Open vat process

The open vat process relies on surface growth of acetic acid bacteria in vats, barrels, jars, or

Box 11β€”2. The Special Appeal of Specialty Vinegars (or, "You Paid How Much for That Vinegar?")

By far, most of the vinegar produced in the United States is used by the food industry in the manufacture of salad dressings, pickles, catsup, mustard, and a variety of other processed foods. In general, the vinegar used for these products should be inexpensive and plain-tasting. Even the more flavorful wine, malt, and cider vinegars available at the local grocery store and used as table vinegar carry rather modest retail prices.

A visit to the gourmet section, however, will reveal that some specialty vinegar products are as expensive as fine wines.Among the most well-known of the specialty vinegars are the sherry and balsamic vinegars.They are made, respectively, in the Jerez region of Spain and the Modena region of Italy. Although versions are now made in the United States and elsewhere, these are protected names in Europe, where their production and authenticity are highly regulated.

Balsamic vinegar is striking in several respects. It has a very dark brown appearance, a strong complex aroma, and a sour, but slightly sweet flavor. Importantly, the fermentation of balsamic vinegar is quite different than traditional vinegars.The starting material is musts obtained from a variety of grapes, mainly Trebbiano, but also Lambrusco and Sauvignon.The musts are concentrated two-fold by raising the temperature to near boiling, followed by simmering until the mixture is syrup-like.Thus, step-wise process can take as long as two to three days and results in a

Box 11β€”2. The Special Appeal of Specialty Vinegars (or, "You Paid How Much for That Vinegar?") (Continued)

sweet solution with a sugar concentration of 20% to 24%. Next, the material is inoculated with a "mother culture" (from a previous batch) and transferred into premium quality wooden barrels (examples include oak, chestnut, mulberry, and cherry wood).These wild cultures contain various osmophilic yeasts, including strains of Saccharomyces and Zygosaccharomyces, and the acetic acid bacterium Gluconobacter. An alcoholic fermentation and the acetic acid fermentation essentially occur at very near the same time, since Gluconobacter can oxidize sugars directly to acetic acid.

Another unique feature of the balsamic fermentation process concerns the manner in which the vinegar is aged. Rather than aging the product in bulk in a single barrel, balsamic vinegar is aged in a series of steps in which the product is transferred, via decantation, from barrel to barrel (Figure 1). Each barrel is usually constructed from different woods, which contributes to the flavor and aroma of the finished product. During each transfer step, about half of the volume from the preceding barrel is moved to the next barrel.

The barrels (usually in sets of five or seven) are held in attics that are exposed both to very warm and very cool temperatures, depending on the season.Warm temperatures stimulate fermentation, whereas cool temperatures promote sediment formation (which improves clarity). The rich and aromatic flavors associated with balsamic vinegar are due, in part, to the grape constituents and the volatile and non-volatile components extracted from the wood, but also to the acids, esters and other metabolic end products produced by the microflora.To achieve De-nominazione di Origine Controllate (DOC) status and Protected Designation of Origin (PDO) certification, the vinegar must be aged for at least twelve years, but some balsamic vinegars are aged for as long as twenty-five to fifty years. Less than 12,000 liters of authentic balsamic vinegar (Aceto Balsamico Tradizionale) are released for sale per year,thus it is not surprising that retail prices for 90 ml bottles can be more than $150.While more moderately priced versions exist (Aceto Balsamico di Modena), they usually are diluted with wine vinegars or aged less.

Sherry vinegars are produced via the "solera" process in a manner similar to that used for Sherry wines (Chapter 10). Specifically, a portion (less than one-third of the total volume) of the vinegar is transferred from cask (or "butt") to cask. Each successive portion is more aged than the preceding material, such that the final product has a uniform and consistent quality. Aging typically occurs for a minimum of six months, to as long as two years or more (to earn "Reserva" status). Flavors are, as for Balsamic vinegar, derived from fermentation, as well as the wood used

Figure 1. Sequential barrel formation used during manufacture and aging of balsamic vinegar. Successive barrels are made from different woods, and as aging progresses, the size of the barrels decreases.

Box 11β€”2. The Special Appeal of Specialty Vinegars (or, "You Paid How Much for That Vinegar?") (Continued)

during aging.The latter contribute important phenolic compounds associated with aged sherry vinegar.

Finally, it should be noted that distinctive specialty wine vinegars are not just produced in Modena, Jerez, or Orleans, but also in the United States.A growing market for varietal vinegars, made from single variety wines, has emerged, especially in the wine-producing regions in California. These vinegars are fermented in barrels; some producers have adopted traditional Orleans-style fermentors and long aging periods.


Cocchi, M., P. Lambertini, D. Manzini,A. Marchetti, and A. Ulrici. 2002. Determination of carboxylic acids in vinegars and in Aceto Balsamico Tradizionale di Modena by HPLC and GC methods.J.Agric. Food Chem. 50:5255-5261.

Garcia-Parilla, M.C., F.J. Heredia, and A.M. Troncoso. 1999. Sherry wine vinegars: phenolic composition changes during aging. Food Res. Int. 32:433-440.

trays. This was undoubtedly the first method used for vinegar production, since it can be easily performed and involves naturally-occurring or wild cultures.Although many variations exist, barrel-type fermentations are typical of the processes that evolved in Europe and that are still used today. The most well-known example is the Orleans process, which takes its name from the French city where this particular method was originally developed. Similar processes also evolved independently in the Far East, but the latter were based on rice wine and other types of alcohol-containing substrates, rather than on grape wine or cider.The principle of the process is simple and straightforward. The ethanolic substrate is placed in a suitable vessel, and the fermentation is initiated either by acetic acid bacteria that naturally contaminate the vessel or by a portion of vinegar from a recent batch.The material is exposed to the atmosphere, but is otherwise left undisturbed (hence, this process is sometimes referred to as the "let alone" method).

In the open vat process, acetic acid bacteria grow only at the surface. Growth is accompanied by production of a polysaccharide-containing film or pellicle that forms at the liquid-air interface. Although most strains of Acetobacter are capable of pellicle formation, this property can occasionally be lost if cells are grown under non-static (i.e., shaking) con ditions (see above). This is important during open vat processes, because an intact film is essential to the success of the fermentation. Any disturbance or disruption of the film may delay the fermentation. Thus, open vat processes may take several weeks before the fermentation is established, and several months before the fermentation is complete.

Traditional open vat or surface film fermentations ordinarily operate in batch mode.After the fermentation is complete, the vessel is emptied, then refilled with substrate. The acetic acid bacteria, left over or added, must then re-establish surface growth. It is possible, nonetheless, to perform this fermentation in a semi-continuous mode, provided steps are taken to maintain the surface film during substrate addition and product removal steps.

In the Orleans process, for example, the barrels are constructed such that a filling device (i.e.,a pipe) extends from just outside the top of the barrel all the way to the inside bottom (Figure 11-5). Aeration is provided by holes (covered with cheesecloth) drilled into the sides of the vessel.A tap is positioned at the bottom end that is used to withdraw the product.The barrel is filled with wine to about 60% to 70% capacity and inoculated with a fresh vinegar culture ("mother of vinegar").When the acetic acid fermentation is completed, it is then possible to remove a portion (from one-third to as much as

Figure 11-5. Schematic illustration of vinegar barrel used for the Orleans process. Adapted from Adams and Moss, 1995.

Figure 11-5. Schematic illustration of vinegar barrel used for the Orleans process. Adapted from Adams and Moss, 1995.

two-thirds) of the fermented vinegar from the tap end, and to replace it with fresh wine or cider, through the filling device, without disrupting the film.The process takes two to three weeks, or longer, for each completed cycle, depending on the temperature and the rate of volume exchange.Although other methods require significantly less time, the quality of vinegar produced by the Orleans and similar barrel methods is considered to be far superior to those produced by more rapid methods.

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