When milk turns sour due to the action of bacteria, which organic acid is mainly produced and responsible for the sour taste?

Introduction / Context:
Food chemistry examines how chemical changes affect taste, texture and safety of food. When milk is left at room temperature, bacteria convert some of the lactose sugar into an organic acid that makes the milk sour. Recognising which acid is produced helps connect everyday observations with basic biochemical processes and is a common question in general knowledge and chemistry exams.

Given Data / Assumptions:

• Milk contains lactose, a milk sugar, and proteins such as casein.
• When milk sours, lactic acid bacteria grow and ferment the lactose.
• The options include lactic acid, acetic acid, tartaric acid, butyric acid and citric acid.
• We assume ordinary souring of milk due to natural fermentation, not contamination by unusual organisms.

Concept / Approach:
Lactic acid bacteria, such as Lactobacillus species, convert lactose into lactic acid in anaerobic conditions. Lactic acid reduces the pH of milk and causes the proteins to coagulate, forming curd. Acetic acid is associated with vinegar, tartaric acid is found in grapes and tamarind, citric acid in citrus fruits, and butyric acid in rancid butter. Therefore, lactic acid is the characteristic acid produced when milk turns sour through normal bacterial fermentation.

Step-by-Step Solution:
Step 1: Recall that lactose is the main sugar present in milk.Step 2: Understand that lactic acid bacteria ferment lactose under suitable conditions, such as warmth and time.Step 3: During fermentation, lactose is converted to lactic acid, which lowers the pH of milk.Step 4: The drop in pH causes milk proteins to coagulate, leading to curd formation and a sour taste.Step 5: Compare with acetic acid, which gives the sharp taste of vinegar, not the typical sourness of curdled milk.Step 6: Note that tartaric and citric acids are associated with fruits, not with milk fermentation.Step 7: Recognise that butyric acid contributes to the smell of rancid butter, which is different from ordinary sour milk.Step 8: Conclude that lactic acid is the main acid produced when milk becomes sour.

Verification / Alternative check:
Dairy technology and microbiology texts consistently state that yoghurt and curd are produced by lactic acid fermentation. The bacteria used in starter cultures are often named lactic acid bacteria because lactic acid is their main metabolic product when fermenting sugars. This strong association between milk and lactic acid further confirms that lactic acid is the correct answer.

Why Other Options Are Wrong:
Option b, acetic acid, is typical of vinegar from fermented alcohol, not fresh sour milk. Option c, tartaric acid, is common in grapes and tamarind. Option d, butyric acid, is present in rancid butter and has a strong unpleasant odour but is not the main cause of ordinary milk souring. Option e, citric acid, is dominant in lemons and other citrus fruits, not produced in significant amounts during milk fermentation.

Common Pitfalls:
Students may mix up acids associated with different sour tastes, thinking that all sourness must come from the same acid. It helps to link each acid with its typical source: lactic with milk, acetic with vinegar, tartaric with tamarind or grapes and citric with citrus fruits. Remembering these associations can quickly guide you to the correct answer in food chemistry questions.

Final Answer:
When milk turns sour, the main acid produced by bacterial fermentation is lactic acid.