Final answer:
An aw (water activity) value of 0.5 is below the threshold needed to support the active growth of bacteria, which generally require an aw of 0.97-0.99. Water activity is critical for controlling microbial growth in food preservation.
Step-by-step explanation:
The water activity value that does not have enough moisture to support the active growth of bacteria is 0.5. Water activity, or aw, measures the available water for microorganisms to grow. An aw of 1.0 is the value for pure water, and most bacteria require high aw (0.97-0.99) to thrive. The growth of fungi like Aspergillus spp. can occur at lower water activity levels, approximately in the range of 0.8-0.75.
Therefore, an aw of 0.5 is too low to support the active growth of bacteria, which is important for food preservation techniques such as drying or adding high concentrations of solutes like salt or sugar to create high osmotic pressure environments unfavourable for bacterial growth.