Lipid Property Tests

• Tests analyze lipid properties and detect

  • Rancidity is a chemical process that leads to the deterioration of fats and oils, resulting in off flavors and odors. This process not only affects the sensory qualities of food but can also indicate spoilage, making it essential to monitor for the presence of rancid compounds.

  Acrolein Test

  • The Acrolein Test is a qualitative method used to detect the presence of glycerol in lipids. This test involves heating lipids, which are composed of glycerol and fatty acids, with potassium bisulfate (KHSO₄), a dehydrating agent.
  • As the lipids are heated, dehydration occurs, converting the glycerol component into acrolein (CH₂=CH-CHO). Acrolein is characterized by its pungent and irritating odor, which serves as a distinctive marker in this testing process.
  • The presence of acrolein after the test is a clear indication that glycerol is present in the sample, making this test a valuable tool in food science for monitoring lipid quality and integrity.

  Fat Saponification

  • Fat saponification refers to the process of converting fats or oils into soap through a reaction with an alkali, typically sodium hydroxide or potassium hydroxide. This process not only highlights the chemical properties of fatty acids but also offers insight into the stability and quality of the fats involved.

  Acrolein Test

  • The Acrolein Test, as previously outlined, emphasizes the importance of identifying glycerol in lipids. The formation of acrolein serves as a critical step in determining lipid quality and the extent of rancidity within food products.

  Fat Saponification

• Triglycerides undergo hydrolysis with a strong base (NaOH/KOH).

• This process produces glycerol and soap (salt of fatty acids).

• This test is for fatty acid detection.

Kreis Test

• Lipid oxidation leads to aldehyde formation.
• Aldehydes react with phloroglucinol to produce a red color.
• This test is for aldehyde rancidity.

Saturated Fatty Acids

• Saturated fatty acids have no double bonds.
• Examples include palmitic acid and stearic acid.

Unsaturated Fatty Acids

• Unsaturated fatty acids have one or more double bonds.
• Examples include oleic acid and linoleic acid.

Fatty Acid Synthesis (Anabolic Pathway)

Fatty acid synthesis occurs predominantly in the cytoplasm of cells, where the necessary enzymes and substrates are readily available for the complex biochemical processes involved in this anabolic pathway.

The precursor for fatty acid synthesis is Acetyl-CoA, a central metabolite in carbohydrate and lipid metabolism. Acetyl-CoA is formed from the breakdown of carbohydrates (via glycolysis), fats, and certain amino acids, making it a crucial link between various metabolic pathways.

Fatty Acid Synthase (FAS) is the key enzyme that catalyzes the elongation of fatty acids. It is a multi-enzyme complex that facilitates the synthesis of long-chain fatty acids by sequentially adding acetyl and malonyl groups. FAS operates through a series of reactions that involve the condensation, reduction, dehydration, and another reduction process, ultimately creating saturated fatty acids, predominantly palmitic acid (C16:0).

Malonyl-CoA, another important metabolite in fatty acid synthesis, provides the essential 2-carbon units required for chain elongation. It is generated from Acetyl-CoA by an enzyme called Acetyl-CoA carboxylase (ACC), which is a rate-limiting step in the synthesis pathway. Malonyl-CoA not only acts as a building block but also plays regulatory roles in lipogenesis and fatty acid oxidation.

The synthesis of fatty acids requires reducing power in the form of NADPH. NADPH is primarily derived from the pentose phosphate pathway (PPP), a metabolic pathway that generates NADPH and ribose-5-phosphate for nucleotide synthesis. This process is vital because NADPH acts as a reducing agent, enabling the reduction steps in fatty acid synthesis to proceed efficiently.

β-Oxidation of Fatty Acids (Catabolic Pathway)

• Occurs in the mitochondria.
• Step 1: Activation: Fatty acid is converted to fatty acyl-CoA by acyl-CoA synthetase.
• Step 2: Transport: The carnitine shuttle brings fatty acyl-CoA into the mitochondria.
• Step 3: β-Oxidation Cycle involves 4 steps: oxidation, hydration, oxidation, and cleavage.
• Produces Acetyl-CoA, FADH₂, and NADH, which enter the TCA cycle and ETC for ATP production.

Ketone Bodies

• Form when acetyl-CoA accumulates (e.g., during fasting or in diabetes).
• Examples include acetoacetate, β-hydroxybutyrate (major circulating ketone), and acetone (exhaled).
• Used by the brain and muscles during energy deficiency.