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Discusses effects of thermal processing on proteins, focusing on (1) the Maillard reaction; (2) heat denaturation of proteins; (3) aggregation, precipitation, gelation, and degradation; and (4) other thermally induced protein reactions. Also discusses effects of thermal processing on muscle foods, egg proteins, fruits and vegetables, and cereal grains. (JN)

Rapidly heated prerigor beef is tender because of incompletely described myofibrillar disruption and tissue fracture. This study was designed to evaluate the effects of heating rate on heat-induced myofibrillar shortening, ultrastructural changes, and fracture behavior in prerigor triceps brachii muscle. Rapid heating (2°C/2 min) to 53°C caused (P < 0·05) more severe myofibrillar shortening in a shorter time and at higher muscle pH and temperature, less muscle weight loss, and shorter sarcomeres than slow heating (2°C/12 min) to 47 or 53°C. Rapid heating caused more extensive degradation of A and I bands, greater loss of the tridimensional pattern of myofibrils, more fragmentation and melting of myofibrils, widened intermyofibrillar spaces, and maximum separation of fiber bundles as compared to slow heating. Slow heating caused extensive shortening but not extensive degradation and disruption of myofibrils. Muscles slowly heated to 53°C sustained greater loss of structural integrity than those slowly heated to 47°C, but fracture behavior was similar. Separation and fracture occurred near the perimysial/endomysial junction in all heated samples, but the perimysium remained affixed to the endomysium at one side of the interface in many rapidly heated samples. Longitudinal fractures showed a granular endomysium and large numbers of supercontraction nodes alternating with areas of sarcolemmal membrane fragmentation and fiber tearing in rapidly heated samples. Alterations of myofibrillar ultrastructure and fiber structure, and separation of bundles, may account for enhanced tenderness of rapidly heated prerigor muscle., (Copyright © 1992. Published by Elsevier Ltd.)

Forty-eight Yorkshire × Chester White crossbred boars, 28 days of age, were assigned to one of four diets differing in crude protein content. Dietary crude protein content was 23% initially and then adjusted to 14, 17, 20 or 23% at 9 weeks of animal age, and 12, 15, 18 or 21% at 14 weeks of age, respectively. The skatole concentrations in serum of the boars was measured at 24, 32 and 40 weeks of age. Results showed that serum skatole concentrations declined (P < 0·05) with increasing age (0·034, 0·027 and 0·021 μg/ml at 24, 32 and 40 weeks, respectively) and also decreased (P < 0·059) with increasing dietary crude protein concentration (0·033, 0·026, 0·022 and 0·021 μg/ml at 12, 15, 18 and 21% in finishing diets, respectively)., (Copyright © 1992. Published by Elsevier Ltd.)

Seven-day-old primary myotube cultures derived from embryonic chicken limb muscles were used to determine the effects of the beta-adrenergic agonist isoproterenol (ISO) on muscle protein metabolism in vitro. Isoproterenol increased (P less than .05) total protein accumulation after 2 h of acute exposure and after chronic exposure for 24 and 48 h. Isoproterenol did not consistently retard rate of protein degradation of the total protein (TP), myofibrillar protein (MFP) pools, and myosin heavy-chain subunit (MHC); degradation of these protein pools tended to be slowed by inclusion of ISO in the culture medium. After acute treatment of 1 X 10(-4) M ISO for 2 h, TP, but not MFP and MHC, synthesis rate was increased, and after chronic exposure to 1 X 10(-4), 1 X 10(-5), and 1 X 10(-6) M ISO, TP, MFP, and MHC synthesis rates and net accumulation of TP, cytoplasmic protein, and MHC fractions were enhanced (P less than .05). The beta-adrenergic antagonist propranolol (1 X 10(-5) M) blocked chronic stimulatory effects of ISO. Furthermore, after 48 h of exposure to ISO, effects on protein synthesis were less pronounced than those observed after 24 h of exposure. Isoproterenol imparted a more pronounced effect on protein synthesis than on protein degradation, indicating that increased muscle protein accretion observed in animals after ISO treatment is likely a function of enhanced protein synthesis.

Skatole-spiked boar serum was used to evaluate the degradation of skatole during frozen storage and to evaluate gas chromatography (GC) and high performance liquid chromatography (HPLC) for measuring serum skatole. Tubes of serum spiked with 1 ppm skatole which had been stored in a freezer (-15°C) for 4 months and tubes of fresh spiked serum (1 ppm skatole) were utilized. No differences (P > 0·05) between GC and HPLC determination in sensitivity and skatole recovery were observed. Measurements of skatole by GC and HPLC were highly correlated (r = 0·93). Both GC and HPLC revealed no difference (P > 0·05) between fresh spiked serum and stored spiked serum samples., (Copyright © 1991. Published by Elsevier Ltd.)

Super-stretched muscle fiber bundles (<10 min post mortem) were incubated at one of four combinations of temperature and pH (pH 5.5, 37°C; pH 5.5, 0-4°C; pH 7.4, 37°C; pH 7.4, 0-4°C) for 1 h prior to fixation and embedding for transmission electron microscopy. These tissues were compared with controls fixed immediately after stretching. Disruption of gap filament ultrastructure was greatest in those muscle bundles incubated at pH 5.5 and 37°C. It was determined that pH had a greater influence upon gap filaments than did temperature and that pH 5.5 caused more gap filament disruption than pH 7.4. Other structural alterations of the stretched muscle included more glycogen granule disruption at pH 7.4 and pulling of groups of thick filaments toward the Z-lines within a sarcomere. A pulling of alternate thick filaments toward opposite Z-lines was not observed. It was concluded that gap filament disruption may occur very rapidly in post-mortem muscle., (Copyright © 1985. Published by Elsevier Ltd.)

Muscle cell cultures were prepared from the leg muscle of 12-d layer and broiler chicken embryos. Cultures were then compared over a 10-d period from their capacity to differentiate into multinucleated myotubes and to synthesize and accumulate protein. Differentiation was qualitatively similar in the two cell types as evidenced by myoblast fusion that occurred rapidly during the first 2 d and remained essentially constant between d 3 and 10. However, several quantitative differences were observed. Even though the number of myotubes per culture was comparable between layers and broilers throughout development, layer muscle cultures usually exhibited a higher percentage fusion and more myonuclei per culture than broiler muscle cultures. Additionally, the nuclear density (i.e., the number of nuclei per myotube segment) was approximately 25% greater in layer cultures than in broilers between d 2 and 10 in culture. The rate of incorporation of 3H-leucine into total protein during pulse labeling experiments was comparable in muscle cultures of layers and broilers; however, broiler muscle cells accumulated approximately 40% more total protein per nucleus between d 6 and 10. Myosin heavy chain synthesis rate was higher in layer than in broiler muscle cultures, but broiler muscle cultures accumulated approximately 30% more myosin heavy chain than layers between d 6 and 10. The half-life of myosin heavy chain was 45 h in layer muscle cultures and 103 h in broiler muscle cultures. Thus, the capacity of broiler cells to accumulate more muscle protein was primarily due to a drastically slower protein breakdown rate.