**1. Introduction**

The most important hop (*Humulus lupulus* L.) compounds for brewing are resins and essential oils, which are responsible for beer bitterness and flavour. Both are synthesized in the lupulin glands of female cones [1,2]. Hop cones also contain other important compounds, such as polyphenols, which contribute to beer flavour, colour, taste and haze formation and have a strong antioxidant power [3,4]. Hop polyphenols include flavonols (e.g., quercertin and kaempferol), flavan-3-ol (e.g., catechins and epicatechins), phenolic acids (e.g., ferulic acid), prenylflavonoids (xanthohumol, isoxanthohumol, desmethylxanthohumol, 6- and 8-prenylnaringenin), multifidus glycosides and resveratrol [1,5,6]. The polyphenolic fraction of hops is so complex that researchers still continue to identify compounds [1,7].

**Citation:** Afonso, S.; Dias, M.I.; Ferreira, I.C.F.R.; Arrobas, M.; Cunha, M.; Barros, L.; Rodrigues, M.Â. The Phenolic Composition of Hops (*Humulus lupulus* L.) Was Highly Influenced by Cultivar and Year and Little by Soil Liming or Foliar Spray Rich in Nutrients or Algae. *Horticulturae* **2022**, *8*, 385. https:// doi.org/10.3390/horticulturae8050385

Academic Editors: Dasha Mihaylova and Aneta Popova

Received: 15 February 2022 Accepted: 26 April 2022 Published: 27 April 2022

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In Portugal, hop plants occur spontaneously along riverbanks, in particular in the north of the country, a region that has been found to have some ecological potential for hop production [8]. The crop was introduced into the country in the early 1960s and currently the national production is located mainly in the Bragança district in the northeast of the country. Nugget, a bitter cultivar, is produced on all the local farms, being destined for a national brewing company. However, local farmers are currently interested in growing aroma cultivars due to the recent growth of the craft beer industry both at home and abroad. According to the Euromonitor [9] report "Beer in Portugal", dozens of craft brewers launched different craft beer products in 2019. Currently Cascade and Columbus are some of the aroma cultivars that regional farmers are starting to experiment with. With the expansion of the craft beer market, new opportunities arise for smallscale growers, producing and supplying desirable cultivars at more favourable prices [10]. The growing of aroma cultivars for the craft beer market is probably also more suited to the Portuguese production structure, which is based on small-sized plots. To reduce the risk of failure, as occurred during recent years with the bitter cultivars grown for the conventional beer industry, greater knowledge should be applied to the Portuguese production system. In other Mediterranean countries, hops also attracted the attention of producers and researchers, and recent work has shown good suitability of different cultivars both for the beer industry [11] and for the production of fresh edible shoots [12–14]. Thus, an important step for Portuguese farmers is comparing the agronomic performance of other cultivars sought by the craft beer industry with the well-established Nugget. The response of hops to soil pH, for instance, is also important to be understood since pH is an important factor in hop production [15,16]. Farmers are currently starting to use foliar sprays to complement their fertilization programmes, given the potential beneficial effects of such products in crop production and quality [17,18]. There is also growing interest in the use of biofertilizer formulations from readily available materials to improve soil conditions and plant yield [19,20]. Over the years, some fields have been showing patches of poorly developed plants that reduce overall productivity and farmers' incomes [21]. Strictly speaking, the cause of these underdeveloped plant is not ye<sup>t</sup> clearly known. There are no signs of phytosanitary problems. Thus, it is important to look at these patches of poorly developed plants and observe the effect of foliar sprays on hop cone quality.

As mentioned above, previous studies have shown that hops can be a promising crop for Mediterranean environments, although it is necessary to improve several aspects of the cropping technique [11]. Thus, this study aims to carry out a set of experimental trials to test important factors (plant vigour, foliar sprays, liming, cultivar and year) that can influence the quality of the cones and particularly phenol concentration and phenolic composition. A high content of phenols is a positive trait of hop cones, due to their bioactive effect, which contributes to beer quality [4–6]. The field trials included four factorial designs exploring the combined effects of (1) plots of different vigour plants and year, (2) plots of different vigour plants, algae- and nutrient-rich foliar sprays and year, (3) plots, liming and year and (4) cultivars and year. From these trials, the concentration of polyphenols in hop cones is reported. The samples presenting the higher polyphenolic content from each of the trials, were selected for phenolic characterization. Furthermore, the relationship between total phenols and nutrient concentration in hop cones was evaluated through a principal component analysis (PCA) and correlation analysis. The results of the elemental composition of hop cones, already reported in previous studies [22–24], were used to evaluate their relationship with hop phenols. Data on dry matter (DM) yield and hop acids from these experiments have also been reported [21–24], but the relevant information to understand the accumulation of phenols in plants in response to the different factors of variation was discussed here. In short, the ultimate objective of this research was to obtain useful data for both hop producers and the craft beer industry.

### **2. Materials and Methods**
