Taste

Taste: Sensations evoked by solutions in the mouth that contact the receptors on the tongue and the roof of the mouth.
Flavor: The combination of true taste (sweet, salty, sour, bitter) and retronasal olfaction.

An odor is perceived when an odorant in the mouth moves up the palate into the nose.
These odor sensations are perceived as originating from the mouth, but the molecules actually contact receptors in the nose (at the olfactory mucosa).
Smell is the stronger contributor to flavor.

Chewing Process:
- Chewing breaks down food into molecules which are dissolved in saliva.
- Saliva-borne molecules flow into a taste pore that contains taste buds.

Function of Taste Buds:
- Create neural signals conveyed to the brain by taste nerves.
- Embedded in structures called papillae (bumps on the tongue).
- Contain taste receptor cells which send information to the brain via cranial nerves.
Human Taste Bud Count: Between 3,000 and 12,000 taste buds, each containing 40-100 taste receptor cells.

Microvilli: Slender projections on some taste bud cells that extend into the taste pore.
- They contain the sites that bind to taste substances.

Tastants can be divided into two large categories:
1. Ionic Tastants: Small, charged particles that taste salty or sour.
- Small ion channels in microvilli membranes allow specific charged particles to enter.
1. Molecular Tastants: Perceived via a ‘lock and key’ process - enter the pore and rotate; if it fits, it causes a chain reaction.
  - These molecules taste sweet or bitter.

The sensory pathway of taste involves:
- Taste buds → Cranial nerves → Medulla (brainstem) → Thalamus → Cortex.
Primary Taste Cortex: Located in the insula; the area that first receives taste information.
Orbitofrontal Cortex: Located above the eyes; receives projections from the insular cortex and processes temperature, touch, smell, and taste, suggesting integration.

Salty:
- Perceives critical for regulating bodily function, cell structure, muscle contraction, and action potentials.
- Salt consists of two charged particles: cation (+) and anion (-).
- Sensitivity to salt increases over time with low sodium diets.
Sour:
- Derived from acidic substances; helps identify unripe or spoiled food.
- At high concentrations, acids can damage body tissues.
Bitter:
- Many bitter substances are poisonous.
- Quinine is a prototypically bitter-tasting substance; caffeine is also bitter.
- The ability to distinguish between different bitter compounds is limited.
- Bitter sensitivity can be affected by hormone levels, intensifying during pregnancy.
Sweet:
- Evoked by sugars, with glucose being the principal source of energy for most animals, followed by fructose (sweeter than glucose) and sucrose (table sugar).
- One single receptor is responsible for all sweet perception; different sweeteners can stimulate various parts of the receptor.

Umami: Considered a candidate for the fifth basic taste but might not be universally recognized as such.
It signals the presence of protein in food, with examples including beef broth.
Monosodium Glutamate (MSG): Acts as a flavor enhancer.
- Safety concerns with large amounts, such as numbness and headache, but typically safe in small doses.

PTC and PROP: Discovered by Arthur Fox in 1931, these compounds taste differently to different people.
- The gene for PTC/PROP receptors was discovered in 2003. Individuals with two recessive genes are nontasters, while those with one or more are tasters.
Supertaster: An individual who is a taster of PTC/PROP and has a high density of fungiform papillae, perceiving the most intense taste sensations.
Cross-modality Matching: Relating the intensity of sensations from different senses, where the bitterness of PROP correlates to:
- Non-tasters relate it to the sound of a watch.
- Medium tasters relate it to the smell of frying bacon.
- Supertasters relate it to the brightness of the sun.

Variations in sensory properties of foods affect food preferences and diets.
- Super-tasters, who may avoid bitter-tasting vegetables, show higher incidences of colon cancer due to reduced vegetable intake.
- Aversion to bitter or fatty tastes often leads to lower rates of overweight and reduced incidences of heart disease.

Omnivore's Dilemma: Refers to modern humans' challenge of finding a healthy diet amidst many choices. Taste and smell assist in dietary choices.
Survival Value of Taste:
- Bitter: May indicate poisons.
- Sour: Detects potentially harmful acidic substances.
- Sweet and Salty: Essential for survival due to the body's need for carbohydrates and sodium.
Infant Behavior: Innate preferences for certain foods can be observed.
- Sweet foods evoke smiling and sucking behavior.
- Sour tastes lead to lip pursing and protrusion.
- Bitter tastes cause gaping, spitting, and vomiting movements.
Specific Hungers Theory:
- Suggests deficiencies in nutrients create cravings specifically for those foods (e.g., salty or sweet).
- This theory has not been widely supported for vitamins.
Modern Theories of Learning: Emphasize the influence of learned responses and the consequences of eating certain foods on future food preferences
- Evaluative Conditioning: The process by which positive or negative food experiences influence liking or disliking of foods.

Fat as a nutrient evokes tactile sensations such as oily or creamy.
Rats possess fatty acid receptors, suggesting humans may have similar receptors, influencing preferences based on the sensory properties of fats.