Phenolic compounds

Phenolic compounds are the most widely distributed secondary metabolites, ubiquitously present in the plant kingdom, even if the type of compound present varies according to the phylum under consideration.

Generic terms ‘phenolic compounds’, ‘phenolics’ or ‘polyphenolics’ refer to more than 8,000 compounds found in the plant kingdom and possessing at least an aromatic ring with one or more hydroxyl substituents, including functional derivatives like esters, methyl ethers, glycosides, etc.

They contain benzene rings, with one or more hydroxyl substituents, and range from simple phenolic molecules to highly polymerized compounds.

Phenolic compounds mainly exist in: soluble phenolic compounds and bound phenolic compounds.

Most soluble phenolic compounds are synthesized in the intracellular endoplasmic reticulum of plants, and stored in vacuoles. Bound phenolic compounds, however, are formed by the transportation of soluble phenolic compounds to the cell wall.

Phenolic compounds possess numerous bioactive properties and, although they are not nutrients, dietary intake provides health-protective effects, therefore postharvest treatments have been used to enhance or preserve the contents of phenolic compounds in fruits and vegetables.

Plant phenolics are considered to have a key role as defense compounds when environmental stresses, such as high light, low temperatures, pathogen infection, herbivores, and nutrient deficiency, can lead to an increased production of free radicals and other oxidative species in plants.

The basic structure of a phenolic compound comprises of an aromatic ring with one or more –OH groups.

Phenolics found in food material can be divided into three major groups: simple phenols and phenolic acids, hydroxycinnamic acid derivatives and flavonoids.
Phenolic compounds

Lettuce

Lettuce (Lactuca sativa L.) is a member of the family Asteraceae (Compositae), a successful and diverse group of plants with a global distribution. Lettuce is native to Southern Europe and Western Asia. It descended from wild lettuce latica scariola, a common weed of roadsides and wastelands in both the Old and New Worlds.

Among the group of lettuce

*The Cos or romaine forms an upright, elongated head and is an excellent addition to salads and sandwiches.

*The Cutting group or leaf group forms no firm heads but instead produces a dense mass of leaves in the center of the plant.

*The Stalk lettuce group does not form heads and has prominent thickened fleshy stems and upright ovate leaves.

*The Butterhead group are generally small, semiheading types that have tender, soft leaves and a delicate flavor.

*The Crisphead group forms tight, dense heads which are comprised of spherical leaves folded upon each other. The leaf is crispy and the veins are prominent. Crisphead lettuce also is commonly referred to as iceberg lettuce. Crisphead has a large solid head similar to a cabbage.

*The Oilseed group is typified by a multistemmed upright growth habit with green leaves that are oblong to oblanceolate.

The three main lettuce types produced in the United States are iceberg (L. sativa L. var. capitata L.), romaine (L. sativa L. var. longifolia L.), and leaf (L. sativa var. crispa L.).

Mature plants of marketable quality are individually harvested. Iceberg lettuce for whole head production has traditionally been harvested at market maturity. Romaine reaches market maturity as fast or faster than iceberg, and leaf reaches market maturity faster than romaine.
Lettuce