Image analysis as a tool to quantify rigor contraction in pre-rigor-filleted fillets

This paper describes and evaluates an economical and efficient online image analysis method for registering length changes in pre-rigor-filleted fillets during rigor contraction. The study aimed to develop image analysis methods for distinguishing between fillet, background and morphological location markers placed on the fillet, to test mathematical shape descriptors for the online registration and to test the image analysis results against measurements of contraction and isometric tension by a rigor-meter. Pre-rigor-filleted fillets of rainbow trout (Oncorhyncus mykiss) were used as a case study to determine whether: (1) changes in whole fillet are indicative of changes in individual parts of the fillet, (2) changes in contraction length measured on the fillet are indicative of changes in contraction observed in isolated (excised) muscle strips and (3) contraction measured in a whole fillet is indicative of isometric tension in isolated muscle strips. The contraction of whole, pre-rigor-filleted, fillet was highly correlated with contraction of its parts (r=0.92 (p<0.0001)) for Scottish Quality Section (SQS) and (r=0.90 (p<0.0001)) for Norwegian Quality Cut (NQC). However, at 20^oC, there were small but significant differences, in that SQS showed a delayed contraction and NQC a more rapid initial contraction than in the whole fillet. Both NQC and SQS also displayed larger maximum contractions than the whole fillet, demonstrating that rigor development in parts of the fillet differs in magnitude from that of the whole. Comparisons with isolated muscle strips showed a similar amount of contraction as that of the same muscle part measured by image analysis of the whole fillet (r=0.70 (p<0.0001)) but with a much more rapid initial contraction of the isolated strip, possibly due to tissue trauma at excision. A clear relationship between contraction curves and temperature was found; in general terms, isometric tension was inversely related to the contraction of whole fillet (r=-0.43 (p<0.0001)), with extreme points in the isometric tension development being associated with changes in the length curve. We conclude that measurements of length changes in whole fillet are indicative of both contraction and isometric tension during rigor as measured in isolated parts of the fillet or in isolated muscle strips.

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