Effects of depuration on subsequent deterioration and shelf life of cultured grooved carpet shell clam Ruditapes decussatus during chilled storage

The grooved carpet shell clam Ruditapes decussatus is one of the most consumed and valuable bivalves in the Mediterranean in terms of both nutrition and market. As with other filter-feeding species there are health risks associated with its consumption and specimens have to be depurated pending assessment of water quality of the environment where they originate. The aim of this study was to examine the effects of depuration on parameters of biological (mortality), commercial/physiological (condition index, CI, and percent edibility, PE), physicochemical (pH and TVB-N content), microbiological (TVC, Enterobacteriaceae and psychrotrophic bacteria) and sensory quality of commercially-sized clams from Ria Formosa (Algarve, south Portugal) stored at chill temperatures. In Summer, the season of peak clam’s consumption, the ‘median time to death’ t50 estimated in the two-parameter logistic model fitted to proportion data of alive depurated and non-depurated clams stored chilled (5ºC) for up to 24 days was similar (13±0.12 vs. 12.1±0.06 days). Moreover, t50 of non-depurated, ‘natural’ clams was found to be much lower in Summer compared to Winter (12.1±0.05 vs. 20.1±0.22 days). In depurated clams, CI decreased, and PE increased steadily during storage, whereas CI and PE of non-depurated clams remained constant (55-61% and 18-20% respectively). pH decreased in the first 1-2 days, markedly in non-depurated clams (from ca. 6 to 5) and then gradually increased to values >7 on days 23-24. Concurrently, TVB-N increased exponentially in both depurated and non-depurated clams, exceeding EU limits by day 20. Initial microbial load was fairly low, 2 to 4 log cfu/g. Expectedly, after depuration microorganisms’ abundance decreased, more pronounced (1-2 log cfu/g) in TVC and psychrotrophic bacteria. Subsequently, abundances grew substantially to 5 log cfu/g at day 24. Enterobacteriaceae abundance remained constant till day 20 and then increased sharply. Similar dynamics were found for non-depurated clams but at comparatively higher abundances. In terms of sensory quality, 50% of the panelists rejected the samples (raw and cooked), t50, on days 7-8 of chill storage. Rejection was elicited by marked changes in appearance and odor. Depuration affected in different ways the level but not the general dynamics of the quality parameters assessed during chilled storage of clams. However, eventual safety issues emerge long after habitual storage time and panelists’ sensory rejection.