Recent increases in production of aquacultured kelps in the United States has driven interest in value-added processes that can be used to extend the shelf life of this highly perishable crop. Although Maine leads the US in production of kelp, processing infrastructure remains decentralized and largely inaccessible to growers. A number of low-input processes, including freezing, salting and fermentation, have been suggested as ways for growers to minimize spoilage of unheated kelps during storage and distribution, but the potential survival of foodborne pathogens in these products is unknown.
In this study, the effects of fermentation and dry salting of seaweed on the survival of bacterial foodborne pathogens Salmonella enterica, Listeria monocytogenes, Staphylococcus aureus and Vibrio vulnificus during refrigerated storage was assessed. A sauerkraut-style seaweed was prepared from varying ratios (25, 50 or 75% wt/wt) of sugar kelp or winged kelp (Saccharina latissima, Alaria esculenta ) and cabbage. Mixtures were inoculated with Lactobacillus plantarum and Leuconostoc mesenteroides as well as relevant pathogens. Pathogen population was monitored using cultural methods during fermentation and sixty days of refrigerated storage. Additionally, winged kelp was combined with various levels of salt (2-2 0% wt/wt) and inoculated with pathogens. Pathogen survival was monitored for up to 90 days during refrigerated storage.
All four pathogens investigated were inactivated within 14 days in fermented sugar kelp regardless of the ratio of kelp utilized. Pathogens were equally susceptible in fermented winged kelp with the exception of Salmonella in the 75% kelp sample, which survived for 49 days. Decreases in pathogen population (from an initial level of approximately 5.0 log CFU/g) roughly mirrored the decrease in pH caused by fermentation. Conversely, Salmonella, L. monocytogenes and S. aureus survived in salted winged kelp for a minimum of 23 days regardless of salt level applied. V. vulnificus was the most susceptible to salting treatments, becoming undetectable after enrichment between days 7 and 14 of storage in all treatments.
Although prior research has documented the acceptable quality of the seaweed products investigated in this study, data generated from this work suggest significantly lesser risk of foodborne illness from consumption of a fermented, as opposed to salted, product. The pH decrease resulting from fermentation inactivated all inoculated pathogens within two weeks of processing. Salting, however, regardless of salt level, was inadequate to mitigate microbial risks. Investigators suggest that processors pursuing this approach adopt a validated microbial kill step.