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Chemisch-analytische en sensorische karakterisering van kettle hoppy aroma van bier
No full textHops constitute an indispensable raw material for beer production. The hop essential oil (-derived) volatiles are at the origin of the highly desired hoppy aroma of beer. (Bio)chemical transformations and losses during the brewing process alter the hop oil volatile fingerprint, resulting in hoppy aroma in beer that is clearly different from the aroma of the hops. Nevertheless, the nature of hoppy aroma is up-to-date far from understood. Especially ‘noble kettle hop’ aroma, which is a typical flavour characteristic of traditional Pilsner-type beers obtained by vigorous boiling of aroma hops, has been a matter of debate and knowledge concerning this controversial issue remains contradictory and fragmentary.
This PhD study aims at providing scientific insights into the impact of the boiling process on the analytical and sensory characteristics of hop essential oil volatiles. Such knowledge is of utmost importance to completely understand the chemical background of ‘kettle hop’ aroma.
In first instance, a literature overview on hops, hop aroma, (bio)chemical transformations of hop oil volatiles during the brewing process and hoppy aroma of beer is presented.
In order to obtain scientific insights on changes in the hop oil volatile profile upon boiling, lab scale boiling experiments (in water) with total hop essential oil (cv. Saaz) were conducted. HS-SPME-GC-MS analysis and multivariate data analysis (PCA, CA) revealed clustering between unboiled and boiled hop essential oil samples, due to decreases in terpene hydrocarbon levels and an increase in the level of particular α-humulene and β-caryophyllene oxidation and hydrolysis products upon boiling. Also a series of volatiles specific for boiled hop oil could be observed, pointing to de novo generation of volatiles as a result of the boiling process. Sensory evaluations of boiled hop essential oil in non-aromatised iso-α-acid bittered lager beer demonstrated that boiled hop oil imparts ‘spicy/herbal’ impressions and ‘kettle hop’ aroma to beer.
Seen the interesting flavour characteristics (spicy, herbal notes) of boiled hop essential oil, the search for hop oil volatiles involved in this flavour impression was further focused by SPE fractionation of boiled hop essential oil. Fractions expressing ‘spicy’ and ‘hoppy’ notes when spiked to water were subjected to comprehensive analytical profiling via HS-SPME-GC-MS. Comparison of the boiled hop oil fractions with the corresponding unboiled hop oil fractions allowed for identification of additional volatiles formed de novo during the boiling process. Sensory evaluation of hop oil fractions in non-aromatised iso-α-acid bittered lager beer suggested that fractions containing high oxygenated sesquiterpenoid levels are most relevant in relation to ‘kettle hop’ aroma. Moreover, an SPE methodology was developed to enrich oxygenated sesquiterpenoids from commercial kettle hopped lager beers. Many compounds formed de novo upon lab scale boiling of hop oil (cv. Saaz) were detected in the commercial beers. Moreover, GC-O analysis demonstrated that several α-humulene and β-caryophyllene oxidation products elute in flavour-active zones of the hop oil-derived fractions as well as in the fractions derived from a commercial lager beer.
Enrichment of a sesquiterpene hydrocarbon fraction from total hop essential oil (cv. Saaz) and subsequent lab scale boiling of this fraction unambiguously proved oxidation of sesquiterpene hydrocarbons. Moreover, SPE isolation of the sesquiterpene oxidation products formed de novo and addition of this fraction to non-aromatised iso-α-acid bittered lager beer demonstrated a cause-and-effect relationship between these constituents and ‘spicy/kettle hop’ flavour. Via GC-O analysis of the oxidation product fraction, two intervals with significant odour-activity were established. Humulene epoxide III, humulenol II, caryophylla-4(12),8(13)-diene-5α/β-ol, (3Z)-caryophylla-3,8(13)-diene-5α-ol, 14-hydroxy-β-caryophyllene and (3Z)-caryophylla-3,8(13)-diene-5β-ol were identified in these intervals and since except for humulene epoxide III all these oxidation products were also found in flavour-active zones of a commercial kettle hopped lager beer, these volatiles are candidate key impact compounds for ‘kettle hop’ aroma.
By boiling increasing hop essential oil (cv. Saaz) concentrations in both water and wort, a positive correlation between the initial hop oil concentration and formation of sesquiterpene oxidation products upon boiling was found. Varietal aspects were also investigated (cv. Saaz, cv. Hallertau Tradition, cv. Perle and cv. Magnum). The results suggest that most of the sesquiterpene oxidation products formed de novo during boiling are identical for the different varieties, and thus point to the importance of the intrinsic hop oil composition for development of ‘kettle hop’ aroma during the brewing process. Moreover, selinenols and related compounds (i.e. cadinols, muurolols, eudesmols) could not be detected upon boiling of a selinene-rich sesquiterpene hydrocarbon fraction cv. Super Pride, supporting the hypothesis that these compounds are biosynthesised by the hop plant.
Finally, scientific insights obtained on lab scale were verified in real brewing practice by brewing eight different pilot-scale lager beers, aromatised with Saaz hop pellets or hop oil (-derived fractions). Analysis of wort samples of an ‘early’ kettle hopped beer indicated de novo formation of sesquiterpene oxidation products during the wort boiling process. This aroma profile of this beer was predominated by ‘spicy/herbal’ impressions. Combination of ‘early’ and ‘late’ kettle hopping resulted in a highly appreciated beer with intense ‘kettle hop’ flavour. A delicate balance between both ‘spicy/herbal’ and ‘floral/citrusy’ connotations appeared to be required in order to obtain the full complex spectrum of ‘kettle hop’ flavour. Aromatisation by post-fermentation addition of the sesquiterpene oxidation product fraction resulted in a beer that approached the flavour of an ‘early’ kettle hopped beer. Using comprehensive multidimensional gas chromatography-mass spectrometry (GCxGC-TOFMS), the previously proposed candidate key impact compounds could be identified in both the fraction and the resulting beer. Moreover, addition of this oxidation product fraction positively influenced the bitterness perception and significantly increased mouthfeel aspects. Post-fermentation aromatisation with boiled hop oil resulted in a beer that received a high score for ‘kettle hop’ aroma and, moreover, the flavour profile of this beer was highly comparable to a conventionally kettle hopped Pilsner-type lager beer. It was concluded that these two new aromatisation technologies impart interesting flavour characteristics to beer and are promising techniques to mimic ‘kettle hop’ flavour by post-fermentation aromatisation.status: Publishe
Het hoppig aroma van bier. Karakterisering van sensorisch gedifferentieerde hoparoma's en brouwtechnische evaluatie
No full textNext to the brewing water, malt and yeast, hops (Humulus lupulus&n bsp;L.) constitute a basic raw material for beer production. Several typ es of secondary metabolites present in hops are at the origin of highly characteristic beer flavour attributes, i.e. beer bitterness and ho ppy aroma. Hop-derived beer bitterness is well defined from a chemi cal point of view and, in current brewing practice, beer bitterness can be adequately dosed and controlled because (advanced) hop products with known composition are available, and optimised analytical methods allow reliable quantification of beer bitterness. In sharp contrast with beer bitterness, the precise nature of hoppy aroma of beer is far from comple tely understood. Although it is generally recognised that mainly volatil e constituents present in, or derived from hop essential oil are involve d, hoppy aroma is still one of the most controversial aspects of beer fl avour. Indeed, flavour-active key compounds causing this highly desirabl e aspect of beer aroma remain to be identified. This gap can be ascribed to the extreme chemical complexity and varietal dependence of hop essen tial oil itself, and to modifications and losses of hop oil constituents that take place along the brewing process. Obviously, introduction of c onsistent and pleasant hoppy aroma continues to be a serious challenge i n view of obtaining a reproducible and sufficiently stable beer flavour. Whereas conventional hopping inherently leads to flavour inconsistency, currently available hop oil products can be adequately dosed at the pos t-fermentation stage in order to attain the desired hoppy aroma or hop-d erived aroma top note in a much more controlled way. In this manner, enh anced consistency of hoppy aroma in the final beer and development of ne w beers with distinct sensory flavour attributes are feasible objectives . However, the spectrum of hop oil products currently available is rathe r limited (in particular, the varietal aspect of hoppy aroma is not full y exploited), and these products often do not give the intended flavour attributes. From an economic point of view, the above mentioned issues a re highly relevant. Beer flavour consistency is of paramount importance for the modern brewing industry (also because of globalisation of the ma rket) and a too variable hoppy aroma will undoubtedly impair brand typic ality and consumer preference.Beer flavour (in)consistency in respec t of hoppy aroma, together with the unresolved issue of the true nature of hoppy aroma of beer, form the rationale of this PhD study. In general , it aimed at (1) preparation of novel hop aromas extracted directly fro m single hop varieties via supercritical fluid extraction, (2) developme nt and implementation of analytical methodologies for determination of t he volatile composition of the hop aromas and reliable measurement of th e yield of aromatisation of beer with the respective hop essences, (3) d etermination of odour impact compounds of hop oil preparations, and (4)investigation of the impact of the novel hop aromas on hoppy character o f pilot beers.First, an extensive literature study is presented (Par t I). An introduction to hops in beer preparation, as well as a comprehe nsive literature review on hop aroma as such and hoppy aroma of beer (th e main topic of this PhD), is provided. The experimental work c omprises 3 different parts (Part II, Part III, and Part IV, respectively ), that are all further subdivided into two chapters.In Part II Pro duction of novel varietal hop aromas by supercritical fluid extraction o f hop pellets the unique properties of supercritical carbon dioxide ext raction were evaluated for preparation of different types of varietal ho p aromas from hop pellets. In chapter 1, extraction conditions were opti mised for preparation of single variety total hop essential oils that we re subsequently fractionated by solid phase extraction (SPE), yielding h op oil fractions highly enriched in water-soluble, oxygenated constituen ts (polar hop essences). Next, in chapter 2, a new methodology based on density programmed supercritical fluid extraction (SFE) and subsequent S PE fractionation of SFE extracts was developed for preparing floral, cit rus, and spicy hop essences. Sensory properties of all novel hop aromas as such, as well as their impact on beer flavour by aromatising referenc e pilot lagers, exclusively bittered with pre-isomerised hop extract, we re clearly demonstrated and described in both chapters of this study.&nb sp;In Part III Comprehensive chemical-analytical and sensory charac terisation of varietal hop oil preparations, analytical methods based o n headspace solid phase microextraction (HS-SPME) and GC-MS were optimis ed and implemented to investigate the volatile composition of varietal f loral and spicy hop essences and total hop essential oils (chapter 3). D etailed analytical characterisation of the typical volatile composition of the different types of (fractionated) hop oils, as well as discrimina tion between varietal hop aromas via principal component analysis, were clearly established. In chapter 4, the focus was on determining odour im pact compounds in floral and spicy hop essences (cv. Spalter Select). By application of advanced gas chromatography in combination with olfactom etric assessment, potent contributors to the odour of floral hop essence were identified and several odour-active regions of hitherto unidentifi ed sesquiterpenoid nature were detected in the aromagram of spicy hop es sence. In Part IV Methodology for determination of hop aromati sation of beer and evaluation of the novel hop oil preparations in brewi ng practice, selective determination of hop oil-derived constituents in the flavour profile of pilot lagers was carried out using HS-SPME in co mbination with gas chromatography and advanced mass spectrometry such as tandem MS and selected ion monitoring (chapter 5). This approach allows monitoring the application of hop oil preparations in beer production and, as a result of this, improved control and specification of hop aroma tisation. Next, in chapter 6, the use of the hop oil preparations was in vestigated in brewing practice. In a comparative study, the impact of th e different types of hop aromas on the hoppy character of pilot lagers, as well as on beer flavour stability, was evaluated through both analyti cal and sensory protocols and compared with hoppy character and flavour stability of conventionally hopped pilot lagers. It was found that the c ontent of analytical markers for hoppy aroma (e.g. linalool and oxy genated sesquiterpenoids) as well as the intensity and the qualitative d escription of hoppy aroma, were dependent on the applied hopping technol ogy and on the hop oil fraction used for beer aromatisation. Interesting ly, sensory evaluation of beer ageing demonstrated that hop aromatisatio n, whether performed in the advanced or the conventional way, significan tly masked beer staling. As a result of our work, an innovative hop aroma technology has bee n developed, allowing the preparation of single variety hop essential oi ls and several refined hop oil fractions with pleasant and distinct sens ory attributes. The impact of the novel hop aromas on hoppy character of pilot lagers was clearly demonstrated. Furthermore, specific analytical and sensory tools were optimised and implemented for detailed character isation of the volatile composition of the novel hop aromas, determinati on of key odour impact compounds in hop essences, and determination of t he yield of aromatisation with the hop oil preparations in brewing pract ice.status: Publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Reproducibility Trials in a Research Brewery and Effects on the Evaluation of Hop Substances in Beer Part 2: Reproducibility in Moderately Aged Beers
No full textThe good reproducibility of the 2 hl research brewery as proven in our previous study has been now reviewed after storing the beers at 0 °C and deep frozen (at – 24 °C) for 240 days. Except for some Strecker aldehydes all other ageing aldehydes, fermentation by-products and particularly the analyzed hop aroma compounds show good stability at 0 °C. The analytical standard deviations of the three-fold determinations are even a little lower than those of the fresh beers. Also the spread (standard deviations) of the results of the repeated brews did not increase. Opposite to that some hop aroma characteristics of the deep frozen dry hopped beers differed significantly from the ones of the fresh beers. On the basis of these findings this kind of sample preservation for considerably hopped beers cannot be recommended.status: Publishe
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