Variability in marbling in Angus steers and an evaluation of subjective marbling assessment

Abstract

Currently there are no marbling grades within the New Zealand beef classification system, yet within some of our major markets (e.g., the United States and Japan) marbling is an important component of the grading system. This paper aims to examine the extent of marbling within Angus steers in New Zealand and relate these data to US and Japanese marbling grades. The amount of intramuscular fat (marbling) in the M. longissimus thoracis at the 12th rib site was estimated for samples from 259 Angus steers using NIRS. Carcass weight ranged from 220 to 540 kg and explained 66% of the variability in chemical fat. For carcasses in the 300 to 400 kg range, intramuscular fat content increased at average rate of 3.5 g kg−1 fresh weight of muscle for each 10 kg increase in carcass weight. Marbling was assessed in 118 ribeye steaks by 11 novice panellists using both the Japanese (JMGA) and United States (USDA) marbling grading standards and the scores obtained were compared against the predicted chemical fat content for each sample as estimated by NIR spectroscopy. Panellists assessed 8 to 13 steaks per session to determine between‐grader variability. Immediately after each session, steaks were re‐randomised and reassessed to determine within‐grader variability. There was sufficient between‐operator variability to indicate that graders should be trained or screened if carcasses are to be graded on the basis of marbling scores. The regression equations of mean panel scores on chemical fat content accounted for approximately 87% of the variation in panel means. Each unit increase in Japanese marbling score was equivalent to an increase in intramuscular fat content of 27 (±1.0) g kg−1. Each unit increase in USDA marbling score was associated with an increase in intramuscular fat of 30 (±1.1) g kg−1. Our results using novice graders differed substantially from earlier calibrations of intramuscular fat on marbling score using trained graders. The longissimus thoracis muscle of heavyweight Angus steers was estimated to contain intramuscular fat contents equivalent to JMGA marbling scores of up to 8 and USDA scores of 6, suggesting that New Zealand has cattle genotypes capable of producing well marbled beef.