Yeast Autolysis in Sparkling Wine - A Review, SUB

Alexandre & Guilloux-Benatier Yeast autolysis in sparkling wine 119

IntroductionSparkling wine manufactured by the traditional or méth-ode champenoise requires two successive fermentations.The first fermentation transforms grape must into basewine. The essence of the champenoise method is thesecond fermentation, which takes place in the bottle andincreases the alcohol content and internal bottle pressure(up to 5–7 atmospheres). After this second alcoholic fer-mentation, the wine is aged on yeast lees. Ageing on leeslasts for at least nine months, depending on the legislationof the wine-producing country. The ageing on yeast leesfor Champagne and Champagne millésimé lasts one andthree years respectively. Autolysis of the yeast occursduring this prolonged contact. Yeast autolysis is a slowprocess associated with cell death, and involves hydrolyticenzymes that act to release cytoplasmic (peptides, fattyacids, nucleotides, amino acids) and cell wall (manno-proteins) compounds into the wine. Low ageing temper-ature causes a low death rate and low enzymatic reactionrates, explaining the slowness of the process. Duringageing on yeast lees, the organoleptic and foam propertiesof the wine are modified, reflecting changes in the winecomposition.

In this review, we focus on yeast autolysis in sparklingwine, which differs from yeast autolysis during ageing ofstill wine (Charpentier and Feuillat 1993, Fornairon-Bonnefond et al. 2001).

Yeast autolysis in sparkling wine production has beenthe subject of many studies. The lees present in still wineduring ageing are composed of tartaric acid salts, organicresidues and microorganisms, whereas sparkling wine leesare mainly composed of yeast with any technological co-adjuvant (riddling aids), such as bentonite, that helps theflocculation and the elimination of yeast lees at the end of

ageing. In-bottle sparkling wine ageing on lees usuallylasts longer than still wine ageing, and autolysis occursunder pressure (6 atmospheres).

For still wine, malolactic fermentation usually takesplace during ageing on yeast lees, whereas for sparklingwine, malolactic fermentation, when desired, occursbefore bottle ageing.

The commercial yeasts that carry out the first andsecond alcoholic fermentation are different. Yeasts for thefirst fermentation are selected for their high fermentationspeed and low acid production, as well as other desirableproperties, whereas yeasts for the second fermentation areselected for other technological properties (Martinez-Rodriguez et al. 2001c). Yeast strains are selected for theirability to grow at low temperatures and under pressures ina medium containing at least 10% (v/v) ethanol, as wellas having desirable flocculating or agglutinating ability. Forsparkling wine production, the yeast strain is alsoselected for its autolytic capacity and its ability to influencefoam quality.

Yeast autolysis mechanismsYeast autolysis can be considered a lytic event in the cells.This is an irreversible process caused by intracellular yeastenzymes. Autolysis generally takes place at the end of thestationary phase of growth and is usually associated withcell death (Babayan and Bezrukov 1985). Babayan et al.(1981) proposed four steps of yeast autolysis.

• First, the cell endostructures degrade, releasing vacuolarproteases in the cytoplasm.

• Second, the released proteases are initially inhibited byspecific cytoplasmic inhibitors, and are then activateddue to degradation of these inhibitors.

Yeast autolysis in sparkling wine – a review

HERVÉ ALEXANDRE1 and MICHÈLE GUILLOUX-BENATIER

Institut Universitaire de la Vigne et du Vin Jules Guyot, UMR INRA-Université de Bourgogne 1232,rue Claude Ladrey, BP27877, 21078 DIJON Cedex- France

1 Corresponding author: Dr Hervé Alexandre, facsimile +33 3 8039 6265, email [email protected]

AbstractSparkling wine produced by the traditional méthode champenoise requires a second in-bottle alcoholicfermentation of a base wine, leading to the sparkling wine. This second fermentation is followed byprolonged ageing in contact with yeast cells (lees). The autolysis of yeast occurs during the ageing ofsparkling wines. During this process, the yeast releases different compounds that modify the organolepticproperties of the wine. The ageing period is required to give these wines their roundness and characteristicaroma and flavour. Autolysis products also influence the foaming properties of sparkling wine. Yeastautolysis is characterised by the hydrolysis of intracellular biopolymers by yeast enzymes activated aftercell death. This results in the release of low molecular weight products. This article reviews the recentadvances in understanding the yeast autolysis mechanism, the factors affecting autolysis, the nature of thereleased compounds and their effects on sparkling wine quality.

Keywords: sparkling wine, yeast, autolysis, méthode champenoise

120 Yeast autolysis in sparkling wine Australian Journal of Grape and Wine Research 12, 119–127, 2006

• Third, intracellular polymer components hydrolyse,with the hydrolysis products accumulating in the spacerestricted by the cell wall.

• Finally, the hydrolytic products are released when theirmolecular masses are low enough to cross pores in thecell wall.

Although this general yeast autolysis process may be validfor most autolysis processes, natural autolysis is differentfrom induced autolysis. Induced autolysis is widely usedin industrial applications, such as for the production of ayeast extract used as a flavour enhancer or for productionof intracellular enzymes (Breddam and Beenfeldt 1991,Kollar et al. 1993, Zambonelli et al. 2000). Yeast auto-lysates are also added to growth culture media as they arerich in vitamins and amino acids. Autolysis in industrialprocesses can be induced by physical inductors (rise intemperature, alternate freezing and thawing, and osmoticpressure), chemical inductors (pH, detergents, and anti-biotics), or biological inductors (aeration and starvation).The autolysis process can be very fast, from 48 h to 72 h,depending on the inducer.

Natural autolysis, however, takes much longer. This isespecially true in wines, in which the autolytic conditions– pH 3 to 4, ageing temperature of 15°C, and the presenceof ethanol (12% v/v) – are far from the ideal of 45°C at pH5. These differences result in different autolysates, andhave been the focus of studies on the autolytic process inwine (Charpentier and Feuillat 1993, Connew 1998).

Yeast autolysis in the production of sparkling winesonly starts two to four months after completion of the sec-ondary fermentation (Charpentier and Feuillat 1993, Toddet al. 2000). Promotion of yeast autolysis has been doneusing a mixture of killer and sensitive yeast for the sec-ondary fermentation; under these conditions a rapid deathof sensitive yeast cells occurs in the presence of killerstrains (Todd et al. 2000).

Biochemical and morphological changesHydrolytic enzymes play a major role in autolysis, and ofall the enzymes involved the activities of proteases havebeen the most extensively studied. Lurton et al. (1989)used specific proteases inhibitors to show that in acidicconditions, protease A was the principal enzyme involvedin proteolysis during autolysis in a model wine system,despite there being numerous proteolytic enzymes presentin yeast. It was suggested that protease A activity may beresponsible for 80% of the nitrogen released during auto-lysis under optimum conditions. Using a ∆pep4 mutantdeleted for protease A, Alexandre et al. (2003) showedthat protease A was responsible for 60% of the nitrogenreleased during autolysis in wine. These results suggestthat other acidic proteases may also be involved in theproteolytic process. Consistent with this, Komano et al.(1999) and Olsen et al. (1999) have identified other acidicproteases (Yapsin proteases).

A study in wine showed that the proteolytic activity ofyeast increased six-fold after sugar exhaustion and thatprotease activity decreased when yeast cell autolysisstarted (Alexandre et al. 2001). This proteolytic activity

also depends on the temperature and pH during ageing.Sato et al. (1997) reported that in wines at pH 3 conservedat 10°C the intracellular protease activity decreased afterthree months ageing, whereas the activity decreased con-siderably during the first two months in the same winestored at 20°C. Under the same conditions, a very lowextracellular protease activity was measured, explainingthe slowness of the process. In sparkling wine, proteolyticactivity decreases during active bottle fermentation and inthe following months, whereas after ageing for ninemonths following fermentation the intracellular prote-olytic activity greatly increases (Feuillat and Charpentier1982). During Champagne ageing, Leroy et al. (1990)reported that proteolytic activity may also vary dependingon the yeast strain.

During autolysis, the yeast cell wall degrades, howeververy few studies have investigated the enzymes involvedin cell wall degradation during autolysis in wine. Thisdegradation has been seen in microscopic studies and fromstudies on cell wall composition during autolysis.

The cell wall of Saccharomyces cerevisiae may account forbetween 20 and 30% of the cell dry mass. It primarilycomprises mannoproteins and β-glucans (85–90% of cellwall dry mass). The inner layer of the cell wall is com-posed of glucans in which the mannoproteins are embed-ded and cover this glucan layer (Klis et al. 2002).

Glucanases have been shown to be involved in yeastcell wall degradation (Arnold 1972, Notario 1982,Charpentier and Freyssinet 1989). β-Glucanases, classifiedas endo- and exoglucanases, hydrolyse the β-O-glycosidiclinks of the β-glucan chains, which leads to the release ofglucose, oligosaccharides and mannoproteins trapped inthe cell wall or covalently bound to β-(1→6) and β-(1→3)glucans.

The kinetics of the β-glucanase activity during autolysisand the enological parameters affecting this activity areunknown in sparkling wine. The action of these enzymeshas been deduced from the released compounds. Theyeast cell walls release amino acids during autolysis. Thisreflects the proteolytic activity that may be occurring inthe cell wall (Hien and Fleet 1983). Cell wall degradationduring autolysis releases both amino acids and macro-molecules (see below).

Charpentier and Freyssinet (1989) and Feuillat et al.(1989) showed that cell wall degradation could be sum-marised as follows.

• First, glucans are hydrolysed by glucanases, thusreleasing mannoproteins trapped or covalently linked tothe glucans.

• Second, the glucans are released due to either residualactivities of cell wall glucanases or solubilised glucanasesin the medium.

• Finally, the protein fraction of the mannoproteins isdegraded by proteolysis.

Microscopy has also been used to study the changestaking place in the cell wall of yeast.

Although proteases and glucanases degrade the cellwall, there is no break-down of the cell wall (Vosti and


Joslyn 1954, Avakyants 1982). The cell wall of yeastgrown in a synthetic medium for 24 h is thick and smoothand can be easily distinguished from the plasma mem-brane. After autolysis, the yeast cells are much smaller andhave wrinkles or folds and ridges (Avakyants 1982,Charpentier et al. 1986, Kollar et al. 1993). These wrinklesare thought to be due to plasmolysis, with the increasedvacuole size due to solubilisation of the cytoplasmic con-tent supporting this suggestion (Martinez-Rodriguez et al.2001a). In these studies, the structural and ultrastructuralchanges occurring in yeast cells during autolysis werecompared in a model wine system and in sparkling wines.After 24 h of incubation in a model wine system, the yeastcells were found to have lost most of their cytoplasmiccontent and have a large vacuole, whereas yeasts aged for12 months still had most of their cytoplasmic content andhave a small vacuole. This shows that the autolysis con-ditions during ageing of sparkling wine or champagne arenot optimal.

Many different events occur during yeast autolysis(Figure 1), although the process leading to autolysis is notcompletely understood. Immediately after the secondalcoholic fermentation yeast cells are elongated and ovoid.The cell wall is thick and smooth. Inside the cell a largevacuole is surrounded by spherical bodies (Figure 1a).Between three and six months (Figure 1b), the cell andvacuole are smaller. Spherical bodies are distributedthroughout the vacuole. The cell wall is rough, small wrin-kles or folds can be seen. Between nine and 12 months(Figure 1c), the cell appears to have collapsed, explainingits small size. The cell wall remains unbroken, with manyridges and folds, nevertheless the yeast cells have lost mostof their cytoplasmic content. The fate of the plasma mem-brane during this process is not clear.

During yeast ageing, the biochemical changes are asfollows: at the start there is an excretion or passive exorp-tion (Morfaux and Dupuy 1966) of amino acids. Afterthree to six months, the medium continues to beenriched in amino acids due to peptide and proteinhydrolysis, and there is a significant increase in polysac-charides from the cell wall. Plasma membrane degradationstarts, with lipids being released into the medium. Fromnine to 12 months, the amino acid concentrationdecreases and peptide and protein release dominates. Cellwall polysaccharides, lipids and ribonucleotides increaseslightly.

Recently, autophagy was shown to play a possible role in the release of yeast compounds into the wines(Cebollero et al. 2005). Autophagy is a degradation path-way activated by nitrogen or carbon starvation. It is characterised by the formation of autophagosomes con-taining intracellular structures including mitochondria,which are carried to the vacuole and degraded, asreviewed by Huang and Klionsky (2002). Cebollero et al.(2005) used a yeast mutant defective in the autophagic orthe Cvt pathways to show that autophagy does take placeunder wine production conditions. Therefore, genesrelated to autophagy are good candidates for studying themolecular basis of autolysis or for the genetic engineeringof wine yeast.

Factors affecting autolysisThe principal factors that may affect autolysis are pH, tem-perature, the presence of ethanol and the nature of theyeast strain. As pH and ethanol content effectively cannotbe changed we will not consider these in this discussion.

High temperatures, up to 60°C, have been reported asfavouring autolysis in a model wine system. Molnar et al.(1981) report that the optimal temperature for proteolysisin the champenoise method is between 10 and 12°C.

Amino acidsa

b

c

Vacuole

Cell wall

Ribonucleotide

Nucleus

Sugars

Spherical bodies

Proteins and peptides

Polysaccharides

Plasma membrane

Lipids

Figure 1. Schematic representation of the morphological andbiochemical changes in yeast during autolysis in sparkling wine.Immediately after the second alcoholic fermentation (a), between 3and 6 months (b) and between 9 and 12 months (c).


Autolysis varies greatly with the yeast strain. Suzzi(1990) compared the autolytic capacity of differentstrains and suggested that this criterion could be used toselect yeasts. The autolytic capacity was evaluated by mea-suring the amino acids released by the yeast at differenttemperatures ten days after fermentation. Significant dif-ferences were observed in the autolytic capacity of threestrains. Therefore, the yeast strain affects the amount ofnitrogen released in the medium, which could be poten-tially useful for sparkling wine production (Martinez-Rodriguez et al. 2001b). Martinez-Rodriguez et al. (2001c)suggested that a yeast strain with good autolytic capacitywould produce better quality sparkling wine than yeasthaving a low autolytic capacity, and that autolytic capac-ity together with foam analysis should be used for select-ing yeast for sparkling wine production.

Nunez et al. (2005) recently confirmed that theautolytic capacity of yeast was important for the quality ofsparkling wine. They used a mutant having acceleratedautolysis to show that the second fermentation of wineswith this mutant improved the foaming properties versusa control strain. Similar results with this mutant were alsoobtained when the ageing period was reduced from nineto six months, which could reduce production costs.

Yeast autolysis compounds and their impact onsparkling wine qualityThe autolysis of yeast during ageing in sparkling winereleases different compounds into the medium, whichmodify the physical and organoleptic properties ofsparkling wine. Table 1 summarises these changes.

Evolution of nitrogen compounds at different stages of méthodechampenoise production of sparkling winesNumerous studies have investigated the changes in nitro-gen composition during the ageing of wine with yeasts.Nitrogen release is thought to reflect the autolytic activity

of yeast, especially the proteolytic activity. In the sparklingwine process, amino acids are released into the mediumduring bottle fermentation. After the available glucose hasbeen exhausted, the levels of amino acids in wineincreases (Feuillat and Charpentier 1982). This excretionor passive exorption as described by Morfaux and Dupuy(1966) should not be confused with autolysis.

The autolysis of yeast begins only after three to ninemonths. The time before autolysis starts varies greatly,depending on base wine composition, ageing time andyeast strain (Moreno-Arribas et al. 1996). Different stud-ies agree that the level of total amino acids increasesbefore the level of free amino acids increases. This showsthat peptides are first released into the medium and arethen degraded into amino acids. Moreno-Arribas et al.(1996) studied the evolution of different nitrogen fractionsduring sparkling wine ageing following the champenoisemethod. Between three and nine months after tirage, theyobserved no differences in the concentration of free aminoacids irrespective of the grape variety. After nine months,the concentration of free amino acids increased, indicat-ing the start of autolysis. These results have been recentlyconfirmed by Nunez et al. (2005).

The peptide content fluctuates during ageing, reachinga maximum after 12 to 15 months of ageing with yeastand then decreasing. This behaviour may be due to theinitial release of peptides that are subsequently degraded.Moreno-Arribas et al. (1996) also showed that the dis-tribution of free amino acids is very different from the distribution of amino acids in peptides and proteins – thishas has been confirmed in other studies (Moreno-Arribaset al. 1998a, b, Guilloux-Benatier and Chassagne 2003)

The amount of peptides released during the autolysisof yeast during sparkling wine ageing is highly variableand dependent on grape variety and ageing time(Moreno-Arribas et al. 1998b). The nature of peptides alsochanges with ageing time, being smaller as the ageing time

Table 1. The origin of different compounds released during yeast autolysis and their proven or potential impact onsparkling wine

Origin Compound type Proven or potential impact Referenceson sparkling wine

NucleosideLeroy et al. (1990)

NucleotideFlavouring agent Charpentier et al. (2005)

Courtis et al. (1998)

Amino acid Aroma precursors Feuillat and Charpentier (1982)Peptide Foam quality Moreno-Arribas et al. (2000)

Cell content Protein Sweet and bitter taste Malvy et al. (1994)

ProteinSweet and bitter taste

Polo et al. (1992)Foam quality

Lipids Foam quality Gallart et al. (2002)

Glucan Foam qualityAndres-Lacueva et al. (1997)

Cell wall Moreno-Arribas et al. (2000)Mannoprotein Increase in mouthfeel Bertuccioli and Ferrari (1999)


increases (Martinez-Rodriguez and Polo 2000). The amino acid composition of the peptides present in

sparkling wines has also been investigated (Moreno-Arribas et al. 1996, 1998a, b). These studies showed thatthe yeast was the origin of the peptides. Indeed, the pro-teins from various musts differ. As these proteins are thesubstrates of yeast proteases, we would expect to get dif-ferent wine peptides. Bartolomé et al. (1997) showed thatthe amino acid composition of peptides from differentvarieties of sparkling wines aged with the same yeast over26 months was the same.

The fact that threonine and serine have a major pres-ence in peptides from sparkling wine is also consistentwith yeast being the origin of peptides, as these aminoacids are not found in base wines as the major free aminoacids (Usseglio-Tomasset and Bosia 1990, Acedo et al.1994, Moreno-Arribas et al. 1998b). Threonine and serineare involved in glycosidic linkages between proteins andmannans in the cell wall (Klis et al. 2002).

Curiously, protein concentration and compositionduring autolysis in sparkling wine has been little studied.This may be explained by amino acids being considered asgood markers for following autolysis. The evolution ofprotein content during autolysis seems to depend on theyeast strain. Leroy et al. (1990) compared two differentyeast strains and found that the protein content remainedstable during the first nine months for one strain, whereasit decreased greatly from the end of the second fermenta-tion for the other strain. A similar recent study (Nunez etal. 2005) showed that the protein and polypeptide levelsincreased during the first three months and thendecreased. This was attributed to protease activity. Theprotein and peptide content then increased again after sixmonths.

Protein concentration has been reported to be stablefor 90 days after the secondary fermentation and increasedslightly, by 8 to 13%, thereafter (Todd et al. 2000).

Impact of nitrogen fractions on sparkling wine qualityAmino acid enrichment of the medium may improve thearoma potential of sparkling wines. Amino acids are theprecursors of some aroma compounds by deamination ordecarboxylation reactions (Feuillat and Charpentier1982). Of the lactones, 3-hydroxy-4,5-dimethyl-2(5H)-furanone, so-called sotolon (with a green nut or curryodour), slowly increases in sparkling wine during ageing.Pham et al. (1995) reports that sotolon comes from thre-onine that is transformed into α-ketobutyric acid whichthen reacts with acetaldehyde.

Some peptides have sweet and bitter tastes (Polo et al.1992) and the surfactant properties of peptides are alsothought to play a role in wine. Thus, sparkling wine pep-tides may play a role in foam stability similar to that inbeer. A positive correlation has been reported betweenpolypeptide molecular mass, hydrophobicity and foam stabilising activity in beer (St John Coghlan et al. 1992,Onishi and Proudlove 1994). Moreno-Arribas et al.(1998a) stated that the hydrophobicity of the charac-terised peptides could account for the the foam propertiesof sparkling wine.

Various studies have attempted to determine the com-pounds in wine that affect the quality of the foam(Brissonet and Maujean 1991, 1993, Malvy et al. 1994,Andrés-Lacueva et al. 1996). Most studies investigatedbase wine and the results are therefore difficult to extra-polate to sparkling wines because autolysis causes impor-tant changes during the champenoise method. Moreno-Arribas et al. (2000) found a positive correlation betweenfoam characteristics and most of the free amino acids andproteins, confirming the results of Malvy et al. (1994).However, no relationship was found between foam char-acteristics and the concentration of wine peptides.

PolysaccharidesGlucanase and protease activity results in the release ofpolysaccharides during autolysis in sparkling wines.These macromolecules contain mainly glucose (74%) andmannose (26%). The mannose/glucose ratio increasesduring autolysis possibly due to mannoprotein releaseafter glucan degradation. Indeed, mannoproteins aretrapped in the glucan layer although the very low man-nosidase activity does not explain the increase in mannoseconcentration (Freyssinet et al. 1989). The concentrationof polysaccharides in wines varies greatly and depends onhow the polysaccharide content is measured. Charpentier(2000) reported the level of polysaccharides in a sparklingwine increased from 366 mg/L in the base wine to 602mg/L after nine months of ageing.

There is much evidence that mannoproteins from theyeast cell wall play a key role in wine stability and in theorganoleptic properties of sparkling wine. Mannoproteinshave been shown to reduce haze formation (Ledoux et al.1992, Dupin et al. 2000), presumably by competing withwine proteins for unknown factors. It is hypothesised thatas the concentration of these unknown factors decreasesdue to the presence of the mannoproteins, the particle sizeof the proteins decreases and the turbidity consequentlydecreases (Dupin et al. 2000).

Mannoproteins also prevent the precipitation of tar-taric salt (Lubbers et al. 1993, Gerbaud et al. 1997, Moine-Ledoux et al. 1997). Mannoproteins affect the crystalgrowth rate, by sticking to the growth sites of the crystal,blocking growth of the crystal lattice (Gerbaud et al.1997).

The effect of colloids (macromolecules) on foam qual-ity has also been investigated (Brissonnet and Maujean1991). Material that precipitates in ethanol has beenfound among the compounds present in the foam. Thissuggests the presence of macromolecules. Moreno-Arribas et al. (2000) showed the importance of neutralpolysaccharides on the foam quality of sparkling wines.The optimum time of ageing for the best and most stable foam appears to be 18 months. However after 18months, foam quality decreases and this phenomenon isaccompanied by an increase in monomeric compoundssuch as fructose, most likely due to hydrolysis of plantcomponents by yeast enzymes released during autolysis(Andreas-Lacueva et al. 1997). Finally, mannoproteins arethought to contribute to the mouthfeel of the wine.Bertuccioli and Ferrari (1999) defined an index to evalu-


ate the body of wine. They showed that mannoproteinsincrease the Body Index. Mannoproteins also influencethe intensity and the persistence of the aroma of wine(Lubbers et al. 1994).

LipidsLipids are important components of sparkling winesbecause they are a large source of flavour compounds(Forss 1969) and also influence foam stability. Therefore,the evolution of the lipid content in sparkling wine hasbeen the focus of several studies.

During the second fermentation, lipid contentincreases (Troton et al. 1989). After bottle ageing in con-tact with yeasts, the lipid content increases further andqualitative changes also occur (Piton et al. 1988). Duringageing, the concentration of polar lipids decreases whereasthe concentration of neutral lipids (i.e. mono-, di- andtriglycerides) increases. However, there are data concern-ing the evolution of phospholipids and sterols duringsparkling wine ageing. Experiments in a model winesystem showed that the levels of triacylglycerols, 1,3-diacylglycerols, 2-monoacylglycerols, free fatty acids,sterol esters and sterols increase after two days of autolysisand then decrease, probably due to yeast hydrolyticenzymes (Pueyo et al. 2000). No phospholipids werereleased into the medium, confirming previous resultsfrom Hernawan and Fleet (1995), and it was suggestedthat any phospholipids are degraded.

There have been conflicting results concerning theinfluence of lipids on foam. Maujean et al. (1990) foundthat the addition of octanoic and decanoic acids reducesfoam stability, whereas Dussod et al. (1994) reported thatthe addition of a lipid mixture did not affect the foam.Furthermore, Pueyo et al. (1995) found that linolenic andpalmitoleic acid were the best indicators of foam stability.The influence of fatty acids on wine foaming has beenreinvestigated (Gallart et al. 2002). It was found that freefatty acids such as C8, C10 and C12 acids were negativelyrelated to the quality of the foam, whereas the ethyl estersof hexanoic, octanoic, and decanoic acids were positivelyrelated.

Nucleic acidsAlthough the degradation of proteins during autolysis hasbeen extensively researched, the hydrolysis of RNA andDNA has been less studied. RNA and DNA make up 5 to15% and 0.1 to 1.5% of the cell dry weight, respectively.

During autolysis, DNA from a brewing and bakingSaccharomyces cerevisiae strain was almost completelydegraded (Hough and Maddox 1970, Suomalainen 1975).However, Trevelyan (1978) found no decrease in DNAcontent during the autolysis of baker’s yeast. The extent ofDNA degradation during autolysis appears to depend onthe yeast species (Hernawan and Fleet 1995). The verylow levels of DNA detected in the autolysate reflectsDNase activity. DNA degradation requires several activeenzymes and leads to oligonucleotide, nucleotide andnucleoside degradation products. The predominance ofdeoxyribonucleotides in the autolysate indicates thatendo- and exonucleases are primarily involved in the

degradation process. Zhao and Fleet (2003) reported thatup to 55% of the total DNA was degraded during auto-lysis, releasing 3’-and 5’ deoxyribonucleotides. Evenunder optimum autolytic conditions, some parts of theDNA are resistant to autolytic degradation. However, stud-ies of DNA degradation in oenological conditions are stillneeded. It is expected that the presence of ethanol, andthe lower pH and temperature would result in a muchlower DNA degradation.

More than 95% of the total content of nucleic acidwithin yeast cells is RNA. Zhao and Fleet (2005) suggestedthat RNA degradation is a key reaction of yeast autolysis.They determined several autolytic conditions and showedthat up to 95% of cell RNA was degraded, releasingmainly 3’-, 5’- and 2’-ribonucleotides. The conditions forforming the two flavour-enhancing nucleotides, 5’-AMPand 5’-GMP, were 50°C at pH 7.0 and 40°C at pH 4.0respectively. Although these are far from the optimal con-ditions for sparkling wine production, the degradation ofnucleic acids and the release of nucleotides in sparklingwine during autolysis can affect the organoleptic proper-ties of the wine (Courtis et al. 1998).

Although RNAse activity has been shown to occurduring autolysis in Champagne (Leroy et al. 1990), thelevel of nucleic acids reported should be interpreted withcaution. The results were from spectrophotometric obser-vations, but nucleotides in wine are present in complexmixture with organic acids, phenolic compounds, peptidesetc. that can interfere with the measurement. Recentlyour colleagues have unequivocally identified monophos-phate nucleotides in Champagne wine (Aussenac et al.2001, Charpentier et al. 2005), including three mono-phosphate nucleotides (5’-UMP, 5’-GMP and 5’-IMP) inChampagne aged on lees for 8 years. The concentration ofthe nucleotide monophosphates ranged from 50 µg/L to 500 µg/L, which is considerably different from that previously reported (Courtis et al. 1998).

In the food industry, monophosphate nucleotides arewell recognised as flavour compounds, but further stud-ies are needed to evaluate the impact of nucleotides onwine flavour.

Volatile compoundsVolatile compounds released during yeast autolysis havebeen less well studied than non-aroma compounds. Thefew existing studies have shown that many compoundsare released, some having low perception levels. Chung(1986) reported in a model wine system (12% v/vethanol, pH 3.5) that autolysis of Saccharomyces cerevisiae at15–20°C or 35–40°C releases many different volatile com-pounds after four to six months. Esters are the majorfamily of volatile compounds released during autolysis,both qualitatively and quantitatively. Short chain (C3–C4)and medium chain (C6–C12) acyl esters with characteristicfruity odours appear at the beginning of yeast autolysisand then decrease. Long chain acyl esters have also beenidentified in model wine and sparkling wine (Molnar et al.1981). Terpenic alcohols and higher alcohols are alsoreleased during autolysis. Geraniol, α-terpineol, citronel-lol and farnesol have all been identified. These compounds


have low perception levels ranging from 100 to 300 µg/L.Molnar et al. (1981) suggested that farnesol contributesgreatly to the aromatic quality of sparkling wine andLoyaux et al. (1981) suggested nerolidol in Champagne.Among the higher alcohols, the rapid formation of isoamylalcohol and 2-phenylethanol (rose odour) has beenobserved during autolysis in a model wine system(Chung 1986).

About ten aldehydes have been measured and identi-fied. 3-Methylbutanal is the most abundant, representing40% of the total aldehydes, and may be formed througha mechanism involving isoamyl alcohol oxidation. Most ofthe aldehydes identified are present at levels close to orgreater than the perception level in water. Aldehydes aredescribed as having a grassy odour that negatively affectsthe organoleptic properties, although most disappearedduring ageing (Chung 1986).

Francioli et al. (2003) recently characterised the volat-ile compounds released during autolysis that could serveas age markers in sparkling wines. Acetates appeared todecrease during ageing, whereas diethylsuccinate, vitispi-rane and TDN (1,1,6-trimethyl-1,2-dihydro naphthalene)increased over time. Hexanol and 2-phenylethanol havebeen shown to be released during autolysis. Compoundssuch as vitispirane, TDN and diethylsucinate may be goodage markers and can discriminate between young andaged sparkling wines. Riu-Aumatell et al. (2006) obtainedsimilar results, reporting that some high molecularweight acetates, and ethyl and isoamyl esters are typicalaroma compounds in young cavas (Spanish sparklingwine), whereas vitispirane, diethyl succinate, TDN,hexenol and ethyl acetate are typical aroma compounds inlong-aged cavas.

The origin of the post-fermentative aromas in bottleageing in contact with lees has been studied. Enzymaticrelease (by yeast enzymes) from glycosidic precursors hasbeen suggested as causing changes in aroma compounds,as C13 norisoprenoids including vitispirane can be derivedfrom glycosidically-bound, carotenoid-derived mega-stigmane compounds (Riu-Aumatell et al. 2006). TDNmay be a direct degradation product of carotene (Rapp1998), although precursors linked to a sugar moleculehave also been reported (Winterhalter 1991).

Descriptive analysis is another way to characterise theeffect of the méthode champenoise process on aroma.Changes in aroma occurring during production ofsparkling wine varied for either individual wines or winesof different varieties. The profiles of the base wines do notpermit prediction of the sensory properties of thesparkling wines after 18 months of lees ageing (De LaPresa-Owens et al. 1998). In this study the authorsdemonstrated that descriptive analysis of the base wineallows discrimination among different grape varieties likeChardonnay or Pinot Noir. However, after the secondaryfermentation the sparkling wines were no longer differ-entiated by variety or colour. This study reported for thefirst time that secondary fermentation together with leesageing profoundly modify the aromatic profile of thewine.

ConclusionAccording to Zambonelli et al. (2000), the study of theautolysis of yeast has been so extensive that there is noth-ing more to uncover. However, this review has shown thatseveral questions remain to be addressed. We have mainlyfocused on autolysis during sparkling wine ageing and theinformation available in that context. There have beenextensive studies on yeast autolysis, although the differ-ent conditions used in these studies (fresh yeast, active dryyeast, temperature, pH, model wine system or wine) haveled to contradictory results. Furthermore, not all the studies could be extrapolated to wine. For example, it isnot known whether the autolysis of yeast following thefermentation of must and of yeast after fermentation ofwine, such as for the champenoise method, is similar.

Analytical studies on wine, and especially sparklingwine, have given a clear picture of the different com-pounds released during autolysis. However, the kinetics ofrelease of certain compounds, such as nucleotides,nucleosides and lipids needs to be studied in more depthand to be correlated to enzyme activity.

Currently, the molecular mechanisms responsible forthe induction of autolysis, and what the signal trans-duction is, remain unknown. Understanding such mech-anisms should increase our understanding of autolysis andmay reveal potential targets for accelerating the process.

Finally, the yeast origin of many aroma compoundsneeds to be proven. Volatile compounds that are charac-teristic of a long-aged Champagne may also be found instill wine that has not been aged on yeast lees. The impactof these compounds on the physical and organolepticproperties of sparkling wine is also poorly understood.Many changes occur during autolysis, making it difficultto attribute a specific compound to specific organolepticchanges. We are still unsure as to which componentsformed or released during ageing are odour-active com-pounds. Therefore, the effect of yeast autolysis on theorganoleptic properties of wine should be re-evaluatedusing techniques such as gas chromatography-olfactometry(GC/O) complemented with sensory descriptive analysis.

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Manuscript received: 1 February 2006

Revised manuscript received: 25 April 2006

Yeast Autolysis in Sparkling Wine - A Review, SUB

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