Carbohydrates: Glucose and Taste Threshold

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Practical 1 : Carbohydrates

Introduction : Carbohydrates are important to human life , this practical is to investigate physical , functional and sensual properties of different carbohydrates , including monosaccharides and disaccharides .

Experiment 1
Title : Relative Solubility of Carbohydrates , Glucose , Galactose and Lactose .

Objective : To determine the relative solubility of glucose , galactose and lactose by measuring their saturation point at the same constant temperature.

Principle : With 10g distilled water contained in a beaker , then add small quantities of the carbohydrate being test at a time with constant stirring until no more can dissolve , saturation point is represented at r.t.p.. Test the carbohydrates , glucose , galactose and lactose , relative solubility of them can be compared.

Precaution : 1) When using magnetic stirrer to stir the mixture , or stirring the solution manually , the speed should not be too fast to prevent splash out of solution and give inaccurate result . 2 ) The temperature of which the experiment performed should remain constant , otherwise solubility of the sugars would be affected and give an inaccurate result.

Results :

sugarsInitial reading ( g )After addition of water ( g )Final reading ( g )Sugar added ( g )







water added ( ml )

Saturation point ( mg / ml )


43.87-36.55 = 7.32
( 1.65 x 1000 ) / 7.32 = 225


45.47-37.40 = 8.07

( 0.86 x 1000 ) / 8.07 = 107


44.01-36.56 = 7.45
( 4.96 x 1000 ) / 7.45 = 666

Conclusion : Relative solubility of glucose is the highest , which is 666mg/ml , and lactose is the lowest , which is 107mg/ml.

Discussion : Comparing saturation points of the three sugars and consider how this relates to the use of them in humans .

The saturation point of glucose is the greatest , galactose the second and lactose is the lowest . Glucose has the highest solubility in water , followed by galactose and lactose the lowest .

Glucose is the major source of energy in human , daily energy required by human mainly comes from oxidation of glucose , so the high solubility of glucose in water allows it to be transported easily and readily in our body . Lactose is milk sugar , it is especially important in the early stage of human life when milk is the major source of nutrients . It has low solubility , this make it impossible to be absorbed directly from the human intestine , so it should be first broken down by the enzyme lactase into its constituent – glucose and galactose in order to be easily absorbed from the intestine.

The monosaccharides , glucose and galactose produced by breaking down of lactose can be utilized as energy or building blocks in the body . Glucose is the major source of energy in human body , there is also conversion of galactose to glucose takes place in the liver , As galactose is less soluble than glucose , it can be temporarily stored in the body before it is converted to glucose , because it has less influence on water potential of body cells and so would not upset the osmotic balance of cells .

Experiment 2 and 3
Title : Relative Sweetness and Taste Threshold .

Objective : To determine relative sweetness of sucrose , glucose and maltose and to find out the taste threshold for different sugars of ourselves , then measure the concentration of the sugars at the taste threshold level with digital refractometer .

Principle : Testing few ml of the prepared sugar solutions each time with different concentration ( labeled 1 – 10 ), with 1 being the lowest and 10 being the highest , the first noticeable sweet taste for each solution being detected is the taste threshold of a person. By measuring the refractive index of the sugars , sugar concentration of...
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