Effects of fig and kiwifruit extract injection combined with ultrasound treatment on proteolysis and tenderness of low-grade Holstein beef

Objective: Comparative evidence for fig and kiwifruit extracts standardized to equivalent protease activity and combined with ultrasound treatment remains limited. Therefore, this study evaluated their effects on proteolysis, tenderness, and purge and cooking loss of low-grade (Quality Grade 3) Korean Holstein beef M. longissimus thoracis during refrigerated storage. Methods: M. longissimus thoracis from three Holstein carcasses (n = 3) was injected with double-distilled water, fig extract, or kiwifruit extract (each at 10 U/mL of protease activity) and treated with or without ultrasound (19.8 kHz, 600 W, 30 min), yielding six treatments, then stored at 4°C for 14 days. Myofibrillar fragmentation index (MFI), trichloroacetic acid-soluble peptide content (TCA-SB), protein patterns (SDS-PAGE), Warner-Bratzler shear force (WBSF), peroxide value, and cooking loss were measured on storage days 0, 7, and 14; purge loss on days 7 and 14; and muscle microstructure (scanning electron microscopy, SEM) and fatty acid composition on days 0 and 14. Results: Fig and kiwifruit extract injection increased MFI and TCA-soluble peptide content and reduced WBSF compared with the water-injected control. Ultrasound treatment further reduced WBSF in some comparisons during the early storage period, whereas this effect was not maintained at day 14. Fig and kiwifruit extract-treated samples showed lower peroxide values than the water-injected controls. Fig and kiwifruit extracts did not differ in WBSF, while ultrasound treatment was associated with higher peroxide values during storage. Conclusion: Fig and kiwifruit extract injection increased proteolysis-related indices and reduced WBSF compared with the water-injected control. The additional effect of ultrasound on tenderness depended on storage time.