Survival Arts

"Chemistry of Winemaking", 1973, on sulfites in wine

Yesterday, I published an article by John Sebastian on the amusing topic of homemade wines done on the cheap. John made some assertions about "sulfates" (actually "sulfites") which generated some informative response from James Rogers in refutation. As a chemistry student with a burgeoning personal library on the science and some of its applications, I happened to have a copy of the proceedings of the 12-13 April 1973 "symposium sponsored by the Division of Agricultural and Food Chemistry at the 165th Meeting of the American Chemical Society" held in Dallas, Texas: "Chemistry of Winemaking", A. Dinsmoor Webb, editor (published 1974 by the ACS, Advances in Chemistry Series #137).

I've scanned in several pages of this out-of-print book, pp280-285, from the Webb article "Home Winemaking", which mention sulfite production and supplementation. I've included the section entitled "The Course of Fermentation" below simply because my OCR program flawlessly reproduced it... why waste the material by not including it? I have reproduced "Table I" manually with the published values, and placed it inline, after the first reference to it in the original text.

Those with a chemistry background will also note that this was written 30 years ago, before IUPAC nomenclature standarization.

- Russell

Excerpt follows:

Addition of Sulfur Dioxide

Certain fruits and some of the white varieties of vinifera have a tendency to brown during crushing and other early processing operations because of oxidation. This oxidation may be promoted by enzymes in the fruit, or it may be a direct reaction between phenolic material of the fruit and oxygen from air. Sulfur dioxide (SO₂) is a strong enough reducing agent that it is oxidized in preference to the phenolics of the fruit juice. Sulfur dioxide may also function by denaturing the oxidizing enzymes. Therefore, to prevent browning, add 25-200 ppm SO₂ to the fruit immediately after crushing. The quantity of SO₂ is governed by the ease of browning of the particular Juice being vinified. SO₂ in addition to preventing oxidative browning in juices, inhibits growth of bacteria and wild yeasts. Thus it provides a more nearly sterile field for the action of the desirable yeast starter added by the enologist. The quantity of SO₂ to be added to the juice is varied according to the condition of the fruit-clear, cool, sound fruit fresh from the vineyard requires very little while fruit that is in poor condition and warm needs more. The amounts of SO₂ to be added to a juice can be estimated from Table I.

Table I.  Sulfur Dioxide to be Added to Juice (Mg per liter.)
	Fruit Condition
Browning Tendency	Poor; Warm, Infected, Some Decay	Good; Cool, Fresh, Sound, Clean
High (white juices)	200-300	100-150
Low	75-125	0-25

SO₂ is a pungent and unpleasant smelling, dense gas at normal temperature and pressure. Under moderate pressure it condenses to a liquid which can be stored in steel cylinders. The large winery usually adds SO₂ to the crushed grapes by carefully metering a small stream of the liquid from a cylinder to the inlet line of the pump that transfers the must from the crusher to the fermenting tanks; this ensures that SO₂ is uniformly mixed into the mass of crushed fruit. For the small winery and the home winegrower, however, the relatively small amounts of SO₂ required are difficult to measure and transfer as liquid, so either water saturated with SO₂ or a SO₂-liberating salt is used.

Water saturated with SO₂ gas at room temperature contains 5-6 wt % SO₂ depending on the temperature. While the SO₂-saturated water solution is still very pungent and unpleasant smelling, it does not present the handling and measurement problems of pure liquid SO₂.

The sodium and potassium salts of SO₂ are simpler and more pleasant to use as they do not have the odor of the pure liquid or the 5% water solution. They are rapidly soluble in must [Editor's Note: this is the original wording] where they react with a small portion of the natural acid present to liberate SO₂. There are two sodium salts of SO₂ available, Na₂SO₃ (neutral sodium sulfite) and NaHSO₃ (sodium acid sulfite). The latter compound introduces less sodium into the wine and removes less acid from the wine for an equivalent amount of SO₂ liberated. Potassium acid sulfite and potassium pyrosulfite (potassium metabisulfite) are the two salts of potassium with SO₂ that are readily available, soluble in grape juice, and capable of yielding SO₂ upon reaction with the acid of the juice. Potassium salt is recommended when it is desired to keep the wine low in sodium ion content for diet reasons. The salts should be edible or food product grade, that is, free of heavy metals and other toxic impurities. They must be stored in tightly closed containers or they will react with the water vapor and carbon dioxide of the air to yield sodium or potassium carbonate and SO₂-thus losing their effectiveness as sources of SO₂ when added to the grape juice.

The required dose of SO₂ should be estimated conservatively and measured precisely because excessive amounts of SO₂ destroy the aroma and taste of the wine and can delay the onset of fermentation. Also SO₂ in excess interferes with the natural development of bouquet in red table wines and diminishes the intensity of the red color. One should always use only the minimum amount of SO₂ required to inhibit bacterial growth and counter oxidation-more definitely is not better.

Yeasts and Bacteria

One of the purposes of adding SO₂ is to inactivate bacteria and wild yeast so that the fermentation may be conducted with a chosen desirable strain of yeasts. Fortunately the wild yeast and the bacteria on grape berries (frequently confused in the older literature with the wax-like bloom which is naturally present on some berries) are susceptible to inactivation by relatively low doses of SO₂. A clear field is thus available to the large inoculum of SO₂-tolerant pure culture yeast added by the enologist.

It is true that wines were made for thousands of years before it was known that yeast was responsible for the fermentation. It is also true that in certain regions of the world wines are still made without SO₂ and pure yeast starters. These latter regions are generally those in which the yeast-containing sediments and press residues from the winery are returned to the vineyards and worked into the soil. Over many years it is likely that this procedure has resulted in the natural selection and stabilization of a mixed culture of yeasts which is carried from the vineyard to the winery and back and that the particular mixture contains enough of the desirable types to produce good wines in most years. It is also true that in years of cold summers and rainy harvest seasons many of the wineries normally relying on spontaneous fermentations use SO₂ and pure-culture starters. Today nearly all standard quality wine (vin ordinaire) and probably the majority of fine wines of the world are vinified using SO₂ and pure-culture yeast starters.

The bacteria which are found on sound grapes as they come from the vineyards are few in types and normally no problem in wine production as the acid, tannin, and alcohol of the wine stop their growth. The wild yeasts cannot be trusted to produce a good fermentation, however. In comparison with selected strains of SO₂-adapted yeasts, defects of wild yeasts are the inability to multiply rapidly in the relatively concentrated sugar solution of grape juice, a sensitivity to alcohol which prevents completion of the fermentation, a tendency to form excessive amounts of odoriferous esters or other non-alcohols, and the characteristic of remaining dispersed throughout the wine rather than aggregating and falling to the bottom of the container. The advantages to the home winegrower to be derived from the use of a selected yeast are obvious.

About 3 vol % of actively fermenting pure-culture yeast starter is required. A clean juice which has had a low dose of SO₂ will start and ferment satisfactorily with a lower inoculum, but the 3% level usually results in a quicker starting fermentation. For the home winegrower the simplest way to get the gallon or so of starter required is from a nearby winery. One has no choice of yeast strain and no guarantee of purity by this method, however. Winery supply agencies can usually furnish some strains of desirable wine yeasts such as Montrachet and Champagne in lyophyllized or freeze-dried form. These can be added directly to the SO₂-treated juice and probably represent the optimum solution to the starter problem for the home winemaker. If it is desired to use a yeast strain that is not readily available in either of the above-mentioned forms, a small pure culture of the desired strain will have to be obtained from a biological laboratory supply house or research laboratory maintaining a yeast collection. The small culture next must be multiplied until enough cells are present to inoculate the grape juice in the large fermenting tank. Sterile medium is required for the multiplication. Juice from a white grape variety of low flavor, such as Thompson Seedless, heated 30 min at 15 Ibs per square inch pressure (2 atmospheres) in a pressure canner, serves very well. The small culture is transferred from the original tube to about one pint of the cooled, aerated, sterilized juice contained in a sterilized quart jar or bottle. Avoid contamination from the hands or the surroundings. The sterile jar should be covered or plugged so that air can penetrate but dust and cells of undesirable organisms cannot-a plug of sterile absorbent cotton works well. The jar should be placed in a room or cupboard at 70°-80°F, and it should be shaken gently at intervals. Within a day or two, growth and fermentation should be evident. The juice will foam and bubble, particularly when the jar is shaken. When the culture is actively fermenting, it is transferred into 1-2 gallons of sterile juice containing 100 ppm SO₂ which after a day or two will be actively fermenting and constitutes enough starter for 25-50 gallons of Treated-treated juice. Successive fermentations can be inoculated from large batches that have fermented without difficulty although there is always the possibility of some contamination of the pure culture.

Yeasts, along with the algae, lichens, and other fungi, are known as thallophytes, a term which means they are undifferentiated plants or ones which do not have separate roots, stems, and leaves. Wine yeasts, along with most brewer's, distiller's, and baker's yeasts, are classed in the genus Saccharomyces or sugar fungus. The classification of yeasts is based on microscopic observation of their shape and forms, the way they divide during growth, and the way they respond when subjected to different test solutions of sugars or other chemicals. As scientists develop newer tools, such as the electron microscope, and as they study and classify more and more types of yeasts, it is desirable to develop further and to modify the older classification systems. Most of the wine yeasts are today put into the species cerevisiae with several strains being recognized by enologists. Many of these were formerly known as strains of S. cerevisiae var. ellipsoideus. It is quite likely that further study of the many species, varieties, and strains of wine yeasts will result in further refinements of the classifications.

Conversion of Sugar to Alcohol

Winemaking is basically concerned with the fermentation of the sugar in fruit Juice solutions by yeasts. Some understanding of the chemistry involved in the conversion of sugar to alcohol and carbon dioxide is significant not only because it engenders an appreciation of the beauty of natural processes but because it also lets us understand and control certain factors affecting the quality of the wine.

The suspension of yeast cells will be added to the must a few hours after adding sulfur dioxide-a time long enough to permit most of the SO₂ to react with juice constituents or to volatilize. The low level of SO₂ and the aeration during the mixing in of the yeast starter permit the cells to start their action in an oxygenated environment, a condition which favors their conversion of some of the sugar to carbon dioxide and water with a high yield of energy for building many new yeast cells. The yeast population increases rapidly from the inoculation level of about one million cells per milliliter to about one hundred to two hundred million cells per milliliter, one to two days after inoculation. Then, nearly all of the oxygen will have been taken from the juice by the yeast cells, cell multiplication will slow dramatically, and conversion of sugar to carbon dioxide and ethanol becomes the main chemical reaction.

Fruit juices, depending on the type of fruit, contain one or more of the three sugars, sucrose, glucose, and fructose, in relatively high concentrations. Other sugars are present in trace to small amounts. Most yeasts have an invertase enzyme on the outer layer of their cell walls which rapidly converts the sucrose to glucose and fructose. These simpler sugars are carried rapidly through the cell wall by active transport. This is not understood fully, but it is known that glucose and fructose get into the cell interior faster than they should by simple diffusion.

Inside the yeast cell the hexoses are converted principally to ethanol, carbon dioxide, and adenosinetriphosphate (ATP) with the liberation of waste heat. The ATP is an energy source in cell metabolism; the ethanol and carbon dioxide diffuse across the cell wall to the exterior where the ethanol dissolves in the juice and the carbon dioxide bubbles to the surface. Excess heat must be removed to prevent the self-pasteurization of the wine, as most yeasts cease fermentation at 40°-45°C. Minor amounts of numerous other compounds are formed as by-products.

In addition to the carbon and nitrogen which are necessary to yeast for building enzymes, a few elements such as phosphorus, potassium, magnesium, manganese, and possibly traces of others, and a few vitamins are required for growth and fermentation. Normally, grape or other fruit juice will contain all substances necessary for yeast growth and fermentation. In preparing certain special flavored wines where the main component of the mixture for fermentation may be pure sucrose, it is necessary to add a yeast food-usually a mixture of ammonium acid phosphate with some autolyzed yeast-as a source of materials required for growth and fermentation.

The Course of Fermentation

The fermentation can be followed, in a rough way, by the bubbling in the fermentation tank since carbon dioxide is a product of the reaction. However, this doesn't indicate the extent or degree of completion of fermentation. Under some conditions, fermentation will stop before all the sugar is transformed, leaving the new wine subject to bacterial spoilage; therefore it is desirable to have a simple way to follow the loss of sugar. Water solutions of sugars are more dense than pure water while water solutions of alcohol are less dense than pure water. Density determinations performed daily thus provide one measure of fermentation.

Normally a stem or hydrometer is used to determine density. Hydrometers may be scaled in many different units. In the United States, grape juice and wine densities are usually measured in Brix or Balling degrees which are density units reflecting the weight per cent of sucrose in sucrose-water solutions.

As densities vary with temperature, and as hydrometers are calibrated to be accurate at different temperatures, the fermenting solution should be warmed or cooled to near the calibration temperature for the particular hydrometer used; for precise determinations, the actual temperature should be measured and the measured density should be corrected.

In theory the fermentation could be followed equally satisfactorily by measuring the alcohol content of the solution. In fact, however, alcohol determinations are much slower and more complicated than density determinations, so they are seldom, if ever, used. It is possible for the fermentation to stop-successive density determinations showing the same value-while there is some sugar left in the solution, although this is not normal behavior for fermentations. It is good practice to analyze for low levels of sugars in all wines when they have apparently completed their fermentations.

Posted by Russell Whitaker at October 11, 2003 03:35 PM | TrackBack