Abstract The formation of dimethylamine and form- aldehyde from trimethylamine N-oxide by the enzyme trimethylamine N-oxide demethylase in whole hake dur- ing frozen storage was studied. The objective was to check if there were parts of the muscle with a higher pro- duction of dimethylamine and formaldehyde, and if the presence of kidney during frozen storage had any signifi- cant influence on the production. Three variables were examined through one year storage. The first was ana- tomical location, considering the red muscle and three zones of white muscle, one located right over the kid- neys, the dorsal part over the viscera, and the tail. The second variable was the temperature of storage, –11 °C or –18 °C. Finally, the influence of kidneys during stor- age, comparing fish with and without kidneys, was also evaluated. No differences were found in dimethylamine and formaldehyde production between fish with and without kidneys stored at –18 °C. However at –11 °C the amounts of dimethylamine and formaldehyde detected in fish without kidneys were, in some cases, higher than in those with kidneys. Kidney removal does not have a sta- tistically significant influence on DMA and FA produc- tion in frozen storage hake. Differences in dimethyl- amine and formaldehyde values among different anatom- ical locations were found, especially in those stored over one year. It was found that, in general, the white muscle located right over the kidneys produced more dimethyl- amine than other parts of the fish. Keywords Frozen storage · Hake · Muscle zones · Formaldehyde · Dimethylamine · TMAO-ase Introduction Gadoid fish like cod, pollack, saithe, and hake are com- mercially very important species because, in addition to being commercialized as round fresh fish or fillets, most frozen fishery products like fillets, fish fingers, cakes, and surimis are made from whole or minced muscle of these fish [1]. This group of species (Gadiforms) have the property of breaking enzymatically trimethylamine N-oxide (TMAO) into dimethylamine (DMA) and form- aldehyde (FA) [2]. Responsible for this rupture is the tri- methylamine N-oxide demethylase (TMAOase), an en- zyme present in very large quantities in the kidney and spleen of these species [3, 4]. TMAOase activity is very low in muscle, making its detection difficult, except in red hake (Urophycis chuss) where significant amounts of TMAOase activity has been described [5]. The two types of fish muscle present different TMAOase activities and in general dark muscle exhibits higher TMAOase activi- ty than light muscle [6]. Despite these differences in TMAOase activity within the fish body, significant amounts of DMA and FA can be found both in viscera and muscle [6, 7]. This enzymatic activity has been con- sidered as especially important for the quality of the gad- oid species during frozen storage, because the interaction of produced FA with muscle proteins is the principal cause of the texture deterioration, causing quality loss and decreasing their storage life [4, 8, 9, 10, 11, 12, 13, 14]. A high percentage of this kind of fish is caught on the high seas where they are eviscerated and frozen stored on board. Given that the kidneys of these fish are in a ce- phalic location imbricated in the white muscle, they are not usually eliminated during an ordinary evisceration, so the round fish is normally frozen stored with kidney. Previous works have described that adding kidney tissue to minced flesh of gadoid fish before freezing increases M. del Mar Rey-Mansilla ( ✉ ) · C.G. Sotelo · S.P. Aubourg Instituto de Investigaciones Marinas (CSIC), Eduardo Cabello 6. 36208 Vigo. Spain e-mail: mar@iim.csic.es H. Rehbein · W. Havemeister Federal Research Centre for Fisheries, Institute of Biochemistry and Technology (IBT), Palmaille 9. 22767 Hamburg. Germany B. Jørgensen · M.K. Nielsen Ministry of Agriculture and Fisheries, Danish Institute for Fisheries Research, Technical University, Bldg 221. 2800 Lyngby. Denmark Eur Food Res Technol (2001) 213:43–47 DOI 10.1007/s002170100345 ORIGINAL PAPER Maria del Mar Rey-Mansilla · Carmen G. Sotelo Santiago P. Aubourg · Hartmut Rehbein Wiebke Havemeister · Bo Jørgensen Michael K. Nielsen Localization of formaldehyde production during frozen storage of European hake (Merluccius merluccius) Received: 23 August 2000 / Published online: 18 May 2001 © Springer-Verlag 2001