**1. Introduction**

The quality and safety of food products are compromised by the loss of nutrients, sensory attributes, and microbial growth. The prior can be in terms of off odor, off flavor, discoloration, or production of toxic compounds as metabolic end products of the saccharolytic, proteolytic, pectinolytic, and lipolytic enzymes, which ultimately leads to food poisoning or intoxication. Ensuring food safety and meeting the demand for food without synthetic chemical preservatives has led to increased interest in natural alternatives to inactivate microorganisms and enzymes in food [1,2]. Although attempts have been made to produce additive-free foods, it is unlikely that the current marketing system could exist without the use of antimicrobials. In addition, requirements for toxicology safety have limited the ability of the industry to develop new chemical antimicrobials [3]. Therefore, it is essential for the food industry to find new and natural antimicrobial food alternatives.

Phytochemicals can be recovered from plant products and used as ingredients in food and cosmetics, as healthy antimicrobials, and as alternatives to chemical preservatives. A typical feature of plants is their ability to synthesize a wide range of phyto-compounds (i.e., secondary metabolites), which play essential roles in the interaction of the plant with its environment [4]. They can be structurally divided into five major groups: phenylpropanoids, flavonoids, polyketides, terpenoids, and alkaloids. Furthermore, it is increasingly clear that several phytocompounds in fruits and vegetables of different chemical classes are beneficial to human health [5,6]. Interestingly, several phytochemicals such as simple phenolic acids,

**Citation:** Panda, L.; Duarte-Sierra, A.Recent Advancements in Enhancing Antimicrobial Activity of Plant-Derived Polyphenols by Biochemical Means. *Horticulturae* **2022**, *8*, 401. https://doi.org/ 10.3390/horticulturae8050401

Academic Editors: Dasha Mihaylova and Aneta Popova

Received: 30 March 2022 Accepted: 24 April 2022 Published: 3 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

polyphenols, terpenes, isothiocyanates, polyacetylenes, etc., also exhibit antimicrobial properties. There is sufficient evidence supporting the potential of plant-derived phytochemicals as natural antimicrobial agents [7–9].

However, only a few natural antimicrobials have found practical application in the food industry. Their use in foods as preservatives is often limited due to the need for high concentrations to achieve the desired activity, which may modify the sensory characteristics of food by making it unacceptable [10]. Another limitation is the interaction of natural antimicrobial with complex food matrices, mainly with hydrophobic compounds such as lipids [11]. Nonetheless, polyphenols can interact with proteins through hydrophobic or hydrophilic interactions, leading to the formation of soluble or insoluble complexes [12].

The "antimicrobial potency" of polyphenols can be altered and enhanced by biochemical means that could allow their application as antimicrobial agents. This means that the antimicrobial effect can be improved by reducing the effective concentration of the plant-derived compounds [13]. Biochemical and physiological studies have provided a large body of evidence to surmise that plant-derived polyphenols can be well adapted to achieve promising and potent antimicrobials for use in foods, ensuring microbial safety of foods without chemical additives [14,15]. Such biochemically modified natural ingredients would positively affect food preservation without compromising the sensory attributes and health of the consumers.

### **2. Plant Polyphenols as Antimicrobials**

Recent studies have shown that plant compounds used as natural antimicrobials are safe alternatives to chemical additives [16,17]. Natural antimicrobials' mechanism of action includes cell membrane rupture, defective nucleic acid mechanisms, decay of the proton motive force, and depletion of adenosine triphosphate (ATP). The antimicrobials from plants (polyphenols, essential oils) use the aforementioned mechanisms of action against foodborne bacteria [18]. Amongst all secondary metabolites in plants, polyphenols are the ones that play multiple essential roles in plant physiology, also in addition having potential health-benefiting properties such as having antioxidant, antiallergic, anti-inflammatory, anticancer, antihypertensive, and antimicrobial features [19,20]. Basically, they are divided into flavonoids and non-flavonoids, on the basis of their chemical structure.
