From waste to valuable by-product: Polyphenols, fiber, and proanthocyanidins in Costa Rican guava (Psidium friedrichsthalianum [O. Berg] Nied) press cake

• Costa Rican guava pressed cake is rich in fiber, bioactive compounds and vitamin C. • Freeze-drying better preserves vitamins, polyphenols, and antioxidant activity. • The by-product contains low-DP B-type PACs, stable and bioavailable. • Valorization of guava press cake supports circular economy and food innovation. Costa Rican guava ( Psidium friedrichsthalianum [O. Berg] Nied) press cake (GPC) is an underutilized by-product of juice and pulp production with potential as a source of functional compounds. This study aimed to characterize the nutritional composition, polyphenolic profile, and antioxidant capacity of GPC and evaluate the impact of hot-air drying (AD) and freeze-drying (FD) on its bioactive components. Proximate analysis revealed a high dietary fiber content (70.00±1.50 g/100 g), moderate protein levels (5.74±0.04 g/100 g), and low-fat content (0.15±0.04 g/100 g). Organic acid and vitamin profiling showed higher retention in FD samples, with vitamin C concentrations exceeding 18,000 µg/g in the dried product. Phytochemical analysis indicated high contents of total polyphenol (52–56 mg GAE/g), flavonoids (1.5–2.2 mg QE/g), and soluble proanthocyanidins (6.4–9.2 mg ProA2/g), with FD samples consistently outperforming AD. Antioxidant assays confirmed strong activity, particularly in the FD samples. Mass spectrometry analysis revealed a predominance of low-degree polymerization B-type proanthocyanidins, which are associated with enhanced stability and bioavailability. These findings highlight Costa Rican guava press cake as a promising ingredient for functional food applications and sustainable valorization strategies within a circular economy framework.