By Steve Parkes (Brewing Techniques)
Beerstone is an aggravating problem in a brewery because it is a stubborn blend of organic and inorganic compounds. Most brewers agree that the best practice is to avoid buildup by using proper cleaning techniques regularly. When beerstone does strike, however, modern brewers can now choose between traditional techniques and some new alternatives.
Scientifically speaking, beerstone is a precipitate of calcium oxalate and protein. All surfaces in the brewery that come into contact with wort and beer are susceptible to this deposit, but some of the biggest problems will occur in heat exchangers, fermentation vessels, aging tanks, kegs, and beer dispense lines. Some breweries only have to address the problem once or twice a year, but inadequate cleaning regimens can require more frequent attention.
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Organic compounds such as proteins and polypeptides in wort and beer bind with compounds derived from the brewing water, mainly calcium and magnesium oxalates. The oxalate compounds form when the carbonate compounds of these metal ions react with organic acids in the wort to form a white crystalline precipitate. Ironically, the carbonate compounds can also be formed if caustic soda (NaOH) is used to clean a tank in a carbon dioxide–rich environment. The resulting complex tenaciously adheres to the stainless steel surface of the tank and can only be removed by first breaking down and solubilizing the protein.
Consequences of not removing beerstone: If it’s not removed, beerstone will create a surface that is impossible to sanitize. Beerstone can provide nutrients for contaminating bacteria and can protect them against sanitizing agents. Coatings of stone on a heat transfer surface will also interfere with cooling or heating. Thus, heat exchangers can be particularly troublesome because the deposit can quickly bake onto the metal surface. Furthermore, the surface of the metal below the layer of stone can escape passivation, and in those anaerobic conditions some corrosion can occur, resulting in permanent tank damage.
The easiest way to deal with the problem of beerstone is to prevent it from building up to a level where it becomes difficult to deal with. The traditional methods of tank cleaning and sterilization should take care of beerstone buildup, provided care is taken in choosing the type of cleaner and the type of water used. Home brewers should use soft water for all cleaning purposes, because it is cheaper to soften water than it is to add some expensive chemical additives to your “built” (formulated) caustic cleaner. Micros and brewpubs should still use built caustic cleaners. Because beerstone has an organic (protein) and an inorganic (oxalate) component, a dual-chemical approach has always been taken: The use of an alkaline detergent to “digest” the protein, followed by an acid detergent to dissolve the minerals.
Caustic or other alkaline cleaners: Most brewers will use a built cleaner based on caustic or some other alkali (for example, sodium metasilicate) that contains a number of additives to aid the active ingredient in its cleaning action. Caustic alone works by hydrolyzing peptide bonds and breaking down large, insoluble proteins into smaller, more easily soluble polypeptides. It does, however, react with the carbonate, bicarbonate, and sulfate compounds of both calcium and magnesium, causing the precipitation of water hardness salts. Thus additives, in the form of sequestrants (or chelating agents), are combined with caustic cleaners to help bind up these metal ions, making them unavailable to react with the caustic. The most commonly used sequestrant is ethylenediaminetetraacetic acid (EDTA), which is a recommended component of many brewery caustic and noncaustic cleaners.
Other additives include gluconic acid and polyphosphate compounds. Different sequestrants exhibit different chelating power, and so manufacturers of brewery cleaners often formulate cleaners for specific tasks. The nature of the additives can also differ depending on their form; powdered detergents tend to contain polyphosphates, and liquids often have gluconic acid added.
Chlorine, in the form of sodium hypochlorite, or bleach, is another useful additive. This compound can also hydrolyze protein and so increases the effectiveness of caustic cleaners. Chlorine, however, has a few disadvantages that may cause problems. At a pH below 5.0, detergents with added bleach can give off chlorine gas, which is extremely dangerous to humans and can seriously damage stainless steel. It is unlikely that the pH will get that low in a caustic cleaner itself, but if a strong caustic solution is applied, followed by an acid CIP without a thorough rinse, the conditions may favor chlorine gas production. Its residue can also form strongly flavored chlorophenols and chloramines, so thorough rinsing is essential; great care must be taken when using chlorinated detergents and acid cleaners. In certain chlorinated detergents, however, the chlorine is chemically bound to the detergent, and manufacturers claim that little free chlorine remains behind to cause problems. If you’re not sure about your cleaner, rinsing certainly won’t hurt.
Another additive may be a wetting agent, or surfactant. These compounds reduce the surface tension of a liquid, causing it to flow more evenly across a surface, and also may prevent reattachment of a soil to a surface after it has been removed.
Remember, in a carbon dioxide–rich environment such as the inside of a fermentor, aging tank, or keg, caustic can react with the carbon dioxide to create a solid precipitate. Unless the closed vessel is left open to the outside air, the interaction of carbon dioxide and caustic can create a sudden vacuum that may even collapse the tank. A more mundane but perhaps more important effect is that the effective concentration of the caustic can also be reduced by 50% over a single cycle through reaction with CO2. Thus, caustic cleaners require CO2 purging before using.
Acid cleaners: Most acids will dissolve mineral deposits from the wall of a tank, but the commercial options vary in their suitability. Sulfuric (H2SO4) and hydrochloric (HCl) acids are very effective but are also highly corrosive and so should not be used. Most brewers will choose a formulated acid cleaner consisting of a mixture of phosphoric and nitric acids blended with some surfactants. Nitric has the ability to hydrolyze proteins; the phosphoric is used only to enable solutions to reach temperatures of 160–180 °F (71–82 °C) without having the nitric flashoff, which would cause corrosion. Nitric offers the added benefit of inducing a passivating effect on stainless steel, whereby a protective layer of chromium oxide is created on the surface of stainless steel to prevent the leaching of iron ions into the beer.
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The Chemistry Behind Beerstone |
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Beerstone consists of a blend of organic (proteins) and inorganic (oxalate) compounds, making its removal difficult. Oxalic acids in malt, for example, can combine with free calcium to form calcium oxalate, a type of mineral scale. (COO–)2 + Ca+2 --> Ca(COO)2 If a protein residue in the form of an amino acid or a polypeptide chain becomes incorporated into the oxalate structure, the result is beerstone. 2H2O + 4H2NCO– + 2CaSO4 --> 2Ca(COO)2 + 4NH3 + 2SO2 + O2 water + protein residue + gypsum --> beerstone The use of improperly blended caustic soda (that is, caustic without sequestrants) with hard water can add to a beerstone problem by depositing a mineral scale on the equipment. Ca(HCO3)2 + 2NaOH --> CaCO3 + Na2CO3 + H2O MgSO4 + 2NaOH --> Mg(OH)2 + NaSO4 CaSO4 + NaCO3 --> CaCO3 + Na2SO4 Caustic soda can be very effective on light beerstone buildup, but requires that the vessel be purged of carbon dioxide first. Otherwise, the tank might implode due to the reaction between the two compounds. An acid-first technique or one-step acid cleaner would eliminate the need for that purge. 2NaOH + CO2 --> Na2CO3 + H2O sodium hydroxide + carbon dioxide --> sodium carbonate + water |
The traditional regimen: The traditional thinking has always been to use a well-formulated alkaline cleaner first, then follow with an acid CIP or soak to remove the inorganic scale before a final sanitization step. Here’s an outline of the traditional cleaning sequence:
Step 1: Rinse with warm, soft water to remove loose solids and purge CO2. (Use pulse rinsing to increase effectiveness and save water.)
Step 2: Recirculate 1–4% hot alkaline solution for 20–40 minutes (according to supplier’s instructions).
Step 3: Rinse well with warm water.
Step 4: Recirculate a hot acid solution for 20–40 minutes (or per supplier’s instructions).
Step 5: Cold rinse.
Step 6: Apply sanitizer cold; leave it until the tank is needed, then drain away. No rinse, if appropriate.
This method will remove the organic debris, yeast, protein, lipid, and hop compounds and other matter, as well as removing the beerstone.
Fighting tough beerstone: No matter how effective the cleaners you’re using are, if you find that you already have a solid, visible layer of stone built up on the wall of one of your tanks, it is going to require a lot of work to remove it. You will need to use chemical concentrations at the upper limits of their suggested use rates and for extended periods of time. Even then you may only be able to loosen the deposit and may have to attack the surface with a scrubbing pad to loosen it further. In the past I have taken an entire day to remove the beerstone from a used 20-bbl tank, using alternate manual scrubbing with caustic and acid. Spending the entire day working in all that protective clothing, goggles, gloves, and breathing mask was unpleasant to say the least; by the end of the day I was cursing the previous owner of the tank. It is possible to make a paste with the caustic or acid, using a commercial gel available from your reputable chemical supplier as a base, and coat the surface to increase the contact time. Be careful in your choice of scouring pad — some are capable of scratching stainless steel surfaces, creating new places for bacteria to hide.
Some chemists are looking more closely at beerstone removal, taking heed of experience with similar problems in other industries. Beerstone closely resembles milkstone in its make-up and formation procedure, so looking at how the dairy industry approaches the problem has offered some insight.
As a result of these investigations, an alternative method of beerstone removal has recently been introduced to the microbrewing industry. Basically, this new process reverses the acid and the alkaline cycles, as follows:
Step 1: Rinse out beer and yeast with ambient-temperature water.
Step 2: Use a 1–2 oz per gallon phosphoric/nitric acid mixture (140 °F [60 °C] maximum temperature) for 15–30 minutes.
Step 3: No rinse.
Step 4: Use a noncaustic alkaline cleaner at 1–2 oz per gallon of warm (120–140 °F [49–60 °C]) water to start. CIP for 15–30 minutes, depending on conditions.
Step 5: Rinse with ambient-temperature water until the pH of the rinse water is neutral (that is, the same pH as the tap water coming in).
This method gets around the problems that can be caused by using incorrectly formulated caustic cleaners and hard water. The proponents of this method believe that not rinsing between CIP cycles enhances the solubilization of the proteins in the alkaline detergent. This method is currently in use at a number of microbreweries around the country, with encouraging results.
It should be noted, however, that acid cleaners are expensive, and it is not usually necessary to use them every time to clean a tank. Some manufacturers only recommend acid cleaning a tank every four times it is used, because of the cost of the acid. Regular use of strong caustic formulated with the correct additives is generally just as effective, and more economical, though safety issues and the negative environmental impact of caustic soda, and particularly chlorinated caustic cleaners, must be weighed. Keep in mind, however, that the primary focus of a regular cleaning regimen is to remove all organic and inorganic matter on the surface of a tank, and caustic soda is still the most effective means for achieving that aim. A specifically tailored method geared for beerstone removal such as the “acid first” method would be a useful technique if you begin to notice a beerstone buildup. Remember: If you choose to follow with a chlorinated caustic, you must prerinse, because contact between acid and chlorine can result in the production of chlorine gas.
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Beerstone & Home Brewers |
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Beerstone can often be a problem with home brewing equipment, since home brewers typically lack the chemicals to effectively remove it and often do not notice its gradual build-up. As with professional brewing, the best way to prevent a serious problem is to keep it from building up in the first place. Rinsing the equipment so that it looks clean is not enough, and some periodic scrubbing of the metal surfaces is recommended. Kitchen scouring pads are usually OK, but stay clear of steel scourers, which can leave steel particles embedded in the stainless steel, leading to rust. Homebrew supply stores are beginning to sell some of the chemical cleaners and sanitizers that are available to commercial brewers, so you should be able to ensure that your brewing surfaces are residue-free. If you can’t find caustic soda or nitric–phosphoric acid blends at your local homebrew store, here are a few alternatives to scrubbing. Drain cleaners, such as Liquid Plumber or spray-on oven cleaners, are caustic soda–based organic solvents and will thus aid in the removal of stubborn organic deposits. Rust-removing gel is a strong acid that will also coat a surface, increasing the contact time. The acid will solubilize and loosen an inorganic–protein complex, aiding removal. One world of caution: These gels are HCl-based and will thus destroy soft metals and stainless upon lengthy contact. They also aren’t generally used in food processing because they rinse poorly and thus may contribute off-flavors. Also, stay away from drain cleaners that contain aluminum chips; they can react with the caustic and stainless to produce black spots that are not removable. Also avoid fragranced products. Please remember that all of these compounds contain aggressive chemicals, and so great care should be taken when using them. Always wear gloves and goggles for safety, and use them in a well-ventilated area. For light rinses or extended soaking, consider vinegar, which is an organic acid and will remove light inorganic deposits. Always rinse equipment treated with household chemicals thoroughly. Be aware that strong caustic will literally dissolve aluminum, as my experience with a kitchen strainer will attest, and strong acids will attack copper. |
Traditionally, acid detergents were thought of only in terms of their ability to remove inorganic scale. Recently, however, attention has shifted to using formulated acid detergents to do most brewery cleaning. The main rationale for this line of thinking appears to be time-related. In a busy brewery where the turnaround time for a brewery vessel needs to be as short as possible, it is important that CIP cycles are expedient. As mentioned previously, caustic reacts with carbon dioxide to produce a solid precipitate, risking damage to the tank and reducing the effectiveness of the cleaning solution. It may take up to 5 hours to purge a tank of carbon dioxide, if left to drain naturally; after cleaning, the tank would need to be refilled with expensive carbon dioxide before filling with beer. Acids, on the other hand, undergo no reactions with carbon dioxide and can be formulated to remove organic matter almost as aggressively as alkaline detergents. One drawback is that they rapidly lose effectiveness with repeated use and can thus support the growth of some organisms within the CIP system (changing the acronym’s meaning to Central Infection Process).
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