The skipjack tuna loin that you took out of your freezer to thaw has a complex scientific story behind it — a tale of biochemical battles, microbial wars, and sensory changes that determine whether your fish will be a delicious meal or a disappointing waste.
When we place fish in our freezers, we often assume we've pressed the pause button on spoilage. But the reality is far more complex. For skipjack tuna, one of the world's most popular commercial fish species, the journey through frozen storage involves a fascinating interplay of biochemistry, microbiology, and food science that directly impacts what arrives on our plates.
This article explores what scientific research reveals about how freezing affects skipjack tuna — from the delicate proteins that give it texture, to the microbes that threaten its freshness, and the sensory qualities that determine whether we'll enjoy eating it.
Contrary to popular belief, freezing is not a method of preservation in itself — it's merely the means of preparing fish for storage at low temperatures. The freezing process converts most of the water in fish (which constitutes 60-80% of its composition) into ice, but this transformation happens gradually across a range of temperatures rather than at a single freezing point 1 .
Even at temperatures as low as -30°C, a small proportion of water in fish muscle remains unfrozen 1 . This ongoing physical change sets the stage for the biochemical drama that unfolds during frozen storage.
The most crucial period during freezing is when fish passes through the "zone of maximum ice crystallization" between -1°C and -2°C. During this phase, the rate of protein denaturation (structural unfolding) is at its peak 1 .
Slow freezing means fish spends more time in this critical zone, resulting in larger ice crystals that can damage muscle cells and accelerate quality deterioration.
Quick freezing, defined in some regulations as reducing the temperature from 0°C to -5°C within 2 hours, minimizes this damage and better preserves fish quality 1 . This explains why commercially flash-frozen fish often outperforms fish frozen slowly in home freezers.
In a comprehensive 2020 study published in the Journal of Applied Biology and Biotechnology, researchers designed an experiment to systematically track what happens to skipjack tuna loins during 28 days of frozen storage at -18°C — a typical home freezer temperature 1 .
The research team purchased fresh skipjack tuna from a fishing station in Bangladesh and processed them into skinless loins in a fish processing laboratory. These loins were sealed in vacuum polyethylene bags and stored at -18±2°C — mimicking how consumers might store tuna at home 1 .
The team then analyzed the loins at regular intervals using a battery of tests:
This multi-pronged approach allowed the researchers to correlate measurable scientific parameters with actual eating quality 1 .
The biochemical results revealed a story of steady deterioration, even at frozen temperatures. The data in the table below shows significant changes in the fundamental composition of tuna loins over the 28-day study period 1 .
| Parameter | Day 0 | Day 7 | Day 14 | Day 21 | Day 28 |
|---|---|---|---|---|---|
| Protein (%) | 21.54 | 21.32 | 20.87 | 20.25 | 19.68 |
| Lipid/Fat (%) | 3.66 | 3.42 | 3.15 | 2.82 | 2.53 |
| Moisture (%) | 73.11 | 72.87 | 72.34 | 71.86 | 71.35 |
| Ash (%) | 1.42 | 1.51 | 1.59 | 1.68 | 1.77 |
| TVB-N (mg/100g) | 7.15 | 8.34 | 9.78 | 11.85 | 13.92 |
| TMA-N (mg/100g) | 1.85 | 2.34 | 2.95 | 3.84 | 4.75 |
| pH | 6.15 | 6.24 | 6.35 | 6.48 | 6.59 |
The most significant trends show proteins and lipids breaking down while ash and spoilage indicators increase. TVB-N (total volatile base nitrogen) and TMA-N (trimethylamine nitrogen) are particularly important — these compounds are produced as fish spoils, creating the characteristic "fishy" odors that consumers find unpleasant 1 .
The freezing process had a dramatic effect on the microbial populations in the tuna loins, as shown in the table below. While this might seem purely beneficial, it's important to note that some microorganisms survive freezing, simply becoming dormant rather than dying 1 .
| Microorganism | Day 0 | Day 7 | Day 14 | Day 21 | Day 28 |
|---|---|---|---|---|---|
| Total Bacterial Load | 3.8×10⁵ | 1.2×10⁵ | 8.5×10⁴ | 5.7×10⁴ | 3.3×10⁴ |
| Total Coliforms (MPN/g) | 113 | 75 | 42 | 19 | 5 |
| Fecal Coliforms (MPN/g) | 13 | 8 | 3 | <1 | Undetectable |
| Salmonella sp. (MPN/g) | 1 | <1 | Undetectable | Undetectable | Undetectable |
The most dramatic finding was the complete elimination of Salmonella and fecal coliforms by the end of the study period. This demonstrates freezing's effectiveness at reducing potentially harmful microorganisms, though it's worth noting that freezing doesn't necessarily eliminate all pathogens — some merely become dormant and can reactivate upon thawing 1 .
Total bacterial load decreased by 91% over 28 days of frozen storage, showing freezing's effectiveness at controlling microbial growth 1 .
While biochemical and microbial changes followed steady trends, the sensory qualities told a slightly different story. The trained panelists rated the tuna loins as being in "excellent condition" for the first 14 days of frozen storage, remaining "acceptable" through day 28 1 .
Excellent Condition
Acceptable Quality
This reveals an important insight for consumers: tuna may remain perfectly enjoyable to eat even as measurable biochemical changes occur. Our senses integrate multiple factors in ways that laboratory instruments cannot replicate 1 .
So what does all this science mean for your everyday experience with frozen tuna?
While tuna remains safe beyond this point, the study suggests optimal quality lasts about 28 days in typical home freezer conditions 1 .
If freezing fresh tuna at home, spread portions in a single layer on a baking sheet to freeze before bagging — this mimics commercial "quick freezing" and preserves better texture 1 .
The research confirms that properly frozen tuna remains organoleptically acceptable for weeks, so if it looks and smells good after thawing, it's likely perfectly fine to eat 1 .
Those unpleasant smells that develop in frozen fish? They're likely trimethylamine — a compound that increases steadily during frozen storage, as documented in the study 1 .
The complex interplay of biochemistry, microbiology, and sensory science determines the fate of every piece of tuna that enters your freezer. While freezing doesn't completely pause the clock on quality changes, understanding these processes helps us make better decisions about storing, preparing, and enjoying this valuable protein source.