Northern shrimp (Pandalus borealis): Quality changes, shelf-life, and safety of cooked shell-on products

Commercially, the Northern shrimp (Pandalus borealis) is the most important crustacean of the North Atlantic and has been caught by the Greenlandic fishermen for decades. It is known for its sweet taste and bright red color both alive and after cooking. Today it accounts for more than 30% of the sales of the biggest company in Greenland, Royal Greenland Inc. Several different products are produced from this species, including cooked, whole, shell-on shrimp, a product with a global market value of 3.2 billion DKK. The production of this product mainly occurs onboard the large, off-shore shrimp trawlers. After removal of non-target species and sorting by size, the shrimp are cooked for 3 minutes and cooled in cold seawater before they are individually quick frozen (IQF) and packaged followed by frozen storage. The products are then stored frozen onboard the shrimp trawlers until they reach the shore, sometimes after several weeks. Onshore, the shrimp are further processed, sometimes glazed with ice water, before being repacked and distributed to the retail market. During frozen storage, both during distribution and at retail, the shrimp loses its initial organoleptic properties, such as negative changes in color, odor, and taste. These changes occur within the first six months after production, especially if the frozen product is not glazed.

The aim of this PhD thesis is, therefore, to investigate, and, if possible, inhibit the quality changes occurring to this product during frozen storage. Another aim was to investigate the potential of producing a chilled version of this product, including evaluation of the sensory shelf-life and the safe shelf-life regarding the growth of the human pathogen Listeria monocytogenes.

Available information on the characteristics, catch, quality changes, and safety of products of P. borealis was collected and reviewed. This literature review showed that limited information is available specifically regarding the quality changes of frozen or chilled, cooked, shell-on Northern shrimp products. Therefore, this thesis will fill out some of the knowledge gaps.

During frozen storage in retail, this product is commonly stored with exposure to light. Thus, the effect of light on lipid oxidation and color changes during 84 days of frozen storage at -20 °C was investigated. Two sizes of shrimp were studied. Results showed that a significant increase in peroxide values, i.e., the formation of primary oxidation products, occurred during illuminated storage for both sizes of shrimp. A greater effect of light was seen for the smaller shrimp. Similar tendencies were seen for the formation of volatile compounds. During illuminated storage, the shrimp became lighter and less red compared to shrimp stored in darkness. Furthermore, it was shown that neither of the common endogenous antioxidative compounds, i.e., astaxanthin and α-tocopherol, efficiently protected the product against oxidation. The changes in color could not be correlated with a decrease in the content of free-astaxanthin, despite this compound being responsible for the red color of the shrimp. In conclusion, the results showed that light increased the oxidation of whole, cooked Northern shrimp and that the illuminated frozen storage commonly used in retail displays greatly affects the color of the whole, cooked Northern shrimp. Therefore, protecting these products against light during frozen storage is important to prolong the market shelf-life.

Currently, the only treatment applied to protect the frozen product against oxidation during storage is a thin layer of ice. This ice layer is applied as a glaze during repackaging on land-based facilities before the product is distributed to retail. Due to temperature fluctuations, this glaze evaporates during storage, losing its protective function. To improve the oxidative stability of whole, cooked shrimp products during frozen storage, the application of antioxidants to reduce the changes has been investigated. Two approaches to apply the antioxidants were chosen: i) dipping the cooled, freshly cooked shrimp in the antioxidant solutions and freezing the shrimp in a plate freezer and ii) addition of the antioxidants to the ice glaze applied to the IQF shrimp. Dipping has previously been shown to be effective for other seafood products and is feasible on shrimp trawlers using the existing processing equipment for raw shrimp for sale in the Asian markets. Based on preliminary studies, the commercial antioxidant Duralox Manc 213 (Kalsec Inc.), containing rosemary extract, citric acid, ascorbic acid, and tocopherols, as well as a homemade solution of tocopherols and ascorbic acid were chosen for full-scale studies. Seawater-treated and untreated samples were used as controls to determine the effect of the antioxidants. Treatments were performed onboard the Greenlandic shrimp trawler Avataq, and samples were stored for 369 days, with sampling every three months. Samples were analyzed for sensory changes in color and odor of the whole shrimp, oxidative reactions (peroxide value, content of free astaxanthin, tocopherol consumption, and formation of volatile compounds), changes in color (CIEL*a*b*), as well as thawing loss and water-holding capacity of the meat. Results of the two antioxidant application methods, dipping or glazing, showed that dipping shrimp in the antioxidants did not inhibit oxidative reactions. However, dipping the shrimp in the antioxidant solutions reduced the unwanted color changes during frozen storage. Glazing shrimp with seawater reduced oxidation but did not reduce the unwanted color changes to the same extent as shrimp glazed with antioxidative solutions. The addition of antioxidants in the glazing solution inhibited oxidation and reduced color changes. As dipping did not reduce oxidative reactions, dipped shrimp resulted in a shorter shelf-life in terms of oxidative reactions (73-172 days) than glazed shrimp (271 to >369 days). Glazing shrimp with seawater reduced the quality deterioration compared to untreated shrimp. The shelf-life for seawater glazed shrimp is estimated to be between 271 and 369 days, compared to approximately 172 days for untreated shrimp. Glazing the shrimp with antioxidants prolonged the shelf-life to more than 369 days. Using antioxidants is clearly beneficial to maintain the quality of the shell-on shrimp during frozen storage.

If more than 20% of the catch of the whole, cooked, frozen shrimp has brown/dark-colored heads determined by a company color score, the catch is considered of lower quality and will be sold at lower prices. Therefore, the effect of removing the heads (beheading) and the effect of modified atmosphere storage (MAP, 40% CO₂/60% N₂) on the sensory shelf-life of chilled (5 °C) ready-to-eat (RTE) cooked shrimp was investigated. Furthermore, the microbiota responsible for spoilage was investigated. Results showed that beheading increased the sensory shelf-life by three days, whereas MAP prolonged the shelf-life from 7 and 11 days in air to 18 and 21 days in MAP for shrimp with and without heads, respectively. The spoilage microbiota of aerobically stored shrimp was mainly composed of Pseudoalteromonas spp., whereas the microbiota of the MAP-stored products was dominated by Carnobacterium spp. Species of Pseudoalteromonas were found to have a spoilage potential in both air and MAP.

The food safety of whole, cooked, chilled shrimp regarding the growth of the human pathogen L. monocytogenes, was studied by means of challenge tests. Specifically, the growth of this pathogen on respectively the meat and on the shell was evaluated. Results showed that the pathogen exhibited a faster growth rate at ~2 °C on the whole shell-on head-on shrimp (0.021 1/h) compared to on the meat (0.011 1/h) and the shell (0.013 1/h). Further studies were performed to elucidate the causes for the differences in growth rates, but no clear effects were observed. Additionally, the observed growth rates were compared to predictions of an existing predictive model incorporated in the freely available predictive software Food Safety and Spoilage Predictor (FSSP). This software is available to assist food-producing companies in assessing the risk of growth of this pathogen. Results showed that FSSP predictions underestimated the growth rates in comparison to the experimentally observed growth rates. Based on the results of challenge-tests and available literature, the existing model was calibrated, allowing estimations of the safe shelf-life to be performed. The safe shelf-life regarding the growth of L. monocytogenes was based on the growth rates in the whole shrimp determined to be <10 days at 5 °C. This is consistent with UK guidelines recommending shelf-lives of less than 10 days at 3-8 °C to manage the risk of Clostridium botulinum in MAP foods.

The work in this PhD project has investigated the causes for the quality deterioration of whole, cooked shrimp during frozen storage and has proposed solutions to prolong the shelf-life. Implementing the proposed solutions could help reduce reclamations leading to economic loss for the company and potentially reduce the overall food loss occurring during the production and retail of this product. Furthermore, results showed that very little change occurred during the initial storage and transport of the shrimp. This gives the company the opportunity to glaze or vacuum pack the products on land rather than trying to change production onboard the shrimp trawlers. Future studies should investigate whether the initial quality of this product could be improved, such as reducing the cooking time of the shrimp.

Additionally, it has been shown that removing the head-region of the shrimp increases the sensory shelf-life. Removing the head-region could make it possible to improve the value of products that otherwise are considered of lower quality due to dark-colored heads. Furthermore, results show that the sensory shelf-life of refrigerated shrimp products is longer than the safe shelf-life for the product. These results highlight the need to incorporate safety aspects into all product development to ensure the safety of consumers. This is important as numerous Listeria outbreaks have been caused by cooked RTE foods, indicating that reliance on cooking and hygiene alone is insufficient. Future studies should investigate ways to determine realistic safe shelf-lives or stabilize the products against the growth of L. monocytogenes and other human pathogens.