Carbohydrate Testing Methods in Food
The three reactions above have almost the same principle, namely using an aldehyde group in sugar to reduce the Cu2SO4 compound to Cu2O (brick red enpadan) after being heated in an alkaline atmosphere (Benedict and Fehling) or acid (Barfoed) with the addition of a binding agent (chelating agent) like Na-citrate and K-Na-tatrat.
Iodine reaction
KH (poilisaccharide) + Iod (I2) à specific color (black blue)
Seliwanoff's reaction
KH (ketose) + H2SO4 à furfural à + resorcinol à red color.
KH (aldosa) + H2SO4 à furfural à + resorcinol à negative
Osazon reaction
This reaction can be used both for aldose and ketose solutions, by adding a phenylhydrazine solution, then heated to form a yellow crystal called hydrazone (osazon).
Quantitative Test
For the determination of carbohydrate levels can be done by physical, chemical, enzymatic, and chromatographic methods (not discussed).
Physical Method
There are two (2) types, namely:
Based on the refractive index
This method uses a device called a refractometer, which is by the formula:
X = [(A + B) C - BD)]
Where :
X =% of sucrose or sugar obtained
A = weight of sample solution (g)
B = weight of diluent solution (g)
C =% sucrose in camp A and B in the table
D =% sucrose in thinner B
Based on optical rotation
This method is used based on the optical properties of sugars that have an asymmetrical structure (can rotate the plane of polarization) so that it can be measured using a device called a polarimeter or digital polarimeter (the result can be known directly) called a sacarimeter.
According to Biot law; "The optical rotation size of each individual sugar is proportional to the concentration of the solution and the thickness of the liquid" so that it can be calculated using the formula:
[a] D20 = 100 A
L x C
Where :
[a] D20 = rotation type at 20 oC using
D = yellow light at a wavelength of 589 nm from the Na lamp
A = observed angle of rotation
C = content (in g / 100 ml)
L = tube length (dm)
so C = 100 A
L x [a] D20
Chemical Method
This method is based on the reducing properties of sugars, such as glucose, galactose, and fructose (except sucrose because it has no aldehyde group). Even though fructose does not have an aldehyde group, it has an alpha hydroxy ketone group, so that it can still react.
In this chemical method there are two (2) kinds of ways, namely:
Titration
For the first way, it can see the standardized method by BSN, namely the SNI for food and beverage testing SNI number 01-2892-1992.
Spectrophotometry
The second method uses the principle of CuSO4 reduction reaction by carbonyl groups on reducing sugars which after heating is formed of oxide oxide deposits (Cu2O) and then added Na-citrate and Na-tatrate and phosphomolibdic acid to form a blue compound compound that can be measured by spectrophotometer at a wavelength of 630 nm.
Enzymatic Method
For this enzymatic method, it is very appropriate to be used for determining the sugar casing individually, due to the work of a very specific enzyme. Examples of enzymes that can be used are glucose oxidase and hexokinase. Both are used to measure glucose levels.
Glucose oxidase
D-Glucose + O2 by glucose oxidase à Gluconate Acid and H2O2
H2O2 + O-disianidin by the peroxidase enzyme à 2H2O + brown-oxidized O-disianidin (can be measured at 1540 nm)
Hexokinase
D-Glucose + ATP by hexokinase à Glucose-6-Phosphate + ADP
Glucose-6-Phosphate + NADP + by glucose-6-phosphate dehydrogenase à Gluconate-6-Phosphate + NADPH + H + The presence of fluorescent NADPH (having chromophore groups) can be measured at 334 nm where the amount of NADPH formed is equal to the amount of glucose.