Guide to beer off-flavors: Metallic

It is not normal for Metallic to appear in any flavored beer. If you can taste Metallic or bloody taste in beer, it is recommended not to drink this beer. Under normal circumstances, the source of iron ions is only a very small amount of iron-containing protein in grains, which will enter the beer during the mashing process, but this content is so low that humans cannot detect it.
Iron ions are metal ions necessary for the growth and development of yeast, but excessive iron ions will make beer produce Metallic and accelerate the oxidation and turbidity of beer. Next, Micet Craft will introduce the role and control measures of iron ions in the beer brewing process.

What are the sources of iron ions in beer?

The main sources of iron ions in beer are raw materials (malt, brewing water, etc.) and iron ions in brewing equipment.

Raw and auxiliary materials

The raw and auxiliary materials include malt, rice, brewing water, hops and diatomaceous earth for filtration. The iron content in malt and rice depends on the variety, growing place, and year. The content of iron ions in the brewing water varies according to the region and the water intake layer. The iron ion content in the diatomaceous earth used for filtration is generally 30~70mg/L, which is very easy to dissolve into the liquor in the acidic environment of beer. Iron ion content in raw and auxiliary materials:

Malt: 3~7 mg/kg
Rice: 2~6 mg/kg
Hops: 0.4~1 mg/kg
Brewing water: <0.2 (mg/L)
Diatomite: 30~70 mg/kg

Dissolution of iron ions

In the beer brewing process, when the liquor contacts various metal containers, the pipes or packaging are corroded or rubbed, which will allow iron ions to dissolve into the beer. During the fermentation process, due to abnormal fermentation, too high algebraic number of yeast used, or delayed yeast discharge, the yeast autolyses, or allows the iron ions in the yeast cell wall and cytoplasm to dissolve into the beer.

The nature of iron ions in the brewing process

Iron ions have different properties in different stages of the brewing process. Next, we will introduce the different properties of iron ions in the brewing process.

Redox properties

Iron ions have divalent and trivalent ion forms, and the main thing that exists in beer is divalent iron ions. During the brewing process, the iron element in the metal container can undergo an oxidation reaction and lose two atoms to become divalent iron ions. The divalent iron ions and the polyphenols in the wort undergo an oxidation-reduction reaction, which can accelerate the oxidative polymerization of polyphenols after being boiled at a high temperature.

Catalytic properties

Iron ions can be used as coenzymes or prosthetic groups of certain enzymes to catalyze corresponding biochemical reactions and participate in the metabolism of sugars, proteins, fats and other substances. For example, iron ions serve as the active base of catalase to promote the progress of biochemical reactions. Iron ions can also be used as structure promoters of enzyme proteins to keep the enzyme protein in a certain structure, easily accept the substrate, and accelerate the decomposition of the substrate.

Form a complex

In beer, there is an equilibrium for any kind of metal ion, that is, the proportion of the ion in the free or complex state. This mainly depends on the affinity of different complexing agents to metal ions and the difficulty of reaching the complex state. During mashing and fermentation, iron ions will complex with enzymes, amino acids, phytic acid, melanoids and other substances, which will reduce the concentration of iron ions in the wine. But, the oxidation effect of iron in the ionic state is not great, and when it exists as a complex, its oxidation effect will be greatly enhanced, which will promote the formation of oxygen free radicals. The vast majority of iron in beer is in the form of complexes, and its harmful effects are more serious than copper ions.

The role and harm of iron ions in beer brewing

Iron ions have a promoting effect in beer brewing, but they can also cause certain harm to beer quality. A proper amount of iron ions play an important role in enzyme activity, yeast growth, fermentation speed, and diacetyl reduction. However, excessive iron ions will cause beer turbidity, metallic, beer color darkening and other unfavorable factors.

The role and harm of iron ions in the mashing process

In the mashing process, the decomposition of macromolecular substances (such as starch, protein, etc.) mainly depends on the action of enzymes, but the action of metal ions on enzymes is relative. Under certain circumstances, one ion may be an activator, but in another case it becomes an inhibitor. A proper amount of iron ion is an activator for β-glucanase, which is beneficial to improve the decomposition of β-glucan and make the wort easier to filter. But, the activity of iron ions on α-amylase and β-amylase slows down the decomposition of starch substances. When the iron ion content reaches more than 0.2mg/L, it will also inhibit the activity of other enzymes.

The role and harm in the boiling process

During the boiling process, iron ions can combine with isomeric compounds in the hop component (such as iso-α-acids) to form crystal nuclei that promote foaming. This will reduce the utilization of hops and the bitterness of the beer, and at the same time cause the gushing of finished beer. In addition, iron ions can also combine with proteins to form complexes to promote protein precipitation.

The role and harm of iron ions in beer fermentation

Iron ions are essential metal ions for the growth and development of yeast. For example, there is an appropriate amount of available iron ions in the wort, which plays an important role in the growth and reproduction of beer yeast and the activation and inhibition of various enzymes. However, if the beer contains excessive iron ions, it will have a toxic effect on the yeast and interfere with the yeast’s absorption of other metal ions.
Iron ions are important inorganic ions that constitute yeast cell walls and cell membranes, and can participate in energy transfer with other inorganic ions, control cytoplasmic colloids and cell permeability, and promote the growth and reproduction of yeast. In the fermentation process, insufficient iron ion content will limit the synthesis of yeast respiratory substance convertase. However, if the iron ion concentration in the fermentation broth is higher than 0.3mg/L, it can accelerate the decline of the yeast, affect the decline of the yeast, and affect the growth and fermentation of the yeast.
Iron ions have an activating effect on enzymes, and it is an important prosthetic group that forms the center of enzyme activity. Certain enzymes (cytochrome oxidase and catalase) require the activation of iron ions to form an “enzyme-iron-substrate” complex in order to accelerate the action of the enzyme. For example, catalase requires 0.05~0.2mg/L of iron ions to be catalyzed by activation to achieve its highest activity.

Impact on the quality of finished beer

If the content of iron ions in the finished wine is too high, it will undergo an oxidation-reduction reaction with oxygen, which will make the non-material stability of the beer worse, promote the oxidative turbidity of the beer, and form ferritin turbidity. When the iron content in beer is greater than 0.3mg/L, the beer will appear Metallic, and iron ions will participate in oxidation to produce aldehydes, which will damage the beer flavor.

Control of iron ion content in beer brewing process

In the beer brewing process, a trace amount of iron ions can meet the needs of yeast growth and reproduction. Therefore, it is necessary to control the content of iron ions in the beer to prevent iron ions from affecting the flavor of the finished beer and improve the quality of the finished beer.

Control the iron ion content of raw materials and water

It is best to use high-quality raw and auxiliary materials to reduce the content of iron ions in the wort. Brewing water and CIP cleaning water must be treated by electrodialysis and reverse osmosis to remove iron ions that are unfavorable to beer brewing. When brewing pale beer, the iron ion content of the ideal brewing water is 0.1mg/L or less.

Properly increase the pH value of wort

Properly increasing the pH value of wort will help amino acids become negative and make the charge properties of iron ions reverse. In addition, this can also enhance the complexation between amino acids and iron ions, and iron ions can be precipitated during the boiling of the wort, reducing the content of iron ions in the wort.

Use good and strong yeast

Use good and strong yeast during fermentation, and try not to use yeast with too high algebraic number. Also, the yeast that has settled in the beer should be discharged in time, and the fermentation liquid should be controlled within a reasonable number of days, so that the yeast autolyses, so as to prevent the iron ions in the yeast cell wall and cytoplasm from dissolving into the beer, resulting in bad taste of the beer.

Use diatomaceous earth filter aid with less iron

Try to use diatomaceous earth with less iron content as filter aid, and its content should be controlled below 50 mg/kg. At the same time, it is necessary to avoid long-term circulation of the filter during the filtration process to reduce the reduction of iron ions in the beer into the beer. In addition, increasing the volume of diatomaceous earth filtering wine per ton (or reducing the amount of diatomaceous earth) is also the most effective way to reduce the concentration of iron ions in beer.

Use corrosion-resistant stainless steel containers and pipes

In the beer production process, it is best to use high-quality acid-resistant stainless steel containers and pipes. If you use carbon steel equipment, you need to take corresponding anti-corrosion measures to prevent iron ions from dissolving into the beer. In addition, in containers that are in direct contact with beer, such as fermentation tanks and bright tanks, it is necessary to coat a layer of beer-specific paint on the inner wall, and regularly check whether the coating is peeling off. If the coating falls off, it must be repaired in time. The pipe fittings of the brewery equipment must not use ordinary carbon steel or iron pipes, and stainless steel pipe fittings are recommended. The water tank for storing hot water shall regularly check the iron ion content to prevent the iron ion from exceeding the standard.

The above is the beer off-flavors guide: metallic the entire content. You can learn more about beer off-flavors and how to deal with them by checking other related articles.

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