The use of inert gas has become very common in modern wine making and still little is known of the correct use thereof. The use of these gasses has especially become more prominent with the recent boom in reductive wine making in which the winemaker strives to exclude oxygen from the wine making process. To quote Bradd Webb, " we make wine while at the bottom of an ocean of air" ( Focus on Chardonnay, 1996 ). The purpose of this paper is to suggest the correct use of inert gases and to show more uses.
The main purpose to use inert gas in the headspace ( ullage ) of a wine container is to protect the wine against oxidation and spoilage by yeast and bacteria.
If it is taken into account that one mole of a gas occupies 23.6 liters of space at 15 degrees centigrade then one would use 1.18 kg N2, 1.68 kg Ar and 1.87 kg CO2 to inert a thousand liters of head space ( see table 1 ). As can be seen from Table. 1 the specific gravity of N2 is 0.9669, Ar = 1.38 and that of CO2 is 1.53. One would then naturally assume that since Ar and CO2 are heavier than air that they would displace the air and would settle to the bottom. This is however not really the case. What we are going to explore next is the difference between what your mind tells you and what really happens.
DENSITY GAS ( kg/M3 @ 15°C & 1 atm )
SOLUBILITY ( v/v )
Table 1. Properties of inert gases
The best way to separate what you think from what really happens is to measure the effect of the gas with a reliable and well calibrated oxygen meter. The oxygen meter will express the amount of oxygen either as a percentage of the total amount of air present, mg/l or as a percentage of normal air oxygen content. The last method gives a better picture of the dilution effect that takes place while sparging the head space. The oxygen content will furthermore always be quoted as a percentage of the volume of air.
It was always thought that a burning candle can be used to detect the presence of oxygen. This is however a very ineffective method as the candle stops burning in an atmosphere with a oxygen content less than 16,5% ( Rankine, 1996 ) while air normally has a saturation of 20,9% at 20 °C at sea level.
Before we decide how well a gas blankets we must first decide on a acceptable level of oxygen that we will tolerate in the space above the wine. Allen( 1994 ) thought this to be 1% and Lewis ( 1990 ) thought it to be 0,5%. Rankine ( 1996 ) agrees with Lewis ( 1990) because 0,5% is the level required to prevent the growth of film forming micro-organisms.
Nitrogen gas is the easiest gas to understand since it's specific gravity is less then that of air. It would therefor not form a blanket but would be dispersed in the head space equally without being confined to a certain region and if anything, it would float to the top. It would therefore be thought that if one wanted to displace all the air in the head space one would just have to add an equal amount of nitrogen gas. This seems pretty straight forward until one tries it in the cellar.
As was previously mentioned one would need 1,18 kg of N2 to displace 1000 liters of air. In practice this is closer to three to seven times that amount ( Allen, 1994 ). This is the result of the mixing effect and currents being generated by gas streaming into a tank. The only way to minimise this is to minimise this is by lowering the velocity of the gas ( Allen, 1996 ). Spargers come in several shapes and the most commonly used ones are those made from sintered stainless steel, the floating diffuser ( see figure 1 ), and the gassing bell.
Figure 1. Floating diffuser
A more efficient way to do this is to displace the tanks volume of water or wine with N2 gas . In practice this is difficult because the rate of nitrogen flowing in must be the same as the rate of the liquid flowing out, otherwise the tank could collapse or inflate. The best way of doing this is to use the positive pressure system described in 3.0.
CO2 is one of the most common gases used in wine making. It is thought to be heavy and it is assumed that the gas covers the wine like a solid blanked. One can say that the same amount of oxidation can occur in the headspace of a tank totally exposed to air in one day as in a tank with 1% oxygen in the headspace in 21 days. This is of course not completely true ( because of the kinetics involved ) but it is sufficient to make a comparison. Lewis ( 1990 ) did an experiment to test this and it was found that the percentage oxygen in the head space deteriorated from a acceptable level O2 in the tank to a unacceptable level within 20 minutes. Two mechanisms can be considered, namely that the gas is dissolved in the liquid or that it is dispersed in the space above the blanket ( see the original depth profile of the gas, Figure 2 ).
Figure 2. Distribution of CO2 blanket (Lewis, 1990)
According to Rankine ( 1996 ) this is due to the molecular movement of the gasses which can cause the gas to be evenly distributed throughout the space within a very short time. Trying to inert at least the space above the wine using this technique is therefore only a temporary solution. The answer is to inert the whole space above the wine by dilution and measuring the effect at the top of the headspace with an oxygen meter.
One drawback of CO2 is the fact that it can increase the CO2 content of the wine and by so doing influence it's sensory properties. To overcome this a mixture of gasses can be used as discussed in 3.0.
Argon occurs naturally in the atmosphere and makes up 1% of the air around us. Industrial gasses are made by compressing air and by cooling it down. Because argon makes up such a small percentage of the atmosphere, much less of it can be made and that explains the fact that it can be so expensive.
Argon's specific gravity is very close to CO2 ( Ar =1.53 compared with CO2 = 2.264 ) but is only 38% as soluble in wine as CO2 ( see table 1 ). Allen ( 1996b ) noted that argon is the best gas to use for either alone or in a mixture of gases ( see table 2 ). From table 2 it can be seen that the argon always had a positive effect on the colour, flavour/aroma and shelf life of the wine. Allen ( 1996b ) did however not provide a mechanism to explain this phenomenon.
SHELF LIFE (DEGREE OF OXIDATION)
(Relative Scaling of Effect compared to Oxygen, set to 0)
Table 2. Effect of different gas storage atmospheres (Allen, 1996b)
A good way of applying the gas is by letting it in the tank by means of a flexible hose or sparger attached to a float. A gassing bell ( light stainless steel ball with holes ) which disperses the gas evenly on the surface of the wine is very effective ( Westrick, 1996 ). The floating diffuser is also very effective. This is especially ideal when the tank is being filled or emptied when the gas hose is able to rise or fall with the surface. The amount of argon necessary to inert headspace is usually 2 to 3 times the volume of the headspace ( Allen, 1994 ) and should be monitored by measuring the O2 content.
Because CO2 can dissolve in wine and N2 gas can completely deplete the CO2 content in a wine, a mixture of gasses which keep the level of CO2 in the wine constant is preferable. As a rule of thumb the CO2 content in red wines should be below 900 mg/l and that of white wines below 1400 mg/l. A to high or to low CO2 content can be detrimental to the wine ( Peynaud, 1994 ) and the exact level should be determined by taste. From figure 3 the mixture of gases can be determined at different temperatures. A mixture of twenty percent CO2 and 80% nitrogen should be used at 10°C to maintain a CO2 content of 500 mg/l in the wine. So if you consider both the normal cellar temperature and the amount of CO2 needed in the wine, then you can work out the ideal mixture. Gasses can be bought premixed or can be mixed on site using a mixing panel.
Figure 3. Theoretical composition of a CO2/N2 mixture to maintain initial CO2 content (Allen, 1994)
This method uses a positive gas pressure in the headspace of a tank preventing oxygen from entering the tank. It also compensates for fluctuations in tank level caused by temperature changes. With a temperature fluctuation of between 20 and 25 °C the change in the ullage volume can be up to 8% ( Allen, 1989 ). This means that if the tank is cooled ( by rain or day/night temperature differences ) that air to the quantity of 8% of the tanks volume can enter the tank. When the tank heats up, this air is expelled and next time fresh air is taken in. A positive pressure system would let in gas when the volume of the tank drops and let out gas when the volume expands. The only gas in contact with the wine is therefore the inert gas. Nitrogen is most often used because of it's low solubility in wine and because it is cheap. The Nitrogen will however cause the wine to lower it's natural CO2 content and therefore a mixture of gases ( See 3.0 ) is preferred.
A schematic of the basic system is shown in figure 4. It consists basically of a temperature compensating regulator connected to a tank or series of tanks and a pressure relief system. The pressure relief system can take two forms, namely a mechanical valve or a short hose suspended under water. The mechanical valve is usually used where a positive pressure higher than 0,3 kPa is needed.
Schematic of positive pressure system redrawn from Wilson( 1985a).
The working of the mechanical valve is quite straightforward but the working of the hose is very interesting. The pressure at sea level is one atmosphere. Ten meters under clean ( not salt ) water the pressure is also one atmosphere ( plus of course the atmospheric pressure which is for practice purposes not included ) and therefore 101,3 kPa. Twenty to thirty centimeters under water would then represent 2 to 3 kPa of pressure that the gas in the ullage have to have to flow out via the relief valve ( see figure 5 ). The reason why the water system is so important is to make sure that the pressure the system exerts is not more than the tank can handle. If the pressure is more than that set by the depth of the hose in the water bottle then the gas would escape through the hose so doing protecting the tank.
The system can also be used to displace the whole volume of the tank with nitrogen when the wine is pumped out. When the tank is being emptied the hose in the water bottle should be lifted to a shallower depth to see that the air bubbles come through. This means that a positive pressure is still present and that the rate at which the wine is being drawn from the tank is less than the system is able to provide gas for. If the hose doesn't bubble air then the possibility of the tank being deformed exist. The hose would also suck in water if a negative pressure exist. The pressure that the system would keep the ullage at would then be set just below that of the overflow device.
This system of course allows a wonderful way to make sure that a tank that is being filled is completely oxygen free. This is accomplished by filling the tank with water and then emptying the water and allowing the gas to displace the water.
The pressure that the system would keep the ullage at would then be set at a pressure just below that of the overflow device.
Currently a system like this is installed at the Sonoma-Cutrer and Louis M. Martini wineries in California ( Rieger, 1992 ).
Dry ice can be used both for cooling grapes or wine and also for providing a gas cover for head space.
Dry ice has the advantage of both cooling and protecting against oxidation. The technique is widely applied in Australia and New Zealand.
From figure 5 the amount of dry ice required to cool grapes from an initial to a final temperature can be calculated. If the ambient temperature is 25 degrees centigrade and the temperature required is 10 °C then 85 kg of dry ice is required per ton of grapes.
The amount of dry ice needed to cool down grapes can be determined from this graph (Allen, 1994)
The dry ice come in several forms. It can be in brick form, manufactured on site or bought in insulated containers. The technique for making dry ice on site is however not very efficient since only 53% of the liquid carbon dioxide eventually ends up as dry ice and the rest is released into the atmosphere ( Lewis, 1990). The device needed to manufacture it from liquid carbon dioxide on site can be seen in figure 6. In brick form the ice is handled by wrapping it in newspaper or handling the brick with rubber gloves. The brick should never be put directly into the wine as a layer of ice will form around it which will hamper it's effect. It is therefore better to let the brick float in a plastic bucket or something similar on the surface of the wine.
Figure 6. Dry ice maker
Dr ice can also be available in the form of snow. The snow is formed by a hose the change in pressure the liquid undergoes when it is released from the storage cylinder to atmospheric pressure. The snow shoots out from a cone attached to a hose similar to that employed in fire extinguisher.
Lewis (1990) used dry ice to determine how effective it would be to blanket a storage tank. As can be seen from the graph in Figure 7. the oxygen content above the surface of the liquid did drop but not to the required level of 0,5%. It can also be seen that it took to reach 3% which is usually a bit impractical. The amount of dry ice used was the amount traditionally used and if the amount of ice was increased the results could have been more satisfactory.
Figure 7. Blanket created with dry ice (Lewis, 1990)
Dry ice in brick form can also be placed on a float on the wines surface to form a blanket.
A 2 kg brick takes about 30 minutes to sublime off totally and the layer of gas takes another 15 to 30 minutes to start diffusing and allowing air to come into contact with the wine ( Wilson, 1985b ).
Dry ice ( 20-30 g/Hl ) can be used to drive off undesirable flavours in wine. When added to wine the ice will form fine bubbles that will drive off SO2, H2S as well as flavours formed during high solid fermentations. During the process a lot of foam may form so it is advisable to make a tank no more than three quarters full.
This is a technique which, despite its infrequent usage, is practiced both in local little usage is practiced both in around the world.
SO2, fining agents and any other wine additives can be mixed in a tank by blowing gas in the tank through the sample valve or a suitably attached fitting. A fitting with a sparger is sometimes used. The tank should not be completely full because the turbulence could cause the tank to overflow.
Allen & Day ( 1986 ) found that at 0,3 L/L of nitrogen a 1500 HL tank could be thoroughly mixed within 15 minutes compared to the usual time of 3 hours with a pump.
An argument against this technique is that while mixing the wine or must flavour compounds are blown off. This can be minimised by using gas with relative big bubbles (see 8.0 Sparging wine ). Mixing by means of a large quantity of gas in a short time would then be preferable to using less gas for a longer time to achieve the same mixing effect.
Pigeage is the process of breaking the cap during red wine fermentation. To do this with gas a large amount of gas is needed and a bulk tank of gas is preferable.
The gas should be administered through a fitting with a one-way valve in the bottom of the tank. There is usually a pipe ( about 30 cm in length ) running to the inside if the tank from the valve. Gas can be taken from the gas cylinder through a regulator that reduces the gas pressure to the maximum that the gas line can handle. Gas can then be administered just by opening the valve from the gas supply. Wineries usually position this valve at the top of the tank so that the person who opens the valve can look inside the tank to see it's effect.
The effect is that the cap is slowly tipped because the gas only applied on the one side of the cap. The cap then tips causing a huge eruption which mixes the tank. The technique is however not very effective when the cap is not well formed .
Another version of the same technique is given in Fig. 8. This is the "Magyar" closed anaerobic fermenter and the principle of it's operation can be deduced from the illustration. Wine fermented with this fermenter was preferred to the control samples, having better colour and more extracted tannins ( Allen, 1994 ).
Figure 8. The Magyer fermenting vessel ( Allen, 1994)
It should however be noted that these techniques cause the fermentation to be more reductive than normal and that a huge amount of nitrogen is used.
These methods rely on the pressure of the gas to move the wine from one container to another.
The most common way this today is by making use of what is commonly known as the Bulldog Pup( Boswell company, 1990 ). The device is used to transfer wine from a barrel or stainless steel drum to a receiving vessel.
As can be seen on Fig. 9, it consists of a stainless steel wand with a expandable silicone bung which seals the vessel. Inert gas is then pushed into the vessel via a tube connected to the wand. This then pushes the wine out of the vessel that is being emptied.
Figure 10. The Bulldog pup.
When used to empty a 225 L barrel it takes about 6 minutes at a pressure of about 1.4 bar. Nitrogen gas is preferred because it is cheap and the least soluble of the gases. This method of transfer has the advantage that air contact is limited and that it is very gentle.
The wand has a adjustable screw at the bottom to adjust for different lees levels and is therefor excellent for racking wine out of barrels. Besides it's use for racking and transfer, it is a wonderful way to top up barrels. Stainless steel kegs are well suited for this and besides the obvious advantages, the wine that is left in the keg is also protected by the gas.
The positive pressure a normal tank with a diameter of 1250 millimeters is able to handle before it starts deforming, is as low as 0,04 kPa and that of a tank with a diameter of 2500 millimeters as low as 0,01 kPa. A tank with a diameter of 1250 millimeters would only be able to handle a negative pressure of 50 kPa before it starts deforming, while a tank with a diameter of 2500 millimeters would only be able to handle a negative pressure of 10 kPa. It is therefore not possible to put a normal tank under pressure and thereby transferring it's contents. Special tanks like those used for the making of Charmat wines however, are suitable.
These tanks can handle pressures of higher than 2 bar and can be used for transferring of wine to other tanks or for putting wine through filters. Unfortunately these tanks are very expensive and not worth the expense if not already available.
The purpose of this technique is to either increase the level of a certain gas in the wine or to decrease the level. The technique relies on the principle implied by Dalton's extension of Henry's law which states that the amount of any one gas dissolved in a mixture of gasses is proportional to it's partial pressure, when the gas has reached equilibrium in the liquid.
The effectiveness of the technique depends on several factors namely ( Allen,1991 ):
The contact time between the wine and the gas can be lengthened by using a pump that is connected to the tank and has a long hose on the outgoing side where the sparger is fitted. The temperature of the wine should ideally be as high as possible, but this would also promote flavour loss. A good compromise is a temperature of 15 degrees centigrade.
The sparger is put inline with a pump on the outgoing side. Because the effectiveness of these methods are highly dependent on temperature and the system, there is no recipe and the ideal should be found with trial and error.
CO2 can be increased if necessary to add freshness to wine ( Weik, 1990 ).The ideal amount of CO2 in a wine will depend on the balance of the wines components and should be determined by taste.
Weik ( 1990 ) found that Riesling and Muller-Thurgau wines with low CO2 levels were noticeably improved by the addition of CO2.
In Germany sparging is being practiced by applying little gas at a time and by repeating the operation several times with less gas at a time ( SG Holroyd systems, 1997 ). It is known as the pulse system.
Nitrogen sparging can be used to get rid of O2 picked up during juice handling and has particular use during reductive wine making where the juice going to the settling tank is sparged. The oxygen content of wines before bottling should be 0,5 mg/L or less to insure that the wine will not oxidise excessively after bottling ( Westrick, 1996 ). The excess oxygen can then be sparged away using nitrogen gas.
Table 3. shows the oxygen levels after sparging a wine. It can be seen that double sparging method was the most effective and that doubling the sparging rate did not proportionally increase the removal of oxygen.
Sparging @ 100 L/min
Sparging @ 40 L/min
Double sparged @ 40 L/min
Table 3. Oxygen levels( mg/L ) after sparging at a wine flow rate of 11000 L/hr and a wine temperature of 10,7°C( Wilson, 1985c ).
The wine can be sparged to get rid of excess CO2, H2S and SO2 by sparging with nitrogen.
Huge amounts of gas is sometimes necessary and it should be remembered that just as there is a huge loss of desirable compounds during fermentation this would also happen to some degree when sparging.( Buzke, 1997).
Many of the above uses require quite a large amount of gas. Gas is normally acquired in cylinders but bulk tanks and gas generators should also be considered (Loughron, 1989). The use of these different forms of gas will depend on the amount of gas on the site and a gas company should be consulted for the best option.
From this paper it is clear that there is no such thing as a heavy gas and that the gas or mixture of gases chosen for a task is very important. Argon looks as if it will still make great headway in the future and it's use should be studied more carefully.
There are also several applications that need to be investigated.
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