**Core Concept:** Pasteurisation is a process of heat treatment used to reduce the number of microorganisms in food products, particularly milk, to improve shelf-life, safety, and quality. Bacteria responsible for spoilage and foodborne illnesses, such as Lactobacillus and Streptococcus, are reduced or killed by heat treatment.

**Why the Correct Answer is Right:** Pasteurisation aims to reduce the presence of harmful microorganisms in milk, which can lead to spoilage or foodborne illnesses. Bacteria like Lactobacillus and Streptococcus are heat-sensitive, meaning they are affected by temperature changes. By subjecting milk to a specific temperature and time, these bacteria are killed or significantly reduced, ensuring the safety and quality of the product.

**Why Each Wrong Option is Incorrect:**

A. Bacteria spores (such as Clostridium botulinum) are resistant to heat and cannot be fully eradicated by pasteurisation.
B. This option is incorrect as pasteurisation is primarily designed to kill or inactivate microorganisms, not to preserve the nutritional content.
C. Some bacteria are heat-resistant and can survive pasteurisation, even though it reduces their numbers.
D. Pasteurisation effectively reduces the number of bacteria, but it does not eliminate all microorganisms, leaving some bacteria that can cause foodborne illnesses if consumed.

**Clinical Pearl:** Apart from pasteurisation, other preservation methods include ultra-high temperature (UHT) treatment, which goes beyond the standard pasteurisation temperature and has a longer shelf-life, but may alter the milk's taste and nutritional content. Understanding the limitations of pasteurisation is crucial for safe food handling and consumption.