Magnesium

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Magnesium (Mg²⁺) is a mineral naturally present in water and grain. It is one of the most important ions for fermentation performance. Magnesium is required for the function of over 100 enzymes, making it a necessary nutrient for yeast growth and fermentation. Magnesium is also an important component of beer flavor, imparting a lightly sour and astringent or bitter character. However, as the level of magnesium increases, the flavor may become unpleasant. At excessive levels (higher than 125 mg/L in beer), magnesium can have possible diuretic or laxative effects, although the latter is unlikely. To avoid any negative effects, it is suggested to have no more than 20–40 mg/L magnesium in the brewing water. The presence of some magnesium in the brewing water is widely considered to be beneficial, although there is no consensus on the ideal amount. Magnesium behaves similarly to calcium, and the effects of these two ions have some interesting interactions.

Potential sources of magnesium

• Brewing water - The water used to make beer may contain dissolved magnesium.
• Grain - Magnesium has high extractability compared to that of other metals, up to 80%. A typical wort provides around 50–90 mg/L magnesium extracted from the grain, although a large portion may be bound to organic molecules (limiting the amount available to the yeast).
• Salt additives - Brewers can enrich the magnesium level of the wort by adding brewing salts that contain magnesiumm, such as magnesium chloride or magnesium sulfate.

Effects of magnesium

• Improved pH control - Magnesium beneficially lowers the pH during mashing by precipitating with phosphates, proteins, and other compounds, although not as effectively as calcium because magnesium salts are much more soluble.
• Improved fermentation - Magnesium ions are needed by many yeast enzymes, such as pyruvate decarboxylase. In fact, magnesium is required for yeast growth and metabolism (fermentation), and can help improve yeast ethanol tolerance and improve the ability of yeast to ferment under stressful conditions. The magnesium extracted from grain typically provides the minimum amount of magnesium needed to support fermentation, although the level is usually not optimal. Additional magnesium in the brewing water can sometimes improve fermentation speed, particularly a faster onset of fermentation. Calcium ions inhibit magnesium absorption by the yeast, and therefore magnesium addition may be especially useful in wort with a high calcium level (e.g. from the water or calcium salt additions).
• Flavor - At the moderate levels typically found in wort, magnesium can enhance a beer's character by providing a mild sour or bitter astringency. At excessive levels, the sour and bitter notes can become unpleasant and harsh. These flavor effects appear to depend on a balance between the magnesium and calcium ions, since calcium can reduce the flavor impact of magnesium.

How to adjust the magnesium level

In order to ensure a good fermentation, the addition of some magnesium to brewing water may be desirable. While magnesium sulfate (e.g. Epsom salt) is a common traditional option, magnesium chloride is also a good option for adding magnesium if you would prefer to minimize that amount of sulfate. Magnesium content in yeast extracts (i.e. yeast nutrient products) can also improve the fermentation performance of yeast, although adding a magnesium salt might be more beneficial.

If the magnesium level in your tap water is too high, the only option to decrease it for use as brewing water is to purify it with a reverse osmosis system. Yeast can theoretically absorb a maximum of 30–45 mg/L of magnesium during a typical fermentation, which can help to lower the amount in the final beer.

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Supplementation of both standard (12°P) and high gravity (20°P) worts with Mg (500 ppm) results in higher fermentation rates, increased uptake of maltose and maltotriose and increased production of ethanol, with up to an extra 5 mL ethanol L–1 produced during high gravity fermentation when Mg had been added as a supplement172,174. This promotion of fermentation performance by Mg was observed for both ale and lager strains172,174 and later with wine yeast fermenting grape must. It is probable that most brewery worts do not contain optimal levels of Mg, and even when wort Mg was increased from a basal level of 75 ppm to 135 ppm, improvement in 16°P wort fermentation performance with a lager yeast strain was not observed31. An alternative to wort supplementation is the ‘pre-conditioning’ of yeast cells by propagation in Mgrich wort. The Mg-rich cells produced have greater ethanol productivity in subsequent wort fermentations than their ‘non-conditioned’ counterparts223. Such pre-conditioning can increase cellular Mg levels by several-fold, depending on the Mg concentration of the propagation medium and duration of exposure191. Much of this Mg may be released when cells are pitched into fresh fermentation wort and this phenomenon has been proposed as a measure of the potential fermentation performance of a pitched yeast147. When considering the potential of Mg to improve yeast fermentation performance, the concentration of wort Ca should also be taken into account. Ca and Mg act antagonistically and any observed improvement may be related to the Mg:Ca ratio rather than the absolute concentration of Mg. Rees et al.172 reported faster fermentation rates at Mg:Ca ratios of 17:1 and 11:1 for 12°P and 20°P worts, respectively, compared with unsupplemented worts with Mg:Ca ratios of 3:1 or 2.5:1. Bromberg et al.31 did not observe an improvement in fermentation performance with higher Mg relative to Ca, though in that case the highest ratio present in wort was 4:1. Mg ions can positively influence flocculation and at a concentration of 10 ppm may substitute Ca under experimental conditions195. This characteristic may be relevant to brewery fermentation where the wort Mg concentration can be several fold greater than that of Ca172. Promotion of flocculation in a lager strain by Mg occurred between 12.5 ppm and 25 ppm Mg and was reversed when the Mg concentration was raised to 2,500 ppm. Increased flocculation of cells from a top-fermenting ale strain was observed at 2,500 ppm Mg51. The deflocculation of lager strain cells at 2,500 ppm Mg may be due to competition of Mg for the Ca-lectin binding sites51. Mg-induced flocculation is more sensitive to the presence of metal-chelating agents than Ca-induced flocculation and it is likely that the greater specificity of Ca in yeast flocculation relates to a greater affinity for cell wall-binding sites51,198. Smit et al.190 reported that onset of flocculation was triggered in a lager strain by the depletion of any of a number of different nutrients from the growth media. The exception was Mg, the loss of which did not result in flocculation, suggesting that nutritional Mg has a role in the process. This role is not fully understood but may relate to cell surface hydrophobicity, which is reduced in cells grown in Mg-limited media (0.7 ppm). Inclusion of Mg in the assay medium did not increase the hydrophobicity (or flocculation potential) of the cells in that study190. Apart from the metabolic requirements of yeast for Mg during fermentation, these ions also have a well-documented role in protecting yeast from the toxic effects of ethanol and may therefore influence fermentation performance during higher gravity brewery fermentations. Mg supplementation reduces the toxic effect of ethanol in defined media. Walker222 reported viabilities of 0% and 53% in a wine strain exposed to 10% ethanol for 24 hours in the presence of 50 ppm and 500 ppm Mg, respectively. Likewise, Hu et al.88 showed that 9 hours of exposure of yeast cells to 20% ethanol led to the death of all cells in a population, whereas supplementation of the same medium with 85 ppm Mg resulted in viabilities of between 25 and 55% depending on duration of Mg exposure. Increasing Mg concentration led to increased resistance of brewing and distilling yeast to the toxic effects of ethanol222. It was proposed that Mg exerted its protective influence on the plasma membrane and, interestingly, Mg also protected cells against the effects of heat shock222. Ethanol and heat shock affect cell membranes in a similar manner164 and Mg may therefore have a general protective effect on the yeast cell plasma membrane. Increased membrane permeability in response to ethanol exposure is well documented40,75,103,169,182 and the protective effect of Mg has been attributed to its ability to prevent excessive loss of ions162 and nucleotides88 from the yeast cell. The presence of Mg obviates the requirement for heat shock protein synthesis, which would otherwise be needed to maintain plasma membrane integrity on exposure to ethanol23. Increased resistance to ethanol is conferred regardless of whether Mg is included in the yeast growth medium as a pre-conditioning step or whether it is added as a supplement concomitant with ethanol exposure23,88, raising the question of how exactly it acts to maintain plasma membrane integrity under stressful conditions. Increased availability of Mg during higher gravity brewery fermentations may also benefit fermentation performance by mitigating the effects of osmotic stress on the cells47. Improvement of fermentation performance and stress tolerance of brewing yeast occurring as a result of Mg supplementation is clearly an attractive approach to increase productivity of higher gravity brewery fermentations. The potential negative effects of Mg on flocculation should however be considered, as well as the potential of these ions to influence the uptake of other essential metals such as Mn24 and the ultimate effect of Mg supplementation on beer quality indicators.

See also

• Water - Expert guide to adjusting water minerals
• Brewing pH - Discussion of pH throughout the brewing process, including the benefits of pH control by adjusting calcium.

Potential sources

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