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The cis-resveratrol Concentration is Proposed as a New Indicator of the Hop Freshness
Jerkovic, V. and Collin, S.

trans-Piceid, cis-piceid and trans-resveratrol contents of hop cones and hop pellets from six American varieties (harvest 2004) were monitored by RP-HPLC-APCI(+)-MS/MS over 12 months of storage. trans-Resveratrol, cis-piceid and trans-piceid were found in all samples. After 8 months of storage, the overall stilbene content was decreased in the same range whatever the conditioning. Absent in fresh hop cones or pellets, cis-resveratrol was released from cis-piceid in all stored samples. cis-Resveratrol concentration revealed very interesting for assessing hop freshness. In the brewing industry, female inflorescences of hop plant (Humulus lupulus, L.) are processed in several different ways, yielding cones (leaf hop), pellets, isomerised pellets, organic solvent extracts, supercritical, isomerised and reduced extracts [1]. According to the conditioning, hop can impart bitterness, flavor, antioxidant activity, foam stability and bacteriostatic activity to beer [2, 3]. After harvest in the autumn, hop is dried (from 75C 80 % moisture to below 10 % [4]) at a temperature close to 60C [1, 3, 5]. Even compressed in bales, hops cannot be stored for a long time without protected atmosphere because of flavor oxidation and a decrease in ??-acid content [1, 4?C 9]. It is therefore very usual to decrease the volume by pelletizing dried milled hop [10]. In order to prevent degradation, hop pellets are then stored under vacuum (after nitrogen flushing) between ?C2 and 4 ??C [6, 7, 11?C14]. Under these conditions, the a-acid content remains relatively constant during one-year storage [14], whilst the essential oil content is inevitably affected after a few months (e.g. : farnesene in aromatic varieties) [15]. Polyphenol oxidation [7] may also take place, depending on the variety of hop. Cold storage doesn't prevent deterioration but slows down oxidation [14]. The hop storage index (HSI), defined as the ratio of absorbance at 275 nm to the absorbance at 325 nm of an alkaline methanolic solution of a non-polar extract of hop, is usually used to indicate hop freshness [16]. The absorption values of and alpha-acids extracts are maximal at 325 nm and minimal at 275 nm. Oxidised a- and alpha-acids extracts have a maximum absorption at about 250C 280 nm. Therefore, hop oxidation induces an HSI increase. However, the variation between samples is sometimes very scarce and the specificity of this assay is insufficient. In 2005, Callemien et al. [17] mentioned for the first time the presence of three cardioprotective stilbenes [18] in hop: trans-resveratrol, trans-piceid and cis-piceid (total concentration close to 3.5 ppm). In hop pellets, Jerkovic et al. [19] measured stilbene concentrations from 5 to 16 ppm mg/kg, trans-piceid being in all cases the major constituent (4 to 8.8 mg/kg). Among the hop cultivars investigated, it appeared that the lower the pellets alpha-acid content, the higher the resveratrol potential, except in highly oxygen-sensitive varieties [19]. Recently, Jerkovic and Collin [20] evidenced the huge impact of the harvest year. No stilbenes were detected in supercritical hop extracts. Spent hop thus emerged as a very cheap and delipidated raw material for the production of resveratrol-enriched hop extracts [21]. The aim of the present work was to evidence a new indicator of the hop freshness, taking advantage of the instability of hop stilbenes. American varieties were analyzed through storage in two differently conditioned forms, all under protected atmosphere: leaf hop and derived pellets. Willamette, Cascade, Nugget, Simcoe, Warrior and Tomahawk hop varieties from the harvest 2004 were a kind gift from Yakima Chief (Louvain-la-Neuve, Belgium). Cones and T90 pellets were industrially produced and vertically sampled from the same batches. Pelletisation took place within a week after the harvest. All samples were stored at 4C under inert atmosphere until needed. ...

Descriptors: resveratrol, stilbene, polyphenols, hop conditioning, hop storage, resveratrol stability

BrewingScience - Monatsschrift fr Brauwissenschaft, 62 (July/August 2009), pp. 141-146