Forty crossbred steers of similar birth date and fed the same growing-finishing diet were used to study adipocyte changes in six fat depots during growth from 11 to 19 mo of age. Steers were slaughtered at 2-mo intervals. Adipose tissue samples were obtained from kidney, mesenteric and brisket fat and subcutaneous, intermuscular and intramuscular fat from the 10th to 12th rib section. The osmium tetroxide fixation technique was used for determination of cell size and number. Except for three brisket fat samples, distributions of adipocyte diameters from six different fat depots were monophasic during the age range considered in this study. At 17 mo of age, the mean adipocyte diameter, in decreasing order, was: kidney fat greater than mesenteric greater than subcutaneous greater than intermuscular greater than intramuscular greater than brisket fat. Fat deposition during growth to 19 mo of age occurred mainly by hypertrophy of adipocytes. An apparent cell hyperplasia occurred in the intramuscular fat depot from 11 to 15 mo and in the brisket fat depot after 15 mo of age. Based on cellularity characteristics, evidence exists to classify intramuscular and brisket fat depots as late-developing ones. Cell number/gram of intramuscular adipose tissue was a better predictor of marbling score than was fat cell diameter.

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Rates of fatty acid synthesis from lactate and acetate and activities of selected lipogenic and NADPH-generating enzymes were determined in subcutaneous, intermuscular and intramuscular adipose tissues of cattle that were 11-19 months of age. Fatty acid synthesis from lactate and acetate increased from 11 to 13 months of age in subcutaneous and intermuscular adipose tissues; synthesis from lactate increased until 17 months of age. In intramuscular adipose tissue, synthesis from lactate also increased until 17 months of age while that from acetate continually increased. Activities of NADPH-generating enzymes increased in all three fat depots from 11 to 13 months of age, and little change occurred thereafter. Acetyl-CoA carboxylase activity was constant over entire growth period in all depots. Activity of ATP-citrate lyase increased from 11 to 13 months of age in subcutaneous and intermuscular adipose tissues, but did not increase until 19 months of age in intramuscular adipose tissue. In all cases, activities of ATP-citrate lyase were sufficient to support synthesis from lactate; therefore, lactate conversion to fatty acids in bovine adipose tissues seems to use the citrate cleavage pathway for generation of cytosolic acetyl-CoA.

Samples of subcutaneous, intermuscular and mesenteric adipose tissues from beef steers were fixed with osmium tetroxide, and freed adipocytes were counted with an automatic particle counter to determine whether a lognormal distribution function would describe adipocyte size distributions more accurately than a normal distribution function. Modes and medians of size distributions generally were larger than means for adipocyte size distributions modeled with a lognormal distribution function. Normalized third and fourth moments of predicted lognormal distributions often were close to 0 and 3, respectively, which are expected values for a normally distributed population. Considerable variation was observed in the skewness of adipocyte size distributions. Both normal and lognormal models for adipocyte size distribution yielded similar means. The lognormal model yielded a greater standard deviation than the normal model for adipocyte size distributions. Smaller chi-square values were found for size distributions modeled with a lognormal than with a normal distribution function. Results suggest that a lognormal distribution function more accurately models the size distributions of bovine adipocytes.

Twenty-seven bulls (10 Holstein, nine Charbray and eight Brahman) grown under grazing conditions were harvested at 31 to 35 months (598 kg) to study the effect of breed and muscle upon sensory characteristics of tenderness, juiciness and overall acceptance. Hindquarter samples from the psoas major (PM), semimembranosus (SM), semitendinosus (ST), longissimus dorsi (LD), rectus femoris (RF), gluteus medius (GM), and biceps femoris (BF) muscles were taken after a 24-h aging period. Shear force, sensory panel, and intramuscular fat (IF) analyses were performed. Beef quality characteristics were not significantly affected (P > 0.05) by breed, and were considered acceptable in tenderness as determined by Warner-Braztler (WBS) and Texture Analyzer (TA.XT2) shear force, and slightly tender, juicy, and acceptable by sensory evaluation. The seven hindquarter muscles of Brahman bulls had a higher (P < 0.05) content of intramuscular fat (1.2%) than those of Holstein (0.8%) and Charbray (0.9%). The PM was the most tender hindquarter muscle by WBS, TA.XT2, and sensory evaluation and the most juicy and acceptable (P < 0.05). According to WBS, ST was more tender than SM, LD, and BF (P < 0.05). At the same time, GM and RF were more tender than LD and BF (P < 0.05). Conversely, TA.XT2 determined that SM was similar in tenderness to BF (P > 0.05) but tougher than the other muscles (P < 0.05). Strategies to reduce the slaughter age of cattle grown under grazing and a better understanding of the mechanisms associated with the deposition of intramuscular fat might result in a greater palatability of the locally produced beef.