nutrients Review Hop Compounds: Extraction Techniques, Chemical Analyses, Antioxidative, Antimicrobial, and Anticarcinogenic Effects Maša Knez Hrnˇciˇc 1,†, Eva Španinger 2,†, Iztok Jože Košir 3, Željko Knez 1 and Urban Bren 2,* 1 Laboratory of Separation Processes and Product Design, Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova ulica 17, SI-2000 Maribor, Slovenia; [email protected] (M.K.H.); [email protected] (Ž.K.) 2 Laboratory of Physical Chemistry and Chemical Thermodynamics, Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova ulica 17, SI-2000 Maribor, Slovenia; [email protected] 3 Slovenian Institute of Hop Research and Brewing, Cesta Žalskega Tabora 2, SI-3310 Žalec, Slovenia; [email protected] * Correspondence: [email protected]; Tel.: +386-2-2294-421 † These authors contributed equally to this work. Received: 7 December 2018; Accepted: 18 January 2019; Published: 24 January 2019 Abstract: Hop plants comprise a variety of natural compounds greatly differing in their structure and properties. A wide range of methods have been developed for their isolation and chemical analysis, as well as for determining their antioxidative, antimicrobial, and antigenotoxic potentials. This contribution provides an overview of extraction and fractionation techniques of the most important hop compounds known for their health-promoting features. Although hops remain the principal ingredient for providing the taste, stability, and antimicrobial protection of beer, they have found applications in the pharmaceutical and other food industries as well. This review focuses on numerous health-promoting effects of hops raging from antioxidative, sedative, and anti-inflammatory potentials, over anticarcinogenic features to estrogenic activity. Therefore, hops should be exploited for the prevention and even healing of several prevalent diseases like cardiovascular disorders and various cancer types. New ideas for future studies on hops are finally presented: computational investigations of chemical reactivities of hop compounds, nanoencapsulation, and synergistic effects leading to a higher bioavailability of biologically active substances as well as the application of waste hop biomass from breweries for the production of high-added-value products in accordance with the biorefinery concept. Keywords: hops; extraction; antioxidative effects; antimicrobial effects; anticarcinogenic effects 1. Introduction Over the last decades, the scientific world has turned its focus on exploring the biological effects of plants used in folk/traditional medicine. Such interest seems justified as almost all described effects of healing plants were also demonstrated through several in vitro and even in vivo studies. Therefore, it is reasonable to continue exploring in this field as gained knowledge represents the basis for development of new food supplements for disease prevention or even for the design of novel drugs, especially for the most prevalent threatening diseases of our time such as cardiovascular disorders, diabetes, Alzheimer’s, and even cancer [1–4] Increased demand for material goods is related to an increased occurrence of certain diseases that can be mostly ascribed to inappropriate dietary habits and to a stressful rapid way of life [5,6] The food industry is in its prime time; however, along with obvious advantages this also brings Nutrients 2019, 11, 257; doi:10.3390/nu11020257 www.mdpi.com/journal/nutrients Nutrients 2018, 10, x FOR PEER REVIEW 2 of 40 Nutrients 2019, 11, 257 2 of 37 numerous harmful consequences. Plain, organically grown food is becoming a privilege of the wealthy. Whereas, the diet of the poor and the middle class increasingly consists of almost exclusively numerousanti-nutritional harmful aliments consequences. filled with Plain, synthetic organically sugars, grown sensory food is becomingproperties aenhancers, privilege ofgrowth the wealthy.hormones, Whereas, and preservatives the diet of the [7]. poor Listed and thesubstances middle class are undoubtedly increasingly consistsconnected of almostwith the exclusively occurrence anti-nutritionalof several most aliments prevalent filled diseases with synthetic as cardiovascular sugars, sensory or neurological properties disorders enhancers, and growth cancer hormones, [7]. and preservativesHop plant ( [Humulus7]. Listed lupulus substances L.), presented are undoubtedly in Figure connected1, has been with in a continuous the occurrence use for ofseveral centuries mostor even prevalent a millennium diseases asmostly cardiovascular as an ingredient or neurological of beer, although disorders some and of cancer its medicinal [7]. properties have beenHop known plant as (Humulus well. Nowadays lupulus L.), its presented sedative effect in Figure is the1, hasmost been well-known, in a continuous certain use food for supplements centuries oron even its abasis millennium already exist mostly for as treating an ingredient sleep disorders of beer, although [8]. Several some in vitro of its and medicinal in vivo propertiesstudies also have show beenthat known certain as hop well. compounds Nowadays itscarry sedative a potential effect is for the becoming most well-known, novel anticancer certain food agents supplements as they onexert itssignificant, basis already numerous, exist for treating and diverse sleep beneficial disorders biological [8]. Several activities.in vitro and Is itin therefore vivo studies of utmost also show importance that certainto pursue hop compounds the investigation carry a potentialof in vivo for potential becoming of novel hop anticancer and hop agentsconstituents as they as exert novel significant, drugs or numerous,anticancer and agents. diverse beneficial biological activities. Is it therefore of utmost importance to pursue the investigation of in vivo potential of hop and hop constituents as novel drugs or anticancer agents. (a) (b) FigureFigure 1. 1.(a )(a Hop) Hop plant plant (Humulus (Humulus lupulus lupulusL.) L.) growing growing on on a trellis.a trellis. (b )(b Tissue) Tissue cultured cultured hop hop plant plant (Humulus(Humulus lupulus lupulusL.). L.) Photos. Photos were were taken taken by by Dr. Dr. Zala Zala Kolenc Kolenc at theat the premises premises of Slovenianof Slovenian Institute Institute of of HopHop Research Research and and Brewing. Brewing. NumerousNumerous publications publications on on isolation isolation of of active active compounds compounds have have appeared appeared over over the the past past two two decadesdecades regarding regarding a widea wide range range of of natural natural flavor flavor formulations formulations that that are are readily readily available available including including essentialessential oils, oils, herb herb and and spice spice extracts, extracts, flavor flavor substances substances fractionated fractionated from from essential essential oils, oils, or or botanical botanical extractsextracts and and natural natural flavor flavor substances substances produced produced by by fermentation fermentation [9]. [9]. The The composition composition of of these these formulationsformulations depends depends on on the the nature nature of of the the solvent solvent and and on on the the applied applied extraction/isolation extraction/isolation method. A A considerableconsiderable amountamount ofof efforteffort hashas been been invested invested in in producing producing with with hop hop extracts extracts high high flavonoid flavonoid or or polyphenolpolyphenol content. content. Polyphenols Polyphenols in in hops hops have have been been studied studied for for various various reasons, reasons, especially especially due due to to theirtheir visible visible role role in industrialin industrial applications applications as naturalas natural additives additives with with antimicrobial antimicrobial and/or and/or antioxidant antioxidant properties.properties. Technologies Technologies used used to isolateto isolate compounds compounds with with a certain a certain biological biological activity activity and and to to produce produce naturalnatural flavor flavor formulations formulations are are well well established, established, but but several several involve involve the the use use of of conventional conventional solvents solvents likelike alcohols alcohols (methanol (methanol and and ethanol), ethanol), acetone, acetone, diethyl diethyl ether, ether, or or ethyl ethyl acetate acetate and and the the use use of methodsof methods thatthat are are energy energy intensive. intensive. Solvents Solvents that that are are produced produced from from fossil fossil fuels fuels need need to to be be tightly tightly controlled controlled withwith respect respect to to their their residual residual levels. levels. TheThe residues of the the applied applied solvents solvents may may also also remain remain in inthe the final finalproducts products bearing bearing a detrimental a detrimental effect effect on onhuman human health. health. This This requires requires additional additional purification purification steps stepsthat that are aretime-consuming time-consuming and and increase increase the thetotal total process process cost cost [10]. [10 In]. applying In applying the theideologies ideologies of green of greenchemistry, chemistry, water water and and carbon carbon dioxide dioxide clearly clearly represent represent safer safer solvents andand thethe use use of of a renewablea renewable feedstockfeedstock possesses possesses the the widest widest acceptability.