antioxidants Review Red Fruits: Extraction of Antioxidants, Phenolic Content, and Radical Scavenging Determination: A Review Gádor-Indra Hidalgo and María Pilar Almajano * Chemical Engineering Department, Universitat Politècnica de Catalunya, Avinguda Diagonal 647, Barcelona E-08028, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-934-016-686 Academic Editor: Stanley Omaye Received: 19 December 2016; Accepted: 13 January 2017; Published: 19 January 2017 Abstract: Red fruits, as rich antioxidant foods, have gained over recent years capital importance for consumers and manufacturers. The industrial extraction of the phenolic molecules from this source has been taking place with the conventional solvent extraction method. New non-conventional extraction methods have been devised as environmentally friendly alternatives to the former method, such as ultrasound, microwave, and pressure assisted extractions. The aim of this review is to compile the results of recent studies using different extraction methodologies, identify the red fruits with higher antioxidant activity, and give a global overview of the research trends regarding this topic. As the amount of data available is overwhelming, only results referring to berries are included, leaving aside other plant parts such as roots, stems, or even buds and flowers. Several researchers have drawn attention to the efficacy of non-conventional extraction methods, accomplishing similar or even better results using these new techniques. Some pilot-scale trials have been performed, corroborating the applicability of green alternative methods to the industrial scale. Blueberries (Vaccinium corymbosum L.) and bilberries (Vaccinium myrtillus L.) emerge as the berries with the highest antioxidant content and capacity. However, several new up and coming berries are gaining attention due to global availability and elevated anthocyanin content. Keywords: red fruit; berry; antioxidant extraction; scavenging assay; phenolic content 1. Introduction In developed countries, alimentation is more focused on complimentary aspects than merely covering major component needs. Because of this, the so called red fruits, or berries, have recently attracted a lot of attention for their antioxidant properties, which are related to the high concentration of polyphenols present in them. In addition, their consumption worldwide has notoriously increased, and red fruits are nowadays not only consumed fresh but also used in cosmetics and dietary supplements. To benefit from these molecules in nutraceuticals, creams and functional foods, an extraction needs to be performed in order to obtain an antioxidant-rich concentrate from a variety of edible berries. The habitual aim is to obtain the maximum extraction yield of the compounds of interest, those that have more antioxidant activity, and, therefore, are capable of being more beneficial to human health, as well as being substitutes for synthetic preservatives, the latter having gained bad press over recent years, especially when part of the final product. In the last few years, several studies analyzing the composition and the antioxidant properties of typical red fruits have been published frequently, and wide research has been taking place all over the world to find the optimal extraction methods to obtain richly antioxidant products for a range of berries. Although conventional solvent extraction is the most widespread technique for the Antioxidants 2017, 6, 7; doi:10.3390/antiox6010007 www.mdpi.com/journal/antioxidants Antioxidants 2017, 6, 7 2 of 27 extraction of antioxidant compounds from red fruits, new non-conventional methods have surfaced as environmentally friendly alternatives to the former method, such as ultrasound [1], microwave [2], and pressure assisted extractions [3], applied alone or together with solvent use, to reduce the energy and solvent requirement. Although extraction techniques seem to have received much attention from researchers, the effects of cultivar [4], storage [5,6], and drying techniques [7,8] have also been studied. This review gathers some of the latest results published in scientific journals about antioxidant extractionAntioxidants and 2017 activity, 6, 7 of red fruits, in order to facilitate a wider vision of this topic. 2 of 28 2. Phenolicberries. AcidsAlthough and conventional Anthocyanidins solvent in Redextraction Fruits is the most widespread technique for the extraction of antioxidant compounds from red fruits, new non-conventional methods have surfaced Berries are characterized by the high amount of antioxidant molecules. These chemical as environmentally friendly alternatives to the former method, such as ultrasound [1], microwave compounds are a group of secondary metabolites that prevent the fruit from oxidation due to [2], and pressure assisted extractions [3], applied alone or together with solvent use, to reduce the environmentalenergy and factors,solvent requirement. such as light, air, oxygen, and microbiological attacks. Phenolic antioxidants interfere withAlthough the oxidationextraction processtechniques as freeseem radical to have terminators received much and attention sometimes from also researchers, as metal chelators. the Phenoliceffects of cultivar compounds [4], storage or polyphenols [5,6], and dryi areng techniques a group of [7,8] hydroxylated have also been molecules studied. very susceptible to oxidation.This review Several gathers studies some haveof the foundlatest results them published to have in various scientific biological journals about properties, antioxidant such as anti-proliferative,extraction and anti-diabetic,activity of red fruits, anticancer, in order anti-microbial, to facilitate a wider anti-inflammatory, vision of this topic. antiviral, and especially important for this review: antioxidant [9]. They have different structures but in general contain 2. Phenolic Acids and Anthocyanidins in Red Fruits an aromatic ring with one or more hydroxyl groups. TheBerries radical are scavenging characterized capacity by the of phenolichigh amount antioxidant of antioxidant molecules molecules. is based These on thechemical ability to becomecompounds radicals are that a aregroup more of secondary stable compared metabolites to thethat majorityprevent the of freefruit radicalfrom oxidation species, due due to to the stabilizationenvironmental of the factors, free electron such as by light, delocalization air, oxygen, and on the microbiological aromatic ring attacks. of the Phenolic phenolic antioxidants compounds. interfere with the oxidation process as free radical terminators and sometimes also as metal chelators. A classification of phenolic antioxidants can be made, the most important being phenolic acids Phenolic compounds or polyphenols are a group of hydroxylated molecules very susceptible to and anthocyanidins,oxidation. Several as studies a subgroup have offound flavonoids. them to have various biological properties, such as Phenolicanti-proliferative, acids cananti-diabetic, be divided anticancer, into two anti-microbial, categories: hydroxybenzoicanti-inflammatory, acid antiviral, derivatives and and hydroxycinnamicespecially important acid for derivatives this review: (Figure antioxidant1). [9 The]. They first have group different includes structures molecules but in general such as hydroxybenzoic,contain an aromatic gallic, ring vanillic, with one and or ellagic more hydroxyl acid (Figure groups.2a). In the second group p-coumaric, caffeic, ferulic, chlorogenicThe radical (Figurescavenging2b), andcapacity hydroxycinnamic of phenolic antioxidant acid can molecules be found. is based on the ability to Thesebecome compounds radicals that can are bemore widely stable found compared in berries, to the and majority each typeof free of radical berrycontains species, due a characteristic to the profilestabilization of phenolic of the molecules. free electron by delocalization on the aromatic ring of the phenolic compounds. AnthocyaninsA classification are water-solubleof phenolic antioxidants plant pigments can be made, responsible the most for important the blue, being purple, phenolic and red acids color of and anthocyanidins, as a subgroup of flavonoids. many plant tissues [10]. Anthocyanidins are based on the flavylium ion, or 2-phenylchromenylium. Phenolic acids can be divided into two categories: hydroxybenzoic acid derivatives and The varietyhydroxycinnamic of chemical acid groups derivatives that can(Figure substitute 1). The thefirst different group includes positions molecules (R1, R2...) such create as the anthocyanidinshydroxybenzoic, found gallic, in nature.vanillic, and A simplification ellagic acid (Fig ofure this 2a). ion,In the focusing second group on the p-coumaric, common caffeic, structures in redferulic, fruits chlorogenic can be seen (Figure in Figure 2b), 3and. hydroxycinnamic acid can be found. Hydroxybenzoic Hydroxycinnamic Hydroxybenzoic Acid DerivativesR1 R1R2 R2 R3 Hydroxycinnamic Acid DerivativesR1 R2 R1R3 R2 R3 Acid Derivatives Acid Derivatives p-Hydroxybenzoicp-Hydroxybenzoic H HOH OH H p-Coumaric p-Coumaric H OH HH OH H ProtocatechuicProtocatechuic OH OH OH OH H Caffeic Caffeic OH OH OHH OH H Vanillic OCH3 OH H Ferulic OCH3 OH H Vanillic OCH3 OH H Ferulic OCH3 OH H Syringic OCH3 OH OCH3 Sinapic OCH3 OH OCH3 Syringic OCH3 OH OCH3 Sinapic OCH3 OH OCH3 Gallic OH OH H Gallic OH OH H FigureFigure 1. 1.Chemical Chemical groupsgroups of of each each acid acid derivative. derivative. Antioxidants 2017, 6, 7 3 of 28 Antioxidants 2017,