05 February 2014

Honey Composition: Acids

An acid can be described as any chemical that donates a proton (note this post will use the Brønsted-Lowry definition). They can be simple ionic compounds like HCl (hydrochloric acid), or complex molecules like CH3COOH (acetic acid). They can be strong or weak (relative terms to describe how easily they disassociate, measured via pK), organic or inorganic (containing carbon, or not), but all have an important role in mead making (and winemaking, even brewing).
Why are they important? Take a glass of cheap pinot gris and add lye (NaOH) to pH 7, taste it. It's terrible; lacking body, aroma, flavor, depth, everything. Acids are flavor enhancers, and they all have aroma or flavor contributions. They can also combine with other chemical to make new aromas and flavors (via esterification, acting as catalysts in reactions, or any other number of changes they themselves can undergo).

TA vs pH
TA stands for Titratable Acidity (not total acidity), and is a measure of how many free Hydrogen ions (H+) are available to react with a strong base (usually Lye (NaOH)) in and acid-base titration. Compare this to Total Acidity which is a measure of all H available in a solution from acids (even if they are not disassociated) using spectrometry or chromatography. Total acidity will always be greater than titratable acidity, but I don't have a gas chromatograph to take advantage of (and I doubt many home wine/mead makers do), so titratable acidity is the common language.
pH is a measure of the concentration of active H+, and it can only give you an estimate of the amount of acid in a solution. Because H+ can react with a number of compounds and get bound up, pH can not accurately account for the true amount of H+ ions.

So how do we quantize this data? How can we put it into scale? Because we are looking to count the number of H+ ions in solution we can use a fancy trick: there are 6.02214129×1023 things per mole. Atoms, molecules, muons, whatever. There are that many per mole. What's a mole? It's6.02214129×1023 things! It's really just a useful conversion tool that allows us to convert from tiny masses, to numbers of things in a larger mass, and some other useful conversions. The atomic weight of H is 1.008, and 6.02214129×1023  H atoms weigh 1.008 grams. See, useful conversions.
Now for something interesting. Tartaric acid has 2 H+ ions it can loose (making it diprotic), so a mole of tartaric acid should contain 2 moles of  H+. Citric acid is triprotic (3  H+), so a mole of citric acid contains 3 moles of  H+ (for those playing along thats 1.80664239 x 1024 H+ ions). One mole of Lye (NaOH) has 1 mole of OH-, and can react with 1 mole of H+ to make water (H+ OH= H2O). So 2 moles of NaOH can neutralize 1 mole of tartaric acid (because it's diprotic), and similar for other acids. So we can just count how many moles of  H+ it takes to get to a certain pH and get a measure of the amount of  H+ there is; this is titratable acidity.
The amount of  H+ in solution can be measure in equivalents, with 1 mole of H+ equaling 1 Eq of H (with the amounts we'll be using, the miliequivalent (mEq) is more practical, being 1/1000th of an equivalent). Once in this measurement it is easy to convert to other acids (saying the amount of H+ in solution is the same as X amount of some acid in solution).
1 mEq/L H = 0.0750435 g/L as Tartaric Acid
1 mEq/L H = 0.0490395 g/L as Sulfuric Acid
1 mEq/L H = 0.06005 g/L as Acetic Acid

1 g/L = 0.1 g/100mL = 0.1 %

In the US it is common to write the acidity as a percent, but it is easily converted to g/L. The US, and most of northern Europe report acid as equivalent to tartaric, while Latin countries, and southern Europe report as sulfuric acid. Volatile acidity (a fault in wines and meads) is reported as acetic acid.

Organic Acids in Honey
Organic acids are those acids that are organic compounds (containing carbon). Honey contains, on average, 0.57% organic acids by weight. They can be categorized as aromatic (containing an aromatic ring like benzene) or aliphatic (non-aromatic). Many of the acids in this group are carboxylic acids (containing a carboxyl -COOH group).

Gluconic Acid HOCH2(CHOH)4COOH - The predominant acid in honey, responsible for much of it's unique flavor. Gluconic acid, and it's related salts, are added as a flavor enhancer in many foods. It is a product of the enzyme glucose oxidase reacting with glucose to produce gluconolactone, which in turn forms an equilibrium with gluconic acid (as a byproduct of this equilibrium reaction, hydrogen peroxide is formed, a powerful antibacterial). This reaction is pH dependent and any change in pH (via titration, or dilution) will shift the balance of the gluconolactone/gluconic acid equilibrium. 

Acetic Acid CH3COOH - The acid responsible for vinegar, it's flavor and aroma are very distinct and detectable at a low threshold. It is generally considered a fault in wine (volatile acidity), but trace amounts will always be present as a metabolic byproduct of yeast.

Succinic Acid HOOC-(CH2)2-COOH - A non aromatic acid that is added as a flavor enhancer to many foods. Commonly found in all fermented products, it is a byproduct of yeast metabolizing nitrogenous compounds. It is very rare in unfermented grape musts; as such mead will have a comparatively higher level than many other fermented beverages. It also produces several esters responsible for general "fruity" aromas in wines.

Mailc Acid HO2CCH2CHOHCO2H - Green apple. That's the flavor, and where it was first isolated from. It's what gives Riesling it's gripping acidity, and what Chardonnay fans want completely gone (via malolactic fermentation). There are trace amounts in honey that add to it's depth of flavor.

Lactic Acid CH3CH(OH)COOH - This is what gives yogurt and sauerkraut their zing. Also responsible for the gripping acidity of a great lambic beer, or the gentle fullness of many red wines. 

Citric Acid  C6H8O7 - The acid responsible for citrus fruits' sour notes. 

Butyric Acid CH3CH2CH2-COOH - An aliphatic acid that has a slight rancid aroma, and is common in milk products (more in goat than cow).

Formic Acid HCOOH - This is what gives ant bites and bee stings their punch. It has a very unique flavor, slightly acetic, chemical, and spicy (?) note.

Amino Acids
An amino acid is simply an organic compound that has a carboxylic acid, and an amine group with a unique side-chain that determines the specific characteristics of the compound compared to others. Honey contains very small amounts of amino acids (0.05-0.1% by weight), with proline being the most commonly abundant (though some varietals have higher levels of glutamic acid or tyrosine).
Amino acids act as a source of assimilable nitrogen for yeast, and can be used in several steps of glycolysis, though the low levels in honey show that mead musts are a nutrient poor environment.
Argine, asparagine, glutamine, serine, aspartic acid, glutamic acid, threonine, glycine, alamine, proline, gamma-Aminobutyric acid, valine, phenylthalanine, isoleucine, leucine, ornithine, lysine,  tyrosine, methiomine,  tryptophan and histidine have all been found in honey, though methiomine, tryptophan, and histidine are very rare.
It has been shown that amino acid profiles are unique to individual monofloral honeys, and determination of the ratios between them can positively identify the primary nectar source of many monofloral honeys.

pH is vital for yeast, as many reaction are catalyzed at low pH, and yeast use this to their advantage metabolically. It is also important to how we perceive the flavors of foods and drinks (we generally prefer acid tastes (low pH)).
pH = - Log [H+]
The above equation shows that pH is a logarithmic function of the concentration of active H+ ions, meaning that there are 10 times more H+ ions (that are not bound, and able to react) at pH 3 than at pH 4. Most people are familiar with the 0-14 scale with 7 being the theoretical pH of distilled water. Common wine musts ranges are 2.8-4.2, with finished wines ranging from 2.8-3.6 (whites lower (3.0-3.3 common), and reds higher (3.3-3.5)). WInes below 3.0 are rare (santorini can have as low as 2.8) because the yeast will struggle as the pH drops below 3.0; whereas above 3.6 oxidation reactions happen very fast, contributing to premature oxidation, and at 4.5-5.0 bacteria rapidly reproduce.
These values are useful only for an estimate. Mead does not act the same as wine. Why? Buffering. Buffering is a substance's ability to resist a change in pH. Wines have good buffering capability due to the high mineral composition (relative to honey) and their large amounts of weak organic acids (such as tartaric acid). Honey does not have high mineral content, or acid levels comparable to grape musts. As such, it is quite common for an all honey must to drop radically during fermentation (yeast will actually lower the pH of the substrate, and due to a lack of buffering it can fall too quick) , to such a point that the yeast struggle and can produce off flavors or not finish a fermentation. This is a big problem, but an easily fixed one: add buffering capacity! It is common to add carbonates for this purpose, and of all carbonates, potassium carbonate is preferable, not only because it is more soluble than calcium carbonate, but it also provides potassium which is a required yeast nutrient. The other additive that serves the buffering purpose is cream of tartar (potassium bitartrate). This is one of the natural buffering substances in wine, and it has been used for mead for some time (Morse recommends it, and latter in his life, Brother Adam also condoned it's use). I will note, that when using these additives your must may actually rise to pH>4 (which wine makers will advise against), but most of the time (almost all the time) it will drop to normal levels during fermentation.

Winemakers will often adjust pH and TA pre fermentation. This is a bad idea for mead as an acid added before fermentation will make the pH drop faster than normal, which is already a problem in mead. For this reason it is recommended to add acid only after fermentation. There are 3 main types of acid that can be added:

Tartaric - the main acid in grapes, it has a general smooth, acid flavor; it adds a generic fruit-like flavor to wines and meads

Malic - the main acid in apples, it is very sharp and angular; responsible for rieslings crisp nature, and adds a fruitiness to the flavor of a wine/mead often reminiscent of green apples

Citric - yep, it's what gives citrus fruits their zing, less sharp than malic, but more than tartaric; it can give a citrus-like impression to wines and meads and can be inappropriate for some styles

Additionally, there are many formulations of blends, some including all 3, other only two. The acid you choose depends on taste preference, style, and end goals. For example, if I made a light 10%abv mead with orange blossom honey, and was inspired by german riesling, I would probably add citric acid to highlight the citrus notes of the honey, and malic acid to give it the crispness of a good riesling. Generally, I prefer tartaric. The main reason is that it's the least obtrusive of the bunch, only adding a slight fruity flavor and not giving the impression of other, more specific fruits. It is also the fastest way to lower the pH, which can add an impression of acid. If tartaric is used, you should cold stabilize the mead to precipitate potassium bitartrate, that way the crystals do not appear when the mead is chilled in the future. This procedure has an added benefit: if the pH is below 3.6(5), then it will drop as the potassium bitartrate precipitates. This means that some of the acid flavor will be lost, but the pH will be lowered giving the impression of acid, making an acid addition even less obvious.

The real problem with TA
Simply put, TA cannot be measured accurately in mead. At least not by using the simple titration that many winemakers take for granted. In a normal titration the pH is adjusted to 8.2 with NaOH, but in homey this will not work like we want. Recall that the predominate acid in honey (and therefore mead) is gluconic acid, which exists in equilibrium with its lactone form gluconolactone. As the pH increases (via NaOH addition for titration) the actual amount of acid changes and the pH is lowered by this continuos equilibrium reaction. This characteristic of honey requires us to measure the free and lactone acidity separately, and add them together to get the TA.
I will be posting the exact procedure for this in the future, but it can be found in USDA Technical bulletin 1261 on p55.

Numbers vs Taste
So, we can't easily measure TA, but we can measure pH. OK, what number should my mead be? That would be too easy, and no fun. The best number to use is the one that works. You can pull some samples of the aging mead and start adding acid to them. Stop when you get to one that tastes great. Figure out how many g/L that is and add 10% less to the batch. Age and taste in a month. As for pH, as long as it's below 4.0 it's fine (though the lower, the slower it ages).

White, J.W., 1962, Composition of American Honeys, USDA Technical Bulletin 1261

Carratu, B., 2011, Journal of ApiProduct and ApiMedical Science, Vol. 3 No. 2, p81-88

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