Extraction, Identification, and Health Benefits of Anthocyanins In

applied sciences

Review Extraction, Identiﬁcation, and Health Beneﬁts of Anthocyanins in Blackcurrants (Ribes nigrum L.)

Lei Cao 1, Yena Park 2, Sanggil Lee 3,* and Dae-Ok Kim 4,5,*

1 Institute of Marine Life Sciences, Pukyong National University, Busan 48513, Korea; [email protected] 2 Department of Smart Green Technology Engineering, Pukyong National University, Busan 48513, Korea; [email protected] 3 Department of Food Science and Nutrition, Pukyong National University, Busan 48513, Korea 4 Department of Food Science and Biotechnology, Kyung Hee University, Yongin 17104, Korea 5 Graduate School of Biotechnology and Institute of Life Science and Resources, Kyung Hee University, Yongin 17104, Korea * Correspondence: [email protected] (S.L.); [email protected] (D.-O.K.); Tel.: +82-051-629-5851 (S.L.); +82-31-201-2662 (D.-O.K.)

Featured Application: The in-depth discussion on the extraction methods of anthocyanins from blackcurrants and the molecular mechanisms behind its health beneﬁts improves the utilization of blackcurrant anthocyanins in academic and industrial ﬁelds.

Abstract: The fruit of the blackcurrant (Ribes nigrum L.) is round-shaped, dark purple, bittersweet, and seed-containing edible berries. The blackcurrant has been used as a traditional medicine in both Asia and European countries. It is known as a rich source of antioxidants, largely due to its high content of phenolic compounds, especially anthocyanins. Studies on anthocyanins from blackcurrants have adopted different extraction methods and a panel of anthocyanins has been identified in them. Research on the health benefits of blackcurrant anthocyanins has also grown. Citation: Cao, L.; Park, Y.; Lee, S.; To present a general overview of research in blackcurrant anthocyanins, this review focuses on the Kim, D.-O. Extraction, Identification, extraction methods of anthocyanins from blackcurrants and the molecular mechanisms underlying and Health Benefits of Anthocyanins their health benefits. in Blackcurrants (Ribes nigrum L.). Appl. Sci. 2021, 11, 1863. https:// Keywords: blackcurrant; anthocyanins; extraction; identification; health benefits doi.org/10.3390/app11041863

Academic Editor: Claudio Medana

Received: 30 January 2021 1. Introduction Accepted: 16 February 2021 The blackcurrant (Ribes nigrum L.) is a branched shrub with dark purple, bittersweet, Published: 20 February 2021 and seed-bearing berries that can grow to about 1 cm diameter [1]. All parts of the plant, including the fruits (skin, flesh, and seeds) and leaves, can be utilized. The fruit is often Publisher’s Note: MDPI stays neutral used to make dark violet dyes and can be eaten raw or in processed forms like juice, jams, with regard to jurisdictional claims in and jellies. Blackcurrant fruit waste has also been studied for its application in renewable published maps and institutional affil- hair dyes [2]. The leaves of the plant have been used as a flavor enhancer in soups due to iations. their strong scent. Oil extracted from the seeds is often used in cosmetics [3,4]. The fruits and leaves of the blackcurrant have a long history as a traditional medicine in both Asia and Europe [5,6]. Nowadays, blackcurrant extract capsules have been commercialized as a dietary supplement and marketed as an immunity booster. Copyright: © 2021 by the authors. Fresh blackcurrant fruit contains a variety of functional and bioactive compounds Licensee MDPI, Basel, Switzerland. such as soluble sugar, organic acid, diverse vitamins, multi-amino acids, diverse minerals, This article is an open access article elements, and unsaturated fatty acids [7–9]. One of the most studied groups of bioactive distributed under the terms and compounds in blackcurrants are anthocyanins. Anthocyanins exist not only in blackcurrant conditions of the Creative Commons berries but also in its seeds and leaves [10,11]. Attribution (CC BY) license (https:// The anthocyanin presents up to 2–4 g/kg of fresh weight in blackcurrants [12]. It creativecommons.org/licenses/by/ contributes to many sensorial properties, such as color, aroma, taste, and astringency. 4.0/).

Appl. Sci. 2021, 11, 1863. https://doi.org/10.3390/app11041863 https://www.mdpi.com/journal/applsci Appl.Appl. Sci. Sci. 20212021, 11, 11, x, 1863FOR PEER REVIEW 2 2of of 14 14

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AA variety variety of of extraction extraction methods methods have have been been applied applied to to blackcurrants blackcurrants for for anthocyanin anthocyanin extraction.extraction. However, However, it itis isdifficult difficult to to compar comparee the the extraction extraction yields yields among among different different meth- meth- odsods since since many many factors, factors, such such as as cultivars cultivars an andd storage storage conditions, conditions, have have an an impact impact on on the the anthocyaninanthocyanin yield yield [8,17]. [8,17 ].These These discrepancies discrepancies could could give give confusion confusion to to the the people people utilizing utilizing blackcurrantsblackcurrants in in academic academic and and industrial industrial fields. fields. Thus, Thus, this this review review analytically analytically organized organized thethe available available information information and and aimed aimed to to facilitate facilitate a abetter better understanding understanding regarding regarding BCA BCA extractionextraction methodologies, methodologies, as as well asas thethe health health benefits benefits and and related related molecular molecular mechanisms mecha- nismsof BCA. of BCA. With anWith emphasis an emphasis on the molecularon the molecular pathways pathways regulated regulated by BCA, by we BCA, wanted we to wantedmake anto in-depthmake an discussionin-depth discussion on the health-promoting on the health-promoting role of BCA. role of BCA. 2. Extraction Methods 2. Extraction Methods Solid-liquid extraction is a classic approach used to extract anthocyanins from plant Solid-liquid extraction is a classic approach used to extract anthocyanins from plant tissues, such as blackcurrants. The polarity of anthocyanins facilitates their dissolution into tissues, such as blackcurrants. The polarity of anthocyanins facilitates their dissolution polar solvents. Besides traditional solvent methods, new technologies have been tested into polar solvents. Besides traditional solvent methods, new technologies have been for a better extraction yield of anthocyanins. These unconventional technologies include tested for a better extraction yield of anthocyanins. These unconventional technologies microwave-assisted extraction, ultrasonic-assisted extraction, enzyme-assisted extraction, include microwave-assisted extraction, ultrasonic-assisted extraction, enzyme-assisted and pulsed electrical field extraction [18]. extraction, and pulsed electrical field extraction [18]. 2.1. Solvent Extraction for Blackcurrant Anthocyanins 2.1. Solvent Extraction for Blackcurrant Anthocyanins The polarity of the targeted compound is the most critical factor for extraction reagent choice.The polarity Given their of the polarity, targeted anthocyanins compound is are the usually most critical extracted factor by for methanol, extraction ethanol, rea- gentacetone, choice. and/or Given water their [polarity,19]. Ethanol anthocyani and methanolns are usually are more extracted efficient by at methanol, extracting etha- antho- nol,cyanins acetone, from and/or blackcurrants water [19]. than Ethanol water and [20 me].thanol A mixture are more of different efficient solvents at extracting showed an- a thocyaninsbetter extraction from blackcurrants ability than than a single water solvent. [20]. A Inmixture total, 50%of different methanol solvents showed showed a better a betterextraction extraction ability ability than waterthan a andsingle methanol solvent. alone In total, [21]. 50% When methanol 40%, 60%, showed and 96%a better aqueous ex- tractionethanol ability were investigatedthan water and for methanol their extraction alone capacity,[21]. When 60% 40%, ethanol 60%, showedand 96% the aqueous highest ethanolanthocyanin were investigated extraction efficiency for their [22extracti]. on capacity, 60% ethanol showed the highest anthocyanin extraction efficiency [22]. Appl. Sci. 2021, 11, 1863 3 of 14

Although acetone and methanol are effective extraction reagents, their application in foods is restricted due to their toxicity. Ethanol is more suitable for food applications but is more difﬁcult to eliminate during the puriﬁcation process [23]. Additionally, the presence of acid improves anthocyanin extractability due to its ability to liberate those bioactive compounds from blackcurrant pomace and skin [24,25]. It is worth noting that hydrochloric acid may hydrolyze acylated anthocyanins [26]. To avoid the breakdown of acylated anthocyanins, certain organic acids, such as formic, acetic, citric, or tartaric acids, whose elimination is easier during the anthocyanin concentration process, are preferred [24]. To concentrate anthocyanins furthermore, liquid–liquid extraction was additionally performed [27]. In a sequential extraction of blackcurrant juice residues that used water followed by methanol, anthocyanin detection increased approximately 2-fold in the methanol extract. [28].

2.2. Non-Conventional Extraction The major challenges of conventional solvent extraction include longer extraction time, the demand for costly and high purity solvent, evaporation of a large amount of solvent, low extraction selectivity, and the thermal decomposition of thermolabile sub- stances [29]. To overcome these limitations, new and promising extraction techniques have been introduced. These techniques are considered non-conventional extraction techniques. Some of the promising techniques include ultrasound-assisted extraction, enzyme-assisted extraction, microwave-assisted extraction, and pulsed electric ﬁeld assisted extraction [30]. Those techniques reduce solvent consumption and shorten the extraction time, while the extraction yields of anthocyanins are similar to or even higher than those procured with conventional methods. However, no single method is considered as standard for extracting bioactive phytochemicals. Due to the involvement of multiple factors in the non-conventional extraction method, factorial designs and response surface methodologies (RSM) are often used to optimize the extraction conditions. The anthocyanins extracted using RSM are mainly affected by solvent concentration, extraction time, and solvent/solid ratio [31]. The studies that used non-conventional extraction methods to extract anthocyanins from blackcurrants are summarized in Table1.

Table 1. Non-conventional extraction methods of blackcurrant anthocyanin.

Method Condition Efﬁciency 1 Reference pH 2, 10 min extraction time, 700 W Microwave-assisted ↑ 20% [32] microwave power 40 kHz frequency, 10 min extraction ↑ 20% from freeze-dried samples, Ultrasonic-assisted [33] time, 70% amplitude → from frozen or over-dried samples 45 kHz frequency, 15 min extraction Ultrasonic-assisted → [34] time, 90 ◦C Ultrasonic bath, 40 min Ultrasonic-assisted → [35] extraction time Pectinolytic enzyme preparations, Enzyme-assisted → or ↑ [36] 30 min treatment, 60 ◦C Pectinolytic enzyme preparations or Enzyme-assisted ↓ or → [37] protease, 10 min treatment, 100 ◦C Pulsed electrical ﬁeld 1318 V/cm and 315 pulses ↑ 6% [38] 1 Compared to the conventional extraction method performed in each study. ↑ Indicates increased extraction yield. → Indicates no change of extraction yield. ↓ Indicates decreased extraction yield. Appl. Sci. 2021, 11, 1863 4 of 14

2.2.1. Microwave-Assisted Solvent Extraction Microwave-assisted solvent extraction (MASE) is a technique that combines microwave and traditional solvent extraction. Microwaves heat up the molecules using ionic conduction and dipole rotation. The heating up of moisture inside plant cells due to the microwave effect results in evaporation and generates enormous pressure on the cell wall, which then ruptures. This process increases the yield of phytoconstituents [39]. When compared with conventional solvent extraction, MASE increased the anthocyanin extraction yield from blackcurrant marc by more than 20% [32,40]. A group of factors, including microwave power, marc/solvent ratio, pH, and time, showed a signiﬁ- cant effect on the yields of anthocyanin extraction Microwave power and time were the most dominant [32,40].

2.2.2. Ultrasonic-Assisted Extraction Ultrasonic-assisted extraction (UAE) is performed using a high-intensity region of ultrasound (10–1000 W/cm2). Sonication and acoustic cavitation take place in the main body for UAE effects [41]. Sonic waves form gas bubbles by creating areas of alternating pressure in the medium. At the critical points of these gas bubbles, rapid condensation occurs and the energy inside the bubbles release. This creates shock waves with a large amount of heat and energy, which then physically affect the sample [42,43]. UAE facilitates solvent penetration and mass transfer. It therefore reduces the solvent usage and extraction temperature and it increases extraction rate and yield [44]. Compared to conventional extraction methods, UAE leads to a higher amount of and more stable polymeric anthocyanins in anthocyanin-rich fruit extracts [45]. Ultrasound pretreatment mixed with conventional extraction showed different results depending on the blackcurrant sample types: frozen, lyophilized, and over-dried [33]. Ultrasonic treatment increased total anthocyanins yield by 20% from freeze-dried samples but showed no effect on frozen or over-dried samples [33]. Additionally, UAE did not show an enhanced extraction efﬁciency of anthocyanins from blackcurrants in another two studies, which used frozen samples [34,35].

2.2.3. Enzyme-Assisted Extraction Cell wall degrading enzymes (e.g., cellulases, pectinases, proteases, and α-amylase) can degrade and break down cell structure, and are thereby used to facilitate the extraction of intracellular contents, including anthocyanins [46]. Enzyme treatment with a panel of pectinolytic enzyme preparations and protease significantly enhance plant cell wall breakdown of blackcurrants [37]. A variety of pectinolytic enzyme preparations were used upon blackcurrant pulp samples during the maceration step and some of them significantly enhanced the anthocyanin yields in blackcurrant juice, while others showed no effect [36]. Inconsistent findings were reported by another group of researchers. A panel of pectinolytic preparations showed no effect on or decreased the extraction yields of anthocyanins from pomace remaining from blackcurrant juice [37]. Given the different types of samples used in the two studies (pulp vs. pomace), the enzymes might have worked differently [36,37].

2.2.4. Pulsed Electric Field Pulsed electric ﬁeld (PEF) treatment is a non-thermal technology that exposes a sample to repetitive short voltage pulses at relatively low energy and moderate intensity. The application of PEF in plant tissues improves the permeabilization of the cell membranes which thus enhances the release of liquid and targeted compounds from the cells [47]. To the best of our knowledge, there has been only one study on the effect of PEF treatment on BCA extraction. It reported that under optimum treating conditions an increment of 6% for total monomeric anthocyanins extracted from blackcurrants was found [38]. Given the limited study on PEF treatment, the effect of PEF on BCA extraction needs further investigation. Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 14

[38]. Given the limited study on PEF treatment, the effect of PEF on BCA extraction needs further investigation. The adoption of non-conventional extraction methods did not show a remarkable increase in the extraction yield of anthocyanins from blackcurrants. However, the reduced extraction time and solvent consumption were significant. For example, MASE greatly reduced the extraction time from 300 min to 10 min and reduced the solvent to sample ratio from 40:1 to 20:1 [32]. UAE also reduced the extraction time from 60 min to 10 min [33]. In addition, the studies on enzyme-assisted extraction showed they should be carried out with great caution given the possible negative effect of cell wall degrading enzymes on BCA extraction.

3. Identification of Blackcurrant Anthocyanins Appl. Sci. 2021, 11, 1863 Four major anthocyanins in blackcurrants were analyzed using high-performance5 of 14 liquid chromatography with a diode-array detector (HPLC-DAD), with mass spectrometry (HPLC-DAD-MS), or with tandem mass spectrometry (HPLC-DAD-MS/MS) (Figure 2). OverThe 70% adoption of the of antioxidant non-conventional capacity extraction of blackcurrants methods came did from not showthose aanthocyanins remarkable increase[22,48]. Four in the dominant extraction anthocyanins—cyanidin-3-O-glucoside, yield of anthocyanins from blackcurrants. cyanidin-3-O-rutinoside, However, the reduced extractiondelphinidin-3-O-glucoside, time and solvent and consumption delphinidin-3-O-rutinoside—accounted were signiﬁcant. For example, for MASE 90% of greatly the to- reducedtal anthocyanin the extraction content time in fromvarious 300 cultivars min to 10 of min blackcurrants and reduced [48,49]. the solvent They to were sample also ratio the frommajor 40:1 anthocyanins to 20:1 [32 ].discovered UAE also reducedin the buds, the extractionleaves, and time seeds from of 60blackcurrants min to 10 min [10,50]. [33]. InAmong addition, the thefour studies anthocyanins, on enzyme-assisted delphinidin-3- extractionO-rutinoside showed was they the should most beprevalent carried outfol- withlowed great by cyanidin-3-O-rutinoside, caution given the possible delphinid negativein-3-O-glucoside, effect of cell wall then degrading cyanidin-3-O-gluco- enzymes on BCAside [48,49]. extraction. The other minor anthocyanins that were identified in various blackcurrant cultivars 3.using Identiﬁcation HPLC-DAD-MS of Blackcurrant included Anthocyaninsthe 3-O-glucosides and the 3-O-rutinosides of pelargonidin,Four cyanidin, major anthocyanins peonidin, delphinidin, in blackcurrants petuni weredin, analyzed malvidin, using cyanidin high-performance 3-O-arabinoside, liquid chromatographyand the 3-O-(6‘‘-p-coumaroylglucoside)s with a diode-array detector of (HPLC-DAD),cyanidin and delphinidin with mass spectrometry [22,51]. (HPLC- DAD-MS),The bioavailability or with tandem of mass anthocyanins spectrometry in blackcurrants (HPLC-DAD-MS/MS) were also ( Figurestudied.2 ). After Over 70%con- ofsuming the antioxidant a 20% blackcurrant capacity of juice blackcurrants (250 mL), the came abundance from those of anthocyaninsconsumed anthocyanins [22,48]. Four in dominanturine was anthocyanins—cyanidin-3-O-glucoside,0.021 ± 0.003% of delphinidin glycosides cyanidin-3-O-rutinoside, and 0.009 ± 0.002% of cyanidin delphinidin-3- glyco- O-glucoside,sides over a 24 and h delphinidin-3-O-rutinoside—accountedperiod [52]. In another study, the recoveries for 90% of ofdelphinidin-3-O-rutino- the total anthocyanin contentside and in cyanidin-3-O-rutinoside various cultivars of blackcurrants in urine samp [48les,49 were]. They 0.040 were± 0.011% also and the 0.048 major ± 0.016%, antho- cyaninsrespectively, discovered over a in period the buds, of 48 leaves, h after and bl seedsackcurrant of blackcurrants juice consumption [10,50]. Among [53]. Although the four anthocyanins,anthocyanins delphinidin-3-O-rutinosidein blackcurrants present a was low the bioavailability, most prevalent their followed metabolites by cyanidin-3-O- may con- rutinoside,tribute to the delphinidin-3-O-glucoside, in vivo protective effects thenobserved. cyanidin-3-O-glucoside [48,49].

FigureFigure 2.2. TheThe structuresstructures of of the the major major anthocyanins anthocyanins in in blackcurrants blackcurrants including including cyanidin-3-O-glucoside, cyanidin-3-O-gluco- cyanidin-3-O-rutinoside,side, cyanidin-3-O-rutinoside, delphinidin-3-O-glucoside, delphinidin-3-O-glucoside, and delphinidin-3-O-rutinoside. and delphinidin-3-O-rutinoside.

The other minor anthocyanins that were identiﬁed in various blackcurrant cultivars using HPLC-DAD-MS included the 3-O-glucosides and the 3-O-rutinosides of pelargonidin, cyanidin, peonidin, delphinidin, petunidin, malvidin, cyanidin 3-O-arabinoside, and the 3-O-(600-p-coumaroylglucoside)s of cyanidin and delphinidin [22,51]. The bioavailability of anthocyanins in blackcurrants were also studied. After consum- ing a 20% blackcurrant juice (250 mL), the abundance of consumed anthocyanins in urine was 0.021 ± 0.003% of delphinidin glycosides and 0.009 ± 0.002% of cyanidin glycosides over a 24 h period [52]. In another study, the recoveries of delphinidin-3-O-rutinoside and cyanidin-3-O-rutinoside in urine samples were 0.040 ± 0.011% and 0.048 ± 0.016%, respectively, over a period of 48 h after blackcurrant juice consumption [53]. Although anthocyanins in blackcurrants present a low bioavailability, their metabolites may contribute to the in vivo protective effects observed. Appl. Sci. 2021, 11, 1863 6 of 14

4. Health Benefits of Blackcurrant Anthocyanins and Related Molecular Mechanisms We have previously discussed the health benefits of blackcurrants [1]. However, we wanted to focus on the anthocyanins in this review; therefore, only studies that used blackcurrant extracts rich in anthocyanin are included. Extracts containing more than 20% anthocyanins are included in the following discussion. In addition, some of the extracts used in certain studies were commercially purchased from fruit extract companies. The concentrations of anthocyanins in those extracts may not have been determined and listed in pertinent articles. Furthermore, the underlying molecular mechanisms through which BCA exerts the health benefits are also discussed.

4.1. Antioxidant and Anti-Inflammatory Activities BCA was widely studied for its antioxidant and anti-inflammatory activities. BCA was shown to alleviate the reactive oxidative species (ROS) production induced by both menadione and H2O2 in SH-SY5Y human neuroblastoma cells [54,55]. In addition, a clinical trial showed that consumption of BCA prior to exercise facilitated recovery from exercise-induced oxidative stress [56]. BCA exerts cellular antioxidant activity, at least partly, through activating the nuclear factor erythroid 2-related factor 2 (NRF2) pathway. As a transcription factor, NRF2 regulates cellular resistance to oxidants in addition to drug metabolism. It controls the basal and induced expression of a group of antioxidant response element (ARE)-dependent genes, including several drug-metabolizing enzymes (e.g., glutathione S-transferase, NAD(P)H, and quinone oxidoreductase) and a group of antioxidant defense genes (e.g., heme oxyge- nase 1) [57]. The elevated expressions of NRF2 by BCA treatment was observed in rats with diethylnitrosamine (DENA)-initiated hepatocarcinoma [58]. Another study that used bone marrow-derived macrophages (BMM) from NRF2+/+ mice and NRF2−/− mice showed that BCA significantly decreased the elevated cellular ROS level in lipopolysaccharides (LPS)-stimulated NRF2+/+ BMM but not NRF2−/− BMM [59]. This study illustrated the critical role NRF2 plays in the antioxidant capacity of BCA. Besides the antioxidant activity, BCA also exerted an anti-inflammatory effect in various models. Pretreatment with BCA or cyanidin-3-O-glucoside inhibited the LPS-induced secretion of interleukin-6 by U937, a human monoblastic leukemia cell line [15]. A similar inhibition effect of BCA on LPS- stimulated cytokine secretion was also observed in THP-1 cells, another human monocytic cell line [60]. A commercially available BCA product attenuated ovalbumin-induced inflammation in a mouse model of acute allergic lung inflammation [61]. Another commercialized BCA product prevented inflammation in diet-induced obese mice, especially macrophage infiltration in the adipose tissue [62]. The widely studied anti-inflammation effect of BCA is mostly contributed to its suppression of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway. NF-κB, as a transcription factor, regulates many genes involved in inflammation and immune responses. These include chemokines, pro-inflammatory cytokines, adhesion molecules, and inducible enzymes (e.g., inducible nitric oxide synthase (iNOS) and cycloxygenase-2) [63]. BCA attenuated LPS-induced NF-κB signaling in multiple macrophage cell lines [59,60]. A recent study showed that consumption of purified anthocyanins isolated from bilberries and blackcurrants inhibited the expressions of proinflammatory genes related to the NF-κB pathway in whole blood samples from human subjects with metabolic syndrome [64].

4.2. Phytoestrogenic Activity Estrogens have an impact on the functions of organs and tissues such as bone, blood vessels, skin, and hair. There are two subtypes of estrogen receptor (ER): ERα and ERβ. ERα is mainly present in female reproductive organs, and ERβ is expressed all over the body regardless of sex [65]. Phytoestrogens are plant-derived compounds with estrogenic effects. The phytoestrogenic activity of BCA was observed in ovariectomized (OVX) rats and mice. BCA increased the levels of extracellular matrix proteins (e.g., collagen, elastin, Appl. Sci. 2021, 11, 1863 7 of 14

and hyaluronic acid) in the skin of OVX rats [66]. Besides the effect in skin, BCA was also effective in mitigating hair loss in OVX rats and attenuating bone loss and bone resorption activity in OVX mice [67,68]. Some studies put forward that this phytoestrogenic activity of BCA could be mediated through ERα and ERβ [69,70]. The four major BCA all induced ERα and ERβ-mediated transcriptional activities, meanwhile they showed a higher affinity for binding to ERβ than ERα. They also upregulated the downstream target genes of ER [69,70]. BCA upregulated the expression of various estrogen signaling-related genes in the human female skin fibroblast cell line. The predicted upstream regulator was estradiol. Microarray profiling of human female skin fibroblasts showed that ERα was strongly activated after it was exposed to BCA [66].

4.3. Anti-Postprandial Hyperglycemic and Anti-Diabetic Effect Postprandial hyperglycemia refers to the rapid rising of blood glucose level following a high carbohydrate meal. The regulatory effect of BCA on postprandial hyperglycemia has been previously reported. Both one-time consumption of BCA before a high-carbohydrate meal and 8-day supplementation of BCA reduced postprandial glycemia in humans [71,72]. The α-amylase and α-glucosidase inhibitory activities of BCA were proposed as major reasons [73,74]. The hydrolysis of starch by the digestive enzymes, including α-amylase and α-glucosidase, is a major cause of postprandial hyperglycemia [75]. Besides the effect on postprandial hyperglycemia, BCA also showed an anti-diabetic effect. Dietary BCA signiﬁcantly improved glucose tolerance in type 2 diabetic mice [76]. Consumption of BCA for 8 weeks alleviated body weight gain and improved glucose metabolism in both low- and high-fat diet-fed mice [77]. The preventive effect of BCA on dyslipidemia and hepatic steatosis, which are strongly associated with type 2 diabetes, was also observed in OVX rats [78]. The studies on the molecular mechanisms underlying the anti-obesity effects suggested that modulation of adenosine monophosphate-activated protein kinase (AMPK) and lipid metabolism- associated genes may contribute to these effects [76]. AMPK pathway, as a part of the energy sensing network, is critical at controlling energy expenditure. The activation of AMPK by BCA was observed in multiple animal models, including diabetic mice, nonalcoholic steatohepatitis (NASH) mice, and high-fructose fed rats [76,79,80]. BCA alleviated metabolic syndrome in those mice. It is noteworthy that an intact gut microbiome was found essential for BCA exerting those effects [77]. Therefore, this study suggested the interactions between BCA and the gut microbiome is critical for the anti-diabetic effect of BCA.

4.4. Cardioprotective Effect BCA was reported effective at maintaining or improving cardiovascular health. A single dose of BCA along with smoking was shown to attenuate the acute endothelial dysfunction induced by smoking and improve the peripheral temperature in young smokers [81]. Short-term BCA intake mitigated central arterial stiffness and central blood pressure in senior subjects [82]. A seven day intake of BCA displayed dose-dependent changes on certain cardiovascular parameters, such as increased cardiac output, increased stroke volume, and decreased total peripheral resistance during supine rest in endurance- trained male cyclists [83]. Six months of BCA consumption also improved the endothelial function in healthy subjects with habitually low fruit and vegetable intake [84]. The underlying pathology for cardiovascular diseases (CVD) is atherosclerosis. This pathology is complex and involves the structural elements of the arterial wall, circulating cells, and some inﬂammatory cells [85]. Given the complex pathology of atherosclerosis, the cardioprotective effect of BCA also involves a panel of action mechanisms. Two major initiations involved in the development of CVD are the production of reactive oxidation species (ROS) via vessels and lipid oxidation [86]. Excessive ROS may promote endothelial dysfunction, a contributing factor to atherosclerosis. The molec- Appl. Sci. 2021, 11, 1863 8 of 14

ular mechanisms of BCA combatting excessive ROS are discussed in this review. On the other hand, the oxidation of low-density lipoprotein (LDL) in the vessel wall induced an inflammatory cascade that activates many atherogenic pathways, which led to the formation of foam cells [86]. The accumulation of foam cells results in the formation of a fatty streak, which is the earliest visible atherosclerotic lesion. The anti- inflammatory activity of BCA and related molecular mechanisms was covered in other parts of this review. Atherosclerotic lesions are initiated through elevated LDL uptake by monocytes and macrophages. The reduction of LDL reduces the risks of CVD [87]. Consumption of anthocyanins from bilberries and blackcurrants decreased LDL-cholesterol levels in dyslipidemic subjects and subjects with metabolic syndrome [88,89]. One of the therapeutic goals to lower circulating LDL-cholesterol is to induce LDL receptor (LDLR) expression and activity in the liver. BCA increased protein levels of LDLR in Caco-2 cells, which led to the enhanced transport of LDL-cholesterol to the apical side of the enterocytes [90]. The BCA-induced increased hepatic LDLR protein expression was also reported in mice fed a high fat and high cholesterol diet [91]. Nitric oxide (NO) is a key cellular messenger in the cardiovascular system [92]. NO maintains vascular integrity through inhibiting platelet aggregation, leukocyte-endothelium adhesion, and proliferation of vascular smooth muscle. Diminished NO bioavailability was shown to cause endothelial dysfunction and increased susceptibility to atherosclerosis [93]. NO is generated by three isoforms of nitric oxide synthases (NOS): neuronal, inducible, and endothelial NOS. BCA activated endothelial NOS in vitro in human endothelial cells and in vivo in blood vessel endothelial cells acquired from OVX rats [94,95]. BCA-treated endothelial cells also showed higher NO levels [95]. Considering the crucial role of inflammation in the pathogenesis of atherosclerosis, the anti-inflammatory activity of BCA also provided a cardioprotective effect. The cardiovascular effect of BCA was not observed in hyperlipidemic rabbits [96].

4.5. Neuroprotection and Cognitive Improvement Studies show that BCA administration delayed cognitive deficit and provided a neuroprotective effect. BCA suppressed the neurotoxic effects of rotenone, which can be used for developing a Parkinson’s disease-like phenotype in a primary cell culture model [97]. Long-term supplementation of BCA reduced the spatial working memory deficit in aged APdE9 mice, which were Alzheimer’s disease model mice [54]. An acute BCA supplementation showed a cognitive benefit in healthy young humans [98]. The mechanisms underlying neuroprotective effects of BCA need further investigation. The proposed mechanisms have included modulation of oxidative stress [99]. However, other studies showed that compounds besides anthocyanins in the extracts may account for their neuroprotective effect [98,100].

4.6. Chemoprevention An anthocyanin-rich fraction of blackcurrant fruit skin extract exhibited a potent cyto- toxic effect on HepG2 human liver cancer cells [101]. It also decreased the incidence, total number, and size of preneoplastic hepatic nodules in rats with DENA-initiated phenobarbital-promoted hepatocarcinoma [5]. Further mechanistic studies suggested that BCA exerted chemopreventive actions against DENA-inﬂicted hepatocarcinogenesis by attenuating oxidative stress through activation of NRF2 pathway along with repressing inﬂammatory responses through the suppression of the NF-κB pathway [58,102]. NRF2 and NF-κB had a modulating role in cancer pathogenesis and progression [103]. Phase II enzymes, whose expressions are regulated by NRF2, detoxify a body of environmental carcinogens and therefore promote their subsequent excretion. The induction of NF-κB pathway thus contributes to tumorigenesis by transactivating several genes that are connected to tumor promotion [103]. Appl. Sci. 2021, 11, 1863 9 of 14

4.7. Vision and Eye Health The positive effects of BCA on vision and eye health were shown on chicks, healthy human subjects, and glaucoma patients. Oral administration of BCA and intravenous administration of major blackcurrant anthocyanins significantly inhibited the elongation of vitreous chamber depth and axial length caused by negative lenses in chicks [104,105]. In healthy humans, acute ingestion of BCA ameliorated dark adaptation and work- induced transient myopic shift [106]. Administration of BCA for 2 weeks significantly decreased the intraocular pressure in healthy subjects [107]. The effect of BCA on vision was also studied on glaucoma patients. It was found that 24 months of administration of BCA decreased the intraocular pressure in glaucoma patients and delayed the visual field loss and elevated ocular blood flow [108]. The mechanisms of vision benefits of BCA have not been thoroughly studied. One proposed mechanism was mediating plasma endothelin-1 (ET-1) [109]. ET-1, serving as a potent vasoconstrictor, has been suggested to play a role in the local autoregulation of blood flow. However, considering the inconsistent conclusions in regards to ET-1 level change on intraocular pressure [110], this mechanism needs to be further tested.

5. Conclusions Blackcurrants are rich in color, nutrients, and flavor. The research on blackcurrants is growing. A comprehensive review of extraction and identification of anthocyanins in blackcurrants has provided a better understanding of the application of anthocyanins for promoting health and a thorough discussion on the related molecular mechanisms elucidated the nutritional values of BCA. Compared with conventional extraction methods, non-conventional extraction techniques include additional factors such as microwaves, ultrasounds, enzymes, and pulsed electric fields, all of which allow the use of less solvents and energy. Not all studies that adopted nonconventional extraction methods significantly increased the extraction yield of anthocyanins from blackcurrants. However, considering the reduced extraction time and solvent usage, non-conventional extraction methods still provided certain advantages for BCA extraction. The health benefits of BCA suggested the potential for BCA use as a key ingredient of functional food or therapeutic products to treat or prevent various chronic diseases. Its antioxidant and anti-inflammatory activities have been widely studied. They protect against oxidative stress, neuron toxicity, and carcinogens. The phytoestrogenic activity also explains certain anti-aging effects of BCA. In addition, the contribution of the single compound and synergistic effect should be considered for the therapeutic application of BCA.

Author Contributions: Conceptualization, S.L., D.-O.K., and L.C.; data curation, L.C. and Y.P.; writing—original draft preparation, S.L., D.-O.K., and L.C.; writing—review and editing, S.L., D.-O.K., L.C., and Y.P.; supervision, S.L. and D.-O.K.; project administration, S.L. and D.-O.K.; funding acquisition, S.L. and D.-O.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Pukyong National University Research Fund—grant number C-D-20180570. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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