Physical and chemical properties

The physical properties of honey vary, depending on water content, the type of flora used to produce it (pasturage), temperature, and the proportion of the specific sugars it contains.

Fresh honey is a supersaturated liquid, containing more sugar than the water can typically dissolve at ambient temperatures.

At room temperature, honey is a supercooled liquid, in which the glucose precipitates into solid granules. This forms a semisolid solution of precipitated glucose crystals in a solution of fructose and other ingredients.

The density of honey typically ranges between 1.38 and 1.45 kg/l at 20 °C.

Phase transitions

The melting point of crystallized honey is between 40 and 50 °C (104 and 122 °F), depending on its composition.

Below this temperature, honey can be either in a metastable state, meaning that it will not crystallize until a seed crystal is added, or, more often, it is in a “labile” state, being saturated with enough sugars to crystallize spontaneously.

The rate of crystallization is affected by many factors, but the primary factor is the ratio of the main sugars: fructose to glucose.

Honeys that are supersaturated with a very high percentage of glucose, such as brassica honey, crystallize almost immediately after harvesting, while honeys with a low percentage of glucose, such as chestnut or tupelo honey, do not crystallize.

Some types of honey may produce few but very large crystals, while others produce many small crystals.

Crystallization is also affected by water content, because a high percentage of water inhibits crystallization, as does a high dextrin content.

Temperature also affects the rate of crystallization, with the fastest growth occurring between 13 and 17 °C (55 and 63 °F). Crystal nuclei (seeds) tend to form more readily if the honey is disturbed, by stirring, shaking, or agitating, rather than if left at rest.

However, the nucleation of microscopic seed-crystals is greatest between 5 and 8 °C (41 and 46 °F). Therefore, larger but fewer crystals tend to form at higher temperatures, while smaller but more-numerous crystals usually form at lower temperatures.

Below 5 °C, the honey will not crystallize, thus the original texture and flavor can be preserved indefinitely.

Honey is a supercooled liquid when stored below its melting point, as is normal. At very low temperatures, honey does not freeze solid; rather its viscosity increases.

Like most viscous liquids, the honey becomes thick and sluggish with decreasing temperature. At −20 °C (−4 °F), honey may appear or even feel solid, but it continues to flow at very low rates. Honey has a glass transition between −42 and −51 °C (−44 and −60 °F).

Below this temperature, honey enters a glassy state and becomes an amorphous solid (noncrystalline).

Rheology

Pouring raw honey.

The sheet-like appearance of the flow is the result of high viscosity and low surface tension, contributing to the stickiness of honey.

The viscosity of honey is affected greatly by both temperature and water content. The higher the water percentage, the more easily honey flows.

Above its melting point, however, water has little effect on viscosity. Aside from water content, the composition of most types of honey also has little effect on viscosity. At 25 °C (77 °F), honey with 14% water content generally has a viscosity around 400 poise, while a honey containing 20% water has a viscosity around 20 poise.

Viscosity increases very slowly with moderate cooling; a honey containing 16% water, at 70 °C (158 °F), has a viscosity around 2 poise, while at 30 °C (86 °F), the viscosity is around 70 poise.

With further cooling, the increase in viscosity is more rapid, reaching 600 poise at around 14 °C (57 °F).

However, while honey is viscous, it has low surface tension of 50–60 mJ/m2, making its wettability similar to water, glycerin, or most other liquids.

The high viscosity and wettability of honey cause stickiness, which is a time-dependent process in supercooled liquids between the glass-transition temperature (Tg) and the crystalline-melting temperature.

Most types of honey are Newtonian liquids, but a few types have non-Newtonian viscous properties. Honeys from heather or manuka display thixotropic properties. These types of honey enter a gel-like state when motionless, but liquefy when stirred.

Electrical and optical properties

Because honey contains electrolytes, in the form of acids and minerals, it exhibits varying degrees of electrical conductivity. Measurements of the electrical conductivity are used to determine the quality of honey in terms of ash content.

The effect honey has on light is useful for determining the type and quality. Variations in its water content alter its refractive index. Water content can easily be measured with a refractometer.

Typically, the refractive index for honey ranges from 1.504 at 13% water content to 1.474 at 25%. Honey also has an effect on polarized light, in that it rotates the polarization plane.

The fructose gives a negative rotation, while the glucose gives a positive one. The overall rotation can be used to measure the ratio of the mixture.

Honey may vary in color between pale yellow and dark brown, but other bright colors may occasionally be found, depending on the source of the sugar harvested by the bees.

Hygroscopy and fermentation

Honey has the ability to absorb moisture directly from the air, a phenomenon called hygroscopy. The amount of water the honey absorbs is dependent on the relative humidity of the air.

Because honey contains yeast, this hygroscopic nature requires that honey be stored in sealed containers to prevent fermentation, which usually begins if the honey’s water content rises much above 25%.

Honey tends to absorb more water in this manner than the individual sugars allow on their own, which may be due to other ingredients it contains.

Fermentation of honey usually occurs after crystallization, because without the glucose, the liquid portion of the honey primarily consists of a concentrated mixture of fructose, acids, and water, providing the yeast with enough of an increase in the water percentage for growth.

Honey that is to be stored at room temperature for long periods of time is often pasteurized, to kill any yeast, by heating it above 70 °C (158 °F).

Thermal characteristics

Creamed honey

Creamed honey: the honey on the left is fresh, and the honey on the right has been aged at room temperature for two years.

The Maillard reaction produces considerable differences in the color and flavor of the aged honey, which remains edible.

Like all sugar compounds, honey caramelizes if heated sufficiently, becoming darker in color, and eventually burns. However, honey contains fructose, which caramelizes at lower temperatures than glucose.

The temperature at which caramelization begins varies, depending on the composition, but is typically between 70 and 110 °C (158 and 230 °F). Honey also contains acids, which act as catalysts for caramelization.

The specific types of acids and their amounts play a primary role in determining the exact temperature.Of these acids, the amino acids, which occur in very small amounts, play an important role in the darkening of honey.

The amino acids form darkened compounds called melanoidins, during a Maillard reaction.

The Maillard reaction occurs slowly at room temperature, taking from a few to several months to show visible darkening, but speeds up dramatically with increasing temperatures.

However, the reaction can also be slowed by storing the honey at colder temperatures.

Unlike many other liquids, honey has very poor thermal conductivity of 0.5 W/(m⋅K) at 13% water content (compared to 401 W/(m⋅K) of copper), taking a long time to reach thermal equilibrium.

Due to its high kinematic viscosity honey does not transfer heat through momentum diffusion (convection) but rather through thermal diffusion (more like a solid), so melting crystallized honey can easily result in localized caramelization if the heat source is too hot or not evenly distributed.

However, honey takes substantially longer to liquefy when just above the melting point than at elevated temperatures.

Melting 20 kg of crystallized honey at 40 °C (104 °F) can take up to 24 hours, while 50 kg may take twice as long.

These times can be cut nearly in half by heating at 50 °C (122 °F); however, many of the minor substances in honey can be affected greatly by heating, changing the flavor, aroma, or other properties, so heating is usually done at the lowest temperature and for the shortest time possible.

Acid content and flavor effects

The average pH of honey is 3.9, but can range from 3.4 to 6.1.Honey contains many kinds of acids, both organic and amino.

However, the different types and their amounts vary considerably, depending on the type of honey. These acids may be aromatic or aliphatic (nonaromatic).

The aliphatic acids contribute greatly to the flavor of honey by interacting with the flavors of other ingredients.

Organic acids comprise most of the acids in honey, accounting for 0.17–1.17% of the mixture, with gluconic acid formed by the actions of glucose oxidase as the most prevalent.

Minor amounts of other organic acids are present, consisting of formic, acetic, butyric, citric, lactic, malic, pyroglutamic, propionic, valeric, capronic, palmitic, and succinic, among many others.

Volatile organic compounds

Individual honeys from different plant sources contain over 100 volatile organic compounds (VOCs), which play a primary role in determining honey flavors and aromas.

VOCs are carbon-based compounds that readily vaporize into the air, providing aroma, including the scents of flowers, essential oils, or ripening fruit.

The typical chemical families of VOCs found in honey include hydrocarbons, aldehydes, alcohols, ketones, esters, acids, benzenes, furans, pyrans, norisoprenoids, and terpenes, among many others and their derivatives.

The specific VOCs and their amounts vary considerably between different types of honey obtained by bees foraging on different plant sources.

By example, when comparing the mixture of VOCs in different honeys in one review, longan honey had a higher amount of volatiles (48 VOCs), while sunflower honey had the lowest number of volatiles (8 VOCs).

VOCs are primarily introduced into the honey from the nectar, where they are excreted by the flowers imparting individual scents.

The specific types and concentrations of certain VOCs can be used to determine the type of flora used to produce monofloral honeys.

The specific geography, soil composition and acidity used to grow the flora also have an effect on honey aroma properties, such as a “fruity” or “grassy” aroma from longan honey, or a “waxy” aroma from sunflower honey.

Dominant VOCs in one study were linalool oxide, trans-linalool oxide, 2-phenylacetaldehyde, benzyl ethanol, isophorone, and methyl nonanoate.

VOCs can also be introduced from the bodies of the bees, be produced by the enzymatic actions of digestion, or from chemical reactions that occur between different substances within the honey during storage, and therefore may change, increase, or decrease over long periods of time.

VOCs may be produced, altered, or greatly affected by temperature and processing.Some VOCs are heat labile, and are destroyed at elevated temperatures, while others can be created during non-enzymatic reactions, such as the Maillard reaction.

VOCs are responsible for nearly all of the aroma produced by a honey, which may be described as “sweet”, “flowery”, “citrus”, “almond” or “rancid”, among other terms.

In addition, VOCs play a large role in determining the specific flavor of the honey, both through the aromas and flavor.

VOCs from honeys in different geographic regions can be used as floral markers of those regions, and as markers of the bees that foraged the nectars.

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