Unless otherwise indicated, the term wine refers to the beverage produced by the alcoholic fermentation of grapes or grape juice by yeasts, followed by ageing. However, wines can be prepared by fermenting the juices of fruits, berries, rhubarb, dandelions, and honey, among others.
The majority of grape wines are either red or white. Red wines fermented on the skins contain the red pigment from the skins of purple or red grape varieties, whilst white wines are prepared from white grapes or the juice extracted from other grape varieties, fermented without the skins. The production of red wine will serve as an illustration of the winemaking process.
Preparation of Juice
- Grapes of a specially suited variety for winemaking are collected when their sugar level is optimal.
- Depending on the grape variety and degree of maturity, the concentration of sugar can range between 15 and 25 percent.
- They are de-stemmed and machine-crushed before being treated with sulphur dioxide (75 to 200 ppm) or potassium metabisulfite in similar proportions to prevent the growth of undesired yeast competitors.
- A “natural inoculum” (yeast present on the grapes) or, more usually, 2 to 5 percent of a particular wine yeast, a strain of Saccharomyces cerevisiae var. ellipsoidus, is added to the must, which consists of crushed grapes.
- Initially, the contents of the tank are stirred twice a day by pounding the “cap” of floating skins, stems, etc., pumping juice over the skins, or mixing in some other manner to aerate and so promote yeast growth and aid in the extraction of colour from the skins (for red wines).
- Alternately, red pigments may be removed from the skins and reintroduced back to the juice using heat. Later, the mixing ceases because anaerobic conditions are optimal for alcoholic fermentation.
- During the active fermentation, which lasts between 3 and 5 days, it is crucial that the temperature be kept within the optimal range, i.e., between 24 and 27 C for red wines and between 10 and 21 C for white wines, i.e., within the optimal range (active fermentation takes 7 to 14 days).
- A temperature that is too high inhibits the wine yeasts and allows competing organisms, such as lactobacilli, to grow and create faults; a temperature that is too low slows the action of the wine yeasts and permits the growth of wild yeasts, lactic acid bacteria, and other organisms.
- Due to the heat produced during fermentation and the high ambient temperature, it may be necessary to artificially cool the must.
- After the primary or active fermentation has progressed sufficiently, the fermented juice is separated from the leftovers (pomace) and placed in a storage tank under a light carbon dioxide pressure for 7 to 11 days at around 21 to 29 degrees Celsius for the secondary fermentation.
- If a dry wine is desired, the leftover sugar is fermented at this stage. The clear wine is separated from the sediment at the bottom of the tank.
Storing and Aging
- To precipitate proteins, the wine may be flash-pasteurized prior to maturing, however this is typically not the case. It is cooled, stored for a few days, filtered, and transferred to hardwood (often white oak or redwood) or concrete tanks with a plastic coating for age.
- The tanks are entirely filled and sealed to exclude air. Periodically, sediment is drained from the wine. There may be some remaining age in the bottle.
- The scent or bouquet that should be one of the wine’s distinguishing qualities is produced by the wine’s body and flavour as it ages for months or years.
- Esters and alcohols are regarded as significant contributions to aroma and flavour. During age, malolactic fermentation of grape juice’s malic acid by lactobacilli or micrococci* may occur, resulting in the creation of lactic acid and carbon dioxide and a decrease in acidity.
- The wine is filtered or otherwise clarified, then stored in barrels or bottles. Some wines are typically pasteurised in the bottle after maturing.
- In general, the final alcohol concentration is between 6 and 9 percent by weight or 8 and 13 percent by volume.
Volatile Acidity of Wines
- The presence of a high concentration of volatile acid in wines is suggestive of an improper fermentation.
- In the United States, the legal limit for volatile acid content in red wine is 0.14 g per 100 ml, expressed as acetic acid, whereas the standard for white wine is 0.12 g per 100 ml.
- When grapes are crushed, a variety of microorganisms, including yeasts and bacteria, populate their surfaces. In addition to the surface flora of the grape, a variety of pollutants from the soil are also present.
- To inhibit the growth of these microorganisms, the winemaker may add sulphur dioxide or sulfite, or, less frequently, pasteurise the must (expressed juice). Added wine yeast predominates throughout the first fermentation phase.
- Initially, aeration of the must promotes the growth of the yeast; subsequently, anaerobic conditions support the alcoholic fermentation of the yeasts, which liberates carbon dioxide and ethyl alcohol, both of which inhibit non-wine yeast species.
- During secondary fermentation, the carbon dioxide atmosphere above the wine hinders the growth of aerobic pollutants such as acetic acid bacteria.
- The subsequent pasteurisation, although not intended for this purpose, minimises the quantity of bacteria that could cause wine to spoil (“diseases”) in the future.
- During the ageing and storage of wine, there should be little or no development of organisms; nonetheless, organisms that can grow may be introduced by contamination from the tanks, barrels, or bottles, and alterations such as malolactic fermentation may occur.
Kinds of Wine
- There will be no attempt to list the dozens of names given to various wine varieties. Instead, we will establish a few general descriptive phrases.
- The vast majority of wines are still wines, meaning they do not retain any of the carbon dioxide produced during fermentation, as opposed to sparkling wines, which contain significant amounts. Other wines may be carbonated artificially.
- As opposed to sweet wines, which have residual or added sugar, dry wines have little or no unfermented sugar. Typically, wines have 11 to 16 percent alcohol by volume, but they can contain as little as 7 percent.
- However, fortified wines, to which a distillate of wine known as “wine spirits” or “brandy” has been added, contain between 19 and 21 percent alcohol by volume.
- Table wines have a relatively low alcohol content and minimal or no sugar, whereas dessert wines are fortified and sweet.
- French dry sherry is interesting because it is prepared from grapes with a high sugar content that have been dried up by an infecting grey mould, Botrytis cinerea; these grapes produce a wine with a high alcohol concentration.
- Following the primary fermentations, Spanish (Jerez) sherry supports the growth of a yeast film, likely of one or more species of Saccharomyces, while the wine is being racked in partially filled barrels. This yeast growth, or “flower,” gives the wine a unique aroma and flavour.
Wine Defects and Microbial Spoilage
Similar to beer, wine can be flawed by nonmicrobial factors and spoiled by germs. Defects include those resulting from the presence of metals or their salts, enzymes, and treatments used to clean the wine. Iron, for instance, can cause a sediment known variably as grey, black, blue, or ferric casse, and in white wine it can cause a precipitate of iron phosphate known as white casse. The salts of tin and copper have been implicated for cloudiness. Peroxidase, an oxidising enzyme produced by certain moulds, may cause white wines to turn brown and red wines to lose their colour. Gelatin, which is used to clarify wines, may induce haziness. Primarily, wild yeasts, moulds, and bacteria of the genera Acetobacter, Lactobacillus, Leuconostoc, and maybe Micrococcus* and Pediococcus are responsible for wine deterioration.
Factors Affecting Growth of Microorganisms in Wine
The following variables are known to affect the susceptibility of wines to microbial spoilage:
- Acidity or pH: The lower the pH, the less likely it is that a substance would spoil. The minimal pH allowing for the growth of microorganisms varies depending on the organism, wine type, and alcohol content. Molds, yeasts, and acetic acid bacteria are not inhibited by typical wine pH, while the majority of lactic acid bacteria can tolerate acidity as low as pH 3.3 to 3.5, which is lower than the pH of most wines (most California table wines have a pH of about 3.5to 4.0).
- Sugar concentration: Due to their low sugar content, dry wines (0.1 percent or less) are rarely ruined by bacteria, but wines with 0.5 to 1.0 percent or higher sugar are susceptible to spoiling.
- Alcohol concentration: Tolerance for alcohol varies depending on the pathogen. Thus, acetic bacteria that spoil musts and wines are inhibited by more than 14 to 15 percent alcohol by volume, while deacidifying cocci are inhibited by approximately 12 percent, Leuconostoc spp. by more than 14 percent, heterofermentative lactobacilli by approximately 18 percent (with the exception of Lactobacillus trichodes, which grows in fortified wines containing more than 20 percent alcohol), and homofermentative lactobacilli by approximately
- Concentration of accessory growth substances: While Acetobacter species can produce their own vitamins, lactic acid bacteria must be supplied with the vast majority of them. The primary source of these compounds in wines is the wine yeast, which, upon autolysis, produces the accessory growth factors. The presence of these compounds increases the risk that lactic acid bacteria may cause spoiling.
- High tannin concentration: Tannins added to gelatin for clarity prevent bacterial growth, but typically insufficient quantities are added to be of practical significance.
- Concentration of sulphur dioxide: The more sulphur dioxide that is added, the more the spoiling microorganisms are inhibited. The 75 to 200 ppm typically added to the requirement is plenty. Efficacy relies on the type of organism to be inhibited and rises with a decrease in pH and sugar concentration.
- Temperature of storage: Typically, spoilage is quickest between 20 and 35 degrees Celsius and slows as the temperature drops below freezing.
- Accessibility to air: Aerobic organisms such as moulds, film yeasts, and Acetobacter cannot survive in the absence of air, whereas lactic acid bacteria thrive anaerobically.
Spoilage by Aerobic Microorganisms
- On the surface of must and wines exposed to air, film yeasts that may oxidise alcohol and organic acids may grow, generating a heavy pellicle known as “wine flowers.”
- They should not be a problem if the must is regularly stirred and air is kept out of the wine.
- Aerobic acetic acid bacteria, typically Acetobacter aceti or Gluconobacter oxydans, oxidise alcohol in must or wine to acetic acid in the presence of air, a process known as acetification.
- They may also convert the glucose in the must to gluconic acid, giving the must a “mousy” or “sweet-sour” flavour.
- Molds, including Mucor, Penicillium, Aspergillus, and others, can develop on plant walls, barrels, tanks, hose lines, corks, and grapes or cold must. Walls and equipment are adequately cleansed to prevent mould growth.
Spoilage by Facultative Microorganisms
- Wild yeasts, which comprise all yeasts other than the wine yeast used as a starter, can cause aberrant fermentations that result in low alcohol content, excessive volatile acidity, unpleasant tastes, and cloudiness in the wine.
- These yeasts, which originate primarily from the grapes used to prepare the must and are typically predominately of the apiculate type, are suppressed or eliminated through the use of an active starter of the wine yeast, sulfiting or pasteurisation of the must prior to fermentation, and temperature control of the must during fermentation.
- Some wild yeasts and slime-producing bacteria thrive at temperatures below 21,1 degrees Celsius.
- Lactic acid bacteria are the primary organisms responsible for must and wine deterioration. There has been considerable confusion in the naming of the many types of bacterial wine spoilage, perhaps because multiple types of bacteria can produce the same defect or the same organism can cause different flaws under different environmental conditions.
- Probably the most prevalent is tourne (sour or fermented), in which acid is produced from sugars, glucose, and fructose in wine, primarily by heterofermentative Lactobacillus species, such as L. brevis, L. hilgardii, L. trichodes*, and possibly L. buchneri.
- The growth of lactobacilli results in silky cloudiness, an increase in lactic and acetic acid, the production of carbon dioxide, the development of “mousy” or other unpleasant flavours, and the deterioration of the wine’s colour.
- Bitterness can come from the fermentation of glycerol in wine. Pousse refers to flatulence coming from any cause, such as the release of carbon dioxide by heterofermentative lactics.
- This is the homofermentative. L. plantarum produces lactic acid mostly from sugars in table wines, so increasing the fixed acidity and imparting a “mousy” flavour.
- Through oxidation of malic, citric, and tartaric acids by Acetobacter (aerobic) or fermentation of malic and tartaric acids by Lactobacillus, Leuconostoc, Pediococcus, or other cocci, spoilage bacteria can lower the acidity of wine.
- The sliminess or ropiness of immature white wines, together with cloudiness and enhanced volatile acidity, has been attributed to Leuconostoc spp., L. mesenteroides, and L. dextranicum*; micrococci*, and lactobacilli. When authorised, the addition of sucrose encourages the formation of dextran and, consequently, sliminess by Leuconostoc.
- Any bacteria or yeasts growing in wine are likely to induce unwanted cloudiness, and any acetic bacteria or heterofermentative lactic developing in wine may increase the volatile acidity to an unsalable level.
- Typically, sugar fermentation leads in an increase in acidity, both fixed and volatile acid if the organism is heterofermentative.
- Oxidation or fermentation of the grape’s organic acids leads to a reduction in the amount of fixed acid. It should be underlined again that the wine’s composition is crucial in determining its susceptibility to bacterial deterioration.
- Thus, white wines with low alcohol content are more susceptible to sliminess and spoilage by cocci than other wines; musts and table wines support the growth of Lactobacillus hilgardii, L. brevis, and Leuconostoc mesenteroides; and Lactobacillus trichodes* is the only known species to spoil California dessert wines that does not grow in musts.
- It is interesting to note that the development of lactic acid from malic acid in extremely sour wines may enhance their quality by decreasing their acidity.
- Lactic acid is a weaker acid than malic acid, and three molecules of malic acid produce just two molecules of lactic acid. Some bacteria in wine can convert tartaric acid and glycerol into lactic acid.
Wine from Other Fruits
- The majority of fruits, including apples (hard cider), peaches, apricots, plums, pears (perry), cherries, and berries, can be used to produce wine.
- Prior to fermentation, berries and the majority of other fruits (with the exception of wine grapes) must be “ameliorated” with sugar to produce a quality wine.
- Otherwise, the winemaking process is comparable to that of grape wine. These items could be dry, sweet, fortified, sparkling, or carbonated.
- Dried fruits such as raisins, dates, figs, and prunes can be used to prepare wines for direct consumption or for distillation into brandy.
- The production and illnesses of apple wine or hard cider, made from apple juice, have attracted great attention.
- Much American hard cider is produced locally from apples that are not particularly suited to its production and is fermented with whatever yeasts are present.
- Consequently, the alcohol content is frequently between 4 and 6 percent, there is a sugar residue, and flavour and scent may be deficient or absent.
- British and French ciders, as well as some American hard ciders prepared industrially, are made from apples with a high sugar and tannin content.
- Methods of production may include sulfiting, the addition of sugars and yeast food, and the inoculation of a proven yeast culture.
- It has been observed that heterofermentative lactobacilli are active during the production of cider, fermenting malic and citric acids, sugars, and glycerol into carbon dioxide, lactic, acetic, propionic, and succinic acids, and mannitol.
- The majority of hard cider problems are comparable to those of grape wine; cloudiness, low alcohol level, and off-flavors created by wild yeasts; cloudiness and poor flavours produced by lactobacilli; sliminess or ropiness owing to bacteria: acetic acid generation, etc.
- Low acidity and low nitrogen levels promote the growth of spoilage organisms in ciders.
Wines from Other Agricultural Products
- Wine can theoretically be made from any culinary foodstuff that contains adequate moisture, sugar, and yeast food.
- Mead is created from honey that has been diluted and to which minerals and food for yeasts have been added.
- Dandelion wine is a handmade beverage produced by the alcoholic fermentation of an extract of dandelion flower water to which sugar, flavourings, and yeast have been added.
- In this context, distilled liquors or spirits are those created by distilling an alcoholically fermented product.
- Rum is produced by distilling fermented sugarcane juice, syrup, or molasses.
- Whiskeys are distilled from saccharified and fermented grain mashes, such as rye whiskey from rye mash, corn whiskey or bourbon from corn mash, wheat whiskey from wheat mash, etc.
- Rums and whiskeys are produced from mashes fermented with high-yielding Saccharomyces cerevisiae var. ellipsoideus yeast strains. Typically, grain mashes are acidified to encourage yeast growth.
- Instead of being a biological process, the ageing of distilled liquors in charred oak barrels or tuns is a chemical one.
- Unless a qualifying term is added, brandy refers to the distillate of grape wine, such as apple brandy (applejack), peach brandy, and apricot brandy.
- There should be no concerns with bacteria causing the deterioration of distilled spirits.