Question

Enzymes involved in the formation of ethyl esters leading to the development of fruity flavour in milk are ________.

• A. Proteases and lipases
• B. Lipases and esterases
• C. Proteases and esterases
• D. Esterases and hydrolases

Hint:

Remember:
- Lipase releases the raw materials (fatty acids).
- Esterase links the fatty acids with alcohol (ethanol) to produce volatile ethyl esters, which cause the fruity flavor.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The development of a "fruity" off-flavor (resembling apple, pear, or strawberry) in pasteurized milk is a common spoilage defect during refrigerated storage.
This defect is caused by the metabolic activity of psychrotrophic bacteria, primarily Pseudomonas fragi.
These bacteria produce extracellular enzymes that synthesize volatile ester compounds.
Detailed Explanation:
The biochemical pathway for the formation of fruity-smelling ethyl esters (such as ethyl butyrate and ethyl hexanoate) requires two enzymatic processes:
1. Lipases (fat-splitting enzymes): These enzymes hydrolyze milk lipids (triacylglycerols) to release free fatty acids, particularly short-chain and medium-chain fatty acids (such as butyric, caproic, and caprylic acids).
2. Esterases (or ester-synthesizing enzymes): These enzymes catalyze the esterification reaction between the released free fatty acids and ethanol (which is present in trace amounts in milk as a byproduct of background fermentation).
This reaction produces volatile ethyl esters.
\[ \text{Ethanol} + \text{Free Fatty Acids} \xrightarrow{\text{Esterase}} \text{Ethyl Esters (Fruity Aroma)} + \text{Water} \] Proteases, which break down milk proteins into peptides and amino acids, lead to bitterness or putrid off-flavors, but they do not participate in ester synthesis.
Hydrolases is a broad category that includes many enzymes, but the specific combination of lipase and esterase activity is required to generate these flavor-active compounds.
Thus, the cooperative action of bacterial lipases and esterases is responsible for the fruity flavor defect.

Step 2: Final Answer:
The enzymes involved are lipases and esterases.