At Penfolds, one of Australia\u2019s most well-known wine brands, the bottling line is an elaborate operation, especially its final stage: the seal. While its white wine range is closed with screw caps, the reds receive different stoppers based on where they are exported.\r\n\r\nIn Australia, the majority of Penfolds\u2019s red wines, which include offerings priced well into triple digits, are sealed by screw cap. In the U.S. market, however, those same wines are sealed under natural cork. Why? Because many Americans still believe screw caps signify low-quality wine.\r\n\r\nAs the saying goes, you shouldn\u2019t judge a book by its cover. But should you judge a wine by its closure? Cultural preconceptions aside, the stopper can tell a great deal about the wine it encloses. It can hint at longevity, quality, culture and history.\r\n\r\nWe spend the most of our time focused on what lies within the bottle. For once, let\u2019s get to know what sits above it. Here\u2019s your guide to the world\u2019s most popular wine closures.\r\n\r\n\r\nCork\r\nThe world\u2019s most widely used wine closure has been around for a while. Evidence of its usage dates back to the ancient Greeks and Romans. However, the close-fitting stopper we\u2019re familiar with wasn\u2019t popularized until the 18th century, in conjunction with the first user-friendly corkscrew.\r\nPros:\r\nRenewable resource. Cork is derived from the bark of Quercus suber, also known as the cork oak tree. These evergreens, the majority of which grow in Portugal and Spain, are abundant and strictly protected. Cork oaks regenerate their outer layer of bark, which allows them to be harvested about once every decade. With a lifespan of up to 200 years, one tree can provide cork for thousands of bottles, which makes it the most ecologically sound material to seal a bottle.\r\n\r\nTradition. There\u2019s something romantic and ceremonial about popping a wine cork. The visceral ritual helps cork retain its dominance, even in the face of stiff competition.\r\n\r\nCellar worthiness. Thanks to its elasticity, cork expands within a bottleneck to seal liquid in and keep oxygen out. Its tiny pores, however, allow minuscule amounts of air to interact with the wine, which can transform the aroma and flavor over time. This makes cork the top choice for producers of ageworthy wines.\r\n\r\n\r\nCons:\r\nSusceptible to taint. The chemical compound 2,4,6-trichloroanisole, or TCA, affects mostly wood-derived materials, which makes it cork\u2019s worst enemy. It\u2019s caused when chlorine comes into contact with certain fungi during the cork\u2019s processing. While harmless, the compound can transfer to the wine and cause aromas of wet cardboard, damp basement or wet dog. Known as \u201ccork taint,\u201d or simply being \u201ccorked,\u201d past studies have estimated that TCA affects up to 10% of wines under cork.\r\n\r\nCork taint, however, could soon be a thing of the past. Recently several companies\u2014including the worlds biggest cork producer, Amorim\u2014have unveiled new methods to remove TCA from their natural cork closures, effectively eliminating cork taint from their production.\r\n\r\nVariability. Cork is a natural product, and each is slightly different. Cork brands and their porousness vary, which affects the rate at which air interacts with the wine in the bottle. Some corks can also impart a woody flavor to the wine. While winemakers choose corks carefully, there\u2019s always an element of the unknown.\r\n\r\nFragility. Cork is made from wood, which dries out and crumbles with time. Wines cellared for long periods of time must be kept on their sides to keep the cork damp. But even with careful cellaring, how many of us have fished the remains of a crumbled cork out of our wine after it breaks on its way out of the bottle?\r\n\r\nCost. Depending on the quality and brand, corks can be up to three times as expensive as screw caps, which can drive up the final price of the wine.\r\n\r\n\r\nFrankenstein corks\r\nNot all corks are cut from the same cloth. Meet natural cork\u2019s quirky cousins.\r\n\r\nMicro-Agglomerated. Demand for micro-agglomerated corks, also known as \u201ctechnical\u201d or \u201ccomposite\u201d corks, is steeply rising. At its simplest, an agglomerated stopper is like a cork-particle board, where granulated cork dust is bound together tightly by glue or a plant-based binder and pressure. Some agglomerated corks should only be used with wines destined to be consumed young as they tend to break down more quickly. But other more premium stoppers stand the test of time. One leading micro-agglomerated cork producer, French-based company Diam, makes corks that are free from TCA, impart no flavor, and control the level of oxygen released into the wine. This consistency makes them an attractive choice for many cork-loving winemakers.\r\n\r\nAgglomerated. It\u2019s like cork-particle board, where granulated cork dust is bound together tightly by glue and pressure. Agglomerated corks should only be used with wines destined to be consumed young as they tend to break down more quickly. Cork producer Amorim suggests wines sealed with these composite closures be consumed within six months of bottling, though premium options do exist that allow for longer aging.\r\n\r\nColmated. Made from medium-grade natural cork, the crevices in these closures are filled in with fine cork powder. It gives the cork a softer texture and smoother exit out of the bottle. Wines can age up to a few years under colmated cork.\r\n\r\nMulti-piece. When two or more pieces of cork are glued together. Examples include Champagne corks, which expand upon removal and cannot be placed back into the bottle. Other producers also affix discs of natural cork to the ends of agglomerated cork in still-wine enclosures, in order to increase durability. These corks are often made from the manufacturer\u2019s bark \u201cscraps.\u201d\r\n\r\n\r\nScrew cap\r\nAfter centuries of unrivaled wine stopper bliss, an unassuming aluminum cap came along and, well, screwed up everything for the humble cork. The Aussies are to blame for stirring the pot.\r\n\r\nIn 1964, Peter Wall, former director of South Australian winery Yalumba, became fed up with the number of tainted corks in circulation. He commissioned a French company to develop an alternative closure. An aluminum cap, the \u201cStelvin,\u201d was born, although it wasn\u2019t patented or used commercially until the late 1970s.\r\n\r\nForty-plus years later, screw caps are the closure of choice for the majority of Australian and New Zealand producers, across all styles and price points. It consists of an aluminum cap lined with plastic, which integrates a metal skirt that hugs the top of the bottleneck, just like a traditional stopper\u2019s foil.\r\nPros:\r\nConsistency. TCA, the taint that affects so many wines under natural cork, is almost nonexistent under screw cap. Also, because there\u2019s less oxygen interaction with wines in comparison to cork, winemakers can theoretically reduce the amount of sulfur dioxide used as an antioxidant before bottling.\r\n\r\nLongevity. Because wines under screw cap reside in a relatively oxygen-free atmosphere, they\u2019re believed to be longer lived. Though long-term studies have shown positive results, it\u2019s a controversial topic within the wine industry. Some argue that with limited oxygen contact, wines under screw cap don\u2019t age at all. Others, like Jeffrey Grosset, owner of Grosset Wines in South Australia\u2019s Clare Valley, say that screw cap enclosed wines age beautifully, just more slowly.\r\n\r\nAffordability. Screw caps can vary in price, depending on quality. Generally, however, they\u2019re cheaper than natural cork.\r\n\r\nEasy to open. Screw caps open with a simple twist of the wrist. There\u2019s no need for any gadgets beyond a free hand and a little muscle.\r\nCons:\r\nNegative environmental impacts. Screw caps are made from aluminum, which is often produced from a strip-mined ore called bauxite. Processing aluminum can be a dirty process, negatively impacting the air and water and generating about 70 million tons of waste annually. Aluminum is non-biodegradable, and though it can be recycled, it\u2019s suspected that most screw caps end up in the trash, and individual waste management companies have their own internal guidelines on whether or not screwcaps are accepted as recyclable. Their plastic liners, if not removed, can also make recycling impossible.\r\n\r\nMost screw cap liners are made from Polyvinylidene chloride\u00a0(PVDC), a plastic that\u2019s unsustainable and toxic when burned. Some, like the California Office of Environmental Health Hazard Assessment, also consider it to pose possible health risks. It\u2019s been banned or restricted in a number of countries in Western Europe. Amcor, the company that makes Stelvin screw caps, recently released a PVDC-free range targeting countries like Germany which have restricted the material. It\u2019s still allowed in the U.S.\r\n\r\nProne to reduction. The opposite of an oxidized wine is a reductive one, or a wine in which there is minimal contact with oxygen during the production process. This can happen when a wine\u2019s sulfur dioxide levels are too high and is characterized by telltale rotten egg/oniony aromas. The topic of reduction in screw-capped wines can be divisive, but it\u2019s generally agreed that the plastic-lined seal itself doesn\u2019t cause these undesirable characteristics. However, the seal can prevent additives like sulfur from being absorbed into the wine the way a more porous cork would, exacerbating the effects.\r\n\r\nQuestionable aging ability. Cork versus screw cap aging has passionate advocates on both sides. Cork proponents say that the interaction between wine and oxygen facilitated by the naturally porous material is essential to the aging process of a cellarworthy wine. Screw cap fans think otherwise, though neither side has proved their enclosure is better.\r\n\r\nAmcor has created multiple liners. One offers more permeability at a higher cost, though the company also offers variations on its original liner with four different levels of \u201coxygen transmission rates\u201d (OTRs).\r\n\r\n\r\nSynthetic cork\r\nSynthetic corks can be made either from petroleum-based plastic or plant-based materials. Plastic corks are made generally from polyethylene, a malleable material that is melted down and turned into \u201cfoam\u201d that imitates the porousness of natural cork. Plant-based stoppers are made similarly, but use bio-polyethylene, a bio-based plastic made from ethylene, a byproduct of processing renewable raw materials like sugarcane.\r\nPros:\r\nConsistency. Synthetic corks are not prone to TCA taint. They provide predictable oxygen transfer rates and a tight, immovable seal.\r\n\r\nDurability. Because they\u2019re not made from wood, synthetic corks don\u2019t degrade or dry out, so there\u2019s no need to store bottles on their sides to keep the cork damp. Synthetic cork also won\u2019t break apart, so there\u2019s no risk of cork crumbs to fish out of your wine.\r\n\r\nAffordability. Synthetic cork can be up to three times cheaper than natural cork. They\u2019re often cheaper than screw caps, too.\r\n\r\n\r\nCons:\r\nNegative environmental impacts. Synthetic corks made from oil-based plastics are not sustainable or biodegradable. They can be recycled, in theory, though it often depends on the materials used to make the cork, whether the producer has imprinted the \u201cchasing arrows\u201d logo on their product, and policies of the recycling company used.\r\n\r\nPlant-based stoppers, however, use renewable resources and have a lower carbon footprint than their synthetic counterparts. Made from sugar cane, Nomacorc, produced by North Carolina-based Vinventions, are recyclable and carbon negative. The company is also releasing a stopper made from recycled plastic.\r\n\r\nHard to open and reseal.\u00a0Many wine lovers groan at the sight of plastic-based synthetic cork, mainly because it tends to be the hardest wine closure to open, and some can be virtually impossible to put back into the bottle once removed. Plant-based corks are often softer than plastic, making them easier to open.\r\n\r\nChemical odor. Some wine professionals claim to detect a chemical odor in wines from oil-based plastic corks, particularly if the wine has been in bottle for a while. While reports of these aromas have abated in recent years and don't affect plant-based stoppers, the potential for off odors may be of concern to some.\r\n\r\n\r\nOther types of wine enclosures\r\n\r\nMore wine closures may pop up, or out, of your bottle.\r\n\r\nVinoseal: Also known as Vinolok, this elegant, expensive glass stopper was developed by the Alcoa Corporation, but later handed over to Czech glass producer Preciosa. It was released in the European market in 2003. The glass is ringed with plastic to create a tight seal.\r\n\r\nHelix: A twist-off cork that requires no corkscrew, created in 2016 by the world\u2019s largest Portuguese cork manufacturer, Amorim, and the world\u2019s largest glass-bottle manufacturer, Owens-Illinois Inc.\r\n\r\nCrown cap: The cap of choice for beer bottles, this closure is used by traditional-method sparkling producers to seal their bubbles before disgorging, thanks to the crown cap\u2019s ability to contain pressure. These bottlings are later resealed with cork, wire cage and foil. Many p\u00e9tillant-naturel (natural sparkling wine) producers who don\u2019t disgorge choose to release their sparklers in all their crown-capped glory. Some still wines, particularly those in the natural camp, are even starting to flaunt the crown cap seal.