Background 

Tenderness in its simplest form is described as how much force is required to bite through a piece of meat, but yet, it is complex as it is influenced by many tenderness contributing components. The overall perception of beef tenderness is dependent on all the tenderness contributing components as well as the interaction among these components, and evaluating 1 or 2 tenderness components does not provide the whole picture. One beef cut may excel in 1 or 2 of these tenderness components, but still failed to be perceived as tender due to failing one single tenderness component. Countless studies over the past 3 decades have evaluated the impact of various individual tenderness contributing components on meat tenderness, such as proteolysis, sarcomere length, muscle fiber cross-sectional area, fat content and fatty acid profile, collagen content and collagen crosslinks…etc. All the tenderness components described above have all shown to influence beef tenderness, but how much does each of these components contribute to a beef muscle’s overall percept tenderness? 

In a beef tenderness ranking study supported by the Beef Checkoff, longissimus lumborum, gastrocnemius and tensor fascia latae received similar rating in tenderness, but their overall tenderness is known be contributed by varying level of various tenderness contributing components. Based on their functionalities and known properties, we know longissimus lumborum (loin) is a muscle of support that exhibits high intramuscular fat, low collagen density and great aging response. Gastrocnemius (heel) is a muscle of locomotion which exhibits low intramuscular fat, high collagen content and unknown response to aging. Tensor fascia latae (tri-tip) functions in both roles of support and locomotion with limiting information on its biochemical properties. As these 3 cuts are categorized in the same tenderness group, but with distinct properties, they serve as the ideal model to evaluate the relative contributions of the proposed tenderness components on overall perception of beef tenderness. 

The objective of this study was to obtain preliminary data to better understand the contribution of each tenderness factor to the perception of tenderness of three specific beef muscles with similar tenderness rating.

Methodology 

Boneless beef striploin (NAMP #180), heel (NAMP #171F) and tri-tip (NAMP #185C) were collected from 10 USDA Choice beef carcasses (n=30) from a commercial beef processing facility in the Midwest and transported back to KSU Meat Laboratory. Steaks were from the anterior to the posterior end of each striploin and dorsal to the ventral end of each tri-tip and heel after 2 and 21 d of aging. Steaks from each aging period from each subprimal were assigned to one of 3 assays: 1) trained sensory analysis; 2) objective tenderness evaluation (Warner-Bratzler Shear Force); or 3) physiochemical analysis (sarcomere length, proteolysis, intramuscular fat content, collagen crosslink and content). 

All data were analyzed as a split-plot using PROC GLIMMIX of SAS (version 9.4, Cary, NC). The model included the whole-plot factor of meat cut, the sub-plot factors of aging time and the cut × aging time interaction. To determine the significance of the correlation between the tenderness contributors and overall tenderness (from trained panel) of each muscle, a multivariate regression model was constructed using PROC REG and the stepwise selection procedure, with variable required to be significant (P < 0.05) to remain in the final model.

Findings 

Biochemical composition of the 3 beef cuts are displayed in table 1. Tri-tip had the longest sarcomere, followed by heel and loin (3.01, 2.59 and 1.71 µm, respectively; P < 0.01). It was interesting to note that heel increased in sarcomere length from 5 to 21 d of postmortem storage (2.49 vs 2.70 µm; P < 0.05). Heel had the greatest relative troponin-T degradation %, followed by tri-tip and loin (68.10, 53.42 and 35.01 % respectively; P< 0.01). As expected, heel had the greatest collagen content, followed by tri-tip and loin (6.06, 3.98 and 2.76 mg/g of muscle tissue, respectively; P< 0.01). It was also worth noting that collagen content decreased for all cuts from 5 to 21 d of postmortem storage (4.64 vs 3.90 mg/g muscle tissue; P < 0.05). Out of the 3 cuts, heel had the highest total mature collagen crosslink density (0.20 mol/mol collagen; P < 0.05), while loin and tri-tip did not differ (0.13 and 0.15 mol/mol collagen, respectively; P > 0.05). It is important to note there was also an aging effect for collagen crosslinks. As collagen content decreased over aging, total mature crosslinks maintained its concentration, resulting in an increase in mature collagen crosslink density from 5 to 21 d of postmortem storage (0.14 vs 0.20; P< 0.01). Heel had lower lipid content than the others (2.68 %; P < 0.01), while tri-tip and loin did not differ in lipid content (8.24 vs. 6.99 %; P > 0.05).    

Loin was ranked by the trained panel to have the highest overall tenderness, while tri-tip and heel did not differ in overall tenderness (P >0.05; table 2). A multivariate regression analysis was conducted to quantify the relative contribution of each of the tenderness factor to overall tenderness evaluated by trained panelists (table 3 and 4). The equations indicated that each beef cut had a unique profile of tenderness contributors. Loin tenderness was driven by lipid content (P< 0.05); tri-tip tenderness was driven by collagen content (P< 0.05). Surprisingly, heel tenderness was driven by proteolysis (P< 0.01). Only collagen content may be casually used as an overall tenderness predictor for all 3 cuts.

Implications 

Our results showed that loin is an inherently tender cut with moderate proteolytic activity, high fat content, low collagen levels, but short sarcomere length. Tri-tip had long sarcomere length, great proteolytic activity, high fat content, and moderate level of collagen. Finally, heel had long sarcomere length, great proteolytic activity, but low-fat content and high level of collagen. Yet, tri-tip and heel were both rated to have similar overall tenderness. The one biochemical trait that could explain this phenomenon was that tri-tip had greater level of a specific mature collagen crosslink -deoxypyridioline compared to heel. The collagen characteristic is the least studied tenderness factor, but it may play the greatest role in meat tenderness regardless of cut. More research is needed to characterize collagen and collagen crosslinks to provide a solution to meat tenderness.