Recent developments in wine packaging have introduced a number of new sources for premium wine bottles.

Along with the benefits of these diverse resources, we have seen an increase in the occurrence of mismatched corks and bottles - often leading to unfortunate consequences.

When most glass companies report internal diameter they refer to the “C dimension”. This is basically the top 5mm from the mouth of the wine bottle. Their typical quality control procedures are targeted at this value. Diameter measurements from further down the neck sometimes exhibit wildly different dimensions. Different bottles have different rates of taper. This can be a matter of design, or sometimes a matter of manufacturing variance.

Irregular diameter or excessive taper can be quite detrimental to long-term wine aging. Problems occur when the bottleneck is too wide at the bottom of the cork. When this case, the seal at the bottom of the cork can be compromised, and wine may seep around the sides. This will weaken the overall sealing capacity of the cork and is likely to cause leakage.

The table below compares the internal dimensions of nine commercially available wine bottles. Bottle “C” starts at 18.4mm and tapers to less than 21mm at a depth of 50mm. Bottle “D” will be a problem as its diameter exceeds 21mm at a depth of only 30mm.

Internal diameter at the bottom of the cork should not exceed 21mm. A maximum diameter of 20.5mm is recommended.

A handy tool for making internal diameter readings is the digital caliper from Fowler (item# 54-554-623). It is priced less than $160 by these three distributors. Coast Tool Company Gaging.com Aronson-Campbell

Leaking wine bottles are often erroneously blamed on poorly performing closures, but unless the closure has serious physical defects, the reason for leakage is usually due to improper bottling practices and excess bottle pressure after bottling.

Wine bottle drawings from glass manufacturers show the suggested fill point for wine at 68ºF. The fill point is measured as the distance from the top of the bottle to the correct wine level in the bottle. These figures do not absolve the winery from their requirement to have a legal fill. They do, however, provide a good idea as to where the correct fill point should be. Generally, the fill point on the
750 ml bottle at 68°F will be approximately 64mm from the top. It is always best, however, to consult the drawing as a +/-3mm variance is possible.

The throat diameter of a standard, American 750ml bottle will vary slightly in the ullage area. On average, however, it is fair to say that the ullage with a 49mm cork and a 64mm fill height will average 4.8 ml. in volume. For a 45mm cork the correct volume would be 6.5 ml.

The key point here is that good bottling is the key to good results with corks. If the winery bottles at legal fill heights and with adequate vacuum to assure that there will be no more than 2 pounds relative pressure in the bottle at 68°F, it is very unlikely that the customer will ever complain about leaking corks.

The classic reason wines develop excess pressure in the bottles is that the bottles were overfilled in the first place. When this is combined with pressures of warming and expanding wine, leaking can become inevitable.

Temperature Effects

Based upon figures from "Principles and Practice of Winemaking” by Boulton et al, the thermal expansion of wine between 20ºC [68ºF] and 40ºC (104ºF] is 0.8%. This doesn't sound like much. It does, however, convert to .166 ml in volume per degree Fahrenheit. Thus, if a winery bottles at
58°F with 4.5 ml in ullage, that ullage will be reduced to under 3 ml at 68ºF and internal bottle pressure will have risen significantly.

There are three ways to achieve proper ullage levels and bottle pressure.

• Bottle wine at 68ºF and fill to the level designated by the bottle manufacturer and confirmed by the winery.
• Adjust the fill level to compensate for temperature differences. A good rule of thumb is to adjust the fill level by 0.55mm for every degree Fahrenheit above or below 68ºF.
• Adjust vacuum levels to compensate for temperature differences. This method seems less reliable than adjusting fill levels because it places so much responsibility on the performance of bottling equipment. Internal bottle pressure needs to be equivalent to less than 2psi (relative) at 68ºF.

Management Suggestions   

One way of dealing with ullage calculations is for bottling managers to chart out target fill heights and internal bottle pressures by bottle type in advance of bottling. Though this will not eliminate their responsibility for a "legal fill'', it will provide an excellent guideline for good bottling.

It is also critical that wineries keep good ongoing records during the bottling day. At a minimum, the following protocols should be observed.

• Freshly corked wines from each corker head should be checked at a minimum every hour for internal pressure [suggested interval is every 30 min].
• Quality control should not rely on the temperature gauge at the filler. A thermometer should be dropped into one bottle ex-filler every half hour.
• If bottling line Q.C. tests bottles that are out of spec for fill or vacuum at a specific temperature, the associated product should be quarantined, [preferably] flipped upright and checked out. Only when the problem is resolved, should cases be returned to regular inventory.
• Q.C. should always check out the readings on cork probe gauges against one another in the morning and again at noon.
• These gauges should also be used to check the functioning of the corker gauge [not vise versa].
• If there is a problem with the vacuum on one or more of the corker heads, the line should be stopped until it is cleared. This should not be done "on the fly".

Legal fill levels are an important requirement. We recommend the following process:

Cork recovery after bottling is rapid but not instantaneous. It takes a compressed cork about 5 minutes to achieve 90% of its expansion in the bottle. The balance of natural expansion takes place in a matter of hours. This is the reason why cork companies strongly suggest that freshly bottled wines remain neck-up for 5-10 minutes after bottling.

When bottles are turned over immediately after bottling, the corks are at risk to develop seepage along the sides of the partially contracted cork. This problem is exacerbated with large diameter bottles or problems with high pressure in the bottle related to ullage calculations or temperature.

Long-term storage is traditionally conducted with bottles pointing neck-down to retain moisture on the cork. Despite the tradition, our surveys show that half of the responding wineries store some portion of their inventory neck-up. The consensus of CQC member companies is that this is perfectly fine. Winery storage is usually less than two years, and is normally practiced in cool, moist environments.

CQC members actively encourage wineries to store wines neck-up if they have any doubts about glass size or bottling conditions.

These specifications outline the general physical and chemical characteristics of cork stoppers as reviewed by the CQC. Specifications are reviewed for compliance by a combination of facilities in California and Europe.

Dimensions - Dimensions are measured to ensure the correct specification is maintained as agreed with the Buyer. It is important to ensure the function of sealing the wine and adequate extraction of the cork stopper. Method: ISO 9727-1

Specification: Diameter ± 0.5 mm Length ± 1.0 mm ± 0.5mm

Moisture content - At low moisture values, the stopper loses its mechanical properties, while high moisture values may promote microbial growth. Method: ISO 9727-3. Specifications for 1+1 corks are measured at the disk.

Specification: Minimum >4.0% ± 0.3% Maximum <7% ± 0.3%

Liquid seal capability - To ensure proper seal capability of the wine to prevent leakage.
Method: ISO 9727-6

Specification: 1.2 Bar (120 kPa) internal pressure

Dimensional recovery after compression - Good resilience enhances good corking impermeability. Method: ISO 9727-4

Specification: Greater than 90% recovery by diameter after five minutes.

Note: Corks are not capable of being compressed by more than 35% of their diameter (at 24 mm diameter the cork stopper must not be compressed to a diameter of less than 15.5 mm before and during insertion).

Extraction force - To ensure adequate extraction force is applied so that the cork stopper can be easily removed. The stoppers' hold should easily allow the normal insertion of the corkscrew. Method : ISO 9727-5

Specification: The amount of force required to remove a 45 x 24 mm cork shall be between 15–45 daN.

Note: extraction measurements taken 24 hours after bottling.

Peroxide residues - Where used, a high level of residual peroxide may impact adversely on the level of sulfur dioxide in the wine. Method: qualitative analysis involving the reaction of potassium iodide with residual oxidants in the presence of a starch indicator.

Specification: No positive results

Dust - High levels of dust may have an impact on equipment performance and appearance.
Method: ISO 9727-7

Specification: Dust as defined by fine particles of natural cork material will not exceed 2mg/cork.

• The 4-segment, sliding jaw type cork compression system is recommended. Roller or iris type jaws tend to cause wrinkles in the cork that can cause leaking.

• Corking machines are maintained to the manufacturer's recommended standards at all times.
• Maintain lubrication schedule.
• Smooth action in compression stage.
• No nicks or other damage to the jaw segments.
• Good alignment and seal of bottleneck in centering bell.
• Properly centered plunger.
• Daily cleaning and sanitation of handling surfaces; i.e. hopper, feed tube, orienter, and jaws.
• A 24mm cork should not be compressed to less than 15.5mm.

Cork Handling and Storage:

• Do not open plastic cork bags until immediately before loading corks into the loading machine. No bags containing corks should be left open for any reason.
• Corks recovered from the corking machine after the bottling is completed should be returned to the plastic bag or another closable container, "dosed" with sulfur dioxide gas (vapor) and sealed tightly.
• Corks should be stored in sealed containers in a cool dry location, not in a bottling room, barrel storage area, or chemical storage area. The temperature should be 55 to 70 ° F and the humidity 50 to 70% and the atmosphere be free of haloanisole contamination.

Moisture Content:

• New shipments of cork as well as corks, which have been stored for extended periods of time, should be checked for moisture content before use. Corks below target moisture levels should be discarded or returned to the supplier for re-hydration and sterile packaging.
• Corks with average moisture content of over 8% should be regarded with suspicion as such a moisture level could support mold growth.
• Inner neck of the bottle must be dry.

Internal Bottle Pressure:

• Wine temperature should be between 60-70 ° F. If lower temperatures are used then the fill point should be adjusted down to compensate for expansion in the bottle when room temperature is reached. Be sure to maintain legal fill volume. Consult bottle specifications and maintain proper fill volume (do not overfill).
• If the fill point is too high, less vacuum can be achieved and internal pressure will increase.
• The vacuum system should be well controlled and maintained. Gauges that continuously display vacuum status at the corking head should be monitored. Frequent (each ½ hour) online QC of corked bottles (pierce test) are highly recommended.
• Bottles should remain upright for 5 to 10 minutes after corking
• It is recommended that the above elements be combined to produce a net effect of no more than 2 psi internal bottle pressure at 68 °F.
• Any suspicions or problems should be addressed immediately, and suspect inventory should be stored neck up.