Auswahl der wissenschaftlichen Literatur zum Thema „Oenology Fermentation“

This paper reviews the main applications of the biopolymer chitosan, the main derivative of chitin, a material usually obtained from natural sources accessible at low cost, i.e., industrial wastes from fisheries. Due to its natural origin, which confers biodegradability and biocompatibility properties, in addition to its low toxicity, chitosan has been gaining attention in numerous sectors, such as agriculture, food, medicine, pharmaceuticals, etc., including also important oenological applications due to its potential as a green alternative to the use of sulphite. Among the many applications that can be generated from these materials in the wine-making area, their use has been reported for the clarification of must; in the preparation of films for the removal of contaminants, whether organics such as ochratoxin A or inorganics such as some metal ions and their salts; the control of turbidity caused by protein precipitation; the encapsulation of yeasts of oenological interest and enzymes for the control of adverse microorganisms such as Brettanomyces; the manufacture of sensors and nanosensors for the quantification of contaminants, the quality control of starting materials and final products, the optimisation of fermentation processes, the monitoring of storage conditions, etc. As a result of this review, significant development of the applications of this material in the oenological area can be expected, especially due to the possibilities of preparing new derivatives, including the great variety of these that have been recently proposed through click reactions, as well as the growing incursion of chitosan in nanobiotechnology.

Contact of wine with wood during fermentation and ageing produces significant changes in its chemical composition and organoleptic properties, modifying its final quality. Wines acquire complex aromas from the wood, improve their colour stability, flavour, and clarification, and extend their storage period. New trends in the use of barrels, replaced after a few years of use, have led to an increased demand for oak wood in cooperage. In addition, the fact that the wine market is becoming increasingly saturated and more competitive means that oenologists are increasingly interested in tasting different types of wood to obtain wines that differ from those already on the market. This growing demand and the search for new opportunities to give wines a special personality has led to the use of woods within the Quercus genus that are different from those used traditionally (Quercus alba, Quercus petraea, and Quercus robur) and even woods of different genera. Thus, species of the genus Quercus, such as Quercus pyrenaica Willd., Quercus faginea Lam., Quercus humboldtti Bonpl., Quercus oocarpa Liebm., Quercus frainetto Ten, and other genera, such as Robinia pseudoacacia L. (false acacia), Castanea sativa Mill. (chestnut), Prunus avium L. and Prunus cereaus L. (cherry), Fraxinus excelsior L. (European ash), Fraxinus americana L. (American ash), Morus nigra L, and Morus alba L. have been the subject of several studies as possible sources of wood apt for cooperage. The chemical characterization of these woods is essential in order to be able to adapt the cooperage treatment and, thus, obtain wood with oenological qualities suitable for the treatment of wines. This review aims to summarize the different species that have been studied as possible new sources of wood for oenology, defining the extractable composition of each one and their use in wine.

Winemaking variables and techniques are known to affect the composition of wines. To obtain a rapid and safe fermentation course, with a reduced risk of proliferation of unwanted microbial species, frequent recourse is made to the use of selected microorganisms, which can greatly simplify the complex management of the fermentation process. In particular, selected strains of lactic acid bacteria are used, which are much more sensitive than yeasts to the operating conditions of the medium. In this regard, the overall aim of this research was to verify whether the early inoculation of homolactic acid bacteria for hexoses (Lactobacillus plantarum) carried out after 24 h, compared with that of saccharomycetes operating alcoholic fermentation, could be advantageous compared with a traditional innoculation with a different heterolactic bacterial strain for hexoses (Oenococcus oeni) operated at the end of alcoholic fermentation. The grape variety chosen was Sangiovese, the protagonist of Tuscan oenology. The evaluation focused on different aspects such as the management of winery operations, and the quality and longevity of the product; was carried out in all phases of winemaking; and analysed both from a chemical and sensory point of view.

In parallel with the development of non-Saccharomyces starter cultures in oenology, a growing interest has developed around the interactions between the microorganisms involved in the transformation of grape must into wine. Nowadays, it is widely accepted that the outcome of a fermentation process involving two or more inoculated yeast species will be different from the weighted average of the corresponding individual cultures. Interspecific interactions between wine yeasts take place on several levels, including interference competition, exploitation competition, exchange of metabolic intermediates, and others. Some interactions could be a simple consequence of each yeast running its own metabolic programme in a context where metabolic intermediates and end products from other yeasts are present. However, there are clear indications, in some cases, of specific recognition between interacting yeasts. In this article we discuss the mechanisms that may be involved in the communication between wine yeasts during alcoholic fermentation.

An increasing interest in novel wine productions is focused on non-Saccharomyces yeasts due to their potential in improving sensory profiles. Although Kluyveromyces marxianus has been originally isolated from grapes and its enzymatic activities are used in oenology, rarely it has been used as co-starter. The K. marxianus Km L2009 strain has been characterized here and selected as a co-starter both at laboratory- and winery-scale fermentation. The Km L2009 strain showed growth of up to 40 (mg/L) of sulfites and 6% (v/v) of ethanol. Gas chromatographic analysis demonstrates that wines produced by mixed fermentation contain remarkably higher quantities of free monoterpenes and aliphatic esters than wines produced only by commercial strains of Saccharomyces cerevisiae. Differences in the volatile organic compound composition produced sensorially distinct wines. In light of these results, it is possible to state that even within the K. marxianus species it is possible to select strains capable of improving the aromatic quality of wines.

Active dry yeasts (ADY, Saccharomyces cerevisiae) are widely used in oenology due to their potential benefits on the control of fermentation and quality reproducibility among other aspects. On the other hand, yeast propagation systems, so called Active Yeast Systems (AYS), can be useful to optimize the alcoholic fermentation (AF) initial lag phase and reduce production costs. The objective of this work was to determine the predominance of an ADY strain propagated by AYS and the impact of this inoculum on cv. Carmenère wine quality. Lalvin ICV D21 ADY strain was inoculate according to the protocol recommended by the manufacturer (T0), and in parallel, it was propagate by the AYS and then used as inoculum (T1). Yeast strain predominance analyzed by restriction fragment length polymorphism (RFLP) technique indicates that nine (out of 9) yeast colonies obtained from a single sample of the ADY, show the same electrophoretic pattern when compared to the ADY. The results show limited significant differences for the fermentation speed and the yeast cell counting. The result of the physicochemical analysis of the musts and resulting wines showed no significant differences between treatments. A triangular test showed no significant sensory differences between wines

The topic of microbial interactions is of notable relevance in oenology, being connected with their impact on microbial biodiversity and wine quality. The interactions among different couples of microorganisms, in particular yeasts and lactic acid bacteria representative of the must/wine microbial consortium, have been tested in this study. This interaction’s screening has been implemented by means of plate assays, using culture medium, grape juice, and wine agar as substrates. Different antagonistic phenomena have been detected, belonging to the following interaction categories: yeast-yeast, yeast-bacteria, bacteria-yeast, and bacteria-bacteria. In general, the inhibitory activity has been observed in all three media agar used as substrates, resulting in more frequent on culture medium, followed by grape juice and, finally, wine. Specifically, the work is one of the first reports demonstrating the reciprocal interactions between non-Saccharomyces yeasts (NSY) and malolactic bacteria. The findings shed new light on the co-inoculation of the yeast starter culture with malolactic bacteria, as well as the biocontrol potential of Lactic Acid Bacteria (LAB) strains. Highlighted microbial interactions are relevant for the management of alcoholic fermentation, malolactic fermentation, and the development of distinctive aroma profiles, control of spoilage yeasts, and the selection of tailored mixed starter cultures. In addition, the plate assay method could be a fast, cheap, and suitable method to exclude negative interactions among Saccharomyces spp., NSY, and malolactic bacteria during trials from regional spontaneous fermentations with the aim to select tailored mixed starter cultures.

Obtaining high-quality wines with a registered designation of origin means making the most of the specific features of the variety and those imprinted with technology and the place of harvesting. The aroma is due to the chemical compounds of terpenic nature that accumulate in the grapes (variety flavors) and the secondary aromas that are formed during the alcoholic fermentation and the aging period of the wines (Amrani, and Glories, 1995). The wine-making of the varieties is followed by the appreciation of the primary grapes. This is achieved by preferential maceration of the must by means of enzymes (Marin et al., 1998). The technology of aromatic wines has two fundamental objectives: extracting the primary aromas of grapes (terpenols) and favoring the formation of secondary fermentation aromas. In order to obtain aromatic wines with variety typology, the preferential stage is decisive. Pectolytic enzyme preparations are used in oenology to accelerate and complete the extraction and clarification processes of the must, extracting and stabilizing the color, highlighting the varietal aromatic potential of the varieties and improving the filterability and maturation of the wines.

Previous studies have proven the potential of antimicrobial plant extracts to delay malolactic fermentation (MLF) in red wines. With the final end of extending their applicability in oenology, the aim of this study was to investigate whether the addition of antimicrobial extracts (from eucalyptus leaves and almond skins) to white wines (i.e., Chardonnay wine), as a way to control MLF, would affect wine organoleptic properties, in particular those related to their volatile and phenolic composition. Although addition of both extracts led to statistically significant changes (p<0.05) in the concentration of several volatile and phenolic compounds, only few of them showed contents higher than their sensory thresholds, meaning that the changes observed in their concentration could slightly affect the final wine aroma and astringency. However, use of the extracts in the elaboration of white wines needs to be assessed in future experiments at winery scale, including wine sensorial analysis.

This article explores the winemaking practices of the Greeks in the Aegean world, from classical times to late antiquity, reading the ancient sources in the light of the latest progress in oenology. Following the process of wine-making from the harvest and its crushing to the moment of the opening of the amphoras and the consumption of wine, it shows that the winemakers used a variety of efficient techniques to supervise fermentation. Resulting from overripe harvests, the must, aged in an oxidative atmosphere, could, if necessary, receive plant or mineral inputs in different forms to correct its sugar content and acidity. These additions also had an antibacterial and antifungal action contributing, like tanning and salting, to the stability of the must. Thermoregulation permitted a long term consumption of unfermented must, especially within the context of γλυκισμοί.

Sulphite is widely used in winemaking for its antimicrobial and antioxidant properties, although its toxic effect on human health is proven. For this reason strategies for reducing chemical preservatives in winemaking is strongly demanded. Wine yeasts can cope with SO2 by different systems, such as acetaldehyde and S-amino acids production or SO2 export. In this study a first screening of SO2 resistance and on plate production of SO2 and H2S have been performed for autochthonous strains, isolated in Veneto vineyards to be used as starter of fermentation in the production of Prosecco di Valdobbiadene DOCG and DOC Piave wines, compared to commercial strains. Then the oenological characteristics of 11 S. cerevisiae strains of have been evaluated. These strains are 4 autochthonous strains isolated during local selection projects in DOCG Conegliano - Valdobbiadene and DOC Piave areas, together with 6 commercial strains, which genome have been recently sequenced, and relative informations are available in the principal genomic databases, and the laboratory strain S288c, the first one that has been sequenced. Main technology and quality characters have been evaluated to determine the suitability of strains for winemaking process. In particular have been studied the production of ethanol and glycerol, the glucose consumption at 2 and 7 days, the production of hydrogen sulphide, acetaldehyde and sulphur dioxide and the resistance to various concentrations of free sulphur dioxide in synthetic must. Sulphite response in yeast has been investigated in order to elucidate factors that affect sulphite production during vinification. Moreover acetaldehyde, another compound produced by yeast, linked with sulphite metabolism or detoxification, has been analysed since it affects wine quality. Genetic characteristics identified after genome sequencing of 4 autochthonous strains (2 from Prosecco area and 2 from Raboso area), such as oenological SNPs, strain-specific genes and important translocations, have been analyzed in Real-time PCR for a large number of autochthonous strains. Then the behaviour towards sulphite of 4 wine yeasts has been investigated, and transcriptome analysis during fermentation has been performed by means of next generation sequencing. For all strains, fermentation rate was monitored together with sulphite production in synthetic must supplemented with different doses of SO2 (0 mg/l and 25 mg/l). Finally, a selection of reference genes for Real-time PCR has been made, and a set of genes suitable for such conditions has been identified. Results point out the importance of verifying strain attitudes towards sulphite at different sulphite concentrations. This study tries to clarify the complex regulative mechanisms of sulphites during fermentation, thus giving new guidelines for critic control of these fermentation parameters in order to maximize effect of sulphite added thus limiting the dose employed during vinification.
I solfiti sono ampiamente utilizzati nella vinificazione per le loro proprietà antimicrobiche e antiossidanti, sebbene il loro effetto tossico sulla salute umana sia dimostrato. Per questo motivo le strategie di riduzione dei conservanti chimici nel processo di vinificazione è fortemente richiesto. I lieviti possono rispondere alla presenza di SO2 con sistemi diversi, come la produzione di acetaldeide e ammino acidi solforati o l’esporto di SO2. In questo studio è stato fatto un primo screening sulla resistenza alla SO2 e sulla produzione di SO2 e H2S in piastra da parte di ceppi autoctoni, isolati nei vigneti del Veneto per essere utilizzato come starter di fermentazione nella produzione di Prosecco di Valdobbiadene DOCG e Vini del Piave DOC, confrontati con dei ceppi commerciali. Inoltre sono state valutate le caratteristiche enologiche di 11 ceppi di S. cerevisiae. I ceppi in esame sono 4 ceppi autoctoni isolati durante i progetti di selezione locale nelle aree Conegliano - Valdobbiadene DOCG e Piave DOC, insieme con 6 ceppi commerciali, il cui genoma è stato recentemente sequenziato, e le informazioni relative sono disponibili nelle banche dati genomiche principali, e il ceppo di laboratorio S288c , il primo che è stato sequenziato. I principali caratteri tecnologici e di qualità sono stati valutati per determinare l'idoneità dei ceppi alla vinificazione. In particolare, sono state studiate la produzione di etanolo e glicerolo, il consumo di glucosio a 2 e 7 giorni, la produzione di idrogeno solforato, acetaldeide e biossido di zolfo e la resistenza a varie concentrazioni di biossido di zolfo libero in mosto sintetico. La risposta ai solfiti nel lievito è stata studiata al fine di chiarire i fattori che influenzano la produzione dei solfiti durante la vinificazione. Inoltre l’acetaldeide, un altro composto prodotto da lievito, collegato con il metabolismo solfito o disintossicazione, è stata analizzata in quanto influisce sulla qualità del vino. Con il sequenziamento del genoma di 4 ceppi autoctoni (2 da zona del Prosecco e 2 dalla zona Raboso) è stato possibile individuare delle caratteristiche genetiche, come ad esempio SNPs enologiche, geni ceppo-specifici e traslocazioni importanti, che sono stati analizzati in Real-time PCR per un gran numero di ceppi autoctoni . Inoltre il comportamento di 4 lieviti enologici nei confronti dei solfiti è stato studiato, ed è stata effettuata l'analisi del trascrittoma durante la fermentazione per mezzo di next generation sequencing. Per tutti i ceppi la velocità di fermentazione è stata monitorata, insieme alla produzione di SO2, in mosto sintetico con differenti dosi di SO2 (0 mg / l e 25 mg / l). Infine, è stata fatta una selezione di geni reference da usare in Real-time PCR, e una serie di geni adatti a queste condizioni è stato identificato. I risultati sottolineano l'importanza di verificare l'atteggiamento del ceppo nei confronti dei solfiti a diverse concentrazioni di SO2. Questo studio cerca di chiarire i complessi meccanismi di regolazione dei solfiti durante la fermentazione, dando così nuove linee guida per il controllo critico di questi parametri di fermentazione, al fine di massimizzare l'effetto dei solfiti aggiunto limitando in tal modo la dose impiegata durante la vinificazione.

Thesis (MScAgric)--University of Stellenbosch, 2004.
ENGLISH ABSTRACT: The production of quality wine is influenced by numerous factors of which grape quality is one of the most important factors. The production of quality wine, however, is not possible without good winemaking techniques and effective quality control. Critical control points (CCP) during the winemaking process must be identified to ensure optimum wine quality. Grape must is a complex medium that contains different micro-organisms which can be either beneficial or negative to wine quality, depending on the physical and chemical conditions that prevail in the must. Yeasts are responsible for alcoholic fermentation, lactic acid bacteria (LAB) for malolactic fermentation (MLF) and acetic acid bacteria (AAB) for the production acetic acid from ethanol. Yeasts and certain LAB can also produce acetic acid and thereby increasing the volatile acidity (VA) of wine. These micro-organisms can influence each other in complex fashions by competing for growth nutrients and by producing inhibitory substances. Most winemakers nowadays use commercial yeast strains to inoculate wine fermentations. This, however, does not assure a problem-free fermentation and cases of stuck and sluggish fermentations are annually reported worldwide. In these or most cases fermentation takes longer than 21 days to complete and the wine contains a residual sugar concentration of more than 4 g/L, which can be utilised by wine spoilage micro-organisms such as certain bacteria and other wild yeasts. Stuck and sluggish fermentations also increase the chances of oxidation due to the absence of the protective CO2 layer on the surface of the wine, which is formed during alcoholic fermentation. Another effect of stuck and sluggish fermentations is that valuable tank space is wasted due to the unexpected time consumption of these fermentation problems. Many factors during the winemaking process can be responsible for stuck and sluggish fermentations. In this thesis the different factors is discussed with the emphasis on the effect of the yeast strain. The way that certain yeast strains influence AAB and LAB numbers during fermentation and MLF through the production of inhibiting by-products such as medium chain fatty acids has not been investigated in detail in the past. Certain fungicides and pesticides that are used in vineyards to control pests (e.g. mildew) contain copper which can be inhibiting to yeast growth and alcoholic fermentation. Legal limits and withholding periods on these sprays are not always strictly obeyed and can lead to stuck and sluggish fermentations. This motivated us to evaluate the growth and fermentation activities of a selection of commercial wine yeasts in the presence of copper levels in the range of maximum legal limits. The effect of these commercial strains on the LAB and AAB numbers during alcoholic fermentation and MLF were also investigated. Our results showed that there was no significant difference on numbers of the AAB obtained from fermentations inoculated with different commercial wine yeast strains. However, with regards to the LAB numbers, one of the strains produced significantly more sulphur dioxide (SO2), which led to the inhibition of MLF in that wine. Our results further indicated which commercial yeast strains were capable of effectively fermenting high sugar musts and which strains were less effective. From the strains tested VIN13, N96 & L2056 were able to utilize fructose more effectively than NT50, RJ11 & D80. We could further distinguish between yeast strains that produced the lowest (VIN13 & RJ11) and the highest (WE372, NT50 & L2056) VA concentrations in must containing high sugar levels. Strains that were more tolerant against high copper levels were also identified. We tested six yeast strains in must with added copper (0.25 mM cu2+) in the form of CuSO4 .H2O. Three Cu2+-tolerant (D80, Collection Cepage Cabernet & NT50) yeast strains were distinguished from three less Cu2+-tolerant yeast strains (VIN13, NT112 & RJ11). This study made a valuable contribution in knowledge gained about commercially available wine yeast strains that can ferment effectively under certain stress conditions. Research such as this, where wine yeasts are evaluated to ferment more effectively during strenuous winemaking conditions, will be very beneficial to winemakers.
AFRIKAANSE OPSOMMING: Die produksie van gehalte wyn word deur verskillende faktore beïnvloed waarvan druifkwaliteit seker die belangrikste is. Die produksie van gehalte wyn is egter nie moontlik sonder goeie wynmaaktegnieke en effektiewe kwaliteitsbeheer nie. Kritieke kontrole punte (KKP) tydens die wynmaakproses moet dus geïdentifiseer word om sodoende ‘n verlaging in wynkwaliteit te vermy. Druiwemos het ‘n komplekse mikrobiologiese samestelling en bestaan uit verskillende mikroörganismes wat vooren nadelig vir wynkwaliteit kan wees, afhangende van die fisiese en chemiese toestande wat in die mos bestaan. Giste is verantwoordelik vir alkoholiese fermentasie, melksuurbakterieë (MSB) vir appelmelksuurgisting (AMG) en asynsuurbakterieë (ASB) vir die produksie van asynsuur vanaf etanol. Asynsuur word egter ook deur giste en MSB geproduseer en dra so by tot die vlugtige suurheid (VS) van ‘n wyn. Hierdie mikroörganismes kan mekaar op komplekse wyses beïnvloed deur o.a. te kompeteer vir voedingstowwe asook deur die produksie van inhiberende verbindings. Die meeste wynmakers maak gebruik van kommersiële gisrasse om alkoholiese fermentasies mee uit te voer. Gevalle van sogenaamde slepende en gestaakte alkoholiese fermentasies, waar suiker nie volledig na etanol en CO2 omgeskakel word nie, kom egter nog gereeld in die wynbedryf voor. In sulke gevalle neem die fermentasie gewoonlik langer as 21 dae om te voltooi met ‘n suiker konsentrasie van meer as 4 g/L wat in die wyn oorbly. Dit is nadelig vir wynkwaliteit aangesien dit nie net die kanse vir bederf deur bakterieë en giste verhoog nie, maar ook die kanse vir oksidasie verhoog a.g.v. die verlies van die beskermende CO2 lagie bo-oor die wyn. Hoe sekere gisrasse, ASB en MSB getalle gedurende fermentasie en AMG beïnvloed deur die produksie van inhiberende verbindings soos medium ketting vetsure en SO2, is ook nie baie in die verlede ondersoek nie. Sommige spuitstowwe wat gebruik word in die bekamping van swamsiektes bevat koper wat inhiberend kan wees vir gisgroei en alkoholiese fermentasie. Wetlike maksimum limiete en onthoudingsperiodes op spuitstofresidue word egter nie altyd gehoorsaam nie en kan lei tot slepende en gestaakte fermentasies. Dit het ons gemotiveer om ‘n seleksie van kommersiële gisrasse te evalueer in terme van gisgroei en fermentasie in die teenwoordigheid van kopervlakke naby die maksimum limiet. Ons resultate het gewys dat daar nie noemenswaardige verskille in AAB getalle tydens alkoholiese fermentasie tussen behandelings met verskillende kommersiële gisrasse was nie. Een van die gisrasse het wel noemenswaardig meer SO2 geproduseer wat gelei het tot inhibering van AMG in hierdie wyn. Ons het verder uitgewys watter kommersiële gisrasse instaat is om meer effektief in hoër suiker mos te fermenteer en watter van die rasse minder suksesvol was. Ons het ook rasse geïdentifiseer wat meer weerstandbiedend is teen hoë kopervlakke in mos en sodoende groter kans op ‘n suksesvolle fermentasie sal hê in mos wat koperresidue bevat wat afkomstig is van sekere spuitstowwe. Die effek van die ASB en MSB getalle gedurende fermentasie en AMG is ook ondersoek. Ons resultate het verder gewys watter kommersiële gisrasse instaat was om mos met hoë suikervlakke meer effektief te fermenteer. Vam die gisrasse wat getoets was het VIN13, N96 & L2056 fruktose meer effektief benut as NT50, RJ11 & D80. Ons kon verder onderskei tussen gisrasse wat die laagste (VIN13 & RJ11) en die hoogste (WE372, NT50 & L2056) vlakke van VS produseer in mos met hoë inisiële suikervlakke. Gisrasse wat meer tolerant was teen koperresidue in mos is ook geidentifiseer. Ons het ses gisrasse getoets in mos met bygevoegde koper (0.25 mM Cu2+) in die vorm van CuSO4 .5H2O. Daar is onderskei tussen drie Cu2+-tolerante (D80, Collection Cepage Cabernet & NT50) en drie minder Cu2+-tolerante gisrasse (VIN13, NT112 & RJ11). Hierdie studie lewer ‘n waardevolle bydrae in die invordering van kennis oor kommersieel beskikbare wyngisrasse wat meer effektief sal fermenteer onder sekere streskondisies wat in mos voorkom. Inligting soos hierdie is belangrik om die wynmaker se keuse uit die reeks bestaande kommersiële gisrasse te vergemaklik.

Thesis (MScAgric (Viticulture and Oenology))--Stellenbosch University, 2008.
The production of good quality wine is essential to ensure competitiveness on an international level. Wine quality is usually evaluated for the visual, olfactory and taste characteristics of that specific wine. The winemaking process starts with the grapes in the vineyard followed by oenological practises in the winery until the final wine is bottled. Factors that could influence wine quality include the grape quality from which the wine is made and different techniques used during wine production. Other factors include the presence as well as the interaction between microorganisms found in the grape juice and wine, and the biochemical effect these microorganisms have on certain chemical compounds in the wine. The different microorganisms found in grape juice and wine can either have a negative or positive contribution to the final quality of the wine. During certain stages of the winemaking process the growth and metabolic activity of certain microorganisms is a necessity to produce good wine. During other stages the presence of certain microorganisms can lead to the development of compounds that is regarded as off-flavours and therefore lead to unpalatable wines of low quality. Yeast strains that naturally present on the grapes and in the winery can also contribute to the final quality of the wine. Brettanomyces yeasts are part of the natural flora of winemaking and can drastically influence the aroma characters of a wine through the production of volatile phenols. The general aroma descriptions of volatile phenols include "smoky", "spicy", "barnyard", "animal" and "medicinal". Although some wine drinkers believe that these characters can add to the complexity of a wine, high levels of volatile phenols is mostly regarded as off-flavours and mask the natural fruity flavours of a wine. With this study we wanted to generate a better understanding of the effect of different winemaking practises on the production of volatile phenols by B. bruxellensis. We evaluated the difference in volatile phenol production when B. bruxellensis was introduced before or after alcoholic fermentation. We have shown that B. bruxellensis could grow and produce volatile phenols during alcoholic fermentation. Results obtained also showed that commercial wine yeast strains could produce the vinyl derivatives that serve as precursors for Brettanomyces yeast to produce the ethyl derivatives. The commercial yeast strains differed in their ability to produce vinyl derivatives. Different malolactic fermentation scenarios were evaluated, namely spontaneous versus inoculated, and with or without yeast lees. Results showed that spontaneous malolactic fermentation had higher volatile phenol levels in the wine than inoculated malolactic fermentation. The treatment with lees reduced the level of volatile phenols, probably due to absorption by yeast cells. The presence of the phenyl acrylic decarboxylase (PAD1) gene and the production of volatile phenols by S. cerevisiae commercial yeast strains were evaluated in Shiraz grape juice and in synthetic grape juice. The results indicated that the yeast strains differ in their ability to produce 4-vinylphenol and 4-vinylguaiacol. All the yeast strains tested had the PAD1 gene. We also evaluated the presence of the phenolic acid decarboxylase (padA) gene and the ability of different lactic acid bacteria strains to produce volatile phenols in synthetic wine media. Although some of these strains tested positive for the phenolic acid decarboxylase gene most of them only produced very low levels of volatile phenols. This study made a valuable contribution on the knowledge about the effect of Brettanomyces yeast on the volatile phenol content of red wines during different stages of the winemaking process and when applying different winemaking practices. It also showed the effect between Brettanomyces yeast and other wine microorganisms and the possible influence it could have on the final quality of wine. Research such as this can therefore aid the winemaker in making certain decisions when trying to manage Brettanomyces yeast spoilage of wines.

Thesis (MScAgric (Viticulture and Oenology))--University of Stellenbosch, 2010.
ENGLISH ABSTRACT: The quality of wine is influenced and determined by various factors, one of which includes the process of malolactic fermentation (MLF). MLF plays an integral role in the flavour and sensory profile of most red wines as well as some white wines like Chardonnay. This process is conducted by lactic acid bacteria (LAB), specifically of the genera Oenococcus, Lactobacillus, Pediococcus and Leuconostoc. Of these, Oenococcus oeni is best adapted to survive in the harsh wine environment. MLF is defined as the conversion of L-malic acid to L-lactic acid and carbon dioxide. The conversion of the dicarboxylic malic acid to the monocarboxylic lactic acid results in a decrease in acidity and an increase in pH, to give a softer mouthfeel and more favourable flavour profile. A further reason for conducting MLF in wine includes the improvement of microbial stability due to the removal of malic acid as a possible substrate for microorganisms. Recently, research focus has shifted to the ability of MLF and LAB to alter the aroma profile of wine via the production and/or modification of certain aroma compounds. In order for wine LAB to conduct MLF, they need to be able to survive the harsh and challenging wine environment. Conditions in South African wines are particularly challenging due to the long, hot ripening seasons resulting in high sugar concentrations which give high ethanol concentrations. Some LAB also struggle to adapt to an environment with high pH and low malic acid concentrations. These factors, combined with the use of sulphur dioxide, cause LAB to struggle in conducting and completing successful MLF. Many of the commercial starter cultures that are currently available contain LAB that have not been isolated from South African wine and are therefore not optimal for use under these challenging wine conditions. Oenococcus oeni is also the single LAB culture present in all commercially available starter cultures. The overriding goal of this study was to create a MLF starter culture containing a mixture of LAB cultures, namely O. oeni and Lactobacillus plantarum, which can successfully convert malic acid to lactic acid, ensure microbial stability, but also make a positive contribution to the wine aroma profile. Lactobacillus plantarum has previously been considered for possible use in a commercial starter culture. The LAB isolates used in this study were selected from the Institute for Wine Biotechnology culture collection as well as isolated from spontaneous MLF. The first objective was to characterise these LAB strains for important traits and for possible use as a MLF starter culture. A total of 23 strains were identified as O. oeni and 19 strains as Lb. plantarum. The identified strains were screened in a synthetic wine medium for their ability to convert malic acid to lactic acid. Based on the LAB strain performance in the synthetic wine medium, seven strains of both O. oeni and Lb. plantarum were selected. These 14 strains were screened for the presence of genes encoding for enzymes responsible for biogenic amine production and were found to contain none of the genes associated with the formation of histamine, tyramine or putrescine. The LAB strains were genetically screened for enzymes associated with aroma modification by LAB during MLF. The enzymes of interest that were screened for included β-glucosidase, esterase, protease and phenolic acid decarboxylase (PAD). The Lb. plantarum strains were found to possess more diverse enzymatic profiles related to aroma than O. oeni. The biggest differences were observed for the presence of β-glucosidase and PAD. The second objective was to perform small-scale fermentations with the individual LAB isolates. The individual isolates were evaluated in Pinotage and based on these results; three strains of each O. oeni and Lb. plantarum were selected for evaluation in mixed culture fermentations. The mixed cultures were evaluated in Pinotage, Shiraz and Cabernet Sauvignon in the 2008 vintage. As a third objective, the wines were also analytically and sensorially evaluated to investigate the changes in the aroma profile that could be attributed to the presence of the mixed LAB isolates. Based on the fermentation data as well as data pertaining to the aroma modification, three mixed cultures were selected for evaluation in the 2009 vintage in Pinotage, Cabernet Sauvignon and Chardonnay. The mixed cultures were able to successfully complete MLF in fermentation periods comparable to that of a commercial culture used as control. The different LAB cultures had distinct and diverse effects on the wine aroma profile. The O. oeni strain played a larger role in the ester concentration present after MLF, while the Lb. plantarum strain had a larger effect on the higher alcohol and volatile fatty acid concentration upon completion of MLF. The results generated by this novel study clearly indicate the potential of a mixed LAB starter culture for conducting MLF. The mixed cultures successfully completed MLF and made a positive contribution to the wine aroma profile.
AFRIKAANSE OPSOMMING: Die kwaliteit van wyn word beïnvloed en bepaal deur verskeie faktore en wynbereidings prosesse, wat die proses van appelmelksuurgisting (AMG) insluit. AMG speel ’n integrale rol in die sensoriese profiel van meeste rooiwyne, sowel as sommige witwyne soos Chardonnay. AMG word gedefinieër as die omskakeling van L-appelsuur na L-melksuur en koolstofdioksied. Hierdie omskakeling kan toegeskryf word aan die teenwoordigheid van melksuurbakterieë (MSB), spesifiek spesies van die genera Oenococcus, Lactobacillus, Pediococcus en Leuconostoc. Vanuit hierdie wyn MSB, is Oenococcus oeni die spesies wat die beste aanpas en oorleef onder stresvolle wyn kondisies. Die omskakeling van appelsuur, ’n dikarboksielsuur, na melksuur, ’n monokarboksielsuur, lei tot ‘n vermindering in suurheid en ’n verhoging in pH. Hierdie vermindering in suurheid gee ’n sagter en meer geronde mondgevoel aan die wyn en dra by tot ‘n meer aangename geurprofiel. ’n Verdere rede vir AMG in wyn is om mikrobiese stabiliteit te verseker deurdat appelsuur verwyder word as ’n moontlike koolstof substraat vir mikroörganismes. Onlangs het navorsing begin fokus op AMG en die vermoë van MSB om die aroma profiel van wyn te beïnvloed deur die produksie/modifisering van sekere aroma komponente. Vir MSB om AMG te kan deurvoer, moet hulle kan oorleef in die stresvolle wynomgewing. Wyntoestande in Suid-Afrika is veral uitdagend vir die oorlewing van mikroörganismes as gevolg van lang, warm somers wat lei tot ’n matriks met ’n hoë suikerkonsentrasie en wyn met ’n hoë etanolkonsentrasie. ‘n Omgewing met ‘n hoë pH en lae appelsuur konsentrasie, kan ook bydrae tot stresvolle kondisies vir MSB. Hierdie parameters, tesame met die gebruik van swaweldioksied, maak dit moeilik vir MSB om AMG te inisieer en te voltooi. Sommige van die kommersiële aanvangskulture wat tans beskikbaar is, bevat nie MSB wat onder Suid-Afrikaanse wyntoestande geïsoleer is nie en daarom is dit nie altyd optimaal vir gebruik nie. Oenococcus oeni is ook die enkele MSB kultuur wat in alle kommersiële kulture gebruik word. Die hoofdoelwit van hierdie studie was om ’n potensiële kommersiële aanvangskultuur te ontwikkel wat ‘n mengsel van MSB bevat. Hierdie aanvangskultuur moet AMG suksesvol kan voltooi, mikrobiologiese stabiliteit bevorder en steeds die wynaroma positief kan beïnvloed. Bakterierasse van O. oeni en Lb. plantarum is geselekteer vir gebruik in hierdie studie. Lactobacillus plantarum het reeds in vorige studies potensiaal getoon as ‘n moontlike aanvangskultuur. Die MSB isolate vir hierdie studie is geselekteer uit die Instituut vir Wynbiotegnologie se kultuurversameling en geïsoleer uit spontane AMG fermentasies. Die eerste doelwit was om hierdie MSB isolate te karakteriseer vir belangrike eienskappe en die moontlike gebruik as ’n kommersiële AMG aanvangskultuur. ‘n Totaal van 23 O. oeni en 19 Lb. plantarum isolate is geïdentifiseer. Hierdie isolate is in ’n sintetiese wynmedium geëvalueer vir hul vermoë om appelsuur na melksuur om te skakel. Op grond van hul reaksie in die sintetiese wynmedium, is sewe isolate van elk van die O. oeni en Lb. plantarum geselekteer. Hierdie 14 isolate is ondersoek vir die teenwoordigheid van die gene wat kodeer vir biogeenamien produksie en daar is gevind dat geen van die isolate enige van die biogeenamien gene wat ondersoek is, naamlik histamien, tiramien en putresien besit nie. Die MSB isolate is geneties ondersoek vir die teenwoordigheid van dié gene wat kodeer vir ensieme wat die aromaprofiel tydens AMG beïnvloed. Dié ensieme sluit β-glukosidase, esterase, protease, fenoliese suurdekarboksilase en sitraatliase in. Daar is gevind dat die Lb. plantarum isolate meer diverse ensiemprofiele as O. oeni besit. Die grootste verskille in die ensiemprofiele kan toegeskryf word aan die teenwoordigheid van β-glukosidase en fenoliese suurdekarboksilase. Die tweede doelwit was om kleinskaalse AMG fermentasies met die individuele MSB isolate uit te voer. Die individuele isolate is in Pinotage geëvalueer. Volgens hierdie resultate is drie isolate van elk van die O. oeni en Lb. plantarum geselekteer om in gemengde kulture getoets te word. Die gemengde kulture is in Pinotage, Shiraz en Cabernet Sauvignon in 2008 geëvalueer. As ’n derde doelwit is hierdie wyne ook analities en sensories geëvalueer om die veranderinge in die aromaprofiele as gevolg van die teenwoordigheid van die MSB te ondersoek. Op grond van die fermentasiedata, sowel as die data oor die aromaveranderinge, is drie gemengde kulture geselekteer vir evaluering in Pinotage, Cabernet Sauvignon en Chardonnay in 2009. Die gemengde kulture kon AMG suksesvol voltooi met fermentasietempo’s wat vergelykbaar was met dié van ‘n kommersiële AMG kultuur wat as kontrole gebruik is. Die verskillende MSB kulture het spesifieke en uiteenlopende uitwerkings op die wynaroma gehad. Die O. oeni isolaat in die gemengde kultuur blyk ‘n belangriker rol te speel in die esterkonsentrasie na AMG, terwyl die Lb. plantarum isolaat ’n groter effek het op die hoër alkohol en vlugtige vetsuurinhoud na AMG. Die resultate wat deur hierdie unieke studie gegenereer is, gee ’n aanduiding van die potensiaal van ’n gemengde MSB aanvangskultuur vir AMG. Die gemengde kulture kon AMG suksesvol voltooi en ‘n positiewe bydrae tot die aromaprofiel van die wyn lewer.

Two pre-fermentation treatments were investigated in Pinot noir (Vitis vinifera L.) wines. The effects of cold maceration and carbonic maceration on the wines' composition, colour parameters and sensory properties were examined. Cold maceration is a winemaking technique used to increase non-alcoholic extraction in Pinot noir winemaking prior to fermentation. It involves holding crushed grapes with approximately 100-150 mg l⁻¹ SO₂ at low temperatures and is thought to increase the colour, aroma and flavour of the resulting wines. Carbonic maceration uses whole bunches that have undergone anaerobic metabolism to produce characteristically fruity and spicy wines. Pre-fermentation cold maceration produces wines that are higher in titratable acidity and monomeric anthocyanin content, but lower in colour density, hue and polymeric pigments. Reducing the maceration temperature below 10°C has little effect. Carbonic maceration produces wines that are lower in titratable acidity, monomeric anthocyanin content, and colour density but are higher in colour hue and amount of polymeric pigments. Quantitative descriptive analysis was used to define the effects of these pre-fermentation maceration treatments on the sensory characteristics of the resulting wine. Trained panel members found that there were no discernable sensory differences in the compositional parameters despite measurable chemical differences. Investigation into the aroma and flavour characteristics of the wines found that carbonic maceration produces wines that were lower in berry aroma and higher in acetate or ester-type aromas than the control wines. These wines were considered to have specific raspberry, floral, sugar, cherry and chemical aromas. This chemical note was also observed in the flavour of the carbonic maceration wines. The temperature of the cold maceration process has no major effect on the aroma and flavour of the resulting wines. However, the 10°C maceration was higher in woody/tobacco aroma than the 4°C maceration, and the 10°C treatment was also higher in bitter flavour than all the other treatments. Cold maceration wines were found to have specific mixed berry, dried fruit and sweet-oxidised aroma characters, together with a blackberry flavour note.

Thesis (MScAgric (Viticulture and Oenology))--University of Stellenbosch, 2010.
ENGLISH ABSTRACT: One of the most striking qualities of wine is its tart, sour taste. The sensory perception of sourness is mainly attributed to the presence of hydrogen ions (protons) at high concentrations. Large amounts of weak carboxylic acids (organic acids) are the main sources of these ions within wine. Once wine enters a person's mouth, the dissociable protons of the weak organic acids within wine are partially neutralized or, in other words, titrated by the saliva secreted inside one's mouth. This explains why the duration and intensity of a wine's sourness is related to its titratable acidity content. The sour taste of wine is usually considered refreshing and it helps balance wine flavour. In fact, wines become watery when its titratable acidity content is too low. After alcoholic fermentation, the titratable acidity of wine will usually be less than that of the grape juice from which was made due to ethanol-induced precipitation of potassium bitartrate crystals and partial consumption of malic acid by fermenting wine yeasts. Occasionally however, increases in titratable acidity are observed during alcoholic fermentation. If wine is produced from grape juice with optimum levels of titratable acidity, unforeseen increases in titratable acidity during alcoholic fermentation can be detrimental to the quality of the final product. Although the net production of malic acid by wine yeasts contributes to increases in titratable acidity seen during grape juice fermentations, the production of succinic acid is regarded as the primary contributor. In fact, succinic acid accounts for approximately 90% of the non-volatile acids produced during fermentation of grape juice. Between 0.5 and 1.5 g/L succinic acid is normally found in wine, but higher concentrations thereof (up to 3.0 g/L) have been detected within certain red wines. Acidity adjustments should preferably be carried out before the onset of alcoholic fermentation to allow better integration of the added compound(s) and to ensure that conditions during fermentation favour the quality and microbial stability of the final product. In doing so unfortunately, winemakers run the risk of ending up with wines that may taste too sour if they are unable to accurately predict and take into consideration the amount of succinic acid produced during alcoholic fermentation. Knowledge with regard to the factors involved in succinic acid's production by fermenting wine yeasts is therefore required in order to manage the titratable acidity of wines more accurately. Ever since Louis Pasteur first noticed succinic acid amongst the by-products of alcoholic fermentation, attempts have been made to determine the metabolic pathways and factors involved in its production by fermenting wine yeasts. Up until now however, it remains unclear why wines sometimes end up with exceptionally high levels of succinic acid. For these reasons it was decided to investigate the possible causes of very high succinic acid concentrations within wine. Due to complexity of grape juice's chemical composition and the problems associated with sterilizing grape juice, fermentation experiments were conducted within a chemically defined grape juice-like medium. Succinic acid production by nine different industrial wine yeast strains was studied under various conditions with regard to the nutrient status of the synthetic grape juice, temperature and availability of molecular oxygen during alcoholic fermentation. The amount of succinic acid produced during alcoholic fermentation was found to depend on the yeast strain, fermentation temperature and chemical composition of the synthetic grape juice. Out of the nine commercial yeast strains selected for this study, strain WE372 produced the largest amount of succinic acid in synthetic grape juice at 28°C. Strain WE372 produced significantly smaller amounts of acetic acid than the other yeast strains of this study and very little acetic acid at 28°C, which indicated that strain WE372 may have less acetaldehyde dehydroganase activity than the other yeast strains of this study under the conditions tested. The effect this has on NAD: NADH balance is the probable cause for its ability to form more glycerol, succinic and malic acid than the other strains. Results from our study show that succinic acid production is influenced primarily by the metabolizable fraction of YAN, which we termed metabolically available nitrogen (MAN). Succinic acid production by fermenting yeasts will be favoured by moderate to high fermentation temperatures (20°C to 28°C) in grape juice with a nicotinic acid and/ or nicotinamide deficiency, high sugar content (200 g/L to 240 g/L), moderate amounts of metabolically available nitrogen (300 ± 50 mg/L MAN), the presence of flavonoids and large supplies of unsaturated long-chain fatty acids. Even higher concentrations of succinic acid were produced when oxygen was made available to fermenting yeasts by aerating the fermenting grape juice. Fermentation temperatures below 18°C, too much metabolizable nitrogen (> 450 mg/L MAN), very high concentrations of fermentable sugar (> 240 g/L), lipid deficiencies and a lack of pantothenic acid, thiamine, biotin or pyridoxine will decrease the amount of succinic acid produced fermenting yeasts.
No Afrikaans summary available.

Thesis (MscAgric (Viticulture and Oenology))--University of Stellenbosch, 2009.
ENGLISH ABSTRACT: Tannins and colour components in red wine are important quality parameters. These factors can be manipulated in the vineyard by grape growing techniques or in the cellar by different winemaking practices. Grape seeds make a significant contribution to tannin concentration in wine when compared to those from the skins and pulp. Tannins contribute to the ageing potential, organoleptic properties and stabilisation of red wine colour. The colour of a red wine is also influenced by malolactic fermentation, the biological process that transforms malic acid into lactic acid which normally leads to an increase in pH. The subsequent change in pH alters the anthocyanin equilibrium, the primary colour components in red wine. Oxygen contributes to the polymerisation of anthocyanins over time to form more stable pigments that are less sensitive to pH fluctuations and sulphur dioxide bleaching. Limited research has been done on the use of oxygen after alcoholic fermentation and the impact it has on the phenolic composition of red wines. Similarly, only a few studies have examined the impact of either the addition or removal of seeds to the phenolic composition of a red wine in combination with oxygen addition. Additionally, little published data seems to exist on the effect of different pHs on red wine’s phenolic and colour development after oxygen addition. In our results we have shown that it is possible to stabilise wine colour by adding supplementary seeds before alcoholic fermentation. This led to an increase in colour intensity in certain cases. Some red wines produced without seeds had significantly lower colour intensities. This clearly suggests that catechins and proanthocyanidins are extracted from seeds and contribute to wine colour as they combine with other pigments to stabilise wine colour. Spectrophotometric and HPLC analyses have shown that the total phenolic content increased with seed concentration. However, we have observed that a wine may possibly become saturated with phenols when supplementary seeds are added. Anthocyanin concentrations often decreased when oxygen was added, while polymeric phenols and polymeric pigments sometimes increased. When applying different oxygen dosages to a red wine on commercial scale with microoxygenation, it was found that monomeric anthocyanins decreased as more oxygen was added and this decrease in anthocyanins led to the formation of stable polymeric pigments. This was reflected in the significant increase in colour intensity for the wines receiving oxygen. Small differences were detected in the total phenol and tannin concentration for the control and oxygenated wines. However, some of these phenolic and colour differences disappeared during subsequent ageing of the wine. When making wines of different initial pHs, we observed that the colour density decreased as the pH increased. The application of oxygen reduced the decrease in colour during MLF, especially at a lower pH. The addition of oxygen did not result in significantly different polymeric pigment concentrations in the various pH treatments, although the results could have been different if the wines were aged. However, pH differences in the range between 3.4 and 4.0 did not significantly influence the phenol composition of the wines under our conditions. This study led to a better understanding on the effect of oxygen additions under different conditions on red wine's phenolic and colour composition.
AFRIKAANSE OPSOMMING: Tanniene en kleur komponente teenwoordig in rooiwyn is belangrike kwaliteit parameters. Hierdie faktore wat wynkwaliteit beïnvloed kan gemanipuleer word in die wingerd deur verskeie verbouingstegnieke toe te pas en in die kelder deur die toepassing van verskillende wynbereidingsmetodes. Die bydrae van sade tot die finale tannienkonsentrasie in rooiwyn is groot in vergelyking met dié van die pulp en doppe. Tanniene dra by tot die verouderingspotensiaal, organoleptiese eienskappe en die stabilisasie van die wynkleur. Die kleur van ‘n rooi wyn word ook beïnvloed deur appelmelksuurgisting (AMG), die biologiese proses wat appelsuur omskakel na melksuur en ‘n gevolglike toename in die pH van die wyn veroorsaak. Hierdie verandering in die pH van die wyn beïnvloed die antosianien ewewig, die primêre kleur komponente teenwoordig in rooiwyn. Suurstof dra by tot die polimerisasie van antosianiene oor tyd om meer stabiele kleur pigmente te vorm met ‘n hoër kleurintensiteit wat minder sensitief is teenoor pH veranderinge en die bleikingseffek van swaweldioksied. Beperkte navorsing is gedoen op die gebruik van suurstof na alkoholiese gisting en die impak daarvan op die fenoliese samestelling van ‘n rooiwyn. Slegs ‘n paar studies het die invloed van die verwydering of byvoeging van sade in kombinasie met suurstoftoediening op ‘n rooiwyn se fenoliese samestelling ondersoek. Dit wil voorkom of beperkte gepubliseerde data beskikbaar is oor die effek wat verskillende pH’s het op rooi wyn se fenoliese en kleurontwikkeling na suurstof byvoeging. Ons resultate het aangedui dat dit wel moontlik is om in sekere gevalle die kleur van ‘n rooiwyn te stabiliseer deur addisionele sade by te voeg voor alkoholiese fermentasie. Hierdie byvoeging het ‘n toename in kleurintensiteit tot gevolg gehad. Sekere wyne wat gemaak is sonder sade het ‘n kenmerkend laer kleur intensiteit gehad. Hierdie bevinding is ‘n duidelike bewys dat katesjiene en prosianidiene geëkstraheer word vanuit die sade en bydra tot wynkleur deurdat hulle met ander pigmente verbind om die kleur sodoende te stabiliseer. Spektrofotometriese en hoë druk vloeistof chromatografie (HDVC) analises het gewys dat die totale fenoliese konsentrasie neem toe met ‘n toename in saad konsentrasie. Daar is egter waargeneem dat ‘n wyn moontlik versadig kan raak met fenole wanneer addisionele sade bygevoeg word. Antosianien konsentrasies het meestal afgeneem wanneer suurstof bygevoeg is, maar polimeriese fenole en polimeriese pigmente het partykeer toegeneem. Met die toediening van verskillende suurstof dosisse tot ‘n rooiwyn op kommersiële skaal het ons bevind dat monomeriese antosianiene afneem wanneer meer suurstof bygevoeg word. Hierdie afname in antosianiene het egter gelei tot die vorming van stabiele polimeriese pigmente. Dié bevinding was gereflekteer in die toename in kleurintensiteit van wyne wat met suurstof behandel is. Klein verskille was waargeneem vir die totale fenol en tannien konsentrasies tussen die kontrole en wyne behandel met suurstof. Sekere van hierdie fenoliese kleur verskille het egter afgeneem tydens die daaropvolgende veroudering van die wyne. Wyne wat gemaak is met verskillende aanvanklike pH’s se kleurintensiteit neem af soos die pH toeneem. Die toediening van suurstof het die kleurverlies tydens AMG verminder, veral by ‘n laer pH. Die toediening van suurstof het nie verskillende polimeriese pigment konsentrasies by verskillende pH’s veroorsaak nie, maar ‘n verskil kon moontlik waargeneem word indien die wyne verouder was. pH verskille tussen 3.4 en 4.0 het egter nie die fenoliese samestelling van die wyne onder ons omstandighede beduidend beïnvloed nie. Hierdie studie het gelei tot meer kennis oor die effek van suurstoftoedienings onder verskillende kondisies op rooiwyn se fenoliese en kleursamestelling.

Thesis (MSc (Wine Biotechnology))--University of Stellenbosch, 2009.
The PGU1 gene encodes an endo-polygalacturonase, an enzyme that degrades pectin. Although the presence and function of this gene is well characterized in Saccharomyces cerevisiae, its regulation is very complex and not yet fully understood. Yeast producing a highly active polygalacturonase (PG) during alcoholic fermentation could potentially improve filtration and turbidity and also enhance extraction of certain aroma compounds. This could replace the addition of expensive commercial enzyme preparations that often contain unwanted enzymes. The first objective of this study was to evaluate PGU1 expression in recombinant strains of S. cerevisiae that originally lacked the PGU1 gene. A functional PGU1 gene and its promoter were successfully re-introduced into their native position in the genomes of five wine strains. Three of these strains recovered PG activity while two did not transcribe the gene and subsequently lacked activity. The three strains that recovered activity were used in microvinification experiments to determine the effect of PG-producing yeast on the aroma profile of the wine. No significant differences were observed in the volatile compounds production between the recombinants and their respective wild types, but some tendencies arose, especially for the monoterpene geraniol. The second objective of this study was to analyze the PGU1 gene and promoter from Saccharomyces paradoxus RO88 (a strain that exhibits high PG activity) and to compare it to those of S. cerevisiae S288C in order to identify differences that could potentially be responsible for the difference in their PG activities. Comparison of the gene sequences revealed several amino acid differences, one of which was in the peptide secretion signal. Analyses of the promoters also indicated some potentially important differences. Furthermore, S. cerevisiae strain VIN13, RO88 as well as two interspecies hybrids (all displaying varying PG activities) were compared under winemaking conditions. Clear differences were observed for the production of certain compounds. RO88 and the hybrids produced higher concentrations of certain volatile compounds, although they were not strong fermenters. Two recombinants, each containing a PGU1-overexpressing plasmid (one with the PGU1 gene from S. paradoxus and the other from S. cerevisiae), were also used in vinification to determine the effects of the different PGU1 gene on the aroma profile of the wine. Unfortunately, the plasmids were unstable and lost during the fermentation. Nevertheless, some tendencies were observed that indicated possible higher production of certain compounds by the recombinants compared to their wild types. This study identified that regulation of the PGU1 gene differs between strains with different genetic backgrounds. Certain differences were observed in the PGU1 gene and promoter sequences between S. cerevisiae and S. paradoxus that could potentially be the reason for the difference in their PG activities. From an oenological point of view, the presence of PGU1 in the genome of a fermenting strain tends to increase the aromatic potential of wine. These results provide a good platform for further studies on the PGU1 gene.

Genus Saccharomyces includes a large number of microorganisms that are important for industrial applications such as the production of fermented beverages, biofuel and baking. Natural selection combined with domestication applied selective pressures to the genome of this yeast producing large numbers of different strains with specialized phenotypes. During the last decades thousand of strains have been phenotypically characterized but correlation between phenotype and genotype is not yet completely unveiled. Genome sequence analysis is a crucial step to obtain a general description of gene content and highlight differences between strains. In this study the homozygous derivatives of four ecotypical Saccharomyces cerevisiae strains isolated from Raboso and Prosecco fermented grape bunch have been successfully sequenced using next generation sequencing, and a variety of tools have been used and developed to solve the complex task of genome finishing. A detailed overview of gene expression in different winemaking and laboratory strains has also been performed using SOLiD RNA-seq. Samples growth in synthetic wine media on controlled bioreactors have been collected during fermentation process. Our results revealed a transcriptional fingerprint characterizing oenological strains adaptation to stressful environment. A comparison between differences in promoter sequences between strains and their downstream effect on gene expression have been performed and the results show a higher influence of tandem repeat variability respect to mutations on transcription factor binding sites. Finally using statistical analysis we correlate the genetic traits of strains with their metabolic properties and we obtained a global overview of fermentation performances in the different genetic groups.
Il genere Saccharomyces comprende un gran numero di microrganismi di interesse tecnologico, utilizzati ad esempio per la produzione di bevande fermentate, biocarburanti e per la panificazione. La selezione naturale unita alla domesticazione ha determinato una pressione selettiva che ha modificato il genoma di questi lieviti producendo un ampio numero di ceppi diversi con fenotipi specializzati. Negli ultimi anni centinaia di ceppi sono stati caratterizzati dal punto di vista fenotipico ma una correlazione tra il fenotipo e il genotipo non è stata ancora completamente chiarita. L’analisi del sequenziamento genomico è un passo cruciale per ottenere una descrizione globale del contenuto genico e per evidenziare le differenze tra i ceppi. In questo studio sono stati sequenziati i genomi degli omozigoti derivati da quattro ceppi ecotipici di S. cerevisiae isolati da grappoli fermentati di Prosecco e Raboso Piave utilizzando sequenziatori di Nuova Generazione. Numerosi strumenti informatici sono stati utilizzati e sviluppati per adempire al complesso compito del finishing. Inoltre una dettagliata panoramica dell’espressione genica in 5 ceppi di vinificazione e 1 di laboratorio è stata effettuata utilizzando la tecnica RNA-seq con la metodologia SOLiD. I lieviti sono stati cresciuti in mosto sintetico in bioreattori controllati e dei campioni sono stati prelevati durante il processo fermentativo. I risultati hanno rivelato un profilo trascrizionale caratteristico dell’adattamento dei ceppi enologici allo stress dell’ambiente di vinificazione. Un confronto tra le differenze nelle sequenze promotoriali tra i ceppi e il successivo effetto a catena sull’espressione genica è stato considerato e i risultati evidenziano una maggior influenza della variabilità delle tandem repeat rispetto alle mutazioni sui siti di binding dei fattori di trascrizione. Infine utilizzando dei modelli statistici siamo riusciti a correlare le caratteristiche genetiche dei ceppi con le loro proprietà metaboliche e ad avere una visione globale dell’abilità di fermentazione dei diversi ceppi.