BOVINE MUSCLE GLYCOGEN CONCENTRATION IN RELATION TO DIET, SLAUGHTER AND ULTIMATE BEEF QUALITY

The aims of this study were to investigate: 1) bovine muscle glycogen concentration on-farm, after transportation to slaughter and after rigor mortis development in relation to various types of diets, 2) the relationship between the ultimate pH and residual glycogen concentration, and 3) the effects of residual glycogen on the physical and sensory quality of beef. The ultimate goal of this work was to gain additional knowledge and tools for integrated beef quality control. The experiments of this study were conducted in two countries: USA, at the University of Wisconsin-Madison, and Finland, at the University of Helsinki. Glycogen concentration in bovine longissimus thoracis et lumborum (LTL) muscle was affected by in vivo sampling location. Cranial locations, which were close to the 10th rib, had less (p≤0.05) glycogen than locations in the mid-section and caudal to the 11th rib. Present results substantiate the metabolic heterogeneity of the LTL muscle, and indicate that special care has to be taken in exercising repeated sampling, particularly, if biochemical analyses are to be performed on fresh-frozen muscle specimen. However, because the overall variation within the LTL muscle was not more than 11.6% and also because the calculation of glycolytic potential (GP) rendered locational concentration differences insignificant, the use of LTL in experiments requiring repeated sampling can be cautiously encouraged. Bovine LTL and gluteus medius (GM) muscle glycogen could not be easily manipulated with diets. Initially high muscle glycogen concentrations were not readily affected by the change in the energy density/energy content of the feed. The more there was glycogen initially the less glycogen increased (r = −0.678 (FIN), r = −0.527 (USA), p<0.0001). Persistence seems to be a characteristic of skeletal muscle glycogen even when a relatively low energy diet is consumed, since cattle were able to gain and maintain high glycogen concentrations with a low energy diet of 100% silage. Furthermore, muscle glycogen decreased minimally when low energy diet was provided following a month on high-energy diet. Corn was superior to alfalfa haylage in increasing glycogen content in the muscles of steers and bulls, when the initial glycogen content was depleted to 50 mmol/kg with one subcutaneous injection of adrenaline. Male status, i.e., bull versus steer, did not consistently affect glycogen concentration nor responsiveness to dietary treatment. However, high-energy diet seemed to protect cattle from potentially glycogen-depleting stressors such as very low and high temperatures as well as transportation to slaughter. In Finland, the cattle on high energy lost 7 ± 4.0 (se) mmol/kg and 23 ± 3.9 mmol/kg during transportation in spring (cool weather) and in summer (hot weather), respectively, whereas the cattle on low energy diet lost 16 ± 3.8 mmol/kg in spring and 33 ± 4.1 mmol/kg in summer. These effects of diet were reflected all the way to ultimate pH values (5.69 ± 0.03 (high energy); 5.93 ± 0.03 (low energy)) and residual glycogen concentrations. As far as the beef production in Finland is concerned, it can be concluded that even a short finishing of two weeks with a concentrate-based high-energy diet is well worth applying, because its clearly protective effects were directed at glycogen depletion and elevation of pH. Thus, it is one of the many appropriate measures to take in the fight against bovine dark-cutting, which is a well-known quality problem causing reduced shelf life and unattractively dark color. The relationship between pH and residual glycogen concentration was curvelinear. At normal pH values (pH≤5.75) the variation in residual glycogen concentration was enormous, from 10 to 83 mmol/kg, and independent of ultimate pH. Low residual carbohydrate serves as an indicator of diminished glycogen content at the time of slaughter. The independent effects of residual glycogen concentration on the physical and sensory quality of normal-pH-beef were somewhat numerous but quite modest. The water holding capacity, characterized e.g., with drip loss, and weight loss in thawing, was affected as were fresh meat redness (Hunter’s a* value), shear force, and Hunter’s b-value of the steak, of which the effects on drip loss, shear force, and b*-value of the steak color can be considered positive. Of all the quality effects of residual glycogen concentration, it can be concluded that it is mainly the beef having either low or very high glycogen concentration that behave slightly differently from each other, the beef having intermediate (somewhat high) residual glycogen concentration mostly follows the pattern of one or the other. The negative effects of high residual glycogen on beef quality ie. , on weight loss in thawing and frying, sensory juiciness, and fresh meat redness, are not of the magnitude to call for action.