Rapid measurement of fish freshness indices by an amperometric flow-injection system with a 16-way switching valve and immobilized enzyme reactors

A novel flow-injection system is proposed for the rapid measurement of the fish freshness indices K-1 and K-2: K-1 = [([HxR + [Hx]) x 100/([IMP] + [HxR] + [Hxl)] and K-2 = [[Hx] x 100/([HxR] + [Hx])], where [IMP], [HxR] and [Hx] are inosine-5'-monophosphate, inosine and hypoxanthine concentrations, respectively.: For the estimation of index K1, 5'-nucleotidase immobilized reactor and nucleoside phosphorylase (NP)/xanthine oxidase (XO) coimmobilized reactor were incorporated in series in the flow-injection line made up by 4 16-way switching valve with two sample loops. For the estimation of index K-2, NP and XO immobilized reactors were also incorporated in the similar flow-line. Two sample portions passed through the flow-line with different residence times so that two peaks were obtained. The first and second peaks obtained in the K-1-determining system corresponded to the total of HxR and Hx and the total of Hx, HxR and IMP, respectively. Similarly, the first and second peaks obtained in the K-2-determining system corresponded to Hx and the total of Hx and HxR, respectively. Therefore, the indices K-1 and K-2 can be estimated by K-1 and K-2 (%) = (i(1)/i(2)) x f x 100 where i(1) and i(2) present the peak current of the first and second peaks, respectively, and f the ratio of the peak currents of the first and second peaks for a Hx standard solution. A sea bream was selected as a model fish and it was stored at 4degreesC after death. A good correlation was found between the index K-1 and the storage time over a period of 400 It, with a correlation coefficient of 0.992, but no correlation between the index K-2 and the storage time. The measurements could be performed at a rate of 22 samples per hour with satisfactory precision (0.6-1.2% R.S.D.), without calibration for each species, accurate weighing of fish meat and any interferences in fish meat. (C) 2002 Elsevier Science B.V. All rights reserved.