Molecular Properties of Post-Mortem Muscle.

SUMMARY— Post-mortem changes in nucleoside triphosphatase activity of bovine myosin B have been studied by using several different modifiers with either 5 mM ATP or 5 mM ITP as substrate at ionic strengths (r/2) of 0.09, 0.19, or 0.52. Enzymic activity was determined by measuring the release of inorganic phosphate. There was very little difference in enzymic activity between myosin B isolated from prerigor, rigor (24 hr post-mortem) or post-rigor (312 hr post-mortem) muscle stored at either 2° or 16°C except that the specific activity of myosin B prepared from muscle stored for 12–24, hr post-mortem was higher than activity of myosin B prepared immediately after death. This increase cannot be explained in terms of rigor shortening, but suggests that a change in myosin conformation or in the nature of the actin-myosin interaction occurs in post-mortem muscle. If an actin-myosin interaction occurs during rigor mortis and if this association remains unchanged during extraction of myosin B, then the very low Mg⁺⁺-modified myosin B enzymic activities obtained at Γ/2 = 0.19 and 0.52 indicate that this interaction is not irreversible. Extraction in the absence of ATP produced a myosin B whose ATPase activity was markedly inhibited by trace amounts of Mg⁺⁺. This may be due to the absence of a-actinin in these myosin B preparations. No consistent differences in activation energies were found either at Γ/2 = 0.19 or 0.52 among the NTPase reactions of myosin B samples prepared from muscle after various times of post-mortem storage.     

Molecular Properties of Post-Mortem Muscle. 7. Changes in Nonprotein Nitrogen and Free Amino Acids of Bovine Muscle

SUMMARY– Proteolysis and its relationship to tenderness were studied by measuring nonprotein nitrogen (NPN), free amino groups, and shear resistance during post-mortem aging of bovine muscle. Both NPN and free amino groups increased during post-mortem aging, indicating some degradation of proteins and/or peptides. However, neither the increase in NPN nor free amino groups was related to post-mortem tenderization since these quantities increased only after most of the improvement in tenderness had occurred. Much of the increase in NPN or free amino groups may originate from degradation of sarcoplasmic proteins or peptides. It is suggested that weakening or breaks at crucial points in the sarcomere, such as at the junction of the Z-line with the thin filaments, occur within the first 48-72 hr post-mortem and that this weakening or cleavage is responsible for tenderization. Cathepsin D may be responsible for this weakening but most of the available evidence is against proteolysis as the primary cause of post-mortem tenderization.     

Molecular Properties of Post-Mortem Muscle. 3. Electron Microscopy of Myofibrils

SUMMARY— A study was made of the fine structure of myofibril suspensions prepared from seven heifers immediately after death and after various times post-mortem. Studies on myofibrils sampled immediately after death showed that sucrose isolation gave the best structural preservation as indicated by maintenance of Z-line structure. Although the appearance of resting muscle was maintained in both sucrose and KCI preparations, several myofibrils from the KCI-treated preparations showed stretched sarcomeres. Glycerol-treated myofibrils usually had shorter sarcomere lengths than myofibrils prepared with the other two solvents. Although fibrillar preservation seemed adequate when glycerol was used, Z-line structure was seldom well-preserved with glycerol.
Myofibrils from muscle sampled 24 hr post-mortem at 2°C were supercontracted with thick filaments pushed against or through the Z-line, and no trace of l-bands remained. Myofibrils from muscle sampled 24 hr post-mortem at 16°C were contracted, but to a much lesser extent than 2°C-24 hr myofibrils. Storage at 2°C for 312 hr after death resulted in myofibrils that were contracted and that were structurally in a much poorer state of preservation than their 16°C counterparts. The 16°C-312 hr myofibrils were slightly contracted as indicated by the absence of H-zones and the presence of prominent, although narrowed, I-bands. All observations showed that shortening accompanying rigor mortis caused changes in banding patterns similar, and probably identical, to those predicted by Huxley's sliding filament model for contracting muscle.     

Influence of the Physical State of Tissue during Rigor Mortis upon Protein Solubility and Associated Properties of Bovine Muscle

SUMMARY— Protein solubility and associated properties were studied in bovine sternomandibularis muscle allowed to pass into rigor in three physical states. Thirty min post-mortem, samples were incubated at 7°C for 48 hr in one of the following conditions: minced through 1/8-in. plate, free to shorten in a vertical position, stretched to 150% of equilibrium length. Stretched muscle exhibited greater protein solubility, higher pH values and longer sarcomeres than the remaining samples. For post-rigor muscle, protein solubility may be related to sarcomere length and moisture press ratio. Variations in sarcomere length may be related to post-mortem changes in pH. Possible relationships between the contractile state of proteins and the chemical, physical and quality characteristics of muscle are discussed.     

Transaminases of Skeletal Muscle. 1. The Activity of Transaminases in Post-Mortem Bovine and Porcine Muscles

SUMMARY: The activity of glutamic-oxaloacetic transaminase (GOT) and glutamicpyruvic transaminase (GPT) of bovine and porcine muscle tissue and muscle press juice was determined. The total GPT activity of muscle tissue is about one tenth of the GOT activity. There are no remarkable differences in the activities of GOT and GPT between these slaughter animals and other species (rat, rabbit and man). The GOT activity of the longissimus dorsi muscle of pigs is significantly higher than that of the same bovine muscle. The mitochondrial (GOT_M) and sarcoplasmic isozymes (GOT_B) of GOT in skeletal muscles of cattle and pigs were determined after electrophoretic separation. The ratio GOT_M:GOT_S in skeletal muscle was found to be about 1:1. There is only a small decrease in GOT activty during storage of muscle tissue at 0 or +4°C for several weeks postmortem. The small activity of GOT_M in the muscle press juice does not substantially change during storage of muscle tissue under these conditions, indicating that there is no drastic change of the mitochondrial structure during aging of meat. Bacterial spoilage of meat, however, results in the release of GOT_M from the mitochondria.     

Effect of Enzymatic Protein Deamidation on Protein Solubility and Flavor Binding Properties of Soymilk

Abstract: The effect of enzymatic deamidation by protein‐glutaminase (PG) on protein solubility and flavor binding potential of soymilk was studied. Treatment of soymilk with PG for 2 h (temperature of 44 °C and enzyme:substrate ratio (E/S) of 40 U/g protein) resulted in high degree of protein deamidation (66.4% DD) and relatively low degree of protein hydrolysis (4.25% DH). Deamidated (DSM) and control soymilks (CSM) did not differ with respect to aroma, but differed in taste characteristics by sensory evaluation. Protein solubility in DSM was enhanced at weakly acidic conditions (pH 5.0), but did not differ from non‐deamidated soymilk at pH values of 3.0 and 7.0. Odor detection thresholds for the flavor compounds vanillin and maltol were approximately 5 and 3 fold lower, respectively, in DSM than in CSM. Dose‐response curves (Fechner's law plots and n exponents from Stevens's power law) further demonstrated that DSM had a lower flavor binding potential than CSM. PG deamidation has the potential to reduce flavor binding problems encountered in high protein‐containing foods and beverages. Practical Application: The findings of this study can help lead to the development of technology to produce protein‐containing foods with improved functional properties, especially protein solubility, and potentially decreased flavor fade problems associated with flavor‐protein interactions, especially with carbonyl containing flavor compounds.     

挤压对小米蛋白溶解性和分子量的影响

本文研究了小米在不同温度和物料水分条件下挤压后各蛋白组分溶解性差异和总蛋白电泳图谱的变化.结果表明：挤压以后水溶性、盐溶性、醇溶性、碱溶性蛋白的含量明显减少.经不同条件挤压后,61.7%～78.9%的小米蛋白需用SDS和SDS＋2-ME才能提取出来,仍有13.2%～22.5%的剩余蛋白.随挤压温度的升高,水溶性、盐溶性、醇溶性蛋白含量增加,而碱溶蛋白的含量减少.SDS可提取蛋白含量随温度和水分的升高而减少,而SDS＋2-ME可提取蛋白含量增加.物料水分变化对蛋白质溶解性影响较小.电泳图谱显示,挤压以后小米蛋白高分子亚基含量减少.醇溶蛋白亚基通过二硫键交链,形成2条新的谱带.也存在蛋白质通过其它共价键交链的可能性.疏水作用和二硫键是导致溶解性降低的主要原因.     

高压处理对大米蛋白溶解性及其分子特征的影响

研究了pH 8.0和pH 10.0条件下500 MPa高压处理对热变性大米蛋白溶解性的影响,并用Sephadex G-100色谱和SDS-PAGE分析了其分子特征的变化,用扫描电镜观察了大米蛋白的表观特征。结果显示,pH 8.0时500 MPa处理可使大米蛋白的溶解性由12.03%提高到19.15%,pH 10.0时高压处理则可由16.60%提高到24.87%。高压处理后的大米蛋白颗粒表面疏松,可使较大的蛋白质分子溶出,也产生了更小的蛋白质分子,且高压与非高压时溶出的蛋白质组分具有不同的紫外吸收特征。高压处理后的可溶性蛋白中均含有14、35 ku和少量22 ku亚基,非可溶性部分中除上述亚基外,还含有12、16、110 ku亚基。表明不同高压处理影响大米蛋白的溶解性能和分子特征,但对亚基的影响不明显。     

Evaluation of Gelation and Solubility of Bovine Plasma Protein Isolates

Gelation and solubility of bovine plasma protein isolates prepared from liquid plasma using different acids (HCl, H₃PO₄, H₂SO₄) and bases (NaOH, Na₂CO₃) were evaluated and compared to commercial plasma. Protein recovery was over 90%. Solubility at 1% (w/v) was low in pH range 4.0 to 5.5 but similar to commerical plasma in pp range 5.5 to 8.0 (p < 0.01). All isolates gelled at a concentration o 6.0%. Only isolate H₃PO₄-Na₂CO₃ and commercial plasma gelled at a minimum concentration of 4.0%. This concentration decreased to 2.4% for the same isolate when 0.025M Ca⁺⁺ ion was added.