Step 1: Understanding the Concept:
Fish spoilage is primarily caused by the metabolic activity of specific spoilage organisms (SSOs) that break down fish proteins and nucleotides post-mortem.
Evaluating nucleotide degradation is a key chemical method used to assess fish freshness.
Step 2: Detailed Explanation:
After fish die, adenosine triphosphate (ATP) degrades through a series of enzymatic steps:
\[ \text{ATP} \rightarrow \text{ADP} \rightarrow \text{AMP} \rightarrow \text{IMP (Inosine Monophosphate)} \rightarrow \text{HxR (Inosine)} \rightarrow \text{Hx (Hypoxanthine)} \]
Inosine monophosphate (IMP) contributes to the pleasant, sweet flavor of fresh fish.
However, specific psychrotolerant spoilage bacteria, primarily *Shewanella putrefaciens*, accelerate the breakdown of inosine into hypoxanthine (Hx).
Hypoxanthine accumulates in the muscle tissue and has a bitter taste.
The buildup of hypoxanthine, along with trimethylamine (TMA) and hydrogen sulfide produced by *Shewanella putrefaciens*, is responsible for the bitter off-flavors and off-odors characteristic of spoiled fish.
Therefore, *Shewanella putrefaciens* is the primary spoilage bacterium responsible for this degradation.
Step 3: Final Answer:
The fish spoilage bacterium that produces hypoxanthine, causing a bitter off-flavor, is *Shewanella putrefaciens*.