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 , 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 . 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 . 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.
Rheology 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 , 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 , has a viscosity around 2 poise, while at , the viscosity is around 70 poise. With further cooling, the increase in viscosity is more rapid, reaching 600 poise at around . 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
mānuka 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. Honey is generally pale yellow and dark brown in color, but other colors can occur, depending on the sugar source. Bee colonies that forage on
Kudzu (
Pueraria montana var. lobata) flowers, for example, produce honey that varies in color from red to purple.
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. The temperature at which caramelization begins varies, depending on the composition, but is typically between . 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.
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.
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. == Classification ==