During the natural fermentation of milk into curd, which of the following microorganisms is mainly responsible for converting lactose into lactic acid?

Introduction / Context:
Curd is a common fermented milk product in many cultures. The transformation of milk into curd involves microbial fermentation, during which lactose, the sugar in milk, is converted into lactic acid. This process causes the milk to coagulate and develop a tangy flavour. This question asks you to identify the main microorganism responsible for curd formation from a list that includes bacteria, fungi, and non microbial organisms.

Given Data / Assumptions:

• The process described is the formation of curd from milk.
• Options include Lycopodium, yeast, Lactobacillus, and a general fungus.
• We assume knowledge that curd formation involves lactic acid bacteria rather than yeast or non microbial agents.

Concept / Approach:
Lactobacillus species are lactic acid bacteria that thrive in milk. They ferment lactose into lactic acid, which lowers the pH of the milk. As the acidity increases, milk proteins such as casein coagulate, leading to the thick, semi solid texture of curd. In many traditional methods, a small amount of previous curd containing live Lactobacillus is added to fresh milk to act as a starter culture. Lycopodium is a plant genus, yeast is mainly used for alcoholic fermentation and bread making, and general fungi are not the main agents in standard curd formation.

Step-by-Step Solution:
Step 1: Identify that curd formation is due to lactic acid fermentation of lactose. Step 2: Recall that lactic acid bacteria, especially Lactobacillus, are responsible for this conversion. Step 3: Recognise that Lycopodium is a club moss plant and therefore cannot be the microbial agent forming curd. Step 4: Note that yeast is typically associated with alcoholic fermentation in bread and beverages, not with curd production, and that a general fungus is too vague and incorrect compared to the specific lactic acid bacterium.

Verification / Alternative check:
Food microbiology resources describe curd and yoghurt production as relying on specific starter cultures of lactic acid bacteria, commonly Lactobacillus species and sometimes in combination with other genera. These bacteria are added to milk, incubated at warm temperature, and allowed to ferment lactose. This method yields consistent curd products and confirms Lactobacillus as the main organism involved in traditional curd making.

Why Other Options Are Wrong:

• Lycopodium: This is a genus of primitive vascular plants and has no role in milk fermentation.
• Yeast: Yeasts such as Saccharomyces cerevisiae ferment sugars to produce carbon dioxide and alcohol and are used in baking and brewing, not in curd formation.
• A general fungus: Although some fermented foods involve moulds or fungi, the standard process of curd formation relies on lactic acid bacteria, not on filamentous fungi.

Common Pitfalls:
Learners may generalise that all fermentation is due to yeast or fungi because of their association with bread and alcohol. It is important to remember that there are many different kinds of fermentation, each with characteristic microorganisms. Lactic acid fermentation in milk is dominated by Lactobacillus and related bacteria, not by yeast. Remembering that curd is a lactic acid fermented product helps in selecting the correct bacterial agent.

Final Answer:
The bacterium mainly responsible for curd formation from milk is Lactobacillus, a lactic acid producing bacterium.