Phenolic acids are plant metabolites widely spread throughout the plant kingdom. Recent interest in phenolic acids stems from their potential protective role, through ingestion of fruits and vegetables, against oxidative damage diseases (coronary heart disease, stroke, and cancers). Phenolic compounds are essential for the growth and reproduction of plants, and are produced as a response for defending injured plants against pathogens. The importance of antioxidant activities of phenolic compounds and their possible usage in processed foods as a natural antioxidant have reached a new high in recent years.

Absorption of Phenolics
The absorption and bioavailability of phenolics in humans are controversial. Data on these aspects of phenolics are scarce and merely highlight the need for extensive investigations of the handling of phenolics by the gastrointestinal tract and their subsequent absorption and metabolism.

Phenolics in Plants
Phenolic acid compounds seem to be universally distributed in plants. They have been the subject of a great number of chemical, biological, agricultural, and medical studies. Phenolic acids form a diverse group that includes the widely distributed hydroxybenzoic and hydroxycinnamic acids.
     Hydroxycinnamic acid compounds occur most frequently as simple esters with hydroxy carboxylic acids or glucose
     Hydroxybenzoic acid compounds are present mainly in the form of glucosides.

Content of Phenolic compounds in Tea, Coffee, Berries and Fruits
The content of total phenolic acids range from 0 (pear cider) to 103 mg/100 g fresh weight (rowanberry). Besides rowanberry, the best phenolic acid sources among berries are chokeberry (96 mg/100 g), blueberry (85 mg/100 g), sweet rowanberry (75 mg/100 g), and saskatoon berry (59 mg/100 g). Among fruits, the highest contents (28 mg/100 g) ared in dark plum, cherry, and one apple variety (Valkea Kuulas). Coffee (97 mg/100 g) as well as green and black teas (30-36 mg/100 g) are the best sources among beverages. Caffeic acid dominates in all of these samples except in tea brews.

Phenolic acids and cholesterol
Drinking a cup of coffee induces an increase in the resistance of LDL cholesterol to oxidative changes, probably as a result of the incorporation of phenolic acids found in coffee into LDL cholesterol.

Chemistry of Phenolics
Plant phenolic compounds are diverse in structure but are characterised by hydroxylated aromatic rings (e.g. flavan-3-ols). They are categorised as secondary metabolites, and their function in plants is often poorly understood. Many plant phenolic compounds are polymerised into larger molecules such as the proanthocyanidins (PA; condensed tannins) and lignins.
     Furthermore, phenolic acids may occur in food plants as esters or glycosides conjugated with other natural compounds such as flavonoids, alcohols, hydroxyfatty acids, sterols, and glucosides.

Phenols
Phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl functional group (-OH) attached to an aromatic hydrocarbon group. The simplest of the class is phenol (C6H5OH). Some phenols are germicidal and are used in formulating disinfectants. Others possess estrogenic or endocrine disrupting activity.

Phenolic compounds
Phenol, the parent compound, used as an disinfectant and for chemical synthesis.
Polyphenols like the flavonoids and tannins.
Capsaicin, the pungent compound of chilli peppers.
Tyrosine, an amino acid.
The neurotransmitters serotonin, dopamine, adrenaline, and noradrenaline.
L-DOPA, a drug to treat Parkinson's disease.
Eugenol, the main constituent of the essential oil of clove.
Chavibetol from betel.
Estradiol and other estrogens. Scientists have formulated the 'phenolic A ring hypothesis' for the neuroprotective effects of estrogens based upon several observations: (i) structure-activity relationships show that a phenolic A ring and at least two additional rings are required for neuroprotection while estrogenicity requirements are more stringent; (ii) neuroprotection with phenolic A ring compounds occurs in cells that lack estrogen receptors and are not antagonized by anti-estrogens; (iii) phenolic A ring compounds rapidly activate a variety of signal transduction pathways that are known to be involved in cell homeostasis; and (iv) in vivo, treatment with estrogens results in a neuronal type-independent neuronal protection from ischemic insult. Potential mechanisms of actions that may be involved in the neuroprotective effects of phenolic A ring compounds are: (i) estrogen redox cycling that potently inhibits oxidative stress; (ii) interactions with signal transduction pathways including the transcription factor cAMP response element binding protein; and (iii) induction of anti-apoptotic proteins. These signaling pathways may individually or collectively contribute to the plethora of neuronal cell types that are protected from a variety of insults by estrogen-like compounds.
Methyl salicylate, the major constituent of the essential oil of wintergreen.
Raspberry ketone a compound with an intense raspberry smell.
Salicylic acid is a phenolic compound
Gallic acid, found in gallnuts.
Ellagic acid
Thymol (2-Isopropyl-5-methyl phenol), an antiseptic that is used in mouthwashes.
BHT (butylated hydroxytoluene), a fat-soluble antioxidant and food additive.
... and many more.

Natural Substances with Phenolic acids
Propolis is one of the few natural remedies that has maintained its popularity over a long period of time. The pharmacologically active molecules in the propolis are flavonoids and phenolic acids and their esters. These components have multiple effects on bacteria, fungi and viruses. In addition, propolis and its components have anti-inflammatory and immunomodulatory activities. Moreover, propolis has been shown to lower blood pressure and cholesterol levels. However, clinical studies to substantiate these claims are required.

Coffee is particularly rich in bound phenolic acids, such as caffeic acid, ferulic acid, and p-coumaric acid.

Purple corn maize

Quince has phenolic acids

Aloe ferox has phenolic acids

Phenolic Acids in Berries
The composition of phenolic acids in several small berries grown in Northeastern Poland, namely, low-bush blueberries, black mulberries, European juneberries, black currants, fruits of blue-berried honeysuckle, and blackberries. The total content of phenolic acids ranged from 2845 (black mulberries) to 5418 (blue-berried honeysuckle). Twenty phenolic acids were identified in the berries. Of these, hydroxycaffeic, m- and p-coumaric, and 3,4-dimethoxycinnamic acids were the major phenolic acids in blackberries and blueberries, m-coumaric acid was the major phenolic acid in blue-berried honeysuckle and black currant fruits, while salicylic, caffeic, and m- and p-coumaric acids were the predominant phenolic acids in European juneberries. Syringic and veratric acids were detected only in blueberries, while p-hydroxybenzoic and sinapic acids were present only in black currants and o-coumaric acid was present in blueberries and black mulberries.

Phenolics in Spices
Spices are known to significantly contribute to the flavor, taste, and medicinal properties of food because of phenolics. Most spices contain phenolic acids such as tannic, gallic, caffeic, cinnamic, chlorogenic, ferulic and vanillic acids. A high amount of tannic and gallic acids are found in black mustard and clove. Caffeic, chlorogenic and ferulic acids are found in a good amount in cumin. Vanillic and cinnamic acids are found in onion seeds.

Other Phenolic compounds
Cannabinoids, the active constituents of Cannabis.
Cresols from coal tar and creosote.
Guaiacol (2-methoxyphenol) from roasted coffee, whisky, and smoke that has a smoky flavor.
Salicylic acid, a plant hormone and analgesic, antipyretic, and anti-inflammatory drug, precursor compound to Aspirin. Aspirin (acetylsalicylic acid) is still the most commonly used salicylate. After oral administration as an aqueous solution aspirin is rapidly absorbed at the low pH of the stomach millieu. Less rapid absorption is observed with other formulations due to the rate limiting step of tablet disintegration - this latter factor being maximal in alkaline pH. The rate of aspirin absorption is dependent not only on the formulation but also on the rate of gastric emptying. Aspirin absorption follows first-order kinetics with an absorption half-life ranging from 5 to 16 minutes. Hydrolysis of aspirin to salicylic acid by nonspecific esterases occurs in the liver and, to a lesser extent, the stomach so that only 68% of the dose reaches the systemic circulation as aspirin. Both aspirin and salicylic acid are bound to serum albumin (aspirin being capable of irreversibly acetylating many proteins), and both are distributed in the synovial cavity, central nervous system, and saliva. The serum half-life of aspirin is approximately 20 minutes. The fall in aspirin concentration is associated with a rapid rise in salicylic acid concentration. Salicylic acid is renally excreted in part unchanged and the rate of elimination is influenced by urinary pH, the presence of organic acids, and the urinary flow rate. Metabolism of salicylic acid occurs through glucuronide formation (to produce salicyluric acid), and salicyl phenolic glucoronide), conjugation with glycine (to produce salicyluric acid), and oxidation to gentisic acid.
Orthophenyl phenol , a fungicide used for waxing citrus fruits.
The pH indicators phenolphthalein, phenol red, bromothymol blue, thymolphthalein, and bromophenol blue.
Psilocin, a hallucinogenic alkaloid of Psilocybe mushrooms.

Phenolic Compounds in Herbs
There are countless herbs that have phenolic compounds, here is one example:
Phenolic components and antioxidant activity of Fernblock, an aqueous extract of the aerial parts of the fern Polypodium leucotomos.
Methods Find Exp Clin Pharmacol. 2006 Apr;28(3):157-60. Industrial Farmaceutica Cantabria, Arequipa 1, Madrid, Spain
Fernblock, an aqueous extract of the aerial parts of the fern Polypodium leucotomos is used as raw material for topical and oral photoprotective formulations. Phenolic compounds were identified as 3,4-dihydroxybenzoic acid, 4-hydroxybenzoic acid, vanillic acid, caffeic acid, 4-hydroxycinnamic acid, 4-hydroxycinnamoyl-quinic acid, ferulic acid, and five chlorogenic acid isomers.