Quality at the top

- inspecting bottle closures safeguards the quality of a product

The quality of a product is used more and more as a sales argument. Bad quality can result in consumers boycotting a product. There have been clear signs in recent months that this is particularly true in the food processing industry. If a product acquires a bad name it is extremely difficult to rectify the situation even with valid arguments.

The consequences are reflected in the medium to long-term sales figures. In the beverage industry it is becoming increasingly difficult to keep track of the possibilities available to check the quality of a product according to latest technological developments. This is due to the large variety of bottled products and the wide choice in packaging. The product and especially its packaging are determining factors regarding the methods of inspection available. The following is an introduction to checking a bottle closure. A selection of possible inspection methods is discussed with regard to the most widely used types of closures: the obvious method of checking that the closure is correctly positioned and how to acquire additional information by analysing the physical parameters available in detail.

Glass bottles with crown corks

Crown corks are (still) the most widely used type of closure in Europe for brewery products. It is precisely in this area that the choice in quality control techniques is the greatest. We distinguish between an inspection immediately following the filling procedure and one after pasteurization.

Detection after the filler

Crown corks can be examined here both optically and acoustically. Ideally a check device should be installed at the outfeed of the filler so that the measuring results of the individual bottles can be allocated to the respective filler valves i.e. closer heads. This correlation provides important information about the operativeness of the filler/closer block and facilitates purposeful maintenance. Not only are faulty containers removed from the stream of production but the filling process can be optimised so that faulty containers are not produced in the first place.

Acoustic check

The O2 residual air analysis evaluates the complex oscillation pattern of a crown cork by means of an acoustic check. In order to produce this signal the closure is stimulated by an electromagnetic pulse. This then oscillates in a characteristic superposition of proper frequencies similar to that of a drum. The following information can be produced by analysing this signal:

1. The presence of a closure - striking does not produce a signal i.e. the closure is missing.

2. The shape of a closure - if the signal is irregular i.e. not an real "taut drum", then the closure is deformed (so-called bullnose). Unfortunately the reverse is not necessarily true. In some cases the oscillation of an irregular closure can be very similar to that of a correct closure.

3. The tautness of a closure - statistical information is provided regarding the correct pressure applied by the individual closer heads. If a closer head shows a significant difference then the correct pressure is probably not being applied. This condition, which is usually only found a few hours later by carrying out a sampling test, can now be quickly corrected.

The following point is particularly important regarding the quality of beer and should therefore be discussed in more detail:

Oxygen content in the headspace

The oxygen content in beer is an important criterion regarding quality but unfortunately this is extremely difficult to measure during high output without disturbing production. The amount of oxygen in the beer can be checked in the tank or in the supply lines. However the amount of oxygen which is produced during the filling process must be added to this. This represents the majority of the total oxygen content. The bottle is inevitably exposed to the normal atmosphere for a short time between the filler and the closer and as a result oxygen contamination occurs. Admittedly an attempt is made to squeeze this out of the bottle by foaming up its contents using a high-pressure injection system. This foaming up process is in itself turbulent and as such very difficult to control. On the one hand the bottle must be completed foamed up in order to squeeze out the last air bubble but on the other hand the product must not be unnecessarily wasted or worse the bottle underfilled. Relevant test results show that it is not always possible to avoid this last air bubble. A foaming up process which has been optimally adjusted can reduce the fault rate to approximately 0.1 per mil. However random sampling cannot determine whether or not the adjustment is correct. For example an optimally adjusted bottling line producing 50,000 containers/hr, operating in two shifts, five days per week, still produces 20,000 containers containing excessive oxygen annually which reach the consumer. This is equivalent to 20,000 potentially dissatisfied customers. The residual air check O2 can "hear" these air bubbles, which are only visible when the bottle is tilted, beneath the closure by accurately analysing the tone.

Optical check

During the optical check the closure is examined for irregularities using a camera. Bullnoses and protruding chips can be identified. Bullnoses which can be distinguished optically are detected. The acoustic check is an ideal way of complementing this because there are certainly bullnoses which are optically very different from a correct closure but sound practically identical acoustically and vice versa.

Detection after the pasteurizer

The most important function of a closure is to seal the container and to reduce the loss of carbon dioxide as much as possible. Crown corks can be examined acoustically to check that they also meet with these requirements. Again the closure is struck in a similar way to that of a drum; the internal pressure changes the tautness of the drum and thus its characteristic proper frequency. This proper frequency depends on many other factors e.g. the finish shape of the bottle, the pressure applied by the closer head, the material of the closure etc. In order to make a reliable distinction between a good and a faulty container, a relatively high difference in pressure between the inside and outside of the bottle is necessary, to achieve a meaningful measuring result. The filling temperature is usually around 5-7ºC and the pressure build-up in the bottle due to the evolution of CO2 gas is a very slow process. Therefore the necessary difference in pressure can generally only be established after the pasteurizer. Without the pasteurizer the bottle has not reached its equilibrium pressure by the time it is on a pallet in the warehouse ready for delivery.

Synergistic effect

An acoustic measurement taken after the filler and another at a later stage provide valuable information about the quality of a closure. This is a very reliable way of detecting faulty bottles. However, for an almost full inspection, it is desirable to have two measuring results from the same bottle in order to compare them. Measuring faults as a result of external conditions can be largely excluded by analysing the development of the pressure build-up in the bottle. The problem is thereby reduced to the container specific transfer of this information from the measurement taken at the filler to another point. Conventional container tracking over multi-lane conveyors and through a pasteurizer is not possible in this case. A "message in a bottle" system is possible whereby the bottle itself is used to carry the information. The first measurement taken after the filler is translated into a code onto the closure (the tinplate crown cork is used as a disk) and this information is then read from the closure at another point and included in the evaluation of the bottle. In this way pressure measurement gains significant clarity.


The magnetic property of the closure is a prerequisite for the acoustic procedures mentioned above. Therefore porcelain cliplocks have to be ruled out for obvious reasons. However it is precisely here that problems with leakages due to faulty or even missing sealing rings exist. The answer is a completely different method. As previously mentioned the build up of the balance between the vapour pressure in the headspace and the carbon dioxide content in the liquid is a very slow process. At the point of closure there is approximately the normal external pressure in the headspace. There is a large amount of carbon dioxide (approximately 5g/l) dissolved in the liquid and the pressure in the bottle now slowly increases until it reaches equilibrium pressure which is strongly dependent on the temperature. At 7ºC this is about 1 bar. We take advantage of the slow speed of this process by engaging the "turbo" in the liquid by the coupling of ultrasonic waves. In this way the build-up in pressure can be accelerated quite considerably. During this process a large amount of foam is produced. This in itself is not an advantage. However the decisive factor in a closed system (= tight containers) is that the state of equilibrium is achieved in a short time and the process stops. By comparison in an open system (= leaking containers) CO2 constantly escapes, the state of equilibrium cannot be achieved and the process continues with a constant liquid loss. This loss of liquid can be easily detected at a later stage and the bottle is automatically removed from the stream of production. This detection method can be used for all types of closures but it is particularly effective for cliplocks.

Plastic closures

This type of closure should be taken into consideration for the sake of completeness even if the filling of beer into plastic bottles is not yet very widespread in the German-speaking countries. A combination of several procedures is advisable for inspecting plastic closures. Leakage tests can be carried out with the foaming up procedure using ultrasound and/or camera techniques. These methods are very useful since most plastic closures are provided with tamper evidence rings. An undamaged ring is a clear sign for the consumer that the container has not been tampered with. If it is damaged the consumer is in doubt as to the genuineness of the product. Therefore it is advisable to check the tamper evidence ring in order to avoid putting the consumer´s trust in the product at risk. Cameras which only examine the closure area show a very small section of an image. If this section of the image contains clearly defined marks (e.g. the neck ring) then very accurate comparative measurements can be made regarding the correct position of the closure and the intactness of the tamper evidence ring irrespective of the actual height above the conveyor belt.

If the body of the bottle is also plastic then a further procedure is possible in order to find leakages. The bottle is taken through a belt drive at production speed as has been customary with the linear bottle inspector for years now. This belt exerts a well-defined pressure on the bottle. If the bottle is tight then the internal pressure produces a counter pressure which only allows the bottle to be slightly compressed. However leaky bottles are by comparison compressed much more. This results in an increase in the fill level. It is now decisive to carry out a comparative measurement again. A fill level measurement without pressure load is carried out first and then a second measurement with pressure load in the belt drive. If the fill level has increased significantly then the bottle is not tight. In this way leakages are found which are impossible to detect optically.

Summary and preview

It is possible with today´s advanced inspection technology to ensure that the consumer receives a perfect product. In order to take full advantage of the possibilities available it is essential to use the method of inspection suitable for the particular product. The wide range of container types and the varying functions required result in many different technical solutions, each with their specific advantages and disadvantages, which have only been briefly mentioned in this article. Before making a decision to buy inspection equipment detailed advice should be sought from specialists in this field so that the most suitable system is chosen.

The possibilities will continue to develop in the future with improved sensor technology and more precise methods of analysis. Research into further physical effects will provide additional information about containers. Consequently the brewing industry will always have better possibilities of guaranteeing that good taste reaches the customer.

Translation of a report published in the "Brauindustrie" journal, July 2001edition, pages 26-29