloned sweet receptor genes.
  • Charles Zuker published in Cell and filed broad patents → led to successful start-up, later acquisition by Swiss firm; example of aligning academic discovery with commercial IP.

Wrap-Up & What’s Next

• Today covered: sensory methodology → peripheral anatomy → molecular identity of taste, chemesthesis, fat sensing.
• Remaining topic (deferred to next lecture): Olfaction (smell receptors, flavour synergy, food design).
• Reminder: VIP speaker + quiz next Tuesday. Bring classmates, arrive on time.