Factors affecting wine closure selection, By Carlos Macku, Ph.D., and Kyle Reed, Ph.D., Cork Supply Group, from PWV
01/18/2011
by Carlos Macku, Ph.D.,
Kyle Reed, Ph.D.,
Department of Technical Services,
Cork Supply, Benicia, CA
Until the early 1980s, natural cork was basically the only material used by the wine industry to close a bottle of wine. Cork was the only choice because it was considered the perfect material, unchallenged by centuries of winemaking practice. Then in 1981 and 1982, Swiss scientist Hans Tanner and his team published research that made the landmark association between 2,4,6-trichloroanisole (TCA) and cork taint.1,2
Winemakers have always been well aware of cork taint, but the problem was considered a very minor annoyance due to its very low rate of incidence. However, anecdotal accounts have reported that, since the end of World War II, the rate of taint climbed close to double-digit figures, particularly during the 1970s and 1980s.
This situation, understandably, created much angst among winemakers, while at the same time leading to an epic struggle to find a satisfactory means of wine packaging. During the ensuing years, innovative and entrepreneurial individuals worked hard to develop reliable and consistent wine closures that kept precious wine vintages away from oxygen with no negative effects.
At the same time, commercial globalization and the renaissance of scientific winemaking have added product convenience and quality consistency to the potential benefits of new and daring closure alternatives to natural cork. Today, producers and consumers can choose from many types of wine closures. Wine packaging (probably one of the most challenging of all food barriers) has certainly evolved from the days when the product was transported, stored, and sold in Egyptian amphorae or medieval wooden barrels.3
For the last ten years, three types of wine closures have clearly dominated the market: natural corks, synthetics, and screw caps or screw tops (also known as Roll On Tamper Evident or ROTE). Other types of closures are also available, such as Vino-Seal, and Zork. Nevertheless, this article will focus on the first three types of closures and establish useful comparisons between the three systems.
Natural Cork
Closures made from cork material come from the bark of Quercus suber, a native oak tree that grows in the Mediterranean region. Most natural cork closures are manufactured in Iberia, primarily in Portugal.
The bark is harvested every nine years from mature trees between the months of June and August. Corkwood, once aged, conditioned, and cleaned, is sliced in sections proportional to the length of the closure and punched parallel to the axis of the trunk of the tree (Figure 1). Then, the corks are rectified (sanded), peroxidewashed (to whiten them), sorted by visual grades,4 and bagged in quantities of usually 10,000 corks (a bale). At this point, the corks are exported to wine-producing regions throughout the world where they are then printed, moisturized, and treated (usually with paraffin and silicone).
The spent corkwood, a byproduct of the natural cork punching, is not wasted but used to manufacture “technical” corks. The material is ground and the cork granules are sorted by particle size. Through various cleaning techniques, the particles are washed to reduce or eliminate any traces of potential taint and/or other unusual aromas. The cork granules are finally glued together to make micro-agglos, 1+1s (dual disk), and champagne cork closures.
Synthetic Closures
Synthetic closures first appeared in the early 1990s and were probably the first wine sealing alternative that significantly competed with natural cork. They are mostly made out of a combination of polyethylene and other trade-secret adjuvants. There are two types of synthetics with two distinct manufacturing technologies: injectionmolded and co-extruded polymer.
Some of the first attempts to create a synthetic closure yielded products that were difficult to extract from the bottle, lacked the resilience of natural cork and therefore created a few problems during and after bottling. Other issues included excessive oxygen transmission rates, resulting in premature wine oxidation. However, by the late 1990s, most of these problems were being resolved and today synthetics are a popular choice for wine products consumed within a short timespan after bottling.
Screw Caps
The idea for sealing wine with a film barrier under an aluminum cap (that emulated a classic bottle capsule) was first developed during the late 1950s and early 1960s. Screw caps with various types of liners had been used on dessert wines and low-cost jug wines during the following years, and were sporadically applied to higher quality wines with mixed results.
Early attempts met much consumer resistance because they had a long association with cheap finished products. Also, the first screw cap liners created some problems during wine ageing, because the barrier requirements were not totally understood and perfected. Even though the screw cap still is an evolving closure, by the turn of the 21st century, it became the wine closure of the new millennium and was gaining wide acceptance in Australia, New Zealand, and Great Britain.
Today, there are basically two types of screw cap liners made with Saran™, (polyvinylidene chloride [PVDC]), an extremely heavy fiber with a remarkable barrier against water, oxygen, and foreign aromas. The Saran/Tin liner is a lamination of Saran™ combined with an impervious thin layer of tin metal. The Saranex™ liner has multilayered films of Saran™, co-extruded and integrally sandwiched between outer layers of other polymers. The Saran/Tin liner provides a greater barrier against oxygen than the Saranex liner.
Considerations in choosing a closure type
Personal, financial, and technical reasons lead winemakers to choose one type of closure over another. Some winemakers are traditionalists and appreciate the qualities of natural cork, even if they have to accept a low rate of tainted bottles. Others will require total product consistency and would probably only look at man-made closures such as synthetics or screw caps. Environmental concerns also play a big role in the decision-making process. Closure cost and sub-type classification differences, particularly for natural corks, are as wide as differences in the prices of wine offered by the market.
Certain technical considerations must be accounted for before, during, and after bottling. Wine style and preparation, expertise during closure application, equipment availability and/or investment, closure robustness, and the logistics of product storage, transportation, and commercialization must all be kept in mind in selecting a closure type.
Comparisons
A systematic approach for looking at closure selection includes five factors.
· Closure Taint
This might not be the most important reason for closure selection, but it certainly has been the original impetus for why, today, there are multiple choices of wine closures. TCA, probably the most significant cork taint, is a biochemical transformation product that starts in the bark of the cork tree.5 The process has been summarized in Figure 2. This pathway of formation was ironically encouraged by the use of chlorine as a bark disinfectant and cork bleaching agent.
However, this practice was discontinued by the mid-1990s, and increased quality control gates and improved processing procedures were developed. Today, taint levels have come down to an average rate of 1% to 2%, depending on what releasable TCA (RTCA) level is considered unacceptable. 6
Therefore, it is important to understand that there will be some degree of TCA taint if natural cork is chosen to close a particular bottling. However, this does not mean that wines closed with other types of closures are totally free of any threat of taint. The raw materials used to manufacture synthetics and screw cap liners have adsorbing capabilities for volatile organic compounds (VOCs) which can migrate from packing materials (cardboard and wood pallets), contaminated warehouses, and cargo containers.7,8
Adsorbed VOCs can be a significant sensory threat to food, food packaging, personal care products, and even medications. 9,10 Therefore, it is recommended that synthetics and screw caps warehoused for prolonged periods of time be tested for potential contaminants, the same way natural cork is QC-tested for TCA upon arrival in the U.S.11
· Oxygen Transmission Rate (OTR)
This is probably the single most important factor to consider during closure selection. Oxygen transmission is the amount of oxygen gas that permeates into the sealed wine through the applied closure. It is typically expressed in cubic centimeters (cc) of oxygen per 24 hours per closure.
Figure 3 gathers and compares OTR values of natural and technical corks, synthetic (extruded and molded), and screw cap (Saran/Tin and Saranex™) closures.12,13 The OTR values in Figure 3 represent only estimated OTR values that were collected from literature references, product specifications, and Cork Supply analysis (mostly on natural and technical corks). The order of closure type (based on OTR values) is more accurate and should be taken into consideration rather than focusing on specific or discrete oxygen transmission values.
It is important to understand that closure OTR values have been determined using various types of analytical techniques causing disagreement between suppliers and scientists. Number of replicates, attention to detail during testing, and closure application on a glass sleeve (if MOCON technique is chosen for measurement) can have a significant impact on the final OTR result. OTR values can be measured based on pure
oxygen or air (20% oxygen). Therefore, measured OTR values based on pure oxygen are five times larger than values obtained with air.
Figure 3 includes a wide range of oxygen transmission rates for natural corks (from about 5.0 x 10-4 to about 1.0 x 10-2 cc/day/closure). As the visual quality of the cork moves from the best to least, the average OTR value will become larger due to a greater incidence of individual outliers (the OTR standard deviation will widen as the number of lenticels and imperfections increases in a lot of natural corks). This is why wine closed in bottles with natural corks from the very same lot might evolve into products with slightly different flavor nuances.
The cork industry is working to develop ways to sort natural corks, not only by visual grades but also by oxygen transmission rates. In the near future, winemakers may see further reduction in cork taint and improved OTR consistency with natural corks.
There are synthetics (extruded and molded), technical corks, and screw caps with Saranex™ liners with decreasing OTR values (values of around 7.0 x 10-3, 3.0 x 10-3, and 1.0 x 10-3 cc/day/closure, respectively). OTR values for technical corks are more discrete (narrow ranges) than natural corks, while there are wide ranges for synthetic and screw cap (Saranex™ liner) closures that are due not to product inconsistency (just the opposite) but rather due to the wide selection of availabile products (with several OTR values).
On the other extreme of the spectrum are Saran/Tin screw caps with impermeable tin layers. Given the low OTR nature of Saran/Tin liners (approximately 1.0 x 10-4 cc/day/closure), screw cap application requires careful consideration due to the potential for wine reduction to occur during storage and cellaring. Reduction is a chemical process which occurs in the absence of oxygen resulting in unpleasant aromas due to the formation of certain sulfur-containing volatile compounds.
It is important to consider how the screw cap liner is shaped during closure application in order to protect the wine from oxygen. As the closure is put in place during bottling, the aluminum cap presses the wafer-shaped liner against the glass bottle by thinning and stretching the circumference of the liner over and around the rim of the bottle neck in a “reform” process. Depending on how much head block pressure is applied, the density of the polymers sandwiched between glass and aluminum will determine the actual oxygen transmission through the molded barrier.
It is sometimes difficult to report a discrete OTR value for screw caps (Saranex™ and Saran/Tin) as product specification. In this respect, it is more appropriate to report a range of OTR values. Nevertheless, if proper QA procedures are in place, screw cap application should provide expected product consistency within a bottling event.
• Closure Application
Equipment for natural cork (which can also be employed for technical cork and synthetic closures) has been available for years, so technical assistance is easy to find when adjustments and repairs are necessary.
In contrast to the equipment built for insertable closures (naturals, technicals, and synthetics), the entrance of screw cap machinery into the wine industry is fairly recent and carries a higher degree of technical sophistication. Head block, thread, and skirt roller pressures, type and height of rollers, and exact dimensions and angles of glass bottle design in relation to the dimensions of the aluminum screw cap are all important factors to observe in order to achieve desired bottling results.
There are differences in performance across the range of natural corks and synthetic closures. Hence it is important that bottling conditions are optimized for each closure type. Otherwise issues such as “suck ins,” “push outs,” and leakage can occur. Improperly maintained equipment (particularly corker jaws) can result in closure damage during bottling leading to leakage and other problems.
Screw cap equipment is not only costly, but requires exact precision during application. Screw cap equipment with improper settings can not only lead to ugly external applications, but seriously compromise the sealing specifications of the delicate top liner. To make things more complex, these mistakes might not be apparent on bottling day but later during product storage. However, like other critical characteristics, these specifications can generally be controlled with good bottling QA protocols.
• Closure Robustness
The true quality of a wine closure is shown when it is submitted to abusive conditions while delivering the expected results. Specification deviation not only can occur as a consequence of the bottling event, but also during final product storage and transportation. Proper bottling temperature, fill height (ullage), and final headspace pressure all need to conform to the specifications of each closure/bottle combination. In the case of inserted closures (natural cork, technical cork, and synthetic closure), appropriate bottle neck diameter and taper are required for an optimum seal.
Periodically, glass bottles with unexpected neck tapers appear in the market. These bottles can seriously affect the seal of some of the longer-length corks, resulting in closures with no or very low compression values at the bottom end, which can cause cork wicking.
It is always advisable to confirm neck diameter and taper with a bottle neck profile analysis. In the case of screw caps, besides the potential defects outlined under closure application, it is important to procure glass bottles with even neck rims and no top surface imperfections; otherwise partial liner compression and cut-through could occur.
If expansion due to elevated wine temperatures during distribution exceeds the headspace volume, seal failure will occur. When wine bottles are stored and/or transported at higher than recommended temperatures, natural cork typically shows signs of wicking or leaking, while synthetic closures often push up or leak. Bottles closed with screw caps require extra care during industrial handling so the top part/circumference is not dented. If so, the seal of the liner could be compromised, resulting in random premature wine oxidation.
• Environmental Considerations
One final consideration is the impact of the closure on the environment. Recently, this impact has been measured as the carbon footprint contributed by one (or one million) unit(s). Studies have indicated that the carbon footprint values of different closure categories claim varying results. However, it is generally accepted that natural corks contribute the least, followed by synthetic closures, and technical corks, with screw caps contributing significantly more.14,15
Bibliography
1. Tanner, H., C. Zanier, and H.R. Buser. 1981 “2,4,6-Trichloroanisol: Eine Dominierende Komponente des Korkgeschmackes.” Schweiz. Zeitschrift für Obst- und Weinbau 117: 97. 2. Buser, H.R., C. Zanier, and H. Tanner. 1982 “Identification of 2,4,6-Trichlroroanisole as a Potent Compound Causing Cork Taint in Wine.” J. Agric. Food Chem. 30: 359.
3. Taber, G.M. 2007 To Cork or Not to Cork. New York: Scribner, Chapter 1.
4. Pereira, H. 2007. Cork: Biology, Production and Uses. Amsterdam, The Netherlands: Elsevier, Chapter 14.
5. Simpson, R.F. and M.A. Sefton. 2007 “Origin and Fate of 2,4,6-Trichloroanisole in Cork Bark and Wine Corks.” Aust. J. Grape Wine Res. 13: 106.
6. Herve, E., S. Price, G. Burns, and P. Weber. 1999 “TCA in Corks, Cork soaks, and Bottled Wine.” ASEV Annual Meeting, Reno, NV.
7. Tindale, C.R., F.B. Whitfiled, S.D. Levingston, and T.H.L. Nguyen. 1989 “Fungi Isolated from Packaging Materials: Their Role in the Production of 2,4,6-Trichloroanisole.” J. Sci. Food Agric., 49: 437.
8. Ramstad, T. and J.S. Walker.1992 “Investigation of Musty Odour in Pharmaceutical Products by Dynamic Headspace Gas Chromatography.” ANALYST, 117: Aug.
9. www.bloomberg.com/apps/news?pid=newsarchive&sid=aFUzlh.mQf84.
10. www.jnj.com/connect/news/all/20100115_100000.
11. Macku, C and Reed, K. 2010. “A Practical Screening Method to Determine the Gradient and Source of Common Halogenated Anisoles in Tainted Indoor Environments” J. Agric. Food Chem. (Submitted)
12. Cork Supply USA (Internal Data).
13. Peck, Jim. G3 Enterprises (personal communication) 2009.
14. Forgues, C. September 2007 “Carbon Appraisal of the Different Methods Used for Closing Still Wines.” Aust. & New Zealand Grapegrower & Winemaker, 524.
15. Walker, L. August 2007 “Cork and Sustainability.” Wines & Vines.
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