During postmortem metabolism, hydrogen ions accumulate in the muscle and gradually lower the pH from 7.2 to an ultimate pH near 5.6. The ultimate pH of meat is widely valued as an indicator of fresh meat quality as it directly affects the quality characteristics of color, texture, and water holding capacity. Therefore, our research was conducted to identify the processes responsible for determining ultimate pH. Pigs harboring the AMPK�•3R200Q mutation produce meat with extremely low ultimate pH (pH ~ 5.3) that is detrimental to quality. This phenomenon is often attributed to a greater glycogen content in muscle from the mutant pigs compared to wild-type pigs. However, our research indicated that greater glycolytic flux in muscle from these pigs causes a lower ultimate pH rather than greater tissue glycogen deposition. On the other hand, however, AMPK�•3R200Q pigs contain more mitochondria and retain greater oxidative capacity. Hence, we hypothesized that mitochondria may contribute to the lower ultimate pH in muscle of these pigs. To test our hypothesis, isolated mitochondria were incorporated into an in vitro system the mimics postmortem glycolysis. Mitochondria enhanced glycolytic flux and pH decline in the in vitro system similar to that of AMPK�•3R200Q pigs. After a series of experiments, we found that the causative agent for enhanced glycolytic flux is a soluble mitochondrial protein. In other experiments, mitochondrial F1-ATPase was found to be responsible for the majority of this effect, principally through promoting greater ATP hydrolysis at lower pH values, thereby allowing for greater flux through glycolysis. These data suggest that variations in ultimate pH may be more thoroughly explained and predicted by the abundance of mitochondria. Broiler pectoralis major muscle, which is a highly glycolytic muscle, possesses high ultimate pH (pH ~ 5.9) compared to pork and beef. We postulated that rapid carcass chilling reduces the flux through glycolysis, thereby causing premature termination of postmortem metabolism. Yet, chilling was only partially responsible for the high ultimate pH of pectoralis major muscle. However, we showed that pectoralis major of broiler chicken exhibits lower phosphofructokinase-1 activity compared to porcine longissimus lumborum muscle, which limits the flux through glycolysis. / Ph. D. / Consumer demand for high quality meat has increased dramatically over the past two decades. In order to meet this demand, it is crucial to understand factors that control the development of fresh meat quality characteristics. Consumers purchase meat based on color, but repeat purchases are also influenced by meat freshness, texture, and juiciness. These quality attributes develop after the animal has been harvested during the conversion of muscle to meat through a series of biochemical reactions. This conversion results in muscle acidification (pH decline) caused by the degradation of stored muscle energy that acidifies muscle. Normally, the muscle pH drops from a neutral value in living muscle to an acid value (~5.6) in fresh meat. In some cases, however, excessive acidification occurs and this can dramatically impact fresh meat quality characteristics. Our research program focuses on the mechanisms responsible for this extended acidification. To that end, we use mutant pigs known as RN– that produce meat with extremely low ultimate pH known as “acid meat”. While most believe that the extended pH decline in muscle of these pigs is a function of elevated energy in the muscle prior to harvesting, we showed that theses pigs have different muscle prior to harvest and this difference cause increased acidification during the transformation of muscle to meat. To investigate this issue further, we also examined the contribution of mitochondria (the powerhouse of the cell) to this process mainly because muscle from the RN– pigs containing around 50% more mitochondria compared to normal pigs. Curiously, we have shown mitochondria participate in this process. Because mitochondria require oxygen to function and harvesting animals disrupts oxygen delivery to the muscle, mitochondria were considered irrelevant to the development of meat quality characteristics. Our studies have definitively proved that mitochondria can contribute to meat quality and may be key in improving fresh meat quality.

Once an animal is harvested for meat, skeletal muscle attempts to maintain ATP at or near antemortem levels. To maintain ATP levels postmortem, stored glycogen is catabolized to produce ATP through glycolysis and possibly oxidative metabolism. Hydrolysis of the produced ATP acidifies muscle until an ultimate pH is reached. The ultimate pH of meat directly impacts the quality characteristics of color, texture, and water holding capacity. Therefore, our research intends to describe the contributions glycolysis and oxidative metabolism play in determining ultimate pH and fresh meat quality. Traditionally, glycogen content at death was thought to be responsible for dictating ultimate pH. This was especially true in oxidative muscle with limited glycogen stores. Yet, our research indicated that in the presence of excess glycogen, oxidative muscle maintains a high ultimate pH. Rather, pH inactivation of phosphofructokinase is responsible for terminating postmortem glycolysis and brackets ultimate pH between 5.9 – 5.5. Meat with a pH below this range is uncommon. However, AMPK γ3R200Q mutant pigs produce meat with an ultimate pH near 5.3. Due to lower AMP deaminase abundance in their muscle, AMP levels are elevated late postmortem. Because AMP is a potent activator of phosphofructokinase, the aberrant meat quality from AMPK γ3R200Q mutant pigs is caused by extended postmortem glycolysis. Combined, these data further our understanding of the factors that contribute to the formation of fresh meat quality. We also characterized AMPK γ3R200Q muscle by investigating antemortem skeletal muscle lactate transport. Lactate is transported in or out of tissues by proton-linked iii monocarboxylate transporters (MCTs). Previous reports indicated that acute activation of AMPK increased monocarboxylate transporter expression in skeletal muscle of other species. Yet, it was unknown the impact chronic activation of AMPK will have on MCT1, MCT2, and MCT4 expression in pigs. Compared to wild-type pigs, the longissimus lumborum of AMPK γ3R200Q pigs increased both MCT2 and MCT4 protein expression. Our data suggest glycolytic skeletal muscle from the AMPK γ3R200Q pigs has increased capacity for antemortem lactate export from muscle and possibly increased pyruvate transport into the mitochondria. / Ph. D.

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Turkey with pale, soft, exudative (PSE)-like condition is one of the growing concerns in the poultry industry as it affects meat quality due to low ultimate pH at 24 h post mortem (pH24). Hence, there is a need for better utilization of PSE-like meat for the preparation of further processed products. In the first two studies, the biochemical, functional, rheological, and textural properties of proteins in turkey breast meat with different pH24 in fresh and frozen conditions were investigated. These studies revealed that low and normal pH meat had similar properties indicating similar extent of protein denaturation, except for lower water holding capacity (WHC) in low pH meat. High pH meat had similar or better functional properties than normal pH meat. In the third study, improvements in WHC, protein solubility, and gel forming ability of low pH meat was achieved by the application of high pressure processing (HPP). / Food Science and Technology

Dans un contexte où la demande mondiale en viande de poulet ne cesse d’augmenter, les qualités technologique et sensorielle de cette viande sont devenues un enjeu majeur pour l’industrie. L’analyse de deux lignées de poulet divergentes pour le pH ultime (pHu) du filet confirme que la génétique est un déterminant majeur de ce caractère. Leur comparaison montre qu’augmenter le pHu du filet par sélection impacte le pHu d’autres muscles et améliore de nombreux critères dont le pouvoir de rétention d’eau, le rendement technologique et la tendreté de la viande. Si l’histologie et la composition biochimique du muscle ne sont pas affectées par la sélection, nos résultats suggèrent un lien défavorable entre un trop faible niveau des réserves énergétiques musculaires et l’incidence de défauts structuraux telle que les striations blanches. Par contre, aucun antagonisme génétique entre le pH ultime et les caractères de production (poids vif, rendement en viande et indice de consommation) n’a été observé. / In a context where the global demand for chicken meat is increasing, technological and sensory qualities of this meat have become a major issue for the poultry industry. The analysis of two chicken lines divergent for ultimate pH (pHu) of breast fillets confirms that genetics is a major determinant of this trait. Between-lines comparison shows that increasing the pHu by selection changes the pHu of other muscles and improves many other quality criteria such as water-holding capacity, curing-cooking yield and meat tenderness. Although histology and biochemical composition of the muscle are not affected by the selection, our results suggest an unfavorable relationship between low muscle energy reserve and the incidence of structural defects such as white striping. On the contrary, no negative association between ultimate pH and production traits (body weight, meat yield and feed conversion ratio) was observed.