Mucic acid test – Principle, Procedure, Result, Uses

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Mucic acid test is a biochemical test used to detect galactose and galactose containing carbohydrates such as lactose. It is also known as galactaric acid test. In this test, carbohydrate sample is heated with concentrated nitric acid which act as a strong oxidizing agent. The nitric acid oxidizes both aldehyde group and primary alcohol group (C1 and C6 position) of sugar molecules to form dicarboxylic acids. Most monosaccharides like glucose forms dicarboxylic acids which are soluble in water but oxidation of galactose produces mucic acid which is highly insoluble in water. Because of this insolubility, a positive result is indicated by formation of white sandy crystals that precipitate at bottom of the solution. These crystals are rod shaped or rectangular. It confirms the presence of galactose or its derivatives in the given sample.

Principle of Mucic acid test

Principle of mucic acid test is based on oxidation of aldose sugars by a strong oxidizing agent which is concentrated nitric acid under high temperature. When galactose or galactose containing sugar like lactose is treated with concentrated nitric acid, both aldehyde group at first carbon (C1) and primary alcohol group at sixth carbon (C6) are oxidized into carboxylic acid groups. The reaction forms a dicarboxylic acid called galactaric acid which is also known as mucic acid. Unlike other saccharic acids formed from other monosaccharides (like glucaric acid from glucose) mucic acid is highly insoluble in cold water and it is precipitated out as white sandy crystals which are rod shaped or clear white. In case of lactose, nitric acid first causes hydrolysis of glycosidic bond releasing glucose and galactose and then galactose is oxidized to mucic acid. Since other sugars forms soluble dicarboxylic acids and remain dissolved in solution, formation of insoluble mucic acid crystals is used as a specific positive indicator for presence of galactose.

Mucic acid test reaction
Mucic acid test reaction

Objective of Mucic acid test

Objectives of mucic acid test are–

  • To detect the presence of galactose and lactose (or other galactose derivatives) in the given sample.
  • To distinguish galactose containing saccharides from other types of sugars.
  • To identify galactose and its derivatives for specific applications like food sample analysis and synthetic manufacturing.

Requirements for Mucic acid test

Requirements for mucic acid test are–

  • Test sample– 1% carbohydrate solution containing substance to be tested like galactose or lactose.
  • Reagent– Concentrated nitric acid (HNO3) which act as mucic acid reagent.
  • Control solution– Distilled water for baseline comparison.
  • Glassware and handling materials– Test tubes, test tube stand, pipette or graduated dropper. In micro scale method clean glass slides and wooden test tube holder are required.
  • Heating equipment– Boiling water bath for standard test tube method or small flame (alcohol lamp) for micro scale slide method.
  • Observation equipment– Microscope with high power objective to examine and confirm morphology of precipitated crystals.

Procedure of Mucic acid test

Procedure of Mucic acid test
Procedure of Mucic acid test

Procedure of mucic acid test are–

Macro scale test tube method–

  1. Take clean test tubes and add 5 to 6 ml of 1% carbohydrate test solution in one tube. Take another tube as control and add distilled water.
  2. Add 1 ml of concentrated nitric acid to each test tube and mix properly.
  3. The test tubes are heated in a boiling water bath for about 1.5 to 2 hours. The liquid is allowed to evaporate until volume is reduced to about 1 to 3 ml. This step is performed in well ventilated fume hood because toxic reddish brown nitrogen dioxide fumes are released.
  4. Remove the tubes from heat and allow it to stand undisturbed at room temperature overnight (12 to 24 hours) for cooling and crystallization.
  5. Observe bottom of test tube for formation of white sandy precipitate which indicates presence of galactose.

Micro scale glass slide method–

  1. Take clean glass slide and mix 3 drops of carbohydrate solution with 3 drops of concentrated nitric acid on the slide.
  2. The mixture is passed carefully over a small flame (alcohol lamp) using wooden test tube holder until the liquid becomes almost dry. Care should be taken not to char or scorch the sample.
  3. Allow the slide to cool to room temperature.
  4. The crystalline residue is examined under microscope using high power objective to see rectangular or rod shaped crystals.

Results of Mucic acid test

Results of Mucic acid test
Results of Mucic acid test

Results of mucic acid test are–

Positive result–

  • Visual appearance– Formation of insoluble white sandy or gritty crystalline precipitate at bottom of test tube.
  • Microscopic appearance– Precipitate is seen as stable rod shaped crystals or rectangular plates or clustered masses under microscope.
  • Interpretation– It confirms the presence of galactose or galactose derivatives like lactose in the given sample.

Negative result–

  • Visual appearance– Solution remains clear or slightly yellow but no crystals or precipitate is formed.
  • Interpretation– It indicates absence of galactose or lactose in the sample. Other carbohydrates like glucose fructose maltose or sucrose gives this negative result because soluble dicarboxylic acids are formed.

Uses of Mucic acid test

Uses of mucic acid test are–

  • It is used to identify the presence of galactose or its derivatives in food samples.
  • It is used to detect galactose containing substances like lactose, agar-agar, carrageenan, and gum arabic.
  • It is used to verify presence of lactose in dairy derived ingredients and to detect adulteration of other sweeteners with galactose containing syrups.
  • It is used as a clinical screening method to detect galactose in biological fluids like urine and it was used in diagnosis of galactosemia.
  • It is used in assisting in determination of blood groups.
  • It is used to identify galactose and its derivatives for application in synthetic manufacturing.
  • It is used as a low cost educational experiment in medical and biochemical training to show chemical properties of galactose.

Limitations of Mucic acid test

Limitations of mucic acid test are–

  • It cannot differentiate between free galactose and galactose containing disaccharides or polymers like lactose because both gives same positive result.
  • False positive can occur due to impurities having terminal carbonyl groups or other organic compounds which forms stable insoluble precipitate with concentrated nitric acid.
  • False negative can occur when concentration of galactose is very low to form visible precipitate or when heating and cooling conditions are not followed properly.
  • It is time consuming test and it may require hours of heating and overnight cooling for proper crystal formation.
  • Concentrated nitric acid is required which is highly corrosive and may react violently with organic matter if not handled carefully.
  • Toxic nitrogen dioxide (NO2) gas is produced so it must be done in fume hood and it limits portability and use outside proper laboratory.
Mucic acid test infographic
Mucic acid test infographic

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