Vol.16, No. 6 November - December, 2001
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061
E-mail: bzoeckle@vt.edu
Web site: http://www.fst.vt.edu/Zoecklein

Table of Contents

I. Microoxygenation Research

A student, Mr. Patrick Sullivan, has been involved in an extensive study on the effects of microoxgenation. Ten different wineries were involved in the study, including Horton Cellars of Virginia. The following is a brief review (and elaboration from Vol. 15, No. 2 Vintner's Corner) and update on Patrick's findings.

Winemakers recognize the beneficial aspects of limited oxygen exposure of young red wines immediately after fermentation and during the early stages of aging. This subject has been, extensively covered in previous editions of the Vintner's Corner and in Enology Notes #33. The process of micro-oxygenation is an extension of this observation and of current understanding of the process of phenolic polymerization as it relates to aging. Table 1 from Ribereau-Gayon and Glories (1983) helps to illustrate the importance of oxygen exposure. These data demonstrate that oxygen exposure causes a decrease in the anthocyanin content in barrel and aerated tank wine, yet an increase in the color.

Table 1.

Source: Ribereau-Gayon and Glories (1983).

Microoxygenation is a process whereby young red wines are continuously exposed to oxygen. Oxygen is supplied in the form of the compressed gas via a micron-size diffuser positioned close to the bottom of a stainless steel tank. Oxygen catalyzes oxidation of ethanol to acetaldehyde which then serves as a bridging agent, linking together phenolics/phenolic-anthocyanins via C4-C8 reactive sites. This is the same process that occurs during barrel aging, except that the time frame is significantly shortened. Many producers add oxygen by splash racking. This frequently results in a variable saturation of the wine with oxygen, likely between 1-4 mL depending on the degree of exposure, the wine and wine temperature. The goal of microoxgenation is to control the oxidative reactions which occur, to enhance their benefits.

The wines produced in Patrick's study were sensorially evaluated using the following aroma categories: fruit, veggy, spicy, oak, herbal and oxidation. Flavor categories included: fruit by mouth, acidity, green tannins, tannin grit, plushness and off-flavor. The results of his work clearly illustrate several common features across all wines evaluated. The microoxygenated wines all had a greater fruit intensity. Additionally, wines which had been treated had a higher perception of plushness and a lower perception of vegetative aromas.

Microoxygenation has several effects on wine including color stability, effecting mouthfeel properties and astringency, and reduction of off-aromas including both reductive and vegetative. What remains are refinements to this technology to determine the optimum degree of oxidation of each wine. Our thanks to Dennis Horton (Horton Cellars) for helping to sponsor this research.

II. Research Winery Renovation Completed

The Enology-Grape Chemistry Group, in the Department of Food Science and Technology, at Virginia Tech, has just completed a $180,000 renovation project, which included the analytical laboratory and the research winery. The bulk of the money went toward improving the research winery. The winery now is established for gravity-flow processing. It has greatly-improved storage facilities, and a specially-coated concrete floor that slopes gently to a full length drain. New heating, ventilation, and air conditioning improve control over processing conditions. Double doors at the entrance of the winery make it much easier to move carts of grapes and wines in and out, and windows along one entire wall open the space visually.

A number of research wines are produced under the direction of Dr. Bruce Zoecklein, head of the Enology-Grape Chemistry Group. Working closely with Virginia Tech's Viticultural Specialist, Dr. Tony Wolf, we are evaluating new cultures and clones for Virginia, varietal training and trellising systems, the influence of crop level on wine quality, etc.

The project is part of the Department of Food Science and Technology's proposed curriculum beverage option - enology, cider, beer, and juice production.

III. Winery Planning and Design Workshop Proceedings Available

Proceedings of the Winery Planning and Design Workshop conducted in July are available. The 104 page proceedings covers establishing a business plan, winery design considerations, including gravity flow, winery tank selection, sanitation, etc. Send $45 payable to:

Dr. Bruce Zoecklein
Foundation Account
Department of Food Science and Technology
Virginia Tech (0418)
Blacksburg, VA 24061.

IV. Winery Sanitation

Uncontrolled growth of microorganisms can eventually lead to product deterioration and spoilage. Traditionally, SO₂ has been an important tool for control of microbiological growth. However, application levels have dropped dramatically in the last few years, leading to spread of microorganisms. Proper cleaning and sanitation is more important than ever be- fore in maintaining wine quality. The following adapted from Wine Analysis and Production (Zoecklein et al., 1995) is a review of winery sanitation and cleaners.

Cleaners

Alkalies
Strong alkalies, including NaOH (caustic soda or lye) or KOH (caustic potash), and sodium carbonate (Na₂CO₃) are the most commonly used detergents. Both NaOH and KOH have excellent detergent properties and are strongly antimicrobial. Unfortunately, they may also be corrosive, even to stainless steel, if recommended application levels are exceeded. Handling strong alkalies requires use of protective gloves and eyeware.

Sodium ortho- and meta-silicates (Na₂SiO₃) are less caustic than NaOH and have better detergency properties. They are also less corrosive. Where the organic load is not heavy, mild alkalies such as sodium carbonate (soda ash), or trisodium phosphate (TSP) find application. Sodium carbonate is an inexpensive, frequently used detergent. Unfortunately, in hard water, Na₂CO₃ contributes to precipitate formation.

Sequestering Agents
Due to their abilities to chelate calcium and magnesium, and prevent their precipitation, polyphosphates are often included in detergent formulations. Examples include sodium hexametaphosphate (Calgon) and sodium tetraphosphate (Quadrofos). The amount included depends on hardness of water. With the exception of TSP (see Alkalies), the group is noncorrosive.

Organic chelating compounds such as EDTA may also be included. Although more expensive than polyphosphates, they have the advantage of being relatively heat stable.

Surfactants

Having both water and oil-miscible properties, surfactants are generally used as wetting agents to reduce surface tension and facilitate contact between a detergent and the surface being cleaned. Various anionic, nonionic, and cationic surfactants are available.

Acids
Acids are used in specialized detergent formulations to reduce mineral deposits and soften water. Maximum effectiveness occurs at pH 2.5. At low pH, acid solutions are very corrosive toward stainless steel (and other metals). Phosphoric acid is preferred because of its relatively low corrosiveness and compatibility with nonionic wetting agents.

Sanitizers

Once deposits are removed and the surface is visibly clean, it can be sanitized. Two general categories of chemical sanitizing agents are currently in use: the halogens, including chlorine and iodine, and Quaternary Ammonium Compounds (QUATS). Additionally, some successfully use sulfur dioxide as a sanitizer agent.

Chlorine-Based Sanitizers
Chlorine in its active form, hypochlorous acid (HOCl), is a powerful oxidant and antimicrobial agent. Molecular hypochlorous acid is present in highest concentration at near pH 4, decreasing rapidly with increased pH. At pHs greater than 5, hypochlorite (OCL) increases whereas, at pH less than 4, chlorine gas (Cl₂) increases. Neither chlorine gas nor hypochlorite have been shown to be active against microorganisms; however, both are very corrosive. Formation of Cl₂ (gas) is a safety issue. Because there are still substantial amounts of HOCl present at pHs greater than 6.5, sanitizing operations are typically carried out in the range pH 6.5 to 7.0.

Chlorine is an oxidant whose activity will prematurely degrade if organic residues (reflecting inadequate cleaning) are present. Therefore, proper cleaning prior to sanitation is essential. Reaction time for chlorine is temperature dependent. Up to 52°C (125°F), the reaction rate (and corrosive properties) doubles for each 18°F (9°C) increase in temperature.

Sanitizing surfaces requires active chlorine concentrations of 100 to 200 mg/L. Although chlorine is compatible with stainless steel surfaces at recommended levels, severe oxidation (pitting) may result from use of larger amounts. Upon completion of the operation, thorough rinsing is required to remove remaining sanitizer. The effectiveness of this operation may be monitored by use of "test kits". There are several forms of chlorine available as shown in Table 2.

Table 2. Forms of Chlorine Available for Use as Sanitizers (expressed as % active chlorine).

Source: York, 1986.

Iodine
Formulations including iodine and nonionic wetting agents are called iodophors. Iodine (I₂) is the active principal, and thus iodophors are most effective in the range pH 4 to 5, where the concentration of I₂ is maximum. To ensure activity, formulations typically include phosphoric acid. The sanitizer has the advantage of low use levels. A concentration of 25 mg/L of iodophore is equivalent to 200 mg/L chlorine. Iodophores are frequently used for bottling sanitation, followed by a cold-water rinse.

Compared with HOCl (hypochlorous acid), iodophores are not as readily degraded by organics (dirt) and are nonirritating at recommended use levels. I₂ becomes volatile at temperatures greater than 49 C (120 F). Formulations containing iodophores may stain polyvinychloride and other surfaces.

Quaternary Ammonium Compounds
QUATS function by disrupting microbial cell membranes. Modern formulations using QUATS are considerably stronger than their predecessors, and have extended activity over a broad pH range. They have the added advantages of being heat stable and noncorrosive. In wineries QUATS find application in controlling mold growth on walls and tanks. The formulation is sprayed on the surface and left without rinsing. Depending on environmental conditions and extent of mold growth, a single application may last for several weeks.

Detergent-Sanitizer Formulations
Suppliers currently market formulations employing both detergents and sanitizer. These typically include a surfactant ("wetting agent") and other necessary adjuncts discussed previously.

Sulfur Dioxide
In some cases, winemakers use SO₂ as a sanitizing agent. The effectiveness of SO₂ against microbes is pH dependent. Depending on the physical properties of the surface and level of organic debris, circulating a solution of 10 g/hL SO₂ (or 20 g/hL potassium metabisulfite) and 300 g/hL citric acid at 60°C (140 F) may be effective.

Physical Sterilants
Hot (82°C/180°F) water or steam is an ideal sterilant. It has penetrative properties, works against all wine/juice microorganisms, is noncorrosive, leaves no residues and is relatively inexpensive. Where hot water is employed to sanitize bottling lines, it is recommended that temperatures greater than 82°C (180°F) for more than 20 minutes be used. The temperature should be monitored at the farthest point from the stream source (i.e., the end of the line, fill spouts, etc.). When steam is used to sterilize tanks, the recommendation is to continue until condensate from valves reaches temperatures greater than 82°C for 20 minutes. Dismantling valves, racking arms, etc., is desirable for cleaning and sanitizing.

Ultraviolet (UV) light is directly effective against microbes. Unfortunately, it has low penetrative capabilities and even a thin film will serve as an effective barrier between radiation and microbes. Thus, its use is generally restricted to laboratory applications for surface sterilization. Skin and eyes must be shielded (by glass) from continued exposure to UV-light.

Ozone (O₃) is being used in water treatment as a sterilant. Ozone degrades rapidly in warm (>35°C/95°F) water. Thus, at present, its primary application is cold water recirculation systems.

Monitoring Cleaning and Sanitation

Following sanitation, all surfaces must be thoroughly rinsed to flush residual sanitizer and solubilized debris. Tank/hose sanitation is typically followed by citric acid rinse to neutralize residual alkali. The final rinse water should be tested for residual oxidants. This may be done in the laboratory or cellar by use of "kits" designed for this purpose, or by a simple procedure available though my office. The presence of residual sanitizer can often be noted by smelling the rinse water. It is essential that all residual sanitizer be removed prior to wine contact. A very small concentration of sanitizer will cause wine oxidation!

The most frequently encountered method for evaluating cleaning operations is sensory. Visually, does the surface appear clean, by touch, does it feel clean, and equally important, does it smell clean? A slippery surface or the presence of "off" odors is indicative of inadequate cleaning or rinsing of detergent. Although a quick sensory review may be adequate for fermenters and storage tanks, other areas (e.g., bottling lines) require further examination. In these cases, follow-up microbiological examination should be conducted to evaluate the effectiveness of sanitation. A variety of tests are used and involve sampling a defined area with sterile cotton swabs, agar surfaces, or special adhesive strips. For more information on monitoring sanitation, contact my office.

V. Upcoming Events

Wine and Juice Analysis Workshops, January 8 - 12, 2002 at Virginia Tech. Contact Terry Rakestraw at rakestra@vt.edu for more information.

Annual winter meeting of the Virginia Vineyards Association at the Omni Hotel in Charlottesville, VA. Conference will include discussions of vineyard nutrition, disease management, research updates, with speakers from New York, Ontario, and Virginia. See Dr. Wolf's newsletter.

Table 3. Characteristics of Sanitizers.

^aBacterial endospores, mold spores.