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- Gravading of mackerel (Scomber sp.): physicochemical and sensory changes during refrigerated storagePublication . Adetokunbo, Mujeeb Adewale; Esteves, E.; Aníbal, J.The Atlantic Mackerel (Scomber colias) is a fast-swimming pelagic fish that can be found in the warm, temperate waters of the Atlantic Ocean, as well as on the shelf and upper slopes of the Mediterranean Sea. Mackerel is a great source of long-chain n-3 polyunsaturated fatty acids, compounds that have health benefits. Mackerel is also a good source of proteins, essential amino acids and other biologically active compounds. However, it is highly vulnerable to lipid oxidation due to the large amount of unsaturated fatty acids it contains. Gravading is a fish processing technique popular in Scandinavian and Nordic countries, which involves applying a mixture of salt and sugar to the fish fillets and then letting it mature for 1-4 days at low temperature (in refrigeration) to produce gravads, a salt-sugar delicacy with a salt content of 3 –6% and a pH greater than 5. This process has been applied to fish such as salmon, whitefish, trout, or Greenland sole. The main objective of this work was to study the effect of the gravading process on mackerel fillets using physicochemical, biochemical and microbiological parameters. Specifically, it was intended to study the effect on the quality and storage stability of mackerel that went through the gravading process; monitor some quality parameters, namely color (CIE Lab), texture (hardness), pH, water activity and moisture content, total volatile basic nitrogen content (TVB-N), or thiobarbituric acid reactive substances content (TBARS) and the presence/abundance of microorganisms evaluating total aerobic and psychotropic bacteria counts; and to study and understand how two different storage temperatures, 4 °C and 9 °C, can influence changes in these quality parameters. Experimental and laboratory work took place from early April to mid-July 2021 at the Chemical and Food Processing Laboratories of the Department of Food Engineering of the Institute of Engineering (ISE) of the University of Algarve (UAlg), Campus da Penha, Faro, Portugal. The fish, S. colias, were purchased from the Municipal Market of Faro, frozen on that date in an air-blast freezer under appropriate conditions (-25°C) and stored frozen (-18°C) until the beginning of the trial, in May 2021. The quality parameters were evaluated for samples (n=3) of fresh fillets on the first day of sampling, before application of the application of the gravad mix, and after 48 h of gravading, the other fillets subjected to gravading were vacuum packed and divided into two groups, one set of packages was stored refrigerated at 4 °C and the other at 9 °C. The same parameters were evaluated in fillets (n=3) of each storage condition, once a week until 4 weeks of storage, after which sampling was done once every two weeks until the 10th week of sampling, in July 2021. The color parameters did not show a particular trend throughout the test, except in the case of L* luminosity, which decreased with time, probably due to the dehydration process. On the other hand, the water activity decreased to about 0.80, a level at which only yeasts and molds and halophyte bacteria can survive, contributing decisively to the stability of the product. The texture of mackerel underwent major changes due to gravading, as excess of sugar and salt increases the osmotic pressure and contributes to the removal of water from fish tissue. This caused the hardness to increase until the fourth week of storage, after which it starts to decrease. Generally speaking, quality parameters did not change with temperature, but all seem to deteriorate after four weeks. In fact, the TVB-N content gradually increased after the fourth week. TBARS values also increased during this period. The total aerobic and psychotropic bacteria counts were around 4 log(UFC/g) which is the common level for other fish species subject to gravading and well below 6 or 7 log(UFC/g), which is considered the critical level for microbiological contamination. This shows a high quality of raw material and product. The fillets subjected to gravading, vacuum-packing and kept in refrigeration, regardless of storage temperature, appear to lose quality after four weeks, a shelf-life that can be partly credited to the vacuum packaging that inhibits the survival and growth of microorganisms.