by Phil Markowski (Brewing Techniques)
Oxygen in the headspace of a bottled beer can alter its flavor and compromise its shelf life. Whether you are in a position to buy a new bottling line with all the bells and whistles or must make due with an older system, proper application of techniques and equipment are essential to ensuring reliably packaged beers.
Many brewers, professional or amateur, have aptly referred to the act of bottling beer as “a necessary evil.” The logistical and mechanical aspects of the task combined with microbiological and general quality control concerns can be daunting. But let’s assume for a moment that you have a dream brewery — a trouble-free, maintenance-free mechanical setup, a perfect aseptic filling operation, and for good measure a tunnel pasteurizer for postbottling sanitation. With all this care and technology, you can rest assured that the beer will be free of microbiologically induced spoilage on the shelves. Even with all of these controls in place, however, one invisible nemesis remains, the very compound that supports life itself: air, or more specifically, the oxygen component of air.
Only about 20% of air is oxygen. The remaining gases are basically inert to us and to packaged beer. This small fraction of air is what gives us life … and eventually stales our beer. In the real world of beer bottling, we accept the fact that even the most sound, most technologically advanced bottling operations invariably introduce small amounts of air to the beer. The goal of engineers and brewers is to keep that amount to a minimum.
It is well known that air negatively affects beer flavor. For centuries, brewers empirically recognized that darker beers held a fresh flavor longer than lighter beers. It is an old brewer’s trick to add a small portion — up to 0.5% — of a dark malt to the grist of a blonde beer to provide some uptake for the air introduced during postfermentation handling. Modern science reveals the explanation in the oxygen-reducing capacity of melanoidins, coloring compounds found in dark malts and, therefore, dark beers.
The problem of oxygen in packaged beer is a relatively recent phenomenon. With draft beer and its fast turnaround in the marketplace, air is of much less concern than with bottled beer. Some breweries want their packaged products’ flavor to remain stable for up to a year. Provided that the beer is properly handled before packaging (that is, that little or no dissolved oxygen is present in the product), the main source of potential oxidation is the headspace between the beer and the bottle cap. This headspace gas is a mixture of carbon dioxide, inert gases (from air), and, with care and attention, only a minuscule amount of oxygen.
Since the advent of mechanized bottling operations, engineers have struggled to minimize the introduction of this life-giving, flavor-spoiling compound to the bottle-filling process. The problem of minimizing levels of air in bottled beer became more of an issue when breweries moved away from bottle-conditioned beers (which contain yeast that uptake oxygen) to filtered, sediment-free precarbonated products. Furthermore, improvements in transportation facilitated the distribution of national and subsequently international brands of beer, which led to longer product life cycles and more challenging warehousing and transportation environments.
Bottling Technologies
Throughout the years, bottle filler designs have incorporated clever features to minimize the amount of air that contacts the beer. Many of these features are standard on new machines but may be absent from older equipment.
The first step in minimizing potential oxygenation of beer is to prevent the introduction of air into the beer during the filling operation. Most, though not all, state-of-the-art fillers incorporate a means of evacuating air from the bottle before filling. One method of evacuating the air is a process called single pre-evacuation, which involves applying a vacuum to the bottle to reduce the volume of air (by approximately 90%) before counter-pressuring with carbon dioxide and filling. Most modern machines perform a double pre-evacuation, which can reduce the volume of air as much as 99%, virtually eliminating air in the bottle before counterpressuring and filling.
Another improvement involves venting the gas displaced from the bottle during filling (a mixture of carbon dioxide and a small amount of air) into the atmosphere. Not all modern fillers use atmospheric venting; many simply redirect the air–carbon dioxide mixture displaced from the bottle right back to the counterpressure above the filler bowl, creating the conditions for potential oxidation of the beer before filling.
Some breweries choose to stick with the simpler single-counterpressure-type fillers and trade off higher air levels in the final product for the lower expense and reduced maintenance of the bottle filler.
Once the beer is in the bottle, the next source of potential oxygenation is the air trapped in the headspace between the top of the beer and the crown. The universal approach for eliminating this air is to induce a controlled foaming, or fobbing, in the bottle to displace the headspace air before applying a seal. Ideally, the foam should be creamy with tiny bubbles, not coarse with large “soda pop” bubbles (see photo). The foam should crest just above the lip of the bottle as the crown is applied. If the foaming is not vigorous enough, a volume of air will be trapped in the bottle, which will probably result in premature oxidation of the beer. If the foaming is too intense, however, an unacceptably high volume of product will be lost from the bottle. Foaming intensity and the quality of the foam produced depend on several factors, primarily the speed of the filler versus the pressure of the jetter. The carbonation level, and to some extent the protein content, of the product also influence foaming quality.
Obsolete fobbing techniques used mechanical knockers (which would occasionally break bottles, resulting in costly downtime for cleanup) or ultrasonic vibrators (which needed frequent readjustment, also resulting in downtime). Also, variations in bottle glass thickness affected these methods, leading to inconsistent foaming from bottle to bottle.
Modern designs use an extremely low-volume, high-pressure jet of sterile de-aerated water that is injected into the bottle as it exits the filler and enters the crowner (see photo). Jetting produces a more controllable and reliable foaming than does knocking or vibrating and is much more conducive for use with today’s high-speed fillers.
State-of-the-art bottling technology is available to craft brewers, and many breweries make this somewhat expensive equipment a necessary part of their production plans. Other craft breweries either lack the capital for new equipment or want to make due with older equipment. Given the mix of business plans and bottling equipment in use today, which bottling process has the most beneficial or profound effect on the amount of headspace air, and what can be done at a reasonable cost and effort? Which technique — purging the bottle with carbon dioxide, evacuation, or fobbing — will give you the best improvement in performance?
Putting the Techniques to the Test
I performed a series of limited experiments to test the effects of some of these techniques.
Testing methodology: Using various techniques and combinations of techniques, 12-oz bottles were filled and tested for the amount of air in their headspace. Each 12-oz bottle was filled using a hand-operated Zahm & Nagel counterpressure bottle filler. Foaming was induced by hand — by swirling the filler tube with the end submerged in the beer — to gently create as consistent a foaming as possible from sample to sample.
Each bottle was tested within two minutes of filling and crowning (to ensure the most accurate reading) using a Zahm & Nagel “airs” tester. The Zahm tester has long been and remains an affordable industry-standard instrument for measuring bottle headspace air. The Zahm tester works by piercing the bottle crown (or can top) and directing the headspace gas through a column of 10–20% sodium hydroxide (caustic soda) solution. The caustic solution reacts with the carbon dioxide component of the headspace gas, forming sodium carbonate. The remaining gas, presumed to be air, displaces the volume of solution in the calibrated cylinder and gives a direct reading in milliliters per total milliliters in the package (a 12-oz bottle is 355 mL). (Note: The Zahm & Nagel airs tester measures headspace air only. There may be an existing dissolved oxygen component in the beer prior to filling, which is difficult to measure; it requires expensive equipment that is beyond the budget of most small-scale brewers.)
Most national breweries, shooting for a shelflife of 6–12 months, target an air level of 0.2 mL and no higher than 0.5 mL air per 12-oz package. Some microbrewers who have older, less suitable equipment would sell their souls for levels in the 0.5 mL range, but often settle for readings two or three times that amount.
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Table I: Air-Level Readings for Various Bottle-Filling Techniques |
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|
Techniques |
No Induced Foaming (mL/355 mL) |
Induced Foaming (mL/355mL) |
|
Single counterpressure and fill |
1.55 |
0.40 |
|
Single purge,* counterpressure and fill |
1.35 |
0.42 |
|
Double purge, counterpressure and fill |
1.35 |
0.35 |
|
Triple purge, counterpressure and fill |
1.25 |
0.35 |
|
Single pre-evacuation, counterpressure and fill |
1.20 |
0.32 |
|
Double pre-evacuation, counterpressure and fill |
1.10 |
0.28 |
|
*Purge indicates counterpressuring with bottled carbon dioxide and venting to reduce the original volume of air. |
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In all tests, the sample bottles were filled with the same product, filtered, and carbonated to a level of 2.70 volumes. All bottles were first counterpressured with bottled carbon dioxide before filling with beer. The bottles were purged — that is, counterpressured and then vented and counterpressured again — to test whether the air component in the bottle would be significantly reduced with successive “purges” of the bottle before filling. Pre-evacuation (vacuuming) of the bottles was performed using a hand-operated vacuum pump, which applied a vacuum pressure of 100 mg Hg to the bottle through the filling device to emulate the level of evacuation typical of modern production-type fillers. Because the bottles were filled by hand, inherent minor deviations in air-level readings were present from bottle to bottle, attributable to small variations in foaming intensity and time (a matter of seconds) between applying crowns.
Test results: The data in Table I show the averages of three bottles tested using each particular method. It is safe to say that the most significant step you can take to reduce the level of air remaining in the headspace is to induce a reliable, repeatable foaming immediately after filling and just before crowning.
As can be seen, successive purging had the effect of reducing the available air in the package. This approach may be feasible for a home brewer doing limited bottling, but would be entirely impractical for a commercial brewer.
In contrast to what the theory would suggest, performing a pre-evacuation on the bottle with no induced foaming had a less profound effect on reducing the air than might be expected. By not fobbing prior to crowning, the advantages to pre-evacuation are lost as air enters the bottle where the filler tube exits. Clearly, inducing foaming before crowning has a significant impact on the final headspace air level in the package.
It is, of course, highly recommended that a small-scale brewery purchase a new bottle filler if at all possible. In situations where a brewery with an older piece of equipment is looking to improve performance relative to package air levels, a jetter could be retrofit to an older machine. We will address that, and other, more detailed, issues in an upcoming article.
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