Taste

15.1 Taste versus Flavor

• Taste (Gustation): Sensations evoked by aqueous solutions contacting receptors on tongue & palate.
• Retronasal olfactory sensation
  • Odorant released during chewing/swallowing travels “up and behind” the palate ➜ contacts olfactory mucosa.
  • Perceived as coming from the mouth even though stimulus–receptor contact is in the nose.
• Flavor = True taste (sweet, salty, sour, bitter) + retronasal olfaction.
• Figure 15.1 contrasts pathways:
  • Orthonasal (through nostrils) vs. Retronasal (from oral cavity) olfaction.
• When taste is lost but smell is intact
  • Clinical case: Patient could smell lasagna but reported “no flavor.”
  • Lab analog: Anaesthetize the chorda tympani (branch of cranial nerve VII) with lidocaine ➜ odor is detected but “flavor” disappears.
• Neuroimaging evidence
  • Brain treats odors differently when they arrive orthonasally vs. retronasally.
• Cross-modal modulation in industry
  • Added sugar ↑ perceived fruitiness (olfactory) in juices.
  • Heirloom-tomato studies: Volatile compounds ↑ perceived sweetness without added sugar ➜ potential low-calorie sweet-flavor enhancement.

15.2 Anatomy and Physiology of the Gustatory System

• Taste buds
  • House gustatory receptor cells; embedded in papillae; convert chemical energy ➜ neural signals carried by cranial nerves.
• Four papilla types
  1. Filiform – tiny, numerous, textured; \varnothing taste buds.
  2. Fungiform – mushroom-shaped (≈ 1 mm Ø), concentrated at tip/edges; ≈ 6 taste buds each.
  3. Foliate – lateral folds near rear; contain many taste buds.
  4. Circumvallate – large mound-in-trench; form inverted V at posterior tongue; 3–5 per side.
• Microvilli
  • Membranous extensions of receptor cells protruding into taste pore; binding sites for tastants (not “hairs”).
• Tastants
  • Category 1: Small ions (e.g., \text{Na}^+,\,\text{H}^+) ➜ salty, sour via ion channels.
  • Category 2: Larger molecules activate GPCRs ➜ sweet, bitter.
• Tongue-map myth
  • Origin: Hänig (1901) measured thresholds; Boring (1942) redrew as “sensitivity” ➜ exaggerated differences.
  • Reality: All four basic tastes detected across entire tongue surface.
• Central pathway (Figure 15.3/15.7)
  • Taste buds → cranial nerves VII (chorda tympani), IX, X → nucleus of solitary tract (medulla) → thalamus → insula (primary taste cortex) → orbitofrontal cortex (OFC).
  • OFC integrates taste with temperature, touch, smell.
• Inhibition
  • Descending cortical signals suppress oral pain to preserve feeding even with mouth injuries.

15.3 The Four Basic Tastes?

• Canonical list: Salty, Sour, Bitter, Sweet.

Salty

• Salts are ionic compounds: \text{cation}^+ + \text{anion}^- (e.g., \text{Na}^+\text{Cl}^-).
• Perception & liking are plastic
  • Low-Na diet ↑ salt sensitivity.
  • Early chloride deficiency or gestational exposure modifies later preferences.

Sour

• Elicited by acids (high \text{H}^+ concentration).
• At high [acid] damages tissue ➜ protective role.

Bitter

• Quinine = prototype.
• Broadly tuned receptors—little ability to differentiate bitters.
• Many bitter molecules are toxic ➜ aversive signal.
• Modulation
  • Hormonal: Sensitivity ↑ in pregnancy.
  • Learned “switch-off” facilitates vegetable acceptance.
• Figure 15.9 lists diverse compounds & their TAS2R receptor affinities.

Sweet

• Evoked by carbohydrates crucial for metabolism.
  • \text{Glucose} (primary energy), \text{Fructose} (sweeter), \text{Sucrose}=\text{Glucose}+\text{Fructose}.
• Single heterodimeric receptor TAS1R2 + TAS1R3 (Figure 15.10).
  • Different ligands bind distinct sites; artificial sweeteners exploit this.

15.4 Beyond Four Tastes

Umami

• Triggered by \text{monosodium glutamate (MSG)}; reflects amino-acid content.
• Safety: Large acute doses ➜ “MSG symptom complex” (numbness, headache, flushing, etc.); small culinary doses generally safe.

Fat taste

• Fatty acids generate tactile & possibly taste signals; rats (and likely humans) express lingual fatty-acid receptors.
• Post-ingestive conditioning: Gut fat digestion conditions preference for fat-containing flavors.

15.5 Genetic Variation in Bitter

• PTC / PROP polymorphism (Fox 1931; receptor gene ID 2003)
  • Genotype \text{TAS2R38}
  • \text{PAV/PAV} or \text{PAV/AVI} ➜ tasters.
  • \text{AVI/AVI} ➜ nontasters.
• Supertasters
  • Taster genotype + high fungiform density ➜ extremely intense taste; see Figure 15.4.
• Cross-modality matching (Figure 1.9)
  • Nontaster bitterness ≈ whisper loudness.
  • Medium ≈ smell of bacon / mild headache pain.
  • Supertaster ≈ sun brightness / worst pain.
• Health links
  • Supertasters may avoid bitter vegetables ➜ lower phytonutrient intake; but also perceive fats as bitter ➜ lower fat consumption → ↓ CVD risk.
  • PROP bitterness intensity correlated with colon polyp prevalence in men.

15.6 Taste, Flavor & Nutrient Regulation

• Omnivore’s dilemma: Balancing varied options with nutritional needs.
  • Smell identifies objects; taste signals nutrient vs. antinutrient.
• Innate survival coding
  • Bitter → possible poison; Sour → potential tissue-damaging acidity; Sweet & Salty → nutrients (\text{Na}^+, sugars).
• Infant evidence
  • Sweet → smiles & sucking; Sour → lip pursing; Bitter → gaping/spitting/vomiting attempts.
• Specific-hungers theory
  • Sodium or sugar deficits provoke targeted cravings; not supported for most vitamins/minerals.
• Evaluative conditioning
  • Hedonic valence of consequences transfers to associated flavors ➜ learned likes/dislikes.
• Food preference study (422 men)
  • Preference ratings for high-fat foods correlated with BMI & waist circumference (r significant), whereas self-reported frequency of eating high-fat foods did not ➜ preferences predict adiposity.

15.7 Nature of Taste Qualities

• Coding theories
  • Labeled lines: Each afferent fiber dedicated to one quality; hearing is example.
  • Pattern coding: Qualities from distributed activity patterns; vision & olfaction examples.
  • Debate persists for taste; evidence exists for both.
• Adaptation & Cross-adaptation
  • Prolonged stimulus ↓ subsequent sensitivity (e.g., own saliva \text{Na}^+ adapts salt taste).
  • Cross-adaptation: One tastant alters perception of another (sweet drink then sour solution tastes extra sour).
• Retronasal vs. Orthonasal pleasure learning
  • Hedonic responses can be route-specific: e.g., fresh-cut grass pleasant orthonasally but not as flavor.
  • Yet aversions learned via retronasal usually transfer to orthonasal (e.g., fish sickness).
• Chili pepper (Capsaicin)
  • Preference acquisition culturally & socially mediated; humans uniquely enjoy.
  • Capsaicin activates pain receptors ⇒ burning; repeated exposure ⇒ receptor desensitization.
  • Individual differences tied to papilla density.
  • Practical tip: If too hot, wait; subsequent bites feel milder due to desensitization.