Gases and beverages
An overview of the most important gas applications for the beverage industry
There are important processes in the beverage industry which are only possible with gases. Carbonation of soft drinks and beer has been the most important application for over a hundred years. Gases also help to protect product quality, particularly during the inertisation of tanks as well as filling operations. Without nitrogen for pressure stabilisation, the storage and transport of non-carbonated beverages would be more complex and expensive. For some years now, there has also been an increasing use of gases in winemaking, where they facilitate the production of top-quality wines.
Carbon dioxide (CO2) has been used in the production of mineral water and beer since 1879, the year in which CO2 started to be industrially produced. In the meantime, the list of beverages that have CO2 added to them – in other words, which are carbonated – has grown significantly. Other gases, such as nitrogen (N2) and argon (Ar), have also become indispensable in beverage production and filling. Besides carbonation, gases are primarily used for inertisation – which means in this case the displacement of oxygen – and for pressure stabilisation. In modern winemaking, they are used for stabilisation, as protection against oxidation and for cooling the mash.
What gases can do
Carbon dioxide can do more than just making beverages sparkle. The gas is colourless and odourless and readily dissolves in water. In high concentrations, it has a bacteriostatic effect and therefore extends the shelf life of beverages. Rising CO2 bubbles displace the air above the surface of the beverage and thus reduce oxidation. When used as dry ice with a temperature of minus 78 degrees Celsius, CO2 can provide over 570 kilojoules of cooling energy per kilogram.
Nitrogen and argon have a very low solubility in water, they are neutral and hardly react or don’t react at all with other substances. Furthermore, they are colourless, odourless, tasteless and non-toxic. They can displace the oxygen from a container and, as inert gases, prevent oxidation of the beverage. Nitrogen and argon are, like carbon dioxide, naturally present in air and are approved as food gases. Precisely dosed oxygen is used for controlled oxidation in winemaking.
Carbonation is the term used to describe the process of dissolving carbon dioxide in liquids such as beer (approximately five grams per litre), soft drinks (five to nine grams per litre) and sparkling wine (approximately 2.5 grams per litre). The degree of carbonation, i.e. the quantity of dissolved CO2, depends on the pressure, the temperature, the air or oxygen content before the process, the surface area and the time period. The liquid should be degassed prior to carbonation. The temperature should be as low as possible during this process so that it can take place at a low pressure. Moreover, it is possible to save time if the area of contact between gas and liquid is as large as possible.
As all beverages consist largely of water, solubility in water is hereafter used as a reference point. It ranges from four grams per litre for beer or mineral water up to fourteen grams per litre for sparkling wine or champagne.
The sparkling effect, which is often highly appreciated, is created precisely because carbon dioxide readily dissolves in water. Alcohol or sweeteners can influence the saturation pressure of CO2. Solubility also depends on pressure and temperature. The equilibrium pressure for seven grams of CO2 in a litre of water is 2.5 bar at a temperature of five degrees Celsius, for example.
In the simplest case, the product container is pressurised with CO2. Depending on the pressure and temperature, the CO2 dissolves in the product until it reaches the saturation point. This principle dates from the time when industrial production of carbonated beverages began and, for technical processing reasons, it is mainly used for beverages with a high CO2 content. Systems with static mixers or nozzle systems, make it possible to dissolve the CO2 in the product in-line. This happens largely irrespective of pressure and temperature. In many cases, it is possible to use the cooling energy provided by the CO2 tank installation’s latent heat of vaporisation to save energy when cooling the product prior to carbonation.
Sparging involves the introduction of gas into the liquid with the aid of a porous metal body, a frit made from sintered metal. This produces fine gas bubbles, increasing the surface area between gas and liquid. In carbonation systems, fine CO2 bubbles can be introduced with the aid of a sparger in order to improve the dissolving process by increasing the surface area.
Inerting involves transforming a reactive state into a non-reactive or inert state by adding inert substances. In beverage processing, the aim is to remove atmospheric oxygen from a container or a liquid using an inert gas in order to protect the beverage against oxidation. Otherwise the oxygen could adversely affect aroma, taste and colour as well as the composition of the ingredients. Nitrogen, carbon dioxide and argon are suitable for using them as inert gases.
One simple method is to purge empty tanks with an inert gas such as nitrogen or carbon dioxide until the desired degree of dilution of atmospheric oxygen is reached. The consumption of inert gas is usually 1.3 to three times the volume of the tank, depending on the degree of inerting required. Slim containers are easier to inert than those with a squat shape. When the diameter of the tank is large in relation to the height, some of the air flows back or circulates, making the inerting process more difficult.
In the case of partially filled containers, the space above the product – the headspace – is inerted. To achieve this, the inert gas has to be introduced through the upper tank opening in such a way that the displaced air can simultaneously escape from the headspace.
In this case, gas consumption is much higher than with tank inerting. At an atmospheric pressure of one bar, for example, the following applies as far as the consumption of nitrogen is concerned: to achieve two per cent residual oxygen, approximately three cubic metres of N2 is required per cubic metre of headspace volume, while for one per cent residual oxygen, the requirement is five cubic metres of N2 per cubic metre of headspace.
Here the objective is to maintain stationary covering of the product with inert gas. This involves striving for a residual oxygen content of zero per cent. When the product filling level changes, the gas needs to be topped up accordingly. In the case of non-pressurised storage, consumption is approximately 1.1 cubic metres of N2 per cubic metre of product withdrawal. When the product is topped up, the displaced gas has to be able to escape in order to prevent overpressure.
In the case of pressurised storage with a tank pressure of 300 millibars, for example, the consumption of inert gas is 1.3 cubic metres per cubic metre of product removal. Here, too, the gas has to be able to escape in equal proportion via suitable venting valves in order to keep the pressure constant when product is added.
Stripping involves fine bubbles of gas being introduced into a liquid, similar to sparging, but the aim here is to displace another gas. In practice, this method is used to remove undesirable oxygen from water, fruit juice or wine with the aid of nitrogen.
Many different systems are used for filling beverage containers. They are constantly being improved by the manufacturers in order to minimise the undesirable introduction of atmospheric oxygen. Two of the most widely used process principles are overpressure filling and pre-evacuation with gas reintroduced subsequently.
With overpressure filling, beverage bottles are pressurised to the product pressure with an inert gas. Filling along the inner wall of the bottle reduces the surface contraction and limits oxygen absorption to just 0.2 to 0.3 milligrams per bottle. In most plants, there is no provision for recovering the pressurisation gas, resulting in less efficient gas use. Gas consumption is two to three times the bottle volume and the priming process requires time.
A more efficient method is to first evacuate the bottles, then re-gas them with nitrogen or carbon dioxide and fill them after that. In this case, gas consumption is no greater than the bottle volume. The residual oxygen content is only about 0.1 to 0.2 milligrams per bottle.
When filling containers with non-carbonated beverages, a simple sparging device can be used to ensure that the headspace in the bottle is oxygen-free.
A frit is used to feed gaseous nitrogen into the product stream being filled. As nitrogen has a low solubility in water, fine N2 gas bubbles rise to the top of the bottle, displacing the atmospheric oxygen before capping. The result is an oxygen-free filling of the beverage container. However, this method cannot be used for carbonated beverages as the nitrogen would also force the carbon dioxide out of the liquid.
Pressure stabilisation for beverage cans and PET bottles
Beverage cans and thin-walled PET bottles containing non-carbonated beverages are not stackable without pressure stabilisation. This can be changed by adding a few drops of liquid nitrogen onto the surface of the beverage prior to sealing. Evaporation of the nitrogen drops causes the pressure inside the sealed container to rise. This gives the containers greater stability, allowing them to be stacked. The nitrogen in the headspace protects the product against oxidation and extends its shelf life.
In winemaking, gases can be used in a variety of ways to improve the quality of the end product. Oxygen can also play a role here.
Already when receiving the grapes, cooling with CO2 dry ice protects the fruit against premature fermentation. The same applies to the mash when it is kept at a low temperature by adding dry ice pellets. This leads to the so-called cold maceration, which slows down fermentation and facilitates extraction of the flavouring substances from the grape skin. As a welcome side effect, rising CO2 keeps atmospheric oxygen away and protects the mash against oxidation. Dry ice has a strong cooling effect and does not leave any meltwater. It ensures controlled beginning of fermentation and until then also provides the mash with microbiological protection.
Inerting of wine tanks
Inerting of wine tanks prevents the damaging effect of oxygen. For white wine, CO2 is usually used as dry ice snow or in the form of pellets. For red wine, nitrogen or a nitrogen-argon mixture is better suited because potential excess carbonation is in this case undesirable. If the CO2 content in the wine is too high, sparging with nitrogen can be used to reduce it to the required level. When putting the wine into, or taking it out of, storage, sparging can be integrated into the transfer process.
Targeted oxidation of tannins with pure oxygen allows to achieve a nicely balanced wine even after shorter storage periods. This involves very small quantities of gas being added over a long period of time. Excessive oxidation, which would destroy the flavouring substances, must, of course, be avoided. Typical quantities used for micro-oxidation range between 0.5 and twelve milligrams per litre of wine and per month.