Yogurt is a fermented dairy product created by the controlled fermentation of milk by specific types of bacteria. These bacteria convert the milk sugar (lactose) into lactic acid.
Step 1: Understanding the Question:
The question asks to identify the key microorganism involved in the fermentation process that turns milk into yogurt.
Step 2: Detailed Explanation:
Let's analyze the roles of the given organisms in food science:
(A) Saccharomyces cerevisiae: This is a species of yeast. It is widely used in baking as a leavening agent and in brewing for alcoholic fermentation.
(B) Lactobacillus bulgaricus: This is a species of lactic acid bacteria (LAB). It is one of the two standard bacteria used in a symbiotic starter culture for making yogurt, along with Streptococcus thermophilus. It is primarily responsible for the acid and flavor development.
(C) Aspergillus niger: This is a type of mold (a fungus). It is used in industrial fermentation to produce enzymes and citric acid, but it is a spoilage organism in many other foods.
(D) Escherichia coli: This is a bacterium commonly found in the lower intestine of warm-blooded organisms. While most strains are harmless, some can cause serious food poisoning, and its presence is often used as an indicator of fecal contamination.
Based on this, Lactobacillus bulgaricus is the correct organism for yogurt production.
Step 3: Final Answer:
Lactobacillus bulgaricus is a primary organism responsible for yogurt production.