molecules Review Pomegranate Peel as Suitable Source of High-Added Value Bioactives: Tailored Functionalized Meat Products Patricia Gullón 1, Gonzalo Astray 2,3, Beatriz Gullón 4, Igor Tomasevic 5 and José M. Lorenzo 6,7,* 1 Nutrition and Bromatology Group, Department of Analytical and Food Chemistry, Faculty of Food Science and Technology, University of Vigo, Ourense Campus, 32004 Ourense, Spain; [email protected] 2 Department of Physical Chemistry, Faculty of Science, University of Vigo (Campus Ourense), As Lagoas, 32004 Ourense, Spain; [email protected] 3 CITACA, Agri-Food Research and Transfer Cluster, Campus Auga, University of Vigo, 32004 Ourense, Spain 4 Department of Chemical Engineering, Faculty of Science, University of Vigo (Campus Ourense), As Lagoas, 32004 Ourense, Spain; [email protected] 5 Department of Animal Source Food Technology, University of Belgrade, Faculty of Agriculture, Nemanjina 6, 11080 Belgrade, Serbia; [email protected] 6 Centro Tecnológico de la Carne de Galicia, Rúa Galicia No 4, Parque Tecnológico de Galicia, San Cibrao das Viñas, 32900 Ourense, Spain 7 Área de Tecnología de los Alimentos, Facultad de Ciencias de Ourense, Universidad de Vigo, 32004 Ourense, Spain * Correspondence: [email protected]; Tel.: +34-988-548-277; Fax: +34-988-548-276 Academic Editor: Lillian Barros Received: 30 May 2020; Accepted: 19 June 2020; Published: 21 June 2020 Abstract: In the last few years, the consumer’s concern with the relationship between health and diet has led to the search of foods with functional properties beyond the nutritional. In this framework, the consumption of pomegranate has increased due to their sensorial attributes and remarkable amounts of bioactive compounds, which generate, at the same time, huge amounts of by-products. A search in the Scopus database for the last 10 years has revealed the rising interest in pomegranate peel (PP), the main residue from this fruit. The meat industry is a food sector that has had to search for new alternatives to substitute the use of synthetic preservatives by new natural additives, to extend the self-life and keep the quality attributes of their processed products. This review sets out the main bioactivities of PP extracts, and their incorporation in meat products is elaborated. PP is a good source of bioactive compounds, including phenolic acids, flavonoids and hydrolyzable tannins, which have beneficial health effects. It can be concluded that the reformulation of meat products with PP extracts is a suitable strategy for enhancing their technological characteristics, in addition to conferring functional properties that make them healthier and potentially more acceptable for the consumer. Keywords: pomegranate; bioactive compounds; meat products; oxidative stability 1. Introduction In western diets, meat and meat products are one of the main sources of high-biological value protein, in addition to containing micronutrients such as minerals (iron, magnesium, potassium, selenium and sodium) and vitamins (A, B12, folic acid, among others) that are highly bio-available [1]. Despite these excellent nutritional properties, the intake of meat and meat products is related with a higher incidence of cardiovascular diseases and obesity, increasing the negative perception attached to Molecules 2020, 25, 2859; doi:10.3390/molecules25122859 www.mdpi.com/journal/molecules Molecules 2020, 25, 2859 2 of 18 the consumers of these food products in recent years [2]. On the other hand, due to its rich nutritional profile, the meat presents a high susceptibility to deterioration due to the microbial growth and oxidation processes that take place when the muscle is transformed in the meat during the meat processing and storage [3–6]. To overcome these drawbacks, and in line with the rising awareness by consumers of the relationship between diet and health [7], in the last few years, the meat industry has had to face the double challenge of offering more healthy meat processed products, and guaranteeing their stability [8–11]. Until now, synthetic antioxidants, such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), have been widely used in the food industry as preservative agents to extend the self-life of food products, but their incorporation should be reduced due to their harmful effects on health, as described in in vitro and in vivo studies [4]. The need to find alternatives to substitute the synthetic additives has led to scientific researchers and meat industries devoting great efforts to using renewable biomass as a natural source of new biomolecules with functional properties [7,12–15]. In this context, the agri-food industry produces large amounts of waste and by-products that usually are underutilized or discarded, but that contain remarkable amounts of bioactive compounds with properties that make them suitable to be used as natural ingredients [16–19]. Therefore, this strategy outlines a promising approach that will allow the removal of a residue without economical value through the recovery of extracts rich in biomolecules, that could then be incorporated in the food chain, contributing to a circular economy model [20–23]. Due to the therapeutic properties associated with pomegranate, its consumption in the form of fresh fruit, juices, jams or dietary supplements has increased significantly in the last decade [5,7,8,12]. The industrial processing of this fruit generates huge amounts of by-products, mainly peels and seeds, which usually are discarded as waste without any valorization [24]. Among these residues, pomegranate peel (PP, around 40–50% of the total fruit weight) is an excellent source of phenolic compounds (flavonoids, phenolic acids and tannins), protein and bioactive peptides, and polysaccharides, among others [7,24]. Several in vitro bioactivities, such as antioxidant, anti-inflammatory, antitumor and antiproliferative activities, have been described for PP extracts in the literature [25–28]. The wide variety of biomolecules with different functionalities suggests that PP extracts could be used in the meat industry as a functional ingredient [5,9,11,12]. In this sense, the reformulation of meat products with extracts from PP could be an alternative for solving the mentioned inconveniences, and it opens the possibility of formulating customizable meat products tailored to the requirements of certain population groups. Due to this, a growing interest in PP is reflected in the research studies concerning this residue. A key point in the obtaining of biomolecules from agri-food residues is the selection of the adequate extraction technology, since it should be efficient and should maintain the bioactivities in order to guarantee their functionality when they are incorporated in a food matrix [2]. Traditionally, conventional methods based on organic solvents and their aqueous mixtures have been widely used in the recovery of phytochemicals from agri-food sources. However, due to the fact that they present disadvantages that negatively affect the bioactive properties of the extracted biomolecules, in the last few years, eco-friendly and sustainable avant-garde technologies have been developed and applied in combination with smart solvents, namely deep eutectic solvents and natural deep eutectic solvents, in the field of the recovery of natural functional ingredients. Among these techniques, microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), pressurized liquid extraction (PLE) and supercritical fluid extraction (SFE) are being used in the recovery of biomolecules from PP [23,25,29,30]. This review collects the research advanced in the last 10 years on the potential use of PP as a suitable source of high-added value biocompounds for formulating tailored functionalized meat products. Aspects such as the main phytochemicals present in PP and their bioactivities are revised. The role of the extracts, in both the technological and health properties of reformulated meat products, are also featured. Molecules 2020, 25, 2859 3 of 18 Molecules 2020, 25, x 3 of 18 2. Main Bioactive Constituents Present in Pomegranate Peel 2. Main Bioactive Constituents Present in Pomegranate Peel It has been described that PP is an excellent source of valuable biocompounds, including phenolic It has been described that PP is an excellent source of valuable biocompounds, including acids (hydroxycinnamic and hydroxybenzoic acids), flavonoids (anthocyanins, catechins and other phenolic acids (hydroxycinnamic and hydroxybenzoic acids), flavonoids (anthocyanins, catechins complex flavonoids) and hydrolyzable tannins (ellagic and gallic acids, pedunculagin, punicalin and other complex flavonoids) and hydrolyzable tannins (ellagic and gallic acids, pedunculagin, and punicalagin), all of them with proven beneficial health effects [26,27,31]. Besides, pomegranate punicalin and punicalagin), all of them with proven beneficial health effects [26,27,31]. Besides, by-products also contain organic acids, minerals (calcium, phosphorus, magnesium, potassium and pomegranate by-products also contain organic acids, minerals (calcium, phosphorus, magnesium, sodium), protein and fatty acids (mainly punicic, linoleic and oleic acids present in the seeds) [27,31]. potassium and sodium), protein and fatty acids (mainly punicic, linoleic and oleic acids present in Some of the main bioactive components are briefly described next. The chemical structures of the main the seeds) [27,31]. Some of the main bioactive components are briefly described next. The chemical biomolecules