Presence of Cane Sugar in Milk

To determine the presence of cane sugar (sucrose) in milk, you can use a combination of chemical tests that exploit the properties of sucrose. Milk naturally contains lactose, a different sugar, so the goal is to identify sucrose specifically, which would indicate added cane sugar. Here’s a straightforward approach based on standard biochemical methods:

Method: Fehling’s Test (or Benedict’s Test) After Acid Hydrolysis

1. Why this works: Sucrose is a non-reducing sugar, meaning it doesn’t directly react with reagents like Fehling’s or Benedict’s solution (which detect reducing sugars like lactose). However, when sucrose is hydrolyzed (broken down) into glucose and fructose—both reducing sugars—it can be detected. Lactose, already a reducing sugar, will react before and after hydrolysis, but a significant increase in reaction after hydrolysis suggests sucrose.

2. Steps:

   • Sample Preparation: Take a small sample of the milk (e.g., 10 mL).

   • Test for Reducing Sugars (Before Hydrolysis):

     • Add 2 mL of Fehling’s solution (or Benedict’s solution) to 2 mL of the milk sample.

     • Heat the mixture in a boiling water bath for 5-10 minutes.

     • Observe: A red precipitate (cuprous oxide) indicates reducing sugars (lactose in milk). Note the intensity of the reaction.

   • Hydrolysis of Sucrose:

     • Take another 10 mL of the milk sample and add 2 mL of dilute hydrochloric acid (HCl, ~10%).

     • Heat gently (e.g., 70-80°C) for 5-10 minutes to hydrolyze any sucrose into glucose and fructose.

     • Neutralize the acid with sodium hydroxide (NaOH) until the pH is neutral (test with litmus paper).

   • Test Again for Reducing Sugars (After Hydrolysis):

     • Repeat the Fehling’s or Benedict’s test on the hydrolyzed sample.

     • Observe: A stronger red precipitate compared to the first test suggests the presence of sucrose, as hydrolysis converts it to detectable reducing sugars.

3. Interpretation:

   • If the reaction before hydrolysis shows a moderate precipitate (due to lactose), but the reaction after hydrolysis is significantly stronger, cane sugar (sucrose) is likely present.

   • If there’s little to no difference, sucrose is likely absent, and the reaction is just from milk’s natural lactose.

Alternative: Resorcinol Test (Selivanoff’s Test Modification)

1. Why this works: This test distinguishes ketoses (like fructose, a sucrose breakdown product) from aldoses (like lactose). Sucrose, after hydrolysis, yields fructose, which reacts quickly to form a red color.

2. Steps:

   • Add 1 mL of milk to 5 mL of resorcinol reagent (resorcinol in dilute HCl).

   • Heat in a boiling water bath for 5 minutes.

   • Observe: A rapid, deep red color within 1-2 minutes suggests fructose from hydrolyzed sucrose. Lactose reacts more slowly and gives a weaker color.

3. Interpretation: A strong, quick reaction indicates sucrose presence after hydrolysis.

Practical Notes

• Controls: Test pure milk (known to have only lactose) and milk spiked with cane sugar as comparisons.

• Limitations: These tests are qualitative, not quantitative. For precise sucrose amounts, advanced methods like HPLC (High-Performance Liquid Chromatography) are needed.

• Safety: Handle acids and heating steps with care, ideally in a lab setting.