In the conversion of milk into curd during fermentation, which enzyme produced by lactic acid bacteria mainly acts on the milk sugar lactose?

Introduction / Context:
The conversion of milk into curd is a very common process in households and an important example of microbial fermentation in biology. Lactic acid bacteria convert the sugar present in milk into lactic acid, which causes coagulation of milk proteins and the formation of curd. This question focuses on the specific enzyme that acts on the milk sugar lactose during this process. Understanding which enzyme is involved helps link everyday food transformations with underlying biochemical concepts.

Given Data / Assumptions:
– The process described is the fermentation of milk into curd by lactic acid bacteria.
– The sugar in milk is lactose, a disaccharide composed of glucose and galactose units.
– Options list four enzymes: Pepsin, Lactase, Invertase, and Diastase.
– We assume that the question is asking for the enzyme that hydrolyses lactose so that fermentation can proceed efficiently.

Concept / Approach:
Lactase is the enzyme that specifically catalyses the hydrolysis of lactose into its monosaccharide components, glucose and galactose. Lactic acid bacteria present in starter cultures produce lactase and then further metabolise the resulting sugars to lactic acid. This acid lowers the pH, leading to coagulation of casein protein and formation of curd. Pepsin is a protein digesting enzyme present in the stomach, not in the fermentation culture. Invertase breaks down sucrose into glucose and fructose, which is more relevant for cane sugar, not lactose. Diastase is a general term for amylase like enzymes that break down starch into simpler sugars. Among the options, lactase is clearly the enzyme that directly targets lactose during curd formation.

Step-by-Step Solution:
Step 1: Identify lactose as the main sugar in milk that needs to be broken down during fermentation. Step 2: Recall that the enzyme that splits lactose into glucose and galactose is called lactase. Step 3: Note that pepsin digests proteins in the human stomach, invertase acts on sucrose, and diastase acts on starch. Step 4: Conclude that lactase is the enzyme most directly involved in the initial breakdown of milk sugar in curd formation.

Verification / Alternative check:
Textbooks on biology and food science explain that lactic acid bacteria, such as Lactobacillus species, ferment lactose. They often mention that these bacteria possess lactase or beta galactosidase enzymes which hydrolyse lactose before further metabolism. References clearly distinguish this from enzymes like invertase and diastase, which have different substrates. Reading labels on commercial lactose free milk, which is treated with lactase, also reinforces the connection between lactase and lactose breakdown. This evidence supports lactase as the correct answer.

Why Other Options Are Wrong:
Pepsin: This is a digestive enzyme in the human stomach that breaks down proteins and is not the primary enzyme acting on lactose in milk fermentation.
Invertase: Invertase catalyses the hydrolysis of sucrose into glucose and fructose and is not specific for lactose, so it does not best describe the enzyme used in curd formation.
Diastase: This term refers to amylase type enzymes that break down starch, not the milk sugar lactose, so it is not the correct choice here.

Common Pitfalls:
Students sometimes memorise enzyme names without linking them to their substrates and may choose invertase simply because it sounds like a sugar related enzyme. Others may be misled by pepsin due to its association with digestion and proteins in milk. To avoid such confusion, always try to pair enzyme names with their key substrates: lactase with lactose, sucrase or invertase with sucrose, amylase or diastase with starch, and pepsin with proteins. Such direct associations improve accuracy in enzyme related questions.

Final Answer:
During the conversion of milk into curd, the enzyme that primarily acts on the milk sugar lactose is Lactase produced by lactic acid bacteria.