Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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FIGURE 1: Perpendicular section of the Quercus suber tree showing wood, cambium zone, and corkwood. Natural corks are punched from the corkwood in a direction parallel to the axis of the tree.
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.
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
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