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Winemaking ... nurturing the natural process...

By definition, wine is a drink made by the partial or complete fermentation of the juice of fresh grapes. Grapes are the only fruit with a high enough level of sugar and with the proper balance of acid and nutrients to sustain a natural fermentation to dryness with stable results. Other fruits or berries may be fermented, but without additions of sugar, acid, or various yeast nutrients, they may readily spoil.

Although the biochemistry of fermentation was a mystery until the late nineteenth century, the results of the process have been known to man for over 9,000 years. Human ancestors in all climates channeled fermentation into making foods that resisted spoilage and kept in storage longer than "fresh" versions. Besides alcoholic beverages, these include such cultural nutritional icons as cheese, kimchee, miso, sauerkraut and yogurt.

Wine creation was thought to be a spontaneous act of Nature, merely set in motion by man. Historically, grapes were crushed to release the juice (must) into a vessel. When the bubbling, heating activity was complete or greatly slowed, the wine was pressed by some mechanical means to separate the liquid from the stems, skins, pips and pulp. It was then stored to age and clarify until it was drunk. While modern technology and methods may have refined and enhanced it, this is still the basic process today.

Fermentation is a natural process. Left alone, a grape would ripen until the skin broke and the juice fermented. The intervention of man is only necessary to increase the clarity and stability of the end product. "Making" wine is mostly a matter of the choices and decisions of the winemaker during each phase of production, from growing the raw material grapes to bottling the finished wine. These choices determine the wine's style, flavors, and aromas to a great extent, in addition to its chemical stability.

Yeast is the microscopic, single-celled fungi which causes fermentation. Yeast cell populations are present in the air, especially in and around vineyards. This indigenous demographic is known as "wild" or "ambient" yeast. At one time, the bloom, that hazy film that covers ripe grape berries, was thought to mostly consist of yeast cells but this has been proven incorrect. Instead, cells are concentrated around the berry stem (peduncle) and much fewer in concentration than thought, in the dozens rather than the thousands.

When yeast comes in contact with the grape juice, it begins to feed on it, grow and reproduce. There are approximately 6000 yeast cells per ounce of actively fermenting must. An enzyme (zymase) within them converts sugar in the grape juice into roughly equal parts of alcohol and carbon dioxide and also releases energy in the form of heat.


Theoretically, this process could continue naturally until the sugar is used up, which is most often the case. Occasionally, fermentation continues only until the yeast cells are no longer able to tolerate the level of their waste products: alcohol, carbon dioxide and/or heat, thus leaving very small amounts of residual sugar.

It is noteworthy there were no significant changes in the methods and practices of viticulture and enology from about 1000 BC until about 1860 AD. Other than small improvements such as using more metal and less wooden parts in presses and equipment, the French vignerons of 1850 knew little more of the scientific principals involved than did the ancient Romans.

Certain events did occur, primarily regarding wine storage, which together marked the beginnings of serious wine collecting. An English print from 1778 is the first known evidence of a corkscrew. In 1797, it was first noticed at Chateau LaFite that wine that aged in bottles improved. A few years later, in 1815, came the first documented Declared Vintage of Port.

Louis Pasteur in the laboratory.WEIRD SCIENCE
Although theorized by German chemist Georg Stahl in 1697, the scientific basis of fermentation was first proven and explained by Louis Pasteur, in 1857. Pasteur demonstrated fermentation was caused by living organisms and he developed a germ theory of fermentation in 1861. He was the first to isolate and distinguish types of yeast (saccharomyces apiculatus and saccharomyces elipsoideus). Pasteur found that some yeasts are efficient converters of sugar to alcohol and some are not. Some yeast will stop at about six percent alcohol and some continue until a level of sixteen or seventeen percent. Even among efficient types, there are several different strains and each viticultural region seems to have a specific native strain. He also noted that high temperature kills off yeast

Pasteur's discovery made it possible to sterilize the must, killing off the wild strains by heating it to below the boiling point and then inoculating the Pasteurized must with the best strain, so that it has no competition. Modern wine techniques do not use Pasteurization, but may add a small amount of sulfur to prevent the wild yeast from starting up (see "Related Links" below).

Modern cultured strains of wine yeast give predictable and dependable results and come in many versions specialized for different purposes. In addition to various tolerances to temperature and alcohol, there are strains that produce aromas with a range of characteristics, strains that produce more body and texture, those that end up with finer and more compact lees, etc. Ambient or feral yeasts became in vogue in the past decade, but the risk of failure or spoilage is enough that most volume producers use only cultured yeasts.

Like all living things, yeast cells have a primary drive to reproduce. In the first and most vigorous stage of fermentation (2 to 4 days), the yeast action mainly produces more yeast. This is the aerobic (contact with air) fermentation. The anaerobic (without air) fermentation follows and produces most of the alcohol. Under optimum conditions, a wine fermentation will last approximately three weeks, but this may take as much as several months, sometimes for no apparent reason.

Fermentation can continue until the wine is dry (without residual sugar), or be stopped at some mid-point to make wines at levels of sweetness ranging from the barest hint to extremely sweet. Stopping fermentation can be accomplished by killing or removing the yeast cells by one of several methods: adding alcohol to raise the level to 15% or more (as in port or sherry), adding sulfur dioxide or sorbate (sorbic acid - not considered a good practice and illegal in many countries), chilling the must, or by simply filtering out the yeast cells using a sterile filter. Wines with residual sugar risk re-fermentation unless filtered to remove any remaining yeast.

With a very few exceptions, grapes have clear juice, whether the skins are white (green) or black (purple). Pigmentation (anthocyanin) comes from the skins. White wine can be made from black grapes by simply pressing the grapes and separating the clear juice from the pigmented skins before fermentation. This is the basis of "White Zinfandel" table wines and "Blanc de Noirs" sparkling wines. The longer the contact between skins and must, the more color is extracted from the skins into the must.

Basket Press (photograph).BRIGHTER WHITES
To make most white wines, winemakers crush and press the juice from the grapes and add sulphite (75-150 parts per million) to the must to kill the wild yeast and bacteria and prevent oxidation. The stems, skins, pips and pulp are separated from the juice. One variation has the fruit go directly into the press, without going through the crusher, to avoid some degree of oxidation. In another variation the grapes, after crushing, go into a settling tank where they are chilled and may sit for up to 24 hours of skin contact before being pressed.

Juice separated before the press is called free-run. It is of higher quality, with less bitterness and oxidation. The leftover skins, pips and pulp is called the marc, pommace, or cake. The resulting juice usually has a lower fixed acidity, but higher volatile acidity, as well as higher tannin than the free-run. Both the speed and the pressure of the press affect the quality. Sometimes the marc is pressed first with a modern bladder-type cylindrical press and then with a traditional basket press. Some portion of the pressed wine may be added back to the free-run, but it most often is used for bulk wine production or distillation. Note that whites are pressed as unfermented must, while reds are pressed after the fermentation has begun or completed. One ton of grapes will yield 155 to 195 gallons of must, with 120 to 160 gallons being free-run juice. The leftover solids are often returned to the vineyard and ploughed back into the soil.

1900 Basket Press (photograph).

Photo courtesy of Fleet Irvine Photomurals, a viewable collection of wine and other theme photos that may also be purchased.

The three-man basket press in France, c. 1900.

Pressing the grapes extracts juice as well as additional color and tannins. Too much pressure will also begin leaching bitter tannins from the seeds, so care must be taken. Modern presses use rubber compound bladders inflated by air pressure as a way to avoid the bitterness.

After inoculating with a selected yeast culture, the must is allowed to ferment for from 2 to 45 days at a temperature usually between 45° and 65° F.

The lower the temperature, the longer the fermentation continues and the more fruitiness in the resulting wine. A fermentation allowed to get too hot produces off flavors and can get stuck. Over the years, temperature control was accomplished by either fermenting small batches in a cooled environment, pumping the must through a heat-exchanger, or inserting refrigerated coils in the tanks.

Casks of oak, vats of concrete, or tanks made of stainless steel are the most common fermentation vessels. Smaller containers of plastic or fibre glass resin are also common. In the 1960s, science began having a greater impact upon commercial winemaking and much more attention was paid to hygiene, especially keeping vats and tanks spotlessly clean. Materials such as concrete and wood are not as easy to clean or to temperature control, but are still in use at many wineries.

Wood allows a small exchange between the oxygen outside and the wine inside the cask or barrel, which some winemakers feel helps to round and soften tannin and astringency. Wood can also be coopered into many sizes, but expensive, perishable and requires a great deal of labor to clean and maintain.

Concrete is inexpensive, easier to clean and maintain than wood, holds temperature well, and can be custom-fitted into square shapes and rows that share common walls and occupy less space than individual casks or tanks. Many are lined with ceramic tile, enamel, or food-grade epoxy. The walls may also contain built-in pipes or conduits for regulating temperature.

Stainless tanks at Cambria (photograph). SS tanks at Cambria (click to enlarge)

Stainless steel equipment has major advantages over wood and concrete. Chemically neutral, stainless steel neither adds nor takes away flavors in wine. Second, it is easily fitted with temperature controls, including jackets through which refrigerant can be pumped, thermostats, internal stirring components and computerized controls that can maintain the temperature of the must within one or two degrees. Third and most significant, stainless does not provide a medium for bacterial growth and can be easily cleaned and sterilized to avoid contamination. Fourth, stainless steel provides both more durability and more portability than the other materials.

For most red wines, winemakers crush, sulphite and inoculate the grapes and allow the juice to ferment in contact with the skins and pulp for 2 to 5 days at temperatures between 70° and 80° F (pinot noir is often allowed to go to 95° F). During this initial period, color is extracted from the skins as more sugar is converted to ethanol.

The skins and solids in the must will float to the top of the fermenter, forming a cake that the carbon dioxide cannot escape from. This leaves only the portion of juice underneath the cake in contact with the skins. Methods used to break up this cake and insure more color extraction include punching down by hand several times a day, pumping-over the juice to the top of the cake using a must pump, Rotary presses and fermentors (photograph).or stirring by some other mechanical means, including the relatively new roto-tanks that turn the entire contents over, like a cement mixer. After enough color and tannin are extracted, the must is pressed and the juice separated for clarification, finishing and aging.

Rotary presses and fermentors at Gallo of Sonoma (click to enlarge)

Centuries ago, wineries in cold climates noticed wine that had completed alcoholic fermentation in the Fall would sometimes begin bubbling again as Spring temperatures began to climb. This secondary fermentation, now called malolactic fermentation, takes place in most wine.
Malolactic fermentation is natural, but does not always occur, which made it a problem for most wineries.

Bacteria, rather than yeast, converts some of the malic acid naturally present in grapes into lactic acid, along with the by-product of carbon dioxide. Renowned wine maker and scientist Emile Peynaud identified and isolated the bacteria responsible (oenococcus oeni) in the 1950s. It could be prevented by adding sulfur, but prior to the 1980s, "ML" was essentially and all-or-nothing proposition.

Malic acid has a strong tartness; it is naturally present in apples and grapes and many unripe fruits. Lactic acid is weaker, naturally found in dairy products.1 Diacetyl, a compound produced during malolactic fermentation, also gives butter its distinctive flavor. Malolactic fermentation, therefore, can have the effect of "softening" the wine, taking some of the sharp edge off, sometimes imparting a "creamy" texture and leaving "buttery" aromas and flavors. This is desirable with certain wines (Chardonnay, Pinot Blanc), undesirable with others (Riesling, Gew├╝rztraminer).

Sometimes, a little goes a long way to change the style or appeal of a particular wine. Controlling the speed and completeness of ML can produce a variety of results. With modern methods and equipment, winemakers can control the pace and degree of ML in their wines. Some wineries inoculate new wine with a malolactic culture.2 They may also heat up the wine slightly to encourage ML. Some wineries do all they can to avoid it, even physically separating white wine operations completely from red, where ML is more likely to occur due to the normally extended aging of red wines.

Before the arrival of modern sterile filtration (in the 1980s), an occasional wine would wait until after it was in the bottle to begin ML. The bottled wine would become cloudy and fizzy, sometimes developing enough pressure to blow the cork or break the bottles, and developing a flavor like sour milk. Long before they understood the science of ML, traditional vintners knew to wait to bottle their wines until after Spring when the wine would warm up and ML would often occur naturally.

Wine can be aged in barrels of oak or other woods to impart and mature flavors. Wooden barrels leech tannins into wine and can also impart "smoky" flavors if the barrels have been toasted, short of actual charring. Oak aging also tends to allow for slow oxygenation of wine that has the effects of allowing more complex aromas and flavors to develop, "softening" tannin, and darkening or stabilizing color, as well as encouraging clarification by permitting time for particulate matter to settle out of solution.

Different wood species from different forests impart differing flavors and to a degree of strength depending upon the age of the barrel, relative to how much use it has seen. The average barrel will add wood flavor to wine for three years. As long as they remain watertight and uncontaminated by spoilage bacteria or yeast (such as TCA or brettanomyces), older barrels are "neutral" in terms of imparting flavors, although the porosity of the wood does allow a slow exchange of oxygen that mellows wine to some degree.

The newer the barrel, the stronger the oak flavor. Flavor can also vary, depending on the manner of cooperage, or barrel-making. Wooden barrels also allow a degree of oxidation that can mellow a wine. Some loss occurs through evaporation and wine in barrels, even when kept in a relatively humid environment, must be topped occasionally with more wine.

Oak-indicated flavors can also be introduced by several methods without the use of barrels. Oak planks or staves can be submerged in tanks of aging wine. Porous bags filled with oak "chips", natural or "toasted" to various levels, can be suspended inside tanks. Similarly-treated oak "dust" can be introduced at the crush pad. Not all of the benefits of barrel aging can be short-cutted using any of these methods, but the oaky-buttery-vanilla flavors popular with consumers are much more economical to gain than aging in increasingly-expensive barrels.

As wine ages, natural settling and clarification will occur to some degree, although it is inefficient and inconsistent. The public, however, is usually unwilling to accept cloudy wine or wine with crystals or other particles in it, so various methods are used for "cleaning-up" and finishing wine after fermentation, either before, during, or after aging. These processes also insure a level of stability or shelf-life for wines shipped to retail or restaurant outlets where the bottles may spend some time "on the shelf" before purchase and consumption.

Clarification methods are similar for both white and red wines. All methods of clarification remove unsightly particles from wine, but may also strip wine of pleasant aroma and flavor elements, body, and color.

Racking is the oldest technique of clarification that is just one step beyond natural settling. This is simply siphoning off the relatively clear wine after the lees have settled to the bottom, leaving them behind to discard. The lees are the insoluble matter including dirt and dust, cellulose, dead yeast cells, bacteria, tartrates and pectin. Racking may be done only once or several times before a wine is bottled. Red wines, especially those barrel-aged, are sometimes bottled after racking without further processing.

Cold stabilization may be considered an adjunct or enhancement to racking. This process removes excess tartaric acid that, if untreated, might later form potassium bitartrate crystals, which can show up in wine bottles or on corks. Although these tartrates dissolve easily and are edible (cream of tartar, commonly used in cooking) and harmless, they can alarm the uninformed consumer who thinks there is "broken glass" in his wine. Cold stabilization is accomplished by allowing the wine to warm up to "room temperature" and then chilling it down to about 40° F. The tartaric acid crystallizes in the tank and the wine drawn off by racking.

Fining is a method of clarifying or chemically stabilizing wine. The procedure begins by stirring into the container of wine a fining agent that is heavier than both water and alcohol and does not dissolve in either. The agent ultimately settles to the bottom of the vessel (tank or barrel), causing small suspended particles to precipitate out along with the agent. The clarified wine is then separated by siphoning (racking) off the settlings (lees).

Fining can lower high levels of tannin, remove haze, and reduce color. Care needs to be taken to chose the proper fining level that conforms the wine style that winemaker wants to achieve. Over-fining can result in thin wines that lack aroma complexity, flavor depth, viscosity, and aging potential.

Physical agents work by absorbing tiny particles and dragging them. Chemical agents work by forming chemical bonds with hydrogen elements in the undesired particles. Fining agents include egg white, milk, blood, gelatin, carbon, casein (the principal protein constituent of milk and cheese) and isinglass (an extract of sturgeon bladders). Heat stabilization is a fining process that uses bentonite (a clay of hydrated magnesium silicates) to remove protein, which may cloud a wine.

Filtering means passing the wine through a filter small enough to remove undesirable elements. Various filtering technologies allow great flexibility to winemakers to make stable wines of varying styles. As with fining, filtering can also remove elements that contribute to flavors and aromas, so winemakers need to be judicious and conservative with this technique to avoid "collateral damage" that leaves their wine clean but lifeless.

Depth or sheet filtration uses a relatively thick layer of fine material (diatomaceous earth, cellulose powder, perlite, etc.) to trap and remove small particles. Surface or membrane filtration passes wine through a thin film of plastic polymer with uniformly-sized holes that are smaller than the particles.

Sterile filtration uses micropore filters, which are fine enough to remove yeast cells, to prevent further fermentation. This is especially significant when residual sugar is allowed to remain in the wine at low levels. Prior to the advent of modern micropore filtration, slightly sweet wines were endangered by the possibility of revived fermentation in the bottle.

Bottling California wine in 1890 (photograph).

Bottling an early Northern California vintage, c. 1890.

Although most wineries have either their own mechanized bottling line or hire a portable bottler do the chore, the steps are still nearly the same: sterilizing, filling, corking, foiling (or not), labeling and casing the bottles. Sterility methods have greatly improved since the nineteenth century and especially over the past three decades.

The ancient Romans invented glass blowing and made the first bottles. An English company patented a machine to mold bottles that were uniform in size and shape in 1821, but selling wine that was already bottled was illegal in England until 1860. Wine was customarily sold by the measure; customers provided their own bottles which were often identified with a personal seal. Paper labels identifying the contents developed in the late 1800s. Until the 1970s, wine bottle sizes varied from about 650 to 850 milliliters. A world standard size wine bottle is now 750 milliliters (26.7 oz.).

Bottling equipment can vary from the primitive, using siphon hoses, funnels, hand corking and labeling machines, to the modern, very sophisticated, sterile "hospital conditions" of a totally automated bottling line. Either process must include methods for sterilizing the bottles, standardizing the fill level, inserting the corks, covering them with capsules or foils, attaching the labels and boxing the bottles for storage or shipment.

Before bottling, the winemaker conducts blending trials, combining small samples of cuvées or batches of wine from different grape varieties, or vineyards, or of different vintages, in varying combinations until the wine tastes best. When the final blend is determined, the "recipe" is made and the wine is blended accordingly and bottled.

Wines that are intended for early consumption, where freshness and fruity, floral characteristics are of prime importance, may be kept for extended periods in large refrigerated tanks where these qualities are best preserved. The wine is bottled in batches at various points during the year, as needed to replenish depleted store shelves or restaurant stocks.

Among the most dynamic and civilization-altering changes of the 20th Century are the methods of preserving and packaging foodstuffs. At the turn of the 19th Century, a typical general store's shelves might have a stock of dried or canned goods, bulk grain and flour. Meat and poultry, fish, dairy, produce and baked goods all came from specialty stores or straight from the production source. Food shopping was an errand run several times per week.

A modern market has a wide variety and large inventory of fresh, packaged, prepared and frozen foods and many shoppers go but once a week or even less. Treatments, additives and refrigeration have made it possible to preserve food in an edible state for greater periods of time and therefore, to cultivate and harvest higher volumes of perishable goods.

Some of these methods also can be applied to winemaking. We have already mentioned the role of refrigeration in temperature control during fermentation. There are also additives, besides yeast and fining agents, that can be used to "doctor" wines. The most common are acids such as citric, tartaric or tannic, used to adjust the balance of wine. Oak chips and powdered oak can add flavor and added tannins can improve color and balance. These treatments and additives are very unusual for fine wine grown in the best appellations, but may be common in attempting to coax palatable wine from grapes grown in marginal climates.

Hanzell Winery, Sonoma, CA (photograph).Until late in the Industrial Revolution, the growth of the wine industry was almost entirely territorial and hardly at all technical. Wine making methods were passed on from mostly European traditions. In 1957, Industrialist-Diplomat James D. Zellerbach opened a new winery in Sonoma, dedicated to and named after his wife Hana and modeled in great detail after the architecture and methods of Clos de Vougeot in Bourgogne ("Burgundy"), France. Mixing innovation with tradition, Hanzell was the first winery to use stainless steel tanks (of his own design) for fermentation, to import French oak barrels, and to have a laboratory on the premises for monitoring and analysis, all of which are common elements of modern wineries.

Mechanical advances such as field crushers, bladder and roto presses, stainless steel tanks, micropore filters, refrigeration, vacuum-bottling and other devices and methods have all evolved in the past four decades. The latest innovation is the Foss Winescan Analyzer which projects a beam of infrared light through wine or juice and quantifies up to 18 different components, based on the way they absorb the light. Conventional chemistry would take hours to perform the same tests.

Invention and innovation have primarily had the effect of allowing the winemaker to have more control over the process and to gain a measure of consistency. This control has stimulated the industry to examine, experiment, and perfect their techniques and methods.

"Modern" methods are often simply extensions of circumstances that have occurred naturally over the history of "primitive" wine making, such as inducing malolactic fermentation. One fairly recent technique is cold-soaking the fruit in tanks chilled to 45° or below for several hours or days to postpone the start of alcoholic fermentation. This technique mimics what may happen during particularly cold harvest seasons. The chilling seems to enhance color and preserve more of the fruit character in the wine before the alcohol reaches a level that causes a high extraction of tannin.

FrankenwineOther developments made in the 21st Century include new chemicals that can "rescue" batches of wine that might formerly have made it to market only as vinegar or industrial alcohol. These same "cures" can also manipulate low-quality wine up to a level of commercial acceptance. Extracts and concentrates of acids, tannin, oak, color, as well as genetically-modified yeasts (see Notes #2 below) can increase efficiency and lower risk of waste, in many cases.

But these products beg to question the definition of "natural" wine production. A movement is growing among consumers to demand ingredient labeling on wine, beer, and other alcoholic beverages and it may not be too far-fetched or unromantic to sympathize with the need for such protection.

It may be that the finest wines are produced with the indigenous natural or wild yeast that balances the various alcohols, which may be more desirable. Natural yeast fermentation also carries risk that the fermentation will not continue to a stable level of alcohol (above 11%) or that the vinegar yeast will take the process beyond wine and into salad dressing. In any case, man seems to be rediscovering Nature to be the greatest wine maker of all.

Jim LaMar

Chemist Alexander Pandell's The Acidity of Wine is an excellent discussion of the acidic elements which dominate wine chemistry and how total acids and pH level affect flavors.

Some wine making technology is turning retro, rather than high-tech, including use of indigenous, feral, or Native Yeast Fermentations, as explained by Winemaker David Ramey.

Randy Caparoso's article-within-an-article, The "no-oak" rebellion at Lodi's Acquiesce (and history of barrel usage since ancient Roman days), offers an excellent summary of the sensory ramifications and history of oak in wine.

Science & Wine is the BLOG of Assistant Professor Paula Silva of the Institute of Biomedical Sciences at the University of Porto, where she agglomerates articles, research papers, and other science communications, as well as posting calendars of conferences, seminars, and workshops related to scientific studies of wine.

Dr Merry C posted this article, Does Eating Fermented Foods Have Health Benefits? 21 Research Papers Examined on Healthy But Smart's site.

1. Malic acid [COOH-CH2-COHO-COOH] is the tart natural acid of apples; lactic acid [CH3-COHO-COOH] is the milder-tasting natural acid of milk. RETURN

2. A recent development of genetic engineering is a strain of yeast that can perform both alcoholic and malolactic fermentations simultaneously, saving production time for large producers. The genes to make this possible were transferred from the bacterium into the yeast. RETURN




Page created April 8, 1999; last updated March 28, 2019
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