Faults in Wine Explained
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Faults in Wine Explained

Faults in Wine Explained

Many people ask what the faults are in wine and how to detect them….
outlined below is some research we have done to fill you in on the basics.

Faults in Wine are many…

In view of its complex chemical nature and the microbiological
changes involved, wine is heir to various faults. The winemaker and
the quality control officer need to be aware of these and how to cure
them and, more importantly, how to prevent their occurrence.

The main faults are easy to spot….here are the basics

Excessive sulphur dioxide…..

SO2 is used in wine-making
as an anti-oxidant. If there is too much of it, it
smells sulphury, like hot mineral springs. Since free
sulphur will diminish once the bottle is opened, often
leaving the bottle for a while or pouring it from glass
to glass will help. But if there’s too much of it avoid
the winemaker in future.

Hydrogen sulphide….

H2S, the dreaded fault, with its
offshoot, mecaptan. Smells like burnt rubber or animal
dung, and also prevails on the palate. Even a
slight amount can spoil the freshness and flavor of
any wine.

Acetification…..

A vinegary smell on the nose, a
vinegary taste on the back of the tongue. It comes from
excessive acetic acid, which is present, in small
amounts, in almost any wine. Since this acid is highly
volatile, it is often called v.a. (volatile acidity). The
v.a. of a white can show at only 0.5 a gram per litre.
A soft red with a v.a. of 0.6 to 0.7 g/l can be ta
by it, yet a robust, big flavoured red will not be
affected. Grange Hermitage 1971 for example, c
the top of the series, had a v.a. of over a gram per
litre, yet it is still a marvellous wine.

Oxidation….

A flatness, coarseness, and downright
nastiness coming from the wine being contaminated
at winemaking, maturation, or in bad storage conditions, by too much oxygen. There are different
oxidised flavours but there seems no point in discussing them here. It is sufficient to say that a white wine
will dull and brown; a red will develop colours premature for its age, the nose will be flat and dull, the
palate flat and flabby and the finish coarse and

Corked….

Simply a nasty taste, bitter, acrid, and
totally unfruitlike, which comes from one dis,
cork. You can buy ten dozen of a wine and only
bottle may be corked. If this is excessive the wine will
dull and brown, but even small amounts are obvious.
Once an experienced taster shares a corked bottle
with you and declares the taste, you should never
forget it.

Dirty wood…..

Some wines are stored badly during
wood maturation and various ‘dirty’ flavours
emerge. V.a. is one problem. A flavour not u
corked character (caused by old, diseased wood
another. Some casks have water stored in them
develop a ‘bilgey’ character, redolent of old stag
water. In others this is less obvious, yet a ‘water·
taste may emerge, not quite bilgey but reminiscent of
rain water stored in old barrels. Again, it’s a mat
experience.

Now for some more detailed and scientific views…

Metal haze….

Haze caused by traces of certain metals used to be one of the most
serious problems in bottled wine, and is still encountered sporadically.
The main metals are iron and copper, and occasionally aluminium.
The surprising feature is how small a quantity of metal is
necessary to form a haze-only a few milligrams per litre (parts per
million) are required, depending on the wine.

Metals in wine derive essentially from contamination with metallic
equipment – the metal content of washed grapes is much too low to
contribute to haze. Historically, iron equipment and pipelines were
used widely in wineries, later to be replaced with copper , bronze and
brass. By doing this, iron contamination was replaced with copper,
which resulted in a more serious and insidious problem. Subsequently
stainless steel, plastics and glass were introduced, and their
use has resulted in the virtual disappearance of metal hazes.

One of the main sources of iron contamination is the use of
uncoated iron grape bins and hoppers, which can result in a level of
more than twenty times that which causes haze in wine. White juices
containing this amount of iron turn grey to dark brown. Much of the
metal contamination is removed by the yeast during fermentation,
but enough remains after such heavy iron contamination to result in a
haze in wine.

Iron contamination can also arise from contact with
iron equipment generally, as well as bentonite and unwashed filter
pads. Iron haze is caused by ferric phosphate or, in red wines, ferric
tannate, and aerating the wine hastens formation by converting
ferrous ions to ferric. In fact, the haze forms poorly or not at all
under reducing conditions. Also, the more acid the wine, the more
likely that iron haze (sometimes called casse) will form.

Copper contamination is not usual in grape-handling, and normally
arises from copper-containing winery equipment contacting the
wine after fermentation. Unwashed filter pads can also be a source of
both copper and iron, which are taken up by the first run of wine
through the pads. Thus, bottles from the beginning of a bottling run
may contain enough metal to cause a haze. Copper haze forms under
reducing conditions and is insidious, in that it appears some time
after the wine has been bottled when the oxidation-reduction potential
has fallen. It forms most readily if the wine contains some
protein, because the deposit consists of a cuprous-sulphide-protein
complex. If the wine is free from protein it can tolerate a higher
copper content before forming a haze.

Maximum tolerance limits for iron and copper depend on the wine
type and its composition, but a rule-of-thumb guide is a maximum
level of 0.5 milligrams per litre for copper and 6 milligrams per Iitre
for iron. Measurement is relatively simple and can be carried out
chemically, or more rapidly by atomic-absorption spectroscopy. Removal
of metals in wine is usually achieved with potassium ferrocyanide,
which precipitates them as blue floccules. This treatment should
only be carried out by a qualified oenologist or chemist, and prevention
of metal contamination is much better than trying to remove the
haze once it has formed.

Although aluminium haze is rare, a brief comment is appropriate.
Aluminium is by no means an inert metal and can produce an
intractable haze in wine. Aluminium and its alloys are moderately
resistant to corrosion by wine, and this has been taken to indicate
that they are satisfactory for use in winery equipment. However, this
is not correct and all aluminium equipment and surfaces which come
into contact with wine are potential sources of haze, unless treated
with a permanent and durable coating. Besides haze formation
aluminium has other undesirable effects. It can impart a metallic
taste, and is one of the metals which can form hydrogen sulphide in
wine from sulphur dioxide by reduction with nascent hydrogen. It
also has a bleaching effect.

As far as sources of aluminium in wine are concerned, grapes
contain only traces. The problem arises when must or more particularly
wine contacts aluminium surfaces, which corrode and aluminium
dissolves. Contact with grapes and grape juice is less serious than
contact with wine, because up to 90 per cent of aluminium in grape
juice is removed by the yeast during fermentation. For this reason
aluminium buckets and other utensils are sometimes recommended
for grape transport, but this is unwise. Some grape mills have aluminium
alloy rollers but the contact is too brief to impart sufficient
aluminium to the must. Fining materials are also a source of aluminium,
especially bentonite which is an aluminium-silicate day mineral.
Bentonite fining can increase the level of aluminium in wine up
to 2 milligrams per litre. Other fining materials and some filter aids
are possible sources.

The appearance of aluminum haze in wine can range from faint
opalescence to a cloud with deposit, depending on the extent of
contamination. Microscopically, it appears as rat her uniform
amorphous particles about 1 to 2 microns diameter. The haze dissolves
on addition of hydrochloric acid but is unaffected by addition of
hydrogen peroxide or hydrosulphite. In this way it can be distinguished
from iron and copper hazes. It does not give a positive test
for protein, and heating or chilling the wine does not influence haze
formation.

lt is possible to set an approximate upper limit for aluminium in
wine at I milligram per litre. Aluminium is not an essential nutritional
element in grape juice or wine, nor does it play a role in fermentation
biochemistry. Wines differ in their tolerance to aluminium and pH is
important- maximum haziness occurs al pH 3.8. and the higher the
acid level present, the more the aluminium is complexed and not
available to form a haze.

One of the main sources of aluminium contamination in wine (as
distinct from must) is from aluminium alloy equipment, particularly
plate and frame filters , where the inert coating is damaged or absent.
So if one is concerned about the possibility of aluminium haze, which
incidentally cannot be removed by blue fining, then make sure that
any aluminium surfaces contacting the \Vi ne are properly coated.

Protein haze…..

A typical practical example illustrates the nature of protein haze. A
winery has bottled some white table wine and stacked it for later
labelling and packaging. Imagine the concern of the winemaker when
a haze is observed in the contents of the bottles on the top of the
stack, and, as time passes, the bottles lower down the stack begin to
show this haze.

This is protein haze, and is the result of the wine containing
heal-unstable grape protein, which slowly denatures and precipitates
as the wine is warmed. Due to natural temperature stratification,
the warmer bottles on the top of the stack
develop the haze first. Protein haze is more common in the hotter
areas of Australia, where the grapes are higher in soluble protein.

The grape varieties most frequently involved are Muscat Gordo
Blanco, Traminer and Semillon; the extent depending on climate and
grape maturity-with the more mature grapes containing more protein.
The proteins involved have molecular weights in the range of
40,000 to 200,000, with iso-electric points between pH 4.8 and 5.7.

There are many other nitrogenous compounds in wine, such as amino
acids and peptides, but these are not involved in protein haze. As a
matter of related interest, protein is also linked with copper haze,
which involves a cuprous sulphide-protein complex, and tin, although
the latter is rare in Australian wine.

Hydrogen sulphide…..

Hydrogen sulphide can best be described as
one of the nuisances of winemaking. It can come from more than one
source, which is confusing and is the reason why the occurrence is
sometimes sporadic and difficult to understand and predict.
Hydrogen sulphide itself is a reactive gas which, when formed in
wine, changes into other very smelly substances, such as mercaptans,
organic sulphides and thiols, which can impart garlic or onion-like
smells to wine. These are Iiquids with relatively high boiling points,
and thus are much more difficult to remove than hydrogen sulphide.
Therefore, if hydrogen sulphide is detected it should be removed as
soon as possible, before it changes into these other compounds.
Hydrogen sulphide can be formed in several ways.
If elemental sulphur is present du ring the yeast fermentation
hydrogen sulphide is almost an inevitable outcome, due to reduction
of the sulphur to hydrogen sulphide. The elemental sulphur can come
from sulphur dust used in the vineyard to prevent fungus infections,
or from sulphur residues from burning sulphur discs or wicks in casks
to sterilise them. For this reason the latter practice is not recommended.
The alcoholic fermentation is strongly reductive with the
oxidation-reduction potential less than 100 millivolts at the peak of
fermentation. This encourages the reduction of sulphur to hydrogen
sulphide.

All fermenting yeasts appear to be able to reduce sulphur to
hydrogen sulphide, but they differ in the amount formed. The smaller
the sulphur particles (i.e. the greater their surface a real, the more
hydrogen sulphide is formed. Vineyard sulphur dusting to prevent
powdery mildew (Oidium), if applied dose to harvest, is an important
cause of hydrogen sulphide, because it leaves a residue of finely
divided particles of elemental sulphur on the leaves and fruit. These
are carried over into the fermentation on the grapes and hydrogen
sulphide is formed. Therefore, it is important not to dust or spray
vines with dusting sulphur within about four weeks of harvest.
Certain yeasts can reduce sulphur dioxide to hydrogen sulphide
during fermentation . A lesser number can even reduce sulphate in
grape juice, although this occurs less readily because more chemical
reduction steps are required. A yeast should be selected which will
not reduce sulphur dioxide or sulphate.

lf the grape juice does not contain enough inorganic nitrogen to
meet the nutrient requirements of the yeast, some of the grape
proteins naturally present in the juice are broken down by the yeast
to provide this nitrogen. The sulphur-containing amino acids of the
proteins, such as methionine and cystine, can give rise to hydrogen
sulphide as a by-product of this protein breakdown . For this reason
diammonium phosphate is added before fermentation to increase the
available inorganic nitrogen and, incidentally, to provide phosphorus
which the yeast also needs. This nitrogen addition usually prevents
hydrogen sulphide formation, providing that the other sources
are absent. The amount of diammonium phosphate needed depends
on the amount of inorganic nitrogen (FAN or free amino nitrogen)
naturally present in the juice. Unless this is measured it is not
possible to state how much diammonium phosphate should be added.
The usual addition rate before fermentation is 100 to 200 milligrams
per litre irrespective of the juice.

Certain metals, such as zinc, can produce hydrogen sulphide in
wine by direct chemical reduction. The metal reacts with tartaric and
malic acids in the wine to produce ‘nascent’ hydrogen which reduces
sulphur dioxide to hydrogen sulphide. This is not an important
source, because zinc or zinc-coated equipment does not normally
come into contact with wine. However, it is possible for hydrogen
sulphide to be produced from stainless-steel vessels if they are not
acid-rinsed beforehand. Manganese sulphide forms on the surface of
the stainless steel and liberates hydrogen sulphide when the acidic
wine contacts it. A preliminary rinse with citric or tartaric acid
removes this sulphide layer, thus preventing it from contacting the wine.

Of the various ways of removing hydrogen sulphide from wine, the
simplest and quickest is a small addition of copper as copper sulphate
solution- up to about 1 milligram per litre of copper, depending on
the hydrogen sulphide content of the wine. A laboratory trial is
necessary, using a range of levels of copper addition and selecting the
lowest level wh ich removes the hydrogen sulphide smell. A stock
solution of 400 milligrams of copper sulphate in a Iitre of water is
appropriate, and 1 millilitre of this solution added to 100 millilitres
wine corresponds to an addition of 1 milligram of copper per Iitre of
wine. In the winery an addition of 4 grams of blue copper-sulphate
crystals per 1000 litres of wine corresponds to an addition of 1
milligram per Iitre of copper. The crystals are dissolved in a Iittle
water and added slowly with stirring to the wine. The hydrogen
sulphide combines with the copper to form insoluble copper sulphide,
which settles to the bottom of the vessel as a fine brown dust.
The amount formed is so small that no further steps to remove it are
necessary.

In some winemaking practices, for example, where one
would expect hydrogen sulphide to be present it is not, and winemakers
affirm that they are rarely troubled by it. When one examines
their wineries in detail the reason becomes apparent- there are
always pieces of brass equipment in use (taps, pipelines, pumps etc.).
Where the brass contacts the wine it is invariably black, due to
deposited copper sulphide. What is happening is that hydrogen
sulphide is being formed then removed by brass equipment, which in
principle has the same effect as the current praclice of adding copper
sulphate.

Volatile acidity…..

Volatile acidity or volatility is a jargon term indicating that the wine
(usually dry red) has been infected with acetic acid bacteria, or
possibly lactic acid bacteria or certain yeasts, wh Ich oxidise akohol to
acetie acid and to its ester, ethyl acetate. The level at which these
eonstituents become detectable to taste varies with the style of wine,
but the generally aceepted maximum limits in praclice are about 0.8
grams per litre for acetie acid and 0.15 grams per litre for ethyl
acetate. Big akoholie wines tolerate higher levels of volatility than
light thin wines. Under Australian law acetic acid in wine has a legal
maximum level of 1.5 grams per Iitre, while in New Zealand the limit
is 1.2 grams per Iitre. There is no legal limit for ethyl acetate. Volatile
acidity is an age-old problem in red table wines, and its occurrence
means that the winemaking process needs to be examined. Once a
wine becomes volatile it is not possible to remove or cure the fault
without seriously damaging the wine, so prevention is important.
The detection of volatility is carried out by tasting (smell of
vinegar) and chemically by measuring the acetic acid present, which
constitutes more than 96 per cent of the acids in wine which are
volatile in steam. Ethyl acetate is more important from the taste
viewpoint, but difficult to measure and thus not usually determined.
Ethyl acetate is usually produced concurrently with acetic acid and in
the same relative proportion, although this depends on the type of
micro-organisms involved. Some Saccharomyces yeasts produce significant
levels of acetic acid but almost no ethyl acetate, so the wine
may have a high level of volatility analytically, but this is not apparent
on the palate. Measurement of acetic acid by steam distillation is
not the simple determination it is made out to be, and the enzymatic
method is an alternative.

From the winemaker’s point of view the conditions contributing to
the development of volatility are as follows:
• Presence of spoilage micro-organisms-usually these are present
in small numbers and will only grow if the conditions set out
below prevail.
• Warm temperature-growth and formation of volatility by acetic
acid bacteria is twice as fast at 23°C as at 18°C and four times as
fast at 28°C.
• Presence of air-acetic acid bacteria need contact with air (they
are aerobic although they can sometimes be found growing in the
body of the wine). Thus, they can grow on ullaged red wine, on
damaged grapes prior to crushing and in the dry cap of skins
during fermentation.
• Low sulphur dioxide.
• Low acidity (high pH)-encourages the growth of lactic acid
and to a lesser extent acetic acid bacteria.
• Low alcohol-encourages growth of bacteria generally. However,
in practice this is not very effective in preventing volatility
because approximately 14 per cent alcohol by volume is needed
before significant reduction of bacterial growth occurs.
Some methods of preventing volatility are:
• Use effective sanitation during making and c1arifieation of wine.
• For red wines in open-top tanks, keep the cap of skins immersed
during fermentation.
• Keep wines cool and (for table wines) out of contact with air.
For red wines top-up regularly or bung tight and roll the casks to
60′ and maintain the correct pH and sulphur dioxide content.

Casks which have contained volatile wine should be treated with a
hot 1 per cent solution of sodium carbonate, followed by a 1 per cent
solution of tartaric or sulphuric acid, then rinsed out and sulphited.
Inert gas should be used to displace air above table wines in ullaged
vessels, and the oxygen content of headspace should be less than 0.5
per cent to prevent the growth of aerobic micro-organisms.

Mousiness…..

Mousiness is a curious term which applies to microbiological spoilage
of certain wines, resulting in a particularly undesirable smell and
taste reminiscent of mice. lt is an infrequent problem these days
but in the past was not uncommon. Few people appear to be able to
detect this character, but those who do find it very unpleasant . It is
one of the most serious examples of microbiological spoilage and is
associated with advanced cases of lactic spoilage, usually in dessert
wines. The mousy taste becomes apparent late in the evaluation after
the wine has left the mouth, and persists for some time.
The causative organisms are certain lactic acid bacteria and/or
Brettanomytes yeasts. The bacteria may work symbiotically with the
yeasts. Both of these groups are inhibited by sulphur dioxide, and
prevention of mousiness is achieved by proper sanitation and effective
use of sulphur dioxide. This involves an understanding of winemaking
practice as it influences microbiological growth in white
wines-specifically pH control, early racking and maintaining a level
of free sulphur dioxide between 20 and 40 milligrams per Iitre for
white table wines and about half this for dessert wines for the Iife of
the wine. lt is also important to relate the free sulphur-dioxide level
to pH, in that the higher the pH, the more free sulphur dioxide is
required.

Until recently the chemical nature of mousiness was unknown. lt
was (and sometimes still is) referred to as acetamide, which has a
somewhat mouse-like smell when impure, but in fact no smell at all
when pure. The contaminant giving rise to this smell is a tri methyl
triazine, but this has not been found in mousy wines. The causative
compound rejoices in the name 2-ethyl-delta I-piperideine. When
pure its odour is hemlock-Iike (it is chemically related to hemlock),
but when oxidised by exposure to air produces a powerful mousy
smell, persistent and difficult to remove from any surface with which
it comes into contact. This means, of course, that mousiness is
accentuated if the wine is oxidised.
The amino acid lysine is involved if the causative organism is the
yeast genus Brettanomyces, but lysine may not be involved with
bacteria. The essential requirements for production of mousy wines
are presence of certain spoilage bacteria and/or yeasts, alcohol and
air. Consequently, prevention of microbiological growth in the wine
is a prerequisite for preventing mousiness.
One final and disconcerting feature of all this is that when a wine
becomes mousy it is a permanent affliction and cannot be removed
without further damaging the wine.

Geranium smell…

This unusual smell and taste occasionally encountered in dry red wine
is one of the worst misfortunes which can happen to a wine. The
smell has been variously described as putrid, vile and reminiscent of
crushed geranium leaves–whence its name derives. The wine is
usually unsaleable and the smell is difficult to remove. lt arises from
the bacterial decomposition of sorbic acid, which may be added to
wine as a fungieide, meaning that it kills yeast and moulds. It is also
used in various foods. Because of this desirable property the addition
of sorbic acid is permitted in wine, and almost wholly confined to
white table wines containing fermentable sugar. Sorbic acid is not a
bactericide, and in the amounts added to wine, bacteria can still
grow. They actually metabolize sorbic acid and convert it into other
compounds, one of which is 2-ethoxy hexa-3, 5-diene, an unsaturated
compound having the smell found in wine with geranium character.
Since the growth of lactic acid bacteria and the presence of sorbic
acid are the two requirements for formation of this geranium character,
prevention is achieved by inhibiting bacterial growth and not
using sorbic acid in red table wines. Sorbic acid should only be added
to sweet white table wines, and then only when free sulphur dioxide
(20 to 30 milligrams per litre) is present to prevent bacterial growth.

Yeast spoilage…

Yeast spoilage can be one of the most serious technical problems
confronting the bottler of table wine. lt occurs in all winemaking
countries and usually happens unexpectedly. The winemaker is suddenly
confronted with a stock of bottled wine which has become
cloudy without warning, or bag-in-box packages that distort due to
gas pressure. When the cloud is examined microscopically it is seen to
consist of budding yeast cells. In view of its importance the problem
will be considered in some detail.

Repercussions of yeast spoilage can range from a nuisance up to a
problem of disastrous proportions. Financial loss can be considerable,
since the wine has to be disgorged, perhaps treated in some
way, sterile-filtered and rebottled into washed and sterilised bottles.
All this rehandling is a complete waste of time and money, and the
wine is usually lowered in quality.

Yeast growth occurs more often in white than in red table wines and can
be separated into two broad types. If the wine contains residual
sugar, carbon dioxide is usually evolved and the cloudy wine becomes
charged with gas. The pressure of carbon dioxide may force the cork
out of the bottle or swell the bag-in-box to resemble a football. This
type of spoilage is a refermentation of sugar and, depending on the
yeast strain involved, may or may not be accompanied by formation
of off-Havours.

If a non-fermenting yeast is present or the wine is free from
fermentable sugar, a haze and/or deposit will form without noticeable
formation of gas. This type of spoilage is less commonly encountered,
but if a resistant strain of non-fermenting yeast is involved, spoilage
can also be quite serious. It is frequently accompanied by offHavours,
due to formation of acetic acid, ethyl acetate and other
undesirable metabolic products.

Appearance of the bottled wine may or may not be indicative of
yeast spoilage. The deposit can range in appearance from a fine sandy
deposit, which readily disperses to a haze on shaking, to
discrete globular particles which settle to the bottom of the bottle and
do not disperse on shaking. In all cases the haze or deposit appears
under the microscope (about 600 diameters magnification) as yeast
cells, either round, ovoid or cylindrical. If other material is present
besides yeast, the spoilage may have more than one cause, such as
protein, metal haze and so on.

Many yeasts have caused spoilage of table wines in Australia and the
nature of the spoilage is gene rally related to the characteristics of the
yeasts involved. Usually the yeasts are species of Saccharomyces, the
most common being S. cerevisiae, S. bayanus, S. bailii, S. capensis, S.
bisporus and S. rouxii. Of these, S. cerevisiae and S. capensis are
extensively used for primary fermentation and for sherry production
respectively. In fact, the yeasts isolated from spoiled wine are frequently
identical to the yeast which carried out the primary fermentation.
In addition to Saccharomyces species, spoilage may be caused by
poorly or non-fermenting yeasts, such as Pichia membranaefaeiens,
Torulopsis bacillaris, Rhodotorula rubra, Candida mycoderma, C.
sake, C. valida and C. parapsilosis. Surprisingly Brel/anomyces,
which has been reported as causing spoilage in other countries,
appears to be rare in Australia. Sometimes yeasts have been isolated,
particularly those forming pink colonies such as Rhodotorula spp,
that do not appear to multiply in wine and are thus of little significance.

Characteristics of spoilage yeast…

Yeasts isolated from spoilt bottled wine show wide differences in
their ability to grow under different conditions. The alcohol tolerance
can range up to about 17 per cent v/v, sulphur dioxide in excess of 200
milligrams per litre, sorbic acid up to 200 milligrams per litre, sugar
concentration 0 to 10 per cent and temperature range 15 to 30·C. The
main food for the yeasts is sugar, but some are able to grow on other
carbon sources, such as tartaric, citric, malic, lactic and succinic acids
and glycerol. The most resistant yeasts to a range of fungicides are
strains of S. bayanus and S. bailii (S. aeid/faeiens). The latter yeast is
sometimes referred to in the literature as Z. bailli.

Whether a contaminating yeast will or will not grow in bottled wine
depends on many factors , such as the strain (not only the genus and
species), the number of cells present, temperature and the amount of
alcohol, sugar, sulphur dioxide and other fungicides, growth factors
and various nitrogenaus compounds present. Wines may effectively
sterilise themselves in time, while others may maintain a low level of
viable yeasts, of no apparent practical significance, for some time. Yet
others encourage the growth of yeasts and rapidly become c1oudy.
Accordingly, it is not possible to say what is the maximum number of
yeast cells which may be tolerated at the time of packaging without
knowing more about the yeast and wine involved. Some bottlers regard
10 cells per bottle as a rule-of-thumb maximum, but this depends on the
yeast and the wine, and the ideal to aim for is zero count.
While the presence of fermentable sugar generally encourages the
growth of yeasts, certain strains can readily grow in wine containing
only non-fermentable pentose amounting to about 0.1 per cent. They
do this by using other wine constituents, such as acids, as a source of
carbon by oxidative growth. S. bayanus, S. bailii and Pichia membra-
naefaeiens are weil known in this regard. Various other yeasts, such
as Brellanomyees, reported in South Africa and Europe, require
thiamine and biotin in the wine as growth factors. If the wine is
deficient in these two B-group vitamins, Brellanomyees contaminants
will grow only slowly or not at all.

Yeasts as a group are not particularly heat-resistant when compared
with sporing bacteria, for example. In general, they are killed
by exposure to 70·C moist heat for 20 minutes, and heat sterilisation
of tilling equipment is so arranged as to ex pose all parts of the
equipment which contact wine to this temperature or above for at
least 20 minutes.

Sources of spoilage yeasts….

Although yeast contamination can come from various sources, such
as the air, empty bottles, corks, bottling equipment and so on, most
spoilage yeasts come from the wine itself and are not sufticiently
removed by pre-bottling filtration. The types of yeast involved are
listed above, and where Saccharomyces strains are involved it is
probable that they are in the wine as a residue from the primary
fermentation.

Prevention of yeast growth…

Since yeast spoilage in packaged wine is caused by growth of yeasts
after packaging, prevention is best achieved by sterile filtration, hot
bottling or the use of appropriate fungicides. Other sterilising procedures
have been proposed in various parts of the world, such as
dielectric heating, radio frequency, ultraviolet irradiation and alternating
current, but are not alternatives.

Quality control procedures….

In the bottling of table wines, particularly those containing fermentable
sugar, same form of quality control is essential in order to detect
the presence of yeast. The use of membrane filtration has been a
boon, since the small membranes can be directly laid onto nutrient
medium and the appearance of yeast colonies noted after a period of
incubation, Unfortunately this procedure takes at least overnight (if
microcolonies are to be counted) and a rapid and foolproof method
of detecting yeast cells, of the order of 10 cells or less per bottle, is
needed. If a check can be made in less than an hour, this would be of
great help in preventing yeast spoilage on a large scale. Various types
of staining techniques and the use of ultraviolet light microscopy hold
promise here.

To conclude, yeast spoilage of bottled table wines can be a serious
problem, particularly if the wines contain fermentable sugar. A wide
range of yeasts are involved, both fermenting and non-fermenting,
and same of them are resistant to the present legally permitted
germicides-sulphur dioxide and sorbic acid. Sterilising-grade filter
pads used according to the maker’s instructions provide wine free
from yeasts. Infection can occur subsequently from such sources as
non-sanitary-type pressure gauges, contaminated bottles and corking
machines. Airborne infection appears to be relatively unimportant.
Microbiological quality control procedures applied routinely are essential
for detecting yeasts in bottled wine, and these procedures,
coupled with sanitary and well designed filtration, filling and corking
equipment, are necessary for sterile filling.

Other taints and off-flavours….

Various other taints and off-flavours can occur in wines, resulting
from contamination by trace amounts of foreign substances, and we
are concerned here with those regarded as the most serious. Fortunately
for the wine consumer, these problems rarely find their way
into the packaged wine, but they can provide difficult problems for
the winemaker to rectify.

One serious cause of tainting is contamination by chlorophenols.
These compounds in very low concentrations can impart medicinal,
disinfectant, phenolic or antiseptic off-odours and flavours to foods
and beverages. Chlorophenols are most likely to be produced in
wines by the reaction of chlorine-based sterilants on phenols which
may be present in the juice, must or wine. These phenols can be
inadvertently picked up during the winemaking operation by contact
of must or wine with certain rubber products, paints and resins. The
need to prevent chlorophenol formation is highlighted by the fact
that certain chlorophenols can be up to 10,000 limes more effective as
tainting agents than the free phenols from which they are derived. In
some cases, e.g. 6-chloro-ortho-cresol, these compounds can contaminate
the foodstuff or beverage in such low amounts that they
elude analytical detection.

Recommendations to help avoid chlorophenol contamination of
wines involve assessments of the ‘tainting potential’ of winery equipment
made from or coated with organic resins. Resinous linings,
paints, rubber or plastic hoses and gaskets, etc., should be treated
with the normal chlorine sanitising agent and careful attention paid to
the generation of any of the ‘disinfectant-like’ odours du ring the
treatment. Should any problem odours arise the use of chlorinebased
sanitisers should be discontinued.

Another source of tainting is the contamination of bottled wine by
naphthalene present as a contaminant in the corks. Corks are a
common cause of such problems due to their adsorptive properties.
Corks kept in dose proximity to certain chemicals during storage or
shipping can absorb the vapour of the chemical and pass the contaminant
into wine after bottling.

More easily recognisable and hopefully more avoidable problems
have involved contamination with adhesives or oil. The latter can be
introduced into wine from leaking machinery gear-boxes during
filtration and from hydraulic lines on mechanical harvesters. Many
other taints are encountered, such as contaminated water used for
diluting brandy to bottling strength, contaminated carbon dioxide,
leaking refrigerant and so on.

Often a taint is not recognised until the wine is bottled, and this can
be expensive. Furthermore, tainting is commonly caused by such low
concentrations of contaminants that treatment procedures may be
ineffective. Accordingly, it is important for the winemaker to be
aware of and eliminate the likely sources of tainting.

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