All microorganisms require water to grow and carry out metabolic functions. Drying is a preservation method that works by reducing the amount of available water, thus inhibiting microbial growth. The concept of water activity ($a_w$) is more scientifically precise, but moisture content is often used as a practical measure.
Step 1: Understanding the Question:
The question asks for the general moisture content threshold below which the growth of most bacteria is inhibited in dried food products.
Step 2: Detailed Explanation:
Different types of microorganisms have different water requirements:
Bacteria generally have the highest water requirement. Most spoilage bacteria need a water activity ($a_w$) of 0.91 or higher, which corresponds to relatively high moisture content.
Yeasts can tolerate slightly drier conditions, typically down to an $a_w$ of 0.88.
Molds are more tolerant of dry conditions and can grow at an $a_w$ as low as 0.80.
A moisture content of 15% in many foods corresponds to a water activity level below 0.90, which is low enough to prevent the growth and multiplication of most pathogenic and spoilage bacteria. While 5% would be even more effective, 15% is a widely recognized general threshold for controlling bacterial growth. Levels of 30% and 50% are far too high and would permit rapid bacterial spoilage.
Step 3: Final Answer:
The moisture content level below which most bacteria cannot grow is approximately 15%.