Good sashimi is seen as the pinnacle of quality seafood, where the texture and taste of the fish are not disguised by cooking, heavy sauces, or spices. However, fish quickly deteriorates, making it hard to preserve sashimi. This means that high quality sashimi is only available near fishing grounds, which is a restriction on distributors and consumers alike.
The researchers at the School of Public Health in Guangzhou, China wanted to find a better method to preserve fish, with less breakdown of the muscle and less bacterial growth. One method used today to freeze food is called superchilling. In superchilling, the food is first wet and then held at 1 to 2 degrees below freezing, creating a thin layer of ice around the outside of the food without freezing it all the way through. This prevents ice crystals from forming inside the cells of the food, which can negatively affect the texture of the food by rupturing cell membranes. These researchers wanted to see if mixing essential oils into the water coating would reduce the amount of physical decay and prevent bacterial growth, due to the antimicrobial properties of some essential oils.
The essential oils they used came from traditional spice plants that are well-known for their antibacterial and antioxidant properties, including one in the citrus family and one that is used to make sichuan pepper. The researchers extracted essential oils from the dried plants and measured the compounds within them and found 12 main compounds that exhibited antimicrobial or antioxidant properties. These compounds made up more than half of the essential oils, indicating that they would be effective in preventing spoilage caused by microbes or oxidation.
A sashimi chef made fillet samples for the researchers from market-sized Atlantic salmon. The researchers froze some samples to determine the exact freezing temperature of the sashimi itself, which turned out to be -2°C (28.4°F). They separated the other samples into four groups and kept them at -1°C (30.2°F): one group was frozen as is, one was coated in water, then frozen, and the others were coated in a mixture of one of the essential oils and water, then frozen. They removed some of the fillets three times – on the 5th day, the 15th day, and the 25th day – and analyzed them physically and chemically for signs of decay and bacterial content.
The results were consistent: the water-coated samples performed better than the samples that were simply frozen, but the essential oil solution-coated samples did much better than both of those. It did not seem to make a difference, however, which of these two essential oils was used, as they performed the same in all categories. The researchers took thin slices of each sample and looked at them under a microscope to determine the amount of physical decay. The samples coated in essential oils maintained the most similar texture, while the other samples had large holes throughout the tissue by the end of the 25th day.
The researchers measured bacterial counts and indicators of meat deterioration to chemically determine the quality of the sashimi. Both essential oil groups showed similar bacterial levels and meat quality and were consistently higher in quality and lower in bacteria than the frozen and water-coated samples.
They also had a panel of people taste the samples and rate them (out of 5) in terms of texture, taste, smell, color, and acceptability. In the beginning, all samples were close to 5, but by the 25th day, the frozen as-is samples were rated 1, the water-coated samples were around 3, and the essential oil-coated samples were above 3.5.
These results suggest that coating fish fillets, especially ones that are meant to be consumed raw, in essential oils before freezing can dramatically improve the storage time and quality after thawing. This also slows bacterial growth in the food, leading to a lower risk of food-borne illnesses in consumers, which means that people further away from water can eat sashimi that more accurately reflects the quality of the actual fish. This would allow restaurants and stores to sell products at a higher price and would give more consumers access to high-quality fish, a healthy and delicious food.