I have manufactured a product (pharmaceutical product) by spray drying. Batch size is 120 kg. The product passes microbiological limit test (MLT).

When I run a 6 kg batch size, my product is failing in microbiologcal test (MLT). With 120 kg batch size, product passes microbiology test.

Before running every batch, I take microbiology clearance of the spray dryer, but empty spray dryer gives high count, inspite of thorough washing and sterilization (heating at 120 deg C).

Why am I getting high count when I run a 6kg batch and NIL count with 120 kg batch size in a spray dryer, keeping all other conditions same? Can someone help me to understand?