27 January 2014

Honey Composition: Sugars

Sugar; its the thing we care about in zymurgy because it what yeast metabolize to make (in the end) ethanol. Mead is differentiated from other fermented beverages due to the use of diluted honey; so it makes sense that we should care about the exact sugar composition of honey. We'll break things down into a few classes of sugars so that things can be gleamed from a larger perspective.

These are the base units from which other sugars are made through different links. They can generally be classified by the number of carbon atoms in the molecule (pentoses have 5, hexoses have 6, heptoses have 7), and can be further classified by their functional group. Honey is about 69.5% (by weight) monosaccharides.

Glucose (D-Glucose, Dextrose) C6H12O6 - The sugar. This is what the yeast actually prefer (being glucophilic), metabolizing it faster than other monosaccharides and faster than the production of enzymes to break down other sugars. On average honey is ~31% glucose, though it can vary from 22%-40% based on nectar source and age.

Fructose (D-Fructose, Levulose) C6H12O6 - The other sugar, found in almost all fruits. It may have the same formula as glucose but the placement of the atoms are different (it's an isomer of glucose). Yeast will almost never completely consume fructose in the presence of glucose, leaving some residual fructose in most wines. Honey is on average 38%, with a range of 27%-44%.

Fructose/Glucose (F/G) ratio - Much is made about this number in analytical winemaking. In their under ripe form, grapes have higher levels of glucose, and as they ripen the fructose levels increase until picking (with an optimum ratio of 1:1 as seen by most winemakers). As seen by the numbers, the F/G ratio in honey favors the fructose side more than many wine grape varieties. This may result in more residual fructose in mead than in a comparable wine.

Composed of two monosaccharides linked together, these require breaking by different enzymes produced by yeast in order to form monosaccharides that yeast can metabolize.

Sucrose (Saccharose) C12H22O11 - Fructose and glucose held together by a glycosidic linkage. Yeast produce an enzyme called invertase which breaks this bond and allows the yeast to consume the derived monosaccharides. The average found in honey is ~1%, with a range of <1%-8%.

Maltose C12H22O11 - Two glucose molecules joined together. Yeast excrete the enzyme maltase which breaks this molecule down to 2 glucose molecules. Honey has an average of 7% with some having as low as 3%, and others having up to 16%.

Turanose C12H22O11 - Fructose and glucose bonded together. Trace amounts are found in honey

Maltulose C12H22O11 - Fructose and glucose bonded with the same type of bond as maltose (and can be broken down with the same enzyme). It is produced via enzymatic reactions within honey between sucrose and enzymes present. Trace amounts are found in honey.

Kojibiose, Isomaltose, and Nigerose C12H22O11 - These are made from 2 glucose molecules and are commonly found when glucose goes through caramelization and oxidation reaction. Trace amounts are present in honey.

Higher Sugars (Oligosaccharides)
These are the larger sugar compounds that contain more than 2 monosaccharides. The average amount in honey is ~1%, with a range of <1%-9%. Most of these are not fermentable (or partially fermentable in specific circumstances).

Melezitose C18H32O16 - This is found in honeydew (a substance produced by certain insects commonly used by bees for food) and is present in all honey even if it is a nectar honey. It can undergo hydrolysis producing glucose and turanose. Trace amounts.

Erlose C18H32O16 - Trace amounts.

Kestose C18H32O16 - Glucose and 2 fructose molecules. Trace amounts.

Raffinose C18H32O16 - Galactose, glucose, and fructose. Trace amounts.

Dextrantriose C18H32O16 - Trace amounts.

OK. What does this mean? While the complexity for wines usually comes from an abundance of aromatic compounds, it seems that honey has a very complex sugar profile that is not found in many other products.

At the top of the post you'll notice a chart comparing many different varietal honeys. Ken Schramm's book The Compleat Meadmaker has a similar chart on pgs. 96-97, both his chart and mine are compiled from the same raw data (with some differences in which samples were chosen): Dr. White Jr's study Composition of American Honeys.  Why can such a chart exist? Because honey from different nectar sources shows marked differences in the sugar profile, and while this may vary with vintage and location, the difference is less than the difference between different nectar sources.

The length and condition of storing honey has an impact on it's sugar profile. To the right you will see a data table showing how the composition of honey can change over time, however it does not agree with the findings of Dr White Jr.

Here are two graphs of the table above.
In Dr White's findings, he states that there is a decrease in both fructose and glucose as honey is stored. He also notes that the reducing sugars (maltose here) increase, as well the sucrose and higher sugars.

Looking at the graphs, none of these seem to be correct. The data from the above two graphs is from an experiment that used "a honey sample of 1kg . . . collected and mixed with 1kg of winter store after it had been deposited by bees from the syrup (sucrose:water ratio of 3:2) fed to them". What?! It has been common practice for quite some time to remove as much honey as possible from hives and to feed the bees a syrup that is supposed to provide the same benefit as honey. Using this method beekeepers can collect >90% of the honey in the hive, where traditional methods (leaving some honey behind for the bees) cannot come near this.

Why have I reported this data if it is useless? I'll quote from Dr White:
"These Changes [changes in sugar due to storage] are in the direction of increased complexity of sugars. They are probably brought about by two mechanisms – chemical and enzymatic. A high sugar concentration and a considerable acidity, both of which are present in honey, are known to promote a slow combination of simple sugars. It is also thought that the enzymes in honey bring about slow increases in the amounts of more complex sugars."

What this means is that honey increases in complexity as it is stored, while it would seem that techniques used by very large beekeepers deprives honey of this aging potential. This may become useful if you wish to highlight certain aspects of a honey, or are looking for more complexity in your mead; or if you forgot you had some honey, you can use it and know that it will be slightly different than fresh honey. If I were you, I'd get to know my beekeeper and make sure that it is truly quality honey, even if the price is more.

Rybak-Chmielewska, H., 2007, Changes in the Carbohydrate Composition of Honey undergoing During Storage, Journal of Apicultural Science, Vol. 51 No.1, p39

White, J.W., 1961, A Survey of American Honeys 3. Identity of Honey Sugars, Gleamings in Bee Culture

White, J.W., 1961, A Survey of American Honeys 8. Effect of Storage on Honey Sugars, Gleamings in Bee Culture

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

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