Abstract
In hop cultivation, one-third of the crop is a valuable product (hop cones), and two-thirds is unexploited biomass, consisting mainly of leaves and stems, which, in a circular economy approach, can be recovered and, once stabilized, supplied to industrial sectors, such as cosmetics, pharmaceuticals and phytotherapy, with high added value. In this regard, this study aimed to investigate the effects of two different drying methods: oven drying (OD) at 45 °C and freeze-drying (FD), on the overall nutraceutical profile (i.e., total phenols, total flavans and total thiols), pigment content (i.e., carotenoids and chlorophylls) and the antioxidant potential of leaves from five different Humulus lupulus varieties grown in central Italy. Moreover, attenuated total reflectance infrared (ATR-FTIR) spectroscopy was applied to dried leaf powders to study the influence of both the variety and treatment on their molecular fingerprints. The spectral data were then analyzed by principal component analysis (PCA), which was able to group the samples mainly based on the applied treatment. Considering the overall phytochemical profile, FD appeared to be the most suitable drying method, while OD provided higher carotenoid retention, depending on the genotype considered. Finally, unsupervised chemometric tools (i.e., PCA and hierarchical clustering) revealed that the two main clusters contained subclusters based on the drying treatment applied; these subgroups were related to the susceptibility of the variety to the drying conditions studied.
Highlights
Hop plants (Humulus lupulus L.) are grown almost exclusively for the brewing industry, in which resins and essential oils from female cones are used for aroma [1]
This study aimed to investigate the effects of two different drying methods: oven drying (OD) at 45 ◦C and freeze-drying (FD), on the overall nutraceutical profile, pigment content and the antioxidant potential of leaves from five different Humulus lupulus varieties grown in central Italy
Dziedzinski et al [16] showed the possibility of obtaining functional teas using hop leaves and low concentrations of hop cones to provide a product with a pleasant taste and pro-health properties, and they emphasized the role of genotype in determining these functional properties
Summary
Hop plants (Humulus lupulus L.) are grown almost exclusively for the brewing industry, in which resins and essential oils from female cones are used for aroma [1]. While hop waste from brewing, named spent hop, has been studied in-depth in recent years [8,9], little attention has been paid to the postharvest hop biomass, which accounts for about 10–15 t/ha annually (2.6 kg/plant) [10] and can be considered a valuable source of functional molecules and nutrients that are still underexploited [11] In this regard, Rutto et al [12] demonstrated the potential value of hop biomass beyond the cones (i.e., leaves, bines and unrecovered cones) as promising forage to improve feed efficiency in ruminants. Iglesias et al [15] suggested the use of alcoholic extracts of hop leaves to produce an eco-friendly bee pesticide and underlined the influence of genotype on their secondary metabolite content These biological activities are related to the bioactive compounds present in hop leaves, such as polyphenols [16]. Dziedzinski et al [16] showed the possibility of obtaining functional teas using hop leaves and low concentrations of hop cones to provide a product with a pleasant taste and pro-health properties, and they emphasized the role of genotype in determining these functional properties
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