Advances in Honey Adulteration Detection
By Vlasta Pilizota, Ph.D., and Nela Nedic Tiban, Ph.D.
Honey is defined as a naturally sweet mixture produced by
bees (Apis mellifera) from the nectar of flowers, from secretions of
parts of the living plants or excretions of plant-sucking insects on the
living part of plants that the honey bees collect, transform and
combine with specific substances of their own (such as enzymes),
deposit, dehydrate, store and leave in the beeswax honeycombs to ripen
and mature. Physically, honey is a viscous material, where all the
sugars (33.3–43.0% fructose, 25.2–35.3% glucose, 0–2% sucrose, maltose
as well as more complex sugars and trace polysaccharides) are present in
an amorphous, devitrified state.
All components (carbohy-drates, water, enzymes, amino acids, pigments,
variable amounts of sugar-tolerant yeasts, pollen, traces of vitamins,
organic acids and wax and probably crystals of dextrose hydrate) are due
to maturation of the honey; some are added by the bees, and some are
derived from the plants. However, honey from the same floral source can
also vary due to seasonal climatic variations or to a different
geographic origin.
Aside from the definition of honey in the Codex Alimentarius (1981),
there are additional definitions in the regulations of many countries
and the European Union (EU). Various physical types (pressed,
centrifuged and drained) and forms (comb, chunk, crystallized or
granulated, creamed and heat-processed) of honey are on the market.
The European Commission (EC) has adopted a proposal to amend the Council Directive 74/ 409/EEC concerning honey.[
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This Directive, which lays down common rules for the composition and
manufacture of honey, will provide information about the product’s
“floral or vegetable origin, being stated if the product comes
essentially from the indicated source and possesses its organoleptic,
physicochemical and microscopic characteristics; regional, territorial
or topographical origin, if the product comes entirely from the
indicated source; specific quality criteria.” The EC is encouraging the
development of harmonized analytical methods to permit the verification
of compliance with the quality specifications for different types of
honeys.
Among the compositional criteria prescribed in the existing EC honey
directive are requirements relating to the concentrations of acidity,
apparent reducing sugar (calculated as invert sugar) and apparent
sucrose, 5-hydroxymethylfurfural (HMF) content, mineral content (ash),
moisture and water-insoluble solids.
Consumption of honey and honey products has grown considerably during
the last few decades. However, at the present time, the traceability of
this food is limited to the quality of each processor’s documentation.
In case of doubt or fraud, there is no standardized analysis available
that can discriminate or determine the botanical (floral or vegetable)
and geographical (regional or territorial) origin of the honey.
Counterfeiting and product adulteration are now commonly practiced in
the global food marketplace.
Because of its high nutritional value and unique flavor, the price of
natural bee honey is relatively much higher than that of other
sweeteners. Honey is susceptible to adulteration with cheaper
sweeteners; those that have been detected in adulterated honeys include
sugar syrups and molasses inverted by acids or enzymes from corn, sugar
cane, sugar beet and syrups of natural origin such as maple.
Adulteration of pure honey with synthetic honey (based on C4 plant
sugars) has become much more prevalent in recent years. In addition,
there has been a recent major adulteration problem in honey from the Far
East.
It should be emphasized that the adulteration of pure honey is one
issue and concern about the botanical and geographical origin of honey
or its authenticity is another, but the two can overlap, as in the case
of adulteration by honey of other geographical origin, from a country
where quality measures are not as stringent and the honey price is much
lower.
Many foods have the potential to be deliberately adulterated, but those
that are expensive and are produced under wide fluctuations in weather
and harvesting conditions are particularly susceptible; honey is one
such material.
Adulteration usually refers to mixing other
matter (substance) of an inferior and sometimes harmful quality with
food or drink intended to be sold. With companies concerned about the
bottom line, the temptation to cheat is considerable, and unfortunately,
the adulteration of honey is a serious economic and regulatory problem.
As usual, the losers are the consumers and the processor or
re-processor seeking to provide a wholesome product that meets
regulatory standards. From an economic point of view, food product
adulteration can destabilize the market by bringing in unfair
competition.
Authentication of pure honey is of primary
importance for both consumers and honey processors. Additionally, honey
processors do not wish to be subjected to unfair competition from
unscrupulous processors who would gain an economic advantage by
misrepresenting the honey they are selling.
Honey adulteration appeared on the world market in the 1970s when
high-fructose corn syrup was introduced by the industry. As the sugars
(60.7–77.8%) are the major components of honey and the most dominant are
the monosaccharides fructose and glucose (accounting for 85–95%), the
actual proportion of glucose to fructose in any particular honey depends
largely on the source of the nectar. The average ratio of fructose to
glucose is 1.2:1. The amount of glucose in honey is usually at a
supersaturated level at normal temperatures. With reduction in
temperature or water content, the glucose can crystallize out.
Saccharose (sucrose) is present in honey at approximately 1% of its dry
weight. Normally, honey contains 12.4–24.5% moisture. Unless the
moisture content is below 17%, no fermentation takes place.
The processing of honey includes controlled heating to destroy yeast
and dissolve dextrose crystals, combined with fine straining or pressure
filtration. Most honey will crystallize during some period of time
unless action is taken to prevent it. Generally, when honey is stored
below 10 °C, crystallization can be prevented or delayed.
Honey is usually warmed to a temperature of 32±40 °C to lower its
viscosity, which facilitates extraction, straining or filtration. This
temperature is similar to that in beehives and does not affect the honey
very much during the relatively short processing period. However, some
honeys are heated to a higher temperature for liquefaction or
pasteurization reasons.
Adulteration Detection
All food products targeted for adulteration are high-value commercial
products, including honey. The detection of adulteration can pose a
technical problem. The quality of honey is mainly determined by its
sensorial, chemical, physical and microbiological characteristics.
Analytical methods applied to honey generally deal with different
topics: determination of botanical or geographical origin, quality
control according to the current standards and detection of adulteration
or chemical residues.
At present, a variety of analytical techniques have been developed to
detect adulteration of honey, such as isotopic (stable isotope
methodology), chromatographic, spectroscopic, trace elements techniques
and thermal analysis. Some of these methods are time-consuming, and some
are expensive. Although there are powerful methods to prove honey
adulteration, they have to be further improved in order to ensure honey
quality.
Due to the limitations of classical analytical methods, which measure
chemical parameters to detect adulteration, many experiments have been
carried out using new indicators derived from physical analysis, such as
thermal analysis.
Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) is a thermal analysis method
with a broad field of application. It is an efficient method for
characterizing pure food compounds as well as their mixtures. DSC
analysis is fairly rapid and simple, requires a small amount of sample
(< 100 mg) and does not use solvents. It has been used in monitoring
thermal behavior in different foods, as well as in cases where no heat
exchange occurs. DSC can monitor and determine heat flows resulting from
various structural modifications (phase transformations and
transitions, glass transition, etc.) or decomposition of food compounds,
during temperature-programmed scans. These phenomena allow the
determination of the type of transformation occurring in the studied
food product (e.g., granulation in honey) as well as its thermodynamic
and kinetic properties. DSC can be a useful technique to complement
chemical analytical methods which show the limitations of the
physicochemical determinations (i.e., pH, acidity, water activity and
conductivity).
DSC has been investigated for the detection of alteration or
adulteration, and for quality control of food. This technique was used
to determine thermal behavior, energy variation during phase transition
(crystallization or/and fusion), transition temperatures (such as glass
transition temperature) and water content relationship in honey. Proper
understanding of honey’s thermal properties is essential for defining
honey quality and detecting its alteration or adulteration.
Honey is a viscous, heterogeneous material, which consists mostly of
sugars (present in an amorphous, devitrified state) and water (> 95%)
and demonstrates certain thermal behavior during heat exchange. The
thermal behavior of honey is influenced by several factors, including
composition, temperature and amount and size of crystals. Knowledge of
honey’s thermal behavior is critical during processing, handling and
storage. Addition of syrups produces commercial honey of lower quality.
When sugar syrups are added to authentic honeys, adulteration can be
determined easily, since the syrups and honeys show significant
differences in thermal phenomena, as well as in their amplitudes and
positions on the temperature scale.
Honey and sugar syrups demonstrate several thermal or thermo-chemical
parameters (phenomena), such as the glass transition temperature (Tg),
along with their respective changes in enthalpy of fusion (?Hfus), and
heat capacity (?Cp) which could be determined by DSC. The glass
transition temperature (Tg) is an important physical parameter for
determination of food adulteration and has been defined as the midpoint
temperature in the range over which the transformation from liquid to
amorphous state occurs at a given scan rate. This parameter is specific
to each food component and product, although it may vary slightly
depending on the thermal history of the material (as in the case of
honey that has been warmed to a certain temperature to lower its
viscosity). Tg values are strongly dependent on the amorphous phases of
the material and respond to modification caused by the addition of an
exogenous compound.
Water decreases the Tg. Most amorphous food components are miscible
with water, which acts as a plasticizer, causing a decrease in
transition temperature as water content increases. The glass transition
is accompanied by a change in heat capacity, which can be observed as
the base line change (shifts) on the heat flow of the thermo-analytical
DSC curve (thermogram).
Addition of syrups to honey can result in a decrease in glass
transition temperatures and an increase in the enthalpies of fusion. The
Tg position and intensity in honey and syrups are different and can be
used to distinguish between them. Pure substances can be characterized
by a unique and sharp melting point, which is not the case for honey
since it has a complex composition. Experiments showed that the effect
that adulterating honey with syrups has on the enthalpy of fusion
follows a linear relationship.
Figure 1 shows some typical features that may be observed on a DSC temperature scan.
Applied to honey samples artificially adulterated with different
industrial syrups, DSC measurements, under laboratory conditions, showed
a detection level of 5–10% depending on the type of syrup and the
measured parameter.
Conclusion
Different parameters, such as Tg and enthalpy changes, can be used to
detect the effects of certain adulterants. DSC provides valuable
information about the thermal behavior of honey and facilitates
detection of adulteration in commercial honey, as well as honey’s
physicochemical and structural properties. DSC is an analytical
technique that is capable of accurate and precise measurements and can
be applied to routine analysis.
