
variation of 2.8–7.8%. Carbohydrates other than
ketoses do not interfere.
0012 In alkaline solution reducing sugars, which contain
an aldehyde or keto group, can reduce copper, silver,
bismuth, and mercury salts to compounds of lower
valence or to a metallic state. The best known re-
agent, based on the reduction of copper, is Fehling’s
solution (two solutions: (1) cupric sulfate; (2) sodium
potassium tartrate and sodium hydroxide). Depending
on the concentration of sugars in a solution, heating
in the presence of Fehling’s solution gives a yellowish
orange to red solution or precipitate.
0013 The available assay methods for mono- and oligo-
saccharides include chemical, colorimetric, chroma-
tographic, electrophoretic, optical, and chemical
procedures. Today, more and more assay techniques
involve preliminary separation by chromatographic
and electrophoretic techniques prior to actual assay
by classic chemical procedures or colorimetric tests.
Microbiological assays have found relatively little
application, while the use of enzymes as aids in
sugar analysis or in actual assays is gaining in popu-
larity with the commercial availability of pure, select-
ive, and stable preparations.
0014 The separation of sugar by paper chromatography
has been developed into the versatile form of qualita-
tive and quantitative carbohydrate microanalysis.
Qualitative paper chromatography is the simplest
method to distinguish between various sugars in a
food. The development of the chromatography is
done by the application of compound sprays to iden-
tify spots. Separation of 1% sugar solutions contain-
ing a maximum of 60 mm of an individual sugar gives
best results. Resolution is more rapid with phenol–
water or colloidin–water systems. In a phenol-
containing system fructose moves faster than xylose.
0015 In enzymatic methods, selective cleavage to mono-
saccharides and/or enzymatic assay of the mono-
saccharide is specific and widely used. The use of
amylase and amyloglucosidase is an example of the
first type; determination of glucose with glucose
oxidase is an example of the second type. Micro-
organisms and enzymes can also be used in the pre-
treatment of substrate prior to assay by chemical or
physical methods. For example, hydrolysis of sucrose
by invertase is much more specific than acid hydroly-
sis, and invertase is used in various procedures requir-
ing sucrose inversion.
Sensory Properties
0016 Fructose is perceived to be sweeter relative to sucrose.
b-d-fructose in solution has been rated with values
from 100 to 175 compared to a sucrose sweetness
value of 100. In the crystalline form, b-d-fructose
was rated 180 relative to crystalline sucrose at 100.
The most common theories of why some molecules
vary in perceived sweetness is based on the hydrogen
bonding and geometrical shape of the molecule.
Intramolecular hydrogen bonding enhances sweet-
ness, as well as intermolecular bonding with a recep-
tor site that has a complementary configuration to the
molecule to allow for efficient bonding. Typically, a
proton donor site, designated as AH, is located
approximately 0.3 nm from an electronegative site,
designated as B. The fructose molecule meets these
requirements to elicit the sweet taste. The AH is
designated to be the anomeric hydroxyl group and B
is designated as the oxygen atom of the primary
alcohol group. A third site capable of hydrophobic
bonding designated as g is the methylene carbon
atom.
0017The sweetness of fructose has been reported to
decrease with time or age of the solution and is ac-
counted for by mutarotational equilibrium with other
forms which are not perceived as sweet. Increased
temperature of the solution also depresses the sweet-
ness value of a fructose solution: at the same time the
specific rotation is increased and there is a shift in the
equilibrium. The age and temperature of the solution
may account for the reported differences of the sweet-
ness of fructose when compared with sucrose, ranging
from twice to one-third as sweet. The specific rota-
tion of fructose increases with increasing concentra-
tion, although not as rapidly as with increased
temperature. The relative sweetness of different con-
centrations is temperature–concentration-dependent.
At lower temperatures, the relative sweetness of fruc-
tose decreases with increasing concentration but, at
higher temperatures (37
C), the relative sweetness
increases. This reflects a complex dependence on
the shift in equilibria, shift of forms, and other
phenomena.
0018Sweetness is system-dependent, i.e., the perceived
sweetness in a food or beverage system depends on
several factors, including temperature, pH, solids
content, and the presence of other sweeteners. Fruc-
tose exhibits a synergy with other sweeteners: the
relative sweetness of 50/50 fructose–sucrose mixture
is 128. Similar synergies occur when fructose is used
in combination with aspartame, saccharin, and/or
sucralose. This synergy allows for a formulator either
to obtain higher degrees of sweetness in the finished
product without increasing the total level of sweeten-
ers, or to retain a satisfactory degree of sweetness
while reducing the amount of sweeteners used and
the cost. Either way, fructose both sweetens and
improves the sweetness profile.
0019The sweetness intensity profile of fructose is differ-
ent from those of sucrose and dextrose. The sweetness
2750 FRUCTOSE