Sample preservation by freezing: impact on sulfite quantification in cooked shrimp

Sulfites are widely used in shrimp processing to prevent melanosis, yet the effect of freezing on sulfite stability during sample preservation for regulatory monitoring remains unexplored. This study assessed whether frozen storage at −18 °C significantly affects sulfite quantification in cooked shrimp, aiming to validate frozen samples as a reliable alternative to refrigerated samples in official control workflows. A total of 31 paired cooked shrimp samples were collected from commercial establishments in Portugal between January and July 2024 and subjected to refrigeration (4 °C) or freezing (−18 °C) prior to analysis. For each preservation condition, two analytical determinations were performed, and their arithmetic mean was used for statistical analysis. Sulfite levels ranged from 47 to 438 mg/kg in refrigerated samples (mean: 121.68 ± 74.51 mg/kg) and 29–350 mg/kg in frozen samples (mean: 131.68 ± 66.51 mg/kg). No statistically significant difference was found between preservation conditions (paired t-test, p = 0.198). Frozen-storage duration, which ranged from 3 to 64 days, was not significantly associated with sulfite content ( r = −0.107, p = 0.566) or with the within-sample difference between frozen and refrigerated results ( r = 0.07, p = 0.71). Observed shifts in compliance status between paired should be interpreted cautiously as analytical uncertainty, subsampling variability and heterogeneous sulfite distribution may have influenced results. Under the specific conditions evaluated - predominantly commercial Penaeus vannamei , storage at approximately −18 °C for 3–64 days and controlled thawing–the findings provide no evidence that freezing systematically affects sulfite quantification, but they should not be generalized to other species, regions or freezing/thawing protocols without further validation.