Therefore, at present, it is necessary to prevent the biosynthesis of the mousy off-odour-forming compounds, by the elimination or strict control of the yeast and bacteria responsible for their formation. This can be achieved by implementing effective microbial control strategies in the winery [ 77 ]. These aroma attributes are generally considered to contribute negatively to overall wine sensory quality and are considered to be related to different low molecular weight volatile sulphur compounds, such as H 2 S, odour threshold in red wine 1.
Yeast fermentation is frequently associated with the occurrence of reductive off-odours, mainly linked to the formation of H 2 S and mercaptan by the yeast as mentioned by Pereira et al. As nitrogen availability is considered one of the main factors for H 2 S production by yeast, a strategy that could be adopted is the addition of yeast assimilable nitrogen to supplement fermentation [ 80 ].
The production of H 2 S during the AF is normal and the quantity produced is dependent on multifactorial factors such, yeast DNA, grape juice turbidity, level of assimilable nitrogen in the grape juice, levels of methionine and cysteine, fermentation temperature, high levels of SO 2, and sulphates.
This type of aroma sometimes masks completely the positive varietal and fermentative aroma, however, H 2 S is very volatile and usually, simple wine aeration is enough to remove them or can be precipitated with copper sulphate or copper citrate.
The excessive aeration of the wine in the presence of H 2 S could lead, by oxidation, to the production of heavy thiols that could be exceedingly difficult to remove from the wine. On the other hand, mercaptans and the other sulphides, are more intractable. Mercaptans impart off-odours reminiscent of rotten onions and disulphides are formed under similar reductive conditions and generate cooked-cabbage odours.
Related compounds, such as 2-mercaptoethanol XV, Figure 1 and 4- methyl thiol butanol XVI, Figure 1 , produce intense barnyard and chive—garlic odours, respectively. Light-struck refers to a reduced-sulphur odour that can develop in wine during exposure to light [ 62 ]. This defect is associated with the formation of volatile sulphur compounds with unpleasant aroma notes, formed by the methionine degradation catalysed by the photochemically activated riboflavin.
Exposure of wine to light at wavelengths close to or nm is particularly effective in inducing the light-struck taste [ 84 ], manly when clear glass bottles are used [ 85 ]. The preventive strategies are the most efficient as this defect generally develops after wine bottling, and these are mainly related to the reduction of the riboflavin levels in grape juice and wine.
There are classic and authorised fining agents, such as bentonite and AC activated carbon that can be used to remove with relative efficiency riboflavin from white wine [ 86 ]. Also during the AF, the selection of low riboflavin-producing yeasts can be used as it was shown that it is yeast strain-dependent [ 86 , 87 ].
Several herbaceous off-odours may be detected in wines. The presence of excessive sensations of herbaceous off-odour results in a decrease in the fruit notes, normally not appreciated by consumers. The source of this off-odour can generally be due to the presence of alkylmethoxypyrazines or aldehydes and alcohols with C6. The content of methoxypyrazine in the wine depends primarily on grape composition [ 90 ], being observed a complex relationship between viticultural practices and varietal aroma, being difficult to predict the final wine aroma because of the multiple compounds and pathways involved.
This vegetative character is most commonly, although not exclusively, associated with Sauvignon Blanc, Cabernet Sauvignon, and other Bordeaux varietals [ 91 ].
IPMP may also be present in certain grapes and thus found in the derived wine as a varietal character.
The excessive green bell pepper aroma found in red wines containing IBMP is generally considered unfavourable to wine quality. The presence of IBMP can be a positive quality factor when it is not dominant but is in balance and complemented by other herbaceous and fruity aromas [ 93 ]. Aldehydes and alcohols with 6 carbon atoms are volatile, odorous molecules that can contribute to the herbaceous aroma in the wine.
Their cut-grass-like aroma is the characteristic odour of freshly damaged green leaves; therefore, these compounds are often referred to as green leaf volatiles [ 94 ] and may also impart a bitter flavour [ 95 ]. These C6 compounds may be present in a free volatile form or in bound form, as glycosides [ 98 ]. They are mainly generated through the enzymatic breakdown of C18 polyunsaturated fatty acids contained in plant membranes.
The C6 aldehydes and alcohols derive from the oxidation of grape polyunsaturated fatty acids such as oleic acid, linoleic acid, and linolenic acid initiated by the lipoxygenase pathway when the berries are crushed [ 99 ]. Precursors of TDN are carotenoid derived compounds originating from the grapes [ ]. These precursors are slowly converted to TDN in the wine acidic medium.
TDN is an ambiguous aroma compound, defining the varietal character of Riesling wine but also constituting a repelling taint [ ] Comparing wines made of various grape varieties, a perceivable amount of TDN is found mostly in Riesling wines.
Exposing the grapes to more sunlight by defoliation increases both TDN levels [ ]. Low pH and bottle ageing will increase their content likewise due to hydrolytic cleavage of the TDN precursors [ , ].
The sensory perception threshold is very low, of 1. Farina et al. According to Saliba et al. Geranium taint is due to the presence of 2-ethoxy-3,5-hexadiene XXIX, Figure 1 in wine, which has an odour reminiscent of crushed geranium leaves. It is originated from the reduction of sorbic acid carried out by the LAB.
The reduction product sorbitol under wine conditions isomerises to 3,5-hexadieneol that after reaction with ethanol generates the 2-ethoxyhexa-3,5-diene which has a sensory threshold of about 0. Kennison et al. Guaiacol causes a phenolic and medicinal taint in a contaminated wine [ ], its flavour threshold is 0. An aroma threshold of 0. It is very easy to recognise because of its low sensory threshold, which is from 0.
However, the threshold values in wine depend strongly on the kind of wine, the wine style, and the experience of the panellist [ ]. As cork is a natural product from the cork oak it is subject to microbial contamination and its quality is dependent on good agricultural practices and quality control during processing, transport, and storage. Chlorophenolic biocides nowadays forbidden but accumulated in the environment are the common precursors which can be transformed by certain fungi to TCA and different chloroanisoles.
Other pathways of chloroanisoles formation usually include reactions of chlorination and methylation of compounds naturally present in wooden and cork materials [ ]. Moulds are considered the most significant causative organisms of cork taint, with implicated genera including Penicillium , Aspergillus , Cladosporium , Monilia , Paecilomyces and Trichoderma. Nevertheless, the process of wine contamination by haloanisoles is complex.
TCA and other haloanisoles can be formed in different wooden parts inside the cellar barrels, ceiling constructions, pallets and subsequently released into the air.
Besides, cork taint-related flavours were found in wines that were barrel samples or not closed with natural cork stoppers, indicating that natural cork stoppers are not the only source of mouldy off-flavours [ , ].
Therefore, depending on the compound causing the cork taint, the consumer has a different impression of the problem. Several compounds with similar negative flavour attributes were discovered in mouldy and musty smelling wines that were not affected by TCA geosmin, 2-methyl-isoborneol, octaneone, pyrazines, etc. That said, misguided hygiene practices have historically been part of the cork-taint problem. Cleaning using chlorinated bleach was common in wineries until a link to cork taint was found.
Contact between barrels and bleach on cellar floors was a particular pathway for TCA to strike. Flame-retardant paints and fungicides were found to taint wine with TBA. Barrelled wines were particularly badly hit, and some facilities had to be rebuilt. Nowadays most wineries know to avoid chemicals containing tribromophenols. Heat-treated wood is more common, and barrels are rarely cleaned with chlorine. Different approaches were made regarding the removal of TCA and TBA from tainted wines; either by fining with ACs and filtered afterwards, or polyethylene was added as an adsorbent to the wine [ ].
Their presence in wines can impart typical earth, mushroom, fungal and mouldy flavour [ , ]. Geosmin may result from the development in grapes picked in unfavourable weather conditions by microorganisms. MIB is a metabolite of Botrytis cinerea , some Penicillium spp.
MIB and 1-octenone have also been found in musts made from rotten grapes but not in the corresponding wines, indicating that they are not stable during AF [ ]. The findings of both compounds in bottled wine can therefore be linked to the cork stopper and the growing of mould on the cork during the manufacturing process.
MIB olfactory detection threshold has been determined as: 0. Lisanti et al. Wine can accidentally be contaminated with styrene when trace amounts of the styrene XLII, Figure 1 are released during wine storage in polyester tanks reinforced with fibre glass [ ]. Also, occasionally styrene contamination has been detected in wine in contact with synthetic closures [ ].
Wagner et al. The wine imbalances by acidity, astringency, or bitterness, are often the first defects noted in the sensory perception of wine quality [ 13 ]. Organic acids are the main responsible for sourness and able of modifying this sourness sensation in wines producing a pleasant and refreshing sensation [ ].
However, when present at high levels they are responsible for an unpleasant acidity. Therefore, it is generally accepted that too much acidity will taste excessively sour and sharp, while wines with too little acidity will taste flabby and flat and present a less defined flavour profile [ ].
Organic acids contribute to the tartness and mouth-feel properties of wine. However, different organic acids have different sensory properties, and the impact of organic acids is therefore not only linked to total acidity and pH, but to the specific levels of each acid in the wine [ ].
The perceived sourness was imparted by L-tartaric acid, D-galacturonic acid, acetic acid, succinic acid, L-malic acid, and L-lactic acid and was slightly suppressed by the levels of chlorides of potassium, magnesium, and ammonium [ 16 ]. Acidity adjustment is the reduction or increase in titratable acidity so that the resulting wine will be acceptable.
Acidity adjustment can be performed by the addition of an approved acid, the chemical deacidification with approved salts, and using ion exchange resins, either cation, anion or both, electromembrane processes and by biological deacidification.
Tartaric acid is commonly used to increase the titratable acidity and reduce the pH in the wine industry, because of its stability and the fact that yeast and other microorganisms are unable to metabolise it at wine pH [ ]. The reduction of titratable acidity by the addition of carbonate salts such as calcium carbonate, can be done in one of two ways, the first, is a direct addition which is not recommended as it results in wines which are unstable with respect to calcium tartrate, the second is to treat only a wine portion.
This process causes the pH to increase up increase to 4 or 4. The tartaric and malic acids are primarily in the ionised forms. The sample must be monitored continuously to avoid over freezing, and consistent results are difficult to achieve Wilkes Results have not always been reliable in determining cold stability.
Additionally, the sample size, sample shape, freezer, and particulates can affect freezing time and effectiveness of the test Wilkes Cold stability is often considered an essential step in producing quality wine. Various production methods are used in the industry as a means to cold stabilize. Often, winemakers are looking for more economical or efficient solutions when cold stabilizing wines.
Ultimately, the method chosen for cold stabilization can be regarded as a stylistic choice or preference, as each has recognizable benefits and disadvantages. Product additions offer new options for winemakers during cold stabilization. As new products emerge, suppliers may find a secondary function of a product that helps prevent KHT precipitation. Bench trials are always recommended prior to commercial application, as wine chemistry differences can affect the efficacy of many products.
Additionally, analytical methods offer only an estimation of wine's cold stability. It is possible for winemakers to receive a "stable" result using one test, and an "unstable" result using another test for the same wine. For this reason, winemakers may choose to run several different tests or primarily use one test to achieve analytical results. Regardless of this choice, analytical testing ensures the reliability of a wine's cold stability, and should be used as part of the winery's quality control program.
Berg, H. The utility of potassium bitartrate concentration product values in wine processing. Bower, P, C. Gouty, V. Moine, R. Marsh, and T. Bosso, A. Salmaso, E. De Faveri, M. Guatia and D. The use of carboxymethylcellulose for tartaric stabilization of white wines in comparison with other oenological additives.
Butzke, C. Wine Cold Stability: Assessments and Techniques. Purdue Wine Grape Team. Purdue University. Wine Cold Stability Issues. Purdue Extension. Dharmadhikari, M. Methods for Tartrate Stabilization of Wine.
Iowa State. ETS Laboratories. Understanding Cold Stability Testing. Technical Bulletin. Gusmer Enterprises, Inc. Fok, S.
Iland, P. Bruer, A. Ewart, A. Markides, J. Monitoring the winemaking process from grapes to wine techniques and concepts. Lutin, F, and D. Keep it Natural! Adjusting the pH of food products without chemical additives thanks to Bipolar Membrane Electrodialysis. Ameridia, Division of Eurodia Industrie. Cold stabilization is when you reduce the temperature of your wine to nearly its freezing point to purposefully form tartrate crystals you can then remove through racking. These harmless crystals form when tartaric acid precipitates out of the wine.
They have no effect on the flavor but they can put people off because they look like broken glass. Oxidized wine, however, is permanently ruined. By in large the best preventative stabilization method is the addition of sulfur-dioxide SO 2. Whether introduced as a gas or in tablet form this chemical kills off yeast, malolactic bacteria, and many other micro-organisms.
Adding SO 2 should be part of your wine making process. Aging your wine prior to bottling will give many stability issues time to show their faces. Any treatments after bottling will take a lot of work and introduce far too much oxygen. Often winemakers use the same dose each year, yet the amount of protein present in grapes is affected by ripeness and disease and so will differ from year to year. A bentonite trial should be carried out each year to calculate the minimum amount required to reduce protein levels and infer protein stability.
Errors can occur when performing a bentonite fining trial. Often people will perform trials in the lab using a different type or concentration of bentonite, or bentonite made up with a different type of water than that used in the cellar. These all affect the bentonite dosage required and may result in over- or under-fining of the wine.
Cellar conditions should be matched exactly during a lab trial. If wines are fined with protein-based additives such as egg or milk after bentonite fining, or if grape juice concentrate is added to sweeten the wine before bottling, then there is a chance that extra protein will be added to the wine, potentially making it unstable.
If the acidity of a wine is adjusted, or copper is added before bottling, this can also affect stability. Thus, the heat stability should be checked after all additions have occurred in case a final bentonite fining is required.
Interpretation of the heat test is done using turbidity measurement. The standard stability test involves heating 0.
0コメント