Physicochemical, Microbiological, and Antioxidant Characterization of Tapai Ketan Hitam (Fermented Black Glutinous Rice) from West Sumatra at Different Fermentation Periods

Abstract

Tapai ketan hitam fermented black glutinous rice produced through the spontaneous mixed fermentation of glutinous rice (Oryza sativa var. glutinosa, black-pigmented variety) inoculated with ragi, a traditional Indonesian starter containing amylolytic molds (principally Rhizopus spp.), yeasts (Saccharomyces cerevisiae), and lactic acid bacteria is a culturally significant traditional fermented food widely consumed across Sumatra and Java. The black-pigmented variety, rich in anthocyanins (particularly cyanidin-3-glucoside) in the pericarp layer, endows the product with both a distinctive visual profile and potential antioxidant properties that distinguish it from tapai produced from white glutinous rice varieties. Despite its cultural importance, systematic time-course characterization of physicochemical changes, microbial community dynamics, and the evolution of antioxidant properties during tapai ketan hitam fermentation remains limited in the scientific literature. This study evaluated the effects of five fermentation periods 0, 24, 48, 72, and 96 hours at ambient temperature (28–32°C) on pH, total titratable acidity (TTA), ethanol content, reducing sugar content, texture (spreadability), color (L*, a*, b*), total microbial count, lactic acid bacteria count, yeast count, total anthocyanin content (TAC), total polyphenol content (TPC), and DPPH radical scavenging activity of tapai ketan hitam. A one-factor Completely Randomized Design with three replicates was employed. The optimum fermentation time was determined based on the integration of physicochemical safety parameters, maximal anthocyanin and antioxidant retention, and sensory acceptability evaluated by 30 semi-trained panelists. Results indicated that fermentation significantly influenced all measured parameters (p < 0.001). Ethanol increased from 0% to 3.2 ± 0.1% at 96 hours, TTA from 0.12 to 1.84%, pH decreased from 6.8 to 3.6, and reducing sugars from 2.1 to 14.8 g/100 g (peak at 48 hours). TAC declined from 184.2 mg/100 g (0 h) to 86.4 mg/100 g (96 h) as a result of anthocyanin degradation in the acidic fermentation environment. DPPH IC50 improved initially (best at 48 h: 42.1 µg/mL) before declining at 72–96 hours. Overall sensory acceptability was highest at 48 hours (4.2/5.0). Fermentation for 48 hours is recommended as the optimal duration balancing safety, functionality, and sensory quality for tapai ketan hitam.