Classification of Phenolic Antioxidants: History
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Teresa Pinto
,
Alice Vilela

Phenolic compounds are classified as primary antioxidants and originate from one of the main classes of secondary metabolites in plants. They have antioxidant properties through several mechanisms: (i) the ability to remove free radicals and inhibit the formation of reactive species during the normal course of metabolism; (ii) preventing the occurrence of damage to lipids, proteins, and nucleic acids; and (iii) preventing consequent cell damage and death. Thus, they are commonly associated with preventing the development of cardiovascular diseases, neurodegenerative diseases, autoimmune diseases, diabetes, and cancer.

  • nutraceutical properties
  • human health
  • phenolic compounds

1. Natural Phenolic Antioxidants in Beverages

Natural antioxidants can be obtained from plants (fruits, legumes, and vegetables), mushrooms, and algae, and are classified as phenolic compounds, vitamins, and carotenoids [1][2][3].
In recent years, there has been a growing preference for natural phenolic antioxidants over artificial ones, fundamentally due to the increasing demand by consumers for functional foods and beverages with the addition of natural additives, which maintain their nutritional properties and flavor [4]. Besides, this trend results, according to the same authors, from the preference currently given to natural phenolic antioxidants in food stabilization, and the restrictions applied by the responsible entities on the use of synthetic phenolic antioxidants [5][6].
Natural phenolic antioxidants have a great diversity of structures, in which the basic monomer of polyphenols is the phenolic ring [7]. They are divided into several classes, the most representative of which are flavonoids and phenolic acids. For their part, flavonoids are further divided into flavones, flavanones, flavonols, flavanols, isoflavones, and phenolic acids and are generally classified into hydroxybenzoic and hydroxycinnamic acids [8].
Stilbenes are a group of phenolic compounds that share a similar chemical structure to flavonoids (Figure 1). Trans-resveratrol is one of the most recognized stilbenes, present mostly in glycosylated forms. Red wine, as well as the red grapes that originate this fermented drink, are rich in resveratrol. Several studies show that moderate consumption of red wine leads to a reduction in the development of cardiovascular diseases and atherosclerotic plaques, and provides neuroprotective, antidiabetic, anti-inflammatory, antioxidant, anticarcinogenic, and antiviral activity [9][10][11].
Figure 1. Phenolic antioxidants classification. Adapted from Shahidi and Ambigaipalan [12].
When analyzing different types of wines, namely, whites, rosés, and reds, Paixão et al. [13] showed that red wine had significantly higher phenolic levels than the rosé and white wines, and consequently exhibited the highest antioxidant power. According to Fiori et al. [14], the highest concentration of resveratrol is higher in red wine than in white wine because it is present in the skin and seeds of grapes. Thus, the different phenolic composition of wines is related to the grape variety, as well as the edaphoclimatic conditions of the region where the grapes are produced, cultural practices, the stage of ripeness [15][16], maceration [17][18], yeasts used in the vinification process [19], and other winemaking conditions [20].
Cordova and Sumpio [21] also concluded that red wine has more health-promoting activity than beer or spirits due to its richer content of phenolic compounds, hence the increased interest in the nutraceutical value of wines.
Additionally, herbal teas and infusions are rich in natural antioxidants, mainly flavonoids: theaflavins, bis-flavanols, and fulvic acids. Their consumption has increased because they are recognized as having anticariogenic properties and antimicrobial and anticancer activity [22][23]. McCarthy et al. [24] studied the antioxidant potential of plant extracts and compared them with synthetic antioxidants and vitamin E, incorporating them in pork. The catechins present in tea were shown to be more effective in terms of their antioxidant power, compared to butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), and antioxidant activity was evaluated through the thiobarbituric acid reactive substance assay (TBARS assay—reactive species of thiobarbituric acid).
The market for natural antioxidants is growing and new products have appeared on the market with a healthy image, namely, smoothies, functional drinks, and yogurts with green tea, grape seed, lemon balm, and aloe vera, among others [4].
It is important to note that there are natural antioxidants that have lower antioxidant activity than their synthetic equivalents, which can lead to the use of a higher dosage, causing toxicity reactions [25]. Thus, the intensification of toxicity studies of these compounds is necessary to know the limits of their use.

2. Synthetic Phenolic Antioxidants in Beverages

Several synthetic phenolic antioxidants (SPAs) are widely used in industrial and commercial products, but only a few can legally be added to food products [26]. Thus, with a common molecular structure in which the phenolic rings are replaced by alkyls hindered in the ortho position [6], butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), and tertiary-butylhydroquinone (TBHQ) are commonly used as food antioxidants [27]. However, BHA and BHT are the antioxidants most used as additives in food products [28][29].
The poor stability of natural antioxidants has increased the number of SPAs that are preferred for use in food and beverages to prevent and delay lipid oxidation reactions, preventing the formation of foreign flavors and undesirable chemical compounds, such as aldehydes, ketones, and organic acids, and prolong the shelf life of products [30].
SPAs can be used alone or in combination. Thus, BHA is commonly found in dry cereals, derived from potatoes, in cooked foods (boiled or fried and desserts), and in drinks [27][31]. In food supplements, condiments, spices, chewing gum, and oils, BHT can be used alone or in combination with BHA or TBHQ [32]. As a preservative, TBHQ is used in edible animal fats, meat products, and unsaturated vegetable oils [33]. Propyl gallate has been used in the food industry as a stabilizer in fatty foods and as an additive in mayonnaise, fats, edible fats, and baked goods [34].
The presence of a wide range of undesirable compounds in foods and beverages has made the area of food safety increasingly relevant, to provide the population with the necessary high-quality food. Therefore, the growing concern about using safe and environmentally friendly food products has recently led to the realization of several studies to find out if the use of SPAs in food and beverages is safe for health. Their conclusions are contradictory. In some studies, SPAs revealed antimutagenic and antitumor properties [35][36][37][38][39] but, in others, allergic reactions, including asthma and hives [40], toxic effects in some animal tissues [41], liver toxicity, endocrine-disrupting effects, and even carcinogenicity [30][42][43][44] were reported, questioning their use [6][45].
This issue generated concern on the part of the governments of the European Union and most countries to create legislation that regulates the use of SPAs, either individually or in mixtures, as the market for combined natural or synthetic phenolic antioxidants is expected to have a growth rate of around 5% by 2023 [46]. In the European Union, the use of certain SPAs has been restricted and has even been banned in soft drinks [26].

This entry is adapted from the peer-reviewed paper 10.3390/beverages7010012

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