Rancidity Testing of Oils and Fats

Introduction

• Purpose: To check the rancidity of an oil or fat sample.
• Key Steps:
  1. Prepare reagents.
  2. Conduct the crease test.
  3. Determine peroxide value and acid value.
  4. Interpret results.

Required Materials

• Equipment:
  • UV spectrophotometer
  • Quartz cuvette
  • Balance machine
  • Incubator
  • Falcon tube
  • Beaker
  • Pipette
  • Filler
  • Measuring cylinder
• Reagents:
  • Ethanol
  • Hydrochloric acid
  • Acetic acid
  • Chloroform
  • Diethyl ether
  • Fluoro-glucanol
  • Trichloroacetic acid

Preparation of Reagents

30% Trichloroacetic Acid (TCA)

1. Materials:
   • 50 ml glacial acetic acid
   • 15 gm trichloroacetic acid
2. Steps:
   • Measure and transfer glacial acetic acid into a reagent bottle.
   • Weigh and add trichloroacetic acid, shake until dissolved.

1% Fluoro-glucanol Solution

1. Materials:
   • 50 ml glacial acetic acid
   • 0.5 gm fluoroglucinol powder
2. Steps:
   • Measure and transfer glacial acetic acid into a reagent bottle.
   • Weigh and add fluoroglucinol powder, shake until dissolved.

0.1% Fluoro-glucanol Solution (Qualitative Test)

1. Materials:
   • 10 mg fluoroglucinol powder
   • 10 ml diethyl ether
2. Steps:
   • Dissolve fluoroglucinol in diethyl ether and shake until dissolved.

CRIS Test Procedures

Qualitative Test

1. Sample Preparation:
   • Take 5 ml of oil/fat sample in a clean beaker.
   • Add 5 ml of 0.1% protoglucinal solution and shake.
   • Add 5 ml concentrated hydrochloric acid and shake again.
2. Results:
   • Reddish pink color indicates incipient rancidity.
   • Negative control (non-rancid oil) should not develop pink color.

Quantitative Test

1. Sample Preparation:
   • In a clean falcon tube, add 5 ml of oil sample.
   • Add 5 ml chloroform, close and shake.
   • Add 10 ml of 30% TCA solution and 1 ml of 1% fluoro-glycinal solution, close and shake.
   • Incubate at 45°C for 15 minutes.
   • After incubation, add 4 ml of absolute ethanol and mix gently.
2. Measurement:
   • Set spectrophotometer to 545 nm.
   • Zero the blank cuvette, measure absorbance of the prepared sample.
   • Results:
     • Sample absorbance of 0.338 indicates incipient rancidity (above 0.2).
     • Negative control absorbance of 0.126 indicates non-rancidity (below 0.15).

Other Methods to Check Rancidity

• Peroxide Value Determination:
  • Links for additional resources available in the description.
• Free Fatty Acid and Acid Value Determination:
  • Links for additional resources available in the description.

Interpretation of Results

• Qualitative CRIS test shows pink color indicating incipient rancidity.
• Quantitative CRIS test shows absorbance of 0.338 indicating rancid oil.
• Fresh oils: Peroxide values below 10 mEq/kg; noticeable rancidity starts over 20 mEq/kg.
• Higher free fatty acid and acid values indicate rancidity.