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Thesis (PhD (Wine Biotechnology))--University of Stellenbosch, 2011. / Includes bibliography. / ENGLISH ABSTRACT: Alcoholic fermentation, and especially wine fermentation, is one of the most ancient microbiological processes utilized by man. Yeast of the species Saccharomyces cerevisiae are usually responsible for most of the fermentative activity, and many data sets clearly demonstrate the important impact of this species on the quality and character of the final product. However, many aspects of the genetic and metabolic processes that take place during alcoholic fermentation remain poorly understood, including the metabolic processes that impact on aroma and flavour of the fermentation product. To contribute to our understanding of these processes, this study took two approaches: In a first part, the initial aim had been to compare two techniques of transcriptome analysis, DNA oligo-microarrays and Serial Analysis of Gene Expression (SAGE), for their suitability to assess wine fermentation gene expression changes, and in particular to assess their potential to, in combination, provide combined quantitative and qualitative data for mRNA levels. The SAGE methodology however failed to produce conclusive data, and only the results of the microarray data are shown in this dissertation. These results provide a comprehensive overview of the transcriptomic changes during model wine fermentation, and serve as a reference database for the following experiments and for future studies using different fermentation conditions or genetically modified yeast. In a second part of the study, a screen to identify genes that impact on the formation of various important volatile aroma compounds including esters, fatty acids and higher alcohols is presented. Indeed, while the metabolic network that leads to the formation of these compounds is reasonably well mapped, surprisingly little is known about specific enzymes involved in specific reactions, the genetic regulation of the network and the physiological roles of individual pathways within the network. Various factors that directly or indirectly affect and regulate the network have been proposed in the past, but little conclusive evidence has been provided. To gain a better understanding of the regulations and physiological role of this network, we took a functional genomics approach by screening a subset of the EUROSCARF strain deletion library, and in particular genes encoding decarboxylases, dehydrogenases and reductases. Thus, ten genes whose deletion impacted most significantly on the aroma production network and higher alcohol formation were selected. Over-expression and single and multiple deletions of the selected genes were used to genetically assess their contribution to aroma production and to the Ehrlich pathway. The results demonstrate the sensitivity of the pathway to cellular redox homeostasis, strongly suggest direct roles for Thi3p, Aad6p and Hom2p, and highlight the important role of Bat2p in controlling the flux through the pathway. / AFRIKAANSE OPSOMMING: Alkoholiese fermentasie, en veral die maak van wyn, is een van die vroegste mikrobiologiese prosesse wat deur die mensdom ingespan is. Die gisspesie Saccharomyces cerevisiae is gewoonlik grotendeels verantwoordelik vir die fermentasie and verskeie vorige studies het gedemonstreer dat hierdie spesie ‘n baie belangrike rol speel in die uiteindelike kwaliteit en karakter van die voltooide produk. Nieteenstaande die feit is daar steeds baie aspekte van beide die genetiese en metaboliese prosesse wat plaasvind tydens alkoholiese fermentatsie wat nog swak verstaan word, insluitende metaboliese padweë wat ‘n impak het op die smaak en aroma van die fermentasie produk. Om ons kennis van die veld uit te brei het die studie twee aanslae geneem: In die eerste geval is gepoog om twee tegnieke van transkriptoom analiese, nl. DNA oligomikro- arrays en Serial Analysis of Gene Expression (SAGE) te bestudeer vir hul vermoë om geen ekspressie veranderinge tydens wynfermentasie te ondersoek en meer spesifiek om hul potensiaal om ‘n kombinasie van kwantitatiewe sowel as kwalitatiewe data met betreking to mRNA vlakke te produseer. Die SAGE metode kon egter geen betroubare resultate produseer nie en dus word slegs die resultate van die mikro-array eksperimente in die tesis bespreek. Die resultaat is ‘n geheeloorsig oor die geenekspressie veranderinge wat so ‘n wyngis tydens alkoholiese fermentasie ondergaan en dien as ‘n verwysingsraamwerk vir toekomstige studies met geneties gemodifiseerde gis of selfs verskillende fermentasieparameters. Die tweede deel van die studie het gefokus op die identifikasie van gene wat ‘n impak het op die vorming van belangrike, vlugtige aroma komponente, o. a. Esters vetsure en hoër alkohole d.m.v. ‘n siftingseksperiment. Alhoewel daar redelik baie inligting is oor die onderligende metaboliese netwerke wat lei tot die vorming van die verbindings, is daar min kennis van die genetiese regulasie van die netwerk en die fisiologiese rol van individuele padweë wat die netwerk vorm. Verskeie faktore – wat of die netwerk direk of indirek affekteer – is al voorgestel, meer met min konkrete bewyse. Dus het ons gepoog om meer lig op die onderwerp te laat m.b.v. ‘n funksionele genoom aanslag deur ‘n siftingseksperiment te doen op ‘n subgroep (spesifiek gene wat kodeer vir dekarboksilase, dehidrogenase en reduktase ensieme) van die EUROSCARF delesiebiblioteek. Dus is tien gene geïdentifiseer – die delesie waarvan ‘n merkbare effek het op die aroma produksie netwerk en spesifiek die van hoër alkohole. Ooruitdrukkings en enkel en meervoudige delesie rasse van die tien gene is gemaak om d.mv. genetiese analiese, hulle rol in aroma produksie en die Ehrlich padweh uit te pluis. Die resultate toon dat hierdie padweg sensitief is teenoor die sellulêre redoks balans en dui op direkte rolle vir Thi3p, Aad6p en Hom2p, asook dat Bat2p ‘n baie belangrike rol speel in die werking van die padweg.

Thesis (MScAgric (Viticulture and Oenology))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: The total sale of brandy for 2007 in South Africa was R 7 300 000 000 and local statistics indicate that brandy is by far the most purchased spirit beverage. Sales of brandy even outweigh the total sales for whisky and the forecast for the estimated sales of brandy in the next five years is said to increase by 25%. It is therefore crucial to investigate those factors that influence the production of brandy as better understanding and control of these processes leads to the production of a brandy that is consistent and of premium quality. Many factors influence the final outcome of distillates. Of these factors, the distillation technique, the apparatus used for the purpose of distillation together with the low wine is of utmost importance as they influence the sensory profile and the chemical composition of the distillate. The effect of different variations of pot still designs on the chemical composition and the sensory profile of the resultant distillate was investigated. Five different Pot still variations were used and varied with regards to the design of their pot still head and swans neck apparatus. Two low wines were used for the purpose of distillations and were both from 2007 vintage. GCFID was used to identify the volatile compounds found in the distillates and together with Quantitative Descriptive Analysis (QDA) a profile of the distillates was produced which was used to differentiate between the different pot still variations and their effect on the final product. The data generated from the QDA sessions was subjected to Principal Component Analysis (PCA) and together with the chemical analysis a correlation between certain compounds and sensory attributes were found in the distillates. Distillate samples were also subjected to a sensory style classification system and were classified accordingly. The chemical composition of the two low wines prior to distillations differed significantly from each other with low wine one containing a larger amount of total esters and carbonyl compounds whilst low wine two contained a larger amount of total higher alcohols and acids. The distillates of low wine one also contained over all larger amounts of total esters and in the case of the distillates of low wine two, they contained larger amounts of higher alcohols and acids than low wine one. Variation one was based on the Alambic Charentais method of pot still design and it was found that only variation one influenced the chemical composition and the sensory profile of the distillates. This variation produced a distillate with a lower amount of total esters and more specifically ethyl acetate as well containing a lower intensity of the fruit and sweet associated caramel aromas and flavours. The esters, ethyl acetate and the ethyl esters of the long chained fatty acids were found to correlate with the sensory attributes known as fruit associated aroma, soapy aroma, and spicy aroma and therefore indicated that these compounds are responsible for these attributes. There were no correlations found between the chemical compounds, sensory attributes and sensory style classifications in the distillates of both low wine one and two. It was shown that the addition of certain esters, carbonyl compounds, higher alcohols and acids in specific ratios could alter the sensory classification of the distillates. Therefore the chemical composition and the sensory characteristics of distillates are largely dependent on the chemical composition of the low wine prior to distillation rather than the pot still design. Therefore, with further research it could be possible to predict the outcome of the chemical composition of the distillates by analyzing the chemical compounds found in the low wine prior to distillation. / AFRIKAANSE OPSOMMING: Die totale verkope aan brandewyn vir 2007 in Suid Afrika beloop R7 300 000 000 en statistiek wys dat brandewyn by verre die mees gesogte spiritus drank is. Verkope van brandewyn is selfs meer as die verkope van whisky en die voorspelling is dat die verkope van brandewyn met 25% gaan vermeerder in die volgende vyf jaar. Dit is dus belangrik om die faktore te ondersoek wat die produksie van brandewyn beïnvloed om sodoende die verstokingsproses te verstaan en te kontroleer om ‘n konsekwente kwaliteitsproduk op die mark te plaas. Baie faktore beïnvloed die finale produk. Faktore soos die distillasie tegnieke, die apperate wat gebruik word vir distillasie tesame met die rabatspiritus is van die uiterste belang aangesien dit die sensoriese profiel en die chemiese samestelling van die distillaat beïnvloed. Die effek van die verskillende variasies van potketelhelms op die chemiese samestelling van die distillate word ondersoek. Vyf verskillende helms met variasies in die swaannek ontwerp was gebruik. Twee verskillende rabatspiritus, van die 2007 oesjaar, was gebruik vir distillasie. GC-FID was gebruik om die vlugtige komponente van die distillate mee vas te stel. Kwantitatiewe Beskrywende Analise (QDA) is gebruik om ‘n profiel van die distillate op te stel wat weer gebruik is om te differensieer tussen die verskillende potketelhelm variasies en hulle effek op die finale produk. Die data wat deur die QDA sessies gegenereer was, is in die Vernaamste Komponent Analise (PCA) ingevoer en tesame met die chemiese analise is ‘n korrelasie tussen sekere komponente en die sensoriese analise van die distillate gevind. Distillaat monsters was ook aan sensoriese styl van klassifikasie onderwerp en is as volg daarvan geklassifiseer. Die chemiese samestelling van die twee rabatspiritus voor finale distillasie het betekenisvol van mekaar verskil ten opsigte daarvan dat die eerste rabatspiritus het hoë konsentrasies esters en karboniel verbindings gehad terwyl die tweede rabatspiritus meer hoë konsentrasies van sure en hoër alkohole gehad het. Die distillaat van die eerste rabatspiritus het ook hoë konsentrasies esters en karboniel verbindings gehad terwyl die distillaat van die tweede rabatspiritus weer hoë konsentrasies van sure en hoër alkohole gehad het. Variasie een is gebaseer op die Alambic Charentais van potketel ontwerp en daar is ook gevind dat hierdie variasie die enigste een was wat die chemiese samestelling en die sensoriese profiel van die distillate beïnvloed het. Hierdie variasie het ‘n distillaat geproduseer wat lae konsentrasies van totale esters, veral etielasetaat, sowel as laer intensiteit van vrugtige en soet geassosieerde karamel aromas en geure. Die esters, etielasetaat en etiel esters van die lang ketting vetsure, is gevind dat dit goed korreleer met die sensoriese eienskappe wat geassosieer word met vrugtige aromas, spesery-agtige aromas en seperige aromas. Daar is geen korrelasie gevind tussen die chemiese verbindings, sensoriese eienskappe en sensoriese styl van klassifikasie van distillate een en twee nie. Dit was ook bewys dat byvoeging van esters, karboniel verbindings, sure en hoër alkohole, in spesifieke verhoudings, die sensoriese eienskappe kan verander. Dus is die chemiese samestelling en sensoriese eienskappe van die distillate grootliks afhanklik van die chemiese samestelling van die rabatspiritus, voor die tweede distillasie, as wat dit afhanklik is van die potketelhelm ontwerp. Gevolglik, met verdere v navorsing, is dit moontlik om die uitkoms van die chemiese samestelling van die distillaat te voorspel deur die analise van die chemiese verbindings van die rabatspiritus te ontleed.

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.

Thesis (MScAgric (Viticulture and Oenology))--University of Stellenbosch, 2011. / Includes bibliogaphy. / ENGLISH ABSTRACT: Sauvignon blanc wines have become increasingly popular in South Africa as it is a cultivar that can be easily manipulated in the vineyard and cellar to produce a range of wine styles. These wines are usually given aroma descriptors such as green pepper, grassy and asparagus; while other more tropical aromas include passion fruit and guava. These aromas are thought to be mainly caused by methoxypyrazines and volatile thiols. These compounds are known to be character impacting compounds of Sauvignon blanc and are present in the grapes in the aromatic form (methoxypyrazines) or as non‐aromatic precursors (thiols) that can be released by the yeast during fermentation. Other aroma compounds such as esters, higher alcohols, fatty acids and monoterpenes are compounds that could potentially influence the aroma bouquet of a wine significantly. These aroma compounds exist either as precursors in the grapes (monoterpenes) or arise due to yeast metabolism during fermentation (esters, higher alcohols, fatty acids) and often display fruity, floral and pleasant aromas. In the cellar, winemaking practices can be manipulated to a certain extent to achieve the desired wine style. Winemaking tools such as temperature, skin contact, pressing conditions, oxygen (O2), sulphur dioxide (SO2) and yeast strain are only a few factors influencing the outcome of a wine. In general, South African winemakers maintain a very reductive environment during Sauvignon blanc wine production by using inert gasses, thereby causing the production costs to increase. There is sufficient evidence to support the reductive handling of white wine, however there seems to be a lack of information as to why the must should be treated reductively before fermentation. The over all goal of this study was thus to investigate the effect of different O2 and SO2 additions to Sauvignon blanc must before settling, specifically focussing on the typical aroma compounds often found in these wines. Chapter 2 gives an overview of the oxidation reactions occurring in must (enzymatic oxidation) and wine (chemical oxidation). This chapter also reports the origin of the specific Sauvignon blanc aroma compounds and their reaction to different must and wine treatments with a focus on oxidation. Chapter 3 reports research results focussing on the effect of the different must treatments on the character impacting compounds of Sauvignon blanc wines, specifically the methoxypyrazines and the volatile thiols. The effect of the treatments on the polyphenols and glutathione content in the must and wine was also investigated. Oxidation in the absence of SO2 led to a decrease in glutathione and certain phenolic compounds in the must. In general, volatile thiols were protected against oxidation by SO2, even when O2 was present in the must. Methoxypyrazines concentrations were not significantly influenced by the treatments. Chapter 4 elucidates the effect of the treatments on other yeast and grape derived aroma compounds often found in Sauvignon blanc wines, such as the esters, higher alcohols, fatty acids and monoterpenes. In general, the effect of SO2 seemed to have the greatest influence on the produced aroma compounds. The results reported in this thesis could possibly change the way South African Sauvignon blanc musts are handled in future during the winemaking process. It is clear that O2 and SO2 management in the cellar is of critical importance for the winemaker to produce wines of high quality. Future work is important to fully understand the mechanisms and evolution of important aroma compounds of Sauvignon blanc wines during the winemaking process. / AFRIKAANSE OPSOMMING: Sauvignon blanc wyn aroma word gewoonlik beskryf met terme soos groen rissie, grasagtig en aspersie terwyl ander tropiese aromas soos grenadella en koejawel ook dikwels voorkom. Die manipulasie van Sauvignon blanc in die wingerd en in die kelder tydens wynmaak, gee die wynprodusent die vryheid om ‘n wye reeks wyn style te produseer. Dit maak Sauvignon blanc baie populêr in die Suid‐Afrikaanse wynindustrie. Die bogenoemde aromas word waargeneem in die wyn as gevolg van die teenwoordigheid van sekere aroma komponente genaamd metoksipirasiene en vlugtige tiole. Hierdie komponente lewer ‘n unieke bydrae tot die aroma samestelling van Sauvignon blanc wyne en kom voor in die druiwe in die aromatiese vorm (metoksipirasiene) of as nie‐aromatiese voorlopers (tiole) wat tydens alkoholiese fermentasie deur die gis vrygestel kan word. Komponente soos esters, hoër alkohole, vetsure en monoterpene kan ook ‘n potensiële bydra lewer tot die algehele aroma van Sauvignon blanc wyne en kom voor in die druiwe (monoterpene) of ontstaan as gevolg van gis metabolisme gedurende alkoholiese fermentasie (esters, hoër alkohole, vetsure). Hierdie geur komponente word dikwels beskryf as vrugtig, blomagtig en oor die algemeen aangenaam. Tydens wynmaak kan die wyn tot ‘n mate gemanipuleer word om ‘n spesifieke wynstyl te bekom. Hulpmiddels soos temperatuur, dopkontak, pers omstandighede, suurstof (O2), swawel dioksied (SO2) en gisras is slegs ‘n paar faktore wat die algemene uitkoms van ‘n wyn kan beïnvloed. Oor die algemeen word Sauvignon blanc in Suid‐Afrika baie reduktief behandel tydens wynbereiding. Dit vereis sekere hulpmiddels, soos die gebruik van inerte gas, wat die produksiekoste dikwels verhoog. Navorsing ondersteun die reduktiewe behandeling van wit wyn, maar dit wil voorkom asof daar ‘n tekort aan navorsing is wat die reduktiewe behandeling van die sap voor fermentasie regverdig. Die algemene doel van die studie is dus om die effek van verskillende O2 en SO2 byvoegings tot Sauvignon blanc sap (voor afsak) te ondersoek met die fokus op die tipiese aroma komponente wat in die wyn voorkom. Hoofstuk 2 lewer ‘n algemene oorsig van die tipes oksidasie reaksies wat voorkom in sap (ensiematiese oksidasie) en wyn (chemiese oksidasie). Spesifieke Sauvignon blanc aroma komponente word ook ondersoek in terme van die oorsprong van die komponente asook die reaksie wat plaasvind met verskillende mos en wyn behandelings, met ‘n fokus op oksidasie. In hoofstuk 3 word die effek van die verskillende mos behandelings op tipiese Sauvignon blanc aroma komponente, spesifiek metoksipirasiene en vlugtige tiole, ondersoek. Die effek van die behandelings op die polifenole en glutatioon inhoud in die mos en wyn word ook gerapporteer. Oksidasie van die sap in die afwesigheid van SO2, het ‘n afname in glutatioon en sekere polifenol konsentrasies veroorsaak. Dit wil voorkom asof die produksie van vlugtige tiole oor die algemeen beskerm word teen oksidasie indien daar genoegsame SO2 teenwoordig is. Hierdie effek word ondervind selfs as die sap met suursof versadig word. Die effek van die behandelings op die konsentrasies van metoksipirasiene was nie beduidend nie. Hoofstuk 4 rapporteer die effek van die behandelings op ander aroma komponente soos esters, hoër alkohole, vetsure en monoterpene. Oor die algemeen wil dit voorkom asof die effek van SO2 beduidend was en waarskynlik die grootste invloed op die produksie van hierdie aroma komponente het. Na aanleiding van die resultate bevind in hierdie tesis, is daar ‘n moontlikheid dat die manier waarop Sauvignon blanc wyne geproduseer word in Suid‐Afrika, moontlik kan verander in die toekoms. Vir die wynmaker om hoë kwaliteit Sauvignon blanc wyne te produseer, is O2 en SO2 bestuur in die kelder van kardinale belang. Verdere navorsing moet steeds gedoen word om die meganisme en evolusie van belangrike aroma komponente in Sauvignon blanc wyne tydens die wynmaakproses, ten volle te verstaan.

Thesis (MScAgric)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Sauvignon blanc wines have become progressively more important in the commercial market. Extensive research is being done in various countries to gain more understanding about the aroma compounds found in Sauvignon blanc wines and the interactions between them. Sauvignon blanc wines often have either have a green or tropical style. The green style is caused by the methoxypyrazines while the volatile thiols are important contributing compounds to the tropical style. Various international studies have focussed on measuring the chemical composition of Sauvignon blanc wines. However, more research is required on South African Sauvignon blanc wines. Little is known of the volatile thiols content of South African Sauvignon blanc wines, although the methoxypyrazine content has been extensively reported on. Although methoxypyrazines and volatile thiols are seen as the most important aroma compounds contributing to Sauvignon blanc character, other compounds contribute as well. Esters, monoterpenes and phenols have been found to influence Sauvignon blanc aroma and interact with the methoxypyrazines and volatile thiols. The complex interaction between the compounds responsible for the aroma of Sauvignon blanc wines are still not fully understood and further research is thus needed. The first part of the current study investigated the interaction between a specific methoxypyrazine and volatile thiol. Five different concentrations of 2-isobutyl-3- methoxypyrazine (ibMP) and 3-mercaptohexan-1-ol (3MH) were spiked in dearomatize, neutral Sauvignon blanc wine. The single compounds as well as every possible combination of the range of concentrations were evaluated using sensory descriptive analysis. It was found using various statistical approaches that ibMP suppressed the tropical attributes associated with 3MH and that 3MH suppressed the green attributes that correlated with ibMP. The concentrations at which the suppression occurred and the degree of suppression was different for each attribute. The second part of the current study focussed on commercial South African Sauvignon blanc wines. Sensory descriptive analysis and chemical analysis were used to assess the wines and measure the volatile thiol and methoxypyrazine concentrations. The concentrations of volatile thiols and methoxypyrazines were found to be in line with international Sauvignon blanc wines. It was also shown for the first time that the mutually suppressive trend between the volatile thiols and methoxypyrazines can be seen in commercial Sauvignon blanc wines as well. Future work is needed to fully understand the complex interaction between the various compounds in Sauvignon blanc wines. Further research could focus on investigating the mechanism of interaction between the volatile thiols and methoxypyrazines as well as other aroma compounds. / AFRIKAANSE OPSOMMING: Sauvignon blanc-wyne word toenemend belangriker in die kommersiële mark. Omvattende navorsing word in etlike lande gedoen om meer begrip te ontwikkel van die aromaverbindings wat in Sauvignon blanc-wyne teenwoordig is, asook van die interaksies tussen hulle. Sauvignon blanc-wyne het in baie gevalle óf ’n groen óf ’n tropiese styl. Die groen styl word veroorsaak deur metoksipirasiene, terwyl die vlugtige tiole belangrike bydraende verbindings is wat aanleiding gee tot die tropiese style. Verskeie internasionale studies het reeds gefokus op die meet van die chemiese samestelling van Sauvignon blanc-wyne, maar meer navorsing is nodig oor Suid-Afrikaanse Sauvignon blanc-wyne. Min is bekend oor die inhoud van vlugtige tiole in Suid-Afrikaanse Sauvignon blanc-wyne, hoewel daar reeds op groot skaal oor die metoksipirasieninhoud verslag gedoen is. Hoewel metoksipirasiene en vlugtige tiole die belangrikste aromaverbindings is wat tot Sauvignon blanc-karakter bydra, is daar ook ander verbindings wat ’n bydrae maak. Esters, monoterpene en fenole het almal ’n invloed op Sauvignon blanc-aroma en reageer op die metoksipirasiene en vlugtige tiole. Die komplekse interaksie tussen die verbindings wat vir die aroma van Sauvignon blanc-wyne verantwoordelik is, word nog nie volledig begryp nie en verdere navorsing is nodig. Die eerste deel van die huidige studie het die interaksie tussen ’n spesifieke metoksipirasien en vlugtige tiol ondersoek. Vyf verskillende konsentrasies van 2-isobutiel-3-metoksipirasien (ibMP) en 3-merkaptoheksaan- 1-ool (3MH) is by ontgeurde, neutrale Sauvignon blanc-wyn gevoeg. Die enkel verbindings, asook elke moontlike kombinasie van die reeks konsentrasies, is deur middel van beskrywende sensoriese analise geëvalueer. Daar is met behulp van verskillende statistiese benaderings gevind dat ibMP die tropiese eienskappe wat verband hou met 3MH onderdruk het, terwyl 3MH die groen eienskappe wat verband hou met ibMP onderdruk het. Die konsentrasies waarteen die onderdrukking plaasgevind het en die vlak van onderdrukking het vir elke eienskap verskil. Die tweede deel van die studie het gefokus op kommersiële Suid-Afrikaanse Sauvignon blancwyne. Beskrywende sensoriese analise en chemiese analise is gebruik om die wyne te assesseer en die konsentrasies van vlugtige tiole en metoksipirasiene te meet. Die konsentrasies vlugtige tiole en metoksipirasiene was in lyn met dié van internasionale Sauvignon blanc-wyne. Daar is ook vir die eerste keer gewys dat die wedersyds onderdrukkende tendens tussen die vlugtige tiole en metoksipirasiene ook in kommersiële Sauvignon blanc-wyne gevind word. Toekomstige werk sou kon fokus op ’n begrip van die komplekse interaksie tussen die verskillende verbindings in Sauvignon blanc-wyne. Verdere navorsing sou ook kon fokus op ’n ondersoek van die meganisme van interaksie tussen die vlugtige tiole en metoksipirasiene, sowel as ander aromaverbindings. / The South African Wine Industry (Winetech), THRIP and the NRF for financial support

L’objectif de cette thèse est d’étudier le rôle de la fermentation malolactique (FML) sur l’arôme fruité des vins rouges. Les bactéries lactiques (BL) modifient la composition du vin mais il n’existe pas de consensus concernant spécifiquement cette famille aromatique. Contrairement aux idées empiriques sur la FML, ce travail a démontré l’absence à court terme d’un " masque lactique ", cependant l’apparition d’une telle interaction olfactive pourrait être plus tardive. Par contre, il est montré l’existence d’un masque proche de la note de réduction, de type fumé/grillé, dont la caractérisation n’a pas été effectuée dans cette étude.Le suivi des principaux marqueurs fruités du vin (70 molécules) a été rendu possible par le développement des méthodes d’analyse chromatographique en phase gazeuse couplée à la microextraction sur phase solide (esters, C13-norisoprénoïdes, lactones, thiols). En particulier, une " base de données esters " (32 composés) a rendu plus robuste l’ensemble des variations constatées au cours du développement des BL. En effet, les modifications des teneurs en esters sont démontrées comme un processus majeur de la balance de la note fruitée au cours de la FML. Cette fermentation permet à court terme, aussi bien la synthèse que l’hydrolyse des esters grâce aux activités estérases et, à plus long terme, la formation tardive d'esters éthyliques d'acides branchés issus du catabolisme de certains acides aminés. La spécificité des estérases vis-à-vis de la nature et de la longueur de la chaîne carbonée des esters est mise en évidence, ainsi que l'importance de la disponibilité des substrats, liée en partie à l'activité des levures.L’étude de l’influence des souches de BL et de la co-inoculation levures/bactéries a permis de confirmer le rôle clé des interactions entre les microorganismes, ainsi que l’importance de la composition de la matrice vin. / The aim of this thesis is to study the role of malolactic fermentation (MLF) on the fruity aroma of red wines. Lactic acid bacteria (LAB) modify wine composition but there is no consensus concerning this aromatic group specifically. In opposition to empirical ideas on MLF, this work has demonstrated the absence, in short-term, of a “lactic-mask” although this kind of olfactory interaction may still occur in a later stage of wine development. Nevertheless, the existence of a smoked/toasted reduction-like mask note was proved. Its characterization has not been done in this work. The survey of the main fruity markers of wine (70 compounds) was made possible by the development of several gas chromatography coupled with solid-phase microextraction analytical methods (esters, C13-norisoprenoids, lactones, thiols). In particular, the creation of an “ester database” (32 compounds) has improved the detection of variations during LAB development in terms of analysis robustness. In fact, changes on esters contents are proved to be responsible for a major part of fruity notes evolution during MLF. Initially, this fermentation allows both synthesis and hydrolysis of esters as a consequence of esterase activity. Moreover, it promotes late-production of ethylic esters through the catabolism of certain aminoacids. Esterases specificity towards nature and size of the esters carbon chain is pointed out along with substrates availability, partially related to yeast activity.The study of the influence of both LAB strains and yeast/bacteria co-inoculation has confirmed microorganisms interactions and wine matrix composition to be of the great importance.

Afin de contribuer à la préservation du patrimoine laitier libanais, l'objectif principal de cette étude a été de caractériser le fromage Darfiyeh, une variété traditionnelle fabriquée à partir de lait de chèvre cru et affinée dans une outre en peau de chèvre. Parallèlement à cet objectif, et pour mieux comprendre l'affinage du Darfiyeh, le transfert de composés d’arôme à travers la peau de chèvre a été étudié au moyen d’un dispositif expérimental original. Trois lots indépendants de production de Darfiyeh ont été analysés après 20, 40 et 60 jours d'affinage. Les résultats des analyses physico-chimiques ont montré que le Darfiyeh est un fromage à pâte mi-dure, caractérisé par une protéolyse et une lipolyse modérées. Le 3-méthyl butanal, le 1-phényl éthanol et le 1-octanol contribuent à la fraction aromatique particulière du Darfiyeh. Les analyses microbiologiques ont mis en évidence (Log cfu.g-1 fromage) des lactobacilles mésophiles (7,1-10,4), des bactéries lactiques (BL) thermophiles cocciformes (6,6-8,4) et des lactobacilles thermophiles (5,5-7,2). Afin d'explorer l'écosystème naturel du Darfiyeh, une combinaison d'approches classiques et moléculaires a été appliquée. L'identification classique a révélé des espèces appartenant aux genres Streptococcus, Enterococcus, Lactococcus et Lactobacillus. Les profils de Temporal Temperature Gel Electrophoresis (TTGE) ont révélé des bandes communes avec l'identification classique. Des analyses menées par PCR spécifiques de l'espèce bactérienne ont confirmé la présence of S. thermophilus, E. faecium, E. durans, Lc. lactis subsp. lactis et Lc. lactis subsp. cremoris. La PCR en temps réel a permis la quantification de S. thermophilus et d' E. faecium, avec un seuil de détection respectif de 104 ufc.g-1 de Darfiyeh et une gamme de 107-109 ufc.g-1 de Darfiyeh, respectivement. Pour le transfert d'arômes à travers la peau de chèvre, l'absorption et la perméabilité de trois molécules aromatiques modèles (2-butanone, 2,3-butanedione, 2-butanol) dans un système solution aqueuse/peau de chèvre a été suivi, après 20, 40 et 60 jours d'exposition. Jusqu'à 40 j d'exposition, le transfert des molécules aromatiques était régi par leurs propriétés physico-chimiques et par le temps d'exposition. Après 60 j, le système solution aqueuse/peau s’est traduit par une tendance à la migration des molécules modèles depuis la peau vers la solution simulée de Darfiyeh. Autrement dit, le transfert de ces molécules modèles dans le système simulé paraît s’établir de façon cyclique. Ce travail est une première contribution scientifique et technique au secteur de la valorisation durable de la tradition fromagère libanaise / In order to contribute for the preservation of the Lebanese dairy heritage, the aim of this study was to characterize Darfiyeh cheese, a traditional variety made from raw goats' milk and ripened in goat's skin. Parallel to this objective, and for a better understanding of Darfiyeh ripening, aroma transfer towards goat skin was conducted through an experimental device. Three independent batches of Darfiyeh production were analyzed after 20, 40 and 60 days of ripening. Physico-chemical results showed that Darfiyeh is a semi-hard goat's milk cheese which undergoes a moderate proteolysis and lipolysis. 3-methyl butanal, 1-phenyl ethanol and 1-octanol contribute to the distinctive aromatic fraction of Darfiyeh. As for microbiological results, bacterial counts (Log cfu.g-1 cheese) for mesophilic lactobacilli (7.1-10.4), thermophilic coccal-shaped lactic acid bacteria (LAB) (6.6-8.4) and thermophilic lactobacilli (5.5-7.2) were found. In order to explore Darfiyeh natural ecosystem, a combination of classical and molecular approaches was applied. Classical identification revealed members of the genera Streptococcus, Enterococcus, Lactococcus and Lactobacillus. Temporal Temperature Gel Electrophoresis (TTGE) profiles revealed common bands between the classical identification. Species-specific polymerase chain reactions (PCR) confirmed the presence of S. thermophilus, E. faecium, E. durans, Lc. lactis subsp. lactis and Lc. lactis subsp. cremoris. Real time PCR enabled quantification of S. thermophilus and E. faecium, with a detection threshold of 104 ufc.g-1 of Darfiyeh and a range of 107-109 ufc.g-1 of Darfiyeh, respectively. As for aroma transfer towards goat skin, absorption and permeability of three model aroma molecules (2-butanone, 2,3-butanedione, 2-butanol) in aqueous solution/goat skin system were followed, after 20, 40 and 60 days of exposure. Until 40 days, the transfer of aroma molecules was dependent on their physico-chemical properties and exposure time. After 60 days, the molecules migrate from the skin to the aqueous solution. Therefore, in such a simulated system, aromatic molecules seem to move under a cyclic way. This work is a first scientific and technical contribution to the preservation of traditional Lebanese cheesemaking

L'emballage alimentaire est l'un des principaux acteurs de la préservation de la sécurité alimentaire et de la qualité au cours du stockage. Toutefois, de transferts de masse se produisent entre le l'emballage polymère et les denrées alimentaires. Polylactide (PLA) est un nouveau polymère biosourcé utilisé dans les applications d'emballage. Pour cela son interaction avec des denrées alimentaires a été étudiée dans des conditions réelles d'utilisation. Une méthodologie originale basée sur l'extraction multiple de l'espace de tête (MHE) et l'extraction multiple de l'espace de tête couplée à la microextraction sur phase solide (MHS-SPME) pour la mesure de sorption de composés organiques volatils (COVs) dans le PLA a été développé à des activités faibles et dans le domaine vitreux du polymère. Les COVs ayant une structure chimique modérément hydrophobe et des similarités de structure avec le PLA montrent une forte affinité pour le polymère. Plus surprenant, la forte sorption de composés aromatiques, notamment du benzaldéhyde, a été montrée. En mélange avec d'autres COVs le benzaldéhyde favorise leur sorption par un effet synergique. De plus, même à des activités faibles (0,01) et en présence de benzaldéhyde, la sorption de COVs induit une cristallisation du PLA. En contact avec un aliment aqueux, le PLA se montre résistant à l'hydrolyse et la sorption des matières grasses est faible, même en utilisant des conditions de stockage proches de sa transition vitreuse. L'apparition de COVs pendant le procédé de transformation a été étudiée par MHS-SPME. Des lactides, l'acétaldéhyde et le 2,3-pentanedione étaient détectés après extrusion et thermoformage. Les quantités des deux derniers composés diminuaient cependant pendant le procédé de thermoformage. / Packaging plays a major role in the preservation of food but mass transfer between the packaging material and foodstuff occurs during shelf life leading to the quality deterioration. Polylactide (PLA) is a novel packaging material; therefore its interaction with food was investigated at service conditions. The sorption of ethyl esters, benzaldehyde and 2-nonanone at low concentrations and in mixture and the effects on the thermal properties of PLA were studied. Multiple Headspace Extraction (MHE) and Multiple Headspace coupled to Solid Phase MicroExtraction (MHS-SPME) developed here allow to show a high affinity for moderately hydrophilic molecules sharing structure similarities with PLA such as ethyl acetate. However, a very high affinity for the aromatic structure of benzaldehyde was revealed which seems to favor interaction and sorption of the aroma compounds present in the mixture. At 0.01 activities, a plasticization by aroma compounds was observed and a solvent induced crystallization seemed to be correlated to the sorption of benzaldehyde. The contact between a flavored food emulsion and PLA, using conditions near the PLA glass transition showed that PLA hydrolysis was moderate and oil and aroma compound sorption was low. The formation of VOC during the PLA manufacturing process was assessed by MHS-SPME. Lactides, acetaldehyde and 2,3-pentanedione are VOCs present in PLA after extrusion and thermoforming. The two latter compounds increased after extrusion and then decreased or disappeared after thermoforming.

L’expression aromatique fruitée des vins rouges a été le sujet de nombreuses études qui démontrent qu’au moins une composante de cette expression est le reflet d’interactions perceptives impliquant des esters. La synthèse de ces esters peut être affectée par la levure réalisant la fermentation alcoolique mais aussi par d’autres paramètres, plus technologiques, comme le niveau de maturité de la vendange. De plus, peu d’études ont été réalisé afin de valider le rôle physiologique tenu par les esters pour la levure.Dans le but de mettre en évidence plus précisément le rôle de ces facteurs dans la synthèse des esters, des souches de levures délétés des majeures estérases ont été construites, et testées en fermentation en milieu œnologique, dans des moûts de merlot et de tempranillo récoltés à deux maturités différentes. Ainsi, la concentration en chaque esters linéaires et substitués a pu être déterminée dans chaque modalité de vinification.L’étude des différents mutants de délétion, a permis, pour la première fois, de valider le rôle des principales estérases dans la synthèse des esters linéaires chez la levure mais aussi, la mise en évidence de deux gènes, non étudiés jusqu’alors en condition œnologique, sur la synthèse des esters substitués. L’analyse de l’expression génétique de la délétion des estérases chez la levure a permis aussi de valider que ces gènes permettent une véritable stabilité physiologique de la levure en conditions stressante.L’impact du degré de maturité de la vendange a été étudié à la fois chez les vins fermentés avec une levure standard commerciale mais aussi fermentés avec une levure délétée des 4 principales estérases. Une maturité avancée, des raisins de merlot seulement, entraine une baisse de 50% de la teneur en esters linéaire avec la levure standard, ce qui n’est plus observé avec la levure mutante. Cette diminution de la concentration en esters linéaire dans ces vins de merlot de maturité avancée, est bien corrélée à une diminution de leur perception fruitée. De plus, des reconstitutions aromatiques faites dans ces matrices, ont permis de valider l’implication totale des esters dans la perception de l’arôme fruité des vins rouges réalisés avec des matrices de maturité normale. En revanche en ce qui concerne les vins de merlot de maturité avancée, il existerait d’autres composés en plus des esters qui pourraient expliquer les différences sensorielles observées dans ces vins.Enfin, une approche de transcriptomique est mise en œuvre pour tâcher d’éclairer les facteurs à l’origine des modifications de production d’esters en fonction du niveau de maturité des raisins de merlot. Il en est sorti que l’effet maturité n’existe pas seul mais est bien combiné avec l’avancée de la fermentation alcoolique. L’effet global de la matrice qui peut expliquer les différences observées entre les vins de merlot. / A lot of studies highlighted the perceptual role of esters in fruity aromatic expression of red wines, demonstrating that at least partially it was due to perceptive interactions. The synthesis of these esters can be affected by the yeast making the alcoholic fermentation but also by other parameters, more technological, such as the level of maturity of the harvest. In addition, few studies have been conducted to validate the physiological role of esters for yeast.In order to more precisely highlight the role of these factors in the synthesis of esters, deleted yeast strains of the major esterases have been constructed, and tested in fermentation in an oenological environment, in merlot and tempranillo musts harvested at two different maturities. Thus, the concentration of each linear and substituted ester could be determined in each vinification modality.The study of the different deletion mutants allowed, for the first time, to validate the role of the main esterases in the synthesis of linear esters in yeast but also the demonstration of two genes, not yet studied in oenological conditions, on the synthesis of substituted esters. The analysis of the genetic expression of the esterase deletion in yeast has also validated that these genes allow a true physiological stability of the yeast under stressful conditions.The impact of the degree of maturity of the harvest has been studied both in wines fermented with a standard commercial yeast but also fermented with a yeast deleted from the 4 main esterases. Advanced maturity, merlot grapes only, results in a 50% drop in linear ester content with standard yeast, which is no longer observed with the mutant yeast. This decrease in the concentration of linear esters in these mature Merlot wines is well correlated with a decrease in their fruity perception.In addition, aromatic reconstitutions made in these matrices made it possible to validate the total involvement of esters in the perception of the fruity aroma of red wines made with matrices of normal maturity. On the other hand, for mature merlot wines, there are other compounds besides esters that could explain the sensory differences observed in these wines.Finally, a transcriptomic approach is implemented to try to shed light on the factors that cause ester production changes as a function of the maturity level of merlot grapes. It has emerged that the maturity effect does not exist alone but is well combined with the progress of alcoholic fermentation. The overall effect of the matrix that can explain the differences observed between merlot wines.