1 KU Leuven Technology Campus Ghent | Faculty of Engineering Technology | Dept. of Microbial and Molecular Systems (M2S) | Cluster for Bioengineering Technology (CBeT) | Laboratory of Enzyme, Fermentation and Brewing Technology (EFBT) | T +32 9 265 86 13 | F +32 9 265 87 24 | tatiana.praet@kuleuven.be

INTRODUCTION The early addition of

European aroma hops to the boiling kettle imparts highly desirable ‘spicy/herbal’ top notes to lager beers. During ageing of hops and wort boiling, sesquiterpene hydrocarbons may be oxidised into sesquiterpene oxidation products (SOPs), which are presumed to be responsible for this unique flavour characteristic (‘noble kettle hoppy aroma’).

Heat-induced changes in the composition of varietal hop essential oils via wort

boiling experiments on lab scale

UNBOILED HOP OIL SAMPLES

Tatiana Praet 1, Filip Van Opstaele 1, Guido Aerts 1, and Luc De Cooman 1

BOILING OF VARIETAL HOP ESSENTIAL OILS

Principal Component biplot of boiled hop essential oil samples (B) (cv. Saaz, Hallertau Tradition, Perle and Magnum) and unboiled hop essential oils (U) on the basis of standardised peak areas of compound classes

Recovery upon boiling for different compound classes as a function of the initial hop essential oil concentration (ppm)

BOILING OF INCREASING CONCENTRATIONS HOP ESSENTIAL OIL CV. SAAZ

Peak area of spicy fraction in unboiled hop essential oils cv. Saaz, Hallertau Tradition, Perle and Magnum

Low hop oil concentrations during boiling: Recoveries close to 100%, suggesting limited differences between the volatile profiles of unboiled and boiled hop essential oil. High hop oil concentrations during boiling: Increase in level of spicy fraction, due to a significant increase in level of oxygenated sesquiterpenoids upon boiling.

HS-SPME-GC-MS chromatogram of commercial kettle hopped lager beer (hopped with German noble aroma hops)

METHODS Increasing hop essential oil concentrations (cv.

Saaz, range 0.010 g/L to 10 g/L) and hop essential oil cv. Saaz, Hallertau Tradition, Perle and Magnum (10 g/L) were boiled in wort (closed system, lab scale). All samples were analysed by HS-SPME-GC-MS and compared to unboiled hop essential oil samples in order to calculate the recovery of different compound classes and individual hop oil-derived volatiles. Multivariate analysis (PCA) was employed to investigate group structures and flavour-active zones were determined using GC-olfactometry (3 assessors; n=3).

SPICY FRACTION: PRESENCE OF SOPS

→ Oxidation of sesquiterpene hydrocarbons, in particular of α-humulene and β-caryophyllene, into SOPs! → Increase in level of several sesquiterpene hydrocarbons (eluting in spicy region, e.g. cadalene) points to isomerisation reactions. → The level of certain oxygenated sesquiterpenoids (e.g. cubenol, τ-cadinol (g), gleenol) does not increase upon boiling, supporting the hypothesis that they are not formed by oxidation but are instead related to the hop plant metabolism.

Compounds located in the ‘spicy region’ and characterised by an increase upon boiling

N= newly formed upon boiling, not detected in unboiled samples, n.d.= not detected

Compounds determined in flavour-active zones of boiled hop oil samples cv. Saaz via GC-olfactometry (detection frequency ≥ 4/9)

• FORMATION OF SOPS DURING LAB SCALE BOILING OF HOP ESSENTIAL OIL IN WORT • DIFFERENTIATION UNBOILED VS. BOILED HOP OIL ↑ IF INITIAL HOP OIL CONCENTRATION ↑ • RELATIVELY MORE SOP FORMATION UPON BOILING OF HIGHER HOP OIL CONCENTRATIONS • SOP FORMATION DURING BOILING LARGELY VARIETY-INDEPENDENT: FORMATION OF

CHEMICALLY IDENTICAL α-HUMULENE AND β-CARYOPHYLLENE OXIDATION PRODUCTS

The spicy fraction of unboiled hop essential oil consists of oxygenated sesquiterpenoids and ‘miscellaneous’ compounds. The latter compound class contains aliphatic ketones, esters, alcohols etc. However, several sesquiterpene hydrocarbons (e.g. cadalene) also elute in the ‘spicy’ region. Oxygenated sesquiterpenoids can be subdivided into sesquiterpene oxidation products (SOPs) and plant metabolism related sesquiterpenoids. SOPs predominantly comprise α-humulene and β-caryophyllene epoxides and their hydrolysis and rearrangement products. However, literature data suggest that particular compounds do not increase upon storage of hops (e.g. τ-cadinol) and are rather a product of hop plant biosynthesis.

→ The bitter variety Magnum contains relatively low levels of spicy compounds, whereas hop essential oil cv. Saaz is characterised by higher levels of SOPs.

SHIFT UPON BOILING

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