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Factors contributing to the texture of fish flesh, including pH, water content, density of fast muscle fibers, and the concentration of collagen and hydroxylysyl pyridinoline (PYD) cross-links, were investigated post-rigor in commercially farmed Atlantic halibut (Hippoglossus hippoglossus L.). The fish was sampled every quarter for a 12 month period from May 2004 to May 2005. Hydroxyproline (HYP) as a measure of collagen and PYD were determined using a high-performance liquid chromatography (HPLC) method. An ANCOVA model with fork length and season as covariates were used to explore the seasonal effects on texture, pH, muscle fiber density, alkaline-insoluble collagen (a-i HYP), alkaline-soluble collagen (a-s HYP), and PYD cross-links. A multiple linear regression (MLR) showed that the most important factors contributing to texture were PYD>water (%)>a-i HYP>fiber density, while pH and a-s HYP did not show any correlation to texture. The contribution of fast muscle fiber density to texture was found to vary between sexes and with the season, contributing more in males and in the spring. The most important parameter affecting texture was PYD, explaining 64% (p<0.001) of the total variation in a linear regression analysis. It is concluded that cross-linking processes are of great importance for the rigidity and strength of the collagen in Atlantic halibut flesh. Farmed halibut should be harvested in the fall or early winter when texture and nutrition are good to obtain optimal quality.

The platelet injury and loss of viability that has been shown to occur with storage of platelet concentrates (PC) under conditions with increasing or falling pH were examined using scanning and transmission electron microscopy. After storage, samples were taken for measurement of pH value, platelet count and size distribution, release of lactate dehydrogenase (LDH) into plasma, and for SEM and TEM. Increased levels of LDH were observed in PC with pH above 7.3 and below 6.1. In PC with pH above 7.3 this was related to an increased number (23%) of platelets that were lysed or had a swollen disintegrated internal structure (balloons) as seen with TEM. SEM and Coulter counter studies also showed that platelet fragmentation and formation of microvesicles were prominent in PC with pH above 7.3. The electron microscopic pictures confirmed previous suggestions that platelet disc-to-sphere transformation and cytoplasmic swelling occur when pH falls below 6.7-6.8 during storage. SEM studies showed that concomitant with this change, folds and bulky projections appeared on the platelet surface. In PC with pH below 6.1 the morphological change was irreversible with the appearance of more than 90% lysed and balloon platelets. In conclusion, these studies suggest that the loss of viability observed with PC with pH above 7.3 or below 6.1 after storage is related to an increased percentage of lysed and balloon platelets.

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