**1. Introduction**

Polyphenols are secondary plant metabolites consisting of an aromatic ring to which one or more hydroxyl groups are attached [1]. These compounds are synthesized through plant development and/or as a plant's response to environmental stress conditions [2]. Even when they are at low concentrations in plants, polyphenols protect them from predators or ultraviolet damage [3]. They are known as natural antioxidants and therefore have many beneficial effects on health (e.g., antimicrobial, anti-inflammatory, antioxidant effect, etc.) [4–6]. The health benefits of polyphenols are influenced by the matrix in which they are processed and ultimately consumed [7]. One well-known property of polyphenols is the positive influence on diabetes and obesity due to the possibility of inhibition of digestive enzymes such as α-glucosidase and α-amylase [8]. Anthocyanins are a group of polyphenols responsible for the red, blue and purple color of fruit and vegetables. The major anthocyanin found in most plants is cyanidin-3-glucoside, correlated with reduced reactive oxygen species (ROS) levels and antioxidant potential in in vitro conditions [9,10]. Flavan-3-ols are a subgroup of flavonoids and their main representatives are catechin, epicatechin, epigallocatechin and epigallocatechin-3-gallate. These compounds are abundantly found in green tea, strawberries and black grapes. Studies showed a positive effect of catechin in Alzheimer's and Parkinson's diseases, diabetes and in cancer treatment [11]. Gallic acid, a polyphenol from a group of phenolic acids, a subgroup of hydroxybenzoic acids was the subject of many studies that have proven its significant antioxidant, anticarcinogenic, antimicrobial and antimutagenic effects [12].

Due to the presence of unsaturated bonds in their structures, polyphenols are sensitive to various environmental conditions such as the presence of oxygen, light and water [2]. The presence of water is the most important factor, due to its essentiality in most chemical

**Citation:** Buljeta, I.; Pichler, A.; Šimunovi´c, J.; Kopjar, M. Polysaccharides as Carriers of Polyphenols: Comparison of Freeze-Drying and Spray-Drying as Encapsulation Techniques. *Molecules* **2022**, *27*, 5069. https://doi.org/ 10.3390/molecules27165069

Academic Editor: Nour Eddine Es-Safi

Received: 21 July 2022 Accepted: 7 August 2022 Published: 9 August 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/).

reactions [13,14]. In the food industry, thermal processes are mostly used to obtain edible, microbiologically safe foods, to improve digestibility, and to modulate their textures, flavors and colors [14]. During these processes, structural changes occur leading to the degradation of polyphenols which are often ignored [14]. In order to maintain their stability, they need to be protected, and one of the possible ways is encapsulation in which polysaccharides, proteins, lipids or combinations thereof can be utilized as carriers. In that way, the preservation of polyphenolic properties is achieved over longer periods because the carrier materials represent a barrier to oxygen and water [2]. By encapsulation of polyphenols, besides increased stability, mitigation of unpleasant tastes or flavors, controlled release, improved aqueous solubility and bioavailability can be achieved [3]. Drying has effect on the material's appearance and chemical composition. It also prolongs shelf life and inhibits enzymatic degradation and microbial growth of materials or foods [15]. Adequate selection of a drying method and operating conditions yields foods with slight changes in appearance and maximum retention of bioactive compounds [15].

Spray-drying is a commonly used method for encapsulating due to its simple regulation and control, limited cost, and continuous operation [16]. Freeze-drying is also often used for encapsulation of thermosensitive compounds and materials with some disadvantages such as higher unit cost and long processing time [17]. Suitable selection of carrier and encapsulation technique leads to successful incorporation and retention of bioactive compounds [2]. Food enriched with encapsulated polyphenols can be a versatile and cost-effective approach [14]. In addition, this approach enables other features such as controlled release, improved bioaccessibility and bioavailability for absorption [14].

This paper will provide the literature review of spray-drying and freeze-drying for the encapsulation of polyphenols from different sources. Moreover, with emphasis on polysaccharides, the influence of carrier materials on polyphenol encapsulation will be reviewed.
