By Bertus Fourie

We are well aware of the influence of oxygen (O2) on various dynamics as far as wine is concerned. We know that excessive amounts at critical times will deteriorate the quality of wines, but the contrary is also true: too little oxygen might have a detrimental effect on the quality of some wines.

Controlled exposure to oxygen especially during primary fermentation has several benefits, which vary from increasing the viability of yeast and alcoholic fermentation, decreasing reductive compounds such as H2S (and of course further reduction to mercaptans), alcohol reduction as a function of aerated pump overs, reduced CO2 content and finally “softer” wines with an enhanced colour stability as a function of polymerization reactions.

Several colour extraction techniques during red wine production are known to us, such as pump overs, rack-and-returns, punch downs, submerged cap fermentations with header boards, and more unconventional, and very expensive technological considerations like Flash détente, carbonic maceration, thermovinification and many others. Controlled and uncontrolled oxygen dosages are applied during these extraction or mixing actions to achieve these favourable outcomes. Of course most techniques have their advantages, and possibly disadvantages in some cases. Open fermentors for example may induce alcohol evaporation, bacterial spoilage risks, etc. These types of fermenters give the winemaker enough flexibility to do punch downs combined with pump overs, as well as having a continuous exposure to oxygen. Fortunately for closed fermentation tanks, an aerated pump over can obtain the same effect. During such an operation fermenting must flow in contact with air into a drain tank and then pumped back into the upper part of the tank, over the fermenting pomace or skin mass.

This technique has several benefits, which range from the ease of execution to yeasts that better resist elevated temperatures, when aerated, but there is a certain amount of confusion (and many, many opinions and recipes) as to the most opportune time to aerate. Early aerations at the beginning of alcoholic fermentation help to prevent stuck fermentations, the yeasts are in their growth phase, and oxygen is utilized to improve their growth and produce survival factors (Ribereau-Gayon et al).  Early pumping-over operations also have the advantage of avoiding alcohol loss by evaporation, particularly where lower sugars may induce unbalanced wines (Ribereau-Gayon et al).

An aeration carried out on the second day of alcoholic fermentation is the most effective, while the effect is greatly diminished because the yeast does not make use of oxygen during the final stages of fermentation, since ethanol and other toxic metabolites hinder its nitrogen assimilation (Ribereau-Gayon et al).

The action itself homogenizes the temperature, sugar concentrations and yeast population of the fermenting mass, facilitates extraction of compounds from the pomace (anthocyanins and tannins) and enhances maceration (Ribereau-Gayon et al).

Considering certain “logistical” variables during such an extraction procedure is also extremely important.  Pumping must into a closed system or “narrow” container may eliminate effective aeration. Running the must over a flat surface is also recommended, to increase air contact and to remove skins, stems and pedicels.

The quantity of dissolved oxygen in must exposed to air may vary between 6–8 mg/l and may fluctuate as a result of the medium’s temperature.  According to research, the quantities necessary to avoid stuck fermentation are approximately 10–20 mg/l, which can be obtained by pumping-over with aeration twice, 24 hours apart (Riberaeu-Gayon).

The ease and effectiveness of Puls Air, and their technique known as “pneumatage” thus plays an important role in controlling the dosages of oxygen which are beneficial for the process.

Must circulation during a pump over do not assure sufficient break up of the pomace constituents by leaching, but it does impregnating the pomace cap with must taken from the bottom of the fermentor. Approximately two-thirds of the cap is immersed in the fermenting must and one-third floats above the liquid, depending on the parameters of the tank. All of the must should be pumped over and the entire cap should be soaked to obtain satisfactory results. These conditions can be difficult to realize in tall, narrow paralleled piped fermentors, especially if the lid is not located in the center of the tank.

Another challenge to overcome during a pump over is the existence of “preferred passages or channels” in the pomace cap, even under ideal conditions. Close monitoring is indispensable, as the winemaker might not even be aware of these passages, which might result in insufficient extraction.

Pump delivery rate must be sufficient, as s sudden increase in the flow rate during a pumping-over operation may lead to an excessive tannin concentration, and consequently aggressive, disagreeable wines (Ribereau-Gayon).  The rule of thumb is that a volume of juice corresponding to one-third to one-half of the tank volume should generally be pumped over. The frequency of pumping-over operations should be increased, opposed to their duration.

Pneumatage is an alternative method of managing the pomace or cap during alcoholic fermentation. A large “bubble” of compressed air generates vertical liquid flow in the tank that turns the cape and floods the solids with juice. Unlike any other conventional methods of extraction, pneumatage breaks up the cap in individual berries and circulates them in the liquid, resulting in very efficient colour and flavor extyraction. This action also forces CO2 and reductive compounds up, and out of the fermentor.

Various options are available from handheld models to fully programmable, automated muti-tank systems. The equipment is designed for various tank sizes and tank configurations from barrels to 300-ton fermenters. The “wine cart” is a stainless steel trolley on which the control panel is amounted, as well as an air filter cartridge that provides food grade air after treatment. A storage reel is mounted on the back of the trolley, and four hoses and tri-clover check valves are used to connect the system to the injection ports. A handheld, stainless steel probe for smaller fermentation lots are connected to the cart via a 25 m hose. The number of valves, pulse rate, injection time, duration of pneumatage and number of pulses are easily selected and controlled by the program.

The probe that physically releases the compressed air can be used from the top of the tank, or the racking or drain valve. The adjustable program on the larger, automated systems can control managing the cap during alcoholic fermentation. The winemaker has precise control over the entire fermentation cycle of each, individual tank as turning the cap can be programmed and accomplished for any hour continuously for up to 21 days. Less physical input from the winemaker relieves him to focus on various other demands. It also saves time, labour and energy and provides greater efficiency, reliability and lower cost. A “mixing mode” on the system enables the winemaker to stop the “Pneumatage-cycle” momentarily, make additions, and mix it thoroughly without interrupting the program. A “blending mode” enables the winemaker to select a pulse rate, injection time and number of pulses necessary for efficient mixing of wine. Nitrogen replaces the oxygen at this time. This mode provides thorough mixing of the blending tank, with minimal nitrogen consumption.

Wines produced by applying pneumatage, show more vibrant fruit, enhanced colour and better overall quality.