ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaenko 2005

Chapter 5. BIOCHEMICAL BASIS OF TASTE AND AROMA FORMATION IN PLANTS CONSUMED BY MAMMALS

One of the key issues in ecological biochemistry is understanding the principles by which herbivores and humans select plant-based foods.

The interaction between animals and plants as a food source was partially examined in previous chapters (see Chapter 4). In this chapter, we will focus on The Development of taste and organoleptic properties in plants consumed by wild and domestic animals, as well as humans.

The Selection of plants for consumption by animals is determined not only by their nutritional value, but also by their taste and odor (aroma). The absence of an appealing scent, or the presence of an unpleasant odor caused by volatile compounds released by a plant, can cause an animal to reject a plant even if its nutritional value is high. This factor is also closely tied to the palatability of plants. The choice of food items by mammals, including humans, is likewise influenced by color, texture, and overall appeal.

Taste perception is mediated by taste buds located on the papillae of the Tongue, and to a lesser extent, on the palate, Pharynx, and epiglottis. In children, and occasionally in adults, taste buds are also present on the Lips. Humans have approximately 2·10³ taste buds. They are semicircular in shape and consist of elongated gustatory and supporting Cells that are tightly packed together. Taste cells bear 40 to 50 microscopic microvilli. Nerve fibers originate On the surface of these cells, with their terminal branches intertwining around them.

It is generally believed that food compounds dissolve and settle on the microvilli of the taste cells, triggering stimulation. Excitation from the taste receptors is transmitted via the fibers of the lingual nerve to the Pons and Medulla Oblongata, and subsequently to the Thalamus and Cerebral cortex. This is where various gustatory sensations are formed. Together, this entire system constitutes the gustatory analyzer.

The sensitivity of taste receptors varies across different Regions of the tongue. For instance, receptors most sensitive to sweet tastes are located at the tip of the tongue, those sensitive to bitter tastes at the base, sour tastes along the edges, and salty tastes at both the tip and edges.

Olfactory ciliated cells (receptors) are located in the mucous membrane of the upper-posterior and partially middle PARTS OF THE Nasal cavity, where a complex system of turbinates develops within the Ethmoid bone. These cells feature short (15 — 20 µm) peripheral processes and long central ones. The peripheral processes of the olfactory cells terminate in club-shaped swellings, which bear 10–12 tapered hairs composed of 9 pairs of filaments at their apex. These olfactory hairs act as biological antennas that interact with odoriferous substances. Humans possess about 4·10⁷ olfactory cells, whereas dogs, for example, have 2.25 · 10⁹. Consequently, dogs can detect odors much more acutely than humans.

It is thought that molecules of odoriferous substances settle on The surface of the nasal concha mucosa and dissolve in the secretions of the glands located in the nasal mucosa. These substances stimulate the olfactory hairs and club-shaped swellings. The resulting excitation travels via the olfactory nerves to the olfactory centers of the Brain, located in the Diencephalon and Cerebral Cortex. There, the sensation of the perceived odors is formed. The system comprising olfactory receptors, nerves, and centers constitutes the olfactory analyzer.

When classifying taste sensations, as previously noted, four primary groups are distinguished: sweet, bitter, salty, and sour. Within each group, humans can perceive and differentiate various shades and nuances. In mammals, including humans, the primary taste attractant is sweetness. Bitter, pungent, and astringent tastes are generally unappealing.

Humans also perceive aromas as pleasant or unpleasant, spanning a wide spectrum of nuances (Table 5.1).

Class="center">Table 5.1

CERTAIN PLANT ODORS AND THE COMPOUNDS RESPONSIBLE FOR THEM

(after Harborne, 1985, with modifications)